| Lanthanum hexaboride（LaB6）is an excellent cathode material due to its characteristics of low work function,high melting point,strong toxic resistance and good chemical stability.It is widely used in high-resolution electronic analysis instruments,electron beam welders,high-energy accelerators and vacuum electronic devices.However,with the rapid development of high-power and long life vacuum electronic devices,higher requirements are put forward to the cathode materials.LaB6operates at high work temperature,which limits its further application.How to improve the thermionic emission performance of LaB6,making it meet the requirement of the devices at a lower temperature range,is an urgent issue to be solved at present.Therefore,in this paper,LaxCa1-xB6（x=0.5～1）cathode materials（polycrystal and single crystal）were prepared by doping alkaline earth Ca into the LaB6.The LaxCa1-xB6polycrystals were prepared by spark plasma sintering（SPS）and the LaxCa1-xB6single crystals were prepared by optical floating zone method using the polycrystal LaxCa1-xB6.The phase,crystal structure,microstructure,mechanical properties and thermionic emission properties of these materials were characterized and analyzed.The specific conclusions are as follows:Firstly,the LaxCa1-xB6 polycrystalline bulk materials were prepared by SPS at temperature of 1900℃ for 5 min,and under the pressure of 50 MPa.The densities of La0.9Ca0.1B6,La0.8Ca0.2B6,La0.6Ca0.4B6 and La0.5Ca0.5B6 polycrystals were 96.77,95.14,93.47 and 93.32%,respectively,and the corresponding Vickers hardness values were18.37,21.58,22.90,23.73 GPa,respectively.The microstructures of LaxCa1-xB6polycrystalline were characterized using SEM.The results show that the structures of the La0.9Ca0.1B6,La0.8Ca0.2B6 and La0.6Ca0.4B6 polycrystals are single-phase when the sintered temperature was 1800℃,that is,the LaB6 and Ca B6 are completely solid solution.The solution temperature of La0.5Ca0.5B6 polycrystalline is 1850℃.The thermionic emission results show that with the increase of Ca content,the thermionic emission current density of LaxCa1-xB6 polycrystal increases at first and then decreases,and the La0.6Ca0.4B6 polycrystal has the best thermionic emission performance.The thermionic emission current density is 6.08A/cm2 and the effective work function is2.91e V,which are better than that of LaB6 polycrystalline cathode at 1600℃ and applied voltage of 1 k V.Then,using LaxCa1-xB6 polycrystal as the feeding rod and[100]LaB6 single crystal as seed crystal,LaxCa1-xB6 single crystal with high quality and high performance was successfully prepared by optical floating zone method.The precise[100]LaxCa1-xB6single crystal was obtained after directional line cutting and the direction was determined by X-ray Laue diffractometer.The SEM-EDS analysis reveals that no obvious subgrain boundaries and defects were observed in LaxCa1-xB6 single crystal.La,Ca and B elements are uniformly distributed,and there is no enrichment of rare earth elements or alkaline earth elements.The rocking curve of La0.6Ca0.4B6 single crystal is smooth,symmetrical.The diffraction peak of the curve is sharp,and the full-width at half-maximum is very small,only 0.1108°,which indicates that as-growth La0.6Ca0.4B6single crystals have a good quality.The thermionic emission test results show that the thermionic emission performance of LaxCa1-xB6 single crystal is better than that of LaB6single crystal at low temperature（1100～1400℃）,especially La0.6Ca0.4B6 single crystal.Therefore,alkaline earth Ca doping method can effectively reduce the working temperature of LaB6 cathode and improve its thermionic emission performance.The LaxCa1-xB6 cathode,especially the single crystal cathode material,has a great application prospect in the field of vacuum electronic devices. |
