4. SODIUM INDIUM SILICATES
4.4 Discussion
4.4.2 Nucleation and Growth
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the glasses, except for the 2 mol% In2O3 glass, are well above the hydroxyl for saturation in typical geologic glasses. It is unlikely that more hydroxyl could be stable in these glasses. It is also noted that Q3 groups are less reactive with water than Q4 groups and water will always prefer to react with Q4 silica. If the reduction reaction is forming the concentrations of hydroxyl calculated, there will be a propensity for the hydroxyl to recombine and diffuse out of the structure. With that in mind, the IR absorbance data is incredibly difficult to analyze in terms of the tarnishing model as it is most assuredly lower in intensity than it should be.
This saturation concentration is a byproduct of water formed by melting in high pressures of water vapor and it is possible that these hydroxyl sites are different than the hydroxyl sites formed by reduction. It has been stated in literature that some of the hydroxyl formed by reduction is more difficult to remove than hydroxyl formed by melting in water vapor, i.e. some hydroxyl formed by reduction is more stable than hydroxyl formed by equilibrating a melt in water vapor.12 Some of the water formed by reduction in these glasses appears to leave the structure with relative ease. This is why saturation concentrations are being considered here. If the water forming is more thermodynamically unstable due to the high concentrations which drive the reaction towards the development of molecular water, it is reasonable to assume that the hydroxyl concentrations should be more variable due to this reaction. Molecular water is capable of reacting and diffusing out of the glass which, as mentioned previously, can also complicate the reactions in this study. Without more concrete data on the saturation concentrations and for the diffusion of water in these glasses, it is difficult to say more.
This reverse reaction clearly complicates application of the tarnishing model to this glass sytem, as well as the dramatic composition change.
indium to contribute to the diffusion limited growth. Distinct layers of particles were reported in literature that move periodically into the bulk of the glass and were not seen here. A possible reason is that the surface viscosity of the reduced glasses is so low that the particles were not contained as much as they may have been had all treatments occurred below the soda silicates Tg. It is also apparent that the viscosity of the glass has a large impact on the size and shape of the clusters. A more viscous glass structure potentially limits the growth of the clusters due to strain, and a less viscous structure allows for much larger particles to develop. If the viscosity is low enough the particles do appear to take on shapes based on the metallic structure. In typical nucleation and growth discussions, the viscosity of the glass is often related to diffusion coefficients through the Stokes-Einstein relationship.42,49
The glass treated above the Tg of the indium containing glass crystallized noticeably. This is normal since typical nucleation and growth temperatures exist above Tg. This system also offers reduced indium atoms and clusters as potential nucleation sites, which could explain the low indium peak intensity in the GIXRD data due to crystals of different compositions forming around indium or absorbing indium to grow.
Since indium melts around 156°C, the clusters are forming as liquid droplets within a glassy matrix, in some respects quite similar to phase separation. The indium clusters either push aside the silicate network, form around the silicate network, or some sort of transport mechanism is involved with the diffusion of ions away from the cluster to allow it to grow. It is also well established that hydroxyl can greatly affect the properties in a glass such as Tg.101 This implies that the glass structure where indium has been reduced could have a locally lower Tg than the rest of the glass and this could mean the structure may deform more readily to accommodate a growing droplet than one would normally expect. There were instances where clusters appeared to contain pieces of glass and that may indicate that the particles form around the silica network. There is one image of a number of small particles coalescing to form a much larger particle. This potentially could surround the silicate network containing it in indium metal. Regardless the UV-Vis data collected indicates that the particles are forming after very short treatment times and the number volume and sizes are most likely contributing to the spectra collected. It was also determined that the UV-Vis data can be interpreted by
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considering the agglomeration of many smaller particles to make larger particles. This would potentially reduce the UV-edge changes associated with a Plasmon resonance absorption of very small particles and potentially could reduce the scattering as the number volume of particles is changing drastically.115 This combination of smaller particles into larger clusters is believed to occur due to the much lower viscosity of the surface layer allowing clusters to aggregate and form larger clusters.
It is interesting to consider why the indium atoms agglomerate. It is typically considered to minimize thermodynamic energy as long as there is enough energy to surpass the activation energy for the atoms to diffuse together. In consideration of the thermodynamic energy term, there are a number of potential driving forces for energy minimization. An atom has a very high surface area, so clustering may occur to minimize the ratio of surface area to volume of the particles. Discreet atoms would exhibit discrete energy levels, and it may be more thermodynamically favorable as a cluster would exhibit much fewer energy levels due to the development of an electronic band structure. Atoms also have three degrees of freedom, and it may be more favorable from a thermodynamic standpoint if the system does not have a large concentration of particles with three degrees of freedom. This could lead to agglomeration as atoms in a cluster would have fewer degrees of freedom due to bonding.