Refractory metal filaments

In the case of hot-filament CVD, refractory metal filaments (e.g., W, Ta, Re, etc.) are electrically heated to very high temperatures (between 2000 and 2700°C) to produce the necessary amount of atomic hydrogen that is necessary for the reasons mentioned above for the synthesis of diamond. Except for combustion flame CVD, hot-filament CVD is considered the simplest of all of the methods and also the most inexpensive. Plasma-jet and laser-assisted CVD methods rely on a plasma torch or laser to attain the very high temperatures that are needed to... 

In thermal-ionization mass spectrometry (TIMS), samples consist of fg-ng quantities of chemically separated and purified analyte dissolved in a small volume (typically 1-10 pi). The solution is deposited on a refractory metal filament (e.g., high-purity W or Re), where it is evaporated to dryness. The filament is then heated to temperatures of 1,000-2,500°G in the ion source by resistive heating. If the ionization potential of the analyte is low compared to the work function of the filament, some of the analyte atoms will be ionized and emitted from the filament surface. TIMS ion sources commonly incorporate a second filament arranged... 

Refractory metals are associated with powder metallurgy because these metals are not easily melted. Therefore in smelting the ores, the metal is recovered in powder form rather than melted. Refractory metals are used mainly to produce filament wire for incandescent lamps. 

Filaments are usually refractory metals such as tungsten or iridium, which can sustain high temperatures for a long time (T > 3000 K). The lifetime of filaments for electron sources can be prolonged substantially if an adsorbate can be introduced that lowers the work function on the surface so that it may be operated at lower temperature. Thorium fulfills this function by being partly ionized, donating electrons to the filament, which results in a dipole layer that reduces the work function of the tungsten. In catalysis, alkali metals are used to modify the effect of the work function of metals, as we will see later. 

A needle source consists of a hairpin filament (M80 pm diameter), usuafly of a refractory metal such as tungsten, with a short length of smaller diameter (M25 pm) wire spot-welded to it, Figure lb. The tip of the latter wire, the emitter, is electrochemically etched to a point with a radius of curvature at the apex of 2-5 pm the etching technique for tungsten has been described in detail by others (7,29). As quickly as possible after the assembly is thermally cleaned under vacuum (n<10 " Pa), the emitter is dipped into a molten pool of liquid metal and then withdrawn. If done correctly, the junction formed by the bend in the filament and the emitter wire will hold a small bead of metal, and the emitter will appear shiny from the thin film of metal on its surface. 

The most important polymeric matrices are linear and cross-linked polyesters, epoxy resins and linear and cross-linked polyimides the most important reinforcements are high-performance polymeric fibres and filaments (for polymeric composites), filaments of refractory metals and inorganic materials (E-glass, A12C>3, B, BN, SiC and Carbon) and whiskers (fibrillar single crystals of A1203, B4C, WC, SiC and C, exclusively for reinforcement of metals). 

Two methods for the evaporation of precursors may be employed - resistance heating and electron beam collision. The first method employs a simple alumina crucible that is heated by a W filament. Temperatures as high as 1,800°C may be reached inside the chamber, which is enough for some metals or metal salts to vaporize. Deposition rates for this method are 1-20 A s . The use of an electron beam to assist in the precursor evaporation results in temperatures on the order of 3,000°C, being more suited for the deposition of refractory metals/alloys and metal oxides such as alumina, titania, and zirconia. Since the temperature of the chamber interior is much higher than the walls, the gas-phase ions/atoms/molecules condense on the sidewalls as well as the substrate this may lead to film contamination as the nonselective coating flakes off the chamber walls. 

Use Additive to tungsten- and molybdenum-based alloys, electronic filaments, electrical contact material, high-temperature thermocouples, igniters for flash bulbs, refractory metal components of missiles, catalyst, plating of metals by electrolysis and vapor-phase deposition. 

Use Incandescent filaments, abrasive, cermet component, high-temperature electrical conductor, refractory, metal cladding, cutting tool component. 

The flash filament experiment as first described by Becker and Hartman (14) has since been used extensively in studies of the adsorption of gases onto refractory metals, particularly in association with other techniques. The basic method is to allow gas introduced at a known input rate to adsorb for a measured time onto a previously cleaned wire or ribbon. The gas is then desorbed by heating the sample, and the resulting pressure bursts monitored. The pressure versus time curve is referred to as a desorption spectrum, as illustrated in Fig. 4 and 5. Sticking probabilities can then be obtained from the relative adsorption times and desorption quantities. Methods of analysis of these desorption spectra (15, 16) and of the variation in thermal resolution by different heating schedules such as linear or reciprocal increase in temperature with time, have been discussed extensively by a number of authors... 

E. Saunders, M. Weinstein, A. I. Mlavsky, Radiant Energy Reactor Technique for the Deposition of SiC onto Quartz Filaments, Proc. Conf. Chemical Vapor Deposition of Refractory Metals, Alloys, and Comp., American Nuclear Society, Hinsdale, IL, 1967, pp. 1X1-21. 

This jar contains a cmcible composed of a filament of refractory metal, generally tungsten. This filament, whose diameter is 0.05 mm, is shaped to form a cone made of an unjointed turn of coil. 

Evaporation by resistance heating usually takes place from aboatmade with a refractory metal, a ceramic crucible wrapped with a wire heater, or a wire filament coated with the evaporant. A current is passed through the element, and the generated heat heats the evaporant. It is somewhat difficult to monitor the temperature of the melt by optical means due to the propensity of the evaporant to coat the inside of the chamber, and control must be done by empirical means. 

Electrothermal atom cells have changed radically since their inception in the late 1950s. The majority of electrothermal devices have been based on graphite tubes that are heated electrically (resistively) from either end. Modifications such as the West Rod Atomizer (a carbon filament) were also devised but were later abandoned. Tubes and filaments made from highly refractory metals such as tungsten and tantalum have also been made, but they tend to become brittle and distorted after extended use and have poor resistance to some acids. Their use continues, however, in some laboratories that need to determine carbide-forming elements. For example, silicon reacts with the graphite tube to form silicon carbide, which is both very refractory and very stable. The silicon is therefore not atomized and is lost analytically. Use of a metal vaporizer prevents this. 

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