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.
Symposium on material transition
The great transition
The importance of critical metals for green energy technologies
A one-day symposium will take place on Thursday, July 13, 2017, from 10 am to 5 pm, at the Orangerie in Darmstadt.
Symposium Materialwende am 13. Juli 2017
Die Energiewende ist eine Materialwende
Substitutionspotentiale für kritische Metalle in Funktionsmaterialien
Zu diesem Thema findet ein eintägiges Symposium am Donnerstag, den 13. Juli 2017, 10.00 – 17.00 Uhr, in der Orangerie in Darmstadt statt.
Most Recent Highlights
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 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 09.09.2018 to 13.09.2018 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
Announcement REPM 2016
The 24th International Workshop on Rare-Earth and Future Permanent Magnets and Their Applications (REPM 16) will be held from 28 August to 1 September 2016 in Darmstadt, Germany.
The workshop provides a platform for scientists and engineers from all over the world working in the field of permanent magnets to get together and discuss their research and ideas on topics including:
• High performance magnets
• Rare-earth free magnets
• Mining, raw materials, recycling
• Advanced synthesis
• Modelling and characterisation
• Coercivity mechanisms
Find additional informations in our flyer.
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.
REPM 2016 in Darmstadt
The 24th REPM (Rare-Earth Permanent Magnets and their Applications) workshop in 2016 will be held in Darmstadt! This decision was taken in Nagasaki at the this year´s conference. We are proud to host REPM 2016. Further information will follow soon…
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.