Oxide melts expansivity

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. 

BaO—Si02 55). At equimolar compositions above 20 mole-% metal oxides, the expansivity is in the descending order of K, Na, Li, Ba, Sr, Ca, and Mg and is inversely proportional to the so-called ion-oxygen attraction defined as I = 2ze /r2 where z is the cationic valence, e is the electronic charge, and r is the separation between the metal and oxygen ions 55). The ionic nature of these melts is further confirmed. Relationships were also obtained between the cationic radii and molar volumes which suggest that the volumes of the silicon-oxygen anions at any composition are comparable for all the binary systems 55). 

Properties Colorless crystals or white powder odorless and tasteless. D 2.2-2.6 thermal conductivity about half that of glass, mp 1710C, bp 2230C, high dielectric constant, high heat and shock resistance. Insoluble in water and acids except hydrogen fluoride soluble in molten alkai when finely divided and amorphous. Combines chemically with most metallic oxides melts to a glass with lowest known coefficient of expansion (fused silica). Noncombustible. 

The oxidation (1) includes 80% volume expansion of the condensed phases. As a result, the space between crack walls is filled with the formed oxide. Moreover, the reaction (I) exhibits huge exothermic heat. The heat makes the matrix and the formed oxide melt at once, and to form the strong bond. For this phenomenon, the healed cracks gains higher strength than the base... 

It also has potential use in ceramic and glass formulas, as the oxide has a high melting point and imparts shock resistance and low expansion characteristics to glass. 

The viscosity of liquid silicates such as drose containing barium oxide and silica show a rapid fall between pure silica and 20 mole per cent of metal oxide of nearly an order of magnitude at 2000 K, followed by a slower decrease as more metal oxide is added. The viscosity then decreases by a factor of two between 20 and 40 mole per cent. The activation energy for viscous flow decreases from 560 kJ in pure silica to 160-180kJmol as the network is broken up by metal oxide addition. The introduction of CaFa into a silicate melt reduces the viscosity markedly, typically by about a factor of drree. There is a rapid increase in the thermal expansivity coefficient as the network is dispersed, from practically zero in solid silica to around 40 cm moP in a typical soda-lime glass. 

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). 

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