Boric acid, boron carbide

Neutron absorption coefficient (cm ) Boric acid, boron carbide 3.08 0.06 Encapsulation of nuclear materials... 

A number of boron chemicals are prepared directly from boric acid. These include synthetic inorganic borate salts, boron phosphate, fluoborates, boron ttihaHdes, borate esters, boron carbide, and metal aHoys such as ferroboron [11108-67-1]. 

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 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. 

Hexane (99% n-hexane) was obtained from Aldrich. Silica was obtained as a 30% Colloid (LUDOX-HS, 30 wt% SiOj) from DuPont. Aluminum hydroxide, boron oxide, and sodium metaborate were obtained from Alfa Products. Boric acid was obtained from J. T. Baker. Commercial samples of mordenite and zeolite Y were obtained from the Norton and Union Carbide Companies, respectively. A specific example which illustrates the typical procedure used for preparing mordenite and zeolite Y from gels is given below. 

Boron Trichloride. Boron trichloride is prepared commercially by the chlorination of boron carbide (equation 15). Direct chlorination of boric acid or a sodium borate in the presence of carbon is an alternative method. Most of the boron trichloride produced is converted to filaments of elemental boron by chemical vapor deposition (CVD) on tungsten wire in a hydrogen atmosphere. Numerous laboratory preparations of boron trichloride have been reported. One of the most convenient is the halogen exchange reaction of aluminum chloride with boron trifluoride or a metal fluoroborate. 

Sodium borates are the most important industrial boron compounds. They are mainly utilized as such, but are also used as starting materials (in addition to calcium borates) for the manufacture of industrially interesting boron compounds (boric acid, di-boron trioxide, inorganic borates, refractory boron-derivatives, boron carbide, boron... 

In the industrial manufacture of boron carbides, boron(lll) oxide(l) or boric acid (2) are reacted with carbon in resistively heated furnaces at 2400°C (as in SiC-manufacture) ... 

Boron carbide (B4C) is one of the hardest known materials with excellent properties of low density, very high chemical and thermal stability, and high neutron absorption cross-section. Bulk B4C is conventionally synthesized by high temperature (up to 2400 °C) reactions, such as the carbothermal reduction of boric acid or boron oxide. Nanocrystalline B4C was solvothermally synthesized in CCI4 at 600 °C (Reaction (32)). 

A mixture of acetylene black, or sugar with ethylene glycol, is heated with boric acid at 1873-2073K. The resulting boron carbide is fine grained 1-5/tm (cf. ref. 3 in... 

Sodium borohydride is marketed in powdered or pellet form, and in solution, for use in fuel cells. Boron nitride can withstand temperatures of up to 650°C (1,202°E) when subjected to high pressures and temperatures, it forms cubic crystals whose hardness rivals that of diamond. Boron carbide, produced by reacting coke and boric acid at 2,600°C (4,712°E), is a highly refractory material and one of the hardest substances known. It has both abrasive and abrasion-resistant applications, and is used in nuclear shielding, see ALSO Davy, Humphry Gay-Lussac, Joseph-Louis Nuclear Chemistry. 

Gaseous boric acid removes a boron oxide film. The rates of formation and removal of the B2O3 film are equal at 550-600°C in air with a dew point of 25-70°C and at 650°C with a dew point of 88°C. At higher temperatures, B2O3 is formed at a higher rate than it is removed by the interaction with water vapor. Therefore, at low temperatures boron carbide is oxidized with water vapor more rapidly than with dry air, at high temperatures the situation is quite the opposite [2]. 

Another reason for the poor sinterability is the extraordinarily high vapor pressure of boron oxides and suboxides. Since boron carbide powders are generally coated by a B2O3 layer [172] which quickly reacts to form boric acid, H3BO3, in humid atmosphere, vapor phase reactions are active at higher temperatures, in particular above 1500"C, providing a fast transport of boron compounds. Redox reactions such as... 

Boron carbide is either prepared from boron ores or from pure boron. The process involves the reduction of a boron compound. Usually, boron carbide is obtained by reacting boric acid or boron oxide and carbon at ca. 2500°C in an electric-arc furnace. 

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