Research On Preparation, Structure And Properties Of Polycrystalline Rare Earth Hexaboride Cathodes

Rare earth hexaborides (RB₆) are widely used in modern technology as excellent thermionic electron emission sources. At the present time, the researches on RB6 are mainly focused on the preparation, structure and application of the binary hexaboride LaB₆, and there are only a few sudies on other binary and ternary hexaborides. The traditional sintering methods of the RB6 have some disadvantages, such as, high sintering temperature, long sintering time, coarse grain of the sintered crystal, low emission properties, and so on. In this paper, the spark plasma sintering (SPS) method was used to prepare several binary and ternary hexaborides. The relationships between structure and properties were studied on the binary RB6(R= La, Ce, Sm, Nd), and ternary hexaborides of (La_xR_1-x)B₆ and (Ba_xR_1-x)B₆(R=Sm, Ce, Nd).LaB₆ powders prepared by boron carbide reduction method were solid state sintered by SPS to obtain LaB₆ cathode material with high density and good properties. The optimal parameters useful in technological applications are temperature of 1650℃, pressure of 50 MPa and holding time of 10 min. The relative density of LaB₆ sintered by SPS can be as high as 96.2%, and its hardness reaches 17.2 GPa. The highest bend strength is 203.2 MPa. The pulse thermionic emission density of densest LaB₆ is 27.67 A cm^-2 at 1600℃with voltage up to 1000 V. TheRichardson work function(Φ_R) and emission constant(A_R) are 2.24 eV and 2.3 A·cm^-2·K^-2, respectively. It is shown that, the mechanical and emission properties of this sintered LaB₆ bulk is much better than that prepared by other methods. In adition, the SPS method decreases the sintering temperature and shortens the sintering time in the densification process. Reactive liquid phase SPS method was also used to prepare the binary RB₆(R= La, Ce, Sm, Nd). The EBSD analysis indicates that, the RB6 sintered bulks prepared in certain condition resulted in (100) preferred orientation, which will enlarge the emission property. Compared to the traditional sintering method, reactive liquid phase SPS method combined the synthesis and densification processes, and then accelerated the densification. For LaB₆ and CeB₆, the emission current density is 36.1 and 40.5 A·cm^-2 at 1873 K when the voltage reaches 1 kV. TheirΦ_R is 1.98, 1.94 eV, and the A_R is 1.24, 0.46 A·cm^-2·K^-2. The results indicate that the properties of LaB₆ and CeB₆ sintered by SPS exceed those of RB6 prepared by traditional sintering methods.Reactive liquid phase SPS method was also used to prepare the ternary (La_xR_1-x)B₆ and (Ba_xR_1-x)B₆. The XRD analysis indicates that, the (La_xSm_1-x)B₆ and (Sm_xBa_1-x)B₆ were single phase solid solution, and the others (Ba_xR_1-x)B₆ were two phases mixed materials. The (La_0.6Sm_0.4)B₆ and (La_0.6Ba_0.4)B₆ sintered by SPS have excellent emission properties. In middle and low temperature zone, most ternary RB6 have emission properties superior to those of LaB₆. Therefore, they are suitable cadicates for the middle and low temperature zone cathodes. In adition, the resistivitis of ternary RB₆ are much higher than that of LaB₆, and this makes them suitable cadicate for the direct heated cathode.Nano polycrystalline samples of LaB₆ and (La_0.6Ba_0.4)B₆ were prepared by the reactive liquid SPS method, using boron nanopowders and RH₂(R=La, Ba) synthesized by hydrogen dc arc plasma. The sintering pressure was 80 MPa, and the sintering temperature was 1300 and 1400℃for the LaB₆ and (La_0.6Ba_0.4)B₆, respectively. The EBSD analysis indicates that, the Nano polycrystalline LaB₆ and (La_0.6Ba_0.4)B₆ had (310) and (210) preferred orientations which will enlarge their emission property. The emission current density of the LaB₆ and (La_0.6Ba_0.4)B₆ is 56.8, 43.7 A·cm^-2 respectively, at 1873 K when the voltage reaches 1 kV. TheirΦ_R is 1.55, 1.83 eV, and their A_R is 0.09, 0.48 A·cm^-2·K^-2, respectively.The results of this paper indicate that, the relative density, mechanical properties and emission properties of RB6 sintered by SPS exceed those of the same materials prepared by traditional sintering methods.