【作者】 姜妮；

【导师】 王为民；

【作者基本信息】 武汉理工大学 ， 复合材料学， 2010， 硕士

【摘要】 LaB6热阴极陶瓷材料具有高熔点、高硬度、化学稳定性高等特点,此外还具有许多特殊的功能性,包括：电子逸出功低(2.6ev)、发射电流密度大、高温蒸气压低,使其在制作现代仪器中的电子元器件等民用、国防工业中广泛应用。LaB6粉体是制备单晶、多晶和复合材料的重要原材料,它的质量将严重影响器件的性能。目前制备LaB6粉体的技术都存在能耗高、产物纯度低、工艺复杂等缺点,不适宜工业化生产。本文选取价格低廉的原料,采用自蔓延高温合成技术成功制备了高纯度、晶粒微细的LaB6粉体。本文选取La2O3-B2O3-Mg反应体系,采用燃烧法点火模式,在高温自蔓延合成设备中合成LaB6粉体。系统探讨了不同的合成条件如成型压力、原料配比等因素对产物的物相组成、微观形貌和化学组成的影响,优化了制备过程的工艺参数。研究表明：原料中Mg粉含量对产物的纯度影响重大；成型压力的增大,有利于晶粒尺寸减小。当Mg过量15%、预成型压力20MPa时,在1600℃可以制备出纯度大于98%、平均粒径小于1μm的LaB6粉体。为了控制晶粒长大,掺加NaCl作为稀释剂,有效的降低了反应温度,晶粒尺寸减小且大小分布均匀。NaCl加入量为30%时,体系的合成温度低至900℃,所得的产物LaB6粉体是单一物相,平均粒径为200nnm。为了提高产物的质量,选取低熔点的LaCl3-B2O3-Mg体系,利用自蔓延燃烧模式,优化工艺参数在1540℃制备了纯度大于99%、平均粒径小于1μm的LaB6粉体。稀释剂NaCl加入量为30%时,体系的合成温度降低至1100℃,所得产物的平均粒径为100nm。基于放电等离子体烧结设备(SPS)具备升温降温快、烧结时间短的优点,有利于减小晶粒尺寸,本文探索了热爆法点火模式对La2O3-B2O3-Mg体系自蔓延反应的影响。在SPS中720℃恒温起爆合成纯度大于98.5%,平均粒径小于1μm的LaB6粉体。加入30%NaCl稀释剂,热爆起始温度降低至570℃,晶粒粒径减小至500nm。研究表明：改变点火模式,产物的物相组成没有变化,对产物的微观形貌有一定的影响,热爆模式合成的产物粒径大小分布均匀。更多还原

【Abstract】 Lanthanum hexaboride (LaB6) is a refractory compound characterized by the high melting temperature, excellent thermal stability and high hardness. At the same time, it has been widely used in modern technology as an excellent thermionic electron emission source which can offer high brightness and long service life, for its low work function (2.6 eV), high current and voltage capability, and low vapor pressure at high temperature. LaB6 powders are the most important raw material for the preparation of single crystal materials, polycrystalline materials and composite materials. And its quality will exert a great influence on the performance of device. At present, the technique of LaB6 powders preparation can hardly satisfy the requirement of commercial run for the defects of high power wasting, low purity product and complicated craft. In this paper, high purity LaB6 fine powders were successfully prepared by self-propagating high-temperature synyhesis (SHS) process using raw materials at low price.La2O3-B2O3-Mg system was used to synthesize LaB6 powders by employing combusition mode in SHS equipment. Whereafter the effects of mixture ratio and compact pressure on combustion reaction mechanism, combustion product microstructure and powder characteristic were investaged systemtically. Results showed that the influence of Mg content on purity of LaB6 powders was very important. High pressure on green compact induced close packing arrangement of green powders and resulted in forming fine powders as a product by an efficient reaction. At the condition of excessive 15% Mg and 20 MPa compact pressure, the purity of LaB6 powders is 98%, and mean grain size is less than 1 urn. Some diluents NaCl (0-30%) in reactant could adjust combustion temperature and LaB6 grain size.With the increase of diluents NaCl content, the grain size of LaB6 powders decreased. When addition of diluent NaCl was 30%, grain size was the lowest at 200 nm, combustion temperature decreased from 1600℃to 900℃.To improve the properties of final product, LaCl3-B2O3-Mg system was also adopted to prepare LaB6 powders by employing combusition synthesis mode. We optimized the process for synthesizing high quality LaB6 powders. Its purity is 99%, and average grain size is less than 1 um. When addition of diluent NaCl was 30%, grain size was the lowest at 100 nm, combustion temperature decreased from 1540℃to 1100℃.Owing to the spark plasma sintering of fast heating rate and short heating time, the synthesis of LaB6 powders via the reaction of La2O3-B2O3-Mg system was also carried out by using thermal explosion mode of SHS in SPS. The product obtained in this process has 98.5% purity and 1μm in grain size. The initial temperature of thermal explosion synthesis decreased from 720℃to 570℃with 30% NaCl addition. It was revealed from particle size distribution measurements that LaB6 powders obtained by 30% NaCl addition contain particles mostly finer than 500 nm. After analyzing the SHS reaction under two models of comusition synthesis and thermal explosion synthesis, it was founded that ignition model had no effect on the composition of the product, although the microstructure were different.更多还原