Tracing the Reactive Melting of Glass-Forming Silicate Batches by In Situ ²³Na NMR

Abstract

The kinetics of the reaction of batches of powdered quartz and sodium carbonate was studied by in situ (23)Na nuclear magnetic resonance (NMR) spectroscopy using a laser-heated probe. We show for the first time that the technique allows one to study solid-state reactions at high temperatures with good time resolution and without the risk of quenching artifacts. The reaction is controlled by solid-state Na(+) diffusion across the grain interface. Independent of the batch composition, the first reaction product is crystalline sodium metasilicate, Na(2)SiO(3), even if the temperature is high enough for much of the composition space between silica and metasilicate to be above the equilibrium liquidus. Fast Na(+) diffusion allows the reaction front to cross the grain interface and form the solid product before liquid intermediate equilibrium products can be formed. This purely solid-state reaction slows down as the thickness of the interface increases; the reaction is more deceleratory than published models suggest. If excess quartz is present, it reacts in a second step involving a liquid film wetting the excess grains. Once this reaction has started, it pulls the reaction into the thermodynamic regime, which leads to an increase even in the rate of the first step leading to intermediate solid metasilicate.

In situ high temperature 27Al NMR study of structure and dynamics in a calcium aluminosilicate glass and melt

“…One-(1D) and two-dimensional (2D) in situ high temperature NMR spectra have been obtained to investigate the structures of a variety of oxide glasses and melts from the line shapes, the peak positions and the line widths for several nuclides, e.g. 29 Si [13][14][15][16][17][18][19][20], 27 Al [20][21][22][23][24][25], 11 B [26,27], 23 Na [18][19][20][28][29][30][31][32], 7 Li [31,33], 25 Mg [32,34].…”

MAS-NMR studies of carbonate retention in a very wide range of Na2O-SiO2 glasses

“…The neutron diffraction pattern of the sample with 70 mol% Na 2 O shows some small Bragg peaks, arising from particles of crystalline -Na 2 CO 3 of diameter ~100 Å, forming ~2% of the sample, see Therefore, we propose that for smaller Na 2 O contents the CO 3 groups occur as individual structural entities that are isolated from each other, but for 70 mol% Na 2 O there is a significant number of CO 3 groups that occur in a cluster within the glass network, separated from each other only by Na + ions, with a similar arrangement at the local scale to that in crystalline Na 2 CO 3 , but without the occurrence of long range order. Potentially, longer melting times may reduce or eliminate these inclusions 27 , or perhaps between 65 and 70 mol% Na 2 O represents the maximum solubility limit for CO 3 2-ions under the preparation conditions used here.…”

Importance of the Atmosphere on the Mechanisms and Kinetics of Reactions Between Silica and Solid Sodium Carbonate

“…The reaction kinetics between sodium carbonate and silica powders have been extensively studied, using various techniques such as thermogravimetric analysis, in situ X‐ray diffraction or in situ 23 Na NMR . Reactions are known to be accelerated above the melting point of sodium carbonate at 865 ∘ C, when the molten carbonate rapidly wets silica grains .…”

“…There is general agreement that reaction between silica and solid sodium carbonate first produces crystalline Na 2 SiO 3 : However, different authors used different grain sizes, atmospheres, or bulk composition, making results on reaction kinetics difficult to compare: published reaction rates at 800 ∘ C differ by one order of magnitude. Furthermore, activation energies ranging from 100 kJ/mol to 490 kJ mol −1 have been described.…”