Internal strain in the magnetocaloric alloy MnCoSi

Sammanfattning: Cooling systems based on magnetic refrigeration are a promising and environmentally friendly alternative to commonly used gas-compression refrigeration. Magnetic refrigeration is based on the magnetocaloric effect. This effect denotes a temperature change of a magnetic material due to a change in its intrinsic magnetic properties, induced by a change in an external applied magnetic field. MnCoSi is such a magnetocaloric material that has been under investigation for several years. It has potential for practical use in magnetic refrigeration owing to its structural and magnetic properties. These properties are influenced by the synthesis process, which consists of alloying pure elements by arc melting followed by a subsequent temperature treatment process. It has been found that the annealing and cooling conditions of the temperature treatment affect the material and can induce lattice strains in MnCoSi samples. Studying the correlation between the synthesis conditions and induced microstrain in a systematic manner is the topic of this project work. Although the synthesis process was optimised for the given working setup, it was found that the structural properties of MnCoSi are highly sensitive to the given characteristic setup itself. This intricates the synthesis of pure samples and hinders a detailed microstrain analysis from powder x-ray diffractograms. Nevertheless, a suitable model to describe microstrain in MnCoSi was identified to be Lorentzian isotropic strain. This holds for the various cooling conditions investigated in the present work. Furthermore, the expected changes in the induced microstrain for cooling rates in between a slow and a fast cooling process were observed. The induced microstrain seems to increase with increasing cooling rate, although the quantitative values do not coincide entirely with previous results and the effect was not observed for all analysed samples. Possible reasons for the deviations from increasing strain with increasing cooling rate lie in the setup and the used methods and are discussed. The results obtained here add to the understanding of the magnetocaloric alloy MnCoSi and emphasise the importance of a uniform synthesis process to allow the development of a tailored working material for magnetic refrigeration.