Preparation And Sintering Performance Study Of Lanthanum Hexaboride Powders

Lanthanum hexaboride has been receiving more and more attentions as a high performance material because of its unique properties, such as high electric conductivity, good chemical stability, low rate of evaporation at high temperature and high emission current density. LaB₆ is widely used as cathode parts in demanding huge emission current field instrument, such us electron microscope, electron beam welding industry, discharge tube industry and so on. Due to its special properties and broad application prospect, the type of materials research have been carried out by many institutions. However, in order to preparation high performance LaB₆ polycrystalline, high purity and uniform particle size distribution of LaB₆ powder is been needed at first. So the low cost preparation technology of high purity and good sintering performance of LaB₆ powder were investigated in this paper, which may establish the academic base for the preparation of high performance LaB₆ polycrystalline and composites.The lanthanum hexaboride powders （LaB₆） were synthesized under atmospheric pressure by using La₂O₃ （D50<0.2μm） and B4C （D50=3.70μm） powder as the precursors. The reaction thermodynamic data for preparing LaB₆ was calculated. The effects of temperature, holding time and different particlesize of B4C on the phase composition, morphology, particle size and distribution of LaB₆ powders were characterized by X-ray diffraction（XRD）, scanning electron microscope（SEM） and Laser particle size anslyzer. The reaction mechanism was also investigated. Results showed that LaB₆ powders can be obtained under atmospheric pressure at 1650℃for 2h with 99.22% purity, D50=15.86μm, uniform particle size distribution and cube structure. When the particle size of La₂O₃ powder is much smaller than that of B4C particles, the growth of LaB₆ starts on the surface of B4C particles, then the residual La₂O₃ and LaBO3 will diffuse through LaB₆ shell onto B4C core until the reaction is completely. The influence of temperature and holding time on the synthesis of LaB₆ powder were not obviously than B4C raw materials, The initial shape and particle size of LaB₆ powders were depended on that of B4C particles.The LaB₆ polycrystalline was produced by using hot-pressing method. The influence of temperature, holding time, hot-pressure and powder particle-size on the LaB₆ polycrystalline performance were system characterized. The LaB₆ polycrystalline hot-pressing sintering mechanism was also investigated. Results show that high performance LaB₆ polycrystalline obtained under 30MPa hot-pressure, at 2050℃for 1h by using self-made LaB₆ powder with 99.22% purity and D50=15.86μm, which relative density reached 96.32%, bending strength and microhardness achieved respectively 153.2MPa and 16.27GPa. With the increasing of temperature, holding time and hot pressure are in favor of LaB₆ polycrystalline sintering densification. However, exorbitant sintering temperature and prolong holding time would lead to grain abnormal grow and material properties sharply decline, which is disadvantage of materials further densification. It is benefit of increasing sintering densification rate, decreasing the stomata size and porosity by using smaller particle-size LaB₆ powders as sintering materials.