Melting, directed metal oxidation

The Hf-B-C system presents a situation that falls somewhat between the Ti and Zr systems [60]. Although the HfB phase is stable in the Hf-B binary system, it melts at 2100°C, below the melting point of the Hf parent metal (2227°C). During a directed metal oxidation reaction of molten Hf with B4C at just above the melting point of Hf, e.g., 2400°C, the Hf-B-C isothermal ternary cross section (Fig. 18) indicates that the molten metal is... 

As for other classes of composite materials, there are many processes that can be used to make CMCs. Key considerations in process selection are porosity and reactions among reinforcements, reinforcement coatings, and matrices. The most important processes for making CMCs at this time are chemical vapor infiltration, melt infiltration, preceramic polymer infiltration and pyrolysis (PIP), slurry infiltration, sol-gel, hot pressing, and hot isostatic pressing. In addition, there are a number of reaction-based processes, which include reaction bonding and direct metal oxidation ( Dimox ),... 

Direct reduction (DR) is the process of converting iron ore (iron oxide) into metallic iron without melting. The metallic iron product, known as direct reduced iron (DRI), is used as a high quaUty feed material in steelmaking. 

Silicides are usually prepared by direct fusion of the elements hut coreduction of Si02 and a metal oxide with C or A1 is sometimes used. Heats of formation are similar to those of borides and carbides but mps arc substantially lower e.g. TiC 3140°, TiBj 2980°, TiSij 1540° and TaC 3800°, TaB2 3100°, TaSi2 1560°C. Few silicides melt as high as 2000-2500°, and above this temperature only SiC is solid (decomp 2700°C). 

From the preceding discussion, it is easily understood that direct polyesterifications between dicarboxylic acids and aliphatic diols (Scheme 2.8, R3 = H) and polymerizations involving aliphatic or aromatic esters, acids, and alcohols (Scheme 2.8, R3 = alkyl group, and Scheme 2.9, R3 = H) are rather slow at room temperature. These reactions must be carried out in the melt at high temperature in the presence of catalysts, usually metal salts, metal oxides, or metal alkoxides. Vacuum is generally applied during the last steps of the reaction in order to eliminate the last traces of reaction by-product (water or low-molar-mass alcohol, diol, or carboxylic acid such as acetic acid) and to shift the reaction toward the... 

Antimony trioxide occurs in several modifications of which the senarmontite form is the preferred one for polymer uses. This does occur as a natural mineral but the deposits are generally too impure for direct use and virtually all the material used in polymers is made synthetically. The principal synthetic method is to blow air through the molten metal or its sulphide. The metal oxide sublimes from the melt and is recovered by cooling and filtration. Particle size is an important factor governing performance and this is controlled by factors such as the temperature and the cooHng rate. 

Recently, Bockris and co-workers (6, 7, 9) have modified the above model to satisfy the experimental findings outlined in (1) to (5). In place of infinite chains and sheets, smaller anions the sizes of which are directly proportional to the concentration of Si02 in the melt, were postulated. Thus at 33 mole-% metal oxide, the anions present may be (SieOnn)6- or (SisCho)8-- Although this later ionic model is entirely compatible with available physicochemical data, it must be emphasized that, at present, direct proofs of the existence of the anions postulated are lacking. 

The reduction of phosphates by carbon is a classical method, but the purity of phosphides obtained this way can be suspect. This also holds for replacement reactions, in which a redox process between a transition metal and a metal phosphide is used for the preparation of phosphides with high thermal stabihty. Binary main-group element and transition metal phosphides hke AIP, CrP, NbP, MoP, or WP can be prepared by the reaction of the powdered metals with a melt of hthium metaphosphate LiP03. Bulk samples of transition metal phosphides hke C02P or NiMoP can be obtained via reduction of metal oxide/phosphate mixtures in a mixture of 5% H2 in N2. MoP, WP, Fc2P, M2P, FeP, and RuP can be synthesized by direct reduction of the transition metal phosphates in hydrogen atmosphere between 670 and 1320 K. ... 

The temperature rise at the surface and sparks is limited by the melting point of the lower-melting-point solid. This limit does not apply if one of the materials can be easily oxidized and the oxidation is highly exothermic. Apart from direct heating because of friction, substances that can undergo a local oxidation sustain additional heating, and can build up much higher temperatures. Thermite reactions are special oxidations of metals by metal oxides. The most common thermite reaction is between aluminum and rust ... 

Recent studies have indicated direct correlations between surface area and catalytic activity of metal oxide sensors (Li et al. 1999). Therefore, it is evident that incorporating catalytic particles by coating techniques may affect surface area and catalysis. Studies by Lee and Bhat (2003) have demonstrated that by incorporating small amounts of electrospun nanoflbers in spun-bond and melt-blown nonwovens, barrier properties such as flitration efficiency and air permeability can be improved. 

All the routines described for the determination of the thermodynamic (concentration) parameters in metal oxide solutions include some indirectly obtained values. For example, the equilibrium concentration of metal cations is calculated proceeding from the quantity of the oxide-ion donor consumed for titration (precipitation). Direct determination of the concentration of metal cations in the melt (if it is possible) allows one to obtain more correctly the obtained solubility product values. Our paper [332] reports a method for correction of the solubility product values for oxides on the basis of the potentiometric titration data. The modification of the standard routine consists of the simultaneous use of two indicator electrodes, one of which is the membrane oxygen electrode and the other is a metal electrode, reversible to the cations the oxide consists of. This routine was used to estimate the solubility products of copper(I) and nickel(II) oxides in the molten KCl-NaCl equimolar mixture at 700 °C. Investigation of Cu20 by the proposed method is of considerable importance since, as will be shown further, the process of dissociation/dissolution of copper(I) oxide in molten alkali-metal halides differs from the generally accepted one which was considered, e.g. in Ref. [119]. 

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