Boron oxide melts

The monoclinic AM2(P04)3 actinide phosphates were obtained by the following methods 1) chemical interactions in boron oxide melt at 1200 °C (48 h), 2) precipitation from the A-M-chloride melts by addition of soluble A-phosphates at 700-800°C (approximately 1 h), 3) interactions between the MO2 (M = Np, Pu) oxides and aqueous solutions of alkaline element salts with H3PO4 followed by several stages of heating, resuspension in water, and finally the product was calcined at temperatures up to 1100 C. 

As a first approach, a thermochemical analysis of the complex reaction system was carried out. This means that transport of reactants in the fluid phase was neglected, but a fast reaction kinetics was assumed. The system under study comprised the gas atmosphere, the boron oxide melt, solid and liquid GaAs, the crucible and the graphite heaters. Figure 9.24 shows a scheme of the system. 

Riebling EF (1967) Volume relations in sodium oxide-boron oxide and sodium oxide-silicon dioxide-boron oxide melts at 1300°. J Am Ceram Soc 50 46-53... 

Boron oxide often behaves as a flux. Boron softens glass for easier melting, but unlike alkahes, boron oxide increases expansion only slightly. This is the basis of the easily melted, low expansion commercial glasses known as borosihcates. 

The major use of boric acid is as the starting material for its anhydride, boron oxide, B203. Because it melts (at 450°C) to a liquid that dissolves many metal oxides, boron oxide (often as the add) is used as a flux, a substance that cleans metals as they are soldered or welded. Boron oxide is also used to make fiberglass and borosilicate glass, a glass with a very low thermal expansion, such as Pyrex (see Section 14.21). 

When mixed with sodium carbonate (soda ash) and calcium carbonate (powdered limestone) and heated until the mixture melts, silica (sand) forms glass when cooled. Glass of all types has near limitless uses. One example is Pyrex, which is a special heat-resistant glass that is manufactured by adding boron oxide to the standard mixture of silica, soda ash, and limestone. Special glass used to make eyewear adds potassium oxide to the above standard mixture. 

T. Tokuda, T. Ito, and T. Yamaguchi, Self-Diffusion in a Glassformer Melt Oxygen Transport in Boron Oxide, Z. Naturforschung, 26A, 2058-2060 (1971). 

Quite recently attention was paid to the role of oxides, either as electro-active species, as impurities or as additives in the electro-deposition of transition metals. This may be demonstrated, e.g. in the case of electro-deposition of molybdenum, where the electrolysis of neither pure K2M0O4, nor the KF-K2M0O4 mixture yields a molybdenum deposit. However, introducing small amounts of boron oxide, or silicon dioxide to the basic melts, smooth and adherent molybdenum deposits may be obtained. Also, in the case of niobium and tantalum deposition, the presence of oxygen either from the moisture or added on purpose leads to the formation of oxohalo-complexes, which due to their lowered symmetry and thus lower energetic state, decompose easier at the cathode yielding pure metal. 

From the literature it follows that the electro-deposition of molybdenum from the binary MeF-Me2Mo04 mixtures is impossible. However, a small addition (1 mole %) of boron oxide or Si02 to the electrolyte facilitates the electro-deposition of molybdenum. The presence of boron or silicon oxide most probably modifies the structure of the melt, which results in changes in the cathode process. The survey of electrochemistry of molybdenum deposition was given by Danbk et al. (1997). 

The viscosity of the melts of the ternary system increases very steeply with the increasing content of boron oxide. This observation is, however, not surprising, because of the polymerization ability of the boron oxide. The formation of more voluminous... 

From the viscosity, as well as the phase equilibrium, surface tension, and density measurements it is evident that the system KF-K2M0O4-B2O3 is very complex. Beside the chemical reactions, the polymerization tendency of the melts, especially in the region of higher contents of boron oxide, makes this system difficult to study. 

Among the systems of alkali metal fluorides containing boron oxide, the melts of the system LiF-KF-B203-Ti02 were tested as possible electrolytes in the electro-chemical synthesis of titanium diboride, especially when well-dispersed powders should be... 

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