Silicon self-bonded

Silicon Nitride. SiUcon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, siUcon nitride parts. a-SiUcon nitride, used in the manufacture of Si N intended for hot pressing, can be obtained by nitriding Si powder in an atmosphere of H2, N2, and NH. Reaction conditions, eg, temperature, time, and atmosphere, have to be controlled closely. Special additions, such as Fe202 to the precursor material, act as catalysts for the formation of predorninately a-Si N. SiUcon nitride is ball-milled to a very fine powder and is purified by acid leaching. SiUcon nitride can be hot pressed to full density by adding 1—5% MgO. 

Self-baking electrodes, 12 305, 755 Self-bonded reaction-sintered silicon nitride, 17 210, 211 Self-catalyzed polyols, 25 464 Self-cleaning materials, 22 108-127 problems and outlook for, 22 123-124 surface characteristics of, 22 108-109... 

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. 

Figure 2 is a schematic diagram of the two-section reactor body and accessories. The fluid bed section was made of self-bonded silicon carbide, 16 inches high by 6% inches o.d. with a recessed flange. The recess accommodated a 120-mesh porosity silicon carbide gas distribution plate. The nickel manifold assembly was topped by a heavy support flange. This manifold sup-... 

Silicon Nitride. Silicon nitride is manufactured either as a powder as a precursor for the production of hot-pressed parts or as self-bonded, reaction-sintered, silicon nitnde parts. 

Point Defects. Point defects are defined as atomic defects. Atomic defects such as metal ions can diffuse through the lattice without involving themselves with lattice atoms or vacancies (Figure 9), in contrast to atomic defects such as self-interstitials. The silicon self-interstitial is a silicon atom that is bonded in a tetrahedral interstitial site. Examples of point defects are shown in Figure 9. 

Slip-casting of technical ceramics has been steadily introduced over the past 60 years or so, and now it is standard practice to cast alumina crucibles and large tubes. The process has been successfully extended to include silica, beryllia, magnesia, zirconia, silicon (to make the preforms for reaction-bonded silicon nitride articles) and mixtures of silicon carbide and carbon (to make the preforms for a variety of self-bonded silicon carbide articles). Many metallics and intermetallics, including tungsten, molybdenum, chromium, WC, ZrC and MoSi2, have also been successfully slip-cast. 

Method 2 produces what is usually referred to as self-bonded SiC by a process essentially the same as that developed for nuclear applications [2] and known as Refel silicon carbide. Refel silicon carbide contains a finely dispersed residual silicon phase accounting for between 5 and 10% of the total volume, the precise amount depending upon the porosity of the original preform. In the case of self-bonded silicon carbide elements the residual silicon is volatilized away by a final heat-treatment at over 2500 °C when recrystallization occurs resulting in grain growth and further bonding between primary and secondary carbide phases. 

Forrest, C.W., Kennedy, P. and Shennan, J.V. (1972) The fabrication and properties of self-bonded silicon carbide bodies, in Special Ceramics 5, The British Ceramic Research Association, Stoke-on Trent, UK. 

Silicon carbide, widely employed as an abrasive (carborundum), is finding increasing use as a refractory. It has a better thermal conductivity at high temperatures than any other ceramic and is very resistant to abrasion and corrosion especially when bonded with silicon nitride. Hot-pressed, self-bonded SiC may be suitable as a container for the fuel elements in high-temperature gas-cooled reactors and also for the structural parts of the reactors. Boron carbide, which is even harder than silicon carbide, is now readily available commercially because of its value as a radiation shield, and is being increasingly used as an abrasive. 

Our interest in this area has been in synthesizing a variety of silanetriols where silicon is bonded to carbon, nitrogen, or oxygen in order to assess their relative stabilities as well as the differences in their reactivities. The C-bonded silanetriol tBuSi(0H)3 (12) was synthesized from the commercially available /BuSiCb (Scheme 3) [23]. The bulky t-butyl group afforded the desired steric protection needed to avoid the self-condensation. However, it should be mentioned that the hydrolysis of tBuSiCla in the presence of KOH leads to the isolation of the primary condensation product of the /-butylsilanetriol, [tBuSi(OH)2]20 [2]. 

The Crystal FT filters are made of 100% self-bonded, re crystallised silicon carbide wuth multi-layer SiC membranes. The filter carrier is characterised by high porosity and an open three-dimensional pore structure, Crystar says modules with 4CFT and 5CFT filters showed time and energy savings of 36%. 

The reactions of compounds with silicon-oxygen bonds continue to attract interest. A two-dimensional Si NMR study of the species present in 1.5 mol dm of potassium silicate indicates the presence of 22 different species, clear evidence of the complexity of such systems. The kinetics of the hydrolysis and self-condensa-tion reactions of (MeO)3SiMe have been studied.Iodine catalyzes the forward and back reactions in equation (21). Radicals derived from... 

Self-bonded silicon carbide materials in molded form and of high SiC content have been available only from the late 1960s onwards. These are being increasingly used for structural components in mechanical engineering, and have proved to be highly successful for use under extreme abrasive, corrosive, and thermal conditions due to their excellent mechanical, chemical, and physical properties. 

Silicon Carbide (SiC) (self bonded) room temp. 59.5x10 ... 

Figure 14.2 Schematic flow-sheet of the production of self-bonded, reaction-sintered silicon nitride...

Walker, D.E.Y., 1968. The electrolytic etching of self-bonded silicon carbide. Prakt. Metallogr. 376. 

Silicon carbide refiactories are classified on the basis of the bonds used. Associated-type bonds are oxide or silica, clay, silicon oxynitride and silicon nitride, as well as self-bonded. The dense materials contain 85-99% SiC the clay-bonded contain 75-80% SiC and the semisilicon carbides are stUl lower in SiC content. 

Hardness testing of the refractories is not an everyday procedure, yet it is very useful to remember that the hardness of self-bonded silicon carbide ceramics is in the range of 20-25 GPa, the hardness of nitride-bonded silicon carbide refractories is 16-20 GPa, and the hardness of alumina ceramics is 12-16 GPa, which is three to four times higher than with dense alumina silica dense refractories. 

Because of die rigidity and directionality of die covalent bonds die energies of self-diffusion have been found to be higher diaii diose of metals. In die case of silicon, it appears drat a furdier complication is drat an intersti-tialcy mechanism predominates above 1000°C. Below diis teiiiperamre, bodi elements appear to self-diffuse by atom-vacancy exchange as for die metals. 

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