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

本项目的目的是获得稀土锰氧化物的高温热化学性质。我们可以通过测量等温电导率和热重分析得到这些量随氧分压的变化。当这些锰氧化物相在高温低氧分压下退火时，它们会分解成稀土倍半氧化物和氧化锰。热重分析和电导率测量的结果都与分解时的温度和氧分压吻合得很好。因此，可以得到稀土锰氧化物与分解氧化物之间的平衡的化学势图，其精度令人满意。利用这些结果，在不同的温度下(900-1200℃和1-10℃；-22&gt；atm)，由热力学第二定律处理，得到了LnMnO_3相的自由能、热焓和热变。同时，用DSC方法测量了几种锰氧化物的高温热容以及其中一些锰氧化物和另一种高结构的稀土锰氧化物(如BaN_2Mn_2O_7)的相变焓变化。在获得满意的测量精度后，利用热力学第三定律处理，可以更精确地获得这些稀土锰氧化物的热化学性质。

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.