Research On Composition Design And Emission Mechanism Of Tungsten Thermionic Emission Materials

Hot electron-emitting materials are mainly used in the field of vacuum electronics.The most widely used and most effective are still Th-W electronemitting materials,but the Th metal is a radioactive element that is harmful to humans and the environment.Therefore,researchers at home and abroad have focused on the development of new radiation-free W-electrode materials to replace Th-W electron-emitting materials.Based on this,in this work,Sc-W hot electronemitting materials are studied,and W-based hot electron-emitting materials containing Sc are prepared using a powder metallurgical preparation method,with the auxiliary addition of a small amount of Re to improve their high-temperature performance.The electronic structure properties of the materials arc calculated via first-principles calculations using the VASP software for different doping elements.Established the route of element type and composition design-sample preparation and tissue characterisation-performance testing and mechanism study.Elemental types and composition design:The electronic structure properties of the material for different doping elements are calculated via first-principles calculations using the VASP software,and the largest increase in energy is observed in the case of doping with La.The defect formation energy is the lowest for doping with Sc.After doping with Sc and La,the charge density in the structure of the system increases,the bond between atoms changes from metallic to covalent,and the electron binding of the nucleus is weakened,which is conducive to improving the electron emission performance.Energy band calculations show that when the Sc atoms combine with the surrounding O atoms through sp3 hybrid orbitals,a highenergy impurity level,namely the 3d energy level of Sc,is introduced in the forbidden band region,reducing the band gap;this results in the electrons requiring less energy to jump into the conduction band as well as an enhancement in the electron emission capability.The addition of the reinforcing element Re is to some extent beneficial to the electron emission performance of the material,and the energy band structure is converted from a direct band gap to an indirect band gap with the addition of Re.The calculation results of the work function show that after co-doping with Sc and Re,the smooth curves on both sides of the potential curve bend,the electrostatic potential increases as a whole,and electron transfer occurs between the surface W atoms and the Sc and Re atoms to form a dipole,which reduces the work function.The theoretical calculation results show that Sc and Re are the best alloy doping elements.Sample preparation and tissue characterisation:The evolution of the structure and composition upon heating from room temperature to 1300℃ is studied,and the formation of the active layer on the surface of the electron-emitting materials is analyzed in detail.The results show that the surface of the sample is relatively flat at low temperatures,with no obvious protrusions or pits;at high temperatures,clear grain boundaries appear;as the temperature continues to rise,the surface composition of the material begins to change,with the Sc oxide particles melting or decomposing,thus forming an active layer on the surface.When the temperature rises to 1300℃,the active layer on the surface of the sample is severely damaged;at this point,the surface does no longer have the ability to promote electron emission due to the absence of active elements.When the sample is heated from room temperature to 1100℃,the surface active layer is formed on the surface of the material,and the Sc/W content ratio increases from 0.12 to 0.42.The composition of the surface active layer remains in a relatively stable state,and even at high temperatures,the outward diffusion of the active material from the interior of the substrate.With the further increase in the treatment temperature,the Sc/W content ratio starts to decrease;at this time,the outward diffusion of the active material inside the substrate is no longer able to compensate for the loss of the surface active material,and eventually the ability to emit electrons is completely lost.The elemental content of the surface on which the active layer has formed is measured over a range of depths,and the results show that as the etching time increases,the composition stabilizes,the surface active layer is gradually consumed,and the elemental concentration gradient disappears.Performance testing and mechanistic studies:Based on the study of the effect of the temperature change on the surface of the sample,the zero-field current emission density and electron escape work are measured for samples with different compositions and after different heat-treatment temperatures to investigate the electron emission mechanism.The results show that the addition of Sc increases the zero-field current emission density from 4.5 to 6.8 mA and reduces the electron escape work from 2.96 to 2.65 eV.Emission performance measurements on the samples treated at 1100℃ show a further increase in the zero-field current emission density and a reduction in the electron escape work from 2.65 to 2.2 eV for samples on which a surface active layer has formed.As the treatment temperature is increased to 1300℃,the zero-field emission current density drops sharply to 2.08 mA and the electron escape work increases to 3.23 eV due to the destruction of the surface active layer.The mechanism through which the active elements promote electron emission is as follows:After the formation of the surface active layer,the atoms in the active layer exist in the form of oxygen-deficient oxides and metallic superfine particles.