Dehydration‐driven mass loss from packs of sintering hydrous silicate glass particles

Abstract

Glass sintering involves the densification of packs of particles and the expulsion of the interparticle pore gas. The pore space begins as a convolute interconnected interparticle network, and ends as distributed isolated bubbles; two configurations that are separated by the percolation threshold. Here, we perform experiments in which (i) the particles are initially saturated in H2O at 871 K, and (ii) they are then heated non‐isothermally at different rates to temperatures in excess of 871 K. In step (ii), H2O becomes supersaturated and the particles diffusively lose mass as they sinter together. We use thermogravimetry to track the loss of mass with time. We find that the mass loss is initially well predicted by solutions to Fick's second law in spherical coordinates with the appropriate material and boundary conditions. However, as the sintering pack crosses the percolation threshold at a time predicted by sintering theory, we find that the mass loss deviates from simple diffusional solutions. We interpret this to be the result of an increase in the diffusion distance from the particle‐scale to the scale of the sintering pack itself. Therefore, we conclude that the open‐ to closed‐system transition that occurs at the percolation threshold is a continuous, but rapid jump for diffusive and other transport properties. We use a capillary Peclet number Pc to parameterize for this transition, such that at low Pc diffusive equilibrium is achieved before the sintering‐induced transition to closed system, whereas at high Pcthere is a “diffusion crisis” and disequilibrium may be maintained for longer relative timescales that depend on the system size.

Viscous Sintering of Acid Leached Glass Powders

“…The interaction between diffusing atomic species, which balance concentration gradients and sintering was analysed using the ratio of the time scales of diffusive mass transport τDiff and sintering τSinter. The approach was adopted from the analysis of Vasseur et al [18] for diffusive mass transport during sintering of hydrous glasses. It is based, on the one hand, on the time required for mass transport to equilibrate the concentration gradients tDiff = 1/τDiff = λDiff 2 /D with the effective diffusion length λDiff and D = diffusion coefficient, and, on the other hand, on the time for concurrent sintering, which according to Eq.…”

“…( 1) is tSinter = 1/τSinter = r0η/γ. Thus, for isochronous heating and for λDiff = r0, one can therefore integrate [18]:…”

“…Since viscous sintering is driven by the reduction of the surface energy of the compacted powder and limited by the viscous flow at the surface of the powder particles, corrosion and leaching phenomena are of crucial importance for the sintering progress. In practice, insufficient surface quality of fired products is empirically corrected by parameters at the furnace, while only recently a first knowledge-based study on diffusive mass transfer during sintering of hydrated glass powders was available [18].…”