Factors Affecting the High Temperature and Pressure Adaptability of Barium Tungsten Electrode

Barium tungsten electrode (usually refers to tungsten-based cathode impregnated with barium salt) is widely used in high-power microwave tubes, high-pressure gas discharge lamps, plasma sources and other equipment that require high temperature and high pressure environment due to its excellent electron emission ability and certain high temperature resistance. Its high temperature and high pressure adaptability is subject to the interaction of multiple complex influencing factors, which can be mainly summarized as follows:

I. Material Properties

1. Tungsten Skeleton Performance:

1.1 Density and Porosity: The density and open porosity of the tungsten skeleton determine its mechanical strength, thermal conductivity and ability to accommodate emission active substances (barium compounds). High density provides better mechanical strength and thermal conductivity, but requires appropriate porosity to ensure effective storage and diffusion of barium source.

1.2 Grain Size and Structure: Fine grain structure usually improves the strength and creep resistance of the material. Grain growth at high temperature is one of the important reasons for failure.

1.3 Purity and Impurities: Impurities (such as C, O, Fe, etc.) will reduce the melting point of tungsten, promote grain growth, increase creep rate, affect the diffusion of barium and the formation of the emission layer, and seriously reduce the high temperature stability.

1.4 Creep Resistance: Under high temperature and high pressure, the ability of the tungsten skeleton to resist slow plastic deformation is crucial. Creep can cause deformation of the electrode structure, collapse of pores, and destruction of the emission layer.

1.5 Thermal Expansion Coefficient: The thermal expansion matching with the emission layer and other contact materials (such as sealing materials) affects the size of thermal stress and may cause cracking or peeling.

2. Emission Active Material (Barium Compound):

2.1 Composition and Ratio: Usually barium aluminates such as BaAl₂O₄, Ba₃Al₂O₆, Ba₅Al₄O₁₁. The evaporation rate, barium release characteristics (temperature, rate), and chemical stability of different components are different, which directly affect the cathode life and emission stability. Optimizing the ratio is the key to improving adaptability.

2.2 Evaporation/Consumption Rate: Barium compounds will continue to evaporate and consume at high temperatures. Evaporation too quickly will cause the active material to be exhausted prematurely, and the emission capacity will drop sharply. High-pressure environment may indirectly affect evaporation by changing the gas phase transmission or chemical reaction rate.

2.3 Diffusion Characteristics: The diffusion rate of barium atoms in the tungsten skeleton determines the speed of its replenishment from the storage area to the emission surface. Diffusion is too slow, the surface activity is insufficient; diffusion is too fast, and the storage layer is consumed quickly. High temperature accelerates diffusion.

2.4 Chemical Stability: Under high temperature, high pressure and specific atmosphere (such as residual gas, working gas), whether the barium compound will decompose, oxidize, or react with other substances to generate inert substances, resulting in inactivation.

3. Emission Layer Formation and Maintenance:

3.1 Surface Single Atomic Barium Layer: The excellent emission performance of the barium tungsten cathode depends on the low work function single atomic barium layer formed on the tungsten surface. High temperature and high pressure will accelerate the desorption of barium atoms (evaporation or ion sputtering) and destroy this layer structure.

3.2 Dynamic Balance: Maintaining the emission layer requires a dynamic balance between barium consumption (evaporation, sputtering, reaction) and barium replenishment (diffusion from the inside). High temperature and high pressure intensify consumption and destroy the balance.

II. Environmental Factors (High Temperature and High Pressure)

1. Temperature:

Temperature increase is the core driving force affecting all factors. It will increase the creep rate of the material and reduce the mechanical strength; increase the evaporation rate of barium compounds and the diffusion rate of barium atoms; accelerate the grain growth process; intensify the chemical reaction rate (such as oxidation, reaction with residual gas); increase electron emission, but may also cause overheating.

2. Temperature Uniformity: Temperature gradients inside or on the surface of the electrode will cause thermal stress, which may cause cracking.

3. Pressure:

3.1 Mechanical Stress: The high pressure environment directly applies huge external mechanical stress to the electrode, which synergizes with the creep caused by high temperature to accelerate structural deformation and failure.

3.2 Atmosphere Effect:

Residual Gas/Working Gas: High pressure means a higher density of gas molecules. These molecules may react chemically with the electrode surface (such as oxidation), physically adsorb, or cause ion sputtering. In particular, active gases such as oxygen, water vapor, and halogens are extremely harmful.

Ion Bombardment: In a discharge environment, high voltage is usually accompanied by high voltage. Gas molecules are ionized into positive ions, which bombard the cathode surface at high speed under the action of the electric field. The bombardment energy is stronger under high temperature and high pressure, causing severe sputtering erosion, directly stripping the barium atomic layer and tungsten material on the surface, which is one of the main mechanisms of cathode failure. Under high pressure, the mean free path of ions is shortened, but the ion density increases, and the bombardment effect is more complicated.

3.3 Gas Thermal Conduction: High-pressure gas has better thermal conductivity and may affect the temperature distribution of the electrode.

4. Atmosphere Composition:

As mentioned above, any gas impurities (O₂, H₂O, CO₂, N₂, H₂, halogens, etc.) in the environment may react with tungsten or barium under high temperature and high pressure to form oxides, carbides, nitrides or volatile compounds, poisoning the surface or consuming active substances.

Inert gases (such as Ar, He) are chemically inert in themselves, but their ion bombardment effect is enhanced under high pressure.

III. Structural Design and Manufacturing Process

1. Electrode Geometry and Size:

The shape affects stress distribution, heat distribution, and current density distribution. Sharp edges or small curvature radii are prone to overheating, electric field concentration, and ion bombardment.

Size affects heat capacity and heat dissipation capacity. Large-sized electrodes are more difficult to dissipate heat and have larger internal temperature gradients.

2. Manufacturing Process:

2.1 Powder Metallurgy: Tungsten powder particle size, pressing pressure, sintering temperature/time/atmosphere directly affect the density, porosity, grain size and purity of the tungsten skeleton.

2.2 Impregnation Process: Barium salt composition, purity, impregnation method (pressure, temperature, time), and heat treatment process after impregnation (decomposition, activation) determine the uniformity of impregnation, filling rate, morphology and distribution of active substances.

2.3 Surface Treatment: Surface finish, coating (such as iridium coating), etc. affect emission, sputtering resistance and anti-poisoning capabilities. Pollution control during the process is crucial.

IV. Working Conditions

1. Current Density:

The working current density directly affects the electrode heating (Joule heat) and ion bombardment intensity (in the discharge tube). Excessive current density leads to local overheating, accelerates evaporation, diffusion, creep and sputtering, and shortens the life.

2. Working Mode:

Continuous or pulsed operation? The peak power and temperature may be high under pulsed operation, but the average power is low. Thermal cycling (rapid temperature change) will produce thermal fatigue stress.

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