Magnetic and Thermal Properties of Perovskite Manganites and the Applications

Perovskite manganites (AMnO3) is an important type of functional material, which exhibit a variety of structural, thermal, electrical and magnetic characteristics. The strong magnetocaloric effect (MCE) of AMnO3 near its Curie temperature (TC) makes it a perfect candidate for magnetic-thermal anti-icing and de-icing materials for high-voltage overhead power transmission lines. When the temperature is below TC, AMnO3 is ferromagnetic. Once composited with the energised transmission conductors, AMnO3 generates extra heat from the everchanging magnetic field induced by the strong AC current, due to MCE, magnetic hysteresis and eddy current. When the temperature is above TC, the material becomes paramagnetic and stop producing heat to avoid unnecessary energy loss at high temperature. In this thesis, we successfully synthesised high purity LaMnO3 doped with alkaline earth metal (Ca, Sr). By alternating doping levels, we are able to adjust the material’s Curie temperature to 0 oC, known as the ice point. We have also investigated the effects of particle size on its MCE performance. The results show that bulk or large-particle-size samples present large magnetic entropy change from a given magnetic field change. The optimised sample shows excellent MCE performance with respect to both magnetic entropy changes and adiabatic temperature change. Prototype experiments in climate simulation chamber show that during the cooling process, a sharp temperature rise can be observed from the AMnO3 composited conductor when its temperature approaches TC. Once energised with 360 A, 50 Hz AC current, the composited conductor temperature is up to 2.2 oC higher than the reference. Different from the MCE that generates heat for anti-icing and de-icing the overhead electrical power transmission lines, AMnO3 can also be used to store heat as a key component of renewable energy systems. More recently, AMnO3 is considered to be a promising energy storage medium for high-temperature thermochemical storage (TCS) systems in concentrated solar power (CSP) plants. Large amount of energy can be stored in AMnO3 ↔ AMnO3-δ reversible redox reactions, as well as the sensible heat of AMnO3-δ. The reaction occurs continually in the temperature ranging from approx. 500 to 1000 oC, which is the perfect operating temperature range for the next generation TCS systems. Results show CaMnO3 has the best performance among four AMnO3 candidates, whose chemical and sensible heat add up to be able to store 717.4 kJ/kg energy from 200 to 1000 oC in air, and 893.2 kJ/kg in ultra-high purity N2. The reaction is highly repeatable as the products have almost no change after 150 cycles from 500 to 1000 oC. System analysis showing that a CaMnO3 based open loop TCS system helps a CSP plant to reach 48.7% overall energy conversion efficiency and 15-hour energy storage capacity. The required storage size is 2~3 times smaller than operating commercial CSP plants.