Interpretation of the peculiar temperature dependence of surface tension for boron trioxide

The surface tension of boron trioxide was measured with uncertainty less than ±0.5 mN/m in the temperature range between 500 °C and 900 °C, using the pendant drop method coupled with a video enhanced image digitization technology and a best-fitting algorithm. These accurate surface tension data showed a peculiar temperature dependence: a mild increase at temperatures between 500 °C and 800 °C followed by a strong increase at T > 800 °C. To explain this unusual temperature coefficient of the surface tension, a change in the boron oxide structure while raising the temperature was proposed in this study. The main component of the vitreous B2O3 network, the planar boroxol ring, breaks up when the temperature increases. However, hypothesizing an analogy with the superstructures that are present in the binary borate glasses, this breakdown is compensated by the formation of new groups characterized by four-fold coordinated borons and three-dimensional structure. The presence of these groups causes a general strengthening of the B2O3 structure, especially noticeable at the complete breakdown of the boroxol rings. This proposed model of the structure changes successfully interprets the singular behavior of the surface tension temperature coefficient of the boron trioxide.

► The B2O3 surface tension is measured in a temperature range 500 < T < 900 °C.
► The data show a low increase at T < 800 °C followed by a strong increase at T > 800 °C.
► A structural change is proposed for interpreting the surface tension behavior.
► The planar boroxol ring breaks up when the temperature increases.
► Following this breakup, the formation of 3D structures is hypothesized.