The Effect of Fe Substitutions on the Microstructure of Gd5Si2Ge2
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1032-I11-09
The Effect of Fe Substitutions on the Microstructure of Gd5Si2Ge2 Benjamin Podmiljsak1, Paul McGuiness1, Spomenka Kobe1, and Irena Skulj2 1 Josef Stefan Institute, Ljubljana, 1000, Slovenia 2 Institute of metals and technology, Ljubljana, 1000, Slovenia
Abstract Gd5Si2Ge2 exhibits the so-called giant magnetocaloric effect, making it a potential material for next-generation near-room-temperature permanent-magnet-based refrigeration devices. In the as-cast form this material is relatively complicated in terms of its structure and its metallurgy. Recent reports have suggested that small additions of iron can substantially improve the problems associated with hysteresis losses, so enhancing the material’s potential for real-world applications. We have looked at iron substitutions in the range X=0 to X=1 for Gd5Si2-XGe2 to investigate the macrostructures and microstructures of cast and homogenized materials, with particular emphasis on the amount of iron substituting for silicon in the Gd5(SiGe)4 phase and the effect of the substituting iron on the Ge/Si ratio. The larger iron substitutions were found to eliminate the unusual macrostructures, and conventional ingots with smooth surfaces were the result. The iron was also found to dissolve at levels of 1-3% in the Gd5(SiGe)4 phase and encourage the formation of the Gd5(SiGe)3 phase at the expense of the magnetocaloric Gd5(SiGe)4 phase. A substitution of X=1 was found to be sufficient to produce as-cast microstructures consisting almost entirely of the Gd5(SiGe)3 phase. With a scanning electron microscope (Jeol 840A) and an analytical transmission electron microscope (Jeol 2010 F with a field-emission gun) and an EDXS system from Oxford Instruments (Link Isis 300) we performed a detailed microstructural investigation of the processed materials. Special emphasis was placed on the intergrain regions and an assessment of the amount of Fe substituting in the main magnetocaloric phase.
Introduction Magnetocaloric materials are known for their cooling capabilities at low temperatures. And in this area they have been used for over 80 years. These materials show a magnetocaloric effect (MCE) which is a change in temperature of the material as a result of the alignment of its magnetic spins when it is exposed to an external magnetic field. Because the MCE was extraordinary only at low temperatures it was not interesting for any room-temperature applications. Until 1997, when Gschneidner et al1 discovered a quite high magnetocaloric effect in the Gd5Si2Ge2 alloy, which they called the giant magnetocaloric effect (GMCE). Gd5Si2Ge2 is a member of the family of line-compound materials that stretches from Gd5Si4 to Gd5Ge4. Morellon et al2 and Choe et al3 showed that the structural and
magnetic properties in this system are strongly interrelated. X-ray diffraction studies revealed that Gd5Si2Ge2 was structurally different to the end members of the series, Gd5Si4 and Gd5Ge4. Using single-crystal techniques the Gd5Si2Ge2 material was found to be monoclinic at room temperat
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