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Nanocrystalline Permanent Magnets Based on Hybrid Metal-Ferrites

Final Report Summary - NANOPYME (Nanocrystalline Permanent Magnets Based on Hybrid Metal-Ferrites)

Executive Summary:
NANOPYME has addressed the design and development of high quality ferrite permanent magnets without rare-earths as opposed to those magnets used in most of nowadays technological applications. Rare earth-based permanent magnets (RE-PMs) are used in a large number of nowadays technological applications. The rapidly increasing market of green technologies demands increased amounts of this type of magnets. This demand in combination with the strategically geographical distribution of REs (especially critical for heavy REs) make necessary the search of alternatives to these PMs in as many applications as possible.

NANOPYME has developed nanocrystalline ferrites-based magnets to compete with rare-earth magnets in a large portion of applications where the latter are simply used because standard ferrites do not fulfill the magnetic energy product required. For this purpose NANOPYME has made use of two complementary approaches. The first one is based on the possibility of achieving improved magnetic properties for ferrites different from those of bulk or microcrystalline materials by going to the nanometer scale. The achievements done have been translated to a second route allowing the synthesis of novel hybrid nanocomposites ferrites with the largest coercivity reported to date for isotropic ferrite powders.

Main achievements done in NANOPYME:
(a) Development of a new generation of rare earth-free permanent magnets based on ferrites.
(b) Successful scaling from the laboratory to the mass production under considerations of:
- Competitiveness.
- Safety.
- Recyclability (with development of a new procedure implemented in industry).
- Eco-efficient production.
(c) Proof of workability through the construction of two different types of motors integrating NANOPYME magnets:
(i) Electric motor for e-scooter.
(ii) Two stepper motors for high precision movement integrating magnets fabricated from: recycled ferrite material and nanocomposite ferrite powders with record coercivity values

New products that have been developed:
(i) NANOPYME magnets: Improved recycled ferrites and hybrid permanent magnets based on ferrites nanocomposites. In addition to the good permanent magnet properties, these materials have an additional advantage: by comparison with rare-earth magnets which need to be coated to avoid rapid oxidation, they are based on oxides and therefore they are not sensitive to air exposure. 1 patent has been submitted (P201600092) and 2 additional patents are under preparation.
(ii) Motors integrating NANOPYME magnets. This achievement means to cover the complete cycle from the starting materials to the final products for customize applications, and recyclability of intermediate as well as final magnetic materials.

NANOPYME success is based on the combination of individual expertise by the different participants and through access to research facilities of the different institutions. Flow of experts between institutions has been continuously done to optimize the different steps comprising the synthesis-processing-product chain.

Project Context and Objectives:
Three milestones were defined in the beginning of the NANOPYME project and all of them have been successfully addressed:
MS1: Preparation of state-of-the-art single-phase for hybrid ferrites-based materials (NANOPYME magnets).
MS2: Development of efficient methods for the manufacturing and scaled production of NANOPYME magnets.
MS3: Integration of NANOPYME magnets in an electric prototype motor.
The work in NANOPYME has been organized according to specific areas, each one comprising well defined objectives.
Synthesis and preparation of magnetic single-phase systems
The nanoscaling of the magnetic materials that constitute the magnets is a driving force to develop alternative rare earth free magnets. Common magnets are composed by micrometric grains which domain-wall drive reversal process occurs always below their characteristic maximum coercive field. This produces that their maximum possible energy product is never reached. In the case of nanoscale powders, the single domain magnetic structure increases theoretically the reversal field and hence improves the capability for magnets. Size effects induce in nanoscaled materials the improvement of the magnetic anisotropy and/or the magnetizations and/or the stabilization of novel phases. In this fashion the properties of standard materials employed for magnets can be improved when they are prepared in the nanoscale.
In the specific case of ferrites, these promising ideas take a particular value if the improvement of the properties were reached in Sr hexaferrites, that constitutes around 1/3 of the value of the market in magnets production. The nanoscaling of these ferrites under classical mechanical processing exhibit the drawback of their decomposition, the growth of secondary phases and the induction of magnetic disorder that reduce the magnetic performances and hence limit both the applicability but also the research.
One of the first objectives in NANOPYME has been the production of single-phase ferrite nanopowders with suitable nanostructure and magnetic properties for further processing as magnets with magnetic properties superior to the commercial ones and to be used in the development of hybrid spring magnets. Objectives related to the research of improved single-phase ferrites have comprised the preparation and study of:
1. Competitive single-phase Co-ferrites.
2. Competitive single-phase Sr-ferrites.
3. Single-phase soft materials for further processing.
Partners in the consortium have collaborated to develop a wide synthetic effort to obtain these nanomaterials exploring standard and novel processing methods for the transformation of commercial powders. Moreover alternatively novel chemical synthesis routes have been employed to produce these nanopowers with fine a tuning of the final properties.
Hybrid nanocomposite magnets
Once optimized single-phase ferrites through proper control of morphological, microstructural and magnetic properties, NANOPYME has focused efforts in the research of the exchange coupling phenomenon of a 'hard' magnetic material having large anisotropy (coercivity, Hc) with high-magnetization 'soft' magnetic species. This constitutes one of the prevalent approaches towards the goal of improving the performance of permanent magnets. As long as the grain size of the magnetically soft phase remains below a certain threshold, exchange coupled hard/soft nanocomposites hold the promise of increasing the energy stored in a permanent magnet. According to early models, soft contents up to 90% could lead to enhanced energy products. However, in reality, an increasing amount of soft phase often leads to a fast decay in coercivity that becomes the limiting factor. The majority of the published reports on polycrystalline samples share a common strategy. The exchange coupling at the hard/soft interface is sought to be maximized and the soft phase content and microstructure is subsequently optimized.
The technological relevance of next generation permanent magnets relies on our ability to enhance the energy product in polycrystalline nanostructured composites whose production is potentially up-scalable. Unfortunately, further complications arise when dealing with polycrystalline nanocomposites. Parameters such as grain shapes, crystallite size distribution or relative orientations of crystallites, which are difficult to quantify and control, play a decisive role. In particular, the structural requirements associated with the effective intergrain coupling, such as interfacial coherency and sizes of soft grains of the order of a few nanometers, are often hard to meet. In this framework, the main objectives dealing with the synthesis and study of ferrites-based nanocomposite permanent magnets have been:
1. Achieve a homogeneous dispersion of the hard and soft phases.
2. Fabricate hybrid composites while controlling their structure.
3. Fabricate fully dense magnets with improved properties.
4. Establishing a deeper understanding of the physical processes involved in hard-soft composites.
Structural and magnetic characterization
Successful achievement of the mentioned objectives could only be managed by establishment of a proper correlation between:
(i) morphology;
(ii) microstructure;
(iii) magnetic properties.
The goal has been to understand the relation between magnetic properties, including size and shape effects in order to predict preparation procedures resulting in magnets with optimized properties.
A full battery of microstructural and morphological characterization techniques have been used in NANOPYME: powder X-ray diffraction, synchrotron X-ray powder diffraction and neutron powder diffraction; high resolution X-ray powder diffraction using SNBL (Swiss-Norwegian Beamlines) for extraction of not only size parameters, but also strain; MAX-lab in Sweden together with PETRA-III in Germany used for in situ studies giving insight into the formation of nanoparticles during hydrothermal synthesis and compaction at various pressures and temperatures. Powder Neutron Diffraction (PND) played a crucial role as neutrons have a magnetic moment, and allows scattering from the atomic magnetic moments in the material. Small Angle Neutron Scattering (SANS) was employed to investigate structural and microstructural features in the range from 1 to 1000 nm. The SANS investigation were carried out at different magnetic fields (0 T < H < 0.5 T). Microstructural characterization was completed by means of high-resolution transmission electron microscopy (HRTEM) complemented by studies using Focused Ion Beam (FIB) and Scanning Electron Microscopy (SEM). Dual FIB/SEM equipment allowed depth profile characterization at the micro- and nano-scale with simultaneously, microstructural and morphological analysis of the sample. Pole figure measurements using X-ray radiation has also been used to established the texture of the microstructure through the orientation distribution function (ODF).
Magnetic characterization has included advanced magneto optical Kerr effect (MOKE) studies for the study of model systems (thin films), magnetometry techniques (VSM) and SQUID and magnetic force microscopy (MFM).
Mechanical and corrosion tests
Mechanical and corrosion tests have been carried out to ensure the integrity of the produced magnets. Various tensile and bending tests have been carried out to assess the best compaction produces with respect the mechanical properties. Likewise was highly accelerated stress test (HAST) and salt spray test performed to established the chemical stability of the prepared magnets.

Simulations and modelling
Theoretical work was performed in close cooperation with the experimental studies focused in single-phase and nanocomposite magnets. The output of this work was also used by computing main characteristics of the final prototype.
In detail, simulations were connected to other activities in NANOPYME in the following way:
1. Within the synthesis and preparation of magnetic single-phase systems, various single-phase magnetic systems - in particular, magnetically 'hard' Sr- and Co-ferrites and magnetically 'soft' nanocrystalline and nanoparticle materials - have been prepared. Magnetic parameters (saturation magnetization, exchange constant, anisotropy type and anisotropy constant) and structural characteristics (e.g. film thickness) of the synthesized materials and corresponding thin films have been used as input for numerical simulations.
2. The same applies to the interconnection with the preparation and study of hybrid nanocomposite magnets and related mate¬rials processing. Experimental data obtained by the fabrication and characteriza¬tion of two-phase materials, have been used as input for the simulations, where magnetic and structural parameters of bilayers were used. Also very important information for simulations was the size and shape of hard and soft nanocrystallites. Inversely, simulation results concerning the optimal size and shape of 'hard' and 'soft' nanograins and the optimal values of intergrain exchange between the 'hard' and 'soft' phases (recommended to achieve the highest energy product) could be used by experimental groups for improving the fabrication procedure in order to obtain better nanocomposite materials.
3. Interaction between simulations and experimental measurement of magnetic properties was organized so that simulation predictions concerning magnetic properties of synthesized materials could be verified experimentally and, using this information, numerical models could be improved in order to include more relevant features of real materials.

Prototypes and testing in working conditions
Magnets fabricated by different routes from the magnetic materials produced by the project partners have been appropriately tested in view of practical applications. This has been done in NANOPYME by integrating magnets in a small prototype motor, which will be additionally evaluated in terms of efficiency against a similar motor containing the more expensive rare earth based magnets of a similar maximal energy product class. To ensure that the motors have realistic application and hence specification, the motors has been designed for an electric scooter/moped. An electric scooter/moped motor was chosen such that a full-scale machine can be constructed, and because it has very similar requirements to direct drive wind turbine generators, namely low rotational speeds and high torque densities. These two applications have been identified as applications with highest need for magnet materials and thus highest value for the success of NANOPYME.
Main objectives related to this activity have been:
1. Fabrication of NANOPYME magnets for prototypes.
2. Design and construction of prototype electric motor using NANOPYME magnets.
Production scale
The key issue considered when translating laboratory and pre-industrial procedures from the lab to the factory has been adaptation and optimization of existing resources (procedures and capabilities at the company) to the NANOPYME raw material.
The specific objectives related to production scale have been:
1. Production of commercial quantities of NANOPYME magnetic materials under reasonable cost conditions.
2. Introducing an up-scaled process for the new magnetic materials at a traditional rare-earth-based manufacturer of permanent magnets.
3. Production of magnets in an environmentally friendly procedure.
NANOPYME magnets integrated in the motorbike prototype have been fabricated in one of the companies of the project by using the industrial equipment and have been manufactured following a process based on existing one used for common ferrite magnets.
Safety and Recycling
There are many activities within Europe to identify all risks and well define the safety limits for manufactured nanoparticles. Nanoparticles may cause hazards to human health and environment. These hazards need to be assessed through the examination of the materials -ferrites and metallic particles- with the advantage that these materials are well known and used in the permanent magnet industry since long time. Size effects are more relevant when submicrometer scale is achieved.
In addition focus on the processes for manufacturing magnets is required; the most common is the powder metallurgy. This process consists on several steps, from powder, compacting, heating and sintering the powder. Studying the manufacturing process of ferrite magnets and the possibilities, the objective has been to produce the magnets in an environmentally friendly process with recycling being an intrinsic part of the entire procedure with the NANOPYME powder.
Based on these premises, the following objectives have been considered:
1. Identification of potential hazards.
2. Development of protocols to ensure workers safety.
3. Fabrication of magnets from production wastes.

Project Results:
[Note: please, refer to PDF file for complete document including Figures and Tables. List of Figures and Tables are provided at the end of the text]
Synthesis and preparation of magnetic single-phase systems
1. Growth of high quality single phase nanometric films
High quality Fe, Co and Fe3O4 soft magnetic materials and Co-ferrite and Sr-ferrite hard magnetic materials were deposited as nanometric films with controlled crystalline orientation, with defined thicknesses and onto different substrates to simulate different interface strains. The knowledge was employed to grow model bicomponent layers and to investigate experimentally the exchange bias process and corroborate the theoretical studies.
2. Nanostructurally improved Sr-ferrite commercial powders by milling
Different ball-milling based processing methods followed by heat treatment applied to Sr-ferrite commercial micrometric powders were determined that give rise to the submicrometric particles with homogenous size distribution and good magnetic properties. Moreover hydrothermal synthesis was employed to obtain directly the nanometric powders. In particular:
1. Ultrafast (dry and surfactant assisted) ball milling for times as short as a few minutes.
2. Cryomilling in air using milling times ranging from a few minutes to several hours.
3. Reactive planetary ball milling under H2 (ca. 20 bar).
4. Hydrothermal synthesis using spiral or autoclave reactors.
Nanopowders produced by the ultrafast high energy milling (particularly case 1) give rise to the smaller average particle size (around 200 nm) and homogeneous particle size distribution. Hydrothermal synthesis (case 4) produces always platelets with an average diameter of 1 micron but the thickness is of around 10 nm. Isotropic nanopowders obtained using the cryomilling technique with long milling times (case 2), reactive milling technique (case 3) and dry ultrafast high energy milling case (case 1) present a homogeneous and fine microstructure while preserving magnetic properties similar to those of the commercially available powders ((BH)max=12 kJm-3). These last materials were used to sinter or develop novel magnets and/or for hybrid magnets, respectively.
Co-ferrite nanoparticles with top of state-of-art energy product for novel magnets
Several preparation methods give rise to Co-ferrite nanoparticles with an energy product above of the actually maximum published (BHmax= 5 kJm-3 reported in Cabral et al. in IEEE Trans. Magn. vol. 44 (2008) pg. 4235) and HC larger than bulk materials at room temperature.

I. Series of Co-ferrite powders obtained employing high energy ball milling with powders synthesized by sol-gel or co-precipitation routes.
I.1 Pre-annealed co-precipitated Co-ferrite oxides (heated at 1000 °C per 1 hour) milled during only 3 min provides nanoparticles with the largest HC= 4.7 kOe, MS=58,6 emu/g and a high BHmax =10.4 kJm-3 [Fig. 1].
I.2 Pre-annealed co-precipitated Co-ferrite oxides (heated at 1000 °C per 1 hour) milled during 3 min and performing a further post annealing at 600 °C during 1 hour result in nanoparticles with the largest (BH)max=18.1 kJm-3. This high (BH)max is the result of combination of magnetic properties: MS=77.0 emu/g, Mr= 47.9 emu/g, HC= 2.4 kOe [Fig. 1]. SEM images shown in Fig. 2 illustrates the refinement in the microstructure induced during milling times as short as 30 and 180 sec. An increased grain size has been observed with increasing the heating temperature during annealing of the milled powders [Figs. 3(d)-(f)].
II. Direct synthesis by thermal decomposition of particles with sizes between 20 to 60 nm. Best properties were measured in octahedral shaped nanoparticles with 40 nm average size prepared using oleic acid solvent at the temperature of 270 °C. The nanopowders exhibit MS= 80.9 emu/g, HC= 1.6 kOe, and (BH)max= 12.3 kJm-3.
Series of Co-ferrite powders obtained by high-energy SPEX shaker ball milled and by cryomilling of oxides prepared from sol-gel or co-precipitation routes and considering different thermal treatments exhibit (BH)max larger than 10 kJm-3 and HC larger than 2 kOe.
Ferrite-based nanoparticles with over Tesla coercive fields
ε-Fe2O3 nanometric particles were dispersed in SiO2 matrix synthesized by chemical route. This was an exploratory route in the project to manage very large coercive materials. It was successful and the obtained composite exhibits a huge HC of 21 kOe (2.1 T) measured at room temperature. The drawback was the low saturation magnetization (5 emu/g). The calculated saturation magnetization of the bare ε-Fe2O3 nanoparticles is of the range of 20-25 emu/g.
Fine size single-phase nanopowders candidates for NANOPYME spring magnets
Hard and soft submicron particles, below the single domain range and with homogenous particle size distribution, were produced by different routes with the aim to prepare hybrid spring magnets. The selection of the preparation method and materials considers the nanostructure characteristics (particle size and homogeneity), magnetic properties (magnetization, coercive field) and also the easy scalability production into the project. In particular were delivered the next materials:
I. Co-ferrite nanoparticles with particle sizes around 11 nm prepared by hydrothermal synthesis in stain-steel autoclave with further thermal annealing. These nanomaterials were subject to total or partial hydrogen-induced reduction to get soft CoFe2 nanoparticles or Co ferrite - CoFe2 spring magnet nanocomposite.
II. Sr-ferrites platelets with thickness of few nanometers and diameters below the micron prepared by hydrothermal synthesis in stain-steel autoclave and further annealing at 1000 ° C.
III. Soft submicron powders prepared by the cryomilling during 5 hours of preheated nanocrystalline Fe rich (FeSiBNbCu) amorphous ribbons (550 °C during 1 hours in N2). The particles distribution of these powders was between 200 nm and 4 micron.
Short time milling as nano-structuration process
Ultrafast mechanical milling allows the decreases of the particle size employed of commercial Sr- ferrite and synthesized Co ferrite powders to the submicrometric range in very short scale of time, 1-5 minutes, differently of times required in all the other ball milling techniques (planetary, mixing, cryomilling or reactive milling) that need hours to reach (or does not reach) similar size reduction (Fig. 3). Moreover this fast milling reduces the probability of the oxide decomposition.

Homogenous nanometric ultra soft powders
Wet milling with oleic acid or cyomilling of pre-heat treated Fe-rich or CoFe-rich (FeCoSiBNbCu) ribbons allow to obtain polydisperse powders with minimum particle sizes of 200 nm and 500 nm respectively and microns of maximum particle size. The soft magnetic properties were conserved even the particle size reduction: magnetization saturation around 140 emu/g for Fe-rich ribbons and 200 emu/g for FeCo-rich ribbons and minimum coercive fields of 3 to 140 Oe and around 40 Oe, respectively.
General progress in understanding the effects of the procedure to modify the structural and magnetic features of nanomaterials considering novel milling techniques and materials.
The study has allowed to understand and rationalize the correlations between the preparation routes and the structural and magnetic properties for each class of materials. Key aspects of the understanding are:
- Ball milling techniques usually require a post-milling annealing to recuperate the magnetic performances of the material. Both milling and annealing processes give rise, with a different weight, to changes in the particle/grain size, morphology and in the crystal phase (secondary phase growth or amorphization) that depends on the considered material, hence a balance between milling conditions – thermal treatments is always required.
The milling of Sr- ferrite powders produces in general the particle size decreasing but also the amorphisation of the oxide or growth of secondary phases that may ruin the magnetic properties. These changes depend critically of the milling processes and energy that were used. Thermal annealing will recuperate crystallinity, will improve morphology (more rounded particles) but will increase particle size and the growth of other secondary phases.
In the Co-ferrites the behavior is different: the milling decreases the particle size, and the structural damages decreases magnetization but increases stress-induced magnetostriction anisotropy and increases energy product. Thermal treatment improves the crystallinity but decreases the stress induced anisotropy and magnetic performance.
Metallic amorphous ribbons have to be nanocrystallized by thermal annealing to get an efficient particle size reduction by milling procedures. Smaller particle size is got with cryomilling because in this technique the agglomeration and welding processes are reduced. Annealing allows the fine tuning of the magnetic properties, but high temperature (>1000° C) should be avoid to limit the welding process.
- The use of surfactants in all the considered milling techniques/materials produces particles with a more homogeneous particle size distribution than the dry route, which is more efficient in the size reduction. Moreover, wet milling reduces the amount of agglomerates in the resulting powders.
-The elimination of the secondary non magnetic phases, like SiO2 and AlO2, present in some of explored materials, like the metallic micro-wires or the ε-Fe2O3 phase, is not completely reached using chemical etching techniques and often it damages the magnetic materials.
Hybrid nanocomposite magnets
Improvement of energy product in isotropic composites
A remanence increase of over 20% and (BH)max increase of over 30% have been achieved in hard-soft isotropic composites based on hexagonal strontium ferrite (SrFe12O19) and a soft metallic phase (FeSiB) with micrometric particle sizes. The best results have been obtained using annealed compositions of 5%pre-milled ribbons-95%- SrFe12O19, where (BH)max enhancement has been observed for the first time in hexaferrite-metal composites. The values of the magnetic parameters associated are very competitive: Composite values: Ms (5T)=82 emu/g; Mr=45 emu/g; Hc=3.3 kOe; (BH)max = 15 kJ/m3.
The structural characterization confirmed an homogenous dispersion of both phases and that average particle sizes are in the micrometer range, which is well above the threshold for effective exchange coupling. The question then is: why do we observe an improvement of the energy product?
The platelet shape associated with the hexagonal structure of the ferrite favors a relative orientation of the easy-axis of the hard phase with respect to the soft particles that supports a substantial degree of alignment towards the applied field direction after its removal –see Fig. 4.
Instead of the hard-soft coupling, the magnetostatic interactions within the material improve the magnetic properties. Besides the very competitive (BH)max values obtained, this constitutes a very important result as it represents a paradigm shift with respect to the prevailing approach that seeks to maximize and exploit exchange-coupling in composite hard-soft systems. The advantages of this approach are: (i) the fact that the absence of coupling preserves coercivity and (ii) the technologically very important circumstance that the structural conditions associated with exchange-coupling do not need to be fulfilled anymore.
This result has led to the application of a patent (P201600092).
Improvement of energy product in anisotropic dense magnets
The best (BH)max values were obtained in 90%Sr-ferrite-10%Co-ferrite composites. Instead of using commercial ferrite, sub-micron-sized SrFe12O19 was produced by hydrothermal synthesis in a stainless-steel autoclave. The sintering method employed was Spark Plasma Sintering, which allows reaching densities up to 94 % of the theoretical one after only a few minutes of sintering (typically between 1-5 min) by applying pressure while sintering occurs. The hydrothermally synthesized particles are platelet shaped with relatively high aspect ratio, which in combination with the applied pressure leads to a partial particle alignment that enhances the squareness of the hysteresis loop (see Fig. 5) and thus (BH)max.
The energy product reached is 32 kJ/m3. This value is comparable to existing commercial ferrites, however, it is important to remark that it is reached by using an advanced sintering method that produces dense magnets after only a few minutes. When compared to conventional sintering techniques that usually require hours of oven operation, it is clear that this result enables the possibility to fabricate ferrite magnets at a reduced energetic cost. In addition, the hydrothermal synthesis method developed in the project constitutes an alternative production method that is up-scalable and cost-efficient. This represents an alternative that reduces the European dependence of exporting ferrites from Asia.
Improvement of coercivity in oxide composite materials
Additionally, we have successfully synthesized Sr-ferrites nanocomposites by ball milling that show a significant increase in coercivity, reaching 6kOe (a 38% increase with respect to commercial ferrites). This coercivity value is, to our knowledge, the largest coercivity reported for Sr-ferrite isotropic powders. This new product is at the moment under patent application. It is worth recalling that for some applications, a large coercivity might be desirable (over a large energy product). Figure 6 shows a comparison of the (second quadrant) hysteresis loops for low-grade (so-called Y30BH), high-grade (UHE10) and NANOPYME nanocomposite isotropic powders.
Enhancing energy product and understanding coercivity mechanisms in hard-soft magnetic composites
Interfacial exchange-coupling is known to improve the permanent magnetic performance in composites of magnetically hard and soft particles. The prevailing strategy, employed in a plethora of compositions, consists in maximizing the coupling between the hard and soft phases and optimizing material parameters such as particle size or phase composition. In CoFe2O4-FeCo nanocomposites, we have experimentally shown that imperfect coupling in combination with the sizes of the soft phase grains below the single-domain threshold leads to enhanced magnetic properties at room-temperature, while maximizing exchange-coupling implies a collapse in coercivity and hence - in the maximal energy product. The results have been corroborated by micromagnetic calculations and the origin of the exchange-induced softening has been understood. Up-scaling towards industrial scale production often implies sacrificing control over the nanostructure, which increases the probability of obtaining sizes of magnetically soft grains above the critical value and interfaces that are not crystallographically coherent. Both effects act against effective exchange coupling. Our results bring forward an appealing alternative to the approach that seeks to maximize the coupling in exchange-spring composite magnets, given the strict requirement that exchange coupling should be as strong as possible does not necessarily need to be fulfilled anymore. This opens opportunities in the development of next generation rare-earth-free permanent magnets for industrial applications.
We emphasize that engineering interfaces in order to optimize, rather than maximize, the degree of exchange coupling is a necessary requirement to improve the energy product in nanocomposite magnets and to successfully develop advanced rare-earth-free permanent magnets. This constitutes an extremely relevant scientific result as it will entail drastic changes in the approach employed to increase energy product in hard-soft composites.
Unravelling the magnetic domain structure of highly-perfect ferrites
We have discovered a route for fabricating highly-perfect cobalt ferrite magnetic nanostructures by combining in-situ low-energy electron microscopy (LEEM), x-ray absorption (XAS), x-ray magnetic circular dichroism in a photoemission electron microscope (XMCD-PEEM) and atomic force microscopy (AFM). The individual ultrathin islands obtained are atomically flat at the surface and, precluding from a single nucleus, free from antiphase boundaries. The extremely low defect concentration leads to a robust magnetic order -even for thicknesses below 1 nm- and exceptionally large magnetic domains. Figure 7 shows the corresponding images. The exceptional level of detail of the characterization and the highly structural perfection of the material allows correlating the location of magnetic domain walls and the presence of structural features such as atomic steps.
This highly innovative result represents a considerable step forward in the possibility to fabricate advanced materials and devices based on ferrites and has been published in Advanced Materials, a high impact factor journal.
Structural and magnetic characterization
One of the premises during the complete duration of the project has been the establishment of a correlation between microstructure and magnetic properties in all materials (thin films, powders, bulk and compacted magnets) produced in NANOPYME.
Spark plasma sintering (SPS) is a modern sintering technology, which has been successfully applied in compaction of SrFe12O19 nanoparticles. Compared with the conventional sintering, high density compacts with improved mechanical and magnetic properties were achieved in very short time using the SPS apparatus shown in Fig. 8 together with the resulting pellets.
The sintering technique is based on passing a pulsed DC current through a graphite die containing the powders, while applying a uniaxial pressure. The current causes internal heat generation at the grain boundaries of the powders, facilitating high heating rates, and therefore minimizing the sintering time and avoiding excessive grain growth of the compacted nanocrystallites.
For the comparison with the cold pressed pellets two pellets were pressed, with and without applied magnetic field, before heating the sample. The two prepared pellets were again cut into two smaller pieces, with one annealed at 850ºC for 2 hours and one unaltered. The samples are named SPS.950 and SPS.950.0.5 where the 0.5 refers to the applied field of 0.5T and ‘a’ in front of the name signifies that the sample was annealed. The hysteresis curves can be seen in Figure 9.
The microstructure can be characterised by measuring pole figure data. In the pole figure measurement the specimen of interested is oriented with different angles of the surface normal with respect to the scattering vector. By varying the orientation it is possible to change the angle between the reciprocal lattice vector and the scattering vector. Figure 10 illustrates the general idea behind the measurements.
As for the cold pressed samples a pole figure measurement was carried out for the reflections (008), (107), (114) and (110) for the unmagnetised sample. The pole figure is shown in Fig. 11 and extracted particle alignment is shown in Fig. 12.
It clear that the 950ºC SPS pressed sample has significantly higher alignment. Almost 60% of the sample is aligned with the first 30º, while for the cold pressed the alignment amounts to about 40% of the sample within the first 30º in comparison a random orientation sample would 12% of the sample aligned along the [00l] within the first 30º. The larger alignment clear improves the remanence magnetisation of the high temperature SPS pressed samples.
Correlation between magnetic characterization and simulations: validity of Henkel plot for nanocomposites
Henkel plot (dependence Jrm vs. Jrd - see below) is often claimed to be a reliable method for the characterization of interaction in various fine-particle and composite magnetic systems basing on the deviation of this plot from the straight line.
The scheme for obtaining this plot is presented in Fig. 13. First, a demagnetized system (Mav = 0) is magnetized applying some external field value Hm. Afterwards this field is reduced to zero and the corresponding remanence value Jrm(Hm) is saved (red path in Fig. 13). This process is repeated starting every time from a demagneti¬zed state and increasing the value of Hm. This way the dependence for the first kind of the remanence on the magnetizing field - Jrm(Hm) - is obtained.
Second (blue path), a similar procedure is applied to the system that is first saturated by the large applied field and then demagnetized by applying a reverse field –Hm. Afterwards, the applied field is removed, so that system is again in a remanent state, but this state is different from that obtain via the 'red' path described above. The remanent magnetization achieved this way is denoted as Jrd(Hm). By definition, the Henkel plot is a plot Jrm(Hm) vs Jrd(Hm).
It is straightforward to show, that the plot Jrm(Jrd) is a straight line, namely, Jrd(Hm) = Jr - 2Jrm(Hm), where Jr is the 'standard' remanence shown in Fig. 13, if the following three criteria apply:
- particles constituting the system do not interact
- these particles possess the uniaxial anisotropy
- the anisotropy axis directions of the particles are randomly distributed in 3D
Magnetodipolar or exchange interactions between particles (e.g. between crystallites in nanocom-posites) in any real system lead to the violation of the first condition, so that the Henkel plot beco¬mes either a concave or convex curve. Basing on the sign of the deviation of this plot from the straight line, the interactions in the corresponding system are usually characterized as 'magnetizing' or 'demagnetizing'.
The major problem by this method is that the presence of interaction is by far not the only reason for the deviation of the Henkel plot from the straight line: such a deviation occurs also for a non-interacting system if one of the two last criteria is violated. For example, if we use the cubic anisotropy instead of the uniaxial one, the Henkel plot looks principally different (see Fig. 14). This result rises very strong concerns of using Henkel plot for the characterization of magnetic interactions in complicated nanocomposite systems.
In light of these theoretical results and experimental evidences obtained in NANOPYME, clear challenges appear when magnetically characterizing and validating the extent of exchange-coupling in hard-soft hybrid systems. Thus, NANOPYME has defined and proven a magnetic protocol for the confirmation of exchange-coupling in hybrid hard-soft permanent magnets (shown in Fig. 15).
Mechanical and corrosion tests
A. Mechanical tests
Mechanical tests of the magnets compacted in NANOPYME have been carried out to guarantee quality of magnets under practical applications. Figure 16 shows the device used for mechanical tests and an image of compression test done on one of the compacted pellets. The mechanical strength for the compacted disc magnets consists in a compression test in a perpendicular direction with respect to applied force during compaction.
By the compression tests, and as seen in data represented in Fig. 17, the conclusion is that the magnets prepared from ball milled powders in the shape of pellets show the best mechanical properties among the different processed materials studied. Magnets shape is also important and that is the reason because project considered realization of tests on magnets with identical shape to those magnets to be used in real applications. Figure 18 shows bending test carried out on a magnet (Fig. 19) prepared from recycled powders.
The values for the bending show that the SFO_CUT BM500 sample is more resistant because the load to fracture is bigger (Fig. 20), but less ductile behaviour (Fig. 21).
B. Corrosion tests
Rare-earths-based magnets lose their magnetic properties as well as structural stability with corrosion. Therefore their resistance to corrosion is a very important factor. The magnet producers as well as the automotive industry usually use two tests in order to acquire data for corrosion resistance. These two tests are the HAST (Highly Accelerated Stress Test) and the Salt spray test.
For the corrosion tests in the project the following setups were used:
- HAST: 130 °C, 95% Humidity, 2.7 bar, 100 hours.
- Salt-spray test: According to ISO 9227 standard (specific salinity of the vapour, 96 hours)
Benchmark corrosion testing was done on ferrite magnets as well as, for comparison, on coated magnetic pieces prepared from commercial neodymium based powders (Figs. 22-25). As expected, the latter showed higher levels of corrosion, especially when subjected to Salt-spray test, while the ferrite magnets seemed to be unaffected by the HAS as well as the Salt-spray test.
The mass gain in neodymium magnet is small (0.03 weight %), but the corrosion has also affected the coating applied on the magnets. The ferrite magnets showed neither the mass gain nor the visual change of the surface.
Figure 26 shows hysteresis loops of a Sr-ferrite NANOPYME magnet (high coercive ferrite nanocomposite) before and after corrosion tests. Room-temperature magnetic measurements have revealed that ferrite magnets produced within the NANOPYME project preserve their magnetic properties
Simulations and modelling
A. Simulations of two-phase (hard/soft) films with nanocrystalline structure
A special tool created in the project has enabled us to simulate both single-phase and nano¬composite films consis¬ting of soft and hard phases, with arbitrary volume fractions (non-magnetic inclusions can also be present); both phases can have arbitrary average grain sizes - see Fig. 27.
Two-phase (ferrite/Fe) films have been simulated, because they could assist in understanding the behavior of bulk composi¬tes. In order to find out the key system parameters for achieving the highest Eprod, we have simulated hysteresis loops varying the following parameters:
a) Structural parameters: (i) film thickness h, (ii) volume concentration of the hard phase ηhard and (iii) average grain size of the hard phase Dav
b) Magnetic parameters: (i) exchange weakening on the grain borders, (ii) exchange stiffness constant of the soft phase; both these parameters can be varied experimentally using nanocomposite films of different purity grades. Other magnetic parameters have not been varied.
We have shown that these parameters have the follo¬wing effect on the energy product Eprod:
1. Effect of structural parameters:
• (BH)max decreases with increasing the film thickness h (about 25% decrease by increasing h from 20 to 50 nm) due to the decrease of Hc. The most probable reason is that the magneti¬zation M of the soft phase (Fe) in this system is much larger than M of the hard phase, what leads to the redistribution of the internal dipolar fields when the film thickness is increased.
• As the function of the hard phase fraction ηhard, (BH)max demon¬strates a pronounced maximum at ηhard ≈ 30 – 40 % (Fig. 28), resulting from interplay between the coercivity increase and the remanence decrease (note that M(ferrite) < M(Fe)) with increasing ηhard.
• By varying the average grain size of the hard phase in the region 10 < Dav < 100 nm we have observed the maximum of Eprod for Dav ≈ 50 nm. Decrease of Eprod towards smaller Dav is due to the better self-averaging of the hard grains anisotropy, while the increase of Dav above 50 nm leads to inhomogeneous magnetization of a single hard grain, what also decreases Eprod.
2. Influence of magnetic parameters:
• Intergrain exchange weakening κ leads to the increase of (BH)max. The reason is the more ‘cooperative’ magnetization reversal for larger exchange (Fig. 29), what results in a more rectangular shape of hysteresis loops. Interestingly, also Hc increases with κ in a nanocomposite film, whereas in single-phase nanocrystalline materials Hc decreases with κ.
• Increase of the exchange stiffness of the soft phase Asoft has the same effect as the increase of κ, because qualitatively this parameter has the same influen¬ce as the intergrain exchange.
B. Simulations of bulk nanocomposites
B.1 Bulk nanocomposites with moderate volume fraction of the hard phase
First, in addition to our initial micromagnetic methodology for simulations of bulk nanocomposi¬tes, we have developed a general method to discretize grains with any shape with polyhedrons (Fig. 30). Due to higher performance of this method we could simulate composites with ‘hard’ grains of any shape and systems containing up to 104 such grains.
Employing this new algorithm, we have systematically studied SrFe12O19/Fe bulk nanocomposite. Following main results have been obtained:
1) Systems with a moderate hard phase volume fraction (≈ 35 vol. %) and a reduced interphase exchange coupling exhibit the two-step magnetization reversal: first, magnetization of the soft phase is switched and subsequently magnetization of ‘hard’ grains is reversed (Fig. 31).
2) Simulations of systems with spherical and ellipsoidal hard grains have revealed the effect of the grain shape on hysteresis loops. The composite SrFe12O19/Fe exhibits a significant dependence of (BH)max on the grain shape, mainly because for this material the soft phase magnetization (Msoft ≈ 1700 G) is much larger than for hard phase (Mhard ≈ 400 G).
Both these results are a clear demonstration of the close relation between the microstructure and the magnetization reversal process of these systems.
B.2 Bulk nanocomposites with high volume fraction of the hard phase
New meshing algorithm. For simulations of nanocomposites with high frac¬tions of the hard phase (ch > 60%) we have also developed a new meshing algorithms. Namely, we place many spheres randomly in space, and then inflate them until their volume fraction reaches the prescribed value. Hard grains created this way do not have any regular shape, what corresponds to real physi¬cal situation. Then we generate a closely packed system of much smaller spheres with the sizes equal to that of a mesh element. Finally, each mesh element inside a large sphere is assigned to the corresponding hard grain and elements outside of large spheres are assigned to the soft phase. Using this algorithm, we have studied SrFe12O19/Fe composites with ch > 60%.
B.2.1 Nanocomposites with randomly oriented anisotropy axes of the hard phase
Effect of the hard phase volume fraction.
For SrFe12O19/Fe, the large magnetization difference between 'soft' and 'hard' phases has led to the strong decrease of the magnetiza¬tion and remanence with increasing chard. In contrast, coercivity increases from ≈ 20 kOe for chard = 60% to ≈ 40 kOe for chard = 90%. The competition between these two opposite trend has led to an appreciable decrease of (BH)max from 23 kJ/m3 to 18 kJ/m3, indicating that materials with the 'hard' phase volume fraction of about 60% should have an optimal performance for this nanocomposite.
Effect of the interphase exchange weakening κ. This effect is very important, because this weakening is unavoid¬able in real systems due to imperfect intergrain boundaries. Simulations of composites discussed above (for chard = 70%, 80% and 90%) and the range of κ from the perfect exchange (κ = 1.0) to the completely decoupled grains (κ = 0) have shown that:
1) (BH)max of SrFe12O19/Fe depends on the interphase exchange weakening much stronger than for SrFe12O19/Ni (also due to the much larger magnetization difference between the phases).
2) Dependence of (BH)max on κ is more pronounced for higher concentrations of the 'soft' phase, because here the hard-soft phase interaction plays a more significant role (Fig. 32).
B.2.2 Nanocomposites with aligned anisotropy axes of the hard phase
We have also studied nanocomposite materials with aligned anisotropy axes of the 'hard' grains (both for uniaxial and cubic anisotropy types). Although such materials are not available at present, this study should answer the question whether it is worth trying to obtain such materials experimentally, e.g. by applying a large external magnetic field during the material processing. In particular, we have simulated the dependence of (BH)max on the 'hard' phase fraction chard for materials with fully exchange coupled and decoupled grains. The main outcome of this study is that the highest values of (BH)max (up to 140 kJ/m3) can be achieved for the perfectly exchange coupled system with hard grains having the uniaxial anisotropy type.
It is also interesting to note that:
1) For exchanged-coupled material, (BH)max rapidly decreases with increasing chard both for cubic and uniaxial anisotropies due to the decrease of the material magnetization and the cor¬responding decrease of jR (we remind that Mhard < Msoft).
2) For exchanged-decoupled material, (BH)max significantly increases with increasing chard due to the very strong increase of Hc. We also point out that here (BH)max is the same for cubic and uniaxial anisotropies of the 'hard' phase, because the energy product is determined by that part of the hysteresis loop, which is dominated by the magnetization reversal of the 'soft' phase.
B.4 Simulations of multi-phase nanocomposites: porous CoFe2O4/FeCo
N-phase generalization of the simulation paradigm. Many real two-phase magnetic nano¬composites have a considerable fraction of pores. In order to study a porous system in frames of our methodology, we have generalized our code to the case of a N-phase system.
For this purpose we have performed two important revisions of our methodology.
1. Site-dependent parameter description. Now all magnetic parameters - material magnetization, anisotropy constants and anisotropy axes directions can be different for different crystallites.
2. Generalization of the exchange interaction treatment for site-dependent calculation of the exchange energy between mesh elements (i) inside the same crystallite and (ii) belonging to different crystallites we have introduced the Nph × Nph matrix (Nph is a number of phases). Diagonal elements of this matrix describe the interaction between mesh elements of the same phase belonging to different crystallites. Off-diagonal elements are responsible for interaction between mesh elements from different phases. The intergrain exchange weakening parameter is also included in this scheme.
As a result of this methodology generalization, we are able to calculate the magnetic response from a nanocomposite consisting of any number of magnetic and non-magnetic phases, including pores.
For the porous nanocomposites CoFe2O4/FeCo we have used the following parameters found in the literature: for the hard phase CoFe2O4: magnetization Ms = 400 G, cubic anisotropy with Kcub = 3.1×106 erg/cm3, average grain size Dav = 15 nm; for the soft phase FeCo: Ms = 1600 G, cubic anisotropy with Kcub = 2×105 erg/cm3, exchange constant Asoft = 3×10-6 erg/cm, Dav = 15 nm. The pore volume fraction was set to 10% and the average pore size to 15 nm (Fig. 33).
For the optimization of energy product we have varied three parameters:
1. The 'hard' phase exchange stiffness seems not to be measured properly. The literature search provided an unusually broad range: Ahard = (0.3 - 3.0) × 10-6 erg/cm.
2. Soft phase volume fraction can be changed experimentally and strongly influences (BH)max. In our studies, csoft was varied between 0 and 50 %
3. Exchange weakening between crystallites can be changed from κ = 0 to 1 using different produc-tion technologies and is also very important for the energy product.
For each set of parameters, hysteresis loops were averaged over 4 independent geometrical realiza-tions of the mesh elements placement. The system with Ahard = 1.0 × 10-6 erg/cm, csoft = 20% and κ = 0.2 was chosen as a reference set; for these parameters (BH)max = 44.1 kJ/m3.
1) Effect of the hard phase exchange constant Ahard. Simulations with different values of Ahard have shown that hysteresis loop almost does not change in the range Ahard = (0.3 - 1.0) × 10-6 erg/cm. Further increase of Ahard led to reduction of the coercivity Hc, what is most probably due to the stronger self-averaging of the anisotropy fluctuations of 'hard' grains at larger exchange.
2) Effect of 'soft' phase volume fraction csoft. The increase of the soft phase fraction naturally leads to the corresponding linear increase of the saturation magnetization of a nanocomposite, but simultaneously results in a decrease of the coer¬civity Hc, because magnetization reversal starts in smaller fields. Simulated dependence of (BH)max on the soft phase volume fraction csoft is shown in Fig. 5.B.5 left panel. The interplay between increasing Ms and decreasing Hc (with increasing csoft) results in the optimal value for csoft ≈ 10%.
3) Effect of the exchange weakening κ. To study this effect, hysteresis have been simulated for κ = 0.0 0.05 0.1 0.2 0.3 0.5 and 1.0. Star¬ting from κ ≈ 0.1 simulated loops 'loose' their two-phase character, what means that for larger values of the exchange weakening the magnetization of the complete system reverses cooperatively. This value of κ also corresponds to the maximum of the energy product as the function of this parameter (see Fig. 34, right panel). With another words, for the 'best' composite material the perfect exchange coupling is fact harmful - due to the too strong decrease of the coercivity for strongly exchange coupled systems.
Prototypes and testing in working conditions
A. Specification of powertrain for electric two-wheeler
The powertrain specification of electric two wheeler is identified based on following requirements
1. The vehicle should be capable of accelerating to maximum speed of 30 kmph in 20-24 s for weight range of 130-150 kg including the weight of passenger and vehicle.
2. The vehicle should be capable of maintaining the maximum speed over inclined road up to 3%
The specification of the powertrain is listed in Table 1.

B. Motor topology
The schematic of the SAT PMBLDC motor is shown in Fig. 35. The SAT motor topology is a variation of a torus slotted north–south axial flux motor (AFM) topology, with a benefit of not needing a stator yoke. The magnetically separated teeth can be wound separately before assembly, and this ensures high fill factor and short end turns resulting in an efficiency improvement. This adds to the improved torque density of the machine. A pole/slot combination of P = Ns ± 2 has been selected to reduce the cogging torque. In addition, a single-layer winding is opted as they are more suitable for the BLDC motor operation.
C. Final designs of the ferrite magnet prototype
A summary of design details of the ferrite magnet motor is shown in Table 2.
D. Fabrication of prototype motor
The fabricated components of ferrite magnet motor are shown in Figs. 36-39.
E. Testing of prototype motor
The prototype has been tested both on test bench and in a driving cycle on board of electric two-wheeler as powertrain. The performance of the ferrite magnet motor is compared to the sintered rare-earth motor that came originally with the electric two-wheeler.
6.1 The results of test bench performance evaluation: The test bench used to test the ferrite magnet prototype motor and sintered RE magnet motor are shown in Fig. 41. The output power is calculated from the torque and speed measurement from torque transducer connected at shaft of motors. The input power is measured at the DC power supply.
The efficiency map of the ferrite magnet prototype and the sintered RE magnet motor is shown in Fig. 42. From efficiency map, it is clear that the ferrite magnet motor is able to achieve higher efficiency in large area of operating region.
6.2 The performance of ferrite magnet motor as a direct drive powertrain: The on board performance of motors is evaluated with the help of acceleration time and energy consumed during eight basic urban drive cycle according to ISO 13064-1:2012 shown in Table 3.
The acceleration time of vehicle with the ferrite magnet motor and the sintered rare earth magnet motor is shown in Fig.43. The acceleration time of vehicle powered by the ferrite magnet motor is 16 s while the acceleration time with the sintered rare earth magnet is 10 s. The acceleration time of the ferrite magnet motor powertrain is well within the target of 20 s. The energy drawn from the battery to cover eight basic urban drive cycle as per ISO 13064-1:2012 is shown in Table 4, and the ferrite magnet based powertrain is drawing marginally lower energy reflecting the improved efficiency from the test bench performance data.
Production scale
The method for production of improved NANOPYME ferrites in industry is economically viable because it uses equipment (furnaces) and processing (temperature of heat treatment) as those typically used in the production process of commercial powders and magnets. Only the different steps (duration of heat treatment) need to be accommodated to obtain the full magnetic properties.
However, an important problem that the project faced was the impossibility of using standard compaction tools due to the different granularity (much finer) for ferrite powders obtained in NANOPYME. This is indeed a very important issue since cost viability also comes when going from free standing powder to compacted magnets. The main question here to allow for a cost efficient procedure was related to the possibility of using the equipment existing at the company with minimum and low-cost modifications. The standard tools were destroyed when using NANOPYME powders due to material shifting between the walls of the compaction moulds. In the frame of the project, new tools were designed, constructed and tested in the lab, with lower tolerance (shown in Fig. 44). These tools were afterwards successfully implemented in the manufacturing process at industry. Therefore, no additional modification in the manufacturing equipment needed to be done and these tools could be use as typically done with standard moulds.
NANOPYME magnets have been fabricated in an industrial scale for the motorbike prototype. These magnets have been manufactured using equipment already existing and used for standard ferrites.
The powder is compacted using a mould with the adequate shape to form the magnet by the wet compaction method (Fig. 45). Once the magnets are formed, the last step on the process is the thermal treatment performed to develop mechanical resistance on the piece.
The next step is to ensure that dimensions are within the requested tolerances. Since the size of the NANOPYME magnets is modified during the sintering process, it is necessary to rectify the segments by a grinding technique (Fig. 46). The magnets (32) are ready to be implemented in the rotor of the NANOPYME e-scooter.
Safety and Recycling
A. Recycling
Sr-ferrite powders have been recycled from magnetic waste generated in industrial partner. This waste is created during the manufacturing process of commercial magnets. Industrial partner pays to a third company through a contract for waste disposal (schematically shown in Fig. 47).
The method is economically viable because it uses equipment (furnaces) and processing (temperature of heat treatment) as those typically used by the company in the production process of commercial powders and magnets. Only the different steps (duration of heat treatment) need to be accommodated to obtain the full magnetic properties. Cost efficiency is thus guaranteed and the company does not need to make use of an extra cost for removal of the residues by a third company. Indeed, and as it can be seen in Fig. 48, the recycled material shows better magnetic properties than the new commercial material used in the fabrication of the commercial magnets. Thus, it is not only a cost efficient procedure, but also it allows offering to customers a product with a quality (high-grade ferrite) by far superior to the original material (low-grade ferrite). A comparison (Fig. 48) of the loop for the dried waste and the starting material shows that the recycling procedure profits, in a very first stage, of the microstructure refinement related to cutting of the magnets during manufacturing. Completion of the recycling process leads to the outstanding magnetic properties (coercivity and remanence) shown in Fig. 48.
Processing (patent pending) of the recycled material in a final step makes possible to obtain a new product with enlarged coercivity (Fig. 48). Coercivity of about 5 kOe for the processed recycle counts as the largest coercivity reported for isotropic Sr-ferrite powders.
Once magnetic properties of powders recycled at research institutions and recycled at industry (following identical procedure) have been compared obtaining practically identical results, the process has been up-scaled to larger amounts of material going from milligrams to tenths of kilograms (see Fig. 49).
B. Safety
Workers that handle and work with nanomaterials either in research or production processes may be exposed to nanoparticles (NPs) through inhalation, ingestion or skin contact. Although the potential health effects of such exposure are not fully understood, scientific studies indicate that at least some of these materials are biologically active and may readily penetrate intact human skin. NANOPYME nanomaterials do not fit into this group of powders.
It is very important to remark that particle size of NANOPYME magnetic materials are not smaller than that of ferrite powders resulting at present in companies in the manufacturing of sintered permanent magnets. This is a very positive consequence of the optimization carried out in NANOPYME project that allows modifying morphological and microstructural properties of powders through short processing times (for example, milling times as short as a few minutes). Thus, the safety actions and measurement of exposure by workers are practically the same that those currently followed in industries working on manufacturing of permanent magnets.
Primary routs of exposure for NANOPYME materials are inhalation and ingestion, less significant is skin contact. Most common tasks that would present a potential for exposure are:
- Handling NPs
- Maintenance on equipment used to produce NPs
- Cleaning of spill or waste material
- Cleaning dust collection system
- Machining of parts made of NPs
Measures taken for better safety should be:
- Engineering measures:
The physical form of the NPs will greatly influence the exposure potential. Other important factors are frequency and duration of exposure. Engineering controls that should be considered are enclosure or insulation of a process, local ventilation systems. Processes involving the generation of NPs and NPs in suspension should be performed in a chemical fume hood to limit the inhalation exposure potential.
- Administrative measures:
Administration controls that should be considered and implemented are education of workers on the safe handling of nanomaterials, restriction and identification of areas where exposure exists, transportation of dry nanomaterials in closed containers and cleaning of work area daily with a HEPA-vacuum or with wet wiping method.
- Personal protective equipment:
Workers should wear latex or nitril gloves when handling NPs, chemical safety goggles when working with suspensions or dry powders, lab coats and respiratory protection.

Potential impact
Socio-economic impact and societal implications
By 2020, the global market for permanent magnets is expected to reach 28,000 million €, and the European market is expected to exceed 2500 million €. First of all, the growth of the automobile industry is expected to be a key driving factor for permanent magnets market as they are largely employed for numerous applications in automobile industry. Various components such as motors, alternators and gearboxes require permanent magnets for their proper functioning. Second, a shift in trend towards developing renewable energy generation including solar and wind energy is also expected to strongly drive the market. Permanent magnets are used in the stator of the wind turbines which are responsible for producing AC electricity. Figure 50 illustrates the main application fields of permanent magnets.
Owing to the price fluctuations originated by the RE crisis, many large scale manufacturers had to replace NdFeB magnets with ferrite magnets for automotive and wind energy generation applications. Ferrite magnets dominate the global market in terms of weight, accounting for over 80% of total volumes. However, in terms of revenue, ferrite magnets account for just over 20% of the total revenue due to their low cost.
The scientific breakthroughs of NANOPYME yield substantial improvements in the energy product of magnetic materials with a completely eliminated CRM content, which will potentially lead to a partial substitution of low-grade RE-magnets in technologies that require moderate energy products. It is however important to point out that these results have yet to be translated onto dense anisotropic magnets.
Advantages of 20% remanence improved magnets made on a ferrite basis can easily be understood when looking at the raw material prices. Ferrites have currently an average price around 1.5 €/kg, whereas the NdFeB prices are around 60 €/kg. (SmCo is even higher, up to 100 €/kg). From the energy product which is assumed to be 40 kJ/m3 (Hitachi NMF12F) and a ferrite density of 5 g/cm3 ,one can calculate specific costs per kJ of 188 €/kJ. For a NdFeB-sintered magnet with 400 kJ/m3 (Vacodym 745HR from Vacuumschmelze) and a density of 7.5 g/cm3 we get a specific price of 1125 €/kJ. If we could apply a 20% remanence improved ferrite and if there were no significant changes in raw material price and density (we estimate that the cost of the hybrid composite magnets will be of the order of 5 €/kg), we would get energy specific costs of around 133 €/kJ only.
In nearly closed magnetic circuits as in motors, actuators or generators, the field energy in the airgap is approximately proportional to the maximum energy product of the magnet. This means, compared to ferrite systems, we would need around 40% less in volume in systems with 20% remanence improved magnets.
In open circuits like in most sensor systems, magnetic fields are proportional to the remanence of the field source as well as to the size and mutual orientation of pole faces. A very simple example should depict the differences between commercial sintered ferrites and possibly improved ones, as well as the difference to rare specimen: Let´s assume we aim to reach, with an axially uniform magnetized cylinder, a flux density of 100 mT at a distance of 4 mm. With a ferrite of quality Hitachi NMF12F, this could be realized with a cylinder diameter of 19.2 mm and an axial length of 10 mm, i.e. its volume is around 2.9 cm3. With a 20% remanence improvement, we would be able to reach the targeted flux with a diameter of 14mm and a length of 8mm, i.e. the needed volume would be reduced to 1.23 cm3. In case we made the magnet from the above mentioned NdFeB material from Vacuumschmelze (Br=1.44T) we would need a cylinder with 6.2 mm diameter and axial length of 4 mm, i.e. the magnet has a volume of only around 0.12 cm3. However, considering costs, the improved ferrite would be by far superior economically: Sintered NdFeB: 5.4 €/100magnets, standard ferrite 2.18 €/100magnets, improved ferrite 0.93 €/100magnets.
For injection molded magnets there is a remanence gap between ferrite and RE magnets. When having 60% percent volume of magnetic powder in the polymer matrix, maximum remanence of ferrites is around 280 mT. With same filling, grade remanence of an isotropic NdFeB powder is around 530 mT. To close the remanence gap one often blends the ferrite mixture with an isotropic NdFeB. To get a 20% improved remanence we have to substitute nearly 40% of Ferrite with such NdFeB powder, so the compound costs would increase to 23 €/kg instead of 10.5 €/kg for the pure Ferrite compound. An improved remanence in ferrites would yield savings of the order of 50% in material costs for respective applications.
Thus, a remanence improved ferrite leads to highly reduced energy specific and field specific costs in comparison to all currently known magnetic materials. In comparison to standard ferrites, which demand relatively high spatial volumes, tremendous reductions of volumes could be reached too. These as well as a few other technical advantages can be predicted both for sintered and bonded magnets.
In addition, the NANOPYME project has been based a strong interdisciplinary integration and transfer of knowledge within and between the relevant fields. Interaction among higher education and training institutions, businesses and research institutes within the consortium has been encouraged in terms of curricula development, mobility programs and access to research and industrial infrastructure such as laboratories, pilot projects and test facilities for practical training in real environment.
We emphasize that, once the viability of the technology will be established for dense magnets, the door to the substitution of RE-magnets in applications in a variety of sectors will be wide open. Ferrite based magnets that will substitute RE-magnets contain elements that are abundant within European borders. Thus, the results from NANOPYME contribute to the EIP main objectives of: reducing CRM imports, improving resource efficiency, putting Europe at the forefront in raw materials.[11]
For instance, in generators for wind turbines, typically used sintered NdFeB magnets do not necessarily provide the highest energy-conversion efficiency, as a generator able of operating efficiently over a wide speed range is required. The extremely high flux density and high coercivity of RE-based permanent magnets can lead to saturation of the generator with flux, leading to significant iron losses (detrimental for the generator efficiency). In addition, RE-based magnets are electrically conductive, leading to eddy current losses and inductive heating of the magnets. These parasitic losses are more pronounced at higher wind speeds resulting in rapid performance degradation. RE-based magnets could be designed with a moderate magnetic flux density and a decreased coercivity but this would not make any sense from the economic point of view. Instead, further development of the much more economical ferrites would decisively better suit the requirements for magnet applications in the electric power system.
Furthermore, many countries have introduced regulations to encourage the manufacturers and users of electric motors fulfilling mandatory minimum efficiency standards. At the moment the inefficient induction motors are being slowly replaced by DC motors; from these latter ones brush-type motors are preferred by manufacturers due to their reduced cost against brushless DC motors (containing permanent magnets), despite the associated efficiency sacrifice. Substitution of RE-based permanent magnets by ferrites with improved performance would lead to a significant price reduction (cheaper raw materials, production processing...) and increased flexibility in both manufacturing and design (sintered arcs, flexible strips, etc.).
While it is not possible to quantify the value of the economic effects in all the involved industries, 5% substitution of current RE-magnets by inexpensive ferrite-based magnets could easily result in savings for Europe of the order of the hundreds of millions of Euros.
The NANOPYME results have the potential to free the permanent magnet industry from its reliance on a single imported source of heavy-rare-earth (HRE) materials, favouring prioritized European industries. The technology gap between Europe and Asia may be shortened as a consequence.
Applications of NANOPYME magnets have been already proven in the development of the project by construction of two very different types of prototypes:
(a) e-scooter containing a motor with 32 improved ferrites arranged in two parallel discs (conforming the rotor) with 16 magnetic poles in each disc (Fig. 51).
The e-scooter was designed, constructed and tested in NANOPYME Project. The prototype (Fig. 52) was shown in operation during the NANOPYME Workshop (September 2015, Madrid).
(b) Stepper motor to control high precision movements.
Workability of the motor was shown by direct substitution of rare earth magnets in the commercial stepper motor by new nanocomposite ferrite magnets (under patent application) developed in NANOPYME project (Fig. 53).
Additional potential application areas profiting of the improved ferrite magnets developed in NANOPYME comprise might include according to industrial partners:
(i) Automotive
The particular ferrite choice depends on the requirements of each application. The materials to be used in automotive applications may be sintered, cast or alloyed with plastic substances. Therefore Sr-ferrite magnets obtained through sintering and Co-ferrite magnets obtained through the polimerization route show important potential applications in electronic appliances in the automotive sector.
(ii) Food Industry Magnets
Magnetic separation systems based on improved ferrites would substitute those based on rare earths. These systems are designed to detect contamination, impurities and any kind of metal particle during the production processes and before the packaging of the final product.
This equipment offers maximum security, as they do not depend on a power supply, together with proper lifelong operation related to the used permanent magnets.
(iii) Support and Robotics
Robot systems are found in many places and everyday objects, such as automatic doors, lifts, elevators, radiology equipment and domestic appliances. Improved ferrites with magnetic properties as those obtained in NANOPYME have use in this broad range of applications.
This kind of magnetic system can be designed for the handling and transport of any kind of metal piece and can be adapted to the special needs of each sector.
- SAFETY AND TRANSPORT: closure devices, automatic doors and fire doors.
- LIFTS AND ELEVATORS: sensors, stairways, ramps, and signalling and transport equipment.
- PACKING AND PACKAGING: labelling and packing machinery and automatic dispensers.
-MACHINERY: industrial robots, motors, presses, domestic appliances, textile machinery and industrial furnaces.

Potential Impact:
An international Workshop entitled “Rare Earth-Free Permanent Magnets and Applications” was held at IMDEA Nanociencia in Madrid on 14th-16th September 2015.
Announcement of the Workshop is shown in Figure 54. The ideas behind this Workshop were:
(i) Promote interaction between the NANOPYME consortium and scientific community and industrial sector working in the search of rare earth-free permanent magnets as alternative to rare earth-based magnets.
(ii) Disseminate achievements done by NANOPYME during its development.
(iii) Establish new cooperation routes to advance in research and development of permanent magnet alternatives.
This Workshop was a scientific event opened to both NANOPYME and external researchers. The main aim was to create a suitable environment to favour scientific interactions and discussions. In addition, consortium intended that this Final Workshop would promote and strengthen European and international research collaborations on different topics related to Rare Earth-Free Permanent Magnets. The main scopes of the Workshop were:
1. RE-Free Nanostructures and Composites for Permanent Magnets
2. Numerical Simulations and Modeling
3. Applications of RE-Free Permanent Magnets
4. Novel RE-Free Hard Magnetic Materials
5. Searching for Alternatives to RE-PMs: EU Projects and Future
Initiatives
A dedicated site was created in IMDEA Website:
http://www.nanoscience.imdea.org/nanopyme2015
Two well-known scientists with a recognized expertise in the area of permanent magnets formed part of the External Advisory Board:
- Prof. George Hadjipanayis (University of Delaware, USA)
- Prof. Spomenka Kobe (JSI, Slovenia)
The Workshop counted with attendees from other EU Projects (ROMEO, MAGDRIVE...). In addition to scientists from well-known international institutions, representatives of industry attended also the event.
The list of Invited Speakers -from different disciplines/topics- shows the relevance of the Workshop:
- Prof. Manuel Vázquez (ICMM-CSIC, Spain).
- Dr. Félix Jiménez-Villacorta (ICMM-CSIC, Spain).
- Prof. Olle Eriksson (Uppsala University, Sweden).
- Prof. Simon Bance (St Pölten University of Applied Sciences, Austria).
- Prof. Antonio Hernando (UCM, Spain).
- Dr. Xavi Marti (Institute of Physics ASCR, Czech Republic).
- Prof. Spomenka Kobe (Jozef Stefan Institute, Slovenia)
"EU Project ROMEO".
- Dr. Santiago Cuesta (ICCRAM, Univ. Burgos, Spain).
- Dr. Erno Vandeweert (European Commission, DG Research & Innovation).
- Prof. George C. Hadjipanayis (Univ. of Delaware, USA)
- Prof. Franca Albertini (IMEM-CNR, Italy)
A picture of attendees was taken during the Workshop (Figure 55).
Sessions and scope were organized to cover a broad range of aspects (from synthesis and characterization to processing and applications) as follows:
DAY 1 (14th Sept)
Session 1: Nanostructures and Composites
The session on Nanostructures and Composites covered recent advances in remanence and coercivity optimization of rare-earth-free permanent magnets via the nanostructuring strategy. Studies seeking to enhance either intrinsic or extrinsic properties of hard magnets by tailoring composition, crystalline structure and microstructure will be presented, from thin-films to bulk powders and dense magnets. Special focus was given to exchange-spring rare-earth-free composites.

Session 2: Simulations and Calculations
The session on Numerical Simulations covered all aspects of recent achievements in computer modeling of problems related to permanent magnets. Invited speakers reported about the recent progress of simulations on all relevant spatial scales – starting from the ab-initio models which aim to compute atomic magnetic moments, exchange interaction strength and anisotropy constants of magnetic materials from the first principles. At the next level, mesocopic models were presented where magnetization states and reversal processes in magnetic nanocomposite materials are addressed on the length scales comparable with the domain wall thickness. Finally, simulation examples produced by macroscopic software packages showed how the input from the previous level models enable to predict and optimize the behaviour of technical devices employing permanent magnets.

DAY 2 (15th Sept)
Session 3: Towards Applications of RE-Free Permanent Magnets
Permanent magnets are indispensable for many commercial and future clean technology applications. Major commercial applications include the electric, electronic and automobile industries, communications, information technologies and automatic control engineering. Virtually in every application, an increase in the magnetic energy density of the magnet, usually presented via the maximum energy product (BH)max, immediately leads to smaller, lighter and more energy efficient devices. The aim of this session was to analyse the possible applications of the “state of the art” research and advances on rare earth-free permanent magnets.

Session 4: In the Search of Alternatives to RE-PMs: EU Projects and Future Initiatives
The criticality of rare-earths -used as fundamental constituents in NdDyFeB-based magnets- has made European Commission to open diverse Calls in recent years aiming an efficient substitution of these elements in permanent magnets.
This Session focused in EU projects dealing with rare earth-free permanent magnets and their applications, and it provided a vision on future EU initiatives in the frame of Horizon 2020.

DAY 3 (16th Sept):
Session 5: Novel Free Hard Magnetic Materials
This session covered the recent investigations on novel or re-discovered single-phase materials in which composition, order and/or structure give rise to large values of the magnetocrystalline anisotropy. Hard single phases containing Rare Earth and Pt or Pd elements were excluded.
During the second day of the Workshop, there was an exhibition where attendees could see materials, synthesis and processing tools, NANOPYME magnetic materials (powders and magnets) and NANOPYME prototypes (stepper motors and electric-scooter) under operation conditions.
Feedback received after NANOPYME Workshop was extremely positive by attendees and external advisors.
Impact on the media was large as indicated by numbers related to people accessing news on the NANOPYME prototype published in different media (see pdf file for images).

Press releases

Nanociencia y tecnología producen imanes permanentes de última generación
(Madri+d, 3 December 2012) (email daily news service of science and technology with about 100 000 suscriptors)

They make magnets that will save the world from China
(Teknisk Ukeblad, 12 December 2013) (Teknisk Ukeblad, a weekly Norwegian leading engineering journal magazine with about 300 000 readers

NANOPYME in the radio
Radio Nacional de España · RNE- “Entre probetas” (4 January 2013)
http://www.rtve.es/alacarta/audios/entre-probetas/entre-probetas-nuevos-imanes-permanentes-14-01-13/1662447

Interview (on-line): Videnskaben.dk Mogens Christensen
http://videnskab.dk/teknologi/nye-nanomagneter-skal-oge-konkurrenceevnen-over-kina
Day of Research Aarhus University, 3 February 2013
Public Talk and booth: Presentation: “Supermagneter”

The media emphasise the prototype bike publicized on September 2015 as one of the European project NANOPYME's achievements, a project coordinated by IMDEA Nanoscience.

http://www.madrimasd.org/informacionidi/noticias/noticia.asp?id=64743&origen=Home_madrimasd

http://blogs.publico.es/kaostica/2015/10/07/ordago-espanol-a-china-imanes-permanentes-libres-de-tierras-raras/

Dr. A. Bollero in Magazine HORIZON “Positive Solutions to Europe Magnets Problem” (16th March 2015):

http://horizon-magazine.eu/article/positive-solutions-europe-s-magnet-problem_en.html

NANOPYME e-scooter Prototype – Video: https://goo.gl/photos/BDDi3MfCT4fzmUaY8

Dr. Alberto Bollero invited speaker as NANOPYME coordinator
1. 2015 MRS Fall Meeting, November 29-December 4, 2015 Boston, Massachusetts (29th November-4th December 2015, Boston, USA). Invited speaker. “Rare Earth-Free Magnetic Powders for Permanent Magnet Applications: From Synthesis to Industrial Recycling”
2. First IMDEA Conference (November 6th 2015, Madrid, Spain). Invited speaker. “Nanostructured permanent magnets without rare-earths: from basics to technology”
3. 5th Trilateral EU-US-Japan Conference on Critical Materials (25th October 2015, Tokyo, Japan). Invited speaker. “Hybrid ferrites for permanent magnet applications: from processing to industrial recycling and proven functionality”
4. Conf. EUROC2015 Workshop “Magnetic Materials for Energy Applications (8th-11th September 2015, Salamanca, Spain). Organizer and Chair in the Workshop
5. International Symposium on Advanced Permanent Magnetic Materials. (9th-11th May 2015, Beijing, China). Invited speaker. “Ferrites-based permanent magnets for technological applications”.
6. NanotechItaly 2014. (26th-28th November 2014, Italy). Invited speaker. “Back to the Future: Revisiting Ferrites with Nowadays Nano-Tools as Alternative to Rare-Earth Permanent Magnets”.
7. REPM2014 - Rare Earth and Future Permanent Magnets and Their Applications Workshop (17th-21st August 2014, Annapolis, USA). Invited speaker. “NANOPYME project: in the search of improved rare earth-free permanent magnets”.
8. Workshop on Energy and Materials Criticality (22-25th August 2014, Santorini, Greece). Invited speaker.
9. 2nd Strategic Innovation Network on Substitution of Critical Raw Materials (13th and 14th May 2014, Brussels, Belgium). Invited Expert.
10. Conference INTERMAG (4th-8th May 2014, Dresden, Germany). Invited speaker and member of the Organizing Committee.
11. 3rd Industrial Technologies exhibition (9 - 11 April 2014, Athens). Exhibitor. Booth 26.
NANOPYME stand chosen by jury among the best 5 stands of the complete event (counting with > 100 stands)]
12. Italian School: "Magnetic Materials for Energy Applications" (13th February 2014, Parma, Italy). Invited speaker.
13. Spanish School: "Novel Frontiers in Magnetism" (10th February 2014, Benasque, Spain) Invited speaker.
14. 1st Strategic Innovation Network on Substitution of Critical Raw Materials (15th April 2013, Brussels, Belgium).
15. National presentation of NMP Call in Madrid (2012). “NANOPYME: a successful case of EU project proposal”

List of Websites:
Contact:
Name (Organization):
Dr. Alberto Bollero (IMDEA-Project Coordinator) Dr. María Jesús Villa (IMDEA)

Email:
alberto.bollero@imdea.org (A. Bollero) marije.villa@imdea.org (M. Villa)
Phone: (+34) 912998758 / (+34) 912998710

Project website:
http://nanociencia.imdea.org/division-permanent-magnets-applications
NANOPYME Consortium:
1 FUNDACION IMDEA NANOCIENCIA - IMDEA Nanociencia - Spain [Coordinator]
2 CONSORZIO INTERUNIVERSITARIO NAZIONALE PER LA SCIENZA E TECNOLOGIA DEI MATERIALI - INSTM - Italy
3 INSTITUTT FOR ENERGITEKNIKK - IFE - Norway
4 AGENCIA ESTATAL CONSEJO SUPERIOR DE INVESTIGACIONES CIENTIFICAS - CSIC - Spain
5 AARHUS UNIVERSITET - AU - Denmark
6 INSTITUT JOZEF STEFAN - JSI - Slovenia
7 UNIVERSIDAD COMPLUTENSE DE MADRID - UCM - Spain
8 DANMARKS TEKNISKE UNIVERSITET - DTU - Denmark
9 INGENIERIA MAGNETICA APLICADA SL - IMA S.L. - Spain
10 MAGNETI LJUBLJANA PODJETJE ZA PROIZVODNJO MAGNETNIH MATERIALOV DD - Magneti - Slovenia
11 VEREIN ZUR FORDERUNG VON INNOVATIONEN DURCH FORSCHUNG ENTWICKLUNG UNDTECHNOLOGIETRANSFER EV - INNOVENT – Germany
12 GENERAL NUMERICS – GNRL - Germany

List of FIGURES
Figure 1. Hysteresis loops of initial CoFe2O4 magnetic properties (black) and its evolution after milling during (red) 30 seconds and (blue) 180 seconds.
Figure 2. SEM images of CoFe2O4 powders after the different procedures applied: (a) starting material (co-precipitated Co-ferrite powders); (b) milled for 30 sec; (c) milled for 3 min; (d)-(e) 3 min milled powders after heating at temperatures of 400, 600 and 1000ºC.
Figure 3. (a) Hysteresis loops for commercial Sr-ferrite [black curve], powders obtained after milling (4.5 min) [blue], milled and heated [red]. SEM images for (b) commercial Sr-ferrite; (c) milled and heated Sr-ferrite powders. Magnetic properties are maintained while microstructure is highly improved (more homogeneous and finer) in view of further use in the preparation of nanocomposites.
Figure 4. a) Strontium ferrite platelets arranged around a soft inclusion. After applying a saturating external field H, the uniaxial anisotropy of the ferrite produces the magnetodipolar field (red lines) that favors a partial spin alignment within the soft phase. b) Scheme of exchange-coupled hard and soft particles portraying the influence of the soft particle size on spin alignment (green arrows). Above a certain threshold size (typically of the order of a few nanometers), reverse domains nucleate that decrease the energy product.
Figure 5. Demagnetization curve of the 90%Sr-ferrite-10%Co-ferrite composites.
Figure 6. Second quadrant of hysteresis loops for low-grade (Y30BH), high-grade (UHE10) and NANOPYME nanocomposite isotropic powders.
Figure 7. a LEEM image of a large island. b AFM image of the same island obtained by combining topography and its derivative in a merged image. The profile associated to the green line is shown as an inset. c and d XMCD-PEEM images of the same island shown in c obtained at the Fe and Co L3-edges respectively, portraying the magnetic domain configuration. Grayscale magnetic contrast according to the depicted arrow symbol, with beam direction indicated by yellow arrow. The green arrow and red line in a and c highlight domain walls pinned to an atomic terrace and a twin boundary.
Figure 8. (left) SYNTEX SPS apparatus used in NANOPYME. (middle) SPS pressing taking place, view on the glowing graphite, (right) the produced pellet after SPS pressing.
Figure 9. Hysteresis curves for the SPS pressed samples, with and without applied magnetic field and before and after annealing. Densities of the samples reach above 95% of the theoretical density.
Figure 10. The scattering vector is in the plane of the paper. a) the scattering vector is parallel with the reciprocal lattice vector of (008), b) by tilting chi it is possible to bring the (107) reflecton into diffraction conditions. c) the scattering from an ensample of crystallites, the pole figure measurements allow deducing the orientation distribution function.
Figure 11. Pole figure data measured for the SPS pressed pellet for the reflections (008), (107), (112) and (110). The sample is clearly aligned with the majority of the crystallites orientated with the (008) parallel to surface normal vector.
Figure 12. Crystallite alignment distribution along the [00l], the angular range is chosen to give equidistant volumes.
Figure 13. Magnetization and demagnetization curves for obtaining the Henkel plot (see text for explanations).
Figure 14. Magnetization and demagnetization curves and corresponding Henkel plot for a system of non-interacting crystallites with random distribution of cubic anisotropy axes directions.
Figure 15. Protocol magnetic characterization defined in NANOPYME–exchange-coupling confirmation.
Figure 16. Equipment used for mechanical strength studies and compression test done on one of NANOPYME pellets.
Figure 17. Results of compressive test done at room temperature for recycled powders obtained from cutting manufacturing process (left) and grinding manufacturing process (right) after heat treatment at 1000ºC. The three columns of measurements correspond to recycled material without heating prior to compaction, pre-heated at 1000ºC and ball milled (500 rpm) prior to pressing.
Figure 18. Bending test on NANOPYME magnet
Figure 19. NANOPYME magnets resulting from industrial compaction of different quality powders
Figure 20. Bending tests for magnets prepared from: cutting and milled (blue); cutting powders (red)
Figure 21. Pictures of magnets after mechanical tests.
Figures 22-25. Pictures of ferrite magnets (Fig. 22 and 23) and coated neodymium magnets (Figs. 24 and 25) before and after the salt-spray test.
Figure 26. Room-temperature hysteresis loops of magnet prepared from NANOPYME ferrite-based nanocomposite: before (black) and after (red) corrosion test.
Figure 27. Various grain structures considered in the study. Examples for various hard grain volume fraction and average sizes of hard and soft grains are shown.
Figure 28. Hysteresis loops (left), coercivity (upper right picture) and Eprod (lower right picture) for ferrite/Fe nanocomposite film (h = 20 nm) in dependence on the hard phase fraction ηhard.
Figure 29. Magnetic configurations (grey-scale maps of the Mx-component) for ferrite/Fe films (h = 20 nm) with different intergrain exchange weakening. The more cooperative nature of the magnetization reversal at larger exchange (κ = 0.5) is clearly seen.
Figure 30. Mesh generation stages for a bulk nanocomposites with shape-anisotropic grains
Figure 31. Correspondence between the crystaline structure and magnetization reversal for the SrFe12O19/Ni nanocomposite with weakened intergrain exchange
Figure 32. Dependence of the maximal energy product on the exchange weakening between the hard and soft phases.
Figure 33. Polycrystalline configuration used in simulations of the porous nanocomposite CoFe2O4/FeCo (pore volume fraction 10%). Left: distribution of mesh elements (Nel = 30000) and pores (white spheres). Right: distribution of crystallites (warm colors - soft phase, cold colors - hard phase). Sphere centers correspond to the centers of polyhedron mesh elements (left), crystallites (right) or pores (both panels).
Figure 34. Dependence of (BH)max on the soft phase volume fraction (left) and on the exchange weakening between crystallites (right) for the porous CoFe2O4/FeCo.
Figure 35. Schematic of SAT PMBLDC motor (1. end cover, 2. rotor yoke, 3. magnets, 4. stator winding, 5. stator tooth and 6. wheel rim).
Figure 36. Revised stator tooth and holder
Figure 37. Completed stator assembly of ferrite magnet motor
Figure 38. Ferrite magnets assembled on rotor yoke
Figure 39. Completed ferrite magnet motor
Figure 40. Ferrite magnet motor assembled on the NANOPYME vehicle
Figure 41. The test bench used for performance test. The sintered RE motor that came with vehicle is connected to the test bench.
Figure 42. The efficiency map of motor for the operating range corresponding to 0 to 30 kmph. (a) Ferrite magnet prototype motor. (b) The sintered RE magnet motor shown for comparison.
Figure 43. Acceleration time of vehicle with the ferrite magnet motor and sintered RE magnet motor.
Figure 44. Compaction tools (left) fully designed and constructed to be used with NANOPYME ferrites and implemented at industrial partners during production process (right).
Figure 45. Image (a) showing the filling in procedure of powders into the mould. The resultant segments for NANOPYME rotor can be observed on image (b).
Figure 46. Rectifying procedure used for the NANOPYME magnets.
Figure 47. Routine followed at IMA for removal of ferrite waste generated during manufacturing process through contract with a third company for waste disposal.
Figure 48. Second quadrant hysteresis loops for same materials shown in Figure 3 plus inclusion of loop for recycled powders after milling (full square symbol). This latter material constitutes a NEW product consisting of a Sr-ferrite nanocomposite.
Figure 49. Images taken at IMDEA (left) and IMA (right) showing the ferrite material resulting of scaling up (milligrams to tenths of kilograms) the recycling process developed in the laboratory (IMDEA) and moved to industry (IMA).
Figure 50. Permanent magnets in nowadays technological applications: from energy generation, going through energy transformation and to devices.
Figure 51. NANOPYME magnetic poles (16 compacted magnets) installed in the prototype motor (a second disc with additional 16 magnets facing these ones comprised the rotor installed in the motor).
Figure 52. NANOPYME e-scooter
Figure 53. NANOPYME stepper motor
Figure 54. Announcement of the NANOPYME Workshop “Rare Earth-Free Permanent magnets and Applications” (IMDEA Nanociencia, Madrid, 14-16 Sept. 2015).
Figure 55. Picture of attendees to the NANOPYME Workshop.

List of TABLES
Table 1. The powertrain specification
Table 2. Geometrical dimensions of prototype ferrite motor
Table 3. A single basic urban drive cycle according to ISO 13064-1:2012
Table 4. Energy drawn from the battery to cover eight basic urban drive cycle as per ISO 13064-1:2012