Refractory carbide powders

Once initiated, zirconium and carbon powders react exothermically in a vacuum or inert atmosphere to form zirconium carbide. With the greater availabiHty of relatively pure metal powders, this technique is coming into common use for the production of several refractory carbides. Zirconium carbide is not a fixed stoichiometric compound, but a defect compound with a single-phase composition ranging from ZrCQ to ZrCQ at 2400°C. 

According to the last mechanism an electrochemical active particle is carbon dioxide. Therefore direct electroreduction of carbon dioxide, dissolved in the salt melts must also give a carbon as a cathode product. It was confirmed in the [6, 7], but there were no any data about morphology and structure of obtained carbon powders. The electrodeposition of carbon from carbon dioxide was taken as the base process for high-temperature electrochemical synthesis (HTES) of refractory carbides [8, 9]. 

Silicon combines with carbon to form silicon carbide or carborundum, SiC, and forms are known that have the wurtzite and zinc blende structures. It is a very hard, tough material that is used as an abrasive and a refractory. The powdered material is crushed after mixing with clay and heated in molds to make grinding wheels. It is prepared by the reaction... 

Processing of Refractory Carbides and Nitrides (Powder, Bulk, and Fibers)... 

The bulk/monolithic and thermal spray processes for refractory carbides and nitrides are two-step operations which first require the preparation of powders. Powders are also used as such in grinding and polishing applications (mostly silicon carbide and boron carbide). 

Refractory carbide and nitride powders are usually produced by chemical reaction between the elements or compounds of the eluents. These processes are moderate in cost but do not generally produce materials of the highest purity and consistency. 

Refractory Carbides. The various chemical preparation reactions for refi tory carbide powders are summarized in Table 14.2.1 1... 

High-quality powders of refractory carbides and nitrides can be obtained by the more recent processing techniques of chemical-vapor deposition (CVD), RF-plasma torch, combustion synthesis, and sol-gel.l H l... 

Ultrafme refractory carbide and nitride powders can be produced in an inductively coupled radio-frequency (rf) plasma torch.a schematic drawing of a plasma torch for the production of silicon carbide powder is shown in Fig. 14.1. 

With the proper conditions, highly exothermic reactions, once initiated, will spread as a combustion wave throughout the reactants mix-ture. " These reactions, originally developed in Russia, are used to produce powders of almost all the refractory carbides and nitrides. 

The fabrication of refractory carbides and nitrides (and ceramics in general) entails two major steps (a) pressing the powder to the required shape and (b) sintering (or firing) it at high temperature. 

In the previous chapter, the processing of refractory carbides and nitrides in the form of powders, bulk/monolithic shapes, frbers, and whiskers was reviewed. These materials have one other major form, i.e., coatings, and the review of coating processes is the objective of this cluster. 

The coating material in the form of powder is metered into a compressed-gas stream and fed into the heat source where it is heated to its melting point and projected onto the substrate. Refractory carbides and nitrides have very high melting points and, at these temperatures, they are extremely reactive and must be sprayed in an inert atmosphere to avoid detrimental chemical reactions such as oxidation. 

Composite materials with Sn02. CdO, carbon, Ni, and refractory carbides as dispersoids are base materials of electrical (Tables 3.1-163, 3.1-164, and Fig. 3.1-213) [1.226,231,243]. Extruded powder composites show preferred alignment of the dispersoid particles along the rod axes. Silver-nickel flber composites are magnetic. Their coercivity increases with decreasing diameter of the Ni fibers [1.244]. 

SHS is particularly suited to the synthesis of refractory ceramic powders and compacts such as carbides of Ti, Si, Cr, Ta, and B, borides of Ti, V and Cr, nitrides of B, Ti, Al, Si, sihcides of Mo, Ti and V, or even more complex compounds such as YBa2Cu307-,. The thermodynamic basis of the process, the individual types of SHS processing techniques, and the equipment and post-synthesis processing to obtain powder compacts have been reviewed by Yukhvid (1992). 

Yas] Yasinskaya, G.A., The Wetting of Refractory Carbides, Borides and Nitrides by the Molten Metals , Sov. Powder Metall. Met. Ceram., (7), 557-559 (1966), translated from Poroshk. Metall, (7), 53-56 (1966) (Experimental, Phase Relations, 5)... 

Copper and silver combined with refractory metals, such as tungsten, tungsten carbide, and molybdenum, are the principal materials for electrical contacts. A mixture of the powders is pressed and sintered, or a previously pressed and sintered refractory matrix is infiltrated with molten copper or silver in a separate heating operation. The composition is controlled by the porosity of the refractory matrix. Copper—tungsten contacts are used primarily in power-circuit breakers and transformer-tap charges. They are confined to an oil bath because of the rapid oxidation of copper in air. Copper—tungsten carbide compositions are used where greater mechanical wear resistance is necessary. 

Besides the chemical industry, sihcon is used as a powder in the ceramics (qv) industry for the production of sihcon carbide and sihcon nitride parts (see Advanced ceramics). Sihcon powder is also used as an explosive for defense apphcations and in the refractory industry for plasma spraying with other oxide mixtures (see Refractory coatings). 

Iron carbide (3 1), Fe C mol wt 179.56 carbon 6.69 wt % density 7.64 g/cm mp 1650°C is obtained from high carbon iron melts as a dark gray air-sensitive powder by anodic isolation with hydrochloric acid. In the microstmcture of steels, cementite appears in the form of etch-resistant grain borders, needles, or lamellae. Fe C powder cannot be sintered with binder metals to produce cemented carbides because Fe C reacts with the binder phase. The hard components in alloy steels, such as chromium steels, are double carbides of the formulas (Cr,Fe)23Cg, (Fe,Cr)2C3, or (Fe,Cr)3C2, that derive from the binary chromium carbides, and can also contain tungsten or molybdenum. These double carbides are related to Tj-carbides, ternary compounds of the general formula M M C where M = iron metal M = refractory transition metal. 

Of a series of powdered refractory compounds examined, only lanthanum hexa-boride, hafnium carbide, titanium carbide, zirconium carbide, magnesium nitride, zirconium nitride and tin(II) sulfide were dust explosion hazardous, the 2 latter being comparable with metal dusts. Individual entries are ... 

Die compaction of simple shapes can be carried out at elevated temperature using carbide, superalloy, refractory metal, or graphite dies. An inert gas atmosphere or vacuum is often used to protect the die and/or the powder. For example, beryllium powder is compacted at about 1350°C in a graphite die under vacuum with pressures of 2-4 MPa. 

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