Setup Implementation for a Direct Measurement Technique of the Magnetocaloric Effect

University essay from KTH/Materialvetenskap

Abstract: This project presents an attempt to construct a setup and implement a reliable technique for measuring the magnetocaloric effect (MCE) on various materials via a direct method for the acquisition of the data. The main objective of the latter is to produce a ∆Tadiabatic vs T graph over a reasonable temperature span (-100◦C up to 220◦C) by thermal monitoring of a magnetic material exposed to an oscillating magnetic field with a maximum strength of 1.2T. The setup consists of a vacuum-insulated glass tube containing the sample placed between two electromagnets of a vibrating-sample magnetometer (VSM) and increasingly heated by a resistance wire, while the temperature is recorded directly by a thermocouple. The first experiments are performed on Gadolinium (Gd) samples as a reference material in order to verify the overall reliability of the system. The obtained results on Gadolinium show that meaningful data can be acquired with this direct method, although the initially-extracted ∆Tadiabatic near room temperature stands at the accuracy limit (25%) generally accepted with this method. Unexpected interference signals from the thermocouple are encountered for high temperatures and are shown to be due to magnetic dependence from one of its constituents. Data from high temperatures can however be reliably corrected with respect to a baseline signal from a neutral nonmagnetic material. As such magnetocaloric properties of two Manganese-rich high entropy alloys are investigated with one exhibiting at most ∆Tadiabatic = 0.2◦C at its Curie temperature TC = 60◦C. Suggestions regarding the possibility of operating the setup at sub-zero temperatures are put forward and promising results from a new spot- welded thermocouple show a significantenhancement of the initial setup accuracy.

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