Tungsten heating elements

The construction materials of all the heated parts of the holder are of utmost importance, as not all materials remain stable when exposed to gas atmospheres of varying composition at elevated temperatures. For example, the tungsten heating element breaks when used in oxidizing gases, and heaters constructed from other materials (e.g., steel) and with different thermal connections to the cup must be applied instead. Moreover, it is also important to choose the materials for construction and for the holder design so that drift of the cup (and drift of the sample) is minimized when the temperature of the holder is changed. 

Radiation shields in high-temperature furnaces (W). Shields closest to the tungsten heating element are made of tungsten sheets. 

The short-wave IR radiation can only be generated by an electric IR heater employing a tungsten heating element. A tungsten filament has small mass as well as low electrical resistance, which allows heavy current flow and very rapid heat up and cool down. The rate of response of various types of radiant heat sources can be an important criterion in the selection of a proper source for coating drying applications. 

Chrome—nickel alloy heating elements that commonly ate used in low temperature furnaces are not suitable above the very low end of the range. Elements commonly used as resistors are either silicon carbide, carbon, or high temperature metals, eg, molybdenum and tungsten. The latter impose stringent limitations on the atmosphere that must be maintained around the heating elements to prevent rapid element failure (3), or the furnace should be designed to allow easy, periodic replacement. 

Tungsten with the addition of as much as 5% thoria is used for thermionic emission cathode wires and as filaments for vibration-resistant incandescent lamps. Tungsten—rhenium alloys are employed as heating elements and thermocouples. Tantalum and niobium form continuous soHd solutions with tungsten. Iron and nickel are used as ahoy agents for specialized appHcations. 

The first commercial use of tantalum was as filaments ia iacandescent lamps but it was soon displaced by tungsten. Tantalum is used ia chemical iadustry equipment for reaction vessels and heat exchangers ia corrosive environments. It is usually the metal of choice for heating elements and shields ia high temperature vacuum sintering furnaces. In 1994, over 72% of the tantalum produced ia the world went iato the manufacturiag of over 10 x 10 soHd tantalum capacitors for use ia the most demanding electronic appHcations. 

Unalloyed tungsten has several major applications. An important use is in the electric lamp filaments for light bulbs. Also, it is used as electrodes in arcwelding, in heating elements for high-temperature furnaces, in electron and television tubes, in glass-to- metal seals, and in solar energy devices. 

About 90 percent of all tungsten alloys are used in mining, construction, and electrical and metal-working machinery. These alloys are used to make high-speed tools heating elements in furnaces parts for aircraft and spacecraft equipment used in radio, television, and radar rock drills metal-cutting tools and similar equipment. 

Many of the first papers which discussed the use of (selective) CVD of tungsten for IC applications used conventional hot wall tube CVD reactors [Broadbent et al.44, Pauleau et al.45, Cheung47]. This type of reactor was and still is the workhorse in IC fabs. Excellent films such as TEOS based oxides, thermal silicon-nitride and poly-silicon can be grown in such equipment. Hot wall tube reactors are suitable for these films because such materials stick very well to quartz tubes and are quite transparent to IR radiation of the heating elements. Thus neither particle nor temperature control is a problem. One other major advantage is that high throughputs are typically obtained. 

The thermal conductivity detector (TCD), which was one of the earliest detectors for gas chromatography, still finds wide application. This device consists of an electrically heated source whose temperature at constant electric power depends on the thermal conductivity of the surrounding gas. The heated element may be a fine platinum, gold, or tungsten wire (Figure 31 -9a) or, alternatively, a small thermistor. The electrical resistance of this element depends on the thermal conductivity of the gas. Twin detectors are ordinarily used, one located ahead of the sample injection chamber and the other immediately beyond the column alternatively, the gas stream can be split. The detectors are incorporated into two arms of a simple bridge circuit (see Figure 31 -9) such that the thennal conductivity of the carrier gas is... 

FIGURE 7.9. Spacer parts, heating elements, and heat radiation shields of tungsten and molybdenum for a high-temperature furnace height of the heating zone, approximately 440 mm. By courtesy of Plansee AG, Austria. 

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