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Turning the concept of magnetocaloric cooling on its head

Periodic Reporting for period 2 - cool innov (Turning the concept of magnetocaloric cooling on its head)

Reporting period: 2019-04-01 to 2020-09-30

The overall contribution of the refrigeration sector to climate change is frequently overlooked. Nevertheless, it is huge: approximately 12% of total greenhouse gas emissions stem from cooling alone. This is a combined impact of the direct emission of fluorocarbons and the electricity production needed to operate devices. Today a total number of roughly 3 billion devices associated with cooling applications are in use globally. That covers household, commercial and transport refrigeration, air conditioning systems and heat pumps. Studies predict that the global energy demand for cooling purposes will soon prevail over the energy that is needed for heating. In 30 years, two-third of all households worldwide are expected to be equipped with air-conditioning systems, while China and India will account for half of them. We are entering, possibly have entered already, a cryogenic age.
It is truly astonishing to recognize that over a long period of time, in fact now more than 120 years, our cooling technology has not changed in its underlying principles. Today’s refrigeration systems based on the vapour compression cycles are inefficient and use refrigerants with a high global warming potential: in total, more than 4 billion tons of CO2-equivalent are emitted either indirectly through high energy consumption (contributing about 70% to this number) or directly through the leaking refrigerants (the rest). As a consequence, new regulations aim to phase down eco-unfriendly refrigerants drastically by 2030 in Europe and around the globe. The problem is that in many cases there are no effective replacements for these soon-to-be outlawed refrigerants, and in some cases explosive or toxic alternative refrigerants are being considered.
New solid-state based cooling technologies provide higher energy efficiency from the thermodynamic point of view and use simple heat exchange liquid like water. The largest heating and cooling effects are found in materials that undergo a spontaneous phase transition at the desired temperature. For those special materials, a driving stimulus such as magnetic field, electric field or pressure can be applied or released around that temperature and tip the thermodynamic balance between one state and another. During this process the material can heat up or cool down, which is called the magnetocaloric effect when caused by a magnetic field application. The reduced temperature of a caloric material can be used for a temperature exchange with the cooling target (the inside of a refrigerator, for example) by a heat exchanger setup. Magnetic refrigeration is probably the most developed of the solid state cooling technologies and has been in focus of intense research since the discovery of giant magnetocaloric effects in the 1990’s.
Solid refrigerants are non-explosive, non-toxic and easier to recycle. They are tunable to different temperatures which makes it possible to build refrigeration systems that are universal in terms of their general setup (including housing, magnetic field source, motor, pump etc.) and customise the specifications in terms of temperature, temperature span and power capacity at a later stage. The material can be adjusted throughout the year, making it able to adapt to a given season’s ambient temperatures, ensuring that the efficiency is maximized. The reversibility of the magnetocaloric effect promises an increase in energy efficiency of up to 40–50% compared to today’s best refrigeration system. Today's problem of the magnetocaloric refrigerants is that the largest effects are shown by materials with first-order phase transitions: exhibiting a large change in temperature, they have a certain lag when switching between their magnetic states, the hysteresis that hinders the efficiency of the material.
Magnetocaloric refrigeration has struggled to break into mass markets because it is a relatively expensive technology, and so far the research in magnetocaloric cooling has not provided the necessary breakthrough that can initiate a commercial realisation of this technology. We strongly believe that the technology needs some highly unconventional approaches. Cool Innov rethinks the whole concept of the conventional caloric cooling. Instead of trying to squeeze the best out of magnetocaloric materials in relatively low magnetic fields by attempts of the reduction of the thermal hysteresis at a first-order phase transition, we intend to make use of the hysteresis by introducing a second stimulus in the form of mechanical load. We are aiming at a revolution in cooling technology which will have a significant impact on global energy consumption.
The main objective of the project is to present an efficient cooling device based on this multi-stimuli approach, which will serve a true alternative to conventional cooling devices based on vapour compression cycles.
The key component of the device is the refrigerant material that has to show both caloric effects under the magnetic field application as well as the mechanical load. The material needs to be mechanically stable and to be composed of non-critical and non-toxic elements. For practical devices, an ideal material should have large caloric effects near room temperature, high mechanical stability and should contain little or no critical or toxic elements. To accelerate the search for novel and optimized magnetocaloric materials, we are performing a high-throughput search for new compounds and detailed calculations of phase transitions from first principles. We focus on Heusler compounds - a wide variety of magnetic intermetallics with tunable properties. Once an appropriate material is identified and the properties are developed for best performance, the next objective is to shape it into the geometry that will allow the most efficient heat exchange during the operation of the thermodynamic cooling cycle. Inspired by nature and supported by the new technology of 3D printing, bionic heat exchangers will become the working body of the new cooling device. A part of the project focuses on the simulation of the proposed multi-stimuli concept in order to support the experimental efforts in constructing the novel cooling device. The model needs to take into account the magnetic properties and to be temperature dependent. The key feature is the description of the stress- and field-induced phase transitions from a martensite to an austenite state of Heusler alloys, and vice versa. The objective is to implement such a model within a finite-element framework, so that it can be applied to complex microstructures and manufacturing processes like selective laser melting. Once the model is refined, it is more energy efficient, more resource efficient, and less time consuming to test new geometries and microstructures. Finally, combining the obtained breakthroughs in material science with advanced engineering, a demonstrator based on the new multi-stimuli principle can be assembled.
As the first step, we have been very excited to be able to demonstrate the proof of concept for a multi-stimuli cooling cycle applied to a Ni-Mn-In Heusler alloy in the magnetic field of 1.8 T and under the moderate uniaxial pressure of 75 MPa. This work has been done in cooperation with the University of Barcelona (T. Gottschall et al., Nature Materials 17, 2018). To avoid the mechanical failing of the material, relatively low uniaxial pressure was applied. Therefore, the multicaloric effect was not maximised. However, inducing a full magneto-structural transformation was expected to result into the maximal effect. That has been proven when the magnetocaloric properties of the materials synthesised for the project were measured in high magnetic field up to 30 T. For measurements under much higher uniaxial loads (up to 1GPa), we equipped a conventional universal testing machine with a temperature chamber to test the materials depending on their transition temperature. Isostress, isofield and isothermal characterization of the first-order magnetostructural phase transformation are performed to create 3-dimensional (magnetic field, stress, temperature) phase diagrams, which enables us to simulate the exploiting hysteresis cycle with optimized parameters. We also monitor the behaviour of a multicaloric material under the cyclic application/removal of uniaxial stress. This brings us the best parameters to build the test bed with a combination of a high magnetic field and a sufficient uniaxial pressure, at the same time avoiding the mechanical failing of the material.

In terms of material properties, all parameters of the magnetostructural transition such as transition shift, width and hysteresis for both stimuli are investigated in order to ensure a proper implementation of the multi-stimuli cooling cycle. A part of the project is dedicated to discovery and synthesis of new Heusler-type alloys with tailored transition properties for the multi-stimuli refrigeration concept. To reduce the time for the experimental search for possible new alloys, we used a high-throughput approach to identify stable compounds of different stoichiometries. For that, first-principles calculations are carried out for two systems: all-d metal Heusler and MM’X alloys. The screening is done based on the stability in respect to two key points: decomposition to elements and decomposition into other known alloys. Conventionally, a Heusler alloy has a stoichiometry of X2YZ, where X and Y are transition metals and Z is a p-block element. However, the all-d metal Heusler family (where Z is also a transition metal) can combine a large MCE effect with improved mechanical properties, for example in the Ni-CoMn-Ti system. Therefore, we searched for Heusler alloys with all sites occupied by d elements, with a least one site being a 3d magnetic atom meaning V, Ni, Cr, Mn, Fe and Co. We found a few dozen compounds that are energetically stable. Experimental validation synthesis of several selected compounds are done.

In parallel, a systematic study of optimized heat treatment and varied stoichiometry of Ni-Co-Mn-Ti alloys has been done and large magnetocaloric effects were observed in the modest magnetic field change of 2 T. The maximal effect of 20 K was achieved in higher magnetic field around 15 T. Temperature and field dependent in-situ studies reveal strong influence of the microstructure (grain size) and homogeneity on the martensitic transition behaviour. By tailoring the martensitic transition temperature and the Curie temperature using different heat treatments and by variation of Co and Ti stoichiometries, we could set up a universal phase diagram and general design rules for this material system according to the desired properties. The evolution of Curie temperature has been supported by theoretical calculations. Based on the methodologies developed above, we describe the Curie temperature dependence with composition in the Ni-Co-Mn-Ti system, along with an understanding of the origin of the remarkable composition sensitivity in the system.

Furthermore, we have studied the maximum magnetocaloric effect and the structural responds of Ni-(Co)-Ni-In/Sn during phase transition in pulsed magnetic fields. We could show that the transition can follow the high field change rate up to 865 Ts-1 yielding a temperature change of more than 10 K in 5 T could be achieved. This prompted a use of pulsed fields as magnetic stimulus. To improve the mechanical stability suction casting was done to achieve a grain boundary strengthening. By optimization of the synthesis route, we obtain higher resistance to mechanical fatigue by consistent excellent magnetocaloric properties. A solidification texture in the suction cast samples provides low critical stresses for the stress-induced transformation. Additionally, we are studying a promising possibility to tailor the thermal hysteresis and enhance the mechanical stability in Heusler alloys by using dopant elements which promote the formation of finely distributed secondary phases.

Bridging micro-magnetic simulations and experimental science, we developed a methodology for calculating the magnetocaloric effect in second-order-type materials. It describes the magnetization behaviour in the vicinity of the Curie temperature and allows to model different microstructures like grain sizes and grain boundary phases. The calculation helps to find the optimized microstructure and therefore maximize the caloric effect. In addition to this methodology for second-order-type materials, we are developing a model for describing the first-order phase transition in magnetic Heusler alloys. First benchmarks are completed, showing that the model is capable of simulating stress and magnetic field induced transformation of the martensite variants.

To simulate the stress- and field-induced phase transformation of Heusler alloys with complex microstructure, a model is implemented within a finite-element framework. These models should be capable of describing also additive manufacturing processes, like selective laser melting or powder bed fusion. In parallel we have established a cooperation with the Institute for Production Management at the TU-Darmstadt and Institute for Materials Technology/Metallic Materials at the University of Kassel to investigate the magnetic and elastic properties of additively manufactured Heusler alloys.
The fundamental part of research during the project will discover new materials in the known class of Heusler alloys. Instead of searching by experimental trial and error, high-throughput screening for new compounds and detailed calculations of phase transitions from first principles are powerful tools to estimate transition temperatures and caloric effect. These calculations will help optimizing the material itself at the atomistic level.

The central part of the multi-stimuli device will be the novel mechanically stable heat exchanger fabricated from the best optimized materials by additive manufacturing. This will have an impact on both the engineering of more efficient heat exchangers with fine structures and material science of tuning a magnetocaloric material by different methods affecting the microstructure.

Presently there is no model capable of calculating the magnetocaloric effect in materials with a first order phase transition. Until the end of the project, we will develop and implement and phase field model coupled with micromagnetism in a finite-element framework for simulating the magnetocaloric effect and the multi-stimuli concept proposed. It will be capable of describing the martensite-austenite transition induced by the application of mechanical stress or an external magnetic field and to simulate the thermal hysteresis. This model will support the experimental effort in development of an efficient magnetocaloric cooling device as an alternative to the conventional cooling systems. Beside the general applicability and scientific advance in the thermodynamic description of magnetostructural materials, it will help to optimize the construction and design of the active material and therefore the overall performance of the device.

Supported by advanced technologies in the field of engineering and material science on one hand, and by computational power at atomistic and micro scale, by the end of the project we expect to produce the multicaloric cooling device that takes the state of the art in the magnetic cooling one huge step further and takes an advantage of the usual obstacle of the solid state cooling, the hysteresis, by adding the mechanical load as a second stimulus. The concept of the multi-stimuli device will drastically reduce the so far expensive cost of this alternative cooling technology and will have a potential of opening the market for large-scale solid-state refrigeration.