The Measurements of High Temperature Thermochemical Properties of Rare Earth Manganites with Electrical Conductivity

本课题的目的是获得稀土锰矿石的高温热化学性质。我们可以从等温电导率的测量和热重分析中得到这些量作为氧分压的函数。当这些锰矿相在低氧分压下高温退火时，它们分解成稀土稀土氧化物和一氧化锰。热重分析和电导率测量的结果与分解时的温度和氧分压一致。由此得到了稀土锰酸盐与分解氧化物平衡的化学势图，得到了满意的精度。在900 ~ 1200℃的温度和1 ~ 10^<-22>大气压的氧分压下，用热力学第二定律处理可以得到LnMnO_3相的自由能、焓和熵的变化。用DSC法测定了几种锰酸盐的高温热容和部分锰酸盐与另一类结构较高的稀土锰酸盐（如BaLn_2Mn_2O_7）的相变焓变。在以满意的精度完成这些测量后，利用热力学第三定律处理，可以更精确地得到这些稀土锰矿石的热化学性质。

The aim of this project is to obtain the high temperature thermochemical properties of rare earth manganites. We could obtain these quantities from the measurements of isothermal electrical conductivity and thermogravimetric analysis as a function of oxygen partial pressure. When these manganite phases is annealed under low oxygen partial pressure at high temperature, they decompose into rare earth sesqui oxides and manganese monoxide. Both results from thermogravimetric analysis and electrical conductivity measurements were in good agreement for the temperature and oxygen partial pressure at the decomposition. Therefore the chemical potential diagram for the equilibria between rare earth manganites and the decomposed oxides could be obtained in satisfactory precision. By using these results at several temperatures (900 - 1200 C of temperature and 1 - 10^<-22> atm of oxygen partial pressures), the free energies, enthalpy and entropy changes for LnMnO_3 phases could be obtained from the second law treatment of thermodynamics.Also heat capacities of several manganites at high temperatures and enthalpy changes of phase transitions of some of these manganites and another type of rare earth manganites with higher structure, e.g. BaLn_2Mn_2O_7, were measured by DSC method. After these measurements are accomplished with a satisfactory precision, the thermochemical properties of these rare earth manganites are to be obtained with more precise from by using the third law treatment of thermodynamics.