The properties required of metals for use in vacuum tube construction are rather numerous. In general, no one metal meets all the requirements, but each metal in turn has its distinctive advantages.
Mechanically, a metal to be useful in vacuum tube construction should have a strength and ductility that permit easy forming of electrode shapes. The strength must be retained at high temperature without excessive crystallization to avoid deformation during degassing and subsequent use. The stiffness and damping factor of the metal should be high, to reduce vibration effects.
Thermally, the coefficient of expansion should be relatively low and except for special applications, quite constant. Good thermal conductivity is generally sought. Depending upon the application, metals should have either a high reflectivity or a high thermal emissivity. The vapour pressure at degassing temperatures should be low, while the melting temperature itself should be well above the highest degassing or operating temperature.
Electrically, a moderate conductivity is desired. Too low a conductivity introduces appreciable resistance and attendant losses, while too high a conductivity makes spot welding difficult. Except for cathodes, the primary and secondary emission should be low. Except for shielding applications, the magnetic permeability should be low, and the metal should be one that is readily demagnetized by a magnetic field.
Chemical freedom from oxidation at high temperatures simplifies construction processes immensely. Resistance to corrosion, by various cleaning agents should be low. Most important of all, the metal should absorb only a small amount of gas and give this up easily when heated in vacuum.
In addition, materials should be-relatively inexpensive and generally available. Alloys having a wide range of physical characteristics as determined by their chemical content are especially useful.
In the construction of vacuum tubes the majority of small metal-to-metal joints are formed by spot welding. Basically the process of spot welding consists in passing a large current through the joint to be welded. The joint is heated by the large current density, of the order of thousands of amperes per square inch, to the point where the metals melt and dissolve into one another, forming a weld.
Spot-welding machines consist of a set of pointed jaws supported by a mechanical arrangement that brings the jaws together by the operation of a foot pedal. The materials to be welded are placed between the jaws, and pressure is applied by the foot pedal. Care must be taken in supporting the work between the jaws to see that current will flow from the jaws through the work and through the point to be welded. The jaws are connected to a step-down transformer that gives a large current through a closed circuit when the primary is closed by means of another foot pedal. For most operations the jaws are made of copper and because of their resulting high conductivity will have relatively little heat developed at their point of contact with the work. Where welding operations are at all critical, an electronic circuit should be used to control the amount of current and the time duration of current flow. Many welding operations require a current flow of hundreds of amperes for a fraction of a second. Not all metal combinations will spot-weld readily. Difficulties are encountered with metals of high conductivity, high melting temperature, and high oxidation tendencies.