Oxide melts

Y. Waseda and J.M. Toguri The Structure and Properties of Oxide Melts. World Scientific, Singapore (1998) ISBN 98I-02-33I7-5. 

Borides are relatively inert, especially to non-oxidizing reagents. They react violently with fluorine, often with incandescence. Reaction with other halogens is not as violent and may require some heat. Resistance to oxidation, acids, and alkalis is summarized in Table 17.5. In oxidation conditions, a layer of boric oxide is formed on the surface which passivates it to some degree. Boric oxide melts at 450°C and vaporizes at 1860°C. It offers good protection up to 1500°C in a static environments but it has low viscosity at these temperatures and tends to flow under stress and the protection it offers is limited.f k l... 

Oxides Melt spinning Sol-gel Medium strength Good oxidation resistance High cost Ceramic composites High-temp insulation Filtration... 

Whereas S1O2 melts at 1710 °C, other nonmetal oxides melt at much lower temperatures. For example, P4 Ofi melts at 25 °C. Referring to the accompanying bonding pictures, describe the forces that hold these solids together. 

Since Arrhenius, definitions have extended the scope of what we mean by acids and bases. These theories include the proton transfer definition of Bronsted-Lowry (Bronsted, 1923 Lowry, 1923a,b), the solvent system concept (Day Selbin, 1969), the Lux-Flood theory for oxide melts, the electron pair donor and acceptor definition of Lewis (1923, 1938) and the broad theory of Usanovich (1939). These theories are described in more detail below. 

The Lux-Flood theory relates to oxide melts. Geologists have often used acid-base concepts for the empirical classification of igneous silicate rocks (Read, 1948). Silica is implicitly assumed to be responsible for acidity, and the silica content of a rock is used as a measure of its acid-base balance ... 

Lux (1939) developed an acid-base theory for oxide melts where the oxide ion plays an analogous but opposite role to that of the hydrogen ion in the Bronsted theory. A base is an oxide donor and an acid is an oxide acceptor (Lux, 1939 Flood Forland, 1947a,b Flood, Forland Roald, 1947) ... 

Oxide Melting point C Boiling point °C Heat of fusion Heat of decomposition kJ mol Heat of vaporization... 

The oxide exhibits two crystalline modifications, the reddish or orange-red alpha form, known as litharge, and the yellow beta form, massicot. The alpha form constitutes tetragonal crystals while the beta modification is a yellow amorphous powder of orthorhombic crystal structure. The alpha form is stable at ordinary temperatures, converting to the beta form when heated at 489°C density 9.35 g/cm (beta form) Moh s hardness 2 (alpha form) the oxide melts at 888°C vaporizes at 1,472°C with decomposition vapor pressure 1 torr at 943° C and 5 torr at 1,039°C practically insoluble in water (the solubdity of alpha form is 17 mg/L at 20°C and that of beta form 23 mg/L at 22°C) insoluble in ethanol soluble in dilute nitric acid and aUtahes. 

The emphasis of the present chapter is on the correlation of the physical properties and structures of oxide melts. Since long-range order is destroyed in the process of fusion, the meaning of structure is necessarily different for the crystalline solid and its melt. For the latter, structural information is often only obtainable at the present by indirect means such as the comparison of certain properties at a particular temperature. Here, a meaningful interpretation may become doubtful because of the lack of a corresponding temperature. For instance, if the melting points of two oxides differ by 1000°C, on what basis can a property of their respective melts be compared For such reasons, some of the conclusions regarding structure discussed below must be considered as qualitative and treated with reservations. 

Instead of adhering to the sequence of the periodic table, the pure oxide melts discussed in this section are being broadly divided into three main liquid types. These are the network liquids, the electrically conducting melts and the molecular liquids. It is emphasized that this distinction is not definitive in every case and serves only to illustrate the wide range of liquid properties and structures encountered. 

A summary of most of the early work on conducting oxide melts has been prepared by Van Arkel et al. (57). These results were based primarily on the fusibility of an oxide in an electric arc furnace. Values of the specific conductance of some of these melts at the fusion temperature, with an uncertainty of 50%, estimated by these authors are shown in Table IV. 

The specific conductivity of a large number of oxide melts is in excess of 1 ohm-1 cm-1 and the temperature coefficient of conductance is positive. Two types of experiments from which conclusions are drawn regarding the mechanism of conduction are to be found in the literature. In one, the applicability of Faraday s laws is directly tested. In the second case, the conductivity of both the crystalline solid up to the melting point and that of the melt is measured if the change in specific conductivity on fusion is negligible, and if the so-called activation energy EK defined by... 

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