Doctor of Philosophy
Institute for Superconducting & Electronic Materials
Debnath, Jyotish Chandra, Novel magnetocaloric materials and room temperature magnetic refrigeration, Doctor of Philosophy thesis, Institute for Superconducting & Electronic Materials, University of Wollongong, 2011. http://ro.uow.edu.au/theses/3449
Modern society relies on cooling technology for food safety, comfort and medical applications. For example, in the US about 34% of the electricity is consumed by cooling appliances and 15% of the total worldwide energy consumption involves the use of refrigeration (air conditioning, refrigeration, freezing, chilling, etc.). Nowadays, most cooling devices are based on vapour-compression technology, which uses strong greenhouse gases and the energy efficiency has reached its limit, the increased international concerns over global warming due to ever increasing energy consumption calls for a change. Currently, several solid-sate cooling technologies, such as optical refrigeration, thermoelectric refrigeration, electric refrigeration, and magnetic refrigeration, are being considered as viable alternative techniques because they are becoming increasingly efficient and affordable. However, for a technique to become a fully mature alternative, clever engineering and more detailed studies on the fundamental physical properties are needed. Roomtemperature magnetic cooling has attracted attention in recent years as a promising environmentally friendly alternative to conventional gas-compression cooling. Using solid magnetic materials as coolant, magnetic cooling does not involve any gases that cause ozone-depletion or global warming, which can help to meet the demands of the recently implemented Kyoto treaty. Furthermore, it has been demonstrated that magnetic cooling is energetically 20% more efficient than gas-compression cooling, which is of particular interest in view of the global energy shortage.
The working materials for room-temperature magnetic cooling are required to possess Curie temperatures around room temperature. Having large magnetization is a prerequisite for achieving a large magnetocaloric effect (MCE). For a long time, Gd was the best room temperature magnetic cooling material with a Curie temperature of 293 K. In comparison with rare-earth-based compounds that exhibit large MCE but are expensive, perovskite manganites exhibit smaller magnetic moments but are far less expensive. The magnetic properties of manganites, the Curie temperature, and the saturation magnetization are strongly doping-dependent. Perovskite manganites are more convenient to prepare and exhibit higher chemical stability, as well as the higher resistivity that is favorable for lowering eddy current heating. In addition, the manganites possess much smaller thermal and field hysteresis than any rare earth and 3d-transition metal based alloy. So, these materials may be good candidates for MR at various temperatures. Many investigations have therefore concentrated on perovskite manganites. The work presented in my thesis is a study of various perovskite manganites magnetocaloric materials for refrigeration applications. We also investigated the MCE of alloys.
The structural, magnetic, and magnetocaloric properties of the manganite La0.7Ca0.3MnO3 have been studied. Significant magnetic entropy change of 5.27 J/kg K was observed at 251 K under the magnetic field of 1.5 T with out noticeable magnetic hysteresis and tiny thermal hysteresis loss. This value is about twice as large as those for other perovskite manganites and is even larger than for Gd-based magnetic materials at low fields.
The influence of first and second order magnetic phase transitions on the magnetocaloric effect (MCE) and refrigerant capacity or relative cooling power (RCP) of La0.7Ca0.3MnO3 and La0.7Ca0.3Mn0.95Co0.05O3 materials has been investigated. The La0.7Ca0.3MnO3 material experiences a large entropy change with the first-order magnetic phase transition at the Curie temperature, TC. On the other hand, La0.7Ca0.3Mn0.95Co0.05O3 displays a smaller entropy change with a second order phase transition. While the first-order magnetic transition material induces a larger MCE (7.528 J/kg K at 5 T) at TC, which is limited to a narrow temperature range, resulting in a relatively small RCP (218 J/kg), while the Co-doped second-order magnetic transition material induces a smaller MCE (7.14 J/kg K for 5 T), but it is spread over a broader temperature range, resulting in a larger RCP (308 J/kg). The maximum magnetoresistance under a field of 5 T is about 206 % and 333% for La0.7Ca0.3MnO3 and La0.7Ca0.3Mn0.95Co0.05O3, respectively. The refrigeration capacity (RCP) is enhanced in La0.7Ca0.3Mn0.95Co0.05O3 (by about 41%) due to small changes from Co doping. The magnetocaloric features of these materials at lower magnetic fields (MCE = 3.163 J/kgK for La0.7Ca0.3Mn0.95Co0.05O3 and 4.63 J/kgK for La0.7Ca0.3MnO3 at 1 T), and the high RCP and MR can provide some ideas for exploring novel magnetic refrigerants that can be operated with permanent magnets rather than superconducting ones as the magnetic field source.
The magnetic properties and magnetocaloric effect of La0.7(Ca1−xAgx)0.3MnO3 (x = 0, 0.1, 0.2, 0.7, and 1) powder samples were studied. The Curie temperature, TC, has been found to increase from ∼250 K for x = 0 to ∼270 K for x = 1. Ag doping weakens the first order phase transition, and at higher Ag doping, the phase transition is of second order. For the La0.7(Ca0.27Ag0.03)MnO3 composition, the maximum magnetic entropy change (|ΔSM|) from change in the applied magnetic field of 0-2 T and 0-5 T is about 4.5 and 7.75 J/kgK, respectively, at the Curie temperature of ∼263 K. The relative cooling power (RCP) values without hysteresis loss are about 102 and 271 J/kg for applied field change of 0-2 T and 0-5 T, respectively. Due to the large ΔSM, large RCP, and high Curie temperature, La0.7(Ca0.27Ag0.03)MnO3 may be a promising candidate for application in potential magnetic refrigeration near room temperature.
Magnetic properties and the magnetocaloric effect have been investigated in La0.7Ca0.3MnO3 single crystal. Upon application of a 0-5 T field change, the magnitude of the magnetic entropy changes, reaching values of 7.668 J/kg K and 6.412 J/kgK for both the ab-plane and the c-direction, respectively. A magnetic entropy change of 3.3 J/kgK was achieved for a magnetic field change of 1.5 T at the Curie temperature, TC = 245 K. Due to the absence of grains in the single crystal, the ΔSM distribution here is much more uniform than for gadolinium (Gd) and other polycrystalline manganites, which is desirable for an Ericsson-cycle magnetic refrigerator. For a field change of 0-5 T, the relative cooling power, RCP, reached 358.17 J/kg, while a maximum adiabatic temperature change of 5.33 K and a magnetoresistance (MR) ratio of 507.88% at TC were observed. The spin fluctuation parameters were estimated by adapting Takahashi’s developed spin-fluctuation theory, and the reciprocal susceptibility was calculated. Evidence is presented that the magnetic property of La0.7Ca0.3MnO3 is weakly itinerant ferromagnetic. The large reversible MCE and lack of any hysteresis loss, combined with a considerable value of refrigerant capacity, indicate that La0.7Ca0.3MnO3 single crystal is a potential candidate as a magnetic refrigerant.
Epitaxial grown La0.8Ca0.2MnO3/LaAlO3 (LCMO/LAO) thin film exhibited a paramagnetic-to-ferromagnetic second order phase transition at 249 K. The lack of any hysteresis loss also confirmed that the material is intrinsically reversible. In addition, the large magnetization of the thin film results in a total entropy change larger than those of all other perovskite type materials and the same as that of Gd. Consequently, the relative cooling power is significantly enhanced. Improved film morphology would be the main reason for the remarkable values of entropy change and relative cooling power of the film. This indicates that thin film processing might provide an alternative pathway in searching for efficient magnetic refrigerators for microscale systems.
The effect of frozen spin on the magnetocaloric properties of La0.7Ca0.3CoO3 polycrystalline and single crystal samples has been studied. Interestingly, an anomalous magnetic field memory effect, an exchange-bias-like effect, and a large inverse irreversible magnetocaloric effect have been observed in this system. It is found that the frozen spins have a significant influence on the ΔSM. The ΔSM – T curves in the low temperature range show totally different features between the ZFC and FC cooling procedures. The ΔSM shows a very large inverse irreversibility value for the ZFC process, since there are a large amount of unfrozen spins aligned under the external field in the low temperature range, while the ΔSM shows a normal positive value and a slightly larger ΔSM value, which indicates that a small amount of unfrozen spins still exist. It is proposed that compositional inhomogeneity is the predominant source of the magnetic properties and the magnetocaloric effect.
Reduction of hysteresis loss in LaFe11.7Si1.3 Hx hydrides with significant magnetocaloric effect has been investigated. The influence of hydrogen absorption on the MCE and especially on the magnetic hysteresis in LaFe11.7Si1.3 Hx (x = 0, 1.37, and 2.07) compounds is discussed from the viewpoint of magnetic refrigerants, and a large MCE which is still higher than for Gd and a small hysteresis loss are achieved near room temperature are favourable for the practical application of these materials near room temperature.