Hydrogen silicid

Silicides of groups I and 2 are generally much more reactive than those of the transition elements (cf. borides and carbides). Hydrogen and/or silanes are typical products e.g. ... 

Limitations of Plasma CVD. With plasma CVD, it is difficult to obtain a deposit of pure material. In most cases, desorption of by-products and other gases is incomplete because of the low temperature and these gases, particularly hydrogen, remain as inclusions in the deposit. Moreover, in the case of compounds, such as nitrides, oxides, carbides, or silicides, stoichiometry is rarely achieved. This is generally detrimental since it alters the physical properties and reduces the resistance to chemical etching and radiation attack. However in some cases, it is advantageous for instance, amorphous silicon used in solar cells has improved optoelectronic properties if hydrogen is present (see Ch. 15). 

The nozzle of original design was fabricated from a niobium alloy coated with niobium silicide and could not operate above 1320°C. This was replaced by a thin shell of rhenium protected on the inside by a thin layer of iridium. The iridium was deposited first on a disposable mandrel, from iridium acetylacetonate (pentadionate) (see Ch. 6). The rhenium was then deposited over the iridium by hydrogen reduction of the chloride. The mandrel was then chemically removed. Iridium has a high melting point (2410°C) and provides good corrosion protection for the rhenium. The nozzle was tested at 2000°C and survived 400 cycles in a high oxidizer to fuel ratio with no measurable corrosion.O l... 

Silicon, like carbon, is relatively inactive at ordinary temperatures. But, when heated, it reacts vigorously with the halogens (fluorine, chlorine, bromine, cmd iodine) to form halides and with certain metals to form silicides. It is unaffected by all acids except hydrofluoric. At red heat, silicon is attacked by water vapor or by oxygen, forming a surface layer of silicon dioxide. When silicon and carbon are combined at electric furnace temperatures of 2,000 to 2,600 °C (3,600 to 4700 °F), they form silicon carbide (Carborundum = SiC), which is an Importeint abrasive. When reacted with hydrogen, silicon forms a series of hydrides, the silanes. Silicon also forms a series of organic silicon compounds called silicones, when reacted with various organic compounds. 

Contact with liquid hydrogen fluoride causes violent evolution of silicon tetraflu-oride. (The same is probably true of metal silicides and other silicon compounds generally.)... 

The flammability and explosive hazard of ferrosilicon powder is increased substantially during grinding in a vibratory mill [1], Explosion hazards from air-hydrogen,—acetylene, or—propane mixtures formed during preparation of ferrosilicon containing alkaline earth additives are attributed to contact of barium or magnesium carbide or silicide additive with atmospheric moisture [2],... 

Palladium and platinum are also used as carrier lifetime controllers in Si. Pd creates an electron trap at Ec - 0.22 eV and a hole trap at Ev + 0.32 eV in Si (Chen and Milnes, 1980). Pt induces a single electron trap at Ec + 0.28 eV (Chen and Milnes, 1980). All of these levels are passivated by atomic hydrogen (Pearton and Haller, 1983) suggesting that hydrogen might be profitably used during silicide formation to passivate electrically active levels near the silicon-silicide interface. 

Defects in a SCR, which is present under reverse bias, can be tested in a similar way. Figure 10.6 c shows the same wafer as in Fig. 10.6 e after removal of the oxide and under cathodic polarization in the dark. Hydrogen bubbles caused by the dark current now decorate nickel silicide precipitates that short-circuit the SCR. Nickel precipitates are known to increase the dark current of a p-type Si electrode under reverse bias by orders of magnitude [Wa4]. If the bias is increased the copper silicide precipitates also become visible, as shown in Fig. 10.6 d. This method, like defect etching (Fig. 10.4f), is only sensitive to precipitated metals. Metals that stay in solution, like iron, do not show up in defect mapping and have to be determined by other methods, for example diffusion length mapping. 

If a mixture of metallic sodium and aluminium silicide is placed in water, hydrogen is evolved, with the production of sodium silicate and aluminium hydrate, in accordance with the following equation. —... 

We also observed growth of SiNW from Au nanoparticles. In this case, H2 was also necessary. Although Au silicides can form at moderate temperatures and can also be reduced by hydrogen to Au nanoparticles and silane, the growth temperature for SiNW was still above 1000°C. It seems that silane was produced in both Au and Co catalytic growth cases. 

FIGURE 10.23. Proposed catalytic processes to make SiNW from Co nanoparticles and hydrogen. Si wafers act as the source of silicon. Silane is produced, which then reacts with Co or Co silicide catalysts to make SiNW. [Chem Comm 2005]—Reproduced by permission of The Royal Society of Chemistry, (ref 54)... 

A schematic of the proposed growth model is shown in Fig. 10.23. In this model, Co nanoparticles play a dual catalytic role. On the one hand, they catalyze silane formation by reacting first with silicon to form Co silicides, and then react with hydrogen to form silane while being reduced to Co metal. The second role of Co nanoparticles is their classic catalytic ability of making nanowires by first dissolving the silane and precipitating out Si nanowires. 

With hydrogen, a series of silanes having a general formula SinH2n+2 are obtained. Sdicon forms binary silicides with several metals when heated at very high temperatures. 

Silicon hydrides can be prepared by several methods. A few methods are outlined below. Silane and its higher homologs can be made by treating magnesium silicide, Mg2Si with 20% hydrochloric acid in an atmosphere of hydrogen. An equation for monosilane is given below ... 

Another preparative method involves treating magnesium silicide with ammonium bromide in liquid ammonia in a current of hydrogen. The process forms 70 to 80% yield of mono- and disilanes. The reaction is shown below ... 

Berzelius heated a mixture of silica, iron, and carbon to a very high temperature, and obtained iron silicide. When he decomposed this with hydrochloric acid, silica was precipitated, and the amount of hydrogen evolved was in excess of the iron, indicating that some other metal must have been present (9). Berzelius finally showed in 1824 that this other seemingly metallic substance was derived from the silica, and succeeded in preparing the amorphous form of it by two methods. In the first of... 

Silicon carbide is comparatively stable. The only violent reaction occurs when SiC is heated with a mixture of potassium dichromate and lead chromate. Chemical reactions do, however, take place between silicon carbide and a variety of compounds at relatively high temperatures. Sodium silicate attacks SiC above 1300°C, and SiC reacts with calcium and magnesium oxides above 1000°C and with copper oxide at 800°C to form the metal silicide. Silicon carbide decomposes in fused alkalies such as potassium chromate or sodium chromate and in fused borax or cryolite, and reacts with carbon dioxide, hydrogen, air, and steam. Silicon carbide, resistant to chlorine below 700°C, reacts to form carbon and silicon tetrachloride at high temperature. SiC dissociates in molten iron and the silicon reacts with oxides present in the melt, a reaction of use in the metallurgy of iron and steel (qv). The dense, self-bonded type of SiC has good resistance to aluminum up to about 800°C, to bismuth and zinc at 600°C, and to tin up to 400°C a new silicon nitride-bonded type exhibits improved resistance to cryolite. 

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. 

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