Standard preparation

Titanium AA standard solution

A 1000ppm Ti AA standard in water with trace amounts of HCl was used as the titanium source for the gels. The latter was accurately weighed out for each standard to achieve the desired weight percentages of Ti for 100ppm, 500ppm and 1000ppm in the final total 10g gel yield. The calculated amounts for starting materials showed 1g of the AA solution added to 34.66973268g of TEOS gave the correct wt% of Ti for 100ppm in the final silica gel yield. 5g and 10g were added to 34.6558634g and 34.6385268g, respectively, of TEOS to achieve 500ppm Ti and 1000ppm Ti for the other two standards.

Tetraethyl Orthosilicate (TEOS)

TEOS is used to supply the silica component of the gels. It weighed out from the bottle to a Teflon beaker using a pipette and then made up more accurately with a syringe. The weight of TEOS needed to achieve a 99.99wt%, 99.95wt% and 99.9wt% of SiO2 was calculated using the molecular weight of TEOS using its chemical formula: Si(OC2H5)4 and the molecular weight ratio of the total value for TEOS and the value for just SiO2. This was altered for the amount of Ti which was being added for each gel, and was found to be 34.66973268g, 34.6558634g and 34.6385268g, of TEOS for corresponding Ti concentrations of 100ppm, 500ppm and 1000ppm, respectively, in the individual gels. A blank standard of pure silica was also made which required 34.6732g of TEOS for the desired 10g final gel yield.

The weight and volume of solutions required for each gel were calculated. These amounts were weighed into Teflon beakers using a pipette and making up the final decimal places with a syringe. A small amount of de-ionised water was added to the weighed Ti standard before 20ml of nitric acid was added whilst stirring. Next, 15ml of ethanol was added to the TEOS and mixed thoroughly before being combined with the Ti solution, the beaker which previously contained TEOS was swilled with de-ionised water, which was also added to the Ti solution to ensure as little of the starting components were lost. The combined TEOS and Ti solution was stirred vigorously to ensure complete homogenisation. As soon as possible after the addition of ethanol, 20ml of ammonia was added, again, with vigorous stirring, thereby forming the gel precipitates. The beakers were covered with loose-fitting Teflon lids to prevent loss of TEOS by evaporation and then left to stand overnight before the drying procedure could begin.

Drying the gels

The gels were transferred from Teflon beakers to Pyrex for heating. The beakers were placed over a hot plate at 70°C for 24 hours, and then the temperature gradually increased to 160°C at around 10 degrees every 2-3 hours. The gels were left at 160°C for 24hours. Any vapour crystals which formed on the side of the beaker were knocked back into the beaker by gently tapping the sides.

The gels were then transferred into Palladium-gold dishes with lids and dried in an open furnace to remove the last traces of NH4NO3. These dishes were placed in a furnace at 100°C, and the temperature gradually increased, over several hours, until a temperature of 400°C was achieved. The samples were left at this temperature overnight, and on removal, if necessary between drying and roasting, they were stored in a desiccator.

Next, the gel containers were loaded into a closed furnace at 400°C. The temperature was raised over several hours to 900°C, and the gels were roasted at this temperature overnight. On removal from the furnace, the gels were stored in a desiccator.  Once cooled to room temperature, they were then ground to a fine powder in an agate mortar and stored in bottles until required.

Small quantities of the dry gels were placed in platinum crucibles and fused in air at atmospheric pressure. The fusions took place at 1735°C for 10minutes, and on removal, were quenched in water. Three glasses were initially produced for each of the four gels, and for the three gels containing Ti, a fusion for 1 hour at 1735°C was produced, making a total of 15 samples.

After fusion, the glasses were checked for homogeneity by electron microprobe analysis (EMPA) to test if there was a difference in homogeneity between the 10minute and 1hour fusions, a very subtle difference was found, which wasn’t detrimental to the 10minute fusions. As a result, all 15 glasses were analysed on the Cameca ims-4f SIMS instrument at EMMAC, University of Edinburgh. Wet chemical analysis of 1 10minute fusion from each of the gels (0ppm, 100ppm, 500ppm and 1000ppm) was carried out using ICE OES again at the University of Edinburgh.