Metallic impurities, silicon carbides

In the drying of compound intermediates of refractory and reactive metals, particular attention is given to the environment and to the materials so that the compound does not pick up impurities during the process. A good example is the drying of zirconium hydroxide. After the solvent extraction separation from hafnium, which co-occurs with zirconium in the mineral zircon, the zirconium values are precipitated as zirconium hydroxide. The hydroxide is dried first at 250 °C for 12 h in air in stainless steel trays and then at 850 °C on the silicon carbide hearth of a muffle furnace. 

Semiconducting Properties. Silicon carbide is a semiconductor it has a conductivity between that of metals and insulators or dielectrics (4,13,46,47). Because of the thermal stability of its electronic structure, silicon carbide has been studied for uses at high (>500° C) temperature. The Hall mobility in silicon carbide is a function of polytype (48,49), temperature (41,42,45—50), impurity, and concentration (49). In n-type crystals, activation energy for ionization of nitrogen impurity varies with polytype (50,51). 

Furnaces are constructed using an induction heated crucible of graphite or silicon carbide, contained within a refractory lined vacuum vessel. The zinc condenser may be external, in the vacuum train, or may be contained within the lid of the containment vessel. Seals are by rubber O rings in water-cooled flanges and the ability to maintain vacuum is a critical aspect of efficient operation. Target operating pressure is around 80 mbar absolute. The final temperature of the retort bullion (impure silver) formed within the retort at the end of a batch is in the range of 1100 to 1200°C. The detection of the end point of the batch distillation is also an important aspect of operation and usually is determined by the vacuum pressure and the temperature of the residual metal. Typical performance is shown in Table 12.2. 

The volatile silicon monoxide which is first produced from silicon dioxide reacts with carbon in the upper, cooler part of the furnace to form silicon carbide. This reacts with carbon dioxide to form elemental silicon. The furnace is operated continuously and is rotated slowly. It is emptied of metallic silicon, in the form of compact chunks, every 1 to 2 hours. The natural impurities, mainly iron and aluminium, constitute about 2% of the product and are conducive to the synthesis of organic silicon chlorides. 

Carbides are binary compounds in which carbon is the more electronegative partner. We shall come across silicon carbide, SiC, again later when we look at the chemistry of silicon this is a very hard material, manufactured in large amounts as an abrasive known as carborundum. Its hardness can be accounted for by the fact that its crystal structure is the same as that of diamond, with alternate carbon atoms replaced by silicon (Figure 10.13). A major industrial use of SiC is in steel refining where its addition to the molten metal removes metal oxide impurities with production of CO and a silicate slag, which is skimmed off. 

The Badische AniKn- und Soda-Fabrik prepared a nitride of undetermined composition by heating a mixture of silica and carbon in an atm. of nitrogen. The reaction proceeds at a relatively low temp, if a hydroxide or salt of a metal be added. The product contains silicon nitride mixed with the nitride of the metal. The Badische Anilin- und Soda-Fabrik also removed many of the impurities—iron, carbon, silicates, carbides, silicides, and phosphides—by treatment with acids or mild oxidizing agents which do not affect the silicon nitride. A. S. Larsen and O. J. Storm prepared the nitride by the action of nitrogen on molten silicides—e.g. ferrosilicon. 

Ignition or explosive reaction with metals (e.g., aluminum, antimony powder, bismuth powder, brass, calcium powder, copper, germanium, iron, manganese, potassium, tin, vanadium powder). Reaction with some metals requires moist CI2 or heat. Ignites with diethyl zinc (on contact), polyisobutylene (at 130°), metal acetylides, metal carbides, metal hydrides (e.g., potassium hydride, sodium hydride, copper hydride), metal phosphides (e.g., copper(II) phosphide), methane + oxygen, hydrazine, hydroxylamine, calcium nitride, nonmetals (e.g., boron, active carbon, silicon, phosphoms), nonmetal hydrides (e.g., arsine, phosphine, silane), steel (above 200° or as low as 50° when impurities are present), sulfides (e.g., arsenic disulfide, boron trisulfide, mercuric sulfide), trialkyl boranes. 

Abrasive materials are usually classified into two groups, natural and manufactured ones. The natural abrasives are generally referred to as those that have been produced by the uncontrolled forces of nature and because of that, they can contain many impurities and vary in quality. Emery, corundum, quartz, flint, garnet, diamond, tripoli, diatomaceous earth, sandstone, pumice, and natural sharpening stones are some of them (Krar 1995 Jacobs 1928). On the other hand, artificial abrasives were first developed in the late nineteenth century and overcame the problems of impurities and inconsistencies, since their manufactore could be carefully controlled. Some manufactored abrasives are carbide of silicon, aluminum oxide, glass, and the metallic abrasives such as steel wool and steel shot and grit (Krar 1995 Jacobs 1928) (Table 1). 

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