Copper, Steel, metallized ceramics
Vacuum tubes are widely used in industry, due to the fact that electrons can move freely within the vacuum. Depending on the application the electron beam can be used for different purposes.
X-ray imaging in industrial and medical applications are example applications of vacuum-tube technology: a beam of high-energy, free moving electrons generated in vacuum is aligned by special arrangement of focusing electrodes and hits a target, resulting in an X-ray beam that can be used to penetrate organic and non-organic materials.
Another example is travelling wave tubes (TWT): emitted electrons are used for signal amplification, e.g. radar waves.