Rheological investigations in the decomposition ränge of the system Na₂O-B₂O₃-SiO₂

Abstract

Three glass melts from the Vycor-type widely distributed along the mixing gap of the pseudobinary line SiO₂-(B₂O₃:Na₂O = 84:16) were invesdgated each one with three different thermal histories. The following rheological properties were determined by the cylinder compression method: the Newtonian and non-Newtonian flow behaviour, the stress generadon modulus as a measure of the stiffness, the high-temperature tensile strength and the critical deformation rate at which the first crack appears. The influences of the fundamental glass compositions (SiO₂ content) as well as the thermal pretreatments on these properties are demonstrated and discussed on the basis of isochomal conditions (equal Newtonian viscosities). The results can be interpreted by means of the knowledge about the demixing rules and processes which lead to various rheological two-phase Systems. Most drastical changes of the rheological properties are observed in the middle of the mixing gap where spinodal decomposidon has its optimum and where he differentation between the matrix phase and decomposed phase is lost.