Metals used in Valve Manufacture
Taken from Vacuum Tubes by K Spangenberg (1948) {1001}
Properties
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 vapor 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.
Nickel
Nickel is the metal that is most extensively used in forming receiving-tube electrodes. It is easily drawn and formed. It stretches easily and does not exhibit any sharp break at its yield point. Its hardness and strength at high temperatures are good. It has thirteen times the mechanical damping factor of iron and molybdenum. It spot-welds well to almost all metals. Its expansion coefficient is nearly constant with temperature, and its thermal and electrical conductivity are good.
When polished, nickel has an emissivity which ranges from 5 to 20 per cent of that of a black body, i.e. it makes a good reflector. When carbon-coated, the thermal emissivity ranges from 80 to 94 per cent of that of a black body, i.e. it makes a good radiator.
Anodes formed of nickel are usually carbon-coated to increase their radiation. Vapor pressure is low at all but very high temperatures, 10¯6 mm of mercury at a red heat. The work function of nickel is high, 5 volts, but commercial nickel may have appreciable thermionic emission due to barium contamination. Alloying about 4.5 per cent manganese reduces both primary and secondary emission. Others of the desirable properties are likewise present. As a result, nickel is an ideal metal for tube construction in all applications except those where a high temperature is involved.
Copper
The outstanding physical characteristics of copper are its high thermal and electrical conductivity. Copper can be sealed to all glasses by the Housekeeper technique. It is extensively used as an anode material in water- and air-cooled tubes.
It is moderately porous and requires a thick wall to withstand atmospheric pressure when hot. Likewise, it oxidizes readily and so cannot be allowed to assume temperatures above a few hundred degrees centigrade. It must be used in the oxygen-free form in all applications that involve heating for red heat. Even within a vacuum, copper must be protected from high temperatures, for it softens and vaporizes at relatively low temperatures. Its high ductility and low yield point make it easy to draw, form, and spin.
Aluminum
Aluminum is easy to work and is fairly noncorrosive to other materials encountered in vacuum-tube construction. One valuable property is that it does not sputter easily. However, it melts at too low a temperature and absorbs too much gas to be very useful in sealed-off tubes.
Molybdenum
Molybdenum has most of the excellent properties of nickel except that it is somewhat harder to work and is more expensive. Its relatively high melting temperature and low vapor pressure make it useful in low-power transmitting tubes. It is readily spot-welded to iron or nickel but not to tungsten. It absorbs oxygen when heated to a dull red heat. Molybdenum is used in applications that involve temperatures in the range of 200 to 500°C.
Tantalum
Next to tungsten, tantalum has the highest melting temperature of all the metals. Its vapor pressure is very low. It is easily formed and drawn. The metal is expensive as a result of the relatively complicated vacuum processing required to put it into form suitable for vacuum-tube construction. It is extensively used in radiation-cooled transmitting tubes, where the electrodes are often run at a red heat. It has a getter action that causes it to absorb gases, particularly hydrogen, the maximum absorption occurring at 1000°C (cherry red). The gases that have been absorbed are given off again at temperatures of 1300°C and higher. Minimum temperature for getter action is approximately 800°C. Tantalum is also used as an emitter in applications requiring specially shaped cathodes. Its work function is lower than that of tungsten, with the result that its emission is greater at the same temperature. Tungsten can, of course, achieve higher emission because it can be heated to higher temperatures without melting.
Tungsten
Reference has already been made to some of the numerous applications of tungsten in vacuum-tube construction; as an emitter and filament wire and in some lead-sealing applications it has virtually no substitute. Its high melting temperature makes it especially useful in some vacuum-tube construction processes. It is used as a filament wire for silver-soldering operations. It is likewise used as a filament heater in numerous metal-evaporation processes. It is one of the few metals that can be used as a target in X-ray tubes. Numerous gauges and control devices make use of its large change of resistance with temperature.
Tungsten is not readily drawn or formed. It must be hammered or swaged into shape. As a result, it is principally available in wire or rod form. Tungsten has a pronounced crystalline structure, which is accentuated by heating.
Tungsten filaments therefore become brittle if overheated for appreciable periods of time. Tungsten is relatively inactive chemically, which reduces contamination problems. It is sometimes alloyed with molybdenum (W/Mo = 49:51> to give a material that is more workable than tungsten itself and yet retains a high melting temperature.
Spot Welding
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.