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Sol-gel processing and crystallization of lithium tetragermanate
Journal of Non-Crystalline Solids 152 (1993) 137-142 North-Holland
~
J O U R N A L OF
~l~
Sol-gel processing and crystallization of lithium tetragermanate A. Aronne, M. Catauro, P. P e r n i c e a n d A. M a r o t t a Department of Materials and Production Engineering, Universit?l degli Studi di Napoli Federico II, Piazza& Tecchio, 80125 Naples, Italy
Received 9 April 1992 Revised manuscript received 14 September 1992
Lithium tetragermanate gel was synthesized by hydrolitic polycondensation of germanium ethoxide with lithium hydroxide monohydrate in alcoholic medium. Crystallization behaviour of the gel, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. Lithium tetragermanate gel crystallizes in two steps. In the primary transformation, at about 560°C, microcrystallitesof Li2Ge409 and GeO 2 are crystallized in an amorphous matrix. In the second transformation, at about 600°C, well shaped Li2Ge409 and Li2Ge7015 are formed. The values of activation energy for the two stages are found to be 557 and 405 kJmol -~, respectively. Activation energies are comparable with those reported with conventional melt glasses from oxides.
1. Introduction The sol-gel method of making inorganic glasses has been intensively studied in recent years [1]. Interest in this process has been stimulated, in part, by the low preparation temperature. The preparation involves hydrolysis and polycondensation of organometallic compounds. A gel forms which is dried to a porous particulate material. Proper thermal treatments are, therefore, required to convert the gel into the glass. Binary lithium silicate gel has been p r e p a r e d in a variety of ways and with a variety of lithium precursors [2]. No attention has been devoted to binary lithium germanate gels. The ionic size and the ionic charge of Ge 4+ are very similar to that of Si 4÷ and, therefore, the chemistry of silicates and of the germanates somewhat resemble each other. However, the ionic radius of Ge 4÷ (0.53 .~) is very close to c a t i o n / a n i o n radius ratio limit that separates teCorrespondence to: Dr A. Marotta, Department of Materials
and Production Engineering, Universit~ degli Studi di Napoli Federico II, Piazzale Tecchio, 80125 Naples, Italy. Tel: + 39-81 768 2410. Telefax: + 39-81 768 2413.
trahedral and octahedral co-ordination and, therefore, Ge 4+ can assume both fourfold and sixfold co-ordination. Investigations of binary lithium germanate gel preparation and crystallization behaviour are of interest for elucidating the nature crystallizing phases and for research on glass-ceramic materials. In this work, a lithium tetragermanate gel was prepared and crystallization behaviour was studied with X-ray diffraction and differential thermal analysis. This composition was chosen since the crystallization of the lithium tetragermanate oxide glass has been previously studied [3], so that the results obtained for the gel could be compared with those of the conventionally prepared glass.
2. Experimental Lithium tetragermanate gel was prepared using G e ( C 2 H 5 0 ) 4 ( T E O G ) and L i O H . H e O analytical grade reagents as starting materials. Water-free ethanol, obtained by distillation with metallic sodium of commercial anhydrous ethanol, was used, since the T E O G is a very water-sensi-
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
138
A. Aronne et al. / Lithium tetragermanate
TEOG :C2H50H
LiOH, H20 :H20 :C2H50H
i : 2 clear solution 20°C dry box
1 : 0.25 : 30 lactescent solution 20°C dry box
0oC 15 min
0oc 15 min
I Stirring
2 days 20°C
L Opaque wet gel
I 1 day 20°C
f 1 day 50°C
I dry gel
Fig. 1. Flow diagram of the gel preparation procedure. tive reagent and to permit control of T E O G / H20. Bidistillated water was used for hydrolysis reaction. The alcoholic solutions were prepared in a dry-box at room temperature. The w a t e r - a l coholic solution of lithium hydroxide appeared milky as the L i O H . H 2 0 was not initially entirely dissolved. A flow-chart indicating the preparation procedure and the compositions employed is given in fig. 1. The alcoholic solution of T E O G was mixed at 0°C with a water-alcoholic solution of lithium hydroxide. Under these conditions complete gelation occurred at room t e m p e r a t u r e in
two days. The gel obtained appeared opaque and quite homogeneous. The gelled system was held for one day more at room temperature before drying. The gel was fully dried in air at 50°C in an electric oven for one day. After these treatments an amorphous powder was obtained. The lithium content in the dried gel was verified using atomic absorption analysis. Samples of 63.9 mg of dried gel were placed in a platinum holder which was introduced into an electric oven at 1200°C in air. After 3 h at this temperature, the melted gel was air-quenched, weighed, dissolved in concentrated H F and diluted in bidistillated water. A P e r k i n - E l m e r spectrophotometer 300 was used. The nature and temperatures of the various reactions that occur during the heating of the dried gel was determined using simultaneous thermogravimetric ( T G A ) and differential thermal analysis (DTA). Powder sample, 20 mg of the dried gel, was subjected to a T G A / D T A run in N 2 at a heating rate of 10°C/min from room t e m p e r a t u r e to 700°C. A S t a n t o n - R e d c r o f t analyzer STA-780 was used, and powdered A120 3 was used as the reference material. The amorphous nature of the dried gel and identification of the phases crystallizing during the D T A runs were ascertained by X-ray diffraction using a Philips diffractometer. Powders of each sample were scanned from 20 = 5 - 6 0 ° using
15
o 10
a
5
i 200
i 400
i 600
Temperature
o
°C
Fig. 2. DTA and TGA curves of the dried gel, recorded in N 2
at
10°C/min.
-* I-
139
A. Aronne et al. / Lithium tetragermanate
CuKtx radiation. Non-isothermal crystallization of the dried gel was examined by differential thermal analysis (DTA). Powder samples of 50 mg of the dried gel, diluted with A120 3 in the ratio 1/5, were subjected to DTA runs in air at heating rates of 2, 3, 5, 10, 15, 20°C/min from room temperature to 700°C. A Netzsch high temperature DSC 404 thermoanalyzer was used, and powdered AI20 3 was used as the reference material. 3. Results
Chemical analysis of the melted gel showed that good composition control was provided by
Table 1 Chemical analysis of the melted gel Constituent
Analyzed wt%
Theoretical wt%
Li20 GeO 2
6.46 93.54
6.66 93.34
the sol-gel process (table 1). The analyzed and theoretical values are in fairly good agreement. Figure 2 shows the TGA and DTA curves of the dried gel. A large endothermic peak, from room temperature to about 250°C, appears on the DTA curve, with a maximum at about 150°C, and a simultaneous weight loss occurs in the TGA curve. The weight loss was 14.9%. These effects
• Li2Ge409 @Li2Ge7015 ®GeO 2 D I
•
c
ill
•
•
~eo
a
i
i
I
i
i
10
20
30
40
50
2~ Fig. 3. Powder X-ray diffraction pattern for (a) a dried gel, (b) a sample of dried gel heated in DTA furnace up to the temperature of the first exo-peak, and (c) a sample of the dried gel after a DTA run from room temperature to 700°C.
A. Aronne et al. / Lithium tetragermanate
140
were due to evaporation from open pores of the water and alcohol physically trapped in the gel. No appreciable effects were observed on T G A and D T A curves in the range of 250-500°C. That is due to the absence of organic substances which can be produced in sol-gel processing [1,4]. This conclusion is confirmed by the absence of exothermic effects on the D T A curve in air (see fig. 4). The D T A curve of the gel exhibits a slope change that may be attributed to the glass transition. In this work, the inflection point of the D T A curve was taken as the glass transition temperature (Tg = 532°C). A high and sharp exothermic peak appears, just above the Tg, on the D T A curve at the temperature of 565°C. At a higher temperature, 604°C, the D T A curve exhibits a second exo-peak, smaller than the first one. The presence of two exothermic effects on the D T A curve of the studied gel suggests a crystallization process in two steps. Figure 3 shows diffraction patterns of (a) a dried gel; (b) a dried gel heated in the D T A furnace up to the temperature of the first exopeak; and (c) a dried gel after a D T A run carried out from room temperature to 700°C. Trace (a) has broad humps characteristic of the amorphous state of the dried gel. The broad reflections on trace (b) of fig. 3 were attributed to Li2Ge409 and GeO2 microcrystaIlites. The reflections of trace (c) of the fig. 3 correspond to Li2Ge409 and LizGe7Oa5 crystals.
4. Discussion Gelation is the result of hydrolysis and condensation reactions according to the following equations: - G e - O E t + H - O H -- --- G e - O H + H - O E t ; (hydrolysis)
(1)
----G e - O E t + H O - G e - = = G e - O - G e ---+ H-OEt;
(2) (de-alcoholation)
= G e - O H + H O - G e = = = G e - O - G e -= Ge + H-OH;
(3) (dehydratation)
LiOH + ----G e - O H = -= G e - O - L i + + H - O H .
(4) At room temperature, the hydrolysis reaction (eq. (1)) is much faster than the condensation reactions (eqs. (2) and (3)), so that the number of G e - O - G e bridges formed are insufficient to give gelation, and precipitation of hydrated germanium oxide-alkoxide aggregate occurs. Mixing at a lower temperature (0°C) allows: (a) control of the hydrolytic reactivity of T E O G so that soluble polymeric intermediates are obtained, which then undergo further polymerization to form gel; and (b) the introduction of modifier cation, Li+, into germanate network according to eq. (4). The above reactions can be catalyzed by acids or bases. The reaction mechanisms are not known in every detail; however, it is generally accepted that they proceed through a second order nucleofic substitution [5]. The interaction between the electrophilic metal, Ge, and the nucleofic agent ( H 2 0 or O H - ) gives rise to addition (eq. (1)) or substitution (eq. (4)) reactions, respectively. Lithium hydroxide works simultaneously as reagent and catalyst in substitution reactions. The diffraction pattern of a gel sample heated in the D T A to the temperature of the first exopeak (see trace (b) in fig. 3) indicates the presence of few broad peaks in an amorphous background. These reflections are attributed to L i z G e 4 0 9 crystals and hexagonal G e O 2. This result, in spite of the strong thermal effect on D T A curve, suggests precipitation of a large number of microcrystaIlites dispersed in a non-crystalline matrix. The diffractogram obtained for a gel sample after the D T A run (trace (c) of fig. 3) exhibits a high number of sharp lines that correspond to LizGe409 and LizGe7015 crystals, while the strongest line of GeO 2 disappears. The crystal structure of LizGe409 contains rings of GeO 4 tetrahedra linked by G e O 6 octahedra forming a three-dimensional network [6]. This structure can be characterized by the formula Liz[Ge(GeO3)3].
141
A. Aronne et al. / Lithium tetragermanate
Voxide glass
+1
gel
I
I
I
l
100
300
SO0
700
Tempe reture oC
Fig. 4. DTA curves recorded in air at 10°C/min.
dehydroxylation occurs on the surface of the gel. The non-crystalline gel phase might be chemically inhomogeneous, in extremely fine scale, with segregation of a GeO2-rich phase. During the thermal treatment, a rearrangement occurs due to the dehydroxylation and the GeO2-rich phase crystallizes [8]. At higher temperatures the chemical reaction predominates and the formation of Li2Ge7Ols, having a composition closer to the starting gel composition, is favoured. However, as in the oxide glass, the major crystalline phase is Li2Ge409 in both crystallization steps. The DTA curves of lithium tetragermanate gel and oxide glass are compared in fig. 4. Both the curves exhibit a very strong exothermic effect followed by a second weak exothermic effect. This behaviour suggests a two-step crystallization mechanism. The activation energy, E, of each crystallization step was evaluated from DTA curves by [9] ln/3 = - (E/R)(1/Tp) + const.
The oxygen atoms in the network are either bridging atoms between two tetrahedrally coordinated Ge atoms or bridging atoms between one tetrahedrally and one octahedrally coordinated Ge atoms. This structure is very similar to that of Li2GevO15 which contains layers of GeO4 tetrahedra linked by GeO 6 octahedra forming a three-dimensional network and is characterized by the formula Liz[Ge(Ge2Os) 3] [7]. These results suggest that the non-isothermal crystallization of gel occurs in two steps. First, a high number of microcrystallites of Li2Ge409 and GeO 2 are formed and then, at higher temperature these are converted into well shaped Li2Ge409 and Li2Ge7015. This crystallization behaviour is quite similar to that of Li2Ge409 glass which was found to devitrify in two steps. In the primary transformation, microcrystallites of Li3Ge409 are crystallized in an amorphous matrix. In the second transformation, well shaped L i E G e 4 0 9 crystals are formed [3]. The formation of GeO 2 (which is not an equilibrium phase), in the first gel transformation, is most likely due to the decomposition or rearrangement of the amorphous gel phase when
This equation is based on the shift of the temperature, Tp, of the DTA crystallization peak as the DTA heating rate, /3, is changed. Multiple DTA runs were recorded in air at different heating rates on powder samples of gel and oxide glass. Since the first exo-peak is very sharp, the heat of crystallization is released in a very short time, making the heating rate non-linear in the crystallization temperature range. To minimize this el-
3
2 _c
1 I 1.18
I 1.20
I 1.22
) / T p , l o o o (K -~)
Fig. 5. Plot of In/3 against 1/Tp: zx, gel; e, conventional glass from oxide melts.
142
A. A r o n n e et al. / L i t h i u m tetragermanate
= 2
1 I
1.12
I
I
I
1.14
1.16
1.18
1/Tp * 1000 (K -1)
Fig. 6. Plot of In/3 against 1/Tp: zx, gel; o, conventional glass
from oxidemelts.
Table 2 Activation energy, E (kJ/mol), for crystallization Composition
1st step
2nd step
Gel Oxide glass
557 562
405 445
fect, gel and oxide glass samples were diluted with powdered A1203. Plots of ln]3 against 1 / T o (figs. 5 and 6) give straight lines in all cases. The values of activation energy calculated from their slopes are reported in table 2. It can be observed that the values of E, for the two crystallization steps, of the gel are very close to those of the conventional glasses from oxides. Moreover, the values of E for the the second crystallization steps of both gel and oxide glass are lower than those for the the first crystallization steps. These results are consistent with diffraction analysis. Li2Ge409 is the major crystalline phase in both gel and oxide glass. The second crystallization step involves only the growth of the microcrystallites formed during the first step.
5. Conclusions
The gel preparation involves hydrolysis and polycondensation of germanium ethoxide with lithium hydroxide monohydrate. The gel thus prepared is an amorphous solid containing water and
organic residues that are lost on heating. Processing temperatures are well below the glass transition temperature of the glass being formed and, therefore, during the heat treatment required for the gel into glass conversion, the gel is kinetically stable to crystallization. The lithium tetragermanate gel heated at constant heating rate during a DTA run crystallizes, as the oxides glass of the same composition, in two steps. Li2Ge409 and GeO 2 microcrystallites are initially formed and then converted at higher temperature into well shaped Li2GeaO 9 and Li2Ge7015 crystals. The values of activation energy for each step are consistent with the crystallization mechanism and very close to those obtained for lithium tetragermanate oxide glass. The authors wish to thank Mrs M. Palumbo and Mr A. Annetta for the X-ray measurements. The present work was supported by CNR 'Proggetto finalizzato: Materiali Speciali per Tecnologie Avanzate'.
References [1] C.J. Brinker, G.W. Scherer, Sol-Gel Science (Academic Press, San Diego, CA, 1990). [2] S.-P. Szu, M. Greenblatt and L.C. Klein, J. Non-Cryst. Solids 124 (1990) 91. [3] P. Pernice, A. Aronne and A. Marotta, Mater. Chem. Phys. 30 (1992) 195. [4] A. Osaka, M. Yuasa, Y. Miura and K. Takahashi, J. Non-Ci'yst. Solids 100 (1988) 409. [5] C. Sanchez, J. Livage, M. Henry and F. Babonneau, J. Non-Cryst. Solids 100 (1988) 65. [6] A. Winmann and E. Modern, Monatsh. Chem. 96 (1965) 581. [7] H. Vollenkle, A. Wittmann and H. Nowotny, Monatsh. Chem. 101 (1970) 46. [8l S.P. Mukheriee, J. Non-Cryst. Solids 82 (1986) 293. [9] T. Ozawa, Polymer 12 (1971) 150.
~
J O U R N A L OF
~l~
Sol-gel processing and crystallization of lithium tetragermanate A. Aronne, M. Catauro, P. P e r n i c e a n d A. M a r o t t a Department of Materials and Production Engineering, Universit?l degli Studi di Napoli Federico II, Piazza& Tecchio, 80125 Naples, Italy
Received 9 April 1992 Revised manuscript received 14 September 1992
Lithium tetragermanate gel was synthesized by hydrolitic polycondensation of germanium ethoxide with lithium hydroxide monohydrate in alcoholic medium. Crystallization behaviour of the gel, examined by differential thermal analysis and X-ray diffraction, is reported and discussed. Lithium tetragermanate gel crystallizes in two steps. In the primary transformation, at about 560°C, microcrystallitesof Li2Ge409 and GeO 2 are crystallized in an amorphous matrix. In the second transformation, at about 600°C, well shaped Li2Ge409 and Li2Ge7015 are formed. The values of activation energy for the two stages are found to be 557 and 405 kJmol -~, respectively. Activation energies are comparable with those reported with conventional melt glasses from oxides.
1. Introduction The sol-gel method of making inorganic glasses has been intensively studied in recent years [1]. Interest in this process has been stimulated, in part, by the low preparation temperature. The preparation involves hydrolysis and polycondensation of organometallic compounds. A gel forms which is dried to a porous particulate material. Proper thermal treatments are, therefore, required to convert the gel into the glass. Binary lithium silicate gel has been p r e p a r e d in a variety of ways and with a variety of lithium precursors [2]. No attention has been devoted to binary lithium germanate gels. The ionic size and the ionic charge of Ge 4+ are very similar to that of Si 4÷ and, therefore, the chemistry of silicates and of the germanates somewhat resemble each other. However, the ionic radius of Ge 4÷ (0.53 .~) is very close to c a t i o n / a n i o n radius ratio limit that separates teCorrespondence to: Dr A. Marotta, Department of Materials
and Production Engineering, Universit~ degli Studi di Napoli Federico II, Piazzale Tecchio, 80125 Naples, Italy. Tel: + 39-81 768 2410. Telefax: + 39-81 768 2413.
trahedral and octahedral co-ordination and, therefore, Ge 4+ can assume both fourfold and sixfold co-ordination. Investigations of binary lithium germanate gel preparation and crystallization behaviour are of interest for elucidating the nature crystallizing phases and for research on glass-ceramic materials. In this work, a lithium tetragermanate gel was prepared and crystallization behaviour was studied with X-ray diffraction and differential thermal analysis. This composition was chosen since the crystallization of the lithium tetragermanate oxide glass has been previously studied [3], so that the results obtained for the gel could be compared with those of the conventionally prepared glass.
2. Experimental Lithium tetragermanate gel was prepared using G e ( C 2 H 5 0 ) 4 ( T E O G ) and L i O H . H e O analytical grade reagents as starting materials. Water-free ethanol, obtained by distillation with metallic sodium of commercial anhydrous ethanol, was used, since the T E O G is a very water-sensi-
0022-3093/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved
138
A. Aronne et al. / Lithium tetragermanate
TEOG :C2H50H
LiOH, H20 :H20 :C2H50H
i : 2 clear solution 20°C dry box
1 : 0.25 : 30 lactescent solution 20°C dry box
0oC 15 min
0oc 15 min
I Stirring
2 days 20°C
L Opaque wet gel
I 1 day 20°C
f 1 day 50°C
I dry gel
Fig. 1. Flow diagram of the gel preparation procedure. tive reagent and to permit control of T E O G / H20. Bidistillated water was used for hydrolysis reaction. The alcoholic solutions were prepared in a dry-box at room temperature. The w a t e r - a l coholic solution of lithium hydroxide appeared milky as the L i O H . H 2 0 was not initially entirely dissolved. A flow-chart indicating the preparation procedure and the compositions employed is given in fig. 1. The alcoholic solution of T E O G was mixed at 0°C with a water-alcoholic solution of lithium hydroxide. Under these conditions complete gelation occurred at room t e m p e r a t u r e in
two days. The gel obtained appeared opaque and quite homogeneous. The gelled system was held for one day more at room temperature before drying. The gel was fully dried in air at 50°C in an electric oven for one day. After these treatments an amorphous powder was obtained. The lithium content in the dried gel was verified using atomic absorption analysis. Samples of 63.9 mg of dried gel were placed in a platinum holder which was introduced into an electric oven at 1200°C in air. After 3 h at this temperature, the melted gel was air-quenched, weighed, dissolved in concentrated H F and diluted in bidistillated water. A P e r k i n - E l m e r spectrophotometer 300 was used. The nature and temperatures of the various reactions that occur during the heating of the dried gel was determined using simultaneous thermogravimetric ( T G A ) and differential thermal analysis (DTA). Powder sample, 20 mg of the dried gel, was subjected to a T G A / D T A run in N 2 at a heating rate of 10°C/min from room t e m p e r a t u r e to 700°C. A S t a n t o n - R e d c r o f t analyzer STA-780 was used, and powdered A120 3 was used as the reference material. The amorphous nature of the dried gel and identification of the phases crystallizing during the D T A runs were ascertained by X-ray diffraction using a Philips diffractometer. Powders of each sample were scanned from 20 = 5 - 6 0 ° using
15
o 10
a
5
i 200
i 400
i 600
Temperature
o
°C
Fig. 2. DTA and TGA curves of the dried gel, recorded in N 2
at
10°C/min.
-* I-
139
A. Aronne et al. / Lithium tetragermanate
CuKtx radiation. Non-isothermal crystallization of the dried gel was examined by differential thermal analysis (DTA). Powder samples of 50 mg of the dried gel, diluted with A120 3 in the ratio 1/5, were subjected to DTA runs in air at heating rates of 2, 3, 5, 10, 15, 20°C/min from room temperature to 700°C. A Netzsch high temperature DSC 404 thermoanalyzer was used, and powdered AI20 3 was used as the reference material. 3. Results
Chemical analysis of the melted gel showed that good composition control was provided by
Table 1 Chemical analysis of the melted gel Constituent
Analyzed wt%
Theoretical wt%
Li20 GeO 2
6.46 93.54
6.66 93.34
the sol-gel process (table 1). The analyzed and theoretical values are in fairly good agreement. Figure 2 shows the TGA and DTA curves of the dried gel. A large endothermic peak, from room temperature to about 250°C, appears on the DTA curve, with a maximum at about 150°C, and a simultaneous weight loss occurs in the TGA curve. The weight loss was 14.9%. These effects
• Li2Ge409 @Li2Ge7015 ®GeO 2 D I
•
c
ill
•
•
~eo
a
i
i
I
i
i
10
20
30
40
50
2~ Fig. 3. Powder X-ray diffraction pattern for (a) a dried gel, (b) a sample of dried gel heated in DTA furnace up to the temperature of the first exo-peak, and (c) a sample of the dried gel after a DTA run from room temperature to 700°C.
A. Aronne et al. / Lithium tetragermanate
140
were due to evaporation from open pores of the water and alcohol physically trapped in the gel. No appreciable effects were observed on T G A and D T A curves in the range of 250-500°C. That is due to the absence of organic substances which can be produced in sol-gel processing [1,4]. This conclusion is confirmed by the absence of exothermic effects on the D T A curve in air (see fig. 4). The D T A curve of the gel exhibits a slope change that may be attributed to the glass transition. In this work, the inflection point of the D T A curve was taken as the glass transition temperature (Tg = 532°C). A high and sharp exothermic peak appears, just above the Tg, on the D T A curve at the temperature of 565°C. At a higher temperature, 604°C, the D T A curve exhibits a second exo-peak, smaller than the first one. The presence of two exothermic effects on the D T A curve of the studied gel suggests a crystallization process in two steps. Figure 3 shows diffraction patterns of (a) a dried gel; (b) a dried gel heated in the D T A furnace up to the temperature of the first exopeak; and (c) a dried gel after a D T A run carried out from room temperature to 700°C. Trace (a) has broad humps characteristic of the amorphous state of the dried gel. The broad reflections on trace (b) of fig. 3 were attributed to Li2Ge409 and GeO2 microcrystaIlites. The reflections of trace (c) of the fig. 3 correspond to Li2Ge409 and LizGe7Oa5 crystals.
4. Discussion Gelation is the result of hydrolysis and condensation reactions according to the following equations: - G e - O E t + H - O H -- --- G e - O H + H - O E t ; (hydrolysis)
(1)
----G e - O E t + H O - G e - = = G e - O - G e ---+ H-OEt;
(2) (de-alcoholation)
= G e - O H + H O - G e = = = G e - O - G e -= Ge + H-OH;
(3) (dehydratation)
LiOH + ----G e - O H = -= G e - O - L i + + H - O H .
(4) At room temperature, the hydrolysis reaction (eq. (1)) is much faster than the condensation reactions (eqs. (2) and (3)), so that the number of G e - O - G e bridges formed are insufficient to give gelation, and precipitation of hydrated germanium oxide-alkoxide aggregate occurs. Mixing at a lower temperature (0°C) allows: (a) control of the hydrolytic reactivity of T E O G so that soluble polymeric intermediates are obtained, which then undergo further polymerization to form gel; and (b) the introduction of modifier cation, Li+, into germanate network according to eq. (4). The above reactions can be catalyzed by acids or bases. The reaction mechanisms are not known in every detail; however, it is generally accepted that they proceed through a second order nucleofic substitution [5]. The interaction between the electrophilic metal, Ge, and the nucleofic agent ( H 2 0 or O H - ) gives rise to addition (eq. (1)) or substitution (eq. (4)) reactions, respectively. Lithium hydroxide works simultaneously as reagent and catalyst in substitution reactions. The diffraction pattern of a gel sample heated in the D T A to the temperature of the first exopeak (see trace (b) in fig. 3) indicates the presence of few broad peaks in an amorphous background. These reflections are attributed to L i z G e 4 0 9 crystals and hexagonal G e O 2. This result, in spite of the strong thermal effect on D T A curve, suggests precipitation of a large number of microcrystaIlites dispersed in a non-crystalline matrix. The diffractogram obtained for a gel sample after the D T A run (trace (c) of fig. 3) exhibits a high number of sharp lines that correspond to LizGe409 and LizGe7015 crystals, while the strongest line of GeO 2 disappears. The crystal structure of LizGe409 contains rings of GeO 4 tetrahedra linked by G e O 6 octahedra forming a three-dimensional network [6]. This structure can be characterized by the formula Liz[Ge(GeO3)3].
141
A. Aronne et al. / Lithium tetragermanate
Voxide glass
+1
gel
I
I
I
l
100
300
SO0
700
Tempe reture oC
Fig. 4. DTA curves recorded in air at 10°C/min.
dehydroxylation occurs on the surface of the gel. The non-crystalline gel phase might be chemically inhomogeneous, in extremely fine scale, with segregation of a GeO2-rich phase. During the thermal treatment, a rearrangement occurs due to the dehydroxylation and the GeO2-rich phase crystallizes [8]. At higher temperatures the chemical reaction predominates and the formation of Li2Ge7Ols, having a composition closer to the starting gel composition, is favoured. However, as in the oxide glass, the major crystalline phase is Li2Ge409 in both crystallization steps. The DTA curves of lithium tetragermanate gel and oxide glass are compared in fig. 4. Both the curves exhibit a very strong exothermic effect followed by a second weak exothermic effect. This behaviour suggests a two-step crystallization mechanism. The activation energy, E, of each crystallization step was evaluated from DTA curves by [9] ln/3 = - (E/R)(1/Tp) + const.
The oxygen atoms in the network are either bridging atoms between two tetrahedrally coordinated Ge atoms or bridging atoms between one tetrahedrally and one octahedrally coordinated Ge atoms. This structure is very similar to that of Li2GevO15 which contains layers of GeO4 tetrahedra linked by GeO 6 octahedra forming a three-dimensional network and is characterized by the formula Liz[Ge(Ge2Os) 3] [7]. These results suggest that the non-isothermal crystallization of gel occurs in two steps. First, a high number of microcrystallites of Li2Ge409 and GeO 2 are formed and then, at higher temperature these are converted into well shaped Li2Ge409 and Li2Ge7015. This crystallization behaviour is quite similar to that of Li2Ge409 glass which was found to devitrify in two steps. In the primary transformation, microcrystallites of Li3Ge409 are crystallized in an amorphous matrix. In the second transformation, well shaped L i E G e 4 0 9 crystals are formed [3]. The formation of GeO 2 (which is not an equilibrium phase), in the first gel transformation, is most likely due to the decomposition or rearrangement of the amorphous gel phase when
This equation is based on the shift of the temperature, Tp, of the DTA crystallization peak as the DTA heating rate, /3, is changed. Multiple DTA runs were recorded in air at different heating rates on powder samples of gel and oxide glass. Since the first exo-peak is very sharp, the heat of crystallization is released in a very short time, making the heating rate non-linear in the crystallization temperature range. To minimize this el-
3
2 _c
1 I 1.18
I 1.20
I 1.22
) / T p , l o o o (K -~)
Fig. 5. Plot of In/3 against 1/Tp: zx, gel; e, conventional glass from oxide melts.
142
A. A r o n n e et al. / L i t h i u m tetragermanate
= 2
1 I
1.12
I
I
I
1.14
1.16
1.18
1/Tp * 1000 (K -1)
Fig. 6. Plot of In/3 against 1/Tp: zx, gel; o, conventional glass
from oxidemelts.
Table 2 Activation energy, E (kJ/mol), for crystallization Composition
1st step
2nd step
Gel Oxide glass
557 562
405 445
fect, gel and oxide glass samples were diluted with powdered A1203. Plots of ln]3 against 1 / T o (figs. 5 and 6) give straight lines in all cases. The values of activation energy calculated from their slopes are reported in table 2. It can be observed that the values of E, for the two crystallization steps, of the gel are very close to those of the conventional glasses from oxides. Moreover, the values of E for the the second crystallization steps of both gel and oxide glass are lower than those for the the first crystallization steps. These results are consistent with diffraction analysis. Li2Ge409 is the major crystalline phase in both gel and oxide glass. The second crystallization step involves only the growth of the microcrystallites formed during the first step.
5. Conclusions
The gel preparation involves hydrolysis and polycondensation of germanium ethoxide with lithium hydroxide monohydrate. The gel thus prepared is an amorphous solid containing water and
organic residues that are lost on heating. Processing temperatures are well below the glass transition temperature of the glass being formed and, therefore, during the heat treatment required for the gel into glass conversion, the gel is kinetically stable to crystallization. The lithium tetragermanate gel heated at constant heating rate during a DTA run crystallizes, as the oxides glass of the same composition, in two steps. Li2Ge409 and GeO 2 microcrystallites are initially formed and then converted at higher temperature into well shaped Li2GeaO 9 and Li2Ge7015 crystals. The values of activation energy for each step are consistent with the crystallization mechanism and very close to those obtained for lithium tetragermanate oxide glass. The authors wish to thank Mrs M. Palumbo and Mr A. Annetta for the X-ray measurements. The present work was supported by CNR 'Proggetto finalizzato: Materiali Speciali per Tecnologie Avanzate'.
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