Stinus Jeppesen

Title: Magnetocaloric materials

A thesis submitted for the degree of Doctor of Philosophy (PhD) on October 2008.

Niels Bohr Institute, University of Copenhagen
& Fuel Cells and Solid State Chemistry Division
Risø DTU

Supervisors:
Robert Feidenhans'l
Søren Linderoth

Abstract

Magnetocaloric materials

New and improved magnetocaloric materials are one of the cornerstones in the development of room temperature magnetic refrigeration. Magnetic refrigeration has been used since the 1930's in cryogenic applications, but has since the discovery of room temperature refrigerants received enormous attention. This Ph.D. work has mainly been concerned with developing a new technique to characterize the magnetocaloric e®ect and using this technique in the investigations on new and improved magnetocaloric materials. For this purpose a novel di®erential scanning calorimeter (DSC) with applied magnetic fields was developed for measuring heat capacity as function of magnetic field. Measurements using the developed DSC demonstrate a very high sensitivity, fast measurements and good agreement with results obtained by other techniques.

Furthermore, two material systems have been described in this work. Both systems take basis in the mixed-valence manganite system La1¡xCaxMnO3 wellknown from research on colossal magnetoresistance (CMR). The mixed-valence manganite crystallize in the perovskite structure of general formula ABO3. The first material system is designed to investigate the in°uence of low level Cu doping on the B-site. Six di®erent samples were prepared with compositions La0:67Ca0:33Mn1:05CuxO3, x=0, 1, 2, 3, 4 and 5%. All compositions crystallized well in the same perovskite structure, but the morphology of the samples changed drastically with doping. Investigation on the magnetocaloric properties revealed that small levels of Cu up to around 3% could improve the magneto caloric performance of the materials. Furthermore, Cu could be used to tune the working temperature interval of the refrigerant without deteriorating the e®ect, which is a very desired characteristic for potential use in refrigerators. A less comprehensive part of the work investigated substitution of the A-site ions. The possibility of substituting the lanthanum content of the material with a lanthanide mix consisting of La, Ce, Nd and Pr was investigated due to the potential of making more coste®ective materials. Four samples with compositions (La1¡x(La,Ce,Pr,Nd)x)0:67Ca0:33Mn1:05O3, x=0, 0.33, 0.67 and 1 were synthesized to investigate the magnetocaloric properties of these compositions. It was found that the perovskite structure could be maintained even at the highest level of doping (x=1), and that the maximum magnetic entropy change, ¢SM, quantifying the magnetocaloric e®ect was actually enhanced to an optimum at x=0.67. Furthermore, the relative cooling power (RCP) of the materials were investigated, and it was demonstrated that RCP increases continuously with doping and reaches the highest value in the composition, where the entire lanthanum content has been replaced by the lanthanide mix. These observations make promise of compositions, which could be competitive both in terms of cost-e®ectiveness and MCE. The work on the latter materials have been disclosed in a US and UK patent application.