Tungsten furnace

As discussed earlier, tungsten furnaces, as first proposed by Sychra et al. [175a], are useful for the determination of refractory carbide forming elements, which in the case of a graphite furnace may suffer from poor volatilization, but they are more... 

Fig. 13. Tungsten furnace h— tungsten tube s—sight glass st—radiation shields v—vacuum bell z—power input the necessary vacuum connections to the base are not shown.

The products prepared in the above manner are ground as finely as possible and compressed under 2000 kg./cm into rods 3 x 40 or 5 X 40 mm. Successful molding usually requires the addition of 2-5% of metal powder. The rods are embedded in nitride powder (to prevent formation of an oxide coating) and presintered in a small tubular tungsten furnace (cf. Part I, p. 40) at about 2300°C in a stream of Ng the small amount of free metal is converted to nitride in the process. Since the reaction is usually accompanied by considerable shrinkage of the rods and concomitant appearance of porosity, the material must be repulverized, remolded and resintered. This procedure is repeated two to four times, until the presintered rods exhibit some constancy of density. 

Figure 9.8 illustrates a problem that can occur with any furnace element—sample contamination. The sample is a-SiC, which was heated in a tungsten furnace for 12 hours at 1300°C. The dark features are tungsten particles evaporated onto the surface during heat treatment. 

Since detailed chemical structure information is not usually required from isotope ratio measurements, it is possible to vaporize samples by simply pyrolyzing them. For this purpose, the sample can be placed on a tungsten, rhenium, or platinum wire and heated strongly in vacuum by passing an electric current through the wire. This is thermal or surface ionization (TI). Alternatively, a small electric furnace can be used when removal of solvent from a dilute solution is desirable before vaporization of residual solute. Again, a wide variety of mass analyzers can be used to measure m/z values of atomic ions and their relative abundances. 

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. 

Incandescent Lamps, Electronic Tubes, and Resistance Elements. Articles fashioned in any form from molybdenum and tungsten usually fall within the bounds of powder metallurgy. These metals normally are first produced as a powder. Both molybdenum and tungsten are used as targets in x-ray tubes, for stmctural shapes such as lead and grid wires in electron tubes, and as resistance elements in furnaces. 

The same properties that make molybdenum metal effective in high temperature furnace appHcations make it useful as support wires for tungsten filaments in incandescent light bulbs and as targets in x-ray tubes. 

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. 

High density tungsten alloy machine chips are recovered by oxidation at about 850°C, foUowed by reduction in hydrogen at 700—900°C. Typically, the resultant powders are about 3-p.m grain size and resinter readily. There can be some pickup of refractory materials used in furnace constmction, which must be controUed. This process is important commercially. Eor materials that may be contaminated with other metals or impurities, the preferred recovery process is the wet chemical conversion process used for recovery of tungsten from ores and process wastes. Materials can always be considered for use as additions in alloy steel melting. 

The WC leaving the furnace is light gray with a bluish tinge. It is generally caked and must be broken up, milled, and screened before use. It should contain about 6.1—6.25 wt % total C, of which 0.03—0.15 wt % is in the free, unbound state. The theoretical C-content is 6.13 wt %. Annual world production of tungsten monocarbide is 15,000—18,000 metric tons. 

The hardness of carbides can only be deterrnined by micro methods because of britdeness, the usual macro tests caimot be used. Neither can the extremely high melting points of the carbides be readily deterrnined by the usual methods. In the so-called Priani hole method, a small hoUow rod is placed between two electrodes and heated by direct current until a Hquid drop appears in the cavity. The temperature is determined pyrometricaHy. When high temperature tungsten tube furnaces are used, the melting point can readily be estimated by the Seger-type cone method. The sample may also be fused in a KroU arc furnace and the solidification temperature determined. 

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