Crystallization kinetics of binary borosilicate glass composite
- PDF / 796,525 Bytes
- 9 Pages / 576 x 792 pts Page_size
- 75 Downloads / 213 Views
Kinetics of cristobalite precipitation in a binary glass composite, containing a low-softening borosilicate (BSG) and a high-softening high silica (HSG) glass, have been investigated. XRD results show that the pure glasses do not crystallize under the sintering conditions used, but when mixed in appropriate proportions the cristobalite gradually precipitates out of the initial amorphous binary glass mixture as the sintering continues at temperatures ranging from 800 to 1000 °C. Average linear thermal expansion coefficient (TCE) results show that the TCE increases significantly with increasing precipitation of cristobalite as a function of sintering time. Comparing the experimental TCE results with those theoretically calculated, it is concluded that the precipitation originates most likely in the HSG rather than in the BSG. The precipitation kinetics follow the Avrami equation, and the results show an apparent activation energy of 82 kJ/mol which is close to those for the diffusion of alkali ions in silicate glasses, suggesting mass-transport controlled kinetics. The values of the Avrami exponent are 1.7-1.8, which could be interpreted as a 3-dimension diffusional growth at zero nucleation rate. The linear growth rates of cristobalite, calculated from the precipitation curve, are in the range of 4 - 8 X 10~5 fim/min, and show slight temperature dependence from 800 to 1000 °C. The linear growth rates of cristobalite are also calculated theoretically using the equation derived by Turnbull et al.,2 and the data are 0 - 3 orders of magnitude smaller than those observed experimentally. This disparity is attributed to the catalytic effect of the OH and O in air and in the glass network, as well as the diffusion of alkali ions from BSG to HSG. I. INTRODUCTION Devitrification exists commonly in many glass systems, and has been investigated almost since the discovery of glass. The rate and mechanism of devitrification vary with the chemical composition of the glass, temperature, ambient atmosphere, and impurities in the glass network. Important parameters1 to characterize the crystallization and melting processes include the entropy of fusion for crystal-glass interface, interfacial morphology, the isotropy of growth, the variation of growth rate with temperature, and the asymmetry of kinetic data in the vicinity of the liquidus temperature of glass. Interested readers are encouraged to refer to a comprehensive review paper by Uhlmann1 for detailed information. Because of its complexity, only a brief summary of the relevant theory regarding the crystal growth rate is presented here for the later discussion of kinetic analysis. This theory was first proposed by Turnbull et al.2 in relation to the study of the devitrification kinetics of fused silica. Their analysis indicates that, under the experimental conditions of k J > AhfAT/Tnq, a linear growth rate (M) of the interface between crystalline and amorphous states of the compound can be approximated by fAhfAT 3ir(ao)2VTliq
(1)
J. Mater. Res., Vol. 7, No. 11, Nov 1992 http
Data Loading...
