Final Report Summary - SUPER-IRON (Exploring the potential of Iron-based Superconductors)
Executive Summary:
Iron-based superconductors (FeSC), discovered in 2008 in Japan by the group of Prof. Hosono, are the newest entry among the superconducting materials, with the second record critical temperature Tc, after the high-Tc cuprate superconductors (HTSC). FeSCs share several characteristics with HTSCs, such as the layered structure, the occurrence of superconductivity upon doping, the small coherence length, and the nonconventional pairing. Some of these aspects have proved to be unsuitable for practical application. However, FeSCs exhibit several advantages with respect to HTSCs; namely, they are more metallic, and anisotropy is generally smaller.
Moreover, FeSCs are extremely versatile in terms of chemical composition, and their layered structure allows designing new FeSC with composite structures or even artificial multilayers that could allow tailoring the superconducting properties for commercial technologies.
In SUPER-IRON we depict the roadmap for exploring and exploiting the potentialities of FeSCs, pursuing the following objectives:
1. Developing preparation methods of single crystals, thin films, polycrystals and wires.
2. Tuning the superconducting properties.
3. Investigating the nature of grain boundaries.
4. Qualifying the potential of FeSCs for high field applications.
5. Strengthening the European-Japanese cooperation.
Main results achieved within the SUPER-IRON consortium:
- Several new FeSC materials have been discovered with remarkable superconducting properties. These discoveries may be functional for deciphering the nature of superconductivity in FeSCs;
- FeSC films and single crystals with outstanding properties (Tc, upper critical field and critical current) have been grown. These results prove that FeSC are positively affected by doping, defects and lattice strain, and suggest that their performance can be further improved;
- New synthesis methods of the main FeSC families have been developed. The production of pure and well-connected polycrystalline materials is the first step for the implementation of technical conductors like wires and tapes;
- The capability of technical samples (wires and coated conductors) to carry current has been largely improved, and our ultimate target performance (Jc=10^5 A/cm^2 @5 T) has been achieved. This definitely demonstrates the high potentiality of FeSCs for high field applications;
- Two successful workshops (one in Europe, one in Japan) have been organized by students for students. These events contributed to the networking of young researchers that are moving between laboratories, laying the foundation for long-term to the EU-Japan collaboration.
- A collaboration agreement, called SUPER-IRON II, devoted to continue SUPER-IRON, at the beginning without dedicated funding, has been signed by all the European and Japanese institutions involved in the original project.
Project Context and Objectives:
Since the discovery of high-Tc superconductivity of REFeAs(O,F) in 2008 [1], a large number of new layered superconducting compounds having anti-fluorite type layers of Fe-Pn (Pn = P, As) or Fe-Ch (Ch = Se, (Se,Te), (S,Te)), which are responsible for superconductivity, have been successively found. These are the so-called Iron-based superconductors (FeSC). Although the local crystal structure of the superconducting layer is unique, the FeSC have large structural and chemical variety in the blocking layer, which is sandwiched by the mechanically flexible superconducting layers. The following systems are representative of FeSC, 1111 (REFeAs(O,F)), 122 ((AE,K)Fe2As2: AE = Sr, Ba), 111 (LiFeAs), 11 (FeCh) and perovskite-type (Fe2Pn2AEn+1MnO3n-1, Fe2Pn2AEn+2MnO3n: AE = Ca,Sr,Ba; M = Sc, V, (Mg,Ti), (Al,Ti), (Sc,Ti)) systems [2]. The highest Tc ~58 K is found in SmFeAs(O,F), and is the highest value in superconductors except for layered cuprates. In addition, the FeSC generally exhibit high upper critical field, Hc2, outlining their excellent potentials as practical materials generating high magnetic fields [2, 3].
FeSCs share several characteristics with HTSCs, such as the layered structure, the coexistence of different orderings, the occurrence of superconductivity upon doping, the small coherence length, and the nonconventional pairing. Some of these aspects have shown to be unsuitable for practical application. However, FeSCs exhibit several advantages with respect to HTSCs; namely, they are metallic in the parent compounds, the anisotropy is generally smaller and not strongly dependent on the level of doping, the supposed order parameter symmetry seems to be different, and, in principle, not so detrimental to current transmission across grain boundaries, and impurities do not affect Tc significantly.
Moreover, FeSCs appear to be extremely versatile in terms of chemical composition, as they belong to a comprehensive class of materials, in which many chemical substitutions are possible, and their layered structure allows designing new FeSC with composite structures or even artificial multilayers.
This versatility could allow tailoring the superconducting properties for commercial technologies.
However, as the recent history of HTSC has taught us, the discovery of new superconductors always raises euphoric expectations concerning their applications, but there are many issues to overcome before actual devices are fabricated.
Therefore, within SUPER-IRON we depicted the roadmap for exploring and exploiting the potentialities of FeSCs, pursuing the following OBJECTIVES:
1. Developing preparation methods of single crystals, thin films, poly-crystals and wires.
2. Tuning the superconducting properties.
3. Investigating the nature of grain boundaries.
4. Qualifying the potential of FeSCs for power applications.
5. Strengthening the European-Japanese cooperation
OBJECTIVE 1 has been addressed within WP1, devoted to material preparation in form of single crystals, polycrystals, thin films, and within WP2, devoted to advanced characterization, including structural, magnetic, thermal and electrical, also under high magnetic field and/or pressure and also local probe of superconducting properties by visualization of local electric field.
OBJECTIVE 2 has been addressed within WP3, and is related to the tuning of superconducting properties, such as Tc, Hc2, anisotropy, pinning, in order to improve them.
OBJECTIVE 3 has been undertaken within WP4, which deals with the issue of grain boundaries, relying on advanced techniques for the preparation, characterization and modelling of well controlled and clean grain boundaries, and the realization of tapes and wires as well. In WP5, theoretical modelling has been developed to account for the experimental observations of the previous WPs, and to predict possible routes to improving the material performance.
OBJECTIVE 4 has been addressed within WP6, devoted to collect the best results obtained on FeSCs and compare them with data relevant to superconductors with a high application potential, with the aim of assessing the capability of FeSCs to turn into industrial products in the long term. Our ultimate target performance is a high critical current density exceeding 10^5 A/cm^2 under a magnetic field of 5 T, which is a suitable condition to judge whether the FeSC can be practically applicable materials or not.
One major objective of SUPER-IRON was to strengthen the European-Japanese cooperation (OBJECTIVE 5) aware that challenging results can be expected only by integrating and coordinating research activities. The Japanese and European groups, all extremely strong in the superconductivity field, have shared knowledge and available tools, advanced and interdisciplinary competences for the achievement of the outstanding results listed above. Steady information flow, exchange and training of researchers, and organization of common scientific events and of two student workshops have also been pivotal.
The SUPER-IRON project is coordinated by Consiglio Nazionale delle Ricerche (Italy) (CNR) in EU, and by the University of Tokyo (UT) in Japan. As to the European partners besides CNR, it associates the Atominstitut of the Vienna University of Technology (TUW) (Austria), the Institut fur Festkorper und Werkstofforschung in Dresden (IFW) and the Ludwig-Maximilians-Universität (LMU) in München (Germany), and the Ecole Polytechnique Fédérale de Lausanne (EPFL) (Switzerland), whereas, in addition to the University of Tokyo, the Japanese consortium is constituted by the University of Kyushu (KU), as well as the National Institute of Advanced Industrial Science and Technology (AIST) and National Institute for Materials Science (NIMS).
References:
[1] Kamihara Y, Watanabe T, Hirano M and Hosono H 2008, J. Am. Chem. Soc. 130 3296
[2] see for a review: Shimoyama J 2014, Supercond. Sci. Technol. 27 044002
[3] see for a review: Putti M et al 2010 Supercond. Sci. Technol. 23 034003
Project Results:
Our research program, aimed to explore the potential of the FeSC as high field generating materials for the future, was organized in the following work packages:
WP1: Iron-based material preparation,
WP2: Advanced characterization,
WP3: Tuning of the superconducting properties,
WP4: The issue of Grain Boundaries,
WP5: Modelling,
WP6: Assessment of application potential and scientific coordination,
WP7: Project Management,
WP8: Dissemination.
To clarify the intrinsic characteristics of various FeSC, polycrystalline bulks samples and single crystals with high quality were prepared by various methods, such as solid-state reaction at high temperatures with or without applying high pressure, melt-solidification using flux, and other chemical or electrochemical methods. These synthesis methods were also used in the search for new superconductors. Thin films and metal-sheathed tapes prepared for achieving high current carrying properties were fabricated by pulsed-laser-deposition (PLD) and powder-in-tube (PIT) methods, respectively.
Characterizations of superconducting properties of the samples were carried out by conventional magnetization measurements using SQUIDs and vibrating sample magnetometer (VSM), by muSR experiments, and transport, thermal and thermoelectric measurements. In addition, local superconducting properties were examined by scanning Hall-probe microscopy (SHPM) and scanning SQUID microscopy (SSM). Tuning of superconducting properties was accomplished by precise and systematic control of chemical composition of the samples including impurity doping, and by modification of the crystal structure by applying external pressure and strain. In particular, enhancement of flux pinning properties was attempted by introduction of artificial pinning produced by electron or neutron irradiations. Modelling of the superconducting properties was carried out using ab-initio, theoretical and phenomenological models.
In the following we summarize the main achievements concerning the PROPOSED OBJECTIVES.
1. Developing preparation methods of single crystals, thin films, polycrystals and wires.
The activity devoted to single crystal growth has been carried out mainly at EPFL with the new equipment for single crystal growth (High Pressure device) delivered in July 2012. The calibration procedure was carried out in September 2012 and the first SmFeAs(O,F) single crystals were grown from KAs flux with optimal properties and fully characterized in November 2012. The single crystals were delivered on time to TUW for the irradiation steps.
As to polycrystals, CNR developed a multi-step technique for the synthesis of Fe(Se0.5Te0.5) polycrystalline materials with improved superconducting properties. The three steps are the following: solid state synthesis, melting and annealing. The final samples appear dense and homogeneous and exhibit the intra-grain critical current around 10^5 A/cm^2 @5 K in agreement with values measured in the best single crystals. The global current evaluated from remanent magnetization by CNR and by Scanning Hall Probe Microscopy by TUW and KU is around 10^4 A/cm^2. These are record values for the 11 polycrystalline materials.
This technique was used to prepare highly dense 11 targets for thin film deposition used by CNR and delivered to IFW.
LMU explored metathesis reactions for the preparation of LnFeAsO powders, which would inhibit the formation of FeAs wetting phase at the grain boundaries. Moreover, LMU optimized the synthesis of polycrystalline Ba0.6K0.4Fe2As2 to be provided to CNR for the fabrication of wires. In particular, LMU developed and optimized two feasible routes: the synthesis from the elements and the ternary precursor route. The obtained samples are of very high quality prior to wire fabrication and exhibit sharp superconducting transitions at the maximal achievable Tc of this material (38 K). The advantage of the synthesis from the elements is that it is a reliable method to produce lab scale amounts (few g) of high quality powder without impurities, but upscaling to larger amounts is difficult. On the other hand, the ternary precursor route allows to fabricate larger amounts of Ba0.6K0.4Fe2As2 with a homogeneous potassium distribution, but currently some FeAs impurities must be accepted (less than 9 %), which leaves room for further improvement.
As concerns thin films, IFW optimized the pulsed laser deposition of 122 and 11 thin films towards the layer-by-layer growth for the processing of epitaxial artificial multilayers.
CNR optimized the growth condition of Fe1+ySexTe1 thin films with a new NdYAG laser at 1024 nm (purchased within the project) which allowed a much higher deposition rate. Thin films grown on substrates with different to lattice mismatch present different Tc, different critical current values and magnetic field behaviours. These films have been fully investigated (Jc anisotropy measurements by TUW, TEM characterization by IFW, Scanning SQUID and Scanning Hall probe microscopy by KU).
These results confirm the large tunability (in terms of strain and interaction with the substrate) of these materials and are achievements of OBJECTIVE 2: “Tuning of superconducting properties “ .
Concerning the preparation of wires, the first European (Ba,K)Fe2As2 superconducting wires have been realized thanks to the collaboration between the LMU (powder production) and CNR (wire manufacturing) groups. They realized Ag sheathed (Ba,K)Fe2As2 tapes by ex-situ Powder in Tube (PIT) method. Highly pure superconducting cores, with Tc ~38 K and critical current value, Jc, close to 3x10^4 A/cm^2 at 4.2 K in self-field and of the order of 10^4 A/cm^2 K up to 7T, have been tested. Even though the Jc values are still one order of magnitude lower than the record values in the literature, these results are extremely encouraging because the developed method can be further implemented. Moreover, using the LMU powders made with the ternary precursor route, an original in-situ PIT method has been proposed, and the first tapes manufactured with this method have been tested.
2. Tuning the superconducting properties
To tune the superconducting properties (Tc, anisotropy, critical fields and pinning), materials can be modified by realizing artificial superstructures, or by creating defects by irradiation or by the introduction of impurities. Last but not least, the discovery of new FeSC families may be functional for deciphering the nature of superconductivity in FeSC and for improving the superconducting properties. The activities devoted to achieve these objectives have been implemented within WP3; the theoretical work performed within WP5 has supported and stimulated them. The main achievements are listed in the following.
IFW tuned the phase diagram of Co, Ru, and P doped Ba-122 thin films by in-plain strain induced by different substrates and growing multilayers structures such as (Fe/Ba(Fe,Co)2As2 and Ba(Fe,Co)2As2/ Fe(Se,Te)). It turned out that strain is the only possibility to tune (i.e. enhance) the critical temperature of an optimally doped compound: indeed, strained P-doped Ba-122 films exhibit Tc over 30 K and high Jc > 6 MA/cm^2 at 4.2 K, which is among the highest for FeSC. In-field Jc exceeds 0.1 MA/cm^2 at 35 T for B // ab and 18 T for B // c.
CNR improved the phase diagram of the 11 family substantially by optimizing the growth of Fe(Te,Se) on CaF substrates, (Tc >21 K, Bc2 >70 T; Jc > 1 MA/cm^2 in self field with a very weak dependence on the magnetic field and an isotropic behaviour .
TUW performed neutron irradiation of single crystals provided by EPFL (1111), AIST (122). Jc was greatly enhanced in all compounds and its angular dependence was reversed in crystals where pinning was originally weak. The highest ever reported values for 122 single crystals were established in the K-doped system: 7.4 and 5.9 x 10^6 A/cm^2 at 2 and 5 K in self-field (~0.8 and 0.65 T), respectively. Heavy ion irradiation of SmFeAsO0.8F0.15 single crystal increased Jc to up to 2 × 10^7 A/cm^2 at 5 K, the highest ever reported for an FeSC.
Within SUPER-IRON a large number of new compounds have been discovered, including new superconductors.
EPFL discovered a new high-Tc FeAs-based family, Pr4Fe2As2Te1-xO. The first member of this family (42214) is Pr4Fe2As2Te1-xO with Tc = 25 K without additional doping. In a second time the structure of 42214 was modified by replacing Pr for Sm and Gd as well as by doping with fluorine on the oxygen site. The critical temperature of Gd42214 single crystals increased up to 45 K after F doping. Although these new compounds seem to share the same building blocks of the 1111 materials, the presence of Te spacer planes adds new possibilities to modify the structural properties and the charge density of the material: in order to investigate these effects, ab-initio calculation have been performed within WP5.
The soft chemistry hydrothermal approach has been developed by LMU. Using this method the synthesis of the new metastable superconductor Na1-yFe2-xAs2 was successfully optimized. Moreover, by this approach LMU succeeded in the synthesis of the new ~40 K superconductor [(Li,Fe)OH]FeSe, which exhibits an extraordinary coexistence of ferromagnetism in the (Li,Fe)OH layers with superconductivity in the FeSe layers.
A new family (112) (Ca,RE)FeAs2 (RE = La,Ce,Pr,Sm,Eu,Gd) has been developed by a joint research of the UT, AIST and CNR groups. It has a completely new crystal structure with zigzag As-chain plane in the blocking layer. In the early stage of the synthesis of this new phase, the LMU group contributed to improve preparation procedures. Superconducting properties of Ca112 is improved by small amount of Co-doping, resulting in Tc~40 K and a relatively high Jc ~ 2.1 x 10^4 A/cm^2 at 2 K, suggesting bulk superconductivity. This activity is carried out mainly in Japan, but it is important mention here that a group of young researchers (F. Caglieris (CNR), A. Sala (CNR and AIST), F. Ricci (CNR) and F. Hummel (LMU)) is collaborating on this subject and joint papers are in preparation.
The most important achievement regarding the OBJECTIVE 2 within WP5 concerns the development of theoretical framework to predict superconducting properties in real materials. A screened effective electron-electron interaction has been formulated, which includes the effect of low energy spin-fluctuations in a computationally feasible way and completely ab-initio.
This many-body perturbation based effective interaction has been used within density functional theory for superconductors, and the first application on bulk FeSe already gave promising results, predicting Tc=5K, of the same order of the experimental measured one (8K).
3. Investigating the nature of grain boundaries (GB)
OBJECTIVE 3 is one of the critical task to power applications based on FeSC. This investigation has been performed both from the experimental and theoretical point of view within WP4 and WP5, respectively. The related activities were originally scheduled to start in the second year but several important results have been obtained even earlier. In particular, the capability of technical conductors based on the FeSC to carry current has been improved much faster than we expected at the beginning, and this called for a review of the planned activities within WP4. This has been done during the Midterm meeting where important aspects were discussed and some changes have been decided which have been formalized in the amendment of the Grant that have been approved in July 2014.
Given that this OBJECTIVE was one of the most challenging, not all experiments can be thought as completely realised by the end of the project. On the other hand the results obtained are extremely important.
IFW realized biaxially textured BaFe1.8Co0.2As2 thin films on high-quality IBAD-textured MgO-coated technical substrates utilizing additional Fe buffer layers. High Jc values were achieved (Jc > 1 MA/cm^2 in self-field and >10^5 A/cm^2at 10 T at 4 K). This value largely overcomes the “Assessment criteria for power applications” that were assumed when we wrote the project (Jc > 10^5 A /cm^2 at 4 K at 5 T).
Co-doped Ba122 bicrystalline films were successfully deposited within the first period by IFW on MgO bicrystal substrates using the Fe buffer approach (bicrystal angles 18°, 24° and 37°). An exponential decrease in Jc with the bicrystal angle was observed. Similar results were obtained by CNR on 11 on bicrystal substrates with angles of 10°, 24° and 45°. Owing to the limited avalaibility on the market of low angle bicrystalline substrates, detailed information on the Jc dependence on bicrystal angle is not available yet, but the overall results on bicrystalline thin films indicate that intergrain coupling in FeSC is intrinsically much better than in cuprate superconductors.
The nature of grain boundaries in polycrystalline materials has been mainly investigated by the Japanese team, and therefore not reported here. However, some of the results have been obtained in collaborations between Japanese and European partners, and will contribute to joint publications.
Original theoretical approaches have been developed within WP5. A model Hamiltonian was developed to address the grain boundary problem in FeSC. The aim is to provide information on the behaviour of Jc as a function of the misorientation angle. To tackle this problem, different microscopic models for grain boundaries were built up increasing the degree of complexity whilst providing more and more realistic information on the grain boundary issue. The current flow in polycrystalline samples was addressed by the University of Tokyo (UT)and the TUW groups in a phenomenological way. While TUW uses a mean-field approach, UT bases the description on the numerical simulation of non-linear resistor networks.
Finally, the overall theoretical and experimental results obtained on polycrystals and bicrystalline thin films indicate that intergrain coupling in FeSC is intrinsically much better than in cuprate superconductors. This statement represents one of the main achievements of the project.
4. Qualifying the potential of FeSCs for power applications.
This activity has been carried out within WP6, purpose of which has been to survey and collect data on other superconductors with a high potential for power applications and compare them with results obtained in WP2, WP3 and WP4 on FeSCs. While in the first year of the project, an exhaustive report of Survey data on technical superconductors has been collected, in the following years the focus has been mainly on FeSCs only. Crucial parameters (critical temperatures, fields and currents) were assessed with the final aim of evaluating the potential fields of application of FeSCs.
In particular, the worldwide state of the art of the research on FeSCs was monitored since the very beginning of the project. Since month 13, the literature data were more systematically collected, compared and assessed. This activity has yielded the submission of a manuscript by Ilaria Pallecchi and Marina Putti, namely a chapter on FeSCs to be published in the book "Handbook of Applied Superconductivity", Publisher Wiley (http://www.wiley-vch.de/publish/en) edited by Professor Paul Seidel. Moreover, Prof. Shimoyama, project leader for the Japanese twinned project, gave a plenary talk about the application potential and future perspectives of FeSCs at the European Conference on Applied Superconductivity (Eucas) 2013, held in Genoa, 15-19 September, 2013 and wrote a relevant review paper (Supercond. Sci. Technol. 27, 044002 (2014)), which can be considered closely related to the activities of WP6.
During the last period the optimal results achieved on FeSCs both within the SUPER-IRON project and worldwide have been collected and directly compared with the corresponding results obtained on other technical superconductors. Quantitative comparisons have been made. A realistic assessment of the application potential of FeSCs for each specific application in suitable temperature and magnetic field ranges, either at the present moment or in the perspective of the forthcoming years, has been presented. At the end of the project, a final report on the results obtained worldwide and within the SUPER-IRON project on Fe-based superconductors, and a final assessment of their application potential of FeSCs has been submitted as a review paper on “The Superconductivity News Forum-Global Edition”(http://snf.ieeecsc.org/).
5. Strengthening the European-Japanese cooperation
This OBJECTIVE has been implemented by several actions that have been planned and coordinated within WP7.
Student Workshops: At the beginning of the project two young researchers, Dr. Fabian Nitsche from LMU and Dr. Nakajima Masamichi from AIST, were appointed for coordinating EUROPEAN-JAPANESE activities. Among these last they organized the first Student Workshop in Dresden, (May the 5th - May the 6th, 2013). The participants were 21 from Japan and Europe. The second Student Workshop took place in Tsukuba, Japan, April 6th-8th, 2014. The meeting was organized by Dr. Shigeyuki Ishida from AIST, who substituted Dr. Nakajima Masamichi, in collaboration with Dr. Fabian Nitsche. The participants were 24 from Japan and Europe.
The aim of the Student Workshops is training of young scientists in the field of superconducting materials, but also establishing a platform for strengthening personal and working contacts between young European and Japanese scientists. This objective has been fulfilled, as proved by the numerous collaborations established among the students.
- Exchanges: From a factual point of view, a tight collaboration has developed among the European and Japanese partners, as demonstrated by the copious exchange of samples, and by the exchanges of students and visits. Complete lists of both have been inserted in the WP6 report. We mention here the one+one years spent by Alberto Sala (CNR) at the University of Tokyo and at AIST during his PhD course.
- Common publications. Regarding the publications, some 132 papers have appeared since the beginning of the project, written by the European and Japanese partners, and among them 18 are issued by multiple partners. In addition, 160 communications at international conferences (43 of these are from invited talks) authored by the European and Japanese partners are reckoned, out of which 18 are by joint European and Japanese partners
- Meetings. European and Japanese partners met in occasion of four official meetings and in occasion of several international conferences. The official meetings have been the following: the Kick-off meeting in Genoa, December 2011; the 12th-month meeting in Tokyo, Japan, December 2012; the 24th-month meeting in Genoa, Italy, September 19th-20th, 2013 after the closure the EUCAS2013 conference; the “Joint Workshop on Iron-Based Superconductors, by SUPER-IRON and IRON-SEA” on March 10-11th, 2015 at the JST Tokyo Headquarters. This last was organized by Prof. Jun-ichi Shimoyama and Prof. Hiroshi Ikuta, and attended by representatives of all the groups involved in the two projects, in addition to the EC and JST Research Program Officers.
- Final review meeting: In March 2015 the twinned European-Japanese projects ended. Therefore, on March 9th a final review meeting was held in Tokyo, in the headquarters of JST. It was attended by the Japanese and European project Coordinators, by Officers and Expert External Reviewers. This event was preceded by the submission of a review Report. As within SUPER-IRON the European and Japanese twinned projects shared both objectives and implementation plan, the two Coordinators of SUPER-IRON, M.Putti and J-I Shimoyama, decided to co-author one joint report. The final interview has been prepared and given and by the two Coordinators (see in the attached document an extract of their final presentation).
This can be viewed as a clear indication of the close collaboration and synergies established within the project.
- Collaboration agreement. One of the main tasks of the project was the signature of an agreement of collaboration among the European and Japanese institutions, to carry on the common activities beyond the duration of the project. Thus, a collaboration agreement, called SUPER-IRON II, devoted to continue SUPER-IRON without dedicated funding, has been proposed to all the European and Japanese institutions involved in the original project. All the Institutions have agreed to take part in it, and SUPER-IRON II has been ratified during the final meeting of the project, which took place on March 10th -11th 2015.
PROJECT ASSESSMENT
The comparison between the SUPER-IRON OBJECTIVES and the achieved results looks extremely positive. The conceived work plan was grounded on a deep investigation of FeSC, including development of materials, tuning of the properties, and understanding of critical current limitation in order to assess their potential for high field applications.
Many of these aspects have been carefully addressed. As reported above, we proved that FeSCs show great flexibility to defects and/or lattice strain, which affects positively the superconducting properties; several new FeSC materials were discovered both within the SUPER-IRON consortium (including the Japanese team) and outside it, with remarkable superconducting properties; this research will continue with the support and feedbacks coming by the extraordinary theoretical tool developed within SUPER-IRON to predict superconducting properties in real materials; investigation of bicrystalline thin films and polycrystalline materials (most of the results have been obtained by the Japanese team) has indicated that intergrain coupling in FeSC is intrinsically much better than in HTSC. This is proved by the fact that capability of technical samples (wires and coated conductors) to carry current improved very fast, indeed our ultimate target performance had been achieved in the first period.
Potential Impact:
In order to evaluate the societal implication of the SUPER-IRON results some tasks have been scheduled during the project. They concerned the collection of the results achieved on FeSCs both within the SUPER-IRON project and worldwide, the comparison with the corresponding results obtained on other technical superconductors and the assessment the application potential of FeSCs for each specific application in suitable temperature and magnetic field ranges, either at the present moment or in the perspective of the forthcoming years.
The conclusion reached by these investigations is that the FeSC performance in terms of the highest possible operating temperature as well as achievable fields and currents is between those of the HTSC and the conventional superconductors (Nb3Sn, MgB2 and Nb-Ti). Thus, they have to be either cheaper than the coated conductors or outperform the conventional wire technology. Both scenarios do seem to be realistic. We pointed out that the critical currents of inexpensive PIT processed wires are already nearing the performance requirements of many applications. If this trend continues, the FeSCs have good prospect for replacing the conventional superconductors and enabling a comparatively cheap magnet technology for generating high fields at low temperatures or intermediate fields at temperatures achievable with cryocoolers. If not, FeSC coated conductors could compete with cuprate-based wires for extremely high field magnets, since they would be less expensive, but offer enough performance for enhancing the fields currently achievable by superconducting magnets.
SUPER-IRON is a fundamental research project, therefore immediate exploitation of its results was not envisaged, but its aim was to draw a roadmap to industrial application. The project continuation has been planned by the partners for this purpose, and preliminary contacts have been established with industry since the early phases.
The next steps will be the following:
(i) to select the best family/doping/strain which optimize the superconducting phase diagram (Tc, Hc2, Jc);
(ii) to demonstrate the feasibility and reproducibility of optimized properties on large scale;
(iii) to develop a scalable and industrially appealing method for the production of FeSC first generation cables.
SUPER-IRON II, the collaboration agreement signed by all the European and Japanese institutions will continue to work on these objectives.
As to the dissemination plan, several channels have been chosen, i.e.:
• creation of dedicated website;
• presence on superconductivity correlated website;
• publication on academic journals;
• participation in the major superconductivity conferences;
• dedicated session at Eucas 2013;
• Workshops organization.
The project website www.SUPER-IRON.eu set up since the beginning, has been constantly updated, both in the public and the restricted area, so that it has maintained its function of reference point for both Partners and interested people-
Moreover, the activities of SUPER-IRON have been promoted by posting communications on the “SUPERCONDUCTIVITY NEWS FORUM” (SNF), which freely distributes announcements, new papers and event highlights within the scientific community. SNF, having an audience of more than 5000 readers involved in superconductivity, is the worldwide broadest and fastest dissemination tool in this field. It also fulfils the role of promotion, assessment and dissemination towards industrial partners. In particular, the following event highlights have been announced: the kick-off of SUPER-IRON, the PhD Student Workshops, and the Joint Final Workshop.
In addition to them, SNF has accepted for publication a review entitled “Assessment of the application potential of FeSCs”, by I.Pallecchi et al, extracted from corresponding evaluation report activity carried out in the project.
As part of the assessment of the status of project activities, the participation to conferences and publication of papers on international journals have been promoted and monitored.
Regarding publications, 132 papers have appeared since the beginning of the project, authored by European and Japanese partners, and among these 18 are authored by multiple partners. Apart from some theoretical works involving only WP5, most of the papers involve two or more WPs, which indicates the strong interaction, synergy and complementarity of the project activities. 59 out of 132 papers are authored by European partners, 42 of them were published during the second period the end of the project. Hence, the number of publication has increased significantly over the course of the project and likely new publications authored by this consortium will follow in the next future as well.
The publications are distributed among top ranked journals of the field (Applied Physics Letter, Physical Review Letters, Superconducting Science and Technology). These journals are not open-access, but several published papers have been posted on arxiv.org an open access archive that collects more than one million e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.
The results of SUPER-IRON have been presented at several International conferences with broad audiences on record. The most important conferences of the field are the following: The European Conference of Applied Superconductivity (EUCAS) (~1000 participants), The Applied Superconductivity Conference (ASC) (~1500 participants), the International Conference on Superconductivity and Magnetisms (ICSM) (~1500 participants), the International Symposium on Superconductivity (ISS) (~800 participants).
67 communications authored by European partners are reckoned, out of which 18 are authored by joint partners, European and Japanese. 21 of these communications are represented by invited talks. Among them, Prof. M. Putti has been invited to present the results of SUPER-IRON project at MRS 2013 and ICSM 2014.
As a further outreach initiative, a session of Eucas2013, entitled: ”Fe-based Superconductors-Bulks and Tapes” has been hosted by SUPER-IRON EU-Japan project. At the same conference Prof. J. Shimoyama gave a plenary talk on the state of the art of FeSC.
As for the Workshops, three main events have been organized: two Student Workshops, one in Europe and one in Japan, and the “Joint Workshop on Iron-Based Superconductors” at the end of the project, in Tokyo.
These events have been functional to strengthen the European-Japanese cooperation and are described in the section above. They had been advertised as open events, and were attended also by participants external to the project, thus contributing to the dissemination of project results. Moreover, the Student Workshops have also represented a precious training tool.
List of Websites:
http://www.super-iron.eu/
CONTACTS
Marina Putti
Project Coordinator
marina.putti@spin.cnr.it
Mariachiara Lupi
Project Manager
mariachiara.lupi@spin.cnr.it
Elisabetta Narducci
Website Manager
elisabetta.narducci@spin.cnr.it
Barbara Cagnana
Administrative Consultant
barbara.cagnana@spin.cnr.it
Iron-based superconductors (FeSC), discovered in 2008 in Japan by the group of Prof. Hosono, are the newest entry among the superconducting materials, with the second record critical temperature Tc, after the high-Tc cuprate superconductors (HTSC). FeSCs share several characteristics with HTSCs, such as the layered structure, the occurrence of superconductivity upon doping, the small coherence length, and the nonconventional pairing. Some of these aspects have proved to be unsuitable for practical application. However, FeSCs exhibit several advantages with respect to HTSCs; namely, they are more metallic, and anisotropy is generally smaller.
Moreover, FeSCs are extremely versatile in terms of chemical composition, and their layered structure allows designing new FeSC with composite structures or even artificial multilayers that could allow tailoring the superconducting properties for commercial technologies.
In SUPER-IRON we depict the roadmap for exploring and exploiting the potentialities of FeSCs, pursuing the following objectives:
1. Developing preparation methods of single crystals, thin films, polycrystals and wires.
2. Tuning the superconducting properties.
3. Investigating the nature of grain boundaries.
4. Qualifying the potential of FeSCs for high field applications.
5. Strengthening the European-Japanese cooperation.
Main results achieved within the SUPER-IRON consortium:
- Several new FeSC materials have been discovered with remarkable superconducting properties. These discoveries may be functional for deciphering the nature of superconductivity in FeSCs;
- FeSC films and single crystals with outstanding properties (Tc, upper critical field and critical current) have been grown. These results prove that FeSC are positively affected by doping, defects and lattice strain, and suggest that their performance can be further improved;
- New synthesis methods of the main FeSC families have been developed. The production of pure and well-connected polycrystalline materials is the first step for the implementation of technical conductors like wires and tapes;
- The capability of technical samples (wires and coated conductors) to carry current has been largely improved, and our ultimate target performance (Jc=10^5 A/cm^2 @5 T) has been achieved. This definitely demonstrates the high potentiality of FeSCs for high field applications;
- Two successful workshops (one in Europe, one in Japan) have been organized by students for students. These events contributed to the networking of young researchers that are moving between laboratories, laying the foundation for long-term to the EU-Japan collaboration.
- A collaboration agreement, called SUPER-IRON II, devoted to continue SUPER-IRON, at the beginning without dedicated funding, has been signed by all the European and Japanese institutions involved in the original project.
Project Context and Objectives:
Since the discovery of high-Tc superconductivity of REFeAs(O,F) in 2008 [1], a large number of new layered superconducting compounds having anti-fluorite type layers of Fe-Pn (Pn = P, As) or Fe-Ch (Ch = Se, (Se,Te), (S,Te)), which are responsible for superconductivity, have been successively found. These are the so-called Iron-based superconductors (FeSC). Although the local crystal structure of the superconducting layer is unique, the FeSC have large structural and chemical variety in the blocking layer, which is sandwiched by the mechanically flexible superconducting layers. The following systems are representative of FeSC, 1111 (REFeAs(O,F)), 122 ((AE,K)Fe2As2: AE = Sr, Ba), 111 (LiFeAs), 11 (FeCh) and perovskite-type (Fe2Pn2AEn+1MnO3n-1, Fe2Pn2AEn+2MnO3n: AE = Ca,Sr,Ba; M = Sc, V, (Mg,Ti), (Al,Ti), (Sc,Ti)) systems [2]. The highest Tc ~58 K is found in SmFeAs(O,F), and is the highest value in superconductors except for layered cuprates. In addition, the FeSC generally exhibit high upper critical field, Hc2, outlining their excellent potentials as practical materials generating high magnetic fields [2, 3].
FeSCs share several characteristics with HTSCs, such as the layered structure, the coexistence of different orderings, the occurrence of superconductivity upon doping, the small coherence length, and the nonconventional pairing. Some of these aspects have shown to be unsuitable for practical application. However, FeSCs exhibit several advantages with respect to HTSCs; namely, they are metallic in the parent compounds, the anisotropy is generally smaller and not strongly dependent on the level of doping, the supposed order parameter symmetry seems to be different, and, in principle, not so detrimental to current transmission across grain boundaries, and impurities do not affect Tc significantly.
Moreover, FeSCs appear to be extremely versatile in terms of chemical composition, as they belong to a comprehensive class of materials, in which many chemical substitutions are possible, and their layered structure allows designing new FeSC with composite structures or even artificial multilayers.
This versatility could allow tailoring the superconducting properties for commercial technologies.
However, as the recent history of HTSC has taught us, the discovery of new superconductors always raises euphoric expectations concerning their applications, but there are many issues to overcome before actual devices are fabricated.
Therefore, within SUPER-IRON we depicted the roadmap for exploring and exploiting the potentialities of FeSCs, pursuing the following OBJECTIVES:
1. Developing preparation methods of single crystals, thin films, poly-crystals and wires.
2. Tuning the superconducting properties.
3. Investigating the nature of grain boundaries.
4. Qualifying the potential of FeSCs for power applications.
5. Strengthening the European-Japanese cooperation
OBJECTIVE 1 has been addressed within WP1, devoted to material preparation in form of single crystals, polycrystals, thin films, and within WP2, devoted to advanced characterization, including structural, magnetic, thermal and electrical, also under high magnetic field and/or pressure and also local probe of superconducting properties by visualization of local electric field.
OBJECTIVE 2 has been addressed within WP3, and is related to the tuning of superconducting properties, such as Tc, Hc2, anisotropy, pinning, in order to improve them.
OBJECTIVE 3 has been undertaken within WP4, which deals with the issue of grain boundaries, relying on advanced techniques for the preparation, characterization and modelling of well controlled and clean grain boundaries, and the realization of tapes and wires as well. In WP5, theoretical modelling has been developed to account for the experimental observations of the previous WPs, and to predict possible routes to improving the material performance.
OBJECTIVE 4 has been addressed within WP6, devoted to collect the best results obtained on FeSCs and compare them with data relevant to superconductors with a high application potential, with the aim of assessing the capability of FeSCs to turn into industrial products in the long term. Our ultimate target performance is a high critical current density exceeding 10^5 A/cm^2 under a magnetic field of 5 T, which is a suitable condition to judge whether the FeSC can be practically applicable materials or not.
One major objective of SUPER-IRON was to strengthen the European-Japanese cooperation (OBJECTIVE 5) aware that challenging results can be expected only by integrating and coordinating research activities. The Japanese and European groups, all extremely strong in the superconductivity field, have shared knowledge and available tools, advanced and interdisciplinary competences for the achievement of the outstanding results listed above. Steady information flow, exchange and training of researchers, and organization of common scientific events and of two student workshops have also been pivotal.
The SUPER-IRON project is coordinated by Consiglio Nazionale delle Ricerche (Italy) (CNR) in EU, and by the University of Tokyo (UT) in Japan. As to the European partners besides CNR, it associates the Atominstitut of the Vienna University of Technology (TUW) (Austria), the Institut fur Festkorper und Werkstofforschung in Dresden (IFW) and the Ludwig-Maximilians-Universität (LMU) in München (Germany), and the Ecole Polytechnique Fédérale de Lausanne (EPFL) (Switzerland), whereas, in addition to the University of Tokyo, the Japanese consortium is constituted by the University of Kyushu (KU), as well as the National Institute of Advanced Industrial Science and Technology (AIST) and National Institute for Materials Science (NIMS).
References:
[1] Kamihara Y, Watanabe T, Hirano M and Hosono H 2008, J. Am. Chem. Soc. 130 3296
[2] see for a review: Shimoyama J 2014, Supercond. Sci. Technol. 27 044002
[3] see for a review: Putti M et al 2010 Supercond. Sci. Technol. 23 034003
Project Results:
Our research program, aimed to explore the potential of the FeSC as high field generating materials for the future, was organized in the following work packages:
WP1: Iron-based material preparation,
WP2: Advanced characterization,
WP3: Tuning of the superconducting properties,
WP4: The issue of Grain Boundaries,
WP5: Modelling,
WP6: Assessment of application potential and scientific coordination,
WP7: Project Management,
WP8: Dissemination.
To clarify the intrinsic characteristics of various FeSC, polycrystalline bulks samples and single crystals with high quality were prepared by various methods, such as solid-state reaction at high temperatures with or without applying high pressure, melt-solidification using flux, and other chemical or electrochemical methods. These synthesis methods were also used in the search for new superconductors. Thin films and metal-sheathed tapes prepared for achieving high current carrying properties were fabricated by pulsed-laser-deposition (PLD) and powder-in-tube (PIT) methods, respectively.
Characterizations of superconducting properties of the samples were carried out by conventional magnetization measurements using SQUIDs and vibrating sample magnetometer (VSM), by muSR experiments, and transport, thermal and thermoelectric measurements. In addition, local superconducting properties were examined by scanning Hall-probe microscopy (SHPM) and scanning SQUID microscopy (SSM). Tuning of superconducting properties was accomplished by precise and systematic control of chemical composition of the samples including impurity doping, and by modification of the crystal structure by applying external pressure and strain. In particular, enhancement of flux pinning properties was attempted by introduction of artificial pinning produced by electron or neutron irradiations. Modelling of the superconducting properties was carried out using ab-initio, theoretical and phenomenological models.
In the following we summarize the main achievements concerning the PROPOSED OBJECTIVES.
1. Developing preparation methods of single crystals, thin films, polycrystals and wires.
The activity devoted to single crystal growth has been carried out mainly at EPFL with the new equipment for single crystal growth (High Pressure device) delivered in July 2012. The calibration procedure was carried out in September 2012 and the first SmFeAs(O,F) single crystals were grown from KAs flux with optimal properties and fully characterized in November 2012. The single crystals were delivered on time to TUW for the irradiation steps.
As to polycrystals, CNR developed a multi-step technique for the synthesis of Fe(Se0.5Te0.5) polycrystalline materials with improved superconducting properties. The three steps are the following: solid state synthesis, melting and annealing. The final samples appear dense and homogeneous and exhibit the intra-grain critical current around 10^5 A/cm^2 @5 K in agreement with values measured in the best single crystals. The global current evaluated from remanent magnetization by CNR and by Scanning Hall Probe Microscopy by TUW and KU is around 10^4 A/cm^2. These are record values for the 11 polycrystalline materials.
This technique was used to prepare highly dense 11 targets for thin film deposition used by CNR and delivered to IFW.
LMU explored metathesis reactions for the preparation of LnFeAsO powders, which would inhibit the formation of FeAs wetting phase at the grain boundaries. Moreover, LMU optimized the synthesis of polycrystalline Ba0.6K0.4Fe2As2 to be provided to CNR for the fabrication of wires. In particular, LMU developed and optimized two feasible routes: the synthesis from the elements and the ternary precursor route. The obtained samples are of very high quality prior to wire fabrication and exhibit sharp superconducting transitions at the maximal achievable Tc of this material (38 K). The advantage of the synthesis from the elements is that it is a reliable method to produce lab scale amounts (few g) of high quality powder without impurities, but upscaling to larger amounts is difficult. On the other hand, the ternary precursor route allows to fabricate larger amounts of Ba0.6K0.4Fe2As2 with a homogeneous potassium distribution, but currently some FeAs impurities must be accepted (less than 9 %), which leaves room for further improvement.
As concerns thin films, IFW optimized the pulsed laser deposition of 122 and 11 thin films towards the layer-by-layer growth for the processing of epitaxial artificial multilayers.
CNR optimized the growth condition of Fe1+ySexTe1 thin films with a new NdYAG laser at 1024 nm (purchased within the project) which allowed a much higher deposition rate. Thin films grown on substrates with different to lattice mismatch present different Tc, different critical current values and magnetic field behaviours. These films have been fully investigated (Jc anisotropy measurements by TUW, TEM characterization by IFW, Scanning SQUID and Scanning Hall probe microscopy by KU).
These results confirm the large tunability (in terms of strain and interaction with the substrate) of these materials and are achievements of OBJECTIVE 2: “Tuning of superconducting properties “ .
Concerning the preparation of wires, the first European (Ba,K)Fe2As2 superconducting wires have been realized thanks to the collaboration between the LMU (powder production) and CNR (wire manufacturing) groups. They realized Ag sheathed (Ba,K)Fe2As2 tapes by ex-situ Powder in Tube (PIT) method. Highly pure superconducting cores, with Tc ~38 K and critical current value, Jc, close to 3x10^4 A/cm^2 at 4.2 K in self-field and of the order of 10^4 A/cm^2 K up to 7T, have been tested. Even though the Jc values are still one order of magnitude lower than the record values in the literature, these results are extremely encouraging because the developed method can be further implemented. Moreover, using the LMU powders made with the ternary precursor route, an original in-situ PIT method has been proposed, and the first tapes manufactured with this method have been tested.
2. Tuning the superconducting properties
To tune the superconducting properties (Tc, anisotropy, critical fields and pinning), materials can be modified by realizing artificial superstructures, or by creating defects by irradiation or by the introduction of impurities. Last but not least, the discovery of new FeSC families may be functional for deciphering the nature of superconductivity in FeSC and for improving the superconducting properties. The activities devoted to achieve these objectives have been implemented within WP3; the theoretical work performed within WP5 has supported and stimulated them. The main achievements are listed in the following.
IFW tuned the phase diagram of Co, Ru, and P doped Ba-122 thin films by in-plain strain induced by different substrates and growing multilayers structures such as (Fe/Ba(Fe,Co)2As2 and Ba(Fe,Co)2As2/ Fe(Se,Te)). It turned out that strain is the only possibility to tune (i.e. enhance) the critical temperature of an optimally doped compound: indeed, strained P-doped Ba-122 films exhibit Tc over 30 K and high Jc > 6 MA/cm^2 at 4.2 K, which is among the highest for FeSC. In-field Jc exceeds 0.1 MA/cm^2 at 35 T for B // ab and 18 T for B // c.
CNR improved the phase diagram of the 11 family substantially by optimizing the growth of Fe(Te,Se) on CaF substrates, (Tc >21 K, Bc2 >70 T; Jc > 1 MA/cm^2 in self field with a very weak dependence on the magnetic field and an isotropic behaviour .
TUW performed neutron irradiation of single crystals provided by EPFL (1111), AIST (122). Jc was greatly enhanced in all compounds and its angular dependence was reversed in crystals where pinning was originally weak. The highest ever reported values for 122 single crystals were established in the K-doped system: 7.4 and 5.9 x 10^6 A/cm^2 at 2 and 5 K in self-field (~0.8 and 0.65 T), respectively. Heavy ion irradiation of SmFeAsO0.8F0.15 single crystal increased Jc to up to 2 × 10^7 A/cm^2 at 5 K, the highest ever reported for an FeSC.
Within SUPER-IRON a large number of new compounds have been discovered, including new superconductors.
EPFL discovered a new high-Tc FeAs-based family, Pr4Fe2As2Te1-xO. The first member of this family (42214) is Pr4Fe2As2Te1-xO with Tc = 25 K without additional doping. In a second time the structure of 42214 was modified by replacing Pr for Sm and Gd as well as by doping with fluorine on the oxygen site. The critical temperature of Gd42214 single crystals increased up to 45 K after F doping. Although these new compounds seem to share the same building blocks of the 1111 materials, the presence of Te spacer planes adds new possibilities to modify the structural properties and the charge density of the material: in order to investigate these effects, ab-initio calculation have been performed within WP5.
The soft chemistry hydrothermal approach has been developed by LMU. Using this method the synthesis of the new metastable superconductor Na1-yFe2-xAs2 was successfully optimized. Moreover, by this approach LMU succeeded in the synthesis of the new ~40 K superconductor [(Li,Fe)OH]FeSe, which exhibits an extraordinary coexistence of ferromagnetism in the (Li,Fe)OH layers with superconductivity in the FeSe layers.
A new family (112) (Ca,RE)FeAs2 (RE = La,Ce,Pr,Sm,Eu,Gd) has been developed by a joint research of the UT, AIST and CNR groups. It has a completely new crystal structure with zigzag As-chain plane in the blocking layer. In the early stage of the synthesis of this new phase, the LMU group contributed to improve preparation procedures. Superconducting properties of Ca112 is improved by small amount of Co-doping, resulting in Tc~40 K and a relatively high Jc ~ 2.1 x 10^4 A/cm^2 at 2 K, suggesting bulk superconductivity. This activity is carried out mainly in Japan, but it is important mention here that a group of young researchers (F. Caglieris (CNR), A. Sala (CNR and AIST), F. Ricci (CNR) and F. Hummel (LMU)) is collaborating on this subject and joint papers are in preparation.
The most important achievement regarding the OBJECTIVE 2 within WP5 concerns the development of theoretical framework to predict superconducting properties in real materials. A screened effective electron-electron interaction has been formulated, which includes the effect of low energy spin-fluctuations in a computationally feasible way and completely ab-initio.
This many-body perturbation based effective interaction has been used within density functional theory for superconductors, and the first application on bulk FeSe already gave promising results, predicting Tc=5K, of the same order of the experimental measured one (8K).
3. Investigating the nature of grain boundaries (GB)
OBJECTIVE 3 is one of the critical task to power applications based on FeSC. This investigation has been performed both from the experimental and theoretical point of view within WP4 and WP5, respectively. The related activities were originally scheduled to start in the second year but several important results have been obtained even earlier. In particular, the capability of technical conductors based on the FeSC to carry current has been improved much faster than we expected at the beginning, and this called for a review of the planned activities within WP4. This has been done during the Midterm meeting where important aspects were discussed and some changes have been decided which have been formalized in the amendment of the Grant that have been approved in July 2014.
Given that this OBJECTIVE was one of the most challenging, not all experiments can be thought as completely realised by the end of the project. On the other hand the results obtained are extremely important.
IFW realized biaxially textured BaFe1.8Co0.2As2 thin films on high-quality IBAD-textured MgO-coated technical substrates utilizing additional Fe buffer layers. High Jc values were achieved (Jc > 1 MA/cm^2 in self-field and >10^5 A/cm^2at 10 T at 4 K). This value largely overcomes the “Assessment criteria for power applications” that were assumed when we wrote the project (Jc > 10^5 A /cm^2 at 4 K at 5 T).
Co-doped Ba122 bicrystalline films were successfully deposited within the first period by IFW on MgO bicrystal substrates using the Fe buffer approach (bicrystal angles 18°, 24° and 37°). An exponential decrease in Jc with the bicrystal angle was observed. Similar results were obtained by CNR on 11 on bicrystal substrates with angles of 10°, 24° and 45°. Owing to the limited avalaibility on the market of low angle bicrystalline substrates, detailed information on the Jc dependence on bicrystal angle is not available yet, but the overall results on bicrystalline thin films indicate that intergrain coupling in FeSC is intrinsically much better than in cuprate superconductors.
The nature of grain boundaries in polycrystalline materials has been mainly investigated by the Japanese team, and therefore not reported here. However, some of the results have been obtained in collaborations between Japanese and European partners, and will contribute to joint publications.
Original theoretical approaches have been developed within WP5. A model Hamiltonian was developed to address the grain boundary problem in FeSC. The aim is to provide information on the behaviour of Jc as a function of the misorientation angle. To tackle this problem, different microscopic models for grain boundaries were built up increasing the degree of complexity whilst providing more and more realistic information on the grain boundary issue. The current flow in polycrystalline samples was addressed by the University of Tokyo (UT)and the TUW groups in a phenomenological way. While TUW uses a mean-field approach, UT bases the description on the numerical simulation of non-linear resistor networks.
Finally, the overall theoretical and experimental results obtained on polycrystals and bicrystalline thin films indicate that intergrain coupling in FeSC is intrinsically much better than in cuprate superconductors. This statement represents one of the main achievements of the project.
4. Qualifying the potential of FeSCs for power applications.
This activity has been carried out within WP6, purpose of which has been to survey and collect data on other superconductors with a high potential for power applications and compare them with results obtained in WP2, WP3 and WP4 on FeSCs. While in the first year of the project, an exhaustive report of Survey data on technical superconductors has been collected, in the following years the focus has been mainly on FeSCs only. Crucial parameters (critical temperatures, fields and currents) were assessed with the final aim of evaluating the potential fields of application of FeSCs.
In particular, the worldwide state of the art of the research on FeSCs was monitored since the very beginning of the project. Since month 13, the literature data were more systematically collected, compared and assessed. This activity has yielded the submission of a manuscript by Ilaria Pallecchi and Marina Putti, namely a chapter on FeSCs to be published in the book "Handbook of Applied Superconductivity", Publisher Wiley (http://www.wiley-vch.de/publish/en) edited by Professor Paul Seidel. Moreover, Prof. Shimoyama, project leader for the Japanese twinned project, gave a plenary talk about the application potential and future perspectives of FeSCs at the European Conference on Applied Superconductivity (Eucas) 2013, held in Genoa, 15-19 September, 2013 and wrote a relevant review paper (Supercond. Sci. Technol. 27, 044002 (2014)), which can be considered closely related to the activities of WP6.
During the last period the optimal results achieved on FeSCs both within the SUPER-IRON project and worldwide have been collected and directly compared with the corresponding results obtained on other technical superconductors. Quantitative comparisons have been made. A realistic assessment of the application potential of FeSCs for each specific application in suitable temperature and magnetic field ranges, either at the present moment or in the perspective of the forthcoming years, has been presented. At the end of the project, a final report on the results obtained worldwide and within the SUPER-IRON project on Fe-based superconductors, and a final assessment of their application potential of FeSCs has been submitted as a review paper on “The Superconductivity News Forum-Global Edition”(http://snf.ieeecsc.org/).
5. Strengthening the European-Japanese cooperation
This OBJECTIVE has been implemented by several actions that have been planned and coordinated within WP7.
Student Workshops: At the beginning of the project two young researchers, Dr. Fabian Nitsche from LMU and Dr. Nakajima Masamichi from AIST, were appointed for coordinating EUROPEAN-JAPANESE activities. Among these last they organized the first Student Workshop in Dresden, (May the 5th - May the 6th, 2013). The participants were 21 from Japan and Europe. The second Student Workshop took place in Tsukuba, Japan, April 6th-8th, 2014. The meeting was organized by Dr. Shigeyuki Ishida from AIST, who substituted Dr. Nakajima Masamichi, in collaboration with Dr. Fabian Nitsche. The participants were 24 from Japan and Europe.
The aim of the Student Workshops is training of young scientists in the field of superconducting materials, but also establishing a platform for strengthening personal and working contacts between young European and Japanese scientists. This objective has been fulfilled, as proved by the numerous collaborations established among the students.
- Exchanges: From a factual point of view, a tight collaboration has developed among the European and Japanese partners, as demonstrated by the copious exchange of samples, and by the exchanges of students and visits. Complete lists of both have been inserted in the WP6 report. We mention here the one+one years spent by Alberto Sala (CNR) at the University of Tokyo and at AIST during his PhD course.
- Common publications. Regarding the publications, some 132 papers have appeared since the beginning of the project, written by the European and Japanese partners, and among them 18 are issued by multiple partners. In addition, 160 communications at international conferences (43 of these are from invited talks) authored by the European and Japanese partners are reckoned, out of which 18 are by joint European and Japanese partners
- Meetings. European and Japanese partners met in occasion of four official meetings and in occasion of several international conferences. The official meetings have been the following: the Kick-off meeting in Genoa, December 2011; the 12th-month meeting in Tokyo, Japan, December 2012; the 24th-month meeting in Genoa, Italy, September 19th-20th, 2013 after the closure the EUCAS2013 conference; the “Joint Workshop on Iron-Based Superconductors, by SUPER-IRON and IRON-SEA” on March 10-11th, 2015 at the JST Tokyo Headquarters. This last was organized by Prof. Jun-ichi Shimoyama and Prof. Hiroshi Ikuta, and attended by representatives of all the groups involved in the two projects, in addition to the EC and JST Research Program Officers.
- Final review meeting: In March 2015 the twinned European-Japanese projects ended. Therefore, on March 9th a final review meeting was held in Tokyo, in the headquarters of JST. It was attended by the Japanese and European project Coordinators, by Officers and Expert External Reviewers. This event was preceded by the submission of a review Report. As within SUPER-IRON the European and Japanese twinned projects shared both objectives and implementation plan, the two Coordinators of SUPER-IRON, M.Putti and J-I Shimoyama, decided to co-author one joint report. The final interview has been prepared and given and by the two Coordinators (see in the attached document an extract of their final presentation).
This can be viewed as a clear indication of the close collaboration and synergies established within the project.
- Collaboration agreement. One of the main tasks of the project was the signature of an agreement of collaboration among the European and Japanese institutions, to carry on the common activities beyond the duration of the project. Thus, a collaboration agreement, called SUPER-IRON II, devoted to continue SUPER-IRON without dedicated funding, has been proposed to all the European and Japanese institutions involved in the original project. All the Institutions have agreed to take part in it, and SUPER-IRON II has been ratified during the final meeting of the project, which took place on March 10th -11th 2015.
PROJECT ASSESSMENT
The comparison between the SUPER-IRON OBJECTIVES and the achieved results looks extremely positive. The conceived work plan was grounded on a deep investigation of FeSC, including development of materials, tuning of the properties, and understanding of critical current limitation in order to assess their potential for high field applications.
Many of these aspects have been carefully addressed. As reported above, we proved that FeSCs show great flexibility to defects and/or lattice strain, which affects positively the superconducting properties; several new FeSC materials were discovered both within the SUPER-IRON consortium (including the Japanese team) and outside it, with remarkable superconducting properties; this research will continue with the support and feedbacks coming by the extraordinary theoretical tool developed within SUPER-IRON to predict superconducting properties in real materials; investigation of bicrystalline thin films and polycrystalline materials (most of the results have been obtained by the Japanese team) has indicated that intergrain coupling in FeSC is intrinsically much better than in HTSC. This is proved by the fact that capability of technical samples (wires and coated conductors) to carry current improved very fast, indeed our ultimate target performance had been achieved in the first period.
Potential Impact:
In order to evaluate the societal implication of the SUPER-IRON results some tasks have been scheduled during the project. They concerned the collection of the results achieved on FeSCs both within the SUPER-IRON project and worldwide, the comparison with the corresponding results obtained on other technical superconductors and the assessment the application potential of FeSCs for each specific application in suitable temperature and magnetic field ranges, either at the present moment or in the perspective of the forthcoming years.
The conclusion reached by these investigations is that the FeSC performance in terms of the highest possible operating temperature as well as achievable fields and currents is between those of the HTSC and the conventional superconductors (Nb3Sn, MgB2 and Nb-Ti). Thus, they have to be either cheaper than the coated conductors or outperform the conventional wire technology. Both scenarios do seem to be realistic. We pointed out that the critical currents of inexpensive PIT processed wires are already nearing the performance requirements of many applications. If this trend continues, the FeSCs have good prospect for replacing the conventional superconductors and enabling a comparatively cheap magnet technology for generating high fields at low temperatures or intermediate fields at temperatures achievable with cryocoolers. If not, FeSC coated conductors could compete with cuprate-based wires for extremely high field magnets, since they would be less expensive, but offer enough performance for enhancing the fields currently achievable by superconducting magnets.
SUPER-IRON is a fundamental research project, therefore immediate exploitation of its results was not envisaged, but its aim was to draw a roadmap to industrial application. The project continuation has been planned by the partners for this purpose, and preliminary contacts have been established with industry since the early phases.
The next steps will be the following:
(i) to select the best family/doping/strain which optimize the superconducting phase diagram (Tc, Hc2, Jc);
(ii) to demonstrate the feasibility and reproducibility of optimized properties on large scale;
(iii) to develop a scalable and industrially appealing method for the production of FeSC first generation cables.
SUPER-IRON II, the collaboration agreement signed by all the European and Japanese institutions will continue to work on these objectives.
As to the dissemination plan, several channels have been chosen, i.e.:
• creation of dedicated website;
• presence on superconductivity correlated website;
• publication on academic journals;
• participation in the major superconductivity conferences;
• dedicated session at Eucas 2013;
• Workshops organization.
The project website www.SUPER-IRON.eu set up since the beginning, has been constantly updated, both in the public and the restricted area, so that it has maintained its function of reference point for both Partners and interested people-
Moreover, the activities of SUPER-IRON have been promoted by posting communications on the “SUPERCONDUCTIVITY NEWS FORUM” (SNF), which freely distributes announcements, new papers and event highlights within the scientific community. SNF, having an audience of more than 5000 readers involved in superconductivity, is the worldwide broadest and fastest dissemination tool in this field. It also fulfils the role of promotion, assessment and dissemination towards industrial partners. In particular, the following event highlights have been announced: the kick-off of SUPER-IRON, the PhD Student Workshops, and the Joint Final Workshop.
In addition to them, SNF has accepted for publication a review entitled “Assessment of the application potential of FeSCs”, by I.Pallecchi et al, extracted from corresponding evaluation report activity carried out in the project.
As part of the assessment of the status of project activities, the participation to conferences and publication of papers on international journals have been promoted and monitored.
Regarding publications, 132 papers have appeared since the beginning of the project, authored by European and Japanese partners, and among these 18 are authored by multiple partners. Apart from some theoretical works involving only WP5, most of the papers involve two or more WPs, which indicates the strong interaction, synergy and complementarity of the project activities. 59 out of 132 papers are authored by European partners, 42 of them were published during the second period the end of the project. Hence, the number of publication has increased significantly over the course of the project and likely new publications authored by this consortium will follow in the next future as well.
The publications are distributed among top ranked journals of the field (Applied Physics Letter, Physical Review Letters, Superconducting Science and Technology). These journals are not open-access, but several published papers have been posted on arxiv.org an open access archive that collects more than one million e-prints in Physics, Mathematics, Computer Science, Quantitative Biology, Quantitative Finance and Statistics.
The results of SUPER-IRON have been presented at several International conferences with broad audiences on record. The most important conferences of the field are the following: The European Conference of Applied Superconductivity (EUCAS) (~1000 participants), The Applied Superconductivity Conference (ASC) (~1500 participants), the International Conference on Superconductivity and Magnetisms (ICSM) (~1500 participants), the International Symposium on Superconductivity (ISS) (~800 participants).
67 communications authored by European partners are reckoned, out of which 18 are authored by joint partners, European and Japanese. 21 of these communications are represented by invited talks. Among them, Prof. M. Putti has been invited to present the results of SUPER-IRON project at MRS 2013 and ICSM 2014.
As a further outreach initiative, a session of Eucas2013, entitled: ”Fe-based Superconductors-Bulks and Tapes” has been hosted by SUPER-IRON EU-Japan project. At the same conference Prof. J. Shimoyama gave a plenary talk on the state of the art of FeSC.
As for the Workshops, three main events have been organized: two Student Workshops, one in Europe and one in Japan, and the “Joint Workshop on Iron-Based Superconductors” at the end of the project, in Tokyo.
These events have been functional to strengthen the European-Japanese cooperation and are described in the section above. They had been advertised as open events, and were attended also by participants external to the project, thus contributing to the dissemination of project results. Moreover, the Student Workshops have also represented a precious training tool.
List of Websites:
http://www.super-iron.eu/
CONTACTS
Marina Putti
Project Coordinator
marina.putti@spin.cnr.it
Mariachiara Lupi
Project Manager
mariachiara.lupi@spin.cnr.it
Elisabetta Narducci
Website Manager
elisabetta.narducci@spin.cnr.it
Barbara Cagnana
Administrative Consultant
barbara.cagnana@spin.cnr.it
