Functional Magnetic Materials and Hydrides

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Saxony Economic Development Corporation. For the issue 1/11 they interviewed team leader Dr. Oliver Gutfleisch considering research in hard magnets and magnetic refrigeration. saxonmail informs about the latest developments and trends in high technologies in Saxony, represents enterprises, research projects and innovative products.

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Comparison of local and global texture in HDDR processed Nd–Fe–B magnets

The local texture in polycrystalline Nd–Fe–B powder particles processed by hydrogenation disproportionation desorption and recombination (HDDR) has, for the first time, been observed directly by electron backscatter diffraction (EBSD). The  quality of local texture was found to vary strongly with the hydrogen pressure applied during HDDR processing. The local texture within the powder particles was shown to be critical in determining the magnetic properties of compacts produced by aligning the powder in an external magnetic field.

K. Guth, T.G. Woodcock, L. Schultz, O. Gutfleisch, Acta Mater. 59 (2011) 2029-2034, pdf

The role of local anisotropy profiles at grain boundaries on the coercivity of Nd₂Fe₁₄B magnets

We present numerical evidence from atomistic calculations that the coercivity of high-performance NdFeB-sintered-magnets (< 20% of the theoretical Stoner–Wolfarth-limit) can be explained by a distorted region of Nd₂Fe₁₄B at grain boundaries, which has a reduced local magnetic anisotropy. We show that depending on the boundary composition of fcc-NdO and hcp-Nd₂O₃, the thickness of this region of reduced anisotropy varies between 0.4 for fcc and 1.6 nm for the hcp phase.

G. Hrkac, T.G. Woodcock et al., Appl. Phys. Lett. 97 (2010) 232511, pdf

Multi-phase EBSD mapping and local texture analysis in NdFeB sintered magnets

We have extended the study of NdFeB sintered magnets by EBSD to include all of the phases present in the microstructure for the first time. A combination of chemical analysis by EDX and crystal structure determination by EBSD was used to identify the phases unambiguously. The orientation of the Nd-rich grains was shown to be random and the defect density of all phases was qualitatively determined from analysis of the intragranular misorientation angles.

T.G. Woodcock and O. Gutfleisch, Acta Mater. 59 (2011) 1026-1036, pdf

IEEE Magnetics Society Distinguished Lecturer 2011

Oliver Gutfleisch has been selected for…

The IEEE Magnetics Society has selected Oliver Gutfleisch to become a Magnetics Society Distinguished Lecturer for 2011 on the topic 'Magnetic Materials in Sustainable Energy'. The Distinguished Lecturer program is a distinctive contribution from the Society to its members and to others in the magnetics community. Distinguished Lecturers are a small number of outstanding individuals who are provided with travel expenses to deliver a lecture on a topic of special interest and importance to audiences throughout the world.

Review: Magnetic Materials and Devices for the 21st Century: Stronger, Lighter, and More Energy Efficient

A new energy paradigm, consisting of greater reliance on renewable energy sources and increased concern for energy efficiency, has accelerated research in energy-related technologies. Magnetic materials play an important role in improving the efficiency and performance of many devices. The impacts of hard magnets on electric motor and transportation technologies, of soft magnets on electricity generation and conversion technologies, and of magnetocaloric materials for refrigeration are discussed. 

Advanced Materials: : 15 DEC 2010 | DOI: 10.1002/adma.201002180, pdf

Reversibility and irreversibility of magnetocaloric effect in a metamagnetic shape memory alloy

We have studied adiabatic temperature change ΔT_ad in a Ni₅₀Mn₃₆Co₁Sn₁₃ metamagnetic shape memory alloy. An irreversible character of ΔT_ad has been observed in the vicinity of the reverse martensitic transformation. In this region, cyclic application of the magnetic field converts comparatively large inverse magnetocaloric effect (MCE) with ΔT_ad^max=−0.8 K to a weaker conventional MCE (ΔT_ad^max=0.3 K).

V. Khovaylo et al., Appl. Phys. Lett. 97 (2010) 052503, pdf

Novel Design of La(Fe,Si)₁₃ Alloys Towards High Magnetic

Refrigeration Performance

We have shown that in LaFe_11.6Si_1.4 the magnetic phase transition and, consequently, the MCE is greatly infl uenced by the internal constraints imposed by grain boundaries. Removal of these constraints through the introduction of porosity leads to a desirable reduction of hysteresis in the field- and thermally induced magnetic phase transitions and to the marked improvement of mechanical stability during cycling, which results in fully reversible MCE.

J. Lyubina et al., Adv. Mat. 22 (2010) 3735-3739, pdf

Al₃Li₄(BH₄)₁₃: A Complex Double-Cation Borohydride with a New Structure

The new double-cation Al–Li–borohydride is an attractive candidate material for hydrogen storage due to a very low hydrogen desorption temperature (~708°C) combined with a high hydrogen density (17.2 wt%). It was synthesised by high-energy ball milling of AlCl₃ and LiBH₄. The material shows a unique 3D framework structure within the borohydrides (space group=P-43n, a=11.3640(3) Å).

I. Lindemann et al., Chem. Eur. J. 16 (2010) 8707-8712, pdf

Nanoscale Magnetic Materials and Applications

A new book "Nanoscale Magnetic Materials and Applications", edited by Drs. J.Ping Liu, Eric Fullerton, Oliver Gutfleisch and David Sellmyer, has been published by Springer. 2007 Nobel Laureate in Physics, Dr. Peter Grünberg has given his foreword to the book. Details can be found here

Confinement of NaAlH₄ in Nanoporous Carbon: Impact on H₂ Release, Reversibility, and Thermodynamics

Metal hydrides are likely candidates for the solid state storage of hydrogen. NaAlH₄ is the only complex metal hydride identified so far that combines favorable thermodynamics with a reasonable hydrogen storage capacity (5.5 wt %) when decomposing in two steps to NaH, Al, and H₂. The slow kinetics and poor reversibility of the hydrogen desorption can be combatted by the addition of a Ti-based catalyst. In an alternative approach we studied the influence of a reduced NaAlH₄ particle size and the presence of a carbon support.

J. Gao et al., J. Phys. Chem. C 114 (2010) 4675-4682, pdf

Ni–Mn–In–Co single-crystalline particles for magnetic shape memory composites

Single- and oligo-crystalline magnetic shape memory _MSM_ Ni_45.2Mn_36.7In₁₃Co_5.1 particles were prepared by mechanical grinding the melt-spun ribbons. The influence of annealing and grinding on the crystal structure and magnetic properties of the ribbons and particles was systematically investigated. The obtained Ni_45.2Mn_36.7In₁₃Co_5.1 single-crystalline particles exhibit a field-induced martensitic transformation above room temperature, thus are used for MSM-polyester composites.

J. Liu et al., Appl. Phys. Lett. 95 (2009) 152503_1-3, pdf

Interaction domains in high-performance NdFeB thick films

The magnetic domain structure in sputtered NdFeB thick films has been imaged by magnetic force microscopy. The local texture of the films was investigated by electron backscatter diffraction. The average misorientation of the grains was shown to decrease with increasing substrate temperature during deposition. Interaction domains were observed and are discussed with reference (i) to the sample grain size compared to the single domain particle size and (ii) to sample texture.

T.G. Woodcock et al., Scripta Mat. 60 (2009) 826-829, pdf