| Vacuum electronic devices are widely used in civil and military fields.As the emission source of electrons,the performance of diffusion barium-tungsten cathode determines the performance of vacuum electronic devices.With the development of vacuum electronic devices to higher power and higher frequency,the performance requirements of barium-tungsten cathode are also improved.As one of the key components of barium tungsten cathode,porous W matrix plays the role of active material carrier and activator.Therefore,the structure of porous W matrix has a direct impact on the emission performance of barium-tungsten cathode.In this paper,the non-gradient porous W matrix was prepared by spark plasma sintering(SPS)technology using narrow particle W powder(NPW)and spherical W powder(SW)as raw materials.The effects of powder properties on the preparation process,pore characteristics and mechanical properties of tungsten matrix were studied.On this basis,porous W matrix with gradient porosity distribution was prepared by using gradient mold and narrow particle W powder as raw material.The pore characteristics of gradient W matrix and the effects of the mold gradient and holding temperature on the pore characteristics of W matrix were studied.Finally,barium-tungsten cathodes were prepared by gradient and non-gradient matrix.The effect of matrix with porosity gradient distribution on the thermal emission performance of barium-tungsten cathode was studied.Finally,the influence mechanism of gradient structure on cathode performance is briefly analyzed.The details are as follows:The W matrix with uniform porosity distribution was successfully prepared by SPS technology using NPW powder and SW powder as raw materials.The results show that under the same preparation conditions,compared with NPW powder matrix,SW powder matrix has higher open porosity,larger average pore size and larger Vickers hardness value.Based on this,using NPW powder as raw material,porous W matrix with low porosity and small pore size at the emitter end and gradually increasing porosity along the thickness direction of the matrix was successfully prepared by using gradient mold.The results of the contrast experiment show that the higher the holding temperature and the smaller the mold gradient,the smaller the porosity gradient change rate of the prepared tungsten matrix,and the lower the porosity at each thickness position.The thermal emission performance of the cathodes prepared by different matrix was tested.The results show that the emission performance of the gradient matrix cathode is better than that of the non-gradient matrix cathode at different working temperatures.When the working temperature is 1100°C,the zero-field current density of the gradient matrix cathode is 3.56 A/cm~2,and the work function is 2.13 e V.Under the same conditions,the zero-field current density of the non-gradient matrix cathode prepared according to the same theoretical porosity is only 2.36 A/cm~2,and the work function is 2.18 e V.XPS analysis of the cathode emission surfaces showed that the coverage of Ba on the cathode emission surface of the gradient matrix cathode was higher,which reduced the cathode work function and is conducive to electron emission.Therefore,the W matrix with gradient porosity distribution can reduce the cathode evaporation while ensuring the storage capacity of barium-aluminate thereby improving the cathode emission performance. |
