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Science and Engineering at The University of Edinburgh

School of GeoSciences

Research Facilities

Preparation of Rare Earth Glasses

Summary

The finished glass contains SiO2, Rare Earth (R.E) oxide (generally RE2O3), CaO and Al2O3

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 70oC and increasing the temperature slowly over several days to 180oC in an oven. This was followed by heating to around 400oC and finally roasting at 900oC.

When dry, the gels were ground up finely, placed in a large platinum crucible and fused using furnaces at around 1420-1550oC. 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

REE Samples
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 900oC 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 110oC 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 4OoC and then gradually increased, in 10 degree stages, to 95oC. 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

REE Samples

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 70oC for 24 hours and then the temperature was gradually increased to 160oC at around 10 degrees every 2-3 hours. The gels were left overnight at 160oC.

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 100oC and the heating was gradually increased, over several hours, until a temperature of 400oC 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 400oC. The temperature was raised over several hours to 900oC 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

REE Samples
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 1420-1550oC 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.



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

 

Element

R.E

Si

Ca

Al

O

Lanthanum

14.15

26.08

12.20

5.57

41.99

Cerium (III)

16.83

25.11

11.75

5.63

40.68

Praseodymium

13.90

26.19

12.25

5.59

42.06

Neodymium

14.48

25.99

12.16

5.55

41.81

Samarium

17.42

22.70

13.89

6.25

39.74

Dysprosium

18.56

22.38

13.73

6.15

39.19

Gadolinium

18.09

22.51

13.79

6.20

39.42

Yttrium

11.00

24.46

15.01

6.72

42.81

Terbium*

19.36

21.78

13.85

6.20

38.79

Erbium*

19.87

21.15

14.26

6.40

38.33

Holmium*

19.00

22.03

13.85

6.20

38.92

Ytterbium*

19.62

21.98

13.64

6.12

38.64

Europium*

19.94

19.60

15.54

7.01

37.91

Thulium*

22.87

17.44

16.28

7.30

36.11

Lutetium

19.62

22.11

16.51

6.08

38.68


*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)

 

Element

R.E Oxide

SiO2

CaO

Al2O3

Lanthanum

16.60

55.80

17.07

10.52

Cerium (III)

19.71

53.72

16.44

10.13

Praseodymium

16.27

56.02

17.14

10.56

Neodymium

16.89

55.59

17.02

10.49

Samarium

20.20

48.55

19.44

11.81

Dysprosium

21.30

47.87

19.21

11.63

Gadolinium

20.85

48.16

19.29

11.71

Yttrium

13.97

52.33

21.0

12.70

Terbium*

22.28

46.60

19.38

11.74

Erbium*

22.75

45.24

19.95

12.10

Holmium*

21.77

47.13

19.38

11.72

Ytterbium*

22.34

47.02

19.08

11.56

Europium*

23.09

41.93

21.74

13.24

Thulium*

26.12

37.30

22.78

13.80

Lutetium

22.31

47.29

18.91

11.49

 

* 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

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