Barium tungsten electrode is a high-performance thermal electron emission cathode widely used in vacuum electronic devices (such as high-power microwave tubes, X-ray tubes, etc.). Its electron emission mechanism involves complex physical and chemical processes, and the core lies in the single atomic layer coverage of barium (Ba) atoms on the surface of porous tungsten substrate.
1. Basic Structure and Preparation of Barium Tungsten Electrode
Porous tungsten substrate: a porous structure formed by sintering tungsten powder.
Impregnation of active substances: impregnation of barium salts in the pores.
Activation process: heating under high temperature and vacuum environment, the active substances decompose to produce free barium.
2. Electron Emission Mechanism of Barium Tungsten Electrode
The core of the efficient emission of barium tungsten cathode is that barium atoms form a "single atomic dipole layer" on the tungsten surface, which significantly reduces the work function of tungsten.
Barium has an extremely low ionization energy (~5.21 eV) and is a strongly positive element. When barium atoms are adsorbed on the surface of tungsten (W), their valence electrons are partially transferred to the vacancies in the d-band of tungsten. The barium atoms become positive ions (Ba?) due to the loss of electrons, and the tungsten surface becomes negatively charged due to the gain of electrons, forming a "surface dipole layer". The electric field generated by this dipole layer points to the vacuum, offsetting part of the potential barrier that needs to be overcome for the electrons inside the metal to escape.
3. Supply and Dynamic Balance of Barium
Maintaining a stable monoatomic layer requires a continuous supply of barium.
Diffusion path of barium: The impregnated salt continuously decomposes at high temperature to produce barium vapor, which diffuses to the emission surface through the pores of the porous tungsten matrix, and the barium atoms are adsorbed on the tungsten surface to form a monoatomic layer.
Dynamic balance: The surface barium atoms will evaporate or react with the residual gas and be lost, and the internal barium vapor will continue to diffuse and replenish to maintain the surface coverage.
4. Key Factors Affecting the Electron Emission Performance of Barium Tungsten Electrodes
4.1 Pore structure: Pore size and connectivity affect the diffusion rate and uniformity of barium.
4.2 Impregnation salt composition:
Aluminate cathode (4BaO·CaO·Al?O?): Traditional and reliable, with moderate barium activity.
Scandinate cathode (BaO-Sc?O?-W): Contains nano-tungsten particles, provides more diffusion channels and adsorption sites, has higher emission current density and anti-poisoning ability.
4.3 Working temperature: Too low temperature → barium diffusion is slow and coverage is insufficient; too high temperature → barium evaporates too quickly, salt consumption is accelerated, and life is shortened.
4.4 Vacuum environment: Residual gases (O?, H?O, CO?) will react with the surface barium to form oxides and carbonates, poisoning the cathode, resulting in increased work function and decreased emission.
