The Microstructure And Properties Of LaB₆-ZrB₂ Eutectic Composites

Lanthanum hexaboride?LaB₆?is an excellent thermionic emission and field emission cathode material owing to low work function,high melting point and strong ion bombardment resistance,but its brittleness has seriously limited the further application in some electronic devices with high requirements.LaB₆-ZrB₂ composites have good reinforcing effects by adding diboride?MeB2?into LaB₆ matrix to form quasi-binary eutectic alloy which has been proved in many researches.In this work,LaB₆-ZrB₂ eutectic composite materials was prepared by spark plasma sintering?SPS?and optical floating zone melting method.The microstructure,mechanical properties and thermionic emission of LaB₆-ZrB₂ composites were investigated.LaB₆-ZrB₂ eutectic composite with large size was fabricated by SPS under 40 MPa holding for 5 min with the heating rate of 100oC /min.Effects of the sintering temperature on the relative density,microstructure and mechanical properties were studied.With the increase of sintering temperature,the density and hardness of the material increased,the fracture toughness increased first and then decreased.The optimal combination properties were obtained sintered at 1900oC with relative density of 98.97%,the hardness of 19.52 GPa and the fracture toughness of 2.79 MPa·m1/2.In order to further decrease the sintering temperature and improve sintering properties,TiB2 was added in La B₆-ZrB₂ composites to form a solid solution phase with ZrB₂,and apparently promoted the sintering densification due to activation of crystal lattice.The optimal combination properties were obtained at 1800oC and the fracture toughness of 4.43 MPa·m1/2 was improved by 59% comparing with binary eutectic alloy.The fully dense and high quality LaB₆-ZrB₂ in-situ eutectic composite was prepared by optical floating zone melting on the basis of SPS.The influences of solidification rate?50-300 mm/h?on the microstructure and properties were mainly studied.LaB₆-ZrB₂ eutectic composite was uniformly oriented with coupled growing eutectic phases.The diameter and spacing of ZrB₂ fiber both decreased with increasing solidification rate.The bending strength at different solidification rates did not vary and maximum value of 809.04 MPa was obtained at 100 mm/h.The sample along the growth direction and vertical growth was anisotropic.The Vickers hardness in the transverse section was 21.15 GPa and fracture toughness was 3.8 MPa·m1/2.The Vickers hardness in the longitudinal section was 22.16 GPa and fracture toughness was 8.6 MPa·m1/2.The LaB₆-ZrB₂ in-situ eutectic composite had good electrical conductivity and thermal conductivity.The electrical resistivity was proportional to test temperature at 15-350oC and the thermal conductivity decreases with the increasing temperature at 25-500oC.The electrical resistivity was reduced to about half and the thermal conductivity was about doubled at the same temperature compared to the LaB₆.LaB₆-ZrB₂ in-situ composite with LaB₆ matrix?100?crystal oriented had higher thermionic emission properties.Under applied voltage of 1 KV at 1873 K,the thermionic emission current density was 20.39 A/cm2 and the average work function was 2.69 eV.