【作者】 王翔；

【导师】 张久兴；

【作者基本信息】 合肥工业大学 ， 材料工程（专业学位）， 2017， 硕士

【摘要】 六硼化镧（LaB₆）由于低功函数、高熔点、强耐离子轰击性,而被认为是具有优越热电子发射性能的阴极材料,然而B-B共价键的存在,使得LaB₆非常脆,严重的限制了其进一步的工程化应用。研究表明:通过在LaB₆基体中添加具有高弹性模量的MeB2,形成La B₆-Me B2共晶复合材料,不仅能够提高材料的力学性能,而且能够改善La原子的扩散速率,因此是一种非常具有应用前景的结构功能一体化复合材料。本文结合放电等离子烧结技术和光学区熔技术,制备了高性能的LaB₆-ZrB₂共晶复合材料,系统的研究了不同工艺条件下材料的微观组织结构、力学性能及热发射性能。采用放电等离子烧结技术制备了大尺寸LaB₆-ZrB₂共晶复合材料,考察了温度对材料的致密度、微观组织、力学性能的影响规律,发现随着烧结温度增加,材料的致密度、硬度逐渐增大,断裂韧性呈现先增大后减少的趋势。最佳的工艺参数为:烧结温度T=1900oC、保温时间t=5 min、烧结压力位40 MPa,该工艺下,LaB₆-ZrB₂共晶复合材料的致密度为98.97%,硬度为19.52 GPa,断裂韧性为2.79MPa·m1/2。为了进一步降低烧结温度和提高烧结性能,在LaB₆-ZrB₂二元合金的基础上通过添加TiB2促进烧结致密化,TiB2和ZrB₂形成了固溶相活化了晶格大大降低了烧结温度,在1800oC烧结的样品达到了较好的性能,断裂韧性为4.43 MPa·m1/2,相对于二元合金断裂韧性提高了59%。在放电等离子基础上,采用光学区熔制备了全致密、高质量的LaB₆-ZrB₂共晶复合材料,研究了凝固速率对材料的微观组织和性能的影响规律,结果表明:当凝固速率为50-300 mm/h时,LaB₆和ZrB₂形成了共晶两相相互耦合生长的均匀组织,ZrB₂纤维的直径和间距都随着抽拉速率的增加而减小,在不同凝固速率下的抗弯强度都较高,在100mm/h时达到809.04 MPa;材料的维氏硬度和断裂韧性横截面和纵截面上呈现出各向异性:横截面上,硬度为21.15 GPa,断裂韧性为3.8 MPa·m1/2,在纵截面上硬度为22.16 GPa,裂纹沿着纤维方向测试的断裂韧性为3.6 MPa·m1/2,垂直于纤维方向断裂韧性为8.6 MPa·m1/2。LaB₆-ZrB₂复合材料具有很好的导电导热性,电阻率测试温度从室温15oC至350oC时,电阻率和温度成正比关系,相较于LaB₆电阻率降低至原来的一半;导热系数随着温度升高而降低,在同样温度下,相较于LaB₆导热系数提高了约一倍。LaB₆-ZrB₂复合材料中LaB₆基体100晶体取向较强的样品热发射性能最好,在1KV外场加速电压,1873K测试温度下,热发射电流密度20.39 A/cm2,平均有效逸出功为2.69 eV。更多还原

【Abstract】 Lanthanum hexaboride（LaB₆）is an excellent thermionic emission and field emission cathode material owing to low work function,high melting point and strong ion bombardment resistance,but its brittleness has seriously limited the further application in some electronic devices with high requirements.LaB₆-ZrB₂ composites have good reinforcing effects by adding diboride（MeB2）into LaB₆ matrix to form quasi-binary eutectic alloy which has been proved in many researches.In this work,LaB₆-ZrB₂ eutectic composite materials was prepared by spark plasma sintering（SPS）and optical floating zone melting method.The microstructure,mechanical properties and thermionic emission of LaB₆-ZrB₂ composites were investigated.LaB₆-ZrB₂ eutectic composite with large size was fabricated by SPS under 40 MPa holding for 5 min with the heating rate of 100oC /min.Effects of the sintering temperature on the relative density,microstructure and mechanical properties were studied.With the increase of sintering temperature,the density and hardness of the material increased,the fracture toughness increased first and then decreased.The optimal combination properties were obtained sintered at 1900oC with relative density of 98.97%,the hardness of 19.52 GPa and the fracture toughness of 2.79 MPa·m1/2.In order to further decrease the sintering temperature and improve sintering properties,TiB2 was added in La B₆-ZrB₂ composites to form a solid solution phase with ZrB₂,and apparently promoted the sintering densification due to activation of crystal lattice.The optimal combination properties were obtained at 1800oC and the fracture toughness of 4.43 MPa·m1/2 was improved by 59% comparing with binary eutectic alloy.The fully dense and high quality LaB₆-ZrB₂ in-situ eutectic composite was prepared by optical floating zone melting on the basis of SPS.The influences of solidification rate（50-300 mm/h）on the microstructure and properties were mainly studied.LaB₆-ZrB₂ eutectic composite was uniformly oriented with coupled growing eutectic phases.The diameter and spacing of ZrB₂ fiber both decreased with increasing solidification rate.The bending strength at different solidification rates did not vary and maximum value of 809.04 MPa was obtained at 100 mm/h.The sample along the growth direction and vertical growth was anisotropic.The Vickers hardness in the transverse section was 21.15 GPa and fracture toughness was 3.8 MPa·m1/2.The Vickers hardness in the longitudinal section was 22.16 GPa and fracture toughness was 8.6 MPa·m1/2.The LaB₆-ZrB₂ in-situ eutectic composite had good electrical conductivity and thermal conductivity.The electrical resistivity was proportional to test temperature at 15-350oC and the thermal conductivity decreases with the increasing temperature at 25-500oC.The electrical resistivity was reduced to about half and the thermal conductivity was about doubled at the same temperature compared to the LaB₆.LaB₆-ZrB₂ in-situ composite with LaB₆ matrix（100）crystal oriented had higher thermionic emission properties.Under applied voltage of 1 KV at 1873 K,the thermionic emission current density was 20.39 A/cm2 and the average work function was 2.69 eV.更多还原