ADVANCED MATERIALS FOR EFFICIENT USE OF ENERGY
Human development has caused a depletion of natural energy resources and climate changes with non-predictable consequences. New energy concepts are required for the future of our industrial society resulting in e.g. an ever increasing emphasis on improving the efficiency of electricity transmission and utilisation and in the progressive replacement of oil-based fuels in transportation by electric motors.
In this context we work on novel permanent magnets, magnetocaloric materials, magnetic shape memory alloys and materials for solid state hydrogen storage.
The development of new magnetic and hydrogen storage materials with improved properties requires advanced processing and high resolution characterisation techniques.
Most Recent Highlights
Optimization of nanocomposite materials for permanent magnets by means of micromagnetic modelling
D.V. Berkov, S.G. Erokhin, N.L. Gorn
General Numerics Research Lab e.V., Jena, Germany
In this talk we present a new methodology for micromagnetic simulations of magnetic nano-composites. Our methodology combines the main advantages of the finite-difference method (efficient evaluation of the magnetodipolar interaction via the fast Fourier transfo¬mation) and the finite-element approach (flexibility by the discretization of arbitrary shapes). We discuss the results obtain with this method for three very different systems:
(i) Iron-based nanocomposite from the Nanoperm (FeZrBCu) family of alloys, where we compute the magnetization curve and compare our results with the corresponding small-angle neutron scattering (SANS) cross-section data. A remarkable agreement with nontrivial “clover-leaf” SANS cross-sections observed experimentally is demonstrated.
(ii) Nanostructured Sr-ferrite-based composites for permanent magnets, where we study the dependence of most impor¬tant material characteristics (coercivity, remanence, energy product) on the structural and magnetic parameters of a composite. In particular, we demonstrate that – in contrast to the common belief – in systems containing hard grains embedded in the iron matrix, the energy product is a non-monotonic function of the intergrain exchange. The influence of the magnetodipolar interaction between hard crystallites and the soft phase and the role of the grain shape will also be discussed.
(iii) Nanocrystalline Nd–Fe–B (a potential material for future permanent-magnet applications), for which we compare three different models of this material class: (i) description in terms of Stoner–Wohlfarth particles with and without the magnetodipolar interaction; (ii) core-shell representation of the nanograins; (iii) the latter model including a contribution of superpara¬magnetic clusters. We show that only the last – most sophisticated – model is able to provide an excellent agreement with the measured hysteresis loop.
The presented methodology is generally applicable to multiphase magnetic nanocomposites and it highligths the complex interrelationship between the microstructure, magnetic inter-actions, and the macroscopic magnetic properties.
Researchers develop magnetic cooling cycle
As a result of climate change, population growth, and rising expectations regarding quality of life, energy requirements for cooling processes are growing much faster worldwide than for heating. Another problem that besets today's refrigeration systems is that most coolants cause environmental and health damage. A novel technology could provide a solution: refrigeration using magnetic materials in magnetic fields.
A multicaloric cooling cycle that exploits thermal hysteresis
Tino Gottschall, Adrià Gràcia-Condal, Maximilian Fries, Andreas Taubel, Lukas Pfeuffer, Lluís Mañosa, Antoni Planes, Konstantin P. Skokov & Oliver Gutfleisch
International conference on caloric cooling
Thermag VIII took place in Darmstadt and was organized by the functional materials group.
267 experts from 24 different countries visited the conference to exchange their recent research results and discuss the future of the field. The topics covered in the conference were extended for the first time from magnetocaloric refrigeration at room temperature to cover all the different caloric effects that are able to provide cooling at different temperatures.
The next Thermag conference will be held 2020 in College Park (Maryland), USA.
A quantitative criterion for determining the order of magnetic phase transitions using the magnetocaloric effect
J.Y. Law, V. Franco, L. Moreno-Ramírez, A. Conde, D. Karpenkov, I. Radulov, K. Skokov, O. Gutfleisch
In Nature Communications (2018) 9:2680
In cooperation with the University of Sevilla we proposed a quantitative criterion for determining the order of magnetic phase transitions using the magnetocaloric effect. The ideal magnetocaloric material would lay at the borderline of a first-order and a second-order phase transition. Hence, it is crucial to unambiguously determine the order of phase transitions for both applied magnetocaloric research as well as the characterization of other phase change materials. We describe a quantitative fingerprint of first-order thermomagnetic phase transitions and demonstrate the validity of the model for different magnetic materials, composites and inhomogeneous materials.
SAVE THE DATE
Together with EIT Raw Materials and under the umbrella of the European Battery Alliance (EBA), we organise a 2-day workshop on Sustainable Materials for Future Mobility: E-drives. Magnets. Batteries, which will take place 23-24 October 2018 in Darmstadt, Germany. The objectives are the creation of cross-value chain alliances, define and implement demonstration projects, and define an agenda towards sustainable battery and e-drives technologies. Attendance is free of charge. For more information see the event website: https://www.lyyti.fi/p/Sustainable_Materials_for_Future_Mobility_Edrives_Magnets_Batteries_4940
Fundamental aspects of cryogenic gas liquefaction by magnetic cooling
Within the German-Russian Program “Helmholtz-RSF Joint Research Groups” on “Energy Storage and Grid Integration” we will start a three-year project on “Fundamental aspects of cryogenic gas liquefaction by magnetic cooling”. The project is led by the Helmholtz-Zentrum Dresden-Rossendorf (HZDR) and the Russian partner is the University of Chelyabinsk. The aim is to develop alternatives to conventional hydrogen liquefaction using solid state magnetic cooling at low temperatures and with this enable new options in energy storage and electro mobility.
Group Seminar in Manigod, France
As in the previous years also this year an internal group seminar took place. From 25.06. to 29.06. the group went to Manigod in France and stayed in Chalet Giersch. The beautiful surrounding of the Alps was the background for our discussions and presentations. Many new ideas came to life in the workshop during the seminar. The weather was friendly and so the pool could be used to refresh the heads afterwards.
The accommodation offered the possibility to cook together in the group, a task that had to be fulfilled in teams. By this task the solidarity of the group was fostered and new experiences have been made. All kitchen teams provided really delicious food and also a barbecue was organized.
The program was complemented by a nice hiking trip through the mountains.
On Friday all members came back healthily but tired from the long bus trip and the intensive work.
Hysteresis design of novel magnetocaloric materials ‐ from basic mechanisms to applications
F. Scheibel et al., in Energy Technology, Available online 14 June 2018
In this review, we discuss first-order magnetostructural phase transitions in the most relevant magnetocaloric compounds. We present studies under static and dynamic conditions at micro- and mesoscopic scales and elucidate the electronic, magnetic and lattice entropy contributions. We describe transitional hysteresis, a loss‐source limiting reversibility of the adiabatic temperature change, and explain strategies for maximizing efficiency. The review is based on a long term collaboration with our partners from University Duisburg-Essen within the DFG Program Ferroic Cooling.
Manipulation of matter by electric and magnetic fields: Toward novel synthesis and processing routes of inorganic materials
O. Guillon, C. Elsässer, O. Gutfleisch, J. Janek, S. Korte-Kerzel, D. Raabe, C.A. Volkert
in Materials Today published June 2018
Out of the DFG SPP 1959 we wrote a review on the use of external electric and magnetic fields for the synthesis and processing of inorganic materials such as metals and ceramics. Electromagnetic energy can be utilized in different ways to improve or accelerate phase formation and stabilization, chemical ordering, densification and coarsening of particle-based materials (pore elimination and grain growth), and mechanical deformation (plasticity and creep). In these new synthesis and processing routes, the resulting microstructures and macroscopic material behavior are determined by the interaction of the applied fields with defects such as single or clustered point defects, dislocation networks, and interfaces. Multiscale experimental investigations and modeling are necessary to unveil the mechanisms underlying this field-assisted manipulation of matter.
Heavy rare earth free, free rare earth and rare earth free magnets – vision and reality
K.P. Skokov and O. Gutfleisch, in Scripta Materialia View Point, Published September 2018
It is commonly understood that among the intermetallic phases used for permanent magnets, practically none can fully realize its potential based on the intrinsic magnetic properties. We discuss different reasons leading to this limitation, known as the Brown paradox, and propose some possible ways of overcoming it. We compare the intrinsic magnetic properties of (Nd1−xCex)2(Fe1−yCoy)14B single crystals with the extrinsic characteristics of sintered and hot compacted magnets made from the very same alloys. In addition, looking at RE-free materials, our results obtained on Mn- and Co-based RE-free single crystals are compared with the hard magnetic properties of Mn-based permanent magnets.
Group-event Merck-Firmen-Lauf 2018
On Wednesday 16.05.18 our FM running team started together with 5000 other runners at the Merck-Firmenlauf. This was the 7th time the event took place and the 3rd time we were participating. After weeks of preparation behind them, all runners crossed the finishing line much faster than anticipated. The Functional Materials mixed team got position 23 out of 433 teams in total, a position we are very proud of. But more importantly, we gained a better fitness and grew together as a team. After the running event the day was celebrated in the Bayerischer Biergarten in the Bürgerpark Darmstadt. (CM)
New DIN SPEC 91373 Magnetocalorics – Terminology now published
The document is ready to download: https://www.beuth.de/de/technische-regel/din-spec-91373/288638546
Functional Materials at TU Darmstadt is leading a DIN Project with an international working group consisting of developers and researchers from both academia and industry. A new DIN SPEC providing a unified terminology for magnetocalorics has been developed and published. It is aimed at researchers, manufacturers and users. Magnetocalorics has seen a huge expansion in scientific publications and conferences over the last decade. Emerging from these developments a magnetic cooling industry value chain is now taking shape.
PhD-graduation in the FM group
We are happy to congratulate our new colleague Franziska Scheibel to her defense. She did her PhD thesis Influence of hysteresis at magnetostructural transitions on the magnetocaloric properties of Heuslers, Antiperovskites, and Pnictides at the University Duisburg-Essen.
Magnetocaloric materials for refrigeration near room temperature
Anja Waske, Markus E. Gruner, Tino Gottschall, Oliver Gutfleisch
in MRS Bulletin Caloric Effects in Ferroic Materials Vol. 43 (4) – April 2018, pp. 269-273
This article overviews the current status of magnetocaloric materials for room-temperature refrigeration. We discuss the underlying mechanism of the magnetocaloric effect and illustrate differences between first- and second-order type materials starting with gadolinium as a reference system. Beyond the key functional properties of magnetocaloric materials, the adiabatic temperature, and entropy change, we briefly address the criticality of the most promising materials in terms of their supply risk. Looking at practical applications, suitable geometries and processing routes for magnetocaloric heat exchangers for device implementation are introduced.
Towards an Alloy Recycling of Nd–Fe–B Permanent Magnets in a Circular Economy
O. Diehl, M. Schönfeldt, E. Brouwer, A. Dirks, K. Rachut, J. Gassmann, K. Güth, A. Buckow, R. Gauß, R. Stauber, O. Gutfleisch
Journal of Sustainable Metallurgy (2018)
Rare earth permanent magnets are an integral part of many electrical and electronic devices as well as numerous other applications, including emerging technologies like wind power, electric vehicles, fully automized industrial machines, and robots. Due to their outstanding properties, magnets based on Nd–Fe–B alloys are often not substitutable by employing less critical material systems. Our joint paper with Fraunhofer IWKS presents material-to-material recycling approaches maintaining the magnet alloys and using them directly for a new magnet production loop. The recycled magnets compete well with those made from primary materials and have a much reduced environmental footprint; an important step regarding the shift towards a Green Economy.
Microstructural origin of hysteresis in Ni-Mn-In based magnetocaloric compounds
H.Sepehri-Amin, A.Taubel, T.Ohkuboa, K.P.Skokov, O.Gutfleisch, K.Hono
Acta Materialia 147, 342-349 (2018)
In cooperation with NIMS Tsukuba, the microstructural origins of thermal hysteresis in Heusler alloys were analyzed to show that nano-scale control of the microstructure is the key factor to minimize the thermal hysteresis of the Ni-Mn-In based magnetocaloric compounds. Therefore, two model specimen of Ni-Mn-In and Ni-Fe-Mn-In showing a small and large thermal hysteresis respectively were investigated. It can be shown by TEM imaging and 3D atom probe techniques that a homogeneous and continuous phase transition occurs for the sample with small hysteresis showing an increased Ni content within the twin boundaries, which serve as nucleation centers for the transition. For the large hysteresis sample, nano-precipitates of Fe as well as differently ordered structures of the Austenite lead to a non-uniform and discontinuous phase transition throughout the sample, resulting in an increased thermal hysteresis.
PhD-graduation in the FM group
We are happy to congratulate our dear colleague Fabian Rhein to his defense. He worked on his thesis Nanoskalige Magnete und Magnetkomposite auf Ferritbasismainly at the company Siemens. The defense was followed up by a visit of a local brewery and a little after-defense ceremony by his colleagues.
Origin of field-induced discontinuous phase transitions in Nd2Fe17
L. V. B. Diop, M. D. Kuz’min, K. P. Skokov, Y. Skourski, and O. Gutfleisch
Phys. Rev. B 97, 054406 (2018)
Magnetic properties of a trigonal ferromagnet Nd2Fe17 have been studied on single crystals in steady (14 T) and pulsed (32 T) magnetic fields. The easy-magnetization direction lies close to the  axis, deviating from the basal plane by 2.9° (at 5 K). Of particular interest is the low-temperature magnetization process along the high-symmetry axis , which is the hard direction. This process is discontinuous and involves two first-order phase transitions (FOMPs). One of them (at 20 T) is a symmetry FOMP similar to that observed in Sm2Fe17. The second transition (at 10.4 T) is without precedent: as the magnetization turns abruptly towards the applied field, it also changes its azimuthal orientation (the angle φ) by 60°. Both transitions can be reasonably accounted for by the presence of a significant sixth-order trigonal anisotropy term.
Consolidation of cobalt nanorods: A new route for rare-earth free nanostructured permanent magnets
Semih Ener, Evangelia Anagnostopoulou, Imants Dirba, Lise-Marie Lacroix, Frederic Ott, Thomas Blon, Jean-Yves Piquemal, Konstantin P. Skokov, Oliver Gutfleisch and Guillaume Viau
Acta Materialia 145 (2018) 290-297
For the first time the bottom-up approach is convincingly reported to produce bulk, anisotropic magnets without the addition of any matrix. The obtained nanostructured materials exhibit coercivity much higher than the AlNiCo magnets and can fill the performance “gap” between hexaferrites and rare-earth based magnets. In this work, we propose alternative consolidation and densification routes for cobalt nano-rods to yield performant permanent magnets. Cobalt NRs with various diameters were produced and compacted under different conditions. The magnetic properties and the performance of the consolidated magnets are discussed in relationship with their microstructure analyzed by neutron scattering and X-ray diffraction.
Bulk combinatorial analysis for searching new rare-earth free permanent magnets: Reactive crucible melting applied to the Fe-Sn binary system
Bahar Fayyazi, Konstantin P. Skokov, Tom Faske, Dmitriy Yu Karpenkov, Wolfgang Donner and Oliver Gutfleisch
Acta Materialia 141 (2017) 434-443
The reactive crucible melting (RCM) method is known to be an efficient and low-cost bulk combinatorial synthesis technique to search for new phases. In this work, reliability of the RCM method was evaluated by investigating the Fe-Sn binary system. The phase compositions forming in the RCM were compared with phases appearing in conventionally melted samples and the phases represented in the reported Fe-Sn phase diagram. For high-throughput characterization, energy dispersive x-ray spectroscopy and magneto-optical Kerr microscopy were used. It was found that in certain cases, the phase relations expected in equilibrium conditions were not completely reproduced in RCM samples. The problem of ‘missing phases’ is discussed. In addition, in Fe3Sn2 phase with uniaxial anisotropy, we demonstrate the erroneous of the quantitative measurement of K1 using the domain structure by comparing with the measured anisotropy on a single phase sample using conventional magnetometry. Finally, a new structural model is introduced for the Fe5Sn3 compound.
Visiting Professor at the University of Parma
Oliver Gutfleisch has started a Visiting Professorship at the University of Parma for the period of 2017/2020. He is formally enrolled at the Material Science PhD College of tutors at the University of Parma, and will be involved in the teaching activities as well as the joint supervision of a PhD candidate in an effort to expand and strengthen the international profile of the university. The University of Parma is one of the oldest universities in the world and dates back to the year 962.
Influence of magnetic field, chemical pressure and hydrostatic pressure on the structural and magnetocaloric properties of the Mn–Ni–Ge system
Andreas Taubel, Tino Gottschall, Maximilian Fries, Tom Faske, Konstantin P. Skokov, Oliver Gutfleisch
J. Phys. D: Appl. Phys. 50 464005 (2017), https://doi.org/10.1088/1361-6463/aa8e89
For magnetocaloric cooling applications, the MM’X alloys with a sharp phase transition from high temperature paramagnetic hexagonal to low temperature ferromagnetic orthorhombic phase recently came into focus of research. Our study on the isostructural alloying of the Mn-Ni-Ge system by introducing Fe for Mn and Si for Ge shows the benefit of this material family for precisely tunable structural and magnetic properties. The high dependence on sample size and shape connected to the enormous volume change during phase transition of about 3% is demonstrated by high isothermal entropy changes of small pieces and directly measured temperature changes for polymer bonded powder. This large volume difference between the two occurring phases offers a large potential for pressure-assisted magnetocaloric cycles and barocaloric applications.
Save the Date for Thermag VIII
International Conference on caloric cooling
You are hereby cordially invited to attend Thermag VIII – International Conference on Caloric Cooling, which will be held in Darmstadt, Germany on 16.-20. September 2018. The conference provides a platform for scientists and engineers from all over the world working in the field of caloric materials and devices to get together and discuss their research and ideas.
More information on thermag2018.de.
REGINA – Globale Industrien Seltener Erden und neue Anwendungen
Ein Konsortium aus acht deutschen wissenschaftlichen Institutionen und Unternehmen inklusive der TU Darmstadt wird unter der Leitung der Fraunhofer-Projektgruppe IWKS gemeinsam mit brasilianischen Partnern aus Wissenschaft und Wirtschaft daran arbeiten, eine Prozesskette zur Herstellung und Vermarktung von Hochleistungspermanentmagneten aus brasilianischen Ressourcen aufzubauen. Gesamtziel ist es, eine Grundlage für die industrielle Umsetzung der Wertschöpfungskette, ausgehend von gemischten Seltenerdoxiden aus brasilianischen Minen, über die Seltenerdaufbereitung, bis hin zum fertigen Nd-Fe-B-Magneten zu schaffen. Der Aufbau einer brasilianischen Nd-Fe-B-Magnetproduktion soll Brasilien als strategischen und verlässlichen Lieferanten von Seltenen Erden in Deutschland etablieren. Diese Magnete sollen den auf chinesischen seltenen Erden basierenden Magneten leistungsmäßig vergleichbar, jedoch im Hinblick auf Umwelt- und Sozialverträglichkeit überlegen sein und das Prädikat „grüner Magnet“ für den Einsatz in grünen Technologien verdienen.
Die TU Darmstadt unter der Leitung von Prof. Gutfleisch (Fachgebiet Funktionale Materialien) wird sich innerhalb dieses Konsortiums speziell mit der Optimierung der Legierungen sowie der Synthese und hochauflösenden Charakterisierung der Magnete beschäftigten und diese Entwicklung vorantreiben. Das Forschungsvorhaben ist auf drei Jahre angelegt und wird vom Bundesministerium für Bildung und Forschung (BMBF) mit ca. 2,9 Mio. im Rahmen des Förderschwerpunktes »CLIENT II – Internationale Partnerschaften für nachhaltige Innovationen« im Rahmenprogramm »FONA – Forschung für nachhaltige Entwicklung« gefördert. Davon werden 0,57 Mio. € für die Optimierung der Legierungen und Magnete der Arbeitsgruppe Gutfleisch bereitgestellt. Projektstart war im August 2017.
Mehr Informationen im offiziellen Pressebericht.
The 2017 Magnetism Roadmap
TOPICAL REVIEW: The 2017 Magnetism Roadmap
D. Sander et al.
Journal of Physics D: Applied Physics, Volume 50, Number 36 (2017) 363001
This Topical Review gives an accurate snapshot of the world of magnetism in 2017. It focuses e.g. on the significantly growing interest in magnetism and magnetic materials in relation to energy applications. The three core pillars of magnetism, namely magnetic materials, magnetic phenomena and associated characterization techniques, as well as applications of magnetism are addressed by an international group of 14 experts. The 2017 Magnetism Roadmap article provides a frame that will enable the reader to judge where each subject and magnetism research field stands overall today and which directions it might take in the foreseeable future.
PhD-graduation in the group of Functional Materials
We congratulate our colleague Maximilian Fries to his defense. He successfully defended his work entitled Phase transitions of borides and phosphides for application in magnetic energy conversion.
The defense was followed in the evening by a nice barbecue with colleagues and friends of the department and an after-defense ceremony by his colleagues. We thank Max for his effort in the group of Functional Materials which often went far beyond his thesis and are happy that he will continue his work within the group.
The great transition – The importance of critical metals for green energy technologies
On July 13, 2017, we hosted a one-day symposium at the Orangerie in Darmstadt focusing on how the supply situation of strategic metals can directly impact on the development and cost of advanced materials which form the basis for green energy technologies. The efficient utilisation or substitution of these critical elements with more sustainable and earth abundant elements is the big challenge for the Great Transition from a fossil fuel based society to an economy built on renewable energies. Well-known experts from academia and industry gave Key-note lectures addressing the substitutionability of critical metals and its possible side effects in terms of entering new dependencies. With more than 150 participants the symposium found a very positive resonance and we will make the Darmstadt Symposium a regular event in the future.
Ministerbesuch in Hanau
Hessens Wirtschafts- und Energieminister Tarek Al-Wazir informierte sich vor Ort über aktuelle und geplante Tätigkeiten der Fraunhofer-Projektgruppe IWKS
Tino Gottschall, Dimitri Benke, Maximilian Fries, Andreas Taubel, Iliya A. Radulov, Konstantin P. Skokov and Oliver Gutfleisch
Adv. Funct. Mater. 2017, 1606735. https://doi.org/10.1002/adfm.201606735
Solid-state magnetic refrigeration is a high-potential, resource-efficient cooling technology. However, many challenges involving materials science and engineering need to be overcome to achieve an industry-ready technology. Caloric materials with a first-order transition—associated with a large volume expansion or contraction—appear to be the most promising because of their large adiabatic temperature and isothermal entropy changes. In this study, using experiment and simulation, it is demonstrated with the most promising magnetocaloric candidate materials, La–Fe–Si, Mn–Fe–P–Si, and Ni–Mn–In–Co, that the characteristics of the first-order transition are fundamentally determined by the evolution of mechanical stresses. This phenomenon is referred to as the stress-coupling mechanism. Furthermore, its applicability goes beyond magnetocaloric materials, since it describes the first-order transitions in multicaloric materials as well.
Atomic structure and domain wall pinning in samarium-cobalt-based permanent magnets
M. Duerrschnabel, M. Yi, K. Uestuener, M. Liesegang, M. Katter, H.-J. Kleebe, B. Xu, O. Gutfleisch & L. Molina-Luna
Nature Communications 8:54 (2017)
A higher magnetization by increased iron contents can yield larger energy products in rare-earth Sm2Co17-type pinning-controlled permanent magnets, which are of importance for high-temperature industrial applications. We present model magnets with an increased iron content based on a unique nanostructure and -chemical modification route using Fe, Cu, and Zr as dopants. The iron content controls the formation of a diamond-shaped cellular structure that dominates the density and strength of the domain wall pinning sites. Using ultra-high-resolution experimental and theoretical methods, we revealed the atomic structure of the single phases present and established a direct correlation to the macroscopic magnetic properties.
A ‘green’ magnetic cooling device built using ‘upcycled’ NdFeB magnets
Magnetocaloric devices hold the potential to satisfy the rising demand for cooling in the future. One remaining challenge is to reduce the high ecological footprint of the permanent magnets driving the magnetic cooling cycle. Existing devices use neodymium-iron-boron (NdFeB)-type permanent magnets, which account for more than 50% of the ecological footprint of the appliance. To overcome this hurdle, TU Darmstadt and Urban Mining Company have built the first working magnetocaloric device that uses recycled NdFeB as a magnetic field source. Coupling this with optimization of the magnets and their geometry, it is possible to further reduce the ecological footprint. Together, these two approaches help to position magnetic cooling as a realistic and sustainable cooling technology.
For more information see our White Paper on this topic.
Neubau für die Fraunhofer-Projektgruppe IWKS
Im neu ausgewiesenen Fraunhofer-Science-Park in Hanau wurde mit einem Spatenstich am 14.6. 2017 der Neubau für die Fraunhofer-Projektgruppe für Wertstoffkreisläufe und Ressourcenstrategie IWKS eingeleitet. Die Projektgruppe wurde 2011 unter dem Dach des Fraunhofer Instituts für Silicatforschung ISC ins Leben gerufen. Vor dem Hintergrund knapper und teurer werdender Rohstoffe werden hier die Voraussetzungen erforscht, wie die Rohstoffversorgung der Industrie langfristig gesichert werden kann. Dafür werden zusammen mit Industriepartnern Trenn-, Sortier-, Aufbereitungs- und Substitutionsmöglichkeiten erforscht und Strategien zum nachhaltigen Umgang mit kostbaren Ressourcen entwickelt. Prof. Dr. Liselotte Schebek und Prof. Dr. Oliver Gutfleisch von der TU Darmstadt sind als wissenschaftliche Leiter für zwei Bereiche an der Projektgruppe beteiligt.
Hessentag in Rüsselsheim from 9 to 18 June 2017
Scientists from the LOEWE “RESPONSE” project participated at this year's Hessentag in Rüsselsheim.
Interesting experiments and exhibits informed about the research on innovative and high-performance magnetic materials.
TU Darmstadt and DIN work on standardization of Magnetocalorics
In order to increase the technological readiness of magnetocaloric cooling and to motivate more researchers to work on magnetocaloric cooling, the German institute for standardization DIN decided to create standards in this field. Funktionale Materialien was assigned to work on this topic in the DIN Connect Project Calorinorm which will take two years (2017-2018).
In this project two DIN SPEC will be established which will standardize the terminology of magnetic refrigeration and the measurement procedures of magnetocaloric materials.
TU MEET and MOVE
A team of 15 highly motivated runners of the Functional Materials group participated in the Ultramarathon of the annual TU sports festival.
Tino Gottschall wins Best Dissertation Award
The association “Vereinigung von Freunden der technischen Universität Darmstadt” awards every department with an award for the best disseration. This year Tino Gottschall won the award for the Materials Science Department. The award is endowed with 2500€.
The European Research Council (ERC) honors Professor Oliver Gutfleisch with an “ERC Advanced Grant” and promotes him with 2.5 million euros over a period of five years. This recognizes outstanding research at the TU Darmstadt for the substitution of critical raw materials and materials for energy technologies.
Microstructural and magnetic properties of Mn-Fe-P-Si (Fe2P-type) magnetocaloric compounds
M. Fries, L. Pfeuffer, E. Bruder, T Gottschall, S. Ener, L.V.B. Diop, T. Gröb, K. P. Skokov, O Gutfleisch
Acta Materialia 132 (2017) 222-229
Mn-Fe-P-Si-based magnetocaloric compounds were prepared using a powder metallurgical process and their microstructural and thermomagnetic properties were analyzed. XRD, SEM, EDX and EBSD analysis reveal a phosphorous depleted cubic secondary phase in many samples with distinct microstructural properties giving an insight into the phase formation process. A porous morphology was found, hindering the direct application of the materials a magnetocaloric heat exchanger in bulk-like structures (see Figure). Thermomagnetic measurements reveal a difference in transition temperature Tt in comparison to literature values which is attributed to a processing induced deviation from the nominal composition. The effect of secondary phases is discussed and the importance of the metal/non-metal (M/NM)-ratio is shown. The article presents a road map for the preparation of Mn-Fe-Si-P-based alloys with highest quality.
REE Recovery from End-of-Life NdFeB Permanent Magnet Scrap: A Critical Review
Yongxiang Yang, Allan Walton, Richard Sheridan, Konrad Guth, Roland Gauß, OliverGutfleisch, Matthias Buchert, Britt-Marie Steenari, Tom Van Gerven, Peter Tom Jones, Koen Binnemans
Journal of Sustainable Metallurgy vol. 3, issue 1 (2017) 122-149
Recycling of REEs contained in magnets from End-of-Life (EOL) products such as consumer electronics, electric and hybrid vehicles (EVs, HEVs) and modern wind turbines will play an important and complementary role in the total supply of REEs in the future. Within the EU MC-ITN EREAN: European Rare Earth Magnet Recycling Network we reviewed the technologies to recover the REEs from these magnets, including physical processing and separation, direct alloy production, and metallurgical extraction and recovery.
Welt der Materialien
On Saturday, 18.02.2017, the FM group participated in the „Welt der Materialien“– Open Day of the Material Science department. Many interested people including younger and older pupils, i.e. our prospective students, as well as families enjoyed the presentations of our research topics in a playful way. Soft and super strong magnets, levitating graphite, a Gauss canon, insights into electromotors, a solar-powered frictionless Mendocino motor and much more could be experienced and was explained by our committed staff. As a highlight, our self-constructed magnetocaloric demonstrator has been shown to visualize the concept of the magnetic cooling technology. In summary, it was an interesting day with lectures, nice cake and a lot of science at your fingertips for everyone.
Production and properties of metal-bonded La(Fe,Mn,Si)13Hx composite material
I.A. Radulov, D.Yu. Karpenkov, K.P. Skokov, A.Yu. Karpenkov, T. Braun, V. Brabänder, T. Gottschall, M. Pabst, B. Stoll, O. Gutfleisch
Acta Materialia 127 (2017) 389
We developed a new method for the production of La(Fe,Mn,Si)13Hx-based magnetocaloric composites using a metal binder with a low melting temperature. Dehydrogenation of the powder during the net-shaping process can be prevented and good mechanical integrity, high thermal conductivity and corrosion resistance of the heat exchanger is ensured. Our patented method can be applied for production of metal-bonded composite of any other mechanically unstable magnetocaloric material, e.g. Fe2P-based compounds or Heusler alloys.
High-performance solid-state cooling materials: Balancing magnetocaloric and non-magnetic properties in dual phase La-Fe-Si
Y. Shao, J. Liu, M. Zhang, A. Yan, K.P. Skokov, D. Yu Karpenkov, O. Gutfleisch
Acta Materialia 125 (2017) 506-512
In cooperation with Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, we demonstrate how the intrinsic brittleness and relatively low thermal conductivity in magnetocaloric La-Fe-Si-H alloys can be overcome by adding extra α-Fe as a reinforcing phase enabling the fabrication of La-Fe-Si-H blocks and plates with good cyclic stability, large and reproducible adiabatic temperature change and improved thermal conductivity.
Correlation of microchemistry of cell boundary phase and interface structure to the coercivity of Sm(CoFeCuZr)7.19 sintered magnets
H. Sepehri-Amin, J. Thielsch, J. Fischbacher, T. Ohkubo, T. Schrefl, O. Gutfleisch, K. Hono
Acta Materialia 126 (2017) 1-10
In cooperation with NIMS Tsukuba, Danube University Krems, and IFW Dresden we revisited the microstructure of Sm2Co17-type sintered magnets. Detailed magnetic measurements, MFM, high resolution STEM-HAADF and 3D atom probe in combination with micromagnetic simulations revealed how the pinning strength of the cell boundary phase correlates with local anisotropy and macroscopic coercivity upon different heat treatments.
Thermag 2018 in Darmstadt
International Conference on Caloric Cooling
This conference organized by the Department of Functional Materials (chair Prof. Oliver Gutfleisch) at TU Darmstadt together with the International Institute of Refrigeration (IIR) is the 8th event in the Thermag series. The scope of the conference will be extended beyond magnetocalorics and will now include elastocalorics, barocalorics and electrocalorics. In addition to cooling at room temperature, cooling at low temperatures and thermomagnetic power generation are to be considered. The conference will take place from 16.09.2018 to 20.09.2018 in Darmstadt, Germany.
Good NEWS! The Hessen Excellence cluster LOEWE RESPONSE coordinated by Prof. Gutfleisch will be extended into a forth year. For 2017 the programme will be supported by an additional 998.127 Euro. Between the faculties of Material Science, Chemistry and Mechanical Engineering we can continue now our work on resource efficient permanent magnets.
The influence of magnetocrystalline anisotropy on the magnetocaloric effect: A case study on Co2B
M. Fries, K. P. Skokov, D. Yu. Karpenkov, V. Franco, S. Ener, and O. Gutfleisch
Appl. Phys. Lett. 109, 232406 (2016)
In the published work the influence of magnetocrystalline anisotropy on the magnetocaloric effect (MCE) was studied on single crystals of Co2B and compared to measurements on polycrystalline samples. Large differences in the adiabatic temperature change and magnetic entropy change were reported. The magnetocaloric effect differs by 40% in the case of the adiabatic temperature change in a ﬁeld change of 1.9 T when applying the ﬁeld along the hard axis and easy plane of magnetization. In the case of the entropy change, the values differ 50% and 35% from each other in ﬁeld changes of 1 and 1.9 T, respectively (see Figure 1). A simple model was developed to illustrate the possible effect on magnetocrystalline anisotropy, showing large differences especially in application relevant ﬁelds of about 1 T. The results strongly suggest that the MCE could be maximized when orienting single crystalline powders in an easy axis parallel to the applied ﬁeld in active magnetocaloric regenerator structures, and therefore the overall device efﬁciency could be increased.
Grain boundary diffusion of different rare earth elements in Nd-Fe-B sintered magnets by experiment and FEM simulation
Konrad Löwe, Dimitri Benke, Christian Kübel, Tim Lienig, Konstantin Skokov, Oliver Gutfleisch
Acta Materialia 124 (2017) 421-429
In the present work, we explore the influence of a surface-bulk coercivity gradient in Nd-Fe-B magnets produced by the Grain Boundary Diffusion Process (GBDP) on the overall coercivity. In the study we diffused four different rare earth elements (Dy, Tb, Ce and Gd) sintered Nd-Fe-B magnets. We find that Tb diffuses significantly faster than Dy, Ce diffuses slightly slower than Dy, and that the overall coercivity decrease is similar for Ce and Gd. High-resolution scanning transmission electron microscopy shows the nano-scale distribution of Tb around the grain boundaries. Finally, a simple model for the magnetization reversal in grain boundary diffusion processed gradient Nd-Fe-B magnets was developed and implemented into a FEM software. Our calculated demagnetization curves correspond very well for the Dy and Tb samples, but deviate significantly for Ce and Gd.
Engineering perpendicular magnetic anisotropy in Fe via interstitial nitrogenation: N choose K
Hongbin Zhang, Imants Dirba, Tim Helbig, Lambert Alff, and Oliver Gutfleisch
APL Mater. 4, 116104 (2016);
In this work, combining experimental results and first principles calculations, we show that interstitial nitrogen not only serves for inducing tetragonality in α′-Fe8Nx but is also essential for achieving a high degree of perpendicular magneto-crystalline anisotropy, K. Our results demonstrate that the orbital magnetic moments of the iron atoms above and below N in the direction of magnetization are much more susceptible to the applied magnetic field than their in-plane counterparts, leading to a giant value of K as compared to a hypothetical distorted material without N.
The Resource Basis of Magnetic Refrigeration
September 2016, Journal of Industrial Ecology
Emerging economies such as China and India are currently experiencing a “refrigeration revolution.” Energy spent for domestic cooling is expected to outreach that for heating worldwide over the course of the twenty-first century. Magnetic refrigeration is an alternative cooling technology that works without gas-based refrigerants and has the potential to be significantly more energy efficient. We evaluate to what extent the raw materials needed to produce this kind of technology on a mass-market scale are critical in terms of demand and supply, thus identifying potential supply bottlenecks that might hinder the breakthrough of this promising technology. We assess the criticality of three promising magnetocaloric materials, that is, Gd5(SiGe)4, La(FeSi)13, and (MnFe)2P), as well as of Nd2Fe14B, as the candidate permanent magnet material to drive the cooling cycle. The Gd-based alloys are disqualified as a mass-market refrigerant in terms of resource criticality, whereas La- and Mn-based alloys are much less problematic. Given the current state of technology and projected resource supply, Nd in Nd2Fe14B magnets would experience a significant bottleneck only at a later innovation stage, that is, when magnetic cooling technology would largely dominate the domestic refrigerator and air-conditioning market.
Synthesis, morphology, thermal stability and magnetic properties of α'-Fe16N2 nanoparticles obtained by hydrogen reduction of γ-Fe2O3 and subsequent nitrogenation
Acta Materialia, 123 pp. 214-222.(2017)
I. Dirba, C.A. Schwöbel L.V.B. Diop, M. Duerrschnabel, L. Molina-Luna, K. Hofmann, P. Komissinskiy, H.-J. Kleebe, O. Gutfleisch
Typical synthesis of α-Fe16N2 nanoparticles involves reduction of iron oxides by hydrogen at elevated temperatures which is disadvantageous due to the particle coalescence. Here we report on a process for reduction of iron oxides at elevated pressures and show that by increasing hydrogen pressure from atmospheric to 53 MPa, it is possible to reduce the reaction temperature from 663 K down to 483 K, resulting in phase-pure α-Fe nanoparticles without noticeable particle growth. By subsequent nitrogenation in an ammonia flow, fine, 99% phase-pure α″-Fe16N2 nanoparticles could be synthesized. The reduction temperature and the respective particle size has a significant influence on the nitrogenation step. α″-Fe16N2 nanoparticles exhibit semi-hard magnetic properties with Ms(0) = 215 Am2 kg−1, μ0Hc = 0.22 T, TC = 634 K and exchange stiffness Ac = 6.84 pJ m−1, Aa,b = 7.53 pJ m−1. Synthesis conditions, microstructure, chemical composition and thermal stability of the nanoparticles are systematically studied and correlated with the observed magnetic properties.
Six new members of the JEMS International Advisory have been appointed
The newly-appointed members are: Oliver Gutfleisch (Darmstadt), Florence Gazeau (Paris), Larissa Panina (Moscow), Yoshichika Otani (Tokyo), Bert Koopmans (Eindhoven) and Ekkes Brück (Delft). They will start now a six year term.
Magnetic anisotropy of Sm2Fe17 single crystals
L. V. B. Diop, M. D. Kuz’min, K. P. Skokov, D. Yu. Karpenkov and O. Gutfleisch
Phys. Rev. B 94, 144413 (2016)
The previously accepted notion that the spontaneous magnetization of Sm2Fe17 lies in the basal plane of the crystal is true only approximately, and then only around room temperature. At low temperatures the magnetization, whose orientation is not fixed by the symmetry, is found to deviate from the basal plane by as much as 10°. The threefold symmetry axis is a hard direction; to magnetize the crystal in this direction a magnetic field of about 9 T is required. The hard-axis magnetization arrives at saturation discontinuously, by way of a first-order phase transition (FOMP). The behavior is a general one for trigonal ferromagnets where K1< 0 and the leading trigonal anisotropy constant is nonzero, K'2 ≠ 0. Although of universal occurrence, the FOMP is only visible at low temperatures, where it is accompanied by a magnetization anomaly of sufficient size.
Research Infrastructures as an Enabling Platform for Advanced Materials
The project provides a design and characterization platform for new materials in the early stages of the innovation chain. The KAVA partners will offer their research infrastructure and expertise to European partners from (applied) research institutes and industry (large companies and SME’s) working within the KIC Raw MatTERS theme “Substitution of critical and toxic materials in products and for optimised performance”. The project creates an European access point for an all-embracing structural, chemical, optical, electronic and magnetic characterisation protocol for technologically promising materials which may be used in e.g. new substitutional optical, electronic and magnetic devices in whatever form (crystal, thin film, powder, molecule, nanostructure or liquid solution).
TU Darmstadt together with Fraunhofer IWKS will provide sophisticated facilities for comprehensive characterization of magnetic properties in wide temperature, pressure and field ranges.
Poster award at JEMS 2016 conference.
At the 8th Joint European Magnetic Symposia (JEMS) in Glasgow, UK, Alexandru Lixandru received the “Prize for the best student poster” for the poster entitled: “Recycling of rare earth permanent magnet scrap material by hydrogen treatment routes: from waste selection to magnet recycling”. This work is part of his PhD on the recycling of the Nd-Fe-B magnets.
The poster presented a study on different permanent magnet waste streams from electrical and electronic equipment (WEEE) and aims to improve the identification and recovery of the products with high volumes of rare earths. The work combines low and high temperature hydrogen treatments – hydrogen decrepitation (HD) process and dynamic hydrogenation disproportionation desorption recombination (d-HDDR) process – to produce fresh anisotropic Nd-Fe-B powder for resin-bonded magnets from scrap sintered magnets.
PhD-graduation in the FM group
We are happy to congratulate our dear colleague Tino Gottschall to his defense. His work entitled: On the magnetocaloric properties of Heusler compounds: Reversible, time- and size-dependent effects of the martensitic phase transition was rewarded with highest honor (summa cum laude) by the PhD commission. The defense was followed up by a barbecue with nice weather and a little after-defense ceremony by his colleagues. We thank Dr. Gottschall not only for this memorable day but for all the work he has done within the Functional Materials group which often went far beyond his PhD thesis. We are also very happy that he will continue his scientific work in Darmstadt within the group.
His PhD thesis resulted in various articles which can be downloaded here: Publications
Invited talk of Prof. Heiko Wende
On Friday the 8th of July Prof. Heiko Wende from the University of Duisburg Essen gave an invited talk entitled: From molecular spin hybrids to magnetocaloric systems: fundamental understanding by element specific investigations, within the Framework of the LOEWE RESPONSE and DFG SPP Ferroic Cooling projects. The talk of Prof. Wende attracted many people followed by an intense discussion. We thank Heiko Wende for his excellent talk giving an insight on the topic of spin hybrid systems.
Micromagnetic simulations on the grain shape effect in Nd-Fe-B magnets
Min Yi, Oliver Gutfleisch, and Bai-Xiang Xu
Journal of Applied Physics 120, 033903 (2016);
Within the LOEWE RESPONSE project we performed micromagnetic simulations to study the effect of grain shape and defect layer in Nd-Fe-B magnets. It was found that the coercivity can be increased by a factor of ~2 by changing the grain shape from the triangular prism to the spheroid. Both the anisotropy field contribution and the shape contribution to the coercivity, and thus also the final coercivity, were found to decrease in the order: spheroid > circular prism > hexagonal prism > square prism > triangular prism…
Mastering hysteresis in magnetocaloric materials
O. Gutfleisch, T. Gottschall, M. Fries, D. Benke, I. Radulov, K. P. Skokov, H. Wende, M. Gruner, M. Acet, P. Entel, M. Farle
Phil. Trans. R. Soc. A 374: 20150308
Hysteresis is more than just an interesting oddity that occurs in materials with a first-order transition. It is a real obstacle on the path from existing laboratoryscale prototypes of magnetic refrigerators towards commercialization of this potentially disruptive cooling technology. Indeed, the reversibility of the magnetocaloric effect, being essential for magnetic heat pumps, strongly depends on the width of the thermal hysteresis and, therefore, it is necessary to understand the mechanisms causing hysteresis and to find solutions to minimize losses associated with thermal hysteresis in order to maximize the efficiency of magnetic cooling devices. In this work, we discuss the fundamental aspects that can contribute to thermal hysteresis and the strategies that we are developing to at least partially overcome the hysteresis problem in some selected classes of magnetocaloric materials with large application potential. In doing so, we refer to the most relevant classes of magnetic refrigerants La–Fe–Si-, Heusler- and Fe2P-type compounds.
Grain boundary diffusion in nanocrystalline Nd-Fe-B permanent magnets with low-melting eutectics
Simon Sawatzki, Christian Kübel, Semih Ener, Oliver Gutﬂeisch
Acta Mater. 115 (2016) 354-363
In order to combine the good thermal stability of nanocrystalline Nd-Fe-B magnets with the efﬁcient grain boundary diffusion process (GBDP), low-melting eutectics have been mixed with Nd-Fe-B melt-spun ribbons, hot-compacted and subsequently die-upset. Transmission electron microscopy (TEM) analysis revealed the formation of 5-10 nm thick Dy-shells and a crystallography dependent diffusion into the individual solid grains on the nanoscale. Subsequent annealing at 600°C leads to an enhanced diffusion but also some undesired Nd-O phases and induced grain growth depending on the annealing time, degree of deformation, strain rate and position within the sample. An optimized distribution of the low-melting eutectic was realized by milling the precursor powder and by using ternary alloys with reduced melting points. As a consequence a much higher effective increase in coercivity per wt%Dy was obtained compared to a homogeneous Dy-distribution which demonstrates the validity of this new approach.
Contradictory role of the magnetic contribution in inverse magnetocaloric Heusler materials
T. Gottschall, K. P. Skokov, D. Benke, M. E. Gruner, O. Gutfleisch
Phys. Rev. B 93, 184431 (2016)
In this paper, we illustrate the dilemma of inverse magnetocaloric materials using the example of Heusler alloys. For the two paradigmatic Heusler systems of Ni-Mn-In and Ni-Mn-In-Co, we provide a systematic comparison of experimental data under different magnetic fields and hydrostatic pressures with magnetic and the magnetocaloric properties obtained from the Heisenberg model. This allows us to separate the lattice and the magnetic contribution to the total entropy of the martensitic transition. Our analysis reveals that a large magnetization change is parasitic, but at the same time it is necessary to drive the magnetocaloric effect. This contradicting role of the magnetic contribution – the dilemma – is a general characteristic of inverse magnetocaloric Heusler materials.
Functional Materials were at Darmstadt Firmenlauf 2016
The members of the Functional Materials attended to 5k and 10k runs of the 2016 Darmstadt Company run. The runners all enjoyed the fun and the after-run-party.
The search for room temperature tetragonal phases of Fe-Mn-Ga: A reactive crucible melting approach
Semih Ener, Johannes Kroder, Konstantin P. Skokov, Oliver Gutfleisch
Journal of Alloys and Compounds 683, 198 (2016)
A bulk high-throughput screening method is needed for the experimental search of new permanent magnet materials. In this work, the fast, easy and effective reactive crucible melting method was applied to the Ni-Mn-Ga and Fe-Mn-Ga Heusler systems. Energy dispersive X-ray spectroscopy was used for elemental analysis of phases formed in the crucibles which allowed us to construct parts of the room-temperature phase diagrams of Ni-Mn-Ga and Fe-Mn-Ga systems. By means of magneto-optical Kerr microscopy the phases with stripe domains – an essential feature of an uniaxial magnetic anisotropy – were identified and synthesized by conventional metallurgy. The structural/microstructural properties of the synthesized individual sample were investigated in addition to the magnetic properties. A high room temperature coercivity value of 0.33 T is obtained which is comparable with Alnico permanent magnets.
Monoborides doped with iron and cobalt for thermomagnetic generators
M. Fries, Z. Gercsi, S. Ener, K. P. Skokov, O. Gutfleisch
Acta Mater. 113, 213 (2016)
A systematic study focusing on the crystallographic, magnetic and magnetocaloric properties of cobalt and iron substituted MnB was conducted. The pure MnB sample shows a sharp magnetic transition of 567 K yielding a large magnetic entropy change in a 2 T field change. It is shown that both the substitution of Co and Fe elements for Mn were found to effectively modify Tc accompanied by a lower magnetization leading to a reduced magnetocaloric effect. The differences in magnetic properties with substitution are described by calculation of the density of states and interatomic distances. The sharp and significant change of Ms(T), the very stable nature of these refractory borides and abundant availability makes some of these compositions suitable for thermomagnetic power generation applications.
Dynamical Effects of the Martensitic Transition in Magnetocaloric Heusler Alloys from Direct ΔTad Measurements under Different Magnetic-Field-Sweep Rates
T. Gottschall, K. P. Skokov, F. Scheibel, M. Acet, M. Ghorbani Zavareh, Y. Skourski, J. Wosnitza, M. Farle, O. Gutfleisch
Phys. Rev. Applied 5, 024013 (2016)
Large magnetocaloric effects can be obtained in Ni-Mn-based Heusler alloys due to the magnetostructural transition between martensite and austenite. This phase transformation proceeds via nucleation and growth. By direct measurements of the adiabatic temperature change ΔTad using different magneticfield-sweeping rates from 0.01 up to 1500 Ts−1, we study the dynamic behavior of the two Heusler compounds Ni50Mn35In15 and Ni45Mn37In13Co5 transforming near room temperature. From these experiments, we conclude that the nucleation process is rather slow in contrast to the relatively fast movement of the phase boundary between martensite and austenite.
On the S(T) diagram of magnetocaloric materials with first-order transition: Kinetic and cyclic effects of Heusler alloys
Tino Gottschall, Konstantin P. Skokov, Ramon Burriel, Oliver Gutfleisch
Acta Mater. 107, 1 (2016)
In this work we investigate the reversible magnetocaloric effect of Heusler alloys under cycling in terms of the S(T) diagram. For this purpose we selected three different Heusler alloys of NieMneIn with transition temperatures between 200 k and room temperature. The S(T) diagrams of the three materials cover all conceivable shapes and are therefore representative for inverse magnetocaloric Heusler alloys. First we comprehensively analyzed the magnetocaloric properties of this model system using calorimetry, magnetometry and direct measurements of the adiabatic temperature change ΔTad and subsequently combined those results into the S(T) diagram.
PhD-graduation in the FM group
We congratulate our dear colleague Simon Sawatzki to his excellent doctoral work on the “Grain boundary diffusion process in nanocrystalline Nd-Fe-B permanent magnets”, which was rewarded by the PhD commission with highest honor (summa cum laude). We thank him for all his contributions going beyond his project work and the memorable day of his defense! We are glad that he will continue his research in our group.
Parts of his work are already published in journals and can be downloaded from our webpage:
The full dissertation is available via ULB Darmstadt
IEEE Distinguished Lecture: Ludwig Schultz
We thank Prof. Dr. Ludwig Schultz from the IFW Dresden/ TU Dresden for his visit and his lecture on the “Interaction of ferromagnetic and superconducting permanent magnets – superconducting levitation” given on the 4th of December 2015.
Magnetic anisotropy of La2Co7
M.D. Kuz'min, K.P. Skokov, I. Radulov, C.A. Schwöbel, S. Foro, W. Donner, M. Werwiński, J. Rusz, E. Delczeg-Czirjak, and O. Gutfleisch
J. Appl. Phys. 118 (2015) 053905
The magnetic properties as well as the crystal structure of La2Co7 single crystals were investigated. The compound has an easy-axis anisotropy with a K1 as high as 2 MJ/m3 (at a maximum near T = 100 K) and a K2 that is much lower than K1. The anisotropy field amounts to 8 T at T = 10K and 6.7 T at room temperature. A significant (10%) magnetization anisotropy has been observed. Density-functional calculations by our partners at Uppsala University are in very good qualitative agreement with experiment.
Effect of doping by 5d elements on magnetic properties of (Fe1−xCox)2B alloys,
PHYSICAL REVIEW B 92 (2015) 174413
A. Edström, M. Werwinski, J. Rusz, O. Eriksson, K.P. Skokov, I.A. Radulov, S. Ener, M.D. Kuz’min, J. Hong, M. Fries, D. Yu. Karpenkov, O. Gutfleisch, P. Toson , J. Fidler
We investigated the magnetic properties of the (Fe1−xCox)2B alloys both experimentally and computationally. Using DFT calculations showed the effect of substitutional doping the transition metal sublattices by 5d transition metals. The theoretical predictions agree well with the experimental observations on eg Ir and Re substituted single crystals. Doping by 2.5 at.% of Re on the Fe/Co site shows a magnetocrystalline anisotropy energy which is increased by 50% compared to its parent (Fe0.7Co0.3)2B compound, making this system interesting in the context of permanent magnet replacement materials.
Annual group meeting
07.09.15-09.09.15 the annual group meeting took place at the Seminarhotel Kurhaus Trifels, giving everybody the opportunity to present and discuss their results and achievements obtained in the last year. To balance activities, a hike to the castle of Trifels was scheduled in the afternoon in the best sunshine that one could expect from the early September weather. The excellent cuisine that included a great barbeque provided the best support for body and spirit. The nice seminar rooms, refreshments and snacks made sure that motivation was always high. While it was a great group experience for the whole team, many fruitful ideas sparked as the result of discussions and future perspectives.
Visit of Dr. Sepehri-Amin from NIMS Tsukuba
Dr. Sepehri-Amin from Magnetic Materials Unit, National Institute for Materials Science – NIMS, Tsukuba, Japan, works on the development of Dy-free high performance Nd-Fe-B permanent magnets via microstructure design and modification. We are happy to say that he is visiting our Functional Materials group for one month within the RESPONSE LOEWE excellence programme. During his stay he gave seminar talks on 3rd of July in Darmstadt and on 7th of July at Fraunhofer IKWS Alzenau on his recent works. Further cooperation is planned on Dy-free Nd-Fe-B permanent magnets.
Two highlights in the first half of 2015 were plenary talks at the Intl Conf on Magnetism ICM2015 in Barcelona and the Spring Meeting of German Physical Society DPG2015 in Berlin on „Magnetic Materials for Green Technologies “.
European Rare Earths Competency Network (ERECON)
Rare earth elements are a group of 17 speciality metals used in high-tech products such as smart phones or wind turbines. Due to the lack of internal supply the EU needs to import more than 90 % of these metals, mainly from China. As the demand for rare earths is expected to grow, the EU aims to improve access to these metals, reduce their consumption and improve extraction conditions in Europe. To help secure the supply, the European Rare Earths Competency Network (ERECON) was established.
Visit to Heraeus Quarzglas GmbH & Co. KG
02.02.2015 Within the lecture course “Materials Engineering” we visited Heraeus Quarzglas GmbH & Co. KG in Kleinostheim. The students, who are in their first master semester, were able to get an insight into Heraeus’ production line, which included continuous casting and glass forming facilities. Additionally, the students could deepen their understanding of glass technology in two presentations about quartz glass and infrared heating devices.
We thank Heraeus for the hospitality and excellent organization of our excursion.
PhysRevLett: Element-Resolved Thermodynamics of Magnetocaloric LaFe13−xSix
M.E. Gruner, W. Keune, B. Roldan Cuenya, C. Weis, J. Landers, S. Makarov, D. Klar, M. Y. Hu, E.E. Alp, J. Zhao, M. Krautz, O. Gutfleisch, H. Wende
Together with the University Duisburg-Essen the element-resolved vibrational density of states across the first-order transition from the ferromagnetic
low temperature to the paramagnetic high temperature phase of LaFe13−xSix was investigated. We found that the increase in lattice entropy associated with
the Fe subsystem is significant and contributes cooperatively with the magnetic and electronic entropy changes to the excellent magneto- and barocaloric properties.
Visit of Hessian Minister of Environment Mrs. Priska Hinz
31.01.2015 Within the Week of Science the Hessian Minister of Environment Mrs. Priska Hinz visited the LOEWE research cluster RESPONSE at the Technische Universität Darmstadt. Up to now the rare earth elements (REE) have no adequate substitutes in many high tech applications, especially high performance permanent magnets for wind turbines and electro-mobility are difficult to replace by magnet materials using less critical elements. Additionally, the beneficiation of the REE from their ores has a high environmental impact which puts a question mark behind the sustainability of some novel energy technologies. The RESPONSE project is dedicated to the drastic reduction and complete substitution of REE in permanent magnets. During her visit the Minister visited the labs, was updated on current research activities, followed by discussions about future strategic goals.
Large reversible magnetocaloric effect in Ni-Mn-In-Co
Tino Gottschall, Konstantin P. Skokov, Bianca Frincu and Oliver Gutfleisch
Appl. Phys. Lett. 106, 021901 (2015)
We report on the high irreversible adiabatic temperature change of −8 K in a magnetic field change of 1.95 T in the Heusler compound Ni45.7Mn36.6In13.5Co4.2 showing a first-order magnetostructural transition. Due to the large thermal hysteresis of 10 K, this high ΔTad cannot be obtained in a cyclic way but still the reversible magnetocaloric effect amounts to −3 K—an unexpectedly high value which compares to the ΔTad of La(Fe,Si,Co)13. In order to reveal the nature of this high reversible magnetocaloric effect, in-situ temperature dependent optical microscopy of minor loops of thermal hysteresis has been done.
Mechanism of the texture development in hydrogen-disproportionation–desorption-recombination (HDDR) processed Nd–Fe–B powders.
Sepehri-Amin, H. ; Ohkubo, T. ; Hono, K. ; Güth, K. ; Gutfleisch, O.
Microstructure evolution in Nd12.8Fe80.1B6.6Ga0.3Nb0.2 alloy powders at different hydrogen pressures during the hydrogen-disproportionation process has been studied. Transmission electron microscopy showed that Fe2B grains “memorize” the
crystallographic orientation of the initial Nd2Fe14B phase and transfer it to the recombined Nd2Fe14B grains in the highly textured sample. 3-D tomography of backscattered electron SEM images showed that recombined Nd2Fe14B grains nucleate at the interfaces of Fe2B/NdH2 phases, which grow through the interfaces of NdH2/a-Fe phases during the DR process.
A new type of La(Fe,Si)13 based magnetocaloric composites with amorphous metallic matrix
Krautz, Maria ; Funk, Alexander ; Skokov, Konstantin P. ; Gottschall, Tino ; Eckert, Jürgen ; Gutfleisch, Oliver ; Waske, Anja
This work shows a promising route for the production of compact refrigerant bodies consisting of brittle giant magnetocaloric materials whose performance are often affected by their mechanical integrity. Hot compaction at the glass transition temperature, Tg, of the amorphous matrix prevents crack formation as commonly occurs in conventional hot-compacted La(Fe,Si)13 material.
Visit to UMICORE
Within the lecture „Materials Engineering“ in the Master course Material Science at TUDA we visited on 15th of December 2014 Umicore AG & Co. KG in Hanau, Hessen. The students were able to get an insight into Umicore‘s production line of Nickel-based contact materials as well as facilities for the recycling of precious metals like e.g. Silver.
MOOC@TU9 presents the wide variety and different perspectives of German engineering courses at TU9 universities, and offers insights into the central issues, contents, structures and work methods of the various disciplines and courses of study. Join us for nine weeks and discover what Excellence in Engineering and the Natural Sciences – Made in Germany at TU9 is all about. Week 5 is about: Models in Material Engineering – From Devices for Nanotechnology to New Magnetics.
Prof. Oliver Gutfleisch explains: Magnetic Materials for green technologies
Permanent magnets are essential materials for everyday life. Every one of us possesses numerous magnets without necessarily being aware of them as they are hidden in computers, speakers, smart phones or also in modern cars in which at least 50 magnets can be found. However, they are also key components in upcoming energy technologies such as windturbines and electromobility. In the session you will learn more about rare earth elements and how modern magnets benefit of their unusual physical and chemical properties. The most important facts about permanent magnets will be discussed and also a critical reflection on the sustainability of rare earth elements and with this the vulnerability of technologies will be part of the session.
Meeting with Japanese delegation of MagHEM
On November 20th we received a Japanese delegation of MagHEM (Technology Research Association of Magnetic Materials for High-Efficiency Motors) contracted with NEDO (New Energy and Industrial Technology Development Organization) funded by METI (Ministry of Economy, Trade and Industry), which is aiming at development of high-performance magnetic materials without rare-earth and high-efficiency motors. All major Japanese magnet producers and major automotive and automotive parts OEMs were represented. Information on common research interests such as in LOEWE Response were exchanged.
High energy product in Battenberg structured magnets
Bance et al., Influence of defect thickness on the angular dependence of coercivity in rare-earth permanent magnets, Applied Physics Letters, Volume 104, 2014, Pages 182408ff
In the paper we present results from micromagnetic simulations that assess the performance of multi-phase nanostructured permanent magnets, whose cross-section resemble that of a Battenberg cake. By including a super-hard outer shell we are able to counteract the effects of thermal fluctuations and surface defects, both of which are detrimental to the performance of such permanent magnets. Such magnets are important for the motors in electric vehicles and for the generators in wind turbines, and these machines usually operate at elevated temperatures
Temperature-dependent Dy diffusion processes in Nd–Fe–B permanent magnets
Löewe, K. ; Brombacher, C. ; Katter, M. ; Gutfleisch, O.
Nd-Fe-B permanent magnets have been coated with Dysprosium and annealed at various temperatures to study the impact of the temperature dependent Dy diffusion processes on both the magnetic properties and the microstructure. When optimum annealing conditions are applied the improved stability against opposing magnetic fields can be observed in the magnets up to a depth of about 3 mm along the diffusion direction.
After the diffusion treatment, the specific “core-shell” microstructure has been investigated with WDX and STEM EDX. While in the proximity of the Dy – coated surface, each grain has a Dy enriched shell with a Dy – content of around 6 at.%, the Dy concentration decreases exponentially to about 1.8 at.% after a diffusion depth of 400 µm and to about 1 at.% after a diffusion depth of 1500 µm.
An epitaxial relation between Dy – poor core and Dy – rich shell was observed by EBSD. This finding is supported by results obtained with Kerr microscopy.
Prof. Michael Kuzmin
We congratulate our group member Michael Kuzmin to his professorship at the University of Marseille, starting on 1 st of September 2014.
Visit of Prof. Victorino Franco
We welcome Prof. Victorino Franco from Universidad de Sevilla, to our group of Functional Materials for the time of two months. Till end of October 2014 he will stay with us, working on magnetocaloric materials.
Wissenschaftsminister überreicht Förderbescheide
Im Zentrum steht das LOEWE-Projekt zur Schonung seltener Erden
Der hessische Wissenschaftsminister Boris Rhein hat am 05. Juni bei einer Feierstunde an der TU Darmstadt Fördergelder in Höhe von rund sieben Millionen Euro aus der LOEWE-Initiative zur Förderung exzellenter Forschung übergeben – sie gehen an die TU Darmstadt und an drei hessische Unternehmen.
DGM-Fachausschuss Funktionsmaterialien gegründet
Am 23. Mai 2014 kamen etwa 60 interessierte Materialwissenschaftler und Werkstofftechniker zur konstituierenden Sitzung zum neuen Fachausschuss „Funktionsmaterialien“ der Deutschen Gesellschaft für Materialkunde (DGM) an der TU Darmstadt, Fachbereich Materialwissenschaft, zusammen. Prof. Dr. Gutfleisch als Sprecher des FA eröffnete die Sitzung. Teilnehmer waren sowohl Studenten, Doktoranden und Hochschulprofessoren als auch Vertreter aus der Industrie.
G8 Project Workshop at TU Darmstadt
On May 9th, 2014 the second Workshop of the G8 Project was held at Technical University of Darmstadt. Partners from US, Japan and Germany attended the meeting, presenting their latest studies on the topic of “Rare-earth Free Permanent sustainable for the Next Generation”. Prof. Gutfleisch (TUDA) opened the meeting and was followed by Prof. Suzuki (MINT – University of Alabama) who presented the recent experimental results on MnAl and MnBi thin films. Dr. Sepehri-Amin (NIMS) presented the microstructure analysis on the MnAl, MnBi and MnRh thin films. Magnetic properties of melt-spun ribbons and mechano-chemically synthesized MnBi and MnGa were presented by Prof. Hadjipanayis (University of Delaware). Dr. Göring from the Max Planck Institute Stuttgart presented the recent XMCD studies on MnBi samples. Presentations were completed by talks of M.Sc. Jian and Dr. Ener from Technical University of Darmstadt on bulk MnAlC and MnGa.
Moderne Rohstoffe – Raubbau oder Nachhaltigkeit?
Chancen und Risiken für unsere Gesellschaft
Seltene Erden, Phosphor und Wertschöpfungskette: Im Rahmen eines Universitätstags erörtern Experten die Verfügbarkeit und den Gebrauch von modernen Rohstoffen – zum Beispiel aus umwelt- und volkswirtschaftlicher Sicht. Rohstoffe sind für Gesellschaft und Wirtschaft unverzichtbar, ihre Verfügbarkeit beeinflusst zukünftige Technologieentwicklungen.
hoch3 Forschen: “High-performance magnets”
In the current issue of the TU Darmstadt maganzin hoch3 Forschen Prof. Oliver Gutfleisch was interviewed on the topic of permanent magnetic marterials. The whole issue can be downloaded free of charge
Opening ceremony of our new building M3
The name of the new building M3is an acronym for “magnets, molecules and materials” which are the research topics of the scientists from the department of Chemistry and Materials Science who moved in recently. On 29 October 2013 the new laboratory and office building was presented to the public by a festive opening.
IEEE Distinguished Lecture: Rudolf Schäfer
On 21 October the IEEE Distinguished Lecturer Rudolf Schäfer from IFW Dresden visited the TU Darmstadt to present his lecture about “Magneto-Optic Analysis of Magnetic Microstructures”.
Electrical and magnetic properties of hot-deformed Nd-Fe-B magnets with different DyF3 additions
The effect of deformation and DyF3 additions on the electrical resistivity and the magnetic performance has been studied in hot-deformed Nd-Fe-B melt-spun ribbons and correlated with respective microstructures. Despite the nanocrystallinity of hot-compacted magnets, the specific electrical resistivity measured by four-point-method was shown to be comparable with that of sintered magnets. Die-upsetting reduces electrical resistivity within the hard plane because of an enhanced shape anisotropy of the grains. The addition of DyF3 overcompensates this reduction due to the presence of electrically insulating Dy-F rich inclusions and thus reduces eddy current losses within the magnet. Magnetic measurements reveal an increase in coercivity without a change in remanence for die-upset magnets with a total height reduction of 63% and 1.2 wt. % Dy (1.6 wt. %DyF3). Both properties demonstrate an effective reduction in heavy rare earth Dy for Nd-Fe-B magnets.
LOEWE application receives positive evaluation!
On 11 July 2013 the Hessian Ministry of Sciences approved the LOEWE application RESPONSE coordinated by Prof. Dr. Oliver Gutfleisch. RESPONSE deals with the reduction and substitution of rare earth elements in high performance permanent magnets. The research grant amounts to 4.4 million Euros for 3 years. RESPONSE complements the applied research on magnetic materials at Fraunhofer IWKS Hanau with fundamental studies at the university.
IEEE Distinguished Lecture: Koki Takanashi
We are pleased to announce that Prof. Koki Takanashi from Tohoku University, Japan, will give his IEEE Distinguished Lecture about “Advanced Spintronic Materials: For Generation and Control Of Spin Current” in Darmstadt on 10 July. The event will start at 3:00 pm in room L2|01 77.
Visit of Prof. George Hadjipanayis
As part of his sabbatical Prof. George Hadjipanayis, University of Delaware, USA visited our group for one month. Experiments on rare earth free permanent magnets were done and future cooperation was planned.
First Group Seminar
In June the first internal seminar of the division of Functional materials was held in the monastery of Bronnbach (near Wertheim) where the Fraunhofer ISC has a branch. In the old vaults of this historical place the group discussed recent scientific result and focused on future activities.
Workshop on “Energy and Materials Criticality”
It is our pleasure to announce the Workshop on “Energy and Materials Criticality”, taking place on the beautiful Greek Island of Santorini, directly preceding JEMS, August 22nd through the 25th. The aim of this Workshop is to bring together world renown scientists and engineers to discuss current worldwide research on the reduction, substitution, and recycling of critical magnetic materials, and to encourage the international collaborations and interactions that would lead to a significant decrease in our dependence on these critical materials worldwide. Organized by George Hadjipanayis of the University of Delaware (U.S.), and Oliver Gutfleisch of TU Darmstadt (Germany), the workshop will feature invited and contributed presentations from around the globe. Abstracts can be submitted until June 28, 2013. Please visit our website for more information. It is our hope that many of you will wish to attend this workshop before going on to JEMS in Rhodes.
Good marks for Materials Science
In the most recent ranking of the Center for University Development (CHE), the department of Materials Science of the TU Darmstadt is part of the leading group of the national comparison in terms of overall study situation, supervision, completing within standard period of study and reputation of research. About 250 000 students of more than 300 universities participated in this vote.
Tour at Vacuumschmelze in Hanau
On March 18th our Functional Materials group and colleagues from Fraunhofer IWKS visited Vacuumschmelze to get an impression of the production of permanent and soft magnetic materials. We thank the organizers for making it possible to visit the factory.
European School on Magnetism 2013
Feb. 25th to Mar. 8th in Cargèse, France
The topic of the this year´s European School on Magnetism (ESM), coorganized by Prof. Oliver Gutfleisch, will be about Magnetism for energy. It will gather 100 participants coming mainly from Europe, with a few positions open to other countries. In within the 10 days of program, young researchers will have the opportunity to deepen their understanding of magnetism for a efficient scientific work and getting in to contact with the magnetic community.
Workshop on Hydrogen Storage
Feb. 19th at TU Darmstadt
Within the scope of the series of events entitled “Materialinnovationen in der Wasserstoff- und Brennstoffzellentechnologie” of the Hessian Ministry for the Environment, Energy, Agriculture and Consumer Protection, a workshop on Hydrogen is held, covering the topics of hydrogen production, storage and fuel cells. On February 19 the TU Darmstadt is host of the program part about Hydrogen Storage.
Visit to UMICORE
In the framework of our lecture „Materials Engineering“ we visited the Umicore AG & Co. KG in Hanau, Hessen. The students were able to get an insight into Umicore‘s production line of Nickel-based contact materials as well as facilities for the recycling of precious metals like e.g Silver.
IEEE Distinguished Lecture: George C. Hadjipanayis
We are very pleased to announce that Prof. G. Hadjipanayis from the University of Delaware, USA, will give his IEEE Magnetics Society distinguished lecture “Science and Technology of Modern Permanent Magnet Materials” in our institute on 12 December. Time and location will be notified in time.
Sabbatical Prof. Toshiyuki Shima
We welcome Prof. Toshiyuki Shima from Tohoku Gakuin University, Sendai, to our group of Functional Materials for the time of his sabbatical. Till August 2013 he will stay with us, working on hard magnetic thin films.
IEEE Distinguished Lecture: Masahiro Yamaguchi
30 August 2012: Our sincere thanks go to Prof. M. Yamaguchi from Tokyo University for following our invitation to give his IEEE lecture “Soft Magnetic Thin Film Applications at Radio Frequencies” in our department (From left to right: Prof. V. Khovaylo (MISIS Moscow), Prof. O. Gutfleisch, Prof. M. Yamaguchi, Dr. K. Skokov, T. Gottschall and Prof. S. Taskaev (Chelyabinsk State University)).
REPM '12 Poster Award
At the 22nd international workshop on Rare-Earth Permanent Magnets and their Applications REPM '12 in Nagasaki, Simon Sawatzki received the “Young Researchers Best Poster Award” for his contribution. Congratulations!
Opening of IWKS Hanau
On the 29th of June 2012 the second location of the newly founded Fraunhofer group IWKS on Materials Recycling and Resource Strategies was opened in Hanau. Professors Armin Reller from Augsburg, Stefan Gäth from Giessen and Oliver Gutfleisch from Darmstadt will lead the activities on resource strategies, recycling and substitution of critical materials.
European Raw Materials Group Breakfast
The second European Raw Materials Group Breakfast on 10 July 2012 was hosted by MEP Reinhard Bütikofer and focused on the situation regarding the rare earth elements and their state-of-play regarding R&D in Europe. O. Gutfleisch gave a key note lecture on “Rare Earth Elements for Energy Applications – Challenges for Research and Policy Makers”.
The Second Trilateral EU-Japan-U.S. Conference on Critical Materials
- Workshop on Substitute, Reduce, Resource and Recycle of Rare Earth Elements –
28./29. March 2012, Tokyo, Japan
The European Commission organized the event together with the US Department of Energy and the Japanese New Energy and Industrial Technology Development Organization (NEDO) on Research on Critical Materials and in particular on Rare Earth Magnets and their connection to a clean energy future. The trilateral conference dealt with New approaches to Reduce Rare Earths for Permanent Magnets and Phosphors and Environmentally sound, economical separation processes for rare earths in diverse ore bodies and recycling streams. Oliver Gutfleisch presented a perspective on the topic “Beyond NdFeB sintered magnets”.
Viewpoint Set No. 51: Magnetic Materials for Energy
Edited by Oliver Gutfleisch and Victorino Franco
This Scripta Materialia viewpoint set on Magnetic Materials for Energy provides opinionated articles by well-known experts in the field on the perspectives of hard and soft magnets as well as magnetocaloric materials for energy applications.
Exploring La(Fe,Si)13-based magnetic refrigerants towards application
Advanced magnetic refrigerants such as La(Fe,Si)13 materials require large entropy and adiabatic temperature changes based on the control of phase change physics and hysteresis. In order to advance their incorporation in prototypes and industrial applications, processing of single phase materials with graded working temperatures needs to be up-scaled and important engineering properties such as the thermal transport properties, corrosion protection and mechanical stability need to be optimized. These issues, including a last step of near net-shaped manufacturing of complex geometries, are discussed in this Viewpoint paper.
Giant magnetocaloric effect driven by structural transitions
Magnetic cooling could be a radically different energy solution substituting conventional vapour compression refrigeration in the future. We report now for Heusler-type Ni–Mn–In–(Co) magnetic shape-memory alloys, the adiabatic temperature change ΔTad = -3.6 to -6.2 K under a moderate field of 2 T. A phenomenological model is established that reveals the parameters essential for such a large ΔTad. We also demonstrate that obstacles to the application of Heusler alloys, namely the usually large hysteresis and limited operating temperature window, can be overcome by using the multi-response to different external stimuli and/or fine-tuning the lattice parameters, and by stacking a series of alloys with tailored magnetostructural transitions.
The effect of the thermal decomposition reaction on the mechanical and magnetocaloric properties of La(Fe,Si,Co)13
We report on the influence of the Co content in the magnetocaloric system La(Fe,Si,Co)13 on the thermal decomposition (TD) reaction. In the course of the TD reaction, the magnetocaloric La(Fe,Si,Co)13 phase reversibly decomposes in the temperature range of 973-1073K into α-Fe(Co,Si) and the intermetallic LaFeSi phase, thus enhancing the machinability of the compound. The addition of Co significantly speeds up the reaction kinetics. With electron microscopy a lamellar microstructure has been found in the decomposed state, indicating a eutectoid-type phase reaction. The width of the lamellae is ∼26 nm in LaFe12Si and decreases with increasing Co content. We conclude that the addition of Co somehow decreases the lamellar spacing, which is the main reason for the enhanced TD kinetics.
Effect of carbon on magnetocaloric effect of LaFe11.6Si1.4 compounds and on the thermal stability of its hydrides
For application as magnetic refrigerant in a domestic fridge it is vital to increase TC near to the room-temperature range while simultaneously maintaining a large magnetocaloric effect. In this work we studied the effect of interstitial carbon on the microstructure and magnetocaloric effect in LaFe11.6Si1.4Cx (x = 0–0.4). Carbon leads to an increase in TC and a decrease of the thermal hysteresis width. For x > 0.2, the magnetic transition changes from first-order to second-order, with a corresponding reduction in magnetocaloric effect. Up to x =0.2 the magnetocaloric properties of the parent alloy La(Fe,Si)13 are improved. Furthermore, the carbon addition increases the thermal stability of hydrided LaFe11.6Si1.4Cx significantly and shifts the desorption temperature from about 460K to 500K-540K (x=0.1 and 0.2).
Magnetostructural transition and adiabatic temperature change in MnCoGe magnetic refrigerants
Crystal structure, structural and magnetic transitions were investigated in a series of MnCo0.95Gex (x = 0.95, 0.97, 1.0) alloys. A first-order magnetic transition was observed in MnCo0.95Ge0.97 with a large thermal and magnetic hysteresis, while the other two composition alloys undergo a second-order ferromagnetic transition. The adiabatic temperature change for MnCo0.95Ge0.97 strongly relies on the sample history (heating or cooling protocols) due to the competition of structural and magnetic transitions. The structural contribution works against the magnetic contribution to the total magnetocaloric effect.