Preparation And Thermionic Emission Properties Of Composite Rare Earth Hexaboride Single-Crystals

Rare earth hexaboride(RB6),which composed of rare earth atom and boron,is a kind of metal ceramic material.The RB6 has many advantages such as high melting point,low work function and excellent chemical stability.Meanwhile,the RB6 also possesses excellent thermionic emission properties.In this regard,it has been widely used in various fields including national defense and civil utilization.In this investigation,we successfully prepared(Ce1-3XLaxPrxNdx)B6 single-crystals with high quality and large size based on spark plasma sintering(SPS)technology.Effect of the solidification rates on the quality and thermionic emission properties of the(Ce1-3XLaxPrxNdx)B6 single-crystals have been investigated.Meanwhile,the thermionic emission properties and mechanical properties of each plane of these crystals have also been systematically tested.(Ce0.7La0.1Pr0.1Nd0.1)B6,(Ce0.4La0.2Pr0.2Nd0.2)B6 and(Ce0.25La0.25Pr0.25Nd0.25)B6 single-crystals were prepared by SPS and optical float-zone traveling solvent technique.These crystals have large size,high quality,and a homogeneous element distribution.XRD,single crystal X-ray diffractometer and Laue diffractometer further revealed that the orientation of these single-crystals were(100).More attractively,the diffraction spots of these crystals are clear,independent and no spilt.Upon increasing the content of La,the crystal lattice parameters of single-crystals were gradually increased.It is also worth to mention that during the single-crystal growth process,various elements would volatile,and the order of the volatilization rate is:La<Ce<Pr<Nd.We further investigated the effect of second growth rate on the thermionic emission properties and mechanical properties of the single-crystal.The growth rate has negligible effect on the thermionic emission and mechanical properties of(Ce0.7La0.1Pr0.1Nd0.1)B6 single-crystal,while those of other two crystals is increased firstly and then decreased.The optimal mechanical and thermionic emission properties belong to the growth rate reached to V=7 mm/h.In terms of the(Ce0.4La0.2Pr0.2Nd0.2)B6 single-crystal:when the voltage and work temperature reached to 1 kV and 1873 K,the thermionic emission current density was 33.373 A/cm2,the zero-field emission current density was 22.631 A/cm2,the work function was 2.703 eV the Vickers hardness was 29.8 GPa,the bending strength was 417.60 MPa,and the emission current density was 33.27%higher than that of CeB6 single-crystal.In terms of the(Ce0.25La0.25Pr0.25Nd0.25)B6 single-crystal:when the voltage and work temperature reached to 1 kV and 1873 K,the thermionic emission current density was 36.279 A/cm2,the zero-field emission current density was 25.877 A/cm2,the work function was 2.681 eV the Vickers hardness was 27 GPa,the bending strength was 655.74 MPa,and the emission current density was 44.884%higher than that of CeB6 single-crystal.Notably,the thermionic emission and mechanical properties of(Ce0.4La0.2Pr0.2Nd0.2)B6 and(Ce0.25La0.25Pr0.25Nd0.25)B6 single-crystal presents anisotropy.The order of the work function of each crystal plane is followed as:(100)<(310)<(210)<(110)<(111)while the order of the hardness of Vickers hardness is followed as:(100)>(310)>(210).