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Dick C.P.,Cologne University of Applied Sciences | Polak C.,Vacuumschmelze GmbH and Co. KG | Waffenschmidt E.,Cologne University of Applied Sciences
IEEE Journal of Emerging and Selected Topics in Power Electronics | Year: 2015

In inductive wireless power transmission systems, often soft-magnetic shielding is used to avoid lossy eddy currents being induced in electrically conducting components like batteries or ground layers of electronic circuits. Datasheet information on such shielding materials are often limited to magnetic permeability and sometimes exemplary loss information. For designing inductive wireless power systems, e.g., at variable frequency, detailed loss information are of interest. Therefore, it is proposed to measure the impact of these materials on the power transmission in a standardized setup, which is closely related to the real application. This consists of two coils, a transmitting and a receiving coil. Here, a configuration as described in the Qi standard for wireless charging of mobile devices published by the wireless power consortium is used as reference. A figure of merit, i.e., the product of the coupling factor k and the geometric average of the coil's quality factors Q, is proposed to qualify materials concerning both: 1) shielding against conducting components on the backside of the receiving coil and 2) establishing a high mutual inductance of the transformer coils, resulting in higher system efficiencies. Furthermore, considerations on the measurement setup as well as qualifications of shielding materials are presented. © 2014 IEEE.

Loewe K.,TU Darmstadt | Brombacher C.,Vacuumschmelze GmbH and Co. KG | Katter M.,Vacuumschmelze GmbH and Co. KG | Gutfleisch O.,TU Darmstadt | Gutfleisch O.,Fraunhofer Institute for Materials Recycling and Resource Strategies
Acta Materialia | Year: 2014

Nd-Fe-B permanent magnets have been coated with 0.6 wt.% 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 Dy processed magnets with initial coercivity of ∼1100 kA m-1 yield coercivity increases which can exceed 400 kA m-1 without a significant reduction of the remanent magnetic polarization. The improved stability against opposing magnetic fields can be observed up to a depth of ∼3 mm along the diffusion direction, restricting the application of the Dy diffusion process to either thin magnets or magnets with tailored coercivity gradients. While in the proximity of the Dy-coated surface, each grain has a Dy-enriched shell with a Dy content of ∼6 at.%; the Dy concentration decreases exponentially to ∼1.8 at.% after a diffusion depth of 400 μm and to ∼1 at.% after a diffusion depth of 1500 μm, as was found with wavelength dispersive X-ray spectroscopy and scanning transmission electron microscopy-energy dispersive X-ray spectroscopy, respectively. In the vicinity of the Dy-coated surface, the mechanism of the Dy-shell formation is attributed to the melting/solidification of a heavy-rare-earth-rich intermediate phase during high-temperature annealing. This is based on the observation that a constant Dy concentration over the width of the shells was found. Also an epitaxial relation between the Dy-poor core and the Dy-rich shell was observed by electron backscattered diffraction, which is supported by results obtained with Kerr microscopy. © 2014 Acta Materialia Inc.

Liu J.,Leibniz Institute for Solid State and Materials Research | Moore J.D.,Leibniz Institute for Solid State and Materials Research | Skokov K.P.,Leibniz Institute for Solid State and Materials Research | Krautz M.,Leibniz Institute for Solid State and Materials Research | And 5 more authors.
Scripta Materialia | Year: 2012

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. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Pradeep K.G.,Max Planck Institute Für Eisenforschung | Herzer G.,Vacuumschmelze GmbH and Co. KG | Choi P.,Max Planck Institute Für Eisenforschung | Raabe D.,Max Planck Institute Für Eisenforschung
Acta Materialia | Year: 2014

Rapid annealing (4-10 s) induced primary crystallization of soft magnetic Fe-Si nanocrystals in a Fe73.5Si15.5Cu1Nb 3B7 amorphous alloy has been systematically studied by atom probe tomography in comparison with conventional annealing (30-60 min). It was found that the nanostructure obtained after rapid annealing is basically the same, irrespective of the different time scales of annealing. This underlines the crucial role of Cu during structure formation. Accordingly, the clustering of Cu atoms starts at least 50 C below the onset temperature of primary crystallization. As a consequence, coarsening of Cu atomic clusters also starts prior to crystallization, resulting in a reduction of available nucleation sites during Fe-Si nanocrystallization. Furthermore, the experimental results explicitly show that these Cu clusters initially induce a local enrichment of Fe and Si in the amorphous matrix. These local chemical heterogeneities are proposed to be the actual nuclei for subsequent nanocrystallization. Nevertheless, rapid annealing in comparison with conventional annealing results in the formation of ∼30% smaller Fe-Si nanocrystals, but of identical structure, volume fraction and chemical composition, indicating the limited influence of thermal treatment on nanocrystallization, owing to the effect of Cu. © 2014 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Ohnuma M.,Japan National Institute of Materials Science | Herzer G.,Vacuumschmelze GmbH and Co. KG | Kozikowski P.,Japan National Institute of Materials Science | Kozikowski P.,Warsaw University of Technology | And 3 more authors.
Acta Materialia | Year: 2012

Amorphous ribbons of different composition were annealed under tensile stress. This yielded a creep-induced magnetic anisotropy with an easy magnetic plane perpendicular to, or an easy axis parallel to, the ribbon direction, depending on the alloy composition. X-ray diffraction experiments and simple thermal expansion measurements show that the stress-annealed samples reveal a structural anisotropy which is released by post-annealing as a residual strain. This strain increases with the annealing stress and is therefore correlated with the induced magnetic anisotropy. The origin of this frozen-in strain is discussed in terms of structural heterogeneity in the strength of local atomic bonds. It is suggested that the induced magnetic anisotropy is related to the local magneto-elastic coupling in regions with strong bonding forces. © 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Bjork R.,Technical University of Denmark | Bahl C.R.H.,Technical University of Denmark | Katter M.,Vacuumschmelze GmbH and Co. KG
Journal of Magnetism and Magnetic Materials | Year: 2010

The magnetocaloric properties of three samples of LaFe 13-x-yCoxSiy have been measured and compared to measurements of commercial grade Gd. The samples have (x=0.86, y=1.08), (x=0.94, y=1.01) and (x=0.97, y=1.07) yielding Curie temperatures in the range 276288 K. The magnetization, specific heat capacity and adiabatic temperature change have been measured over a broad temperature interval. Importantly, all measurements were corrected for demagnetization, allowing the data to be directly compared. In an internal field of 1 T the maximum specific entropy changes were 6.2, 5.1 and 5.0 J/kg K, the specific heat capacities were 910, 840 and 835 J/kg K and the adiabatic temperature changes were 2.3, 2.1 and 2.1 K for the three LaFeCoSi samples respectively. For Gd in an internal field of 1 T the maximum specific entropy change was 3.1 J/kg K, the specific heat capacity was 340 J/kg K and the adiabatic temperature change was 3.3 K. The adiabatic temperature change was also calculated from the measured values of the specific heat capacity and specific magnetization and compared to the directly measured values. In general an excellent agreement was seen. © 2010 Elsevier B.V. All rights reserved.

Herzer G.,Vacuumschmelze GmbH and Co. KG | Budinsky V.,Vacuumschmelze GmbH and Co. KG | Polak C.,Vacuumschmelze GmbH and Co. KG
Journal of Physics: Conference Series | Year: 2011

Ribbons of originally amorphous Fe73.5Cu1Nb 3Si15.5B7 have been annealed under tensile stress for about 4 seconds at temperatures between 450°C and 750°C. Under such short time conditions the optimum nanocrystalline state is achieved for annealing temperatures between 600°C and 720°C. This is about 100°C higher than in more conventional heat treatments with annealing times in the order of an hour. The stress annealed ribbons reveal an almost perfectly linear hysteresis loop and a transverse domain pattern with zigzag domain walls characteristic for a magnetic easy plane perpendicular to the stress axis. The induced magnetic anisotropy increases proportional to the annealing stress and is comparable to that obtained after more prolonged annealing. We succeeded to induce anisotropy constants as high as Ku 12 kJ/m3 which even exceed the local magneto-crystalline anisotropy (K1 8 kJ/m 3) of the crystalline Fe-Si phase. Nonetheless, the coercivity is still as small as Hc 8 A/m. This is to be compared to Hc 0.5 A/m for field annealed samples where Ku ( 20 J/m3) is smaller by more than two orders of magnitude. The behaviour of Hc can be understood within the framework of the random anisotropy model. © Published under licence by IOP Publishing Ltd.

Herzer G.,Vacuumschmelze GmbH and Co KG
Acta Materialia | Year: 2013

This article surveys amorphous and nanocrystalline alloys for soft magnetic applications. Both materials have much in common, starting from the technique of production and including the key factors that determine their properties. Thus the magneto-crystalline anisotropy randomly fluctuates on a scale much smaller than the domain wall width and, as a consequence, is averaged out by exchange interactions so that there is no net anisotropy effect on the magnetization process, the prerequisite for good soft magnetic behaviour. Superior soft magnetic properties additionally require low magnetostriction, which is true of amorphous Co-based alloys and, more recently, nanocrystalline Fe-based alloys, but at a significantly higher saturation induction and with better thermal stability. Both materials reveal low losses of up to several hundred kilohertz and their B-H loop can be tailored by magnetic field annealing according to the demands of the application. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

Herzer G.,Vacuumschmelze GmbH and Co KG | Budinsky V.,Vacuumschmelze GmbH and Co KG | Polak C.,Vacuumschmelze GmbH and Co KG
Physica Status Solidi (B) Basic Research | Year: 2011

We have investigated the creep induced anisotropy, magnetostriction, domain structure and coercivity of nanocrystalline Fe 73.5Cu 1Nb 3Si 15.5B 7 crystallized by flash annealing under high tensile stress up to 800MPa. The samples reveal a magnetic easy plane perpendicular to the stress axis with anisotropy constants up to K u≈12kJ/m 3 which even exceed the local magneto-crystalline anisotropy (K 1≈8kJ/m 3) of the crystalline Fe-Si phase. Although coercivity increases with K u, it remains reasonably small even for huge induced anisotropy constants. The coercivity mechanism can be understood from the interplay of the induced anisotropy and the random fluctuations of the local magneto-crystalline anisotropy of the crystallites. The tensile stress applied during annealing also affects the saturation magnetostriction constant λ s. Thus, λ s decreases with increasing magnitude of the annealing stress. This behaviour is compared to the elastic stress dependence of magnetostriction. The latter is well-known for amorphous Co-base alloys, but can also be observed in nanocrystalline alloys. The experimental results will be discussed theoretically in terms of the strain dependence of the magnetic anisotropy energy which ultimately provides the physical origin of magnetostrictive phenomena. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

Suzuki K.,Monash University | Herzer G.,Vacuumschmelze GmbH and Co. KG
Scripta Materialia | Year: 2012

Realization of a high-saturation magnetization comparable to that of Fe-Si steel in advanced Fe-rich nanocrystalline soft magnetic alloys is potentially a very effective approach to reducing the emission of greenhouse gasses. This potential has stimulated recent research on the development of new alloys with exceptionally high Fe concentrations. However, some nanocrystalline soft magnetic alloys at the Fe-richest compositions exhibit unexpectedly large values of field-induced magnetic anisotropy (K u ∼ 100 J m -3) which have a detrimental effect on the exchange-softening process in the nanostructures. Our viewpoint is that much attention must be paid to the induced anisotropies in order to utilize the full potential of the exchange-softening effect in Fe-rich nanocrystalline alloys. Possible origins of the large K u value and the approach to suppressing the field-induced effect on K u are discussed. © 2012 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.

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