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Abstract

Magnetocaloric materials are promising materials for magnetic refrigerators which would substitute present compressor refrigerators. They work on the principle of the magnetocaloric effect (MCE), where the magnetocaloric material increases its temperature when exposed to an external magnetic field. These new type of refrigerators are energetically more efficient and environmentally friendly.
One of the promising materials, discovered in 1997, is the Gd5Si2Ge2 alloy with a giant magnetocaloric effect (-ΔSm), because it has a simultaneous structural and magnetic transition. This giant MCE has a disadvantage that it has very high hysteresis losses when cycled through a changing magnetic field. To reduce this, a small amount of germanium was substituted in small amounts with iron. With this an increase in the net refrigeration capacity (NRC) was achieved. The lower hysteresis losses were explained with the suppression of the structural transformation.
The goal in this thesis was to investigate different preparation techniques of Gd5Si2Ge2, such as arc melting, mechanical alloying and melt spinning to improve the properties of these alloys. Also we wanted to investigate more thoroughly the effect of iron on Gd5Si2Ge2. For this we have investigated the magnetic properties as well as the macro-, micro- and nanostructural changes for a wide range of iron substitutions. The affect of iron was researched on three sets of samples. In the first one we substituted germanium with iron by the following formula Gd5Si2Ge2-xFex, where x=0, 0.06, 0.125, 0.25, 0.5, 0.75 and 1. The second we substituted silicon with iron by the formula Gd5Si2-xGe2Fex, where x=0, 0.06, 0.125, 0.25, 0.5, 0.75 and 1. In the third set of samples we substituted silicon and germanium equally with iron by the formula Gd5Si2-x/2Ge2-x/2Fex, where x=0, 0.06, 0.125, 0.25, 0.5, 0.75. The samples were prepared with an arc-melter and homogenized in an inert atmosphere. They were analyzed with an optical microscope, field-emission-gun scanning electron microscope (FEG SEM) and a transmission electron microscope (TEM). The structural changes were analyzed with an X-ray diffractometer at room temperature and at cryogenic temperatures. The magnetic measurements were made with a cryogenic vibrating-sample magnetometer (VSM) and a physical properties measurement system (PPMS), with a vibrating-sample magnetometer attachment. Adiabatic temperature changes where measured with a modified magnetocaloric measurement setup.
The preparation of Gd5Si2Ge2 presented many problems when trying to prepare them with mechanical alloying or melt spinning. Only arc melting produced the desired phase effectively.
The substitutions in all three sets of samples presented us with some unusual macrostructures and a consistent change in the microstructures of the samples. Iron reduces the silicon and germanium concentration in the main matrix phase, by forming a grain boundary phase. Because of the reduction of Si and Ge, the main matrix phase is converted to the Gd5(Si,Ge)3 phase.
Low temperature XRD confirmed the structural transformation in the SI and GESI sample. In the GE sample only a partial transformation is observed suggesting that iron can suppress the structural transformation only in the GE samples.
We also observed that the addition of iron has a pronoun effect on the Curie temperature of the Gd5Si2Ge2 phase. When substituting Si the TC is reduced and when substituting Ge TC increases. If we substitute both the transition temperature does almost not change. In all substitutions -ΔSm is reduced. The difference is that it is reduced faster with GE substitutions then with GESI and SI substitutions. Also the hysteresis losses are reduced faster in the GE samples than in the other two. This has to do with the change in the Si/Ge ratio in the main phase and the reduction of the main phase on account of the new 5:3 phase. We also observe a broadening of the lowered -ΔSm peaks. This results in a increase of the net refrigeration capacity in the GESI samples compared to the unsubstituted Gd5Si2Ge2 sample. Adiabatic temperature change measurements confirmed the reduction of the MCE in these alloys with added iron. The reduction is comparable to the -ΔSm results with a broadening of the peak.
TEM investigations confirmed the formation of twins in the Gd5Si2Ge2 alloy. The results also suggest that additions of Fe produce complex changes to the structure of the Gd5Ge2Si2 material, like amorphous regions, dislocations and planar faults, as well as crystallographically related grains.
We have achieved an increase of the refrigeration capacity by carefully substituting some of the elements with iron. Also with iron we can tailor the transition temperature of the magnetocaloric material. With these findings this material is a promising candidate as a working material in a magnetic refrigerator.