Dynamic glass transition and fragility of lithium borate binary glass

The compositional variations of the fragility of lithium borate binary glasses have been investigated over a wide composition range up to 64 mol% of Li2O. The frequency dependences of the dynamic heat capacity Cp(ω) above the dynamic glass transition temperature Tg have been observed by Temperature-Modulated DSC. The fragility or steepness index m has been determined. The composition dependence has been discussed on the basis of the fragility model proposed by Vilgis, and it has been revealed that the trends of the fragility and the fluctuation of the coordination number of boron atom have the correlation over a wide composition range. The result indicates the origin of the fragility relates to the degree of the fluctuation of the coordination number. The formation of the 4-coordinated boron atom plays the dominant role in the fragility of the borate glassy system.