| Barium-tungsten cathode(Ba-W)is widely used in modern microwave vacuum electronic devices because of its high pulse current emission density,long service life,good ion bombardment resistance and anti-poisoning ability.Ba-W cathode,as the core component of vacuum electronic devices,can provide electrons for gyrotron,high current linear induction accelerator,klystron and other devices.Porous tungsten matrix is an important part of barium-tungsten cathode.The active materials are stored in the porous matrix and reacts with tungsten matrix at high temperature to enhance the thermal electrons emission.In this thesis,the porous matrixes were prepared by Spark plasma sintering(SPS)using the spherical and the irregular tungsten powders as raw materials,respectively.The influences of the particle morphology and porosity on the matrix properties were studied.The barium-tungsten cathodes were prepared after the porous matrixes were impregnated with the active materials,and the effects of the particle morphology,matrix porosity and activation time on the thermal emission properties of the Ba-W cathode were studied.The thermal emission performances of the spherical and the traditional cathode were compared.Finally,the influences of copper(Cu)infiltration process on the property of the matrix and cathode were investigated.SPS is successfully employed to prepare porous tungsten matrix with high percentage of open pores and uniform pore diameter distribution;The optimum sintering process parameters can be determined by orthogonal test,and the optimum parameters are T(sintering temperature)=1600℃,P(pressure)=15 MPa and t(holding time at T)=1min for the porous matrix with porosity of 23%.Compared with irregular matrix,the spherical matrix has more uniform pore diameter distribution and the larger median pore size(1.41μm)and lower values of Vickers hardness and bending strength.For the irregular or the spherical cathode,with the increase of the porosity,thermal emission current density of Ba-W cathode firstly increases and then decreases.The optimum porosity for the best thermal emission current density is about 26%.The zero-field emission current density of the spherical cathode with porosity of 26.3%is 4.02A/cm~2 at 1050℃,and the work function is 2.03 eV.The zero-field emission current density of the irregular cathode with porosity of 26.8%is 2.97 A/cm~2,and the work function is 2.06 eV.Furthermore,the underheated property test showed that the deviation temperature of the spherical cathode is 956℃,which is lower than that of the irregular cathode,and the normalized current density at low temperature is higher,all of which indicate that the spherical cathode has better thermal emission property at low temperature which indicated it is suitable for low-temperature cathode preparation.The Vickers hardness and bending strength of the matrix are greatly improved after Cu infiltration,which indicates that Cu infiltration can effectively improve the mechanical properties of the matrix and the matrixes are easily machined after Cu infiltration.The results of thermal emission of the cathodes prepared by Cu infiltration process show that the zero-field current and work function of the cathode are 3.55A/cm~2,2.04 eV(spherical)and 2.06 A/cm~2,2.10 eV(irregular)at 1050℃,respectively.The emission property is kept at the same level as that of the cathode without Cu infiltration,indicating that Cu infiltration will not affect the thermal emission property of the cathode. |
