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Sodium ionic conductivity in densified silicate glass
SOLID STATE
Solid State Ionics 67 (1993) 209-213 North-Holland
IOIlICS Sodium ionic conductivity in densified silicate glass Z u y i Z h a n g 1, N a o h i r o Soga Department of lndustrial Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan and Teiichi H a n a d a Department of Chemistry, Collegeof Liberal Arts and Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received 26 December 1992; accepted for publication 17 September 1993
The electric conductivity of a densified Na20' 3SIO2 glass with a density of 2.539 g/cm3was measured at temperatures from 90 to 160°C after the heat treatments at various temperatures up to the glass transition temperature. The structural relaxation started at about 200 °C, and correspondingly the isotherm conductivity ( 120°C) increased towards that of undensified glass. But, there was little change in the activation energy. The obtained results are attributed to a change in the pathways for the migration of sodium ions.
1. Introduction The ionic migration in glasses has been studied extensively on various systems, for its interesting phen o m e n a and the prospect o f obtaining this kind of applicable solid electrolyte. The ionic mobility depends on both the characteristics of ions, such as size, charge etc. [ 1 ], and the structure o f glass matrix accommodating them [ 2 - 4 ]. Furthermore, it drops with an introduction of dissimilar mobile ions, due to an interaction between the different ionic species [ 5 ]. As for the structural aspect, the voids present in glass have been considered to play an important role in the ionic migration [ 6 - 1 0 ] , and application of high pressure seems to be a useful way in this study area. The ionic conductivity decreases with increasing applied pressure, and this change was interpreted in terms of an activation volume for the ionic migration [8,9]. Glass manifests an irreversible densification after high-pressure treatment [ 1 1,12 ], and the density inPresent address: New Glass Research Center, Yamamura Glass Co. Ltd., 2-1-18 Naruohama, Nishinomiya 663, Japan. To whom all correspondence should be addressed. Elsevier Science B.V.
creases by ~ 20%. Structural studies o n S i P 2 glass by using R a m a n and neutron scattering methods [ 13,14 ], indicate that the densification takes place by the rotation o f SiO4 units and a resultant decrease in the void volume. Also, a similar structural rearrangement was considered to occur in densified sodium silicate glass [ 15 ]. But, the relaxation of these densified glasses tend to occur at low temperatures far from the glass transition temperatures [ 16 ], due to their low stability compared with corresponding plain glasses. In the present study, a change in the electric conductivity with the structural relaxation was investigated on a densified sodium silicate glass by heat treating at various temperatures, as an experimental approach for the ionic conducting mechanism of oxide glass. F r o m the results obtained, the activation energy and the preexponential term were calculated, and the mechanism o f ionic conduction in densified glass is discussed.
2. Experimental Na20'3SiO2 glass sample was prepared from reagent grade Na2CO3 and SIP2, The batch composi-
210
Z. Zhang et al. /Densified silicate glass
tion was melted at 1400°C for three hours, poured into a stainless steel mold, and then annealed at the glass transition temperature for 15 rain. One piece of the bulk glass was ground to the shape of O 3.6 X 7.0 mm, and placed into a heating cell for the densification treatment. A hexahedra-type hydrostatic pressure apparatus at Osaka University was used for the preparation of the densified glass. Carbon and pyrophyllite were used as a heater and a pressure transmitting medium, respectively. Aftel~ heating to 450°C, a pressure of 2.5 GPa was applied at a rate of 100 MPa/min and kept for 10 min. After cooling to room temperature, the pressure was relieved. The details of the preparation method have been described elsewhere [ 12,15 ]. A chip of the densifted glass with a size of O 3.5 X 0.4 mm, in which there was no visual crack, was employed in the electrical conductivity measurements. As the electrodes, both sides of the glass were ground using 600 grit silicon carbide paper, and deposited with chromium and thereafter with gold by means of vacuum evaporation. After annealing at a treatment temperature for 1.5 min and quenching to room temperature, the impedance was measured at frequencies within 100 Hz-100 KHz at temperatures from 90 to 160°C by using a LCR meter (AG4311 model). The measurement was repeated by changing the treatment temperature from 200 to 450°C with a step of 50°C. The impedance limit of the LCR meter was 20 mfL To check the shunt leakage due to the surface conduction, both the two and three terminal methods were employed on an undensified glass sample. Since no noticeable difference was detected in the temperature region of the present measurements, the surface conduction was discarded. The conductivity was determined from the dc impedance, the thickness as well as the average area of the deposited electrodes.
temperature region for the conduction measurements, the change in density with heat treatments was investigated on another small chip of the densifted glass. After annealing at temperatures from 200°C to 440°C with an interval of 20°C for 15 min and quenching to room temperature in air, the density was determined by a sink-float method [ 18 ]. The results are shown in fig. 1 as a function of the treatment temperature. The relaxation of densified glass started at about 200 ° C, far from the glass transition temperature (450°C), as reported previously for other silicate glasses [ 16,17 ]. Above 300°C, the relaxation became drastic and the density decreased towards that of undensified glass. However, the relaxation was very slow below 160°C. Hence, the temperature region from 90 to 160 °C was employed in the electrical conduction measurements. The isotherm conductivity at 120°C is illustrated in fig. 2 as a function of the treatment temperature. The conductivity decreased by a factor of six after the densification. After heat treatments below 250 ° C, little change was detected. Above 300°C, however, the conductivity increased significantly towards that of undensifted glass with increasing the treatment temperature. That corresponds well to the change in density seen in fig. 1. At 450°C, the sample showed a conductivity slightly higher than that of undensifled glass. The ionic conductivity is expressed by eq. 1 ) ac2.60~
2"55: o rO i
0
0 O000©Oo
E u o2.50 -
0 0 o o 0
g 2.45a
o
3. Results
The density of the densified glass was 2.539 g / c m 2, whereas that of undensifted was 2.437 g/cm 3. The density increased by about 4% after the 2.5 GPa densification. In order to clarify the structural relaxation behavior of the densified glass and also to choose a suitable
2.40 I00
I
I
I
200 300 400 Temperature / °C
500
Fig. 1. Change of the density of 2.5 GPa densified glass with a series of heat treatments. Closed square and circle present the density prior to the heat treatments and that of undensified glass, respectively.
Z. Zhang et al. / Densified silicate glass
10--6
211
106 O
0
T
0
8
O3
(70 Ao
0
"o 10-7 F
0 •
0
x/
T 10
5
0
s o=
Before treatment
i
10-8
I
I
200
I 500
I
I
Before
I
treotme~'t
400
104
T /°C
I
I 200
I
I 300 T /°C
I
I 400
I
Fig. 2. Isotherm conduetivities of densified glass against the treatment temperatures, tro presents undensified glass.
Fig. 4. Plots of calculated preexponential factor A against the heat treatment temperature. Aorepresents undensified glass.
cording to the diffusion theory, where T is absolute temperature, E is activation energy and A is a constant. The activation energy, E, is an energy barrier for the migration of alkali ions. The preexponential term, A, is expressed by eq. (2), in which N is number o f mobile ions, d is j u m p distance, and/t is j u m p trial frequency:
In fig. 3, a T is plotted against the reciprocal of temperature (K) for each treatment temperature. For each treatment temperature, the values of E and A were calculated by at least square method and shown in figs. 4 and 5, respectively, as a function of the treatment temperature. These activation energies ranged from 69,0 to 71.8 k J / m o l close to that o f u n densified glass, and no trend of a change with heat treatments was found within the present experimental error. On the other hand, the preexponential factor, A, monotonically increased towards that o f undensified glass with increasing the heat treatment temperature, like the isotherm conductivity shown in fig. 2.
tr=A/T exp( -E/kT)
,
( 1)
(2)
A = ldVe2d2 / 6 k .
°\0 10 -z
~ o
%"k 4. Discussion T
~X N
t~
10-5 I 2.2
I
I I I 2.4 2.6 I / T x10 3 / K -I
I 2.8
Fig. 3. Arrhenius plots of aT against the reciprocal of T (K). o: undensified glass; a: 2.5 GPa densified glass; before the thermal treatments; b: treated at 200°C; c: treated at 250°C; d: treated at 300°C; e: treated at 350°C; f: treated at 400°C; g: treated at 450 °C.
In silicate glasses, ionic species break the silicate network and take up their preferred sites, where nonbridging oxygens serve as the charge compensating centers. Therefore, the effect o f pressure on the ionic conductivity should depend greatly on the characteristics of the rearrangement of silicate network caused by pressure. A densified glass is a little different from a glass under pressure, where a decrease of ionic conductivity under pressure has been treated exponentially in terms of an activation volume for the migration o f alkali ions [8,9 ]. U p o n unloading the applied pressure, although the glass tend to change to a stable state, this rearrangement cannot occur completely at room temperature. Consequently, the
212
Z. Zhang et al. / Densified silicate glass
80,
75 7
0
~7cL
•
0
O --Eo
0
o
w65 Before treatment
6C
l
I 2OO
I_ T
I_l 500 /°C
J 400
I
Fig. 5. Plots of activation energy E against the heat treatment temperature. Eo represents undensified glass.
volume fails to recover, leading to an increase in the density. Therefore, some bonds in the densifted glass should be subjected to a tensile deformation to keep the overall shrinkage [ 15 ]. Hence, the glass can be considered to exist in a high energy state. Due to the low stability, the structural relaxation takes place at 200°C far from the glass transition temperature, as observed previously [ 16,17 ]. From the infrared spectroscopic results [15 ], this densification is inferred to occur by the rotation of SiO4 units and a resultant decrease in the void volume. Here, the structural change is considered to be responsible for the densiftcation-induced decrease of ionic conductivity. In crystals, N in eq. (2) depends on the defect concentration or the available interstitial sites (a geometric factor). Although such sites are difficult to define in a random glass structure in comparison with crystals, similar ones are thought to exist in glass and contribute to the ionic conduction [ 19,20]. In the mechanism proposed by Charles [ 19 ], it is assumed that there are more than two preferred sites for alkali ions at a non-bridging oxygen and the ionic migration is achieved by the formation of a kind of defects, where two alkali ions take up the sites owing to a non-bridging oxygen. In another conduction mechanism [20], these interstitial sites are also indispensable. To explain the low correlation factor, f, observed in glasses, the conduction was considered to occur by a correlated movement of ions [ 21,22 ]. That implies that the jumps of an ion are limited due to lack of these sites around it. Similarly, the ionic conduction mechanism was also attempted in terms
of the preferential pathways for ionic migration in glass [ 19 ]. That is quite understandable from the viewpoint of the deformation caused by the ionic migration. For each jump, in addition to an electric energy barrier, another energy is necessary for the deformation [ 1,16]. As a great number of bonds (SiO-Si) deviates from their stable states, this deformation energy should change correspondingly. From this viewpoint, the preexponential factor should increase with an increase in the homogeneity of glass. Here, the concept of the preferential pathways is employed to describe the conduction mechanism qualitatively. If the jump distance in eq. (2), d, is proportional to the average distance between sodium ions, it is inferred to have decreased by about 1% in the densifted glass by judging from the increase of about 4% in density. Therefore, the change of preexponential term A seems to be mainly determined by that of N, i.e. the pathways. Namely, less pathways contribute to the conduction in the densifted glass. The subsequent heat treatments cause the structural relaxation and the preexponential term, A, changes towards the original value. As the whole volume recovers with the heat treatments (fig. 1 ), the pathways should be attributed to a kind of relatively open local microstructures in glass. Indeed, it has also been confirmed that an increase in the void volume, which results from high quenching rates, tends to increase the conductivity [6,23 ]. That seems to be very reasonable from the above concept of the deformation energy. At the same time little change in the activation energy suggests that these open microstructures still percolate through the glass and play a dominant role in the conduction. This may result partially from the opposite change of volume during the unloading process. If these structures are reduced to some extent with further densification, especially under pressure, the denser ones may become dominant in the ionic conducting process. In such a case, the activation energy is expected to increase with densiftcation. The change in the preexponential term shows a similar tendency reported previously on silicate glasses [7,10]. But, there is a discrepancy in the change of the activation energy. Biefeld et al. [ 10 ] reported on lithium aluminosilicate glasses, where no noticeable increase was observed even under a
z. Zhang et al. I Densified silicate glass
pressure o f 2 G P a . C h a k r a v o r t y a n d Cross [ 7 ], rep o r t e d that the activation energy increased considerably after a high pressure treatment. F r o m the above discussion, a difference in the densification extent m a y be responsible for this discrepancy. Also, the densiftcation b e h a v i o r changes greatly with the t r e a t m e n t variables, such as a p p l i e d pressure, pressure t r a n s m i t t i n g m e d i u m a n d t e m p e r a t u r e [24,25 ]. Therefore, as p o i n t e d by M a c k e n z i e [ 17 ], the structural arrangement can be different even at a similar density. F u r t h e r m o r e , the relaxation which occurs during the electric m e a s u r e m e n t s at high t e m p e r a tures, should influence the reliability o f the activation energy. T h a t was not taken into account in the previous report [ 7 ]. To clarify this discrepancy, more precise investigations are necessary in b o t h loading a n d unloading processes o f high pressure.
5. Conclusion The ionic c o n d u c t i v i t y o f Na20" 3SIO2 glass was r e d u c e d by a factor o f six after 2.5 G P a densification. By the subsequent heat treatment, structural relaxation t o o k place a n d correspondingly the conductivity increased towards that o f undensified glass. However, the activation energy was essentially unchanged. The results can be explained in terms o f a special k i n d o f pathways, which d e p e n d on the v o i d v o l u m e present in glass. After densification, the pathways decrease a n d consequently the ionic conductivity decreases. But, these pathways can still percolate through the glass, a n d hence the activation energy remains essentially unchanged.
Acknowledgement The authors t h a n k Professor S. K u m e o f O s a k a U n i v e r s i t y for his help in this study. One o f the au-
213
thors (Z. Z h a n g ) thanks Dr. O. O t a k a o f Osaka University for his assistance in the p r e p a r a t i o n o f densifted glass, and also thanks Dr. T. Yoko o f K y o t o University for helpful discussion on the measurem e n t m e t h o d o f electric conduction.
References [ 1] O.L. Anderson and D.A. Stuart, J. Am. Ceram. Soc. 37 (1954) 573. [2] Y.P. Gupta and U.D. Mishira, J. Phys. Chem. Solids 30 (1969) 1327. [ 3 ] S.W. Martin, J. Am. Ceram. Soc. 74 ( 1991 ) 1767. [ 4 ] J.N. Mundy and G.L. Jin, Solid State Ionics 24 (1987) 263. [ 5 ] J.O. Isard, J. Non-Cryst. Solids 1 (1969) 235. [6] R.J. Charles, J. Am. Ceram. Soc. 45 (1962) 105. [7] D. Chakravorty and L.E. Cross, J. Am. Ceram. Soc. 47 (1964) 370. [8] K. Arai, K. Kumata, K. Kadota, K. Yamamoto, H. Namikawa and S. Saito, J. Non-Cryst. Solids 13 (1973) 131. [91 S.D. Hamann, Aust. J. Chem. 18 (1965) I. [101R.M. Biefeld, R.T. Johnson Jr. and R.J. Baughman, J. Electrochem. Soc. 125 (1978) 179. [ 11 ] P.W. Bridgman and I. Simon, J. Appl. Phys. 24 (1953) 405. [12] N. Soga, N. Hirao, M. Yoshimoto and H. Yamamoto, J. Appl. Phys. 63 (1988) 4451. [ 13 ] F.A. Seifert, B.O. Mysen and D. Virgo, Phys. Chem. Glasses 24 (1983) 141. [ 141 S. Susman, K.J. Volin, R.C. Liebermann, G.D. Gwammesia and Y. Wang, Phys. Chem. Glasses 31 (1990) 144. [ 151 Z. Zhang, N. Soga and K. Hirao, J. Non-Cryst. Solids 135 (1991) 55. [ 16] Z. Zhang, Ph.D. thesis (Kyoto University, 1992). [ 17 ] J.D. Mackenzie, J. Am. Ceram. SOc.46 (1963) 470. [ 18] T. Hanada, N. Soga and M. Ohkawa, Adv. Ceram. 24 (1988) 319. [ 191 R.J. Charles, J. Appl. Phys. 32 ( 1961 ) 1115. [20] C. Lira and D.E. Day, J. Am. Ceram. Soc. 60 (1977) 198. [21 ] R. Terai, J. Cerarn. Soc. Japan. 77 (1969) 88. [ 22 ] Y. Haven and B. Verkerk, Phys. Chem. Glasses, 6 (1965 ) 38. [23] M. Yoshiyagawa and M. Tomozawa, J. Phys. Colloq. 43 (1982) 411. [241J.D. Mackenzie, J. Am. Ceram. SOc.46 (1963) 461. [251H.M. Cohen and R. Roy, Phys. Chem. Glasses 6 (1965) 149.
Solid State Ionics 67 (1993) 209-213 North-Holland
IOIlICS Sodium ionic conductivity in densified silicate glass Z u y i Z h a n g 1, N a o h i r o Soga Department of lndustrial Chemistry, Faculty of Engineering, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan and Teiichi H a n a d a Department of Chemistry, Collegeof Liberal Arts and Science, Kyoto University, Sakyo-ku, Kyoto 606-01, Japan Received 26 December 1992; accepted for publication 17 September 1993
The electric conductivity of a densified Na20' 3SIO2 glass with a density of 2.539 g/cm3was measured at temperatures from 90 to 160°C after the heat treatments at various temperatures up to the glass transition temperature. The structural relaxation started at about 200 °C, and correspondingly the isotherm conductivity ( 120°C) increased towards that of undensified glass. But, there was little change in the activation energy. The obtained results are attributed to a change in the pathways for the migration of sodium ions.
1. Introduction The ionic migration in glasses has been studied extensively on various systems, for its interesting phen o m e n a and the prospect o f obtaining this kind of applicable solid electrolyte. The ionic mobility depends on both the characteristics of ions, such as size, charge etc. [ 1 ], and the structure o f glass matrix accommodating them [ 2 - 4 ]. Furthermore, it drops with an introduction of dissimilar mobile ions, due to an interaction between the different ionic species [ 5 ]. As for the structural aspect, the voids present in glass have been considered to play an important role in the ionic migration [ 6 - 1 0 ] , and application of high pressure seems to be a useful way in this study area. The ionic conductivity decreases with increasing applied pressure, and this change was interpreted in terms of an activation volume for the ionic migration [8,9]. Glass manifests an irreversible densification after high-pressure treatment [ 1 1,12 ], and the density inPresent address: New Glass Research Center, Yamamura Glass Co. Ltd., 2-1-18 Naruohama, Nishinomiya 663, Japan. To whom all correspondence should be addressed. Elsevier Science B.V.
creases by ~ 20%. Structural studies o n S i P 2 glass by using R a m a n and neutron scattering methods [ 13,14 ], indicate that the densification takes place by the rotation o f SiO4 units and a resultant decrease in the void volume. Also, a similar structural rearrangement was considered to occur in densified sodium silicate glass [ 15 ]. But, the relaxation of these densified glasses tend to occur at low temperatures far from the glass transition temperatures [ 16 ], due to their low stability compared with corresponding plain glasses. In the present study, a change in the electric conductivity with the structural relaxation was investigated on a densified sodium silicate glass by heat treating at various temperatures, as an experimental approach for the ionic conducting mechanism of oxide glass. F r o m the results obtained, the activation energy and the preexponential term were calculated, and the mechanism o f ionic conduction in densified glass is discussed.
2. Experimental Na20'3SiO2 glass sample was prepared from reagent grade Na2CO3 and SIP2, The batch composi-
210
Z. Zhang et al. /Densified silicate glass
tion was melted at 1400°C for three hours, poured into a stainless steel mold, and then annealed at the glass transition temperature for 15 rain. One piece of the bulk glass was ground to the shape of O 3.6 X 7.0 mm, and placed into a heating cell for the densification treatment. A hexahedra-type hydrostatic pressure apparatus at Osaka University was used for the preparation of the densified glass. Carbon and pyrophyllite were used as a heater and a pressure transmitting medium, respectively. Aftel~ heating to 450°C, a pressure of 2.5 GPa was applied at a rate of 100 MPa/min and kept for 10 min. After cooling to room temperature, the pressure was relieved. The details of the preparation method have been described elsewhere [ 12,15 ]. A chip of the densifted glass with a size of O 3.5 X 0.4 mm, in which there was no visual crack, was employed in the electrical conductivity measurements. As the electrodes, both sides of the glass were ground using 600 grit silicon carbide paper, and deposited with chromium and thereafter with gold by means of vacuum evaporation. After annealing at a treatment temperature for 1.5 min and quenching to room temperature, the impedance was measured at frequencies within 100 Hz-100 KHz at temperatures from 90 to 160°C by using a LCR meter (AG4311 model). The measurement was repeated by changing the treatment temperature from 200 to 450°C with a step of 50°C. The impedance limit of the LCR meter was 20 mfL To check the shunt leakage due to the surface conduction, both the two and three terminal methods were employed on an undensified glass sample. Since no noticeable difference was detected in the temperature region of the present measurements, the surface conduction was discarded. The conductivity was determined from the dc impedance, the thickness as well as the average area of the deposited electrodes.
temperature region for the conduction measurements, the change in density with heat treatments was investigated on another small chip of the densifted glass. After annealing at temperatures from 200°C to 440°C with an interval of 20°C for 15 min and quenching to room temperature in air, the density was determined by a sink-float method [ 18 ]. The results are shown in fig. 1 as a function of the treatment temperature. The relaxation of densified glass started at about 200 ° C, far from the glass transition temperature (450°C), as reported previously for other silicate glasses [ 16,17 ]. Above 300°C, the relaxation became drastic and the density decreased towards that of undensified glass. However, the relaxation was very slow below 160°C. Hence, the temperature region from 90 to 160 °C was employed in the electrical conduction measurements. The isotherm conductivity at 120°C is illustrated in fig. 2 as a function of the treatment temperature. The conductivity decreased by a factor of six after the densification. After heat treatments below 250 ° C, little change was detected. Above 300°C, however, the conductivity increased significantly towards that of undensifted glass with increasing the treatment temperature. That corresponds well to the change in density seen in fig. 1. At 450°C, the sample showed a conductivity slightly higher than that of undensifled glass. The ionic conductivity is expressed by eq. 1 ) ac2.60~
2"55: o rO i
0
0 O000©Oo
E u o2.50 -
0 0 o o 0
g 2.45a
o
3. Results
The density of the densified glass was 2.539 g / c m 2, whereas that of undensifted was 2.437 g/cm 3. The density increased by about 4% after the 2.5 GPa densification. In order to clarify the structural relaxation behavior of the densified glass and also to choose a suitable
2.40 I00
I
I
I
200 300 400 Temperature / °C
500
Fig. 1. Change of the density of 2.5 GPa densified glass with a series of heat treatments. Closed square and circle present the density prior to the heat treatments and that of undensified glass, respectively.
Z. Zhang et al. / Densified silicate glass
10--6
211
106 O
0
T
0
8
O3
(70 Ao
0
"o 10-7 F
0 •
0
x/
T 10
5
0
s o=
Before treatment
i
10-8
I
I
200
I 500
I
I
Before
I
treotme~'t
400
104
T /°C
I
I 200
I
I 300 T /°C
I
I 400
I
Fig. 2. Isotherm conduetivities of densified glass against the treatment temperatures, tro presents undensified glass.
Fig. 4. Plots of calculated preexponential factor A against the heat treatment temperature. Aorepresents undensified glass.
cording to the diffusion theory, where T is absolute temperature, E is activation energy and A is a constant. The activation energy, E, is an energy barrier for the migration of alkali ions. The preexponential term, A, is expressed by eq. (2), in which N is number o f mobile ions, d is j u m p distance, and/t is j u m p trial frequency:
In fig. 3, a T is plotted against the reciprocal of temperature (K) for each treatment temperature. For each treatment temperature, the values of E and A were calculated by at least square method and shown in figs. 4 and 5, respectively, as a function of the treatment temperature. These activation energies ranged from 69,0 to 71.8 k J / m o l close to that o f u n densified glass, and no trend of a change with heat treatments was found within the present experimental error. On the other hand, the preexponential factor, A, monotonically increased towards that o f undensified glass with increasing the heat treatment temperature, like the isotherm conductivity shown in fig. 2.
tr=A/T exp( -E/kT)
,
( 1)
(2)
A = ldVe2d2 / 6 k .
°\0 10 -z
~ o
%"k 4. Discussion T
~X N
t~
10-5 I 2.2
I
I I I 2.4 2.6 I / T x10 3 / K -I
I 2.8
Fig. 3. Arrhenius plots of aT against the reciprocal of T (K). o: undensified glass; a: 2.5 GPa densified glass; before the thermal treatments; b: treated at 200°C; c: treated at 250°C; d: treated at 300°C; e: treated at 350°C; f: treated at 400°C; g: treated at 450 °C.
In silicate glasses, ionic species break the silicate network and take up their preferred sites, where nonbridging oxygens serve as the charge compensating centers. Therefore, the effect o f pressure on the ionic conductivity should depend greatly on the characteristics of the rearrangement of silicate network caused by pressure. A densified glass is a little different from a glass under pressure, where a decrease of ionic conductivity under pressure has been treated exponentially in terms of an activation volume for the migration o f alkali ions [8,9 ]. U p o n unloading the applied pressure, although the glass tend to change to a stable state, this rearrangement cannot occur completely at room temperature. Consequently, the
212
Z. Zhang et al. / Densified silicate glass
80,
75 7
0
~7cL
•
0
O --Eo
0
o
w65 Before treatment
6C
l
I 2OO
I_ T
I_l 500 /°C
J 400
I
Fig. 5. Plots of activation energy E against the heat treatment temperature. Eo represents undensified glass.
volume fails to recover, leading to an increase in the density. Therefore, some bonds in the densifted glass should be subjected to a tensile deformation to keep the overall shrinkage [ 15 ]. Hence, the glass can be considered to exist in a high energy state. Due to the low stability, the structural relaxation takes place at 200°C far from the glass transition temperature, as observed previously [ 16,17 ]. From the infrared spectroscopic results [15 ], this densification is inferred to occur by the rotation of SiO4 units and a resultant decrease in the void volume. Here, the structural change is considered to be responsible for the densiftcation-induced decrease of ionic conductivity. In crystals, N in eq. (2) depends on the defect concentration or the available interstitial sites (a geometric factor). Although such sites are difficult to define in a random glass structure in comparison with crystals, similar ones are thought to exist in glass and contribute to the ionic conduction [ 19,20]. In the mechanism proposed by Charles [ 19 ], it is assumed that there are more than two preferred sites for alkali ions at a non-bridging oxygen and the ionic migration is achieved by the formation of a kind of defects, where two alkali ions take up the sites owing to a non-bridging oxygen. In another conduction mechanism [20], these interstitial sites are also indispensable. To explain the low correlation factor, f, observed in glasses, the conduction was considered to occur by a correlated movement of ions [ 21,22 ]. That implies that the jumps of an ion are limited due to lack of these sites around it. Similarly, the ionic conduction mechanism was also attempted in terms
of the preferential pathways for ionic migration in glass [ 19 ]. That is quite understandable from the viewpoint of the deformation caused by the ionic migration. For each jump, in addition to an electric energy barrier, another energy is necessary for the deformation [ 1,16]. As a great number of bonds (SiO-Si) deviates from their stable states, this deformation energy should change correspondingly. From this viewpoint, the preexponential factor should increase with an increase in the homogeneity of glass. Here, the concept of the preferential pathways is employed to describe the conduction mechanism qualitatively. If the jump distance in eq. (2), d, is proportional to the average distance between sodium ions, it is inferred to have decreased by about 1% in the densifted glass by judging from the increase of about 4% in density. Therefore, the change of preexponential term A seems to be mainly determined by that of N, i.e. the pathways. Namely, less pathways contribute to the conduction in the densifted glass. The subsequent heat treatments cause the structural relaxation and the preexponential term, A, changes towards the original value. As the whole volume recovers with the heat treatments (fig. 1 ), the pathways should be attributed to a kind of relatively open local microstructures in glass. Indeed, it has also been confirmed that an increase in the void volume, which results from high quenching rates, tends to increase the conductivity [6,23 ]. That seems to be very reasonable from the above concept of the deformation energy. At the same time little change in the activation energy suggests that these open microstructures still percolate through the glass and play a dominant role in the conduction. This may result partially from the opposite change of volume during the unloading process. If these structures are reduced to some extent with further densification, especially under pressure, the denser ones may become dominant in the ionic conducting process. In such a case, the activation energy is expected to increase with densiftcation. The change in the preexponential term shows a similar tendency reported previously on silicate glasses [7,10]. But, there is a discrepancy in the change of the activation energy. Biefeld et al. [ 10 ] reported on lithium aluminosilicate glasses, where no noticeable increase was observed even under a
z. Zhang et al. I Densified silicate glass
pressure o f 2 G P a . C h a k r a v o r t y a n d Cross [ 7 ], rep o r t e d that the activation energy increased considerably after a high pressure treatment. F r o m the above discussion, a difference in the densification extent m a y be responsible for this discrepancy. Also, the densiftcation b e h a v i o r changes greatly with the t r e a t m e n t variables, such as a p p l i e d pressure, pressure t r a n s m i t t i n g m e d i u m a n d t e m p e r a t u r e [24,25 ]. Therefore, as p o i n t e d by M a c k e n z i e [ 17 ], the structural arrangement can be different even at a similar density. F u r t h e r m o r e , the relaxation which occurs during the electric m e a s u r e m e n t s at high t e m p e r a tures, should influence the reliability o f the activation energy. T h a t was not taken into account in the previous report [ 7 ]. To clarify this discrepancy, more precise investigations are necessary in b o t h loading a n d unloading processes o f high pressure.
5. Conclusion The ionic c o n d u c t i v i t y o f Na20" 3SIO2 glass was r e d u c e d by a factor o f six after 2.5 G P a densification. By the subsequent heat treatment, structural relaxation t o o k place a n d correspondingly the conductivity increased towards that o f undensified glass. However, the activation energy was essentially unchanged. The results can be explained in terms o f a special k i n d o f pathways, which d e p e n d on the v o i d v o l u m e present in glass. After densification, the pathways decrease a n d consequently the ionic conductivity decreases. But, these pathways can still percolate through the glass, a n d hence the activation energy remains essentially unchanged.
Acknowledgement The authors t h a n k Professor S. K u m e o f O s a k a U n i v e r s i t y for his help in this study. One o f the au-
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thors (Z. Z h a n g ) thanks Dr. O. O t a k a o f Osaka University for his assistance in the p r e p a r a t i o n o f densifted glass, and also thanks Dr. T. Yoko o f K y o t o University for helpful discussion on the measurem e n t m e t h o d o f electric conduction.
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