Silicides density

Vanadium Subsilicide, V2Si, is obtained by fusing a mixture of vanadium trioxide, V2Os, and silicon, with the addition of either a large excess of vanadium or carbon or copper. The carbide or copper alloy produced is decomposed at the temperature employed.11 The silicide forms metallic prisms, of density 5-48 at 17° C., the m.pt. of which is higher than in the ease of the disilicide. It is attacked by the halogens, hydrogen chloride gas, and fused sodium or copper, but hydrochloric acid, nitric acid and sulphuric acid are without action. 

Silicon is a very hard but brittle element having a melting temperature of 1422°C and a density of 2.40. This element is fairly reactive toward the halogens and solutions of strong bases such as potassium hydroxide. Silicon reacts less readily with oxygen to form silicon dioxide and with other elements similarly to form a class of binary compounds known as silicides. 

Cobalt Monosilidde, CoSi, is formed as prismatic needles by heating cobalt and copper silicide in the electric furnace.7 It melts at 1300° C. in hydrogen, and has a density of 6-30 at 20° C. When heated in fluorine it incandesces, yielding fluorides. Chlorine decomposes it only at a dull red heat, but it dissolves slowly in aqua regia and more rapidly in concentrated hydrogen chloride. It melts at 1393° C.6... 

Cobalt Disilicide, CoSi2, results when cobalt is heated in the electric furnace with excess of silicon or with a mixture of copper silicide and silicon.8 It forms dark crystals, probably belonging to the cubic system density 5 3 hardness 4-5. Sulphur has no action on it, and oxygen at 1200° C. only effects a superficial oxidation. It incandesces in fluorine if gently wanned chlorine attacks it at 300° C., and bromine and iodine at dull red heat. Concentrated hydrogen... 

Di nickel Silicide or Nickel Subsilicide, Ni2Si, is obtained by heating nickel and 10 per cent, of silicon in a carbon crucible in an electric furnace.3 It is a stable, steel-grey substance, density 7-2 at 17° C. Fluorine attacks it with incandescence at ordinary temperatures, and chlorine at red heat. It dissolves readily in hydrogen fluoride, less so in hydrogen chloride aqua regia decomposes it completely. 

The freezing-point curve of nickel and silicon indicates the existence of several other silicides, namely, NisSi, Xi3Si2, NiSi, and Xi2Sis.4 A tefranickel silicide, NijSi, has been isolated.6 Density curves,6 however, apparently indicate the existence of three silicides only, namely, Ni2Si, NiSi, and Ni3Si2. 

Ruthenium Silicide, RuSi, results on heating a mixture of finely divided ruthenium and crystallised silicon in the electric furnace.3 The product is crushed and treated successively with alkali and a mixture of hydrofluoric and nitric acids. The silicide together with carborundum remains behind. The two may be separated with methylene iodide on account of the high density of ruthenium silicide, namely, 5-4. 

Palladium Monosilicide, PdSi, is obtained as brilliant bluish grey fragments on treating any Pd-Si alloy, containing above 60 per cent, of silicon, with dilute potash. The free silicon dissolves, leaving the silicide as residue. Density 7-31 at 15° C. 

Platinum Monosilicide, PtSi, may be obtained by igniting a mixture of finely divided silicon and platinum sponge at a high temperature. On treating the melt with potassium hydroxide solution, excess of silicon is removed, leaving a residue of monosilicide.5 When recrystallised from fused silver silicide, the latter being removed by extraction with sodium hydroxide and nitric acid in succession, the monosilicide is obtained as prismatic crystals, melting at about 1100° C., and of density 11-63 at 15° C. 

Lithium silicide, LigSi2.—By heating excess of lithium with silicon, and expelling the uncombined metal at 400° to 500° C., the silicide is obtained as a very hygroscopic, dark-violet, crystalline substance 8 of density 1 12. It is a very reactive product and a powerful reducer. With concentrated hydrochloric acid it yields spontaneously inflammable silicoethane, Si2H3, of which it may be considered a derivative. 

Iron silicides are, in general, hard and brittle, thus forming good abrasives. They are white or grey in colour, and may take a good polish. Unattacked by air and water, they are but slowly decomposed by acids, except hydrofluoric, which readily dissolves them. In general their density falls with rise of silicon content. 

Diferro silicide, as obtained by these methods, occurs as small prismatic crystals, possessed of metallic lustre, magnetic, and of density 7 00 at 22° C. Hydrogen fluoride attacks it readily, and aqua regia decomposes it, yielding silica and ferric chloride. Hot potassium hydroxide is without action on it. It is decomposed by chlorine with incandescence. 

Iron silicide or Iron monosilidde, FeSi, is prepared by heating a mixture of copper silicide and iron filings in an electric furnace. The resulting product is treated with 50 per cent, nitric acid to decompose any copper silicide, and washed. Obtained in this way, iron silicide occurs as tetrahedral crystals, with a brilliant metallic lustre they are extremely hard, and have a density of 6-17 at 15° C. Fluorine attacks them at ordinary temperatures, whilst chlorine and bromine decompose them at red heat. Molten alkali hydroxides attack the silicide, as also do fused mixtures of the alkali nitrates and carbonates.11... 

Two different types of reactors are used depending on the product synthesized. The first type can maintain pressures up to 150 atm, and is widely used for production of powders in gasless and gas-solid systems. Carbides, borides, silicides, intermetallics, chalcogenides, phosphides, and nitrides are usually produced in this type of reactor. The second type, a high-pressure reactor (up to 2000 atm), is used for the production of nitride-based articles and materials, since higher initial sample densities require elevated reactant gas pressures for full conversion. For example, well-sintered pure BN ceramic with a porosity of about 20-35% was synthesized at 100 to 5000-atm nitrogen pressure (Merzhanov, 1992). Additional examples are discussed in Section III. 

The samples prepared at 500 °C, 550 °C and at 600 °C by RDE of 0.6 nm Cr without Si cap layer were analyzed by AFM to characterize the CrSi2 nanoislands. Cr deposition at 500 °C silicide islands have bimodal size distribution (10-20 nm and 40-80 nm) as it is shown in Fig. la. The bimodal size distribution indicates that significant secondary nucleation occurs during Cr deposition at 500 C in parallel to the growth of the Islands. At 550 °C a narrow CrSii island size distribution was observed as it is shown in Fig. lb. Islands had the maximal density (4T0 cm ), minimal sizes (15-20 nm) and heights (2-4 nm). The silicon surface is 30-50% covered by nanoislands. 

PIBT of Cr-implanted Si results in full crystallization of the Si layer at fluencies up to MO cm. Precipitates of chromium silicide with semiconductor type of absorption (probably CrSi2) are formed at the depth more than 20 nm by data of optical and Raman spectroscopy. The increase of implantation fluence up to 6-1 o cm results in an increase of the precipitate density up to 6T0 cm increase of roughness (up to 6.9 nm). The subsequent Si growth was non-epitaxial. 

The densities of the stoichiometric compositions were determined to 4.32 g/cm3 for Ti5Si3 and 4.07 g/cm3 for TiSi2. Both compounds possess lower densities than titanium base alloys. A good correlation between binding energies, melting temperatures and microhardness of the investigated silicides is shown in table I. 

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