Boron, vapor

There was 10 mole % excess metal present. The calculated ARTs are presented in Table 1 for the Zr system, as well as the Ti and Hf analogs, which are discussed in the next section. The theoretical total pressures generated in all systems are greater than 1 atm. The composition of the gas phase varies from system to system, though it is composed primarily of metal and boron vapor in all cases suggesting that the ART is controlled by the highly endothermic vaporization of the metal and boron (A//vap = 450 to 650 kJ/mol). 

Uses Catalyst in organic synthesis source of boron compounds refining of alloys soldering flux electrical resistors extinguishing magnesium fires in heat-treating furnaces mfg. of diborane semiconductor dopant boron vapor deposition raw material (boron fibers)... 

Allen et al [130,131] showed that boron vapor increased the modulus and conductivity (Table 5.16) by increasing the crystallinity and helping to prevent shear in the crystallites. It is believed that the boron atoms hinder dislocation in the graphite lattice by a type of hardening process akin to the solid solution hardening effect in metallurgy. 

Generally, boron fibers are made by means of boron vapor condensing on some carrier materials, such as of tungsten wires, glass, graphite, aluminum, and molybdenum, and the typical fiber diameter with tungsten wires as carrier materials is around 12 pm. Boron fibers are preferable to make some composites. Typical properties of boron products are shown in Table 2.71. 

Boron exists naturally as 19.78% lOB isotope and 80.22% IIB isotope. High-purity crystalline boron may be prepared by the vapor phase reduction of boron trichloride or tribromide with hydrogen on electrically heated filaments. The impure or amorphous, boron, a brownish-black powder, can be obtained by heating the trioxide with magnesium powder. 

The reaction of adipic acid with ammonia in either Hquid or vapor phase produces adipamide as an intermediate which is subsequentiy dehydrated to adiponitrile. The most widely used catalysts are based on phosphoms-containing compounds, but boron compounds and siHca gel also have been patented for this use (52—56). Vapor-phase processes involve the use of fixed catalyst beds whereas, in Hquid—gas processes, the catalyst is added to the feed. The reaction temperature of the Hquid-phase processes is ca 300°C and most vapor-phase processes mn at 350—400°C. Both operate at atmospheric pressure. Yields of adipic acid to adiponitrile are as high as 95% (57). 

Another important function of metallic coatings is to provide wear resistance. Hard chromium, electroless nickel, composites of nickel and diamond, or diffusion or vapor-phase deposits of sUicon carbide [409-21-2], SiC , SiC tungsten carbide [56780-56-4], WC and boron carbide [12069-32-8], B4C, are examples. Chemical resistance at high temperatures is provided by aUoys of aluminum and platinum [7440-06-4] or other precious metals (10—14). 

Isopropylnaphthalenes can be prepared readily by the catalytic alkylation of naphthalene with propjiene. 2-lsopropylnaphthalene [2027-17-0] is an important intermediate used in the manufacture of 2-naphthol (see Naphthalenederivatives). The alkylation of naphthalene with propjiene, preferably in an inert solvent at 40—100°C with an aluminum chloride, hydrogen fluoride, or boron trifluoride—phosphoric acid catalyst, gives 90—95% wt % 2-isopropylnaphthalene however, a considerable amount of polyalkylate also is produced. Preferably, the propylation of naphthalene is carried out in the vapor phase in a continuous manner, over a phosphoric acid on kieselguhr catalyst under pressure at ca 220—250°C. The alkylate, which is low in di- and polyisopropylnaphthalenes, then is isomerized by recycling over the same catalyst at 240°C or by using aluminum chloride catalyst at 80°C. After distillation, a product containing >90 wt % 2-isopropylnaphthalene is obtained (47). 

Diphenylamine can also be produced by passing the vapors of aniline over a catalyst such as alumina, or alumina impregnated with ammonium fluoride (17). The reaction is carried out at 480°C and about 700 kPa (7 atm). Conversion per pass, expressed as parts diphenylamine per 100 parts of reactor effluent, is low (18—22%), and the unconverted aniline must be recycled. Other catalysts disclosed for the vapor-phase process are alumina modified with boron trifluoride (18), and alumina activated with boric acid or boric anhydride (19). 

For adding dopiag impurities duriag vapor-phase growth, a gaseous or easily vaporizable Hquid compound is metered, added to the siUcon source gas stream, and reduced along with the siUcon compound. Typical examples are diborane, 2 phosphine, and boron tribromide, BBr. ... 

Boron filaments are formed by the chemical vapor deposition of boron trichloride on tungsten wire. High performance reinforcing boron fibers are available from 10—20 mm in diameter. These are used mainly in epoxy resins and aluminum and titanium. Commercial uses include golf club shafts, tennis and squash racquets, and fishing rods. The primary use is in the aerospace industry. 

Boron Monoxide and Dioxide. High temperature vapor phases of BO, B2O3, and BO2 have been the subject of a number of spectroscopic and mass spectrometric studies aimed at developiag theories of bonding, electronic stmctures, and thermochemical data (1,34). Values for the principal thermodynamic functions have been calculated and compiled for these gases (35). 

Vapor phases ia the B2O3 system iaclude water vapor and B(OH)3(g) at temperatures below 160°C. Appreciable losses of boric acid occur when aqueous solutions are concentrated by boiling (43). At high (600—1000°C) temperatures, HB02(g) is the principal boron species formed by equiUbration of water vapor and molten B2O3 (44). At stiU higher temperatures a trimer (HB02)3(g) (2) is formed. 

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. 

Reactions of boron ttihalides that are of commercial importance are those of BCl, and to a lesser extent BBr, with gases in chemical vapor deposition (CVD). CVD of boron by reduction, of boron nitride using NH, and of boron carbide using CH on transition metals and alloys are all technically important processes (34—38). The CVD process is normally supported by heating or by plasma formed by an arc or discharge (39,40). 

The vapor-phase esterification of ethanol has also been studied extensively (363,364), but it is not used commercially. The reaction can be catalyzed by siUca gel (365,366), thoria on siUca or alumina (367), zirconium dioxide (368), and by xerogels and aerogels (369). Above 300°C the dehydration of ethanol becomes appreciable. Ethyl acetate can also be produced from acetaldehyde by the Tischenko reaction (370—372) using an aluminum alkoxide catalyst and, with some difficulty, by the boron trifluoride-catalyzed direct esterification of ethylene with organic acids (373). 

The temperature-independent parachor [P] may be calculated by the additive scheme proposed by Quale.The atomic group contributions for this method, with contributions for silicon, boron, and aluminum from Myers,are shown in Table 2-402. At low pressures, where Pi. pc, the vapor density term may be neglected. Errors using Eq. (2-168) are normally less than 5 to 10 percent. 

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