Microstructure And Properties Of Lanthanum Chromites Based Composites

Lanthanum chromites materials have been widely applied in the fields of electric heating materials and solid oxide fuel cells(SOFCs) due to their unique crystal structure and excellent physical and chemical performances.Although the properties (such as density of sintered body and electrical conductivity) of lanthanum chromites materials can be systematically adjusted by Ca or Sr doping at A site to basically satisfy the criteria of interconnector of SOFCs,the problems still remain to be solved, such as CrO₃ volatilization and cation diffusion,etc..The service life of homemade lanthanum chromites heating elements at a constant temperature has international level; however,the cycled usage times of the homemade products are extremely lower than that of the foreign products,fundamentally resulting from the poor thermal shock resistance.In order to solve the problems,such as inconsistent shrinkage ratios of different parts of a lanthanum chromites constant-diameter heating element,poor thermal shock resistance and the serious CrO₃ volatilization in traditional process,the sintering shrinkage ratios were modified by Li doping at B site,the microstructure and thermal shock resistance was improved by aggregate addition,the lanthanum chromites crystal structure was stabilized and the CrO₃ volatilization was inhibited by alumina addition and the mechanical properties of lanthanum chromites were further improved by zirconia addition in this study.The corresponding microstructure and property were analyzed by X-ray diffractometer,scanning electron microscope,universal testing machine, four-point probe arrangement and so on.The whole contents are summarized as follows:(1) Li doped lanthanum chromites superfine powders were prepared by Pechini method.Based on the systematical analysis of the process,the key factors,i.e.,content of citric acid and ethylene glycol,drying and calcining temperature and heating rate, were detected and the optimum processing parameters were determined.Through the process,uniform and rarely aggregative superfine powders of lanthanum chromites were obtained,whose mean particle size was about 40 nm.The influence of Li content and sintering temperature on sintering behavior and microstructure was investigated during sintering process.The results show that the shrinkage ratio of LaCr_1-yLi_yO₃ sintered body rises with the increasing of the Li content y under the condition of y＜0.2, at a certain sintering temperature;and approaches the maximum value near y=0.2;and then decreases with y＞0.2.For the samples of LaCr_1-yLi_yO₃ with the same Li content, the higher the sintering temperature is,the denser the sintered body becomes.The sectional sintered body shows a mixed mode of transgranular and intergranular fracture, which indicates that the strength of grain boundary is close to that of the bulk.The grains grow in stage mode,and merge with each other during sintering process.Li doped lanthanum chromites sintered bodies collapsed after a period of time in air due to the precipitation of La₂O₃.(2) The effect of different aggregate reinforcing schemes on performances of the sintered bodies was investigated for lanthanum chromites micron powders with particular aggregate content and the process for preparing La_0.9Ca_0.1CrO₃ and La_0.974Ca_0.026CrO₃ materials with optimum performance was determined by optimizing the main processing parameters.The results show that the aggregate particles retard the grain growth of the matrix grains and cause higher degree of porosity in the sintered bodies;the larger the particle size is and the higher the content of aggregate is,the more remarkable the effect is.The optimum addition strategy to obtain best thermal shock resistance performance for La_0.9Ca_0.1CrO₃ and La_0.974Ca_0.026CrO₃ materials is adding 5%minus-260-mesh aggregate and 10%minus-40-mesh aggregate, respectively.The improvement of thermal shock resistance lies in the higher degree of porosity and bridge effect of aggregate.With the increasing amount of aggregate content,the relative density of the sample is decreased,the flexural strength at room temperature decreases and the electrical resistivity increases.The grain growth of lanthanum chromites heating elements added with aggregate is restricted during the operation at high temperature,which will be useful to prevent performance degradation after a long time.(3) Investigations were made on the microstructure,sintering characteristics and electrical properties of Al₂O₃/La_0.9Ca_0.1CrO₃ composites.The results show that the system of Al₂O₃/La_0.9Ca_0.1CrO₃ results in thermodynamic unstablity with visible cation diffusion at high temperature,and the final phase composition of sintered bodies depend on the alumina content.When Al₂O₃ content≤20 mol%,the sintered body comprises lanthanum chromite solid solution and Al_2(1-x)Cr_2xO₃ and when Al₂O₃ content≥20 mol%,lanthanum chromite solid solution transfers to rhombohedral structure.When Al₂O₃ content reaches 30 mol%,a new phase of lanthanum hexaaluminate solid solution is observed besides the two phases mentioned above. When Al₂O₃ content is between 50mol%and 80mol%,the sintered body comprises perovskite phase with rhombohedral structure and lanthanum hexaaluminate solid solution.When Al₂O₃ content reaches between 90mol%and 95mol%,the samples comprise corundum phase and lanthanum hexaaluminate solid solution,while perovskite phases are not detected for their low concentration.Cation diffusion between the two components was studied by Energy Dispersive X-ray analysis.It can be considered that when one Al³⁺ transfers to lanthanum chromite,one Cr³⁺ transfers to alumina.Alumina addition improves the sintering activity of La_0.9Ca_0.1CrO₃ powders, restricts the chromium volatilization and thus improves the sintering densification process of the composite when Al₂O₃ content≤20mol%.When Al₂O₃ content is between 30mol%and 70mol%,plate-like lanthanum hexaaluminate is formed by solidstate reaction between the two components,which causes the higher degree of porosity to the sintered bodies.The relative density reaches its minimum value when Al₂O₃ content is 70mol%and gradually increases with the increasing ofα-Al₂O₃ content.The conductive behavior of composites sintered at 1450℃is in conformity with the general effective media(GEM) equation in the whole composition range.However,for the samples sintered at 1650℃,the conductivity mutates at the Al₂O₃ content of 20 mol%because of interdiffusion of the two components.As a result,the conductivity of the composites is partially in conformity with the GEM equation.Porosity is the principal factor affecting the flexural strength of Al₂O₃/La_0.9Ca_0.1CrO₃ composite;the relationship between flexural strength and porosity is in conformity with the Ryskewitsch empirical equation.(4) The influence of zirconia on microstructure and properties of ZrO₂/La_0.9Ca_0.1CrO₃ composites was investigated.The results reveal that the ZrO₂ and La_0.9Ca_0.1CrO₃ can co-exist stably at high temperature and no cation diffusion was detected.The zirconia particles restrict the growth and merging of the lanthanum chromite grains,which results in fine crystalline structure and low density of sintered bodies.The electrical resistivity of the composite increases as the zirconia content increases and the conductive behavior conforms well to the GEM equation,which will benefit its application in electrical heating materials.The flexural strength of ZrO₂/La_0.9Ca_0.1CrO₃ composite decreases with the increasing of zirconia content; however,the thermal shock resistance improves during the process.By adding zirconia, the microstructure of lanthanum chromites was optimized and the thermal shock resistance was improved consequently.Besides being used as high temperature heating elements,the composite of lanthanum chromites added with zirconia can be developed as new type refractory materials as they can meet the requirements of special refractory materials for continuous metal cast process.