Preparation Of Cerate Ln₂Ce₂O₇（Ln=Gd,La） Nano-powders By Microemulsion Method And Their Thermophysical Properties

There exist lots of rich rare earth resources in our country.How to efficiently utilize rare earth resources and develop new rare earth materials has become the research direction and goal of today’s researchers.At present,with the increasing use of rare earth cerate materials,the pursuit of their excellent properties also puts forward to higher requirements for the preparation of nano-powders.Studies have shown that compared with traditional materials,nanomaterials show more better properties.As to the case,a novel microemulsion system was developed in this thesis,namely an inverse microemulsion system composed of oleic acid and 2-amino-2-methyl-1-propanol（AMP）/n-octanol/kerosene/water solution.By control of above reaction medium,Ln₂Ce₂O₇（Ln=Gd,La）nanopowders were successfully synthesized.X-ray diffraction（XRD）,Raman spectroscopy（Raman）,thermogravimetric-differential scanning calorimetry（TG/DSC）,transmission electron microscopy（TEM）,scanning electron microscopy（SEM）and the laser flash method were combined to comprehensively characterize and analyze the phase structure,microstructure and thermal conductivity of the products.In this thesis,the inverse microemulsion system formed by oleic acid and 2-amino-2-methyl-1-propanol（AMP）/n-octanol/kerosene/water solution is used for the first time,and the microemulsion methodconsisted of the double microemulsion.The particle size and morphology of the Gd₂Ce₂O₇nano-powder could be well-controlled by optimizing the reaction conditions.Experimental results show that the concentration of reactants,the amount of n-octanol and the calcination temperature have a significant effect on the particle size of the product,while the stirring time has little effect on it.The initial concentration of reactants is adjusted to be 0.1 mol/L,the amount of n-octanol added is 2.6ml,the stirring time is 12 h,and the calcination temperature is 900℃to prepare fine and uniform nano-powders.The microemulsion system was used to prepare(Gd_1-xLa_x)₂Ce₂O₇（x=0,0.24,0.45）ceramic materials,and three control groups were set up for comparative analysis to explore the effect of doping components on their thermophysical properties.And its density and thermal conductivity were also tested and analyzed.It was found that the densities of the three groups of ceramic sheets were all above 97%,with higher densities.Moreover,after doping with La₂O₃,the thermal conductivity of both components decreases.When x=0.45,the decrease is not obvious;when x=0.24,it decreases significantly,showing extremely low thermal conductivity,which is a component with the best thermal insulation performance,which can be explained by phonon scattering theory.Using the same microemulsion system,La₂Ce₂O₇nanoparticles were prepared,and the effects of different calcination temperatures on the phase stability,particle morphology and size of the powder products were systematically studied.The crystal growth and anti-sintering properties were further explored.The following conclusions are drawn through the research:the average grain size of the prepared La₂Ce₂O₇nano-powders is 10-15nm.The apparent activation energy of grain growth is only 35.53 k J/mol,and the low activation energy can be attributed to the existence of a large number of oxygen vacancies.The powder is pressed into green compacts and calcined at 1600℃for 6 hours to obtain ceramic materials with dense microstructure,clear grain boundaries and no component segregation occurred.However,as the temperature increases,the grain size grows significantly.