Vitreous Boric Oxide

5 STRUCTURAL MODELS FOR BORATE GLASSES 5.1 Vitreous Boric Oxide 

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Table 4. Physical Properties of Vitreous Boric Oxide ...

Takada, A. (1993) Computer Modelling of Crystalline and Vitreous Boric Oxide, PhD Thesis, University of London. 

Values for the physical properties of boric oxide are somewhat confused in the literature due to the tendency of this substance to react readily with water. Many physical properties of boric oxide are sensitive to water content, and the water content of samples used for measurements was not always carefully controlled. For example, at 0.28% water, the density of vitreous boric oxide is 1.853 gmL and the softening point 240-275°C. Reducing the water concentration to 20 p,g g reduced the density to 1.829 gmL and raised the softening point to 300-325°C. Other properties, such as thermal expansion, viscosity, and refractive index, are also significantly affected by moisture content. Water concentrations can be reduced to as low as 10p,gg by prolonged heating in vacuum. 

Figure 5.4 Boroxol ring structures in vitreous boric oxide and alkali borate glasses...

The viscosity of vitreous boric oxide is among the lowest reported for common oxide glasses. Boric oxide melts are also considerably more... 

The effect of water concentration on Tg and isothermal viscosity is most evident for the glass former oxides. Commercial vitreous silica is produced with water contents ranging from < 1 to > 600 wtppm of H2O. The Tg of these glasses decreases from 1200 °C to only 1060 °C over this range. The Tg of vitreous boric oxide ranges from 200 C for glasses containing as much as 1 wt% H2O, to over 300 °C for the driest... 

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