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>>The materials of tungsten heater
The most important heater material is tungsten. In tungsten's natural formis usually obtained from the minerals wolframite (Fe, Mn)WO4 and schoelite (CaWO4). Because 70% of our original tungsten resources have been depleted, methods have been found for purifying relatively poor-grade ores.
The quality of tungsten heater wire depends upon many factors, and the materials and manufacturing process are carefully controlled. The powder used to produce the ductile metal is initially of high purity. For the purposes of inhibiting grain growth, however, very small quantities of partly volatile alkali silicates and non-volatile oxides such as silica, alumina, thoria, or calcia are added to the tungsten powder.
After being mixed, the tungsten powder is pressed into bar ingots. Ingots are then sintered at a temperature of approximately 3000C. The time-temperature relationship at which the ingots are sintered is carefully controlled to assure a dense bar which, in turn, determines many of the properties of the finished tungsten heater wire. At a temperature of about 1300C, the sintered bar is worked into a rod by mechanical hammering or "swaging". During this process the cross-sectional area is reduced by 15% each time the rod is run through a successively smaller die. After the swaging process, the tungsten rods are drawn hot through a tungsten carbide die. The final, smaller wire sizes are drawn through highly polished diamond dies. As the wire is drawn and reduced in area, its tensile strenth increases to as much as 500,000 lbs/sq.in.
The electrical, as well as the chemical and physical properties of tungsten have been intensively investigated. Although the resistivity of tungsten is not as high as that of some other materials, its high melting point of 3400C makes it a desirable heater material.
At room temperature, small variations in resistivity are found among tungsten wires, depending upon their previous treatment. Despite these small variation, however, tungsten wires display similar electrical resistivities at high temperatures. This characteristic is important because it enables the mass production of reproducible tungsten heaters having a uniform current and voltage rating.
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