Preparation And Modification Of LaB₆ Polycrystalline Cathode Material

With the rapid development of electric propulsion technology,there is an urgent need of high performance and long work life cathode materials.Lanthanum hexaboride?LaB₆?polycrystalline is one of the most promising cathode materials due to its excellent properties such as low work function,low evaporation rate,high chemical stability and easy to prepare.However,the commercial LaB₆ polycrystalline is faced with many difficulties including low density,coarse grain and low thermal emission current density,which are caused by drawbacks of La B₆ raw powders,such as large particle size,agglomeration and low purity.Therefore,in this work,LaB₆ nanopowders with uniform size,high dispersion and high purity have been prepared by a molten salt synthesis route.Then,a high density,fine grain and?100?preferred orientation of LaB₆polycrystalline has been prepared using LaB₆ nanopowders as raw materials.Lastly,LaB₆ polycrystalline has been further modified by other rare metal elements to improve its thermionic emission property.The main contents are as follows.A molten salt synthesis route was developed to prepare LaB₆ nanopowders.Firstly,lanthanum trichloride?LaCl₃?and sodium brohydride?NaBH₄?were chosen as lanthanum and boron source,respectively.Lithium chloride-kalium chloride?LiCl-KCl?eutectic salt was chosen as molten salt medium.The feasibility of reaction of La Cl3 and NaBH4 is demonstrated based on the thermodynamics calculation.Secondly,the effects of different process conditions on La B₆ nanopowders fabricated by MSS were investigated.Results showed that with temperature increasing,the particle size of LaB₆ powders increased,and morphology of LaB₆ powders changed from spherical shape to cubic and polyhedron shape.With time prolonging,the particle size of LaB₆ powders also increased,but after a certain time,the powders would be agglomerated and even be dissolved in the molten salt liqud.With molten salt medium increasing,La B6 powders were easy to crystallize and grow up,and the dispersion of LaB₆ powders was better.With boron source content increasing,the particle size of LaB₆ powders decreased,and morphology of La B₆ powders changed from cubic shape to spherical shape.Lastly,the preferential process parameters of MSS to prepare LaB₆nanopowders were:the temperature and time were 800?and 1 h,respectively.The mass ratio of reactants to salts was 1:10.The molar ratio of La Cl₃ to NaBH₄ was 1:6.The powder properties and synthesis mechanism of La B₆ nanopowders synthesized by MSS were studied systematically.In terms of powder properties,the nanopowders had a cubic shape with an average particle size of 90 nm,and the cubic particle was a single crystal.The purity of La B₆ nanopowders reached to 99.60 wt.%.In terms of synthesis mechanism,the reaction mechanism was dominated by NaBH₄ was decomposed into Na,B and H2,and then Na and B reacted with LaCl3.The formation mechanism of LaB₆ nanopowders synthesized by MSS was dominated by“dissolution-precipitation”,and two stages were contained.One was the nucleation stage which was influenced by the degree of supersaturation of La B6 in molten salt liquid.Another was the growth stage which was influenced by the crystal structure characteristics of LaB₆and the growth environment.LaB₆ polycrystallines of high density,fine grain and?100?preferred orientation were prepared.Firstly,HN-La B6 polycrystalline was prepared by hot pressing using LaB₆ nanopowders as raw materials.The relative density and average grain size of HN-LaB₆ were 96.56%and 5.30?m,which were 9%higher and 3.5 times lower than those of commercial HM-LaB₆ polycrystalline,respectively.Besides,a?100?preferred orientation which had a low work function was formed in the direction of sintering pressure of HN-LaB₆.The electrical resistivity,hardness,elastic modulus and work function of HN-LaB₆ were 22.23±0.05??·cm?30.96±1.92 GPa?358.93±20.45 GPa and2.80 eV,which were 2 times,4 times,2 times higher and 11%lower than those of HM-LaB₆,respectively.Secondly,SN-LaB₆ polycrystalline was prepared by spark plasma sintering using LaB₆ nanopowders as raw materials to further reduce the grain size.The relative density of SN-La B6 was 95.75%,which was closely to that of HN-LaB₆.Also a?100?preferred orientation was formed in the direction of sintering pressure of SN-La B₆.Moreover,the average grain size of SN-LaB₆ was only 2.83?m,which was nearly half smaller than that of HN-LaB₆.The electrical resistivity and work function of SN-LaB₆ were 31.63±0.10??·cm and 2.73 eV,which were 1.4 times higher and 3%lower than those of HN-LaB₆,respectively.Lastly,high density,fine grain and?100?preferred orientation were key reasons for strengthening properties of LaB₆polycrystalline.LaB₆ polycrystalline were further modified by Cerium?Ce?and Samarium?Sm?.Firstly,single phase solid solution of ternary La_1-xRe_xB₆ nanopowders were fabricated by MSS.These nanopowders had a spherical or cubic shape with an average particle size of 50 nm?80 nm.Besides,the lattice constant of these nanopowders decreased with Ce/Sm increasing.Secondly,single phase solid solution of ternary La1-xRexB6polycrystalline which had high density uniform distribution of elements were prepared by hot pressing using La_1-xRe_xB₆ nanopowders as raw materials.The relative densities of the two polycrystallines were 97.05%?98.89%,which were closely to that of HN-LaB₆.Also La1-xCexB6 polycrystalline had?100?preferred orientation in the direction of sintering pressure,while the trend of?100?preferred orientation in the direction of sintering pressure of La_1-xSm_xB₆ polycrystalline would decreased with increasing Sm.Thirdly,properties of the two polycrystalline were studied.Results showed that both Ce and Sm were good for increasing the room temperature electrical resistivity.However,only Ce was propitious to the thermal electron emission of LaB₆polycrystalline.The material with optimum Ce content was La0.6Ce0.4B6 polycrystalline,and its work function was 1.85 eV,which was nearly 34%lower than that of HN-LaB₆.Sm was adverse to the thermal electron emission property of LaB₆.