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Determination of the crystallization enthalpies of lithium ion conducting alumino–silicate glasses
Journal of Non-Crystalline Solids 262 (2000) 177±182
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Determination of the crystallization enthalpies of lithium ion conducting alumino±silicate glasses J. Rogez a, P. Knauth b,*, A. Garnier b, H. Ghobarkar c, O. Sch af d b
a CNRS ± Centre de Thermodynamique et de Microcalorim etrie, France Laboratoire de Physico±Chimie des Mat eriaux, Universit e de Provence, Centre de St. Charles, Case 26, 3 Place Victor Hugo, F-13331, Marseille cedex 3, France c Institut f ur Mineralogie, Freie Universit at Berlin, D-14195, Berlin, Germany d Institut f ur Chemie und Biochemie, Universit at Greifswald, D-17489, Greifswald, Germany
Received 24 May 1999; received in revised form 22 October 1999
Abstract An irreversible change of the electrical conductivity of Li ion-conducting lithium alumino±silicate glasses is observed during crystallization. The enthalpies of crystallization of the glasses are measured by solution calorimetry at 298 K in HF/HNO3 mixtures and compared with similar sodium glasses. The observed dierences are related to the alumino±silicate crystal chemistry and the cation radii. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Lithium alumino±silicate glasses are solid lithium ion conductors and tend to form glass ceramics [1]. They have been used as solid electrolytes in CO2 gas sensors [2,3]. The crystallization of the glasses limits the temperature range for application of such systems, because the properties of the solid electrolytes change irreversibly. Several glass compositions were previously investigated [2,3]: schematically, increasing the concentration of alumina in the glass matrix leads to a higher temperature of crystallization and therefore an improved sensor stability. We show here that the irreversible variation of the electrical conductivity is a sensitive indicator
* Corresponding author. Tel.: +33-4 91 10 62 96; fax: +33-4 91 10 62 37. E-mail address: [email protected] (P. Knauth).
for starting devitri®cation. The glass transition (Tg ) and the recrystallisation temperatures (Tx;r ) were previously determined by electrical measurements [2,3]. We de®ne Tx;r as the temperature at which an irreversible change of conductivity is observed. It is worth noting that in glasses of these systems, crystallization starts below the glass transition temperature. Based on this knowledge, we report crystallization enthalpies for the glasses and show that correlations can be found between the crystallization enthalpies for lithium and sodium glasses with the crystal chemistry and the cation radii. The investigated materials are referenced as follows: A and B samples correspond respectively to a composition Li2 O 5SiO2 and Li2 O 2SiO2 . The C and D samples are obtained with partial substitution of silica by alumina: C is Li2 O 3.5SiO2 1.5Al2 O3 and D is Li2 O 1.25SiO2 0.75Al2 O3 . According to the phase diagrams [4,5], sample B corresponds to a line compound and sample A to a
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 8 8 - 2
178
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
two-phase region: it should crystallize as (Li2 O 2SiO2 ) + SiO2 . The ternary phase diagram is incomplete in the sub-liquidus part. One can estimate that the sample C crystallizes into a mixture of a silica-rich lithium alumino±silicate (Li2 O Al2 O3 3SiO2 or Li2 O Al2 O3 4SiO2 ), a binary alumino±silicate (Al2 O3 SiO2 or 3Al2 O3 2SiO2 ) and SiO2 . Sample D should give a silica-rich ternary compound (Li2 O Al2 O3 2SiO2 ), a lithium-rich ternary compound (5Li2 O Al2 O3 4SiO2 or 3Li2 O Al2 O3 2SiO2 ) and lithium aluminate (Li2 O Al2 O3 ). 2. Experimental Glasses of the respective compositions were obtained by mixing commercial products Li2 CO3 (suprapure), SiO2 (a-quartz, p.A.) and, for samples C and D, c-Al2 O3 (suprapure) in a ball mill using methanol. Afterwards, methanol was removed under vacuum conditions and the ground and milled materials were melted in an induction furnace using high purity graphite crucibles at temperatures between 2100 and 2300 K for 10 min. The specimens were then quenched on copper
plates, ground in a mortar and re-heated. This process was repeated twice in order to obtain homogeneous glasses. Powder X-ray diraction (XRD) was used to verify the glassy state. The glasses were cut with a diamond saw, using methanol as lubricant in order to avoid leaching of the alkaline. Electrical conductivity was determined by impedance spectroscopy with sputtered gold electrodes of 100 nm thickness. The investigated frequency range was between 105 and 0.05 Hz with an a.c. amplitude of 15 mV. The measurements were performed generally between ambient temperature and 970 K in a dried mixture of N2 (78%), O2 (21%) and Ar (1%), corresponding to the composition of air. The times and temperatures for a complete crystallization of the glasses were extrapolated from a study of the crystallization of lithium disilicate glass, observed to be total after 144 h at 793 K [6]. The maximum of the nucleation rate lies about 50 K above Tg [7]. For the similar compound Na2 O 2SiO2 , the growth rate presents a maximum 150 K above the maximum nucleation rate, according to the results of Scott and Pask [8]. However, alkaline loss increase with the tempera-
Table 1 Solution enthalpies of glassy (DHdiss gl) and crystalline (DHdiss cr) lithium alumino±silicates in HF/HNO3 at 298 K and enthalpies of crystallization (DHcryst ) at that temperaturea
A
B
C
D
a
Tg (K)
Tx (K)
Tx;r (K)
Heat treatment
DHdiss gl (kJ molÿ1 )
DHdiss cr (kJ molÿ1 )
774
890
700
12 h/973 K
)148.0 )147.5 )147.7 )147.8 0.2
)138.6 )138.4 )138.5 0.1
)9.3 0.3
)147.0 )146.9 )147.0 0.1
)131.3 )131.2 )131.2 0.1
)15.8 0.2
)202.7 )202.6 )202.7 0.1
)179.9 )177.7 )178.8 0.1
)23.9 0.2
)197.4 )198.2 )198.3 )198.0 0.1
)182.0 )182.2
746
1048
881
860
1118
967
700
780
780
93 h/913 K
71 h/1173 K
22 h/1053 K
)182.1 0.1
DHcryst (kJ molÿ1 )
)15.9 0.2
Tg is the glass transition temperature, Tx the maximum crystallization temperature (from Ref. [1]) and Tx;r the temperature at which the electrical conductivity of the glass changes, irreversibly. Time and temperature of the heat treatment of glassy materials are indicated in column 5.
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
ture. Based on these observations, the times and temperatures for the heat treatments are a compromise (cf. column 5 of Table 1); pure platinum crucibles were used. The crystallized materials were analyzed by XRD. The solution calorimeter was previously described [9]. For each run, 50 mg of ground sample were dissolved at 298 K in 50 ml of a solvent containing 50 vol.% HF (6 mol/l) and 50 vol% HNO3 (4 mol/l). This takes about 3 h for the glassy and the crystalline D samples. Crystalline A et B were completely dissolved only after 5 h and more than 5 h were necessary for the complete dissolution of the crystallized specimen C. This large dissolution time is due to the presence of an alumina-rich compound, well known to be hardly soluble in aqueous acidic solvents.
3. Results All glasses were completely amorphous according to XRD. Fig. 1 shows an Arrhenius plot of the electrical conductivity of sample A. One recognizes easily a conductivity anomaly due to the start of crystallization (Tx;r ) below the glass transition temperature. The Tx;r values (Table 1) are about 700 K for the binary Li±Si±O glasses (A
Fig. 1. Temperature dependence of the electrical conductivity of glass A with composition (mol%) Li2 O 16.6, SiO2 83.4.
179
and B) and around 780 K for the ternary ones (C and D). The enthalpies of activation in the region below the crystallization are (0:7 0:1) eV. Fig. 2 shows the XRD pattern of the crystallized sample A. Glass A crystallizes into lithium disilicate Li2 Si2 O5 ; quartz is not observed within the limits of detection of XRD. In the case of sample B, lithium disilicate is detected together with lithium metasilicate Li2 SiO3 . Silica-rich lithium alumino±silicates are the major phases formed by crystallization of samples C and D. Sample C gives Li2 O Al2 O3 3SiO2 and Al2 O3 SiO2 and sample D Li2 O Al2 O3 2SiO2 together with Li2 O Al2 O3 and 5Li2 O Al2 O3 4SiO2 . In Table 1 the experimental solution enthalpies at 298 K and in the last column the calculated enthalpies of crystallization at that temperature are also gathered. 4. Discussion The observed phase equilibria are consistent with the present knowledge of the phase diagrams. Further work is in progress to perform crystallization of the glasses into the equilibrium phases using hydrothermal conditions. The Tx;r values increase with the concentration of alumina and silica. The activation enthalpies of conductivity are consistent with Li ion migration in a glassy matrix [10]. The enthalpy of crystallization of lithium disilicate obtained by Sen et al. [6] from drop-dissolution experiments in a 2PbO B2 O3 melt at 976 K is about 6 kJ/mol more exothermic than the present result [6]. However, the summed experimental uncertainties (nearly 5 kJ/mol for Sen et al. and 1 kJ/mol for our data) are of the same order of magnitude than this dierence and there is no real contradiction to our results. On the other hand, experimental factors may contribute to the discrepancy and need to be considered: (i) the enthalpy variation with solute concentration and (ii) heat capacity corrections. Table 2 shows some experimental data on concentration eects in related silicate systems. Clearly, they can be sometimes important: in our experiments, we therefore worked as far as possible in solutions of the same
180
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
Fig. 2. XRD pattern of the crystallized sample A: # Li2 Si2 O5 (JCPDS n° 40-0376).
®nal concentration. The R T heat capacity correction term can be written: 298 Cpcr ÿ Cpgl dT . The use of a solvent at 298 K has the advantage to avoid possible eects due to a temperature change of the glasses during the drop. For example, if we assume that 50% of glass was heated to the dissolution temperature (976 K) in the experiments of Sen et
al., an estimation based on data for pure silica [11,12] would give an enthalpy variation DH 1 kJ/mol. Altogether, our results are not incompatible with those of Sen et al. [6]. The enthalpies of crystallization of several lithium and sodium silicates and alumino±silicates at 298 K are compared in Table 3. There is no
Table 2 Variation of the dissolution enthalpy (D DHdiss ) with concentration, expressed in mol% in (Bi2 O3 )4 (B2 O3 )6 melts and in mmol/l in HF/ HNO3 Solvent
Solute
T/K
D DHdiss
(Bi2 O3 )4 (B2 O3 )6
CuO Y2 O3 Y2 Cu2 O5
1033 1033 1033
kJ/mol per mol% 0 0 5
50 vol% HF 6M + 50 vol% HNO3 4M
Na2 O 2SiO2 Na2 O Cs2 O 2SiO2
298 298
kJ/mol per mmol/l 2 2
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
181
Table 3 Enthalpies of crystallization (DHcryst ) and enthalpies of melting (DHmelt ) of some alkali silicates and alumino±silicates at 298 K SiO2 Li2 O 5SiO2 Li2 O 2SiO2 Li2 O 3.5SiO2 1.5Al2 O3 Li2 O 1.25SiO2 0.75Al2 O3 Na2 O SiO2 Na2 O 2SiO2 Na2 O Al2 O3 2SiO2 Na2 O Al2 O3 6SiO2
DHcryst (kJ/mol)
DHcryst (kJ/mol) (SiO2 + Al2 O3 )
)7.8 )55.8 )47.4 )53.5 [6] )143.4 )47.7 )17.8 [12] )9.4 [12] )56.0 [12] )119.0 [12]
)7.8 )11.2 )23.7 )28.7 )23.8 )17.8 )4.7 )18.7 )17.1
systematic correlation between the composition of the glasses and their crystallization enthalpy. However, the comparison with literature data reveals that the enthalpies of crystallization per mole of silica + alumina are smaller for sodium than for lithium glasses. The enthalpies of melting seem to change in a similar way. Following the de®nitions of DHcrystal and DHmelt , we can deduce the dierence between the extrapolated enthalpy of the melt and that of the glassy state at 298 K, which is close to 30 kJ/mol for both lithium and sodium disilicates. Furthermore, the relative species abundance in alkaline silicate melts calculated from equilibrium constants as function of composition shows that the dierence between lithium and sodium systems is small. This suggests that the observed enthalpy dierence stems mainly from the crystalline state [13]. To clarify the in¯uence of cations of dierent radius, let us consider the case of disilicates, which form ¯at sheets of tetrahedral silicate entities with cations densely packed between these layers [14]. With smaller cations than cesium, the layer becomes warped for electrostatic reasons. The layers are signi®cantly folded with small lithium ions and more energy is necessary to destroy long range order than with larger sodium ions. Although the balance between oxygen±oxygen and cation±cation repulsions and not radius ratio rules alone control the stability of ionic crystals [15], this is a qualitative explanation for the observed dierence of crystallization enthalpies of sodium and lithium glasses.
DHmelt (kJ/mol)
53 [12] 52 [12] 35 [12]
5. Conclusion The crystallization of Li ion-conducting lithium alumino±silicate glasses degrades the electrical conductivity irreversibly. Measurements of the electrical conductivity can thus be used as an indicator of starting devitri®cation. The dierence of crystallization enthalpies between lithium and similar sodium glasses can be related to the different cation radii and their in¯uence on the alumino±silicate crystal chemistry. Acknowledgements The authors wish to thank M. Decressac (CNRS ± Centre de Thermodynamique et de Microcalorimetrie) for some calorimetric measurements. References [1] P.E. Doherty, D.W. Lee, R.S. Davis, J. Am. Ceram. Soc. 50 (1967) 77. [2] O. Sch af, Ionics 2 (1996) 266. [3] O. Sch af, PhD thesis, Free University Berlin, 1995. [4] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase diagrams for ceramists, Am. Ceram. Soc. Columbus (1969) diagr. 179, 182. [5] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase diagrams for ceramists, Am. Ceram. Soc. Columbus (supplement) (1969) diagr. 2426. [6] S. Sen, C. Gerardin, A. Navrotsky, J.E. Dickinson, J. NonCryst. Solids 168 (1994) 64.
182
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
[7] I. Gutzow, J. Schmelzer, The Vitreous State, Springer, Berlin, 1995. [8] W.D. Scott, J.A. Pask, J. Am. Ceram. Soc. 44 (1961) 181. [9] M. Ganteaume, M. Coten, M. Decressac, Thermochim. Acta 178 (1991) 81. [10] J.L. Souquet, Ionic transport in glassy electrolytes, in: P.G. Bruce (Ed.), Solid State Electrochemistry, Cambridge University, Cambridge, 1995. [11] P. Richet, Y. Bottinga, L. Denielou, J.P. Petitet, C. Tequi, Geochim. Cosmochim. Acta 46 (1982) 2639.
[12] I. Barin, Thermochemical Data of Pure Substances, 3rd Ed., VCH, Weinheim, 1995. [13] A. Navrotsky, Structure, Dynamics and Properties of Silicate Melts, in: J.F. Stebbins, P.F. McMillan, D.B. Dingwell (Eds.), Reviews in Mineralogy, vol. 32, MSA, Washington, 1995 ch. 5. [14] F. Liebau, Structural Chemistry of Silicates, Springer, Berlin, 1985, p. 199. [15] P.C. Hess, in: J.F. Stebbins, P.F. McMillan, D.B. Dingwell (Eds.), Reviews in Mineralogy, vol. 32, MSA, Washington, 1995, ch. 6.
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Determination of the crystallization enthalpies of lithium ion conducting alumino±silicate glasses J. Rogez a, P. Knauth b,*, A. Garnier b, H. Ghobarkar c, O. Sch af d b
a CNRS ± Centre de Thermodynamique et de Microcalorim etrie, France Laboratoire de Physico±Chimie des Mat eriaux, Universit e de Provence, Centre de St. Charles, Case 26, 3 Place Victor Hugo, F-13331, Marseille cedex 3, France c Institut f ur Mineralogie, Freie Universit at Berlin, D-14195, Berlin, Germany d Institut f ur Chemie und Biochemie, Universit at Greifswald, D-17489, Greifswald, Germany
Received 24 May 1999; received in revised form 22 October 1999
Abstract An irreversible change of the electrical conductivity of Li ion-conducting lithium alumino±silicate glasses is observed during crystallization. The enthalpies of crystallization of the glasses are measured by solution calorimetry at 298 K in HF/HNO3 mixtures and compared with similar sodium glasses. The observed dierences are related to the alumino±silicate crystal chemistry and the cation radii. Ó 2000 Elsevier Science B.V. All rights reserved.
1. Introduction Lithium alumino±silicate glasses are solid lithium ion conductors and tend to form glass ceramics [1]. They have been used as solid electrolytes in CO2 gas sensors [2,3]. The crystallization of the glasses limits the temperature range for application of such systems, because the properties of the solid electrolytes change irreversibly. Several glass compositions were previously investigated [2,3]: schematically, increasing the concentration of alumina in the glass matrix leads to a higher temperature of crystallization and therefore an improved sensor stability. We show here that the irreversible variation of the electrical conductivity is a sensitive indicator
* Corresponding author. Tel.: +33-4 91 10 62 96; fax: +33-4 91 10 62 37. E-mail address: [email protected] (P. Knauth).
for starting devitri®cation. The glass transition (Tg ) and the recrystallisation temperatures (Tx;r ) were previously determined by electrical measurements [2,3]. We de®ne Tx;r as the temperature at which an irreversible change of conductivity is observed. It is worth noting that in glasses of these systems, crystallization starts below the glass transition temperature. Based on this knowledge, we report crystallization enthalpies for the glasses and show that correlations can be found between the crystallization enthalpies for lithium and sodium glasses with the crystal chemistry and the cation radii. The investigated materials are referenced as follows: A and B samples correspond respectively to a composition Li2 O 5SiO2 and Li2 O 2SiO2 . The C and D samples are obtained with partial substitution of silica by alumina: C is Li2 O 3.5SiO2 1.5Al2 O3 and D is Li2 O 1.25SiO2 0.75Al2 O3 . According to the phase diagrams [4,5], sample B corresponds to a line compound and sample A to a
0022-3093/00/$ - see front matter Ó 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 3 0 9 3 ( 9 9 ) 0 0 6 8 8 - 2
178
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
two-phase region: it should crystallize as (Li2 O 2SiO2 ) + SiO2 . The ternary phase diagram is incomplete in the sub-liquidus part. One can estimate that the sample C crystallizes into a mixture of a silica-rich lithium alumino±silicate (Li2 O Al2 O3 3SiO2 or Li2 O Al2 O3 4SiO2 ), a binary alumino±silicate (Al2 O3 SiO2 or 3Al2 O3 2SiO2 ) and SiO2 . Sample D should give a silica-rich ternary compound (Li2 O Al2 O3 2SiO2 ), a lithium-rich ternary compound (5Li2 O Al2 O3 4SiO2 or 3Li2 O Al2 O3 2SiO2 ) and lithium aluminate (Li2 O Al2 O3 ). 2. Experimental Glasses of the respective compositions were obtained by mixing commercial products Li2 CO3 (suprapure), SiO2 (a-quartz, p.A.) and, for samples C and D, c-Al2 O3 (suprapure) in a ball mill using methanol. Afterwards, methanol was removed under vacuum conditions and the ground and milled materials were melted in an induction furnace using high purity graphite crucibles at temperatures between 2100 and 2300 K for 10 min. The specimens were then quenched on copper
plates, ground in a mortar and re-heated. This process was repeated twice in order to obtain homogeneous glasses. Powder X-ray diraction (XRD) was used to verify the glassy state. The glasses were cut with a diamond saw, using methanol as lubricant in order to avoid leaching of the alkaline. Electrical conductivity was determined by impedance spectroscopy with sputtered gold electrodes of 100 nm thickness. The investigated frequency range was between 105 and 0.05 Hz with an a.c. amplitude of 15 mV. The measurements were performed generally between ambient temperature and 970 K in a dried mixture of N2 (78%), O2 (21%) and Ar (1%), corresponding to the composition of air. The times and temperatures for a complete crystallization of the glasses were extrapolated from a study of the crystallization of lithium disilicate glass, observed to be total after 144 h at 793 K [6]. The maximum of the nucleation rate lies about 50 K above Tg [7]. For the similar compound Na2 O 2SiO2 , the growth rate presents a maximum 150 K above the maximum nucleation rate, according to the results of Scott and Pask [8]. However, alkaline loss increase with the tempera-
Table 1 Solution enthalpies of glassy (DHdiss gl) and crystalline (DHdiss cr) lithium alumino±silicates in HF/HNO3 at 298 K and enthalpies of crystallization (DHcryst ) at that temperaturea
A
B
C
D
a
Tg (K)
Tx (K)
Tx;r (K)
Heat treatment
DHdiss gl (kJ molÿ1 )
DHdiss cr (kJ molÿ1 )
774
890
700
12 h/973 K
)148.0 )147.5 )147.7 )147.8 0.2
)138.6 )138.4 )138.5 0.1
)9.3 0.3
)147.0 )146.9 )147.0 0.1
)131.3 )131.2 )131.2 0.1
)15.8 0.2
)202.7 )202.6 )202.7 0.1
)179.9 )177.7 )178.8 0.1
)23.9 0.2
)197.4 )198.2 )198.3 )198.0 0.1
)182.0 )182.2
746
1048
881
860
1118
967
700
780
780
93 h/913 K
71 h/1173 K
22 h/1053 K
)182.1 0.1
DHcryst (kJ molÿ1 )
)15.9 0.2
Tg is the glass transition temperature, Tx the maximum crystallization temperature (from Ref. [1]) and Tx;r the temperature at which the electrical conductivity of the glass changes, irreversibly. Time and temperature of the heat treatment of glassy materials are indicated in column 5.
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
ture. Based on these observations, the times and temperatures for the heat treatments are a compromise (cf. column 5 of Table 1); pure platinum crucibles were used. The crystallized materials were analyzed by XRD. The solution calorimeter was previously described [9]. For each run, 50 mg of ground sample were dissolved at 298 K in 50 ml of a solvent containing 50 vol.% HF (6 mol/l) and 50 vol% HNO3 (4 mol/l). This takes about 3 h for the glassy and the crystalline D samples. Crystalline A et B were completely dissolved only after 5 h and more than 5 h were necessary for the complete dissolution of the crystallized specimen C. This large dissolution time is due to the presence of an alumina-rich compound, well known to be hardly soluble in aqueous acidic solvents.
3. Results All glasses were completely amorphous according to XRD. Fig. 1 shows an Arrhenius plot of the electrical conductivity of sample A. One recognizes easily a conductivity anomaly due to the start of crystallization (Tx;r ) below the glass transition temperature. The Tx;r values (Table 1) are about 700 K for the binary Li±Si±O glasses (A
Fig. 1. Temperature dependence of the electrical conductivity of glass A with composition (mol%) Li2 O 16.6, SiO2 83.4.
179
and B) and around 780 K for the ternary ones (C and D). The enthalpies of activation in the region below the crystallization are (0:7 0:1) eV. Fig. 2 shows the XRD pattern of the crystallized sample A. Glass A crystallizes into lithium disilicate Li2 Si2 O5 ; quartz is not observed within the limits of detection of XRD. In the case of sample B, lithium disilicate is detected together with lithium metasilicate Li2 SiO3 . Silica-rich lithium alumino±silicates are the major phases formed by crystallization of samples C and D. Sample C gives Li2 O Al2 O3 3SiO2 and Al2 O3 SiO2 and sample D Li2 O Al2 O3 2SiO2 together with Li2 O Al2 O3 and 5Li2 O Al2 O3 4SiO2 . In Table 1 the experimental solution enthalpies at 298 K and in the last column the calculated enthalpies of crystallization at that temperature are also gathered. 4. Discussion The observed phase equilibria are consistent with the present knowledge of the phase diagrams. Further work is in progress to perform crystallization of the glasses into the equilibrium phases using hydrothermal conditions. The Tx;r values increase with the concentration of alumina and silica. The activation enthalpies of conductivity are consistent with Li ion migration in a glassy matrix [10]. The enthalpy of crystallization of lithium disilicate obtained by Sen et al. [6] from drop-dissolution experiments in a 2PbO B2 O3 melt at 976 K is about 6 kJ/mol more exothermic than the present result [6]. However, the summed experimental uncertainties (nearly 5 kJ/mol for Sen et al. and 1 kJ/mol for our data) are of the same order of magnitude than this dierence and there is no real contradiction to our results. On the other hand, experimental factors may contribute to the discrepancy and need to be considered: (i) the enthalpy variation with solute concentration and (ii) heat capacity corrections. Table 2 shows some experimental data on concentration eects in related silicate systems. Clearly, they can be sometimes important: in our experiments, we therefore worked as far as possible in solutions of the same
180
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
Fig. 2. XRD pattern of the crystallized sample A: # Li2 Si2 O5 (JCPDS n° 40-0376).
®nal concentration. The R T heat capacity correction term can be written: 298 Cpcr ÿ Cpgl dT . The use of a solvent at 298 K has the advantage to avoid possible eects due to a temperature change of the glasses during the drop. For example, if we assume that 50% of glass was heated to the dissolution temperature (976 K) in the experiments of Sen et
al., an estimation based on data for pure silica [11,12] would give an enthalpy variation DH 1 kJ/mol. Altogether, our results are not incompatible with those of Sen et al. [6]. The enthalpies of crystallization of several lithium and sodium silicates and alumino±silicates at 298 K are compared in Table 3. There is no
Table 2 Variation of the dissolution enthalpy (D DHdiss ) with concentration, expressed in mol% in (Bi2 O3 )4 (B2 O3 )6 melts and in mmol/l in HF/ HNO3 Solvent
Solute
T/K
D DHdiss
(Bi2 O3 )4 (B2 O3 )6
CuO Y2 O3 Y2 Cu2 O5
1033 1033 1033
kJ/mol per mol% 0 0 5
50 vol% HF 6M + 50 vol% HNO3 4M
Na2 O 2SiO2 Na2 O Cs2 O 2SiO2
298 298
kJ/mol per mmol/l 2 2
J. Rogez et al. / Journal of Non-Crystalline Solids 262 (2000) 177±182
181
Table 3 Enthalpies of crystallization (DHcryst ) and enthalpies of melting (DHmelt ) of some alkali silicates and alumino±silicates at 298 K SiO2 Li2 O 5SiO2 Li2 O 2SiO2 Li2 O 3.5SiO2 1.5Al2 O3 Li2 O 1.25SiO2 0.75Al2 O3 Na2 O SiO2 Na2 O 2SiO2 Na2 O Al2 O3 2SiO2 Na2 O Al2 O3 6SiO2
DHcryst (kJ/mol)
DHcryst (kJ/mol) (SiO2 + Al2 O3 )
)7.8 )55.8 )47.4 )53.5 [6] )143.4 )47.7 )17.8 [12] )9.4 [12] )56.0 [12] )119.0 [12]
)7.8 )11.2 )23.7 )28.7 )23.8 )17.8 )4.7 )18.7 )17.1
systematic correlation between the composition of the glasses and their crystallization enthalpy. However, the comparison with literature data reveals that the enthalpies of crystallization per mole of silica + alumina are smaller for sodium than for lithium glasses. The enthalpies of melting seem to change in a similar way. Following the de®nitions of DHcrystal and DHmelt , we can deduce the dierence between the extrapolated enthalpy of the melt and that of the glassy state at 298 K, which is close to 30 kJ/mol for both lithium and sodium disilicates. Furthermore, the relative species abundance in alkaline silicate melts calculated from equilibrium constants as function of composition shows that the dierence between lithium and sodium systems is small. This suggests that the observed enthalpy dierence stems mainly from the crystalline state [13]. To clarify the in¯uence of cations of dierent radius, let us consider the case of disilicates, which form ¯at sheets of tetrahedral silicate entities with cations densely packed between these layers [14]. With smaller cations than cesium, the layer becomes warped for electrostatic reasons. The layers are signi®cantly folded with small lithium ions and more energy is necessary to destroy long range order than with larger sodium ions. Although the balance between oxygen±oxygen and cation±cation repulsions and not radius ratio rules alone control the stability of ionic crystals [15], this is a qualitative explanation for the observed dierence of crystallization enthalpies of sodium and lithium glasses.
DHmelt (kJ/mol)
53 [12] 52 [12] 35 [12]
5. Conclusion The crystallization of Li ion-conducting lithium alumino±silicate glasses degrades the electrical conductivity irreversibly. Measurements of the electrical conductivity can thus be used as an indicator of starting devitri®cation. The dierence of crystallization enthalpies between lithium and similar sodium glasses can be related to the different cation radii and their in¯uence on the alumino±silicate crystal chemistry. Acknowledgements The authors wish to thank M. Decressac (CNRS ± Centre de Thermodynamique et de Microcalorimetrie) for some calorimetric measurements. References [1] P.E. Doherty, D.W. Lee, R.S. Davis, J. Am. Ceram. Soc. 50 (1967) 77. [2] O. Sch af, Ionics 2 (1996) 266. [3] O. Sch af, PhD thesis, Free University Berlin, 1995. [4] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase diagrams for ceramists, Am. Ceram. Soc. Columbus (1969) diagr. 179, 182. [5] E.M. Levin, C.R. Robbins, H.F. McMurdie, Phase diagrams for ceramists, Am. Ceram. Soc. Columbus (supplement) (1969) diagr. 2426. [6] S. Sen, C. Gerardin, A. Navrotsky, J.E. Dickinson, J. NonCryst. Solids 168 (1994) 64.
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