Elizabeth Drabarek

Shear-induced restructuring of concentrated colloidal silica gels

The viscometric properties and structure of concentrated sheared colloidal gels, formed at a pH = 8 from a commercial aqueous sol of 7 nm diameter silica particles, were investigated. As the system gels under a constant shear rate, the viscosity first increases, then peaks when the shear stress reaches a shear-rate-independent threshold value of about 350 Pa, and finally decreases asymptotically to approximately one tenth the peak value. This low-viscosity state can apparently be held indefinitely by maintaining the shear, but the system gels once the shear is removed. The structural origins of this viscometric behaviour were studied using in situ small-angle neutron scattering (SANS). An abrupt change in the character of the SANS intensity was observed at the time corresponding to the peak in the viscosity. Samples of the gelling system were extracted and diluted at various times after gel initiation. Effective hydrodynamic diameters of these extractants were estimated by dynamic light scattering, and their Guinier diameters were estimated by SANS when appropriate. We conclude that the drop in viscosity corresponds to a structural densification of growing clusters of the colloidal particles, and that this transformation occurs once the critical stress is reached.

The Immobilization of Cesium and Strontium in Ceramic Materials Derived from Tungstate Sorbents

The effective immobilization of Cs+ and/or Sr2+ sorbed on hexagonal tungsten oxide bronze (HTB) adsorbent materials can be achieved by heating in air at temperatures in the range 500 – 1000 °C. Crystalline powdered HTB materials formed by heating at 800 °C show leach characteristics comparable to Cs-containing hot-pressed hollandites in the pH range from 0 to 12. If the Cs-loaded HTB sorbents are pressed into pellets prior to calcination, ceramic monoliths can be prepared. Heating to temperatures in excess of 1250 °C results in the melting of the sorbent to form millimetre-sized crystals of bronzoid phases. Thermal analysis shows that melting of the cation-exchanged tungstate sorbents is initiated at temperatures as low as 850 °C and concludes by 1300 °C. The absence of any significant mass loss immediately after melting, as well as chemical analyses before and after melting, confirm that Cs is not volatilized to any significant extent at the temperatures required to generate durable, coarsely crystalline products. The melted bronzoid product is a multiphase ceramic in which Cs+ remains bound within, and appears to stabilize, the hexagonal bronze phase, even after complete melting at 1300 °C, while elements such as Sr2+ are present within other tungstate phases. The bronzoid chemical system appears capable of accommodating a wide range of other elements. Here we have demonstrated that modification of the sorbent properties by incorporation Mo does not impact severely on the durability of materials prepared below 1000 °C, even when exposed to strong acid (pH=1) and elevated temperature (150 °C). As an example, one-day MCC1 leach rates lower than 1×10−5g/m2/day were measured using demineralized water at 90 °C for Cs-saturated Mo-doped sorbents that had been heated in air at 900 °C, while the fraction of Cs leached from powdered samples in 0.1 M HNO3 solutions at 150 °C for 4 days is only 4×10−3 g/m2/day.

Effect of processing variables on the structural evolution of silica gels

Constant shear rate and dynamic rheological measurements have been used to investigate the effect of shear on the processes controlling the structural evolution (size and volume fraction of clusters and the extent of cluster–cluster cross-linking) during the gelation of colloidal silica sols. In the absence of shear, the storage and loss moduli (G′ and G″, respectively) initially increase slowly prior to gelation, indicating that cluster growth and network formation are initially proceeding slowly, but then the system evolves rapidly, with cluster growth occurring at a slightly faster rate than network formation. In contrast, sols presheared for 4 h prior to gelation exhibit rapid increases in both G′ and G″ immediately after cessation of the applied shear, reflecting significant differences in the evolution of the gel structure. On aging, the viscoelastic properties of the unsheared and presheared samples are similar, indicating that their structures are comparable on the length scales (several microns) being probed by the frequency range used here. However, their chemical and microstructural properties differ significantly, due to differences in the intercluster bonds. The effect of shear rate, initial pH, colloid particle size, and volume fraction on the structure of the resulting sols and gels is discussed.

Shear-Induced Restructuring of Colloidal Silica Gels

Continuous oscillatory experiments were used to investigate the viscoelastic properties and structural evolution of silica gels obtained from unsheared and presheared sols, at pH 8. The processes controlling structural evolution before, at and after the gel point are discussed. Although the viscoelastic properties of the aged gels are comparable (implying similar structures on length scales of several microns), their chemical properties and microstructures (<1 μm) are significantly different. This is directly related to the different aggregate structures formed in the unsheared and presheared systems (50 and 700 nm, respectively) and the associated concentration of surface hydroxyls available for cross-linking and formation of stable siloxane bonds.

Universal behaviour of silica suspensions gelled under shear

The shear-influenced gelation of three aqueous suspensions consisting of silica particles of nominal diameter 7, 12 and 24 nm, respectively, is reported. It is shown that the viscosity/stress of a gelling system increases with time after gel initiation, reaches a maximum, then falls to a plateau value. A very simple relation between this maximum stress and the precursor volume fraction is verified experimentally.

Sol-Gel Silica Particles for Controlled Release Applications

Using sol-gel technology combined with water-in-oil (W/O) emulsions, we have developed an innovative method for producing ceramic particles with independent control over the release rate and particle size. The average particle size can be varied from 10 nm to 100 μm and is controlled by the emulsion chemistry. The release rate can be independently varied from mg/hours to mg/month, and is controlled by the internal microstructure of the particles and the initial sol-gel chemistry.

Synthesis of Mesoporous Titanium Phosphate

Mesoporous titanium phosphates were synthesised using three different approaches, involving reactions of titanium isopropoxide with long-chain alkyl phosphates, and subsequent aging at 80°C for several days. The resulting powders were characterised using XRD, TEM and N2 sorption. The first approach involved the classical MCM methodology, where the modified titanium precursor was added to an alkylphosphate micellar solution. The resulting solids possess an organised, lamellar mesostructure. However, removal of the surfactant, either by heating or leaching in strong basic solution, leads to the collapse of the lamellar mesostructure. In the second route, titanium alkoxide was pre-reacted with the long-chain alkyl phosphate, then hydrolysed with a large excess of water prior to ageing. Although the structure of the as-precipitated samples could be easily tailored by changing the processing parameters such as the alkyl phosphate chain length or phosphate/titanium ratio, surfactant removal by heating invariably led to the production of microporous samples. The third approach involved the hydrolysis of the acetylacetone-modified titanium isopropoxide with excess water. The alkyl phosphate was then introduced into the resulting suspension. Subsequent ageing under acidic conditions destablished the particles, leading to aggregation and subsequent gelation. In contrast to the previous approaches, the pyrolysed solid contained a significant proportion of mesopores, a high porosity (40%) and a surface area exceeding 300 m2.g-1.