Study On Thermionic Emission Property And Mechanism Of Tungsten Cathode Material Doped With Rare-Earth Oxides

A composite method of liquid-liquid mixing － freezing and drying － SPSsintering was designed and used to prepare a novel kind of tungsten cathode materialdoped with rare-earth oxides. The emission property of the new type material wasstudied, and the microstructures of the powder and sintered products were studied bymeans of x-ray diffraction analysis, scanning electron microscopy, field emissionscanning electron microscopy and transmission electron microscopy. The primarycontradiction of electron emission was analyzed in the view of atomic structure, andthe principle of thermionic emission was discussed. The thermionic emissionmechanism of tungsten cathode material doped with rare-earth oxides was studied. Compared with conventional tungsten cathode material doped with rare-earthoxides, FDL-W, FDC-W, FDY-W have higher zero-field emission current densitiesand lower effective work functions. Microstructure studied by FESEM and TEM shows that the particle size ofFDL-W, FDC-W, FDY-W powder is about 30nm, so that the designed method ofpreparing nano-powder through freezing and drying is an excellent technique and therare-earth elements distribute more uniformly in both the powder and sinteredproducts also by microstructure observation. Tungsten, rare-earth tungsten, rare-earth Molybdenum and barium tungstencathodes all have elements with high energy valence electrons such as W, Mo, Y, La,Ce, Ba. The outer high energy valence electrons of these elements get weaker nucleusaffinity and can easily break away from the force field of nucleus, so that the highenergy valence electrons are the source of thermionic emission. A kind of cathode material with good emission quality should have higher energyvalence electrons and lower work function. Rare-earth ultrafine particles are thesource of thermionic emission, which cooperate with oxygen properly to form lowerwork function emission centers. The emission stability of nanocomposite rare-earth oxide－tungsten cathodematerial was studied. It is considered that the emission stability is decided by two -II-ABSTRCTequilibria: one is the equilibrium between the evaporation and the supply of surfaceactive matter, the other is the equilibrium between the emission and the supply ofsurface electrons. The emission property of nanocomposite tungsten cathode material doped withrare-earth oxides can be described according to the "Patch model". r2/2σ2 is smallerwhen jm increases, which shows this kind of material has both higher current densityand more uniform distribution of emission. Nanocomposite tungsten cathode material doped with rare-earth oxides has highemission ability, good emission stability and uniformity as well as operatingtemperature lower about 350K compared with thoriated tungsten cathode material.Hence, nanocomposite tungsten cathode material doped with rare-earth oxides is anexcellent cathode material.