结构功能一体化的LaB6/ZrB2阴极材料与性能

Directionally solidified LaB6-MeB2 eutectic in situ composite is a new type of composite with high-performance in the recent years, and LaB6-ZrB2 eutectic in situ composite, as compared with LaB6 single-crystal, has low work function, high emission current density, high poison resistance and high thermal shock resistance, which makes the LaB6-ZrB2 eutectic considered as new-type thermal cathode material applied in new generations of high-power vacuum devices. Besides, relatively low melting point for LaB6-ZrB2 eutectic can provide a convenience to prepare the sample with low cost and large size. However, the preparation of functional and structural LaB6-ZrB2 eutectic with low cost and large size and its property is not reported till now. At present, light zone melting technique is best in preparing the high quality single-crystal. In the present proposal, the preform of LaB6-ZrB2 eutectic with large size, high density and high purity is prepared with the high purity LaB6 and ZrB2 powder by using advancedly reactive spark plasma sintering technique, and the preform is then put into the light zone furnace to prepare the LaB6-ZrB2 eutectic in situ composite with low cost, large size and single-crystal matrix. The relations among the microstructure structures, mechanical property and thermal emission characteristic of this eutectic in situ composite are investigated, and the thermal emission mechanism of this composite is revealed. The fundamental investigation of this project is to provide scientific basis of studing the functional and structural LaB6/ZrB2 cathode material with low cost and large size .

LaB6-MeB2共晶自生复合材料是近来年新发展的一类新型高性能复合材料。LaB6-ZrB2共晶自生复合材料相比LaB6单晶具有低逸出功、高发射电流密度、高抗“中毒”性、高抗热裂性，可成为新一代大功率真空器件用的新型热阴极材料；LaB6-ZrB2相对低的熔点为制备低成本、大尺寸试样提供了便利。然而目前低成本、大尺寸且结构功能一体化的LaB6/ZrB2阴极材料的制备及性能未见报道。光学区熔技术是目前制备高质量单晶最好的方法。本项目以高纯LaB6和ZrB2粉末为原料，采用放电等离子烧结炉制备大尺寸、高致密度且高纯的LaB6-ZrB2共晶预制体，然后采用光学区熔炉制备低成本、大尺寸且基体是单晶的LaB6-ZrB2共晶自生复合材料，探明材料微观组织结构、力学性能及热发射特性之间的关系，揭示其热发射机理，为研制低成本、大尺寸且结构功能一体化的LaB6/ZrB2阴极材料提供科学依据。

稀土六硼化镧(LaB6)单晶是工业上具有成熟应用的热电子发射阴极材料，广泛用于应用于卫星、深空探测、等离子体源等国防高技术领域，然而LaB6单晶低的抗弯强度(300MPa)使得单晶在阴极加工中有脆断现象，为改善单晶的力学性能，本项目在LaB6单晶中加入高弹性模量的ZrB2形成共晶复合材料，采用光学区熔技术制备了基体是单晶的LaB6(001)-ZrB2共晶复合材料，系统的考察了凝固速率对共晶组织形貌、界面结构、力学性能、抗氧化性和热发射特性的研究，获得的结果如下：通过优化工艺参数之间的匹配关系，形成了高质量复合材料的制备工艺规范；探明了共晶微观组织特征尺寸随凝固速率的变化规律，其中凝固速率与共晶间距遵循λR0.42=6.42 μm1.42/s0.42，高分辨界面结构表明共晶两相在高温下可以互存，保持各自优异性能；LaB6(001)-ZrB2共晶复合材料的断裂韧性具有强烈的各向异性，最高的断裂强度为5.16±0.3 MPa.m1/2，该值是LaB6单晶断裂韧性的将近3倍（1.8 MPa.m1/2）, 最高抗弯强度为809.04 MPa，而LaB6单晶的抗弯强度只有293.79 MPa。LaB6(001)-ZrB2共晶复合材料具有比LaB6单晶更优异的高温抗氧化特性。在1600oC、1kV条件下，LaB6(001)-ZrB2共晶复合材料热发射电流密度达到了20.39 A/cm2，完全满足霍尔推进系统对阴极材料热发射电流密度的要求。