Study On The Struture And Valence State Of The Rear Earth Tungsten Active Layer

With the development and application of automatic welding,thermal spraying,thermal plasma chemistry,laser technology and gas discharge light source,people in the professional field have increased their interest in special tungsten electrodes once again,material researchers in various countries are working on the manufacture and emission mechanism of various types of electrode materials.Rear earth tungsten electrodes have a range of successful applications in the field of vacuum electronic devices and gas discharge light sources,it replaced the Thorium-tungsten electrode to a certain degree,and achieved significant research results.However,most of the previous reports focused on composition optimization and electrode manufacture methods,the study of the surface structure and valence state of the rear earth tungsten electrode under actual working conditions is rarely reported.Based on the previous research results of the research group,this article selects the commercial cerium tungsten electrode product as the research object and heats the electrode sample in the ultra-high vacuum chamber,simulate the working environment of the electrode.The surface structure of the cathode active layer,the valence states of the components,the types of phases,and their variation with temperature were studied to provide suggestions for the actual manufacture and use of rare earth tungsten cathodes,which has a strong theoretical significance.The paper draws the following main results:the cerium-tungsten cathode active layer material is composed of tungsten metal and a small amount of cerium oxide.The active layer exhibits continuous undulating bulge-like morphology,whose average diameter is 80nm,and the active layer thickness is about 6nm.On the surface of the cathode that forms the active layer,cerium oxides are present in the form of aggregation regions that are present at the junction of the tungsten crystal grains,in addition to the surface of the tungsten matrix,and the size of the aggregation region is between 400 nm and 800 nm.The cerium element in the active layer exists at both Ce³⁺and Ce⁴⁺valence states.Quantitative analysis showed that the relative contents of Ce³⁺and Ce⁴⁺were 65%and 35%respectively.And the corresponding oxygen element also exists in the active layer mainly in the form of O^2-,which combines with cerium to form the cerium oxides.The active layer on cathode surface is gradually formed as the working temperature of the cathode increases,and significant consumption or even disappears at a higher operating temperature.Therefore,for a particular cathode material,there is a corresponding optimal operating temperature.In the process of forming the active layer of the cerium tungsten cathode,the cerium oxide diffuses from the aggregation area to the surface of the tungsten matrix,especially after the heating temperature reaches 800°C.,the diffusion phenomenon is more obvious.When the cathode temperature rises from room temperature to normal operating temperature,the content of Ce⁴⁺drops from 49%to 35%compared to the content of Ce³⁺.The cerium oxide shows a tendency to be reduced,and the observation of the oxygen on the surface of the sample also shows the corresponding result,that is,the decrease in the oxygen content of the lattice combined with Ce⁴⁺.This shows that during the formation of the active layer,an oxygen-deficient cerium oxide was formed on the surface of the cerium-tungsten cathode.Ce-O covalentity decreases and ionicity increases in this compound,among them,the high energy valence electrons are more likely to escape from the cathode surface into vacuum to form electron emission under the effect of high temperature and electric field,that is,the oxygen-deficient cerium oxide in the cathode surface active layer constitutes a good electron emitter,which reduces the work function of the cathode.