Effect of Cooling Rate on Vitrifaction of Condensed Sodium Phosphate Melts

Critical cooling rates (Q) for vitrifaction of condensed sodium phosphte melts were determined experimentally. The values of Q for the melts with the compositions of 1.00, 1.02, 1.04, 1.20, 1.40, 1.50 and 1.67 in mole ratio Na2O/P2O5 were observed to be 3, 2, 20, 20-30, 30, 50 and above 500°C/sec, respectively. Generally Q decreased with an increase of the mean chain length of the phosphate ion. Structural chemical meaning of Q could be clarified in terms of crystal growth rate (u) involving induction time (τ). The thickness (D) of the crystallized layer formed at the surface by cooling the melt was represented as a temperature integral of the isothermal crystal growth rate at the surface layer of the glass with the same composition as the melt. That is, D=∫TgTm∫0tius{1-exp(-t/τ)}dt⋅dT where Tm, Tg, ti and us are melting temperature, glass transition point, duration at temperature Ti and growth rate of the layer in a steady state, respectively. It is concluded that Q is the cooling rate above which no crystallization from the surface of the melt can occur, in other words, D is zero. The crystallization phenomena at the surface of the melts and of the glasses belong to a particular case in a viewpoint of crystal morphology. In the above case, in place of spherulite growth an assembly of small crystals with orientation in a same direction formed, and the crystal growth was restricted to one direction vertical to the surface owing to simultaneous formation of a great number of nuclei at the surface. The growth rate of the layer is considered to be controlled by a second order nucleation rate at the front of growth and can be expressed by the formula: u=us{1-exp(-t/τ)} Therefore the vitrifaction of the phosphate melts depends conspicuously on two heterogeneous nucleation processes, namely, those at the surface of the melt and at the front of growth. The occurrence of bond-exchange reactions among the phosphate anions at temperatures above 600°C was confirmed by the fact that the viscosity of the melts was independent of their mole composition in the concerned temperature range.