Preparation And Properties Of LaB₆ Single Crystal And LaB₆-VB₂ Composite

Lanthanum hexaboride（LaB₆）single crystal is an excellent thermion cathode material due to its characteristics of high melting point,low work function,long life,high brightness and strong toxic resistance.However,there is a lack of unified conclusion on LaB₆ characteristics at present,mainly due to some conclusions obtained by different researchers under different conditions.For this reason,the preparation,characterization and properties of[100]orientation of LaB₆ single crystal were systematically analyzed by using spark plasma sintering and optical floating melting technology.Then,the LaB₆（100）-VB₂ eutectic composite material was formed by adding VB₂ with high elastic modulus on this basis.On the basis of ensuring the thermal emission performance of the material,the mechanical properties of the material were improved to realize the machinability of hollow cathode.The details are as following:High-quality LaB₆ single crystal with different diameters were prepared by the spark plasma sintering and optical floating zone melting technique.The deviation angle of LaB₆ single crystal from theoretical[100]crystal orientation was decreased from 6^oto0^o through improving the quality of the seed and optimizing the growth process.When the growth rate was increased from 8 mm/h to 12 mm/h,the FWHM decreased first and then increased.The minimum value of FWHM of the swing curve of the single crystal means the highest quality of the single crystal belongs to the LaB₆ single crystal grown at the rate of 10 mm/h.Under the condition of T=1873 K and U=1 k V,the thermal emission current density of the single crystal is 43.75 A/cm²,and the work function is2.67 e V.The variational trend of Vickers hardness value of single crystal with different rates is similar to that of thermal emission performance.At the growth rate of 10 mm/h,Vickers hardness of single crystal is 22.6 GPa.As the diameter of the single crystal increased from 2 mm to 10.24 mm,the uniformity of single crystal composition gradually becomes poor,so the quality of the single crystal is also affected.The FWHM of single crystal is only 0.063°at the diameter of 2 mm.On the condition of T=1873 k,U=1 kv,the highest thermal emission current density of single crystal is 46.8 A/cm²,the work function of 2.66 e V is obtained;As the component becomes non-uniform,the Vickers hardness keeps decreasing,and the maximum Vickers hardness of single crystal of D=2 mm is 23.1GPa.With the increase of the melting times from two-step to three-step,the thermal emission performance of the single crystal decreased due to the volatilization of the high-temperature material,but the Vickers hardness increased.A large-size LaB₆-VB₂ eutectic composite was prepared by spark plasma sintering.The effect of sintering temperature on the relative density,microstructure and mechanical properties of the materials were studied.Under the sintering temperature of1900℃,sintering pressure of 40 MPa,and holding time of 5 min,the sintered LaB₆-VB₂ composite has a density of 98.59%and a hardness value of 19.49 GPa.The LaB₆-VB₂ eutectic composite with the matrix of[100]single crystal was prepared by optical floating zone melting on the basis of spark plasma sintering.With the increasing of the solidification rate from 20 mm/h to 300 mm/h,the fiber spacing and diameter of the fiber decrease on the transverse section,the minimum spacing of 1.81 um and the minimum diameter of 0.71 um are obtained.The best thermal emission and mechanical properties belongs to the composite prepared at the rate of 100 mm/h.The Vickers hardness and fracture toughness of the material presented anisotropy on the transverse and longitudinal sections.The hardness value of 23.18 GPa and the fracture toughness of 3.65 MPa·m^1/2 are obtained on the transverse section.The hardness value is 24.8 GPa,and the fracture toughness parallel to the direction of the fiber is 3.45 MPa·m^1/2,perpendicular to the fiber is 8.32 MPa·m^1/2 on the longitudinal section.The highest bending strength is 565.6 MPa.LaB₆-VB₂ has the highest thermal emission property in the system of LaB₆-Me B₂ alloy.The highest thermal emission current density of J_{1k V}=31.67 A/cm²,and the work function of 2.69 e V are obtained at 1873K.