Oxidation boron carbides

In thick samples, a boron oxide/boron carbide crust has been detected on the surface of the polymer. This inorganic surface layer has a shielding effect on the inner polymer layers, further enhancing the thermal stability of the material. Poly(m-carborane-siloxane)s have therefore been considered as surface coatings for organic materials, providing protection from erosion effects. 

The boron carbide process can also start from blends of metal carbides, metal hydrides, boron oxide, boron carbide and carbon black ... 

Natural abrasives can be sand, walnut shells, etc. Artificial abrasives include silicon carbide, aluminum oxide, boron carbide, and boron nitride. Artificial abrasives are generally superior in uniformity to naturally occurring abrasives and provide more consistent abrasive finishing processes. 

Except for siUca and natural abrasives containing free siUca, the abrasive materials used today are classified by NIOSH as nuisance dust materials and have relatively high permissable dust levels (55). The OSHA TWA allowable total dust level for aluminum oxide, siUcon carbide, boron carbide, ceria, and other nuisance dusts is 10 mg/m. SiUca, in contrast, is quite toxic as a respkable dust for cristobaUte [14464-46-1] and tridymite [15468-32-3] the allowable TWA level drops to 0.05 mg/m and the TWA for quartz [14808-60-7] is set at 0.1 mg/m. Any abrasive that contains free siUca in excess of 1% should be treated as a potential health hazard if it is in the form of respkable dust. Dust masks are requked for those exposed to such materials (see Industrial hygene). 

Hafnium Boride. Hafnium diboride [12007-23-7] HfB2, is a gray crystalline soHd. It is usually prepared by the reaction of hafnium oxide with carbon and either boron oxide or boron carbide, but it can also be prepared from mixtures of hafnium tetrachloride, boron trichloride, and hydrogen above 2000°C, or by direct synthesis from the elements. Hafnium diboride is attacked by hydrofluoric acid but is resistant to nearly all other reagents at room temperature. Hafnium dodecaboride [32342-52-2] has been prepared by direct synthesis from the elements (56). 

The third control is by use of a fixed burnable poison. This consists of rods containing a mixture of aluminum oxide and boron carbide, included in the initial fuel loading using the vacant spaces in some of the fuel assembhes that do not have control clusters. The burnable poison is consumed during operation, causing a reactivity increase that helps counteract the drop owing to fuel consumption. It also reduces the need for excessive initial soluble boron. Other reactors use gadolinium as burnable poison, sometimes mixed with the fuel. 

These are made of boron carbide ia a matrix of aluminum oxide clad with Zircaloy. As the uranium is depleted, ie, burned up, the boron is also burned up to maintain the chain reaction. This is another intrinsic control feature. The chemical shim and burnable poison controls reduce the number of control rods needed and provide more uniform power distributions. 

Research-grade material may be prepared by reaction of pelleted mixtures of titanium dioxide and boron at 1700°C in a vacuum furnace. Under these conditions, the oxygen is eliminated as a volatile boron oxide (17). Technical grade (purity > 98%) material may be made by the carbothermal reduction of titanium dioxide in the presence of boron or boron carbide. The endothermic reaction is carried out by heating briquettes made from a mixture of the reactants in electric furnaces at 2000°C (11,18,19). 

Preparation. The simplest method of preparation is a combination of the elements at a suitable temperature, usually ia the range of 1100—2000°C. On a commercial scale, borides are prepared by the reduction of mixtures of metallic and boron oxides usiag aluminum, magnesium, carbon, boron, or boron carbide, followed by purification. Borides can also be synthesized by vapor-phase reaction or electrolysis. 

Preparation. Boron carbide is most commonly produced by the reduction of boric oxide with carbon in an electric furnace between 1400 and 2300°C. In the presence of carbon, magnesium reduces boric oxide to boron carbide at 1400—1800°C. The reaction is best carried out in a hydrogen atmosphere in a carbon tube furnace. By-product magnesium compounds are removed by acid treatment. 

Boron carbide from boron oxide and carbon Calcium silicate from lime and silica Calcium carbide by reaction of lime and carbon Leblanc soda ash... 

Boron carbide from boron oxide and carbon... 

Carbide-based cermets have particles of carbides of tungsten, chromium, and titanium. Tungsten carbide in a cobalt matrix is used in machine parts requiring very high hardness such as wire-drawing dies, valves, etc. Chromium carbide in a cobalt matrix has high corrosion and abrasion resistance it also has a coefficient of thermal expansion close to that of steel, so is well-suited for use in valves. Titanium carbide in either a nickel or a cobalt matrix is often used in high-temperature applications such as turbine parts. Cermets are also used as nuclear reactor fuel elements and control rods. Fuel elements can be uranium oxide particles in stainless steel ceramic, whereas boron carbide in stainless steel is used for control rods. 

Boron carbide (p. 149) is a most useful and economic source of B and will react with most metals or their oxides. It is produced in tonnage quantities by direct reduction of B2O3 with C at 1600° a C resistor is embedded in a mixture of B2O3 and C, and a heavy electric current passed. 

Chemical Resistance. Boron carbide resists oxidation in air up to 600°C due to the formation of a film of B2O3. Its chemical resistance is generally excellent although it reacts with halogens at a high temperature. 

Reduction of a metal oxide or other metal compound using C, B or boron carbide... 

There followed, after the initial study of imidogen (42), papers on boron carbide (43), nitric oxide and its ions (44), as well as fluoronitride and its cation (45). Results from these calculations are still useful since they offer mutually consistent representations of potential curves for a considerable number of states. This successful campaign was furthermore beneficial to the development at Aarhus since several Danes were given the opportunity to participate in the work and to learn to appreciate Yngve s way of scientific quest. 

Magnesium diborate Magnesium perborate Boron oxide Titanium diboride Boron carbide... 

Beryllium oxide Zirconium oxide Aluminum nitride Boron carbide Silicon carbide and nitride Tungsten carbide Beryllium carbide... 

Boron carbide is prepared by reduction of boric oxide either with carbon or with magnesium in presence of carbon in an electric furnace at a temperature above 1,400°C. When magnesium is used, the reaction may be carried out in a graphite furnace and the magnesium byproducts are removed by treatment with acid. 

The other abrasive agents used are tin oxide, chromic oxide, sand, carbides (silicon carbide and boron carbide), zirconium silicate, zinc oxide, garnet, rouge (fine red powder of iron oxide), kieselgurh, tripoli, magnesium oxide, hydrated silica etc. 

Both lithium chlorate and perchlorate have been proposed as oxidizers in explosive formulations [131]. Li nitrate/K nitrate/Na nitrate eutectics (23.5/60.2/16.3) have been proposed by Kruse as oxidizers in illuminating flare formulations [132]. Similarly, LiC104 has been proposed as an oxidizer in obscurant formulations with or boron carbide(B4C) or Si. The obscuring power is mainly due to the presence of hygroscopic LiCl in the aerosol. The formulation B/LiC104 (60/40) possesses the best performance [133-135] compared with Si/Li C104 (35/65) and B4C/Li C104 (30/70) formulations. 

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