【作者】 李士刚；

【导师】 李胜利；

【作者基本信息】 山东大学 ， 材料学， 2009， 博士

【摘要】 铬酸镧材料具有独特的晶体结构和优异的物理、化学性能,已经在高温电热材料和固体氧化物燃料电池等方面得到了广泛应用。A位掺杂Ca或Sr离子可以系统调整铬酸镧材料的性能（如提高烧结体致密度,改善电性能）,使铬酸镧材料基本满足了高温电热元件和固体氧化物燃料电池连接体的使用要求,但仍然存在组分（CrO₃）挥发,阳离子扩散等问题。国产铬酸镧电热元件的恒温持续使用寿命等指标已经达到国际同类产品水平,但元件的循环使用次数远低于国外产品,抗热震性能差是根本原因。针对掺杂铬酸镧等直径电热元件不同部位烧结收缩存在差异,抗热震性能差及传统工艺Cr挥发严重等问题,本研究通过B位Li掺杂调节元件各部分的烧结收缩率;通过添加骨料改善组织,提高元件的抗热震性能;通过添加氧化铝稳定铬酸镧结构,抑制Cr挥发;通过添加氧化锆进一步改善铬酸镧的力学性能。借助X射线衍射仪,扫描电子显微镜,万能试验机和四探针测试仪等设备对材料进行了相应的组织性能测试和分析。具体研究内容如下:（1）用Pechini法制备了掺Li铬酸镧超细粉体,在系统分析Pechini法工艺的基础上,找出了影响粉体形貌的关键因素,即柠檬酸量、乙二醇量、凝胶干燥温度、加热速率和煅烧温度,并确定了最佳工艺参数。使用该工艺参数可以获得均匀、极少团聚的铬酸镧超细粉体,平均粒径约为40nm。研究了掺Li铬酸镧烧结过程中掺Li量和烧结温度对粉体烧结行为及烧结体组织的影响。结果表明,在特定烧结温度下掺Li铬酸镧烧结体收缩率随掺Li量的增加而升高;当掺Li量为0.20时收缩率达到最大;此后随着掺Li量增加而降低。就相同掺Li量的烧结体而言,烧结温度越高组织越致密。烧结体的断口特征为穿晶和沿晶混合断裂,这表明晶界结合强度接近晶内。烧结过程中晶粒不断合并、长大,长大机制为台阶方式。掺Li铬酸镧在空气中放置一段时间后会出现粉化现象,其原因与烧结体中析出La₂O₃有关。（2）在固相法合成铬酸镧微粉中添加一定比例的大尺寸骨料,研究了不同骨料添加方案对材料组织及各项性能的影响,通过对主要工艺参数的优化确定出可以获得最佳综合性能的铬酸镧材料(La_0.9Ca_0.1CrO₃,La_0.974Ca_0.026CrO₃)的制备工艺。研究发现,骨料对基体晶粒的长大存在阻碍作用,并使烧结体气孔率增大,且骨料尺寸越大、添加量越高效果越明显。La_0.9Ca_0.1CrO₃材料添加5%的55μm（260目）以下骨料后抗热震性能最佳,而La_0.974Ca_0.026CrO₃材料的最佳配方则是添加10%的380μm（40目）以下骨料。材料抗热震性能提高的主要原因是气孔率的增大和骨料的桥接效应。随骨料添加量的增加,试样的相对密度减小,致密度下降,室温抗弯强度降低而电阻率增大。添加骨料的铬酸镧电热元件在高温使用过程中骨料会抑制基体晶粒的长大,防止长时间使用后性能劣化。（3）研究了氧化铝对氧化铝/铬酸镧复合材料显微组织,烧结性能及电性能的影响。试验证实Al₂O₃/La_0.9Ca_0.1CrO₃系统是热力学不稳定的,高温烧结过程中阳离子存在显著的扩散现象,系统最终相组成决定于氧化铝含量。当Al₂O₃含量≤20mol%时,烧结体由La_0.9Ca_0.1Cr_1-xAl_xO₃和Al_2（1-x）Cr_2xO₃两相组成,当氧化铝添加量达到20mol%及以上时,试样中的铬酸镧固溶体变为菱形结构。当Al₂O₃含量为30mol%时,烧结体中除了存在La_0.9Ca_0.1Cr_1-xAl_xO₃和Al_2（1-x）Cr_2xO₃两相外,还有六铝酸镧固溶体生成。氧化铝含量为50mol%～80mol%时,试样主要由菱形结构的钙钛矿相和六铝酸镧固溶体组成。氧化铝含量为90mol%和95mol%时,烧结体则主要由刚玉相和六铝酸镧固溶体组成,此时由于钙钛矿相含量较低,在XRD图谱中未检测到。用能谱仪研究了复合材料在烧结过程中发生的阳离子互扩散现象,发现每向铬酸镧中扩散1个Al³⁺,必有1个Cr³⁺扩散到氧化铝中。复合材料中Al₂O₃含量较低时（≤20mol%）可以提高铬酸镧粉体的烧结活性,降低Cr挥发,显著促进致密化;当Al₂O₃量较高时（30mol%～70mol%）,由于生成相互交错的层片状六铝酸镧,复合材料的致密度有所减小,并在Al₂O₃量为70mol%时达到最低值,随后试样的致密度又随Al₂O₃量的增大逐渐增大。1450℃烧结的复合材料导电率与Al₂O₃量的关系符合通用有效介质（GEM）方程;1650℃烧结的试样由于离子扩散导致复合材料的电导率在Al₂O₃摩尔含量为20%时发生突变,Al₂O₃摩尔含量在0～10%与20～50%之间时,复合材料的电导率分段符合GEM方程。添加Al₂O₃后由于材料致密度减小,气孔率增大,材料的抗弯强度有所减小,抗弯强度和气孔率的关系符合Ryskewitsch经验公式。（4）研究了氧化锆对氧化锆/铬酸镧显微组织和性能的影响。发现氧化锆与铬酸镧在高温下能稳定共存,基本不存在阳离子扩散,组织稳定性好。氧化锆颗粒阻碍了铬酸镧晶粒的长大、熔合,使烧结体致密度降低。烧结体的电阻率随氧化锆含量的增加而增大,并且变化规律符合GEM方程,这将有益于其在电热材料等领域的应用。氧化锆/铬酸镧复合材料的抗弯强度随氧化锆含量的增加而下降,但抗热震性能有所提高。该复合材料除可以应用于高温电热元件外,其性能完全满足连铸用特种耐火材料的要求,有望开发成新型高品质耐火材料。更多还原

【Abstract】 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.更多还原