Preparation of Rare Earth Glasses

Summary

The gels were made in a similar manner to that described in Biggar and O'Hara (1969). The gel starting materials were prepared by mixing the required weights of standard solutions of aluminium, calcium and rare earth nitrates to produce a final weight of either 5g or 10g. Ethanol was then added to the nitrate mixture. This was to ensure the miscibility of tetraethyl orthosilicate (TEOS Si(OC₂H₅)₄), used for the silica component, which was added next. Concentrated ammonia (0.88vol NH₄OH) was then added to form a gelatinous precipitate of hydroxides. The mixture was then covered and left for at least 16 hours to ensure the complete hydrolysis of the TEOS.

The gels were then slowly dried, starting at 70^oC and increasing the temperature slowly over several days to 180^oC in an oven. This was followed by heating to around 400^oC and finally roasting at 900^oC.

When dry, the gels were ground up finely, placed in a large platinum crucible and fused using furnaces at around 1420-1550^oC. They were then quenched in ice water. The fusions were carried out in controlled atmospheric conditions when required. Due to the small crucible size the fusion had to be done in several batches. Different oxides underwent a different number of fusions depending on the homogeneity after each fusion. If the homogeneity could be improved by further fusion the batches were bulked, ground and re-fused. This process could be repeated up to 5 times

A detailed description of the Gel making process can be downloaded (by Elizabeth Cairnes)

GEL STARTING MATERIALS

RARE EARTH SOLUTIONS

With the exception of cerium, all of the rare earth solutions used were obtained by dissolving the corresponding rare earth oxide. In the case of cerium, (NH₄)₂Ce(NO₃)₆ was used.

The rare earth solutions were prepared by first igniting the oxides at 900^oC for around 2 hours. When cool, the required amount was weighed accurately into a 250ml beaker and enough water added to cover the powder. This was followed by the addition of  concentrated HNO₃ to dissolve the R.E. oxide. The mixture was heated on a hotplate until the oxide was completely dissolved and then the beaker was transferred to a steam bath. The solution was evaporated to dryness. The resultant R.E. nitrates were taken up in water and a small amount of 50% HNO₃ added (1ml to a 10g batch). The solution was then diluted to the required concentration.

The (NH₄)₂Ce(NO₃)₆ solution was prepared by 'drying' the crystals at 110^oC overnight and dissolving in water with 10ml concentrated HNO₃. This solution was then transferred quantitatively to a volumetric flask and made up to the mark with water.

ALUMINIUM SOLUTION

The Aluminium solution was prepared by accurately weighing the required amount of dry aluminium powder into a 250ml conical flask and adding enough water to cover. Next, around 120ml of a 33 % (approx.) solution of HNO₃ was dripped slowly and with stirring onto the mixture. When reaction had ceased, the temperature of the bath was set to 4O^oC and then gradually increased, in 10 degree stages, to 95^oC. When the aluminium powder was completely dissolved, the cooled solution was transferred quantitatively to a volumetric flask.

CALCIUM SOLUTION

The calcium solution was prepared by accurately weighing the required amount of calcium carbonate into a conical flask an adding approximately 100ml of water. Around 200ml of a 33% (approx.) solution of HNO₃ was then dripped slowly onto the mixture. When the CaCO₃ was completely dissolved, the solution was transferred quantitatively to a volumetric flask.

TETRAETHYL ORTHOSILICATE

TEOS is used to supply the silica component of the gels. It is used 'straight from the bottle' but the purity of each batch was measured prior to use. TEOS is immiscible with water and in order to homogenise it, ethanol must be added to the mixture of aqueous nitrates.

PREPARATION OF GELS

The weight and volume of solutions required for each gel were calculated and these amounts were titrated into Teflon beakers.  Next, 20ml of ethanol was added and mixed thoroughly with the aqueous components before adding the TEOS. As soon as the TEOS was added, the mixture was stirred vigorously to ensure the complete homogenisation of the TEOS. As soon as possible after the alcohol addition, 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 to glass beakers for heating. The beakers were placed in an oven at 70^oC for 24 hours and then the temperature was gradually increased to 160^oC at around 10 degrees every 2-3 hours. The gels were left overnight at 160^oC.

A slow heating process removed the NH₄NO₃ over a period of approximately 36 hours. The heating was slow to avoid gel fragments contaminating other gels if large amounts of NH₄NO₃ were released explosively. 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 roasted in an open furnace to remove the last traces of NH₄NO₃. The beakers were placed in a furnace at 100^oC and the heating was gradually increased, over several hours, until a temperature of 400^oC was achieved. The samples were left at this temperature overnight and, on removal, they were stored in a dessicator.

The gels were finally dried to constant weight. Next, the gel containers were loaded into a closed furnace at 400^oC. The temperature was raised over several hours to 900^oC and the samples were left at this temperature overnight. On cooling, the containers were weighed and the yield of dry gel recorded. The gels were then ground to a fine powder in an agate mortar and stored in bottles until required.

N.B. R.E. oxides are very hygroscopic and so too are the corresponding gels. Because of this, it was difficult to record the yields accurately.

FUSION OF GELS

FUSION PROCEDURE

The dry gels were placed in platinium crucibles and  fusions were carried out in furnaces, in small quantities, under controlled atmospheric conditions (log fO₂ ~ -7.2) where necessary. The fusions took place between 1420-1550^oC and were then quenched in water.

After fusion the glasses were checked for homogeneity by electron microprobe analysis (EMPA). If the glasses' homogeneity could be improved they underwent further fusions. Due to the small quantities used the fusions occurred in batches. Before re-fusing the batches were bulked together, ground and then re-fused. This process could be repeated up to 5 times before the final result was reached.

*Recalculated values as some of the gels produced gave yields of less than the theoretical values. The most likely explanation for this is evaporation of non-hydrolysed TEOS. The compositions of these gels have been recalculated to take this into account.

Theoretical Composition of rare earth glasses (wt. percent oxide)

* Recalculated values as some of the gels produced gave yields of less than the theoretical values. The most likely explanation for this is evaporation of non-hydrolysed TEOS. The compositions of these gels have been recalculated to take this into account.

References

Biggar (G.M) and O'Hara(M.J) 1969- A comparison of gel and glass starting materials for phase equilibrium studies, Mineralogical Magazine, June 1969, Vol 37, No 286