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Diffusion processes in metals: How do gases "penetrate" metals?
In materials science, "diffusion" refers to the process by which molecules of a substance move from areas of high concentration to areas of low concentration, ultimately achieving uniform distribution within a medium. This phenomenon occurs not only in gases and liquids but also within metals.
1. Why Study Diffusion in Metals?
In the manufacture of vacuum electronic devices (such as electron tubes and filaments), the diffusion of gases within metal parts is particularly important.
This is because:
Residual gases in the metal can be released at high temperatures or under vacuum conditions, affecting the device's vacuum level;
A decrease in vacuum can lead to unstable electron emission performance and even device failure.
Therefore, during the manufacturing process, it is necessary to diffuse gases from the metal, using methods such as heating, to achieve "degassing" and achieve high vacuum and stable performance.
2. Gas-Metal Interactions
Most gases (except inert gases) react strongly with metals at high temperatures:
Some form compounds;
Others form solid solutions of the gas dissolved in the metal.
These dissolved or reactive gases must be completely removed before exhaust, otherwise they will affect vacuum quality.
3. Factors Affecting Diffusion
The diffusion rate of gases or atoms in metals is primarily affected by the following factors:
Temperature: The higher the temperature, the faster the diffusion;
Medium viscosity and grain size: The larger the viscosity, the slower the diffusion.
Therefore, during the degassing or exhaust process, metal components are often heated to accelerate gas diffusion.
4. Diffusion in Solids
Not only can gases diffuse, but diffusion can also occur between solids. For example, atoms of some metals form monoatomic layers on the surfaces of other metals, thereby changing the surface properties of the materials.
A classic example is:
When thorium oxide is doped into tungsten, at high temperatures, the thorium atoms diffuse onto the tungsten surface, forming a monoatomic thorium film. This film significantly reduces the electron work function, making the tungsten filament more susceptible to electron emission.
Summary
The diffusion process in metals is not only a fundamental physical phenomenon but also a crucial step in electronic device manufacturing. By controlling temperature and material properties, researchers can effectively remove gas impurities from metals or utilize diffusion effects to optimize the electronic properties of metal surfaces.
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