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The impact of recent developments on the theory of and prospects for ionic conduction in glass
Journal of Non-Crystalline Solids 73 (1985) 247-253 North-Holland, Amsterdam
247
T H E IMPACT O F RECENT D E V E L O P M E N T S ON T H E T H E O R Y O F AND P R O S P E C T S FOR IONIC C O N D U C T I O N IN GLASS Malcolm D. I N G R A M Department of Chemistry, University of Aberdeen, Aberdeen A B9 2 UE, Scotland
The discovery of new materials within the past 10-15 years indicates that the mechanisms of ionic (and electronic) transport in glasses and crystals are closely related. The procedure for determining mobile ion concentrations in glass from the mixed alkali effect is analogous to the established technique of aliovalent doping uoed in halide crystals. This procedure could be used for testing present day theories of ionic conduction in glass. Glasses are now available as Li+-ion and Ag+-ion conducting solid electrolytes for miniature batteries, but further development work is required to produce glasses suitable for the high temperature N a / S batteries to be used (it is hoped) in automotive power sources and load levelling applications.
1. Introduction Important discoveries since 1970 are opening up new areas for the application of glass in electronic devices, batteries, and electrochemical sensors. New types of material are being developed, which include amorphous silicon semiconductors and vitreous electrolytes. This is a convenient time to enquire if progress in the theory of ionic conduction has been keeping pace with experimental advances, and to identify areas where further advances are possible.
2. Recent developments 2.1. New materials and applications
One of the most important of recent developments has been the discovery by Spear and co-workers [1,2] that amorphous silicon can be deposited in thin layers by the action of glow-discharge on silane gas. This material has a low density of defect states, and can be doped with B and P simply by admitting B2H 6 and PH 3 into the gas stream. In this way, p- and n-type semiconductors are produced whose conductivity can be controlled over many orders of magnitude. These materials have several potential applications, and already large numbers of photovoltaic cells [3] are being manufactured for use in wrist watches and pocket calculators, and also for roof-top collectors of solar energy. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
248
M.D. Ingrain / Ionic conduction in glass
T/'C 800 300
100
0
-100
KNO3 melt
rg
,S \ SCSo
fE
-4 3S102\
-6 1
2
-6 3 4 103/T
5
6
Fig. 1. Arrhenius plots of conductivity for: (i) KNO3 melts, (ii) N a 2 0 . 3 S i O 2 melts and glasses, and (iii) AgsI4BO 3 glasses (data from refs. [8], [9] and [10] respectively).
Important progress is also being made in the field of ionic conductors. Thus in 1973, Kunze [4] prepared (largely by accident) a highly conducting AgI/Ag2SeO 4 glass. Subsequently, a whole family of such silver iodide/silver oxysalt glasses was identified (incorporating phosphates, borates, arsenates, dichromates and molybdates [5]). In these glasses, the conductivity can be as high as 10 -2 S cm -1 at 25°C, similar to that of 0.1 M solution of KC1 in H20. It is reasonable therefore to regard these as "optimised ionic conductors" with conductivities at Tg much like those of typical molten salts (aTg = 10 -1 S c m - 1 ), see fig. 1. Batteries incorporating these electrolytes have been made, typically: Ag,AgvAsO414 IAg7AsO414] 12,AgvAsO4I 4 ; (composite) (composite)
E = 0.69 V.
These batteries are of interest for demonstration and teaching purposes [6], but at present do not have commercial value because of their low energy density. The high conductivity of this family of glasses is not surprising in view of the high conductivity of crystalline RbAg4I 5, and indeed of a-AgI itself [5]. As Armstrong and Dickinson explained some years ago [7], the high mobility of Ag ÷ ions can be attributed to the ease with which they can adopt various (e.g. 3, 4 and 5) coordination numbers - a property which also reflects the "soft" covalent interactions between Ag + and I - ions. In solid electrolytes generally, cation mobility increases with increasing polarizability of the anion sublattice. This effect has recently been exploited in the synthesis of a variety of thio glasses (e.g. mixed lithium iodide/lithium thiophosphate glasses, in the system Li2S-P2Ss-LiI ) [11], which exhibit comparable Li+-ion mobility at slightly higher temperatures. These glasses are now being incorporated into batteries like: LiA1]LizS-P2Ss-LiIglass]Cu40(PO4)2,
E = 2.3 V,
which are being developed for various applications, the possibilities including use as reserve power supplies for memory boards in computers.
M.D. Ingram / Ionic conduction in glass
249
3. Mechanisms of ionic mobility The conductivity of any solid can be written as (1)
o = Zn*eozu,
where n* is the number of charge carriers (electrons, holes, etc.) per unit volume, e 0 is the electronic charge, z is the valency (of an ion), and u is the corresponding electrical mobility. For semiconductors, the mobilities of holes and electrons can be determined directly from the Hall effect, or drift mobility experiments - see ref. [12], for example. So far, these techniques have not been applied successfully to ionic glasses. For convenience it is usually assumed that all similar cations in glass (e.g. Na + ions in a sodium silicate glass) are equally mobile. Thus conductivity mechanisms are discussed in the absence of important information relating to the nature and numbers of charge carriers. But how can charge carriers in glass be identified? In this context the phenomenon of doping an amorphous silicon is important. Boron and phosphorus atoms must enter directly into sites normally coupled by silicon atoms (4-coordinate) and satisfy their bonding requirements (i.e. complete their Lewis octet) either by trapping electrons (thus producing positive holes in the valence band) or releasing electrons into the conduction band of the glass. The conducting defects (i.e. the holes and electrons) originate therefore in much the same way as they do in crystalline silicon. In considering an appropriate theory of ionic transport in glasses, we shall see first how progress has been made with crystalline materials. As long ago as 1949, Teltow [13] examined the effects of aliovalent dopants, such as Cd 2+, on the conductivity of silver halide crystals. Cd 2+ ions enter sites normally occupied by Ag + ions in the crystal lattice, and cation vacancies are produced. The resulting steep drop in conductivity which occurs during the initial state of doping, see fig. 2, was attributed to the "trapping" of mobile interstitials (Ag +) by these vacancies. Teltow showed the shapes of these conductivity isotherms could be analysed, and was able to determine values of the concentrations of mobile Ag + cations in pure AgBr, and hence (through eq. (1) above) to calculate the ionic mobilities). Over 30 years were to elapse before similar arguments were to be applied to conduction in glass. Ingrain suggested [14] that the fast-ion conductor B-alumina might be a model for Na + (and other alkali ion) conducting glasses. Within the conduction planes of ]~-alumina alkali cations migrate by a paired or "split"' interstitially mechanism, which for convenients can be represented schematically as: (M2)++M=M+(M2)
+,
(2)
where only "excess" charges on interstitial species are shown for clarity. If the same sort of process occurs in glass, the occurence of the "mixed alkali effect" [15,16] is attributed to reactions like: (K 2)++ Na = K + ( N a K ) +.
(3)
250
M.D. l n g r a m / Ionic conduction in glass r
-5( ~
'200"C
i
i
i
MOYNIHAN - LESIKAR (1981) ]
1.( o
250"C
-4) 0.9
~-9
>
u 0.8
07
(Teltow, 1949 ) ISOTHERMS FOR AgBr CRYSTALS
-11i
ISOTHERMSFOR
I
O.J05 01 tool. */, Cd Br2
0"2
0"4 0"6 K/(K*Na)
J
1
08
Fig. 2. Relative conductivity isotherms for AgBr crystals doped with CdBr 2 (data from ref. [13]). Fig. 3. Conductivity isotherms showing the mixed alkali effect in N a 2 0 - 3 S i O 2 / K 2 0 - 3 S i O 2 glasses (data from ref. [17]).
where (NaK) + pairs are immobilised within the framework of the glass by ion polarisation, or other interactions. Moynihan and Lesikar [17] pointed out that the steepest fall in conductivity (taking the N a 2 0 . 3 S I O 2 / K 2 0 . 3 S i O 2 system as an example) occurs in the dilute foreign alkali region, i.e. at each end of the conductivity isotherm, fig. 3. By making certain assumptions about constant ionic mobility in this region and assuming that reaction (3) above is quantitative, they developed a mathematical treatment [18] very similar to Teltow's and calculated the concentrations of mobile interstitials (or pairs) in the glass. The proportion of mobile K ÷ ions in K 2 0 - 3 S i O 2 glass was found to increase from about 0.6% at 25°C to 5% at 400°C. It appears from their calculations that the conductivity of glass can be increased either by increasing the concentration of interstitial cations, or by increasing the mobility. The conductivity of NaO-SiO 2 glasses is known to increase with increasing N a 2 0 content, but does this occur because the concentration of (Na2) ÷ pairs increases, or because they become more mobile? This is an important but unanswered question, on which opinions in the literature differ. Ravaine and Souquet [19] attributed the rapid increase in conductivity in N a 2 0 - S i O 2 glasses, to changes in the concentrations of "free" Na + ions [19], according to the equilibrium: N a 2 0 ~ N a O - + Na +. (4)
M.D. l n g r a m / Ionic conduction in glass
251
This is often called the "weak electrolyte" theory. If reaction (4) is rewritten in the form: 2(-O-Na+)~-O
+-O-(Na2)
++,
(5)
the "free" Na + ions can be identified with the interstitial pairs of the Moynihan-Lesikar-Ingram (MLI) model. The essence of Ravaine and Souquet's theory is that the mobility of the "free" Na + ions remain constant over the whole range of N a 2 0 . SiO 2 glasses. By way of contrast, Tuller and co-workers [20,21] consider variations in mobile ion concentration to be relatively unimportant, and have looked instead for the influence of glass structure on ion mobility. It seems that the matter could now be resolved quite readily by taking appropriate measurements in the dilute foreign alkali region, and calculating the mobile carrier concentrations using the MLI theory described above. It would be unwise, however, to assume that exactly the same kind of "defect" model necessarily applies to all conducting glasses. The phase transition in AgI at 147°C is accompanied by melting of the Ag + sublattice [5], and the high conductivity is often attributed to quasiliquid motions of Ag + ions, disordered within the loose framework of I ions. If this kind of mechanism operates in AgI-containing glasses, it could explain e.g. a number of unusual phenomena such as curved Arrhenius plots, which occur near the glass transition [22]. The "special" interaction between Ag + and I ions overrides the normal mixed alkali effect. In AgPO 3 glasses, for example, addition of NaI produces similar increases in conductivity to the addition of AgI [23]. A method for determining mobile ion concentrations in these glasses has still to be worked out.
4. Future prospects The main conclusion to be drawn is that ionic conduction processes in glasses and crystals are rather similar, fl-alumina is a good model for alkali ion conduction in alkali silicate and other "conventional" glasses. A variety of mixed-alkali effects are observed in these single-crystal electrolytes [24,25], and these are now being interpreted in the light of cation distributions determined by X-ray diffraction experiments [26]. A comprehensive theory of the mixed alkali effect is now in sight, and so one of the outstanding "mysteries" of glass science [15,16] may soon be solved. It is interesting to recall that the first patents [27,28] taken out on the sodium sulphur battery, fig. 4, specified using Na+-ion conducting glasses as alternatives to the fl-alumina solid electrolyte. However the resistivity of glass is usually considered to be too large for this application. The problem is illustrated in fig. 5. The conductivity at 300°C for a series of simple and "modified" sodium borate glasses [29] is plotted as a function of total modifier
252
M.D. Ingrain / Ionic conduction in glass
Current
cottector
I
r
i
i
i
-3
ii i' '~i'i!,i °,
.:: o
Na+-ion so[id electrolyte
ki',,i~
~.::,
(g [ass )
i
F
E -5 i./I
,:, :o
**,
Marten sutphur
• ~ *,
and carbon fett
o 0
'G
~
[ e
0
10
• NQC[ a NaF ~ Na2 SO/, c3 AI203
-9
I
®
-g203 glasses,
] n/ ~
No[ten sodium J
/~Ct20
I
1
I
20 30 40 mlo NQ2Z
I
50
Fig. 4. Schematic of N a / S cell (beta battery) which operates in range 300-350°C. Fig. 5. Conductivities at 300°C of "modified" sodium borate glasses (data from ref. [29]). The abscissa, m / o Na 2Z = m / o (Na 2° + Na 2C12), etc.
content expressed as a mol per cent, m / o ( N a z Z ) = m / o (Na20 + Na2CI2), etc. We see that the conductivity is strongly influenced by Na+-ion content, and at this temperature, it varies by seven orders of magnitude (from --- 10 -l° to 10 -3 S cm-~). A further enhancement in conductivity, of only one or two orders of magnitude, would make glass a serious competitor with fl-ahimina for this kind of application, especially when certain advantages of glass (e.g. ease of fabrication) are taken into consideration. This problem is under active consideration [29,30] at the present time. It is hoped that by the turn of the century, when oil supplies are less plentiful than now, sodium sulphur batteries will play an important role as energy storage devices either in the electrical supply industry or as automotive power sources. However, this will depend on improving the reliability of existing fl-alumina solid electrolytes, or else finding new vitreous electrolytes to take their place.
The author has presented here a personal assessment of the prospects for ionic conduction in glass in the next twenty years. He acknowledges with pleasure discussions on glass science with his co-workers and with Professor Kreidl, and also his indebtedness to the Science and Engineering Research Council (UK), and to Chloride Silent Power. Ltd, for providing financial support over a number of years.
M.D. l n g r a m / Ionic conduction in glass
253
References [1] W.E. Spear and P.G. Le Comber, Phil. Mag. 33 (1976) 935. [2] W.E. Spear, P.G. Le Comber, S. Kinmond and M.H. Brodsky, Appl. Phys. Lett. 28 (1976) 105. [3] Y. Kuwano and H. Ohnishi, J. de Phys. 42, Supp. C4 (1981) 1155. [4] D. Kunze, in: Fast Ion Transport in Solids, ed., W. van Gool (North-Holland/American Elsevier, Amsterdam, 1973). [5] M.D. Ingram and C.A. Vincent, Chem. Soc. Ann. Repts. (1977) 23. [6] J.S. McKechnie, L.D.S. Turner, C.A. Vincent, M. Lazzari and B. Scrosati, J. Chem. Ed. 55 (1978) 418. [7] R.D. Armstrong, R.S. Bulmer and T. Dickinson, J. Solid St. Chem. 8 (1973) 219. [8] G.J. Janz, Molten Salts Handbook (Academic Press, New York, London, 1967). [9] J.L. Souquet, Ann. Res. Mater. Sci. 1l (1981) 211. [10] A. Magistris~ G. Chiodelli and A. Schiraldi, Electrochim. Acta 24 (1979) 203. [11] J.P. Malugani, B. Fahys, R. Mercier, G. Robert, J.P. Duchange, S. Baudry, M. Broussely and J.P. Gabano, Solid State Ionics 9-10 (1983) 659. [12] P.G. Le Comber and J. Mort, Electronic and Structural Properties of Amorphous Semiconductors (Academic Press, London and New York, 1973). [13] J. Teltow, Ann. Phys. Lpz. 5 (1949) 71. [14] M.D. Ingrain, J. Amer. Ceram. Soc. 63 (1980) 248. [15] J.O. Isard, J. Non-Crystalline Solids 1 (1969) 235. [16] D.E. Day, J. Non-Crystalline Solids 21 (1976) 343. [17] C.T. Moynihan, N.S. Saad, D.C. Tron and A.V. Lesikar, J. Amer. Ceram. Soc. 63 (1980) 458. [18] C.T. Moynihan and A.V. Lesikar, J. Am. Ceram. Soc. 64 (1981) 40. [19] D. Ravaine and J.L. Souquet, Phys. Chem. Glasses 18 (1977) 27. [20] H.L. Tuller, D.P. Button and D.R. Uhlmann, J. Non-Crystalline Solids 40 (1980) 93. [21] D.P. Button, L.S. Mason, H.L. Tuller and D.R. Uhlmann, Solid St. Ionics 9-10 (1983) 585. [22] M.D. Ingram, C.A. Vincent and A.R. Wandless, J. Non-Crystalline Solids 53 (1982) 73. [23] M. Doreau, J.P. Malugani and G. Robert, Electrochim. Acta 26 (1981) 711. [24] J.A. Bruce and M.D. Ingram, Solid St. Ionics 9-10 (1983) 717. [25] J.A. Bruce, C.C. Hunter and M.D. Ingram, Solid St. lonics 9-10 (1983) 739. [26] J.A. Bruce and M.D. Ingram, to be published. [27] J.T. Kummer and N. Weber, US Patent No. 3 404 035 (1 October 1968). [28] W.E. Brown, G. Heitz and C.A. Levine, US Patent No. 3 476 602 (4 November 1968). [29] C.C. Hunter and M.D. Ingram, Solid St. lonics 14 (1984) 31. [30] S. Susman, C.J. Delbecq, J.A. McMillan and M.F. Roche, Solid St. Ionics 9-10 (1983) 667.
247
T H E IMPACT O F RECENT D E V E L O P M E N T S ON T H E T H E O R Y O F AND P R O S P E C T S FOR IONIC C O N D U C T I O N IN GLASS Malcolm D. I N G R A M Department of Chemistry, University of Aberdeen, Aberdeen A B9 2 UE, Scotland
The discovery of new materials within the past 10-15 years indicates that the mechanisms of ionic (and electronic) transport in glasses and crystals are closely related. The procedure for determining mobile ion concentrations in glass from the mixed alkali effect is analogous to the established technique of aliovalent doping uoed in halide crystals. This procedure could be used for testing present day theories of ionic conduction in glass. Glasses are now available as Li+-ion and Ag+-ion conducting solid electrolytes for miniature batteries, but further development work is required to produce glasses suitable for the high temperature N a / S batteries to be used (it is hoped) in automotive power sources and load levelling applications.
1. Introduction Important discoveries since 1970 are opening up new areas for the application of glass in electronic devices, batteries, and electrochemical sensors. New types of material are being developed, which include amorphous silicon semiconductors and vitreous electrolytes. This is a convenient time to enquire if progress in the theory of ionic conduction has been keeping pace with experimental advances, and to identify areas where further advances are possible.
2. Recent developments 2.1. New materials and applications
One of the most important of recent developments has been the discovery by Spear and co-workers [1,2] that amorphous silicon can be deposited in thin layers by the action of glow-discharge on silane gas. This material has a low density of defect states, and can be doped with B and P simply by admitting B2H 6 and PH 3 into the gas stream. In this way, p- and n-type semiconductors are produced whose conductivity can be controlled over many orders of magnitude. These materials have several potential applications, and already large numbers of photovoltaic cells [3] are being manufactured for use in wrist watches and pocket calculators, and also for roof-top collectors of solar energy. 0022-3093/85/$03.30 © Elsevier Science Publishers B.V. (North-Holland Physics Publishing Division)
248
M.D. Ingrain / Ionic conduction in glass
T/'C 800 300
100
0
-100
KNO3 melt
rg
,S \ SCSo
fE
-4 3S102\
-6 1
2
-6 3 4 103/T
5
6
Fig. 1. Arrhenius plots of conductivity for: (i) KNO3 melts, (ii) N a 2 0 . 3 S i O 2 melts and glasses, and (iii) AgsI4BO 3 glasses (data from refs. [8], [9] and [10] respectively).
Important progress is also being made in the field of ionic conductors. Thus in 1973, Kunze [4] prepared (largely by accident) a highly conducting AgI/Ag2SeO 4 glass. Subsequently, a whole family of such silver iodide/silver oxysalt glasses was identified (incorporating phosphates, borates, arsenates, dichromates and molybdates [5]). In these glasses, the conductivity can be as high as 10 -2 S cm -1 at 25°C, similar to that of 0.1 M solution of KC1 in H20. It is reasonable therefore to regard these as "optimised ionic conductors" with conductivities at Tg much like those of typical molten salts (aTg = 10 -1 S c m - 1 ), see fig. 1. Batteries incorporating these electrolytes have been made, typically: Ag,AgvAsO414 IAg7AsO414] 12,AgvAsO4I 4 ; (composite) (composite)
E = 0.69 V.
These batteries are of interest for demonstration and teaching purposes [6], but at present do not have commercial value because of their low energy density. The high conductivity of this family of glasses is not surprising in view of the high conductivity of crystalline RbAg4I 5, and indeed of a-AgI itself [5]. As Armstrong and Dickinson explained some years ago [7], the high mobility of Ag ÷ ions can be attributed to the ease with which they can adopt various (e.g. 3, 4 and 5) coordination numbers - a property which also reflects the "soft" covalent interactions between Ag + and I - ions. In solid electrolytes generally, cation mobility increases with increasing polarizability of the anion sublattice. This effect has recently been exploited in the synthesis of a variety of thio glasses (e.g. mixed lithium iodide/lithium thiophosphate glasses, in the system Li2S-P2Ss-LiI ) [11], which exhibit comparable Li+-ion mobility at slightly higher temperatures. These glasses are now being incorporated into batteries like: LiA1]LizS-P2Ss-LiIglass]Cu40(PO4)2,
E = 2.3 V,
which are being developed for various applications, the possibilities including use as reserve power supplies for memory boards in computers.
M.D. Ingram / Ionic conduction in glass
249
3. Mechanisms of ionic mobility The conductivity of any solid can be written as (1)
o = Zn*eozu,
where n* is the number of charge carriers (electrons, holes, etc.) per unit volume, e 0 is the electronic charge, z is the valency (of an ion), and u is the corresponding electrical mobility. For semiconductors, the mobilities of holes and electrons can be determined directly from the Hall effect, or drift mobility experiments - see ref. [12], for example. So far, these techniques have not been applied successfully to ionic glasses. For convenience it is usually assumed that all similar cations in glass (e.g. Na + ions in a sodium silicate glass) are equally mobile. Thus conductivity mechanisms are discussed in the absence of important information relating to the nature and numbers of charge carriers. But how can charge carriers in glass be identified? In this context the phenomenon of doping an amorphous silicon is important. Boron and phosphorus atoms must enter directly into sites normally coupled by silicon atoms (4-coordinate) and satisfy their bonding requirements (i.e. complete their Lewis octet) either by trapping electrons (thus producing positive holes in the valence band) or releasing electrons into the conduction band of the glass. The conducting defects (i.e. the holes and electrons) originate therefore in much the same way as they do in crystalline silicon. In considering an appropriate theory of ionic transport in glasses, we shall see first how progress has been made with crystalline materials. As long ago as 1949, Teltow [13] examined the effects of aliovalent dopants, such as Cd 2+, on the conductivity of silver halide crystals. Cd 2+ ions enter sites normally occupied by Ag + ions in the crystal lattice, and cation vacancies are produced. The resulting steep drop in conductivity which occurs during the initial state of doping, see fig. 2, was attributed to the "trapping" of mobile interstitials (Ag +) by these vacancies. Teltow showed the shapes of these conductivity isotherms could be analysed, and was able to determine values of the concentrations of mobile Ag + cations in pure AgBr, and hence (through eq. (1) above) to calculate the ionic mobilities). Over 30 years were to elapse before similar arguments were to be applied to conduction in glass. Ingrain suggested [14] that the fast-ion conductor B-alumina might be a model for Na + (and other alkali ion) conducting glasses. Within the conduction planes of ]~-alumina alkali cations migrate by a paired or "split"' interstitially mechanism, which for convenients can be represented schematically as: (M2)++M=M+(M2)
+,
(2)
where only "excess" charges on interstitial species are shown for clarity. If the same sort of process occurs in glass, the occurence of the "mixed alkali effect" [15,16] is attributed to reactions like: (K 2)++ Na = K + ( N a K ) +.
(3)
250
M.D. l n g r a m / Ionic conduction in glass r
-5( ~
'200"C
i
i
i
MOYNIHAN - LESIKAR (1981) ]
1.( o
250"C
-4) 0.9
~-9
>
u 0.8
07
(Teltow, 1949 ) ISOTHERMS FOR AgBr CRYSTALS
-11i
ISOTHERMSFOR
I
O.J05 01 tool. */, Cd Br2
0"2
0"4 0"6 K/(K*Na)
J
1
08
Fig. 2. Relative conductivity isotherms for AgBr crystals doped with CdBr 2 (data from ref. [13]). Fig. 3. Conductivity isotherms showing the mixed alkali effect in N a 2 0 - 3 S i O 2 / K 2 0 - 3 S i O 2 glasses (data from ref. [17]).
where (NaK) + pairs are immobilised within the framework of the glass by ion polarisation, or other interactions. Moynihan and Lesikar [17] pointed out that the steepest fall in conductivity (taking the N a 2 0 . 3 S I O 2 / K 2 0 . 3 S i O 2 system as an example) occurs in the dilute foreign alkali region, i.e. at each end of the conductivity isotherm, fig. 3. By making certain assumptions about constant ionic mobility in this region and assuming that reaction (3) above is quantitative, they developed a mathematical treatment [18] very similar to Teltow's and calculated the concentrations of mobile interstitials (or pairs) in the glass. The proportion of mobile K ÷ ions in K 2 0 - 3 S i O 2 glass was found to increase from about 0.6% at 25°C to 5% at 400°C. It appears from their calculations that the conductivity of glass can be increased either by increasing the concentration of interstitial cations, or by increasing the mobility. The conductivity of NaO-SiO 2 glasses is known to increase with increasing N a 2 0 content, but does this occur because the concentration of (Na2) ÷ pairs increases, or because they become more mobile? This is an important but unanswered question, on which opinions in the literature differ. Ravaine and Souquet [19] attributed the rapid increase in conductivity in N a 2 0 - S i O 2 glasses, to changes in the concentrations of "free" Na + ions [19], according to the equilibrium: N a 2 0 ~ N a O - + Na +. (4)
M.D. l n g r a m / Ionic conduction in glass
251
This is often called the "weak electrolyte" theory. If reaction (4) is rewritten in the form: 2(-O-Na+)~-O
+-O-(Na2)
++,
(5)
the "free" Na + ions can be identified with the interstitial pairs of the Moynihan-Lesikar-Ingram (MLI) model. The essence of Ravaine and Souquet's theory is that the mobility of the "free" Na + ions remain constant over the whole range of N a 2 0 . SiO 2 glasses. By way of contrast, Tuller and co-workers [20,21] consider variations in mobile ion concentration to be relatively unimportant, and have looked instead for the influence of glass structure on ion mobility. It seems that the matter could now be resolved quite readily by taking appropriate measurements in the dilute foreign alkali region, and calculating the mobile carrier concentrations using the MLI theory described above. It would be unwise, however, to assume that exactly the same kind of "defect" model necessarily applies to all conducting glasses. The phase transition in AgI at 147°C is accompanied by melting of the Ag + sublattice [5], and the high conductivity is often attributed to quasiliquid motions of Ag + ions, disordered within the loose framework of I ions. If this kind of mechanism operates in AgI-containing glasses, it could explain e.g. a number of unusual phenomena such as curved Arrhenius plots, which occur near the glass transition [22]. The "special" interaction between Ag + and I ions overrides the normal mixed alkali effect. In AgPO 3 glasses, for example, addition of NaI produces similar increases in conductivity to the addition of AgI [23]. A method for determining mobile ion concentrations in these glasses has still to be worked out.
4. Future prospects The main conclusion to be drawn is that ionic conduction processes in glasses and crystals are rather similar, fl-alumina is a good model for alkali ion conduction in alkali silicate and other "conventional" glasses. A variety of mixed-alkali effects are observed in these single-crystal electrolytes [24,25], and these are now being interpreted in the light of cation distributions determined by X-ray diffraction experiments [26]. A comprehensive theory of the mixed alkali effect is now in sight, and so one of the outstanding "mysteries" of glass science [15,16] may soon be solved. It is interesting to recall that the first patents [27,28] taken out on the sodium sulphur battery, fig. 4, specified using Na+-ion conducting glasses as alternatives to the fl-alumina solid electrolyte. However the resistivity of glass is usually considered to be too large for this application. The problem is illustrated in fig. 5. The conductivity at 300°C for a series of simple and "modified" sodium borate glasses [29] is plotted as a function of total modifier
252
M.D. Ingrain / Ionic conduction in glass
Current
cottector
I
r
i
i
i
-3
ii i' '~i'i!,i °,
.:: o
Na+-ion so[id electrolyte
ki',,i~
~.::,
(g [ass )
i
F
E -5 i./I
,:, :o
**,
Marten sutphur
• ~ *,
and carbon fett
o 0
'G
~
[ e
0
10
• NQC[ a NaF ~ Na2 SO/, c3 AI203
-9
I
®
-g203 glasses,
] n/ ~
No[ten sodium J
/~Ct20
I
1
I
20 30 40 mlo NQ2Z
I
50
Fig. 4. Schematic of N a / S cell (beta battery) which operates in range 300-350°C. Fig. 5. Conductivities at 300°C of "modified" sodium borate glasses (data from ref. [29]). The abscissa, m / o Na 2Z = m / o (Na 2° + Na 2C12), etc.
content expressed as a mol per cent, m / o ( N a z Z ) = m / o (Na20 + Na2CI2), etc. We see that the conductivity is strongly influenced by Na+-ion content, and at this temperature, it varies by seven orders of magnitude (from --- 10 -l° to 10 -3 S cm-~). A further enhancement in conductivity, of only one or two orders of magnitude, would make glass a serious competitor with fl-ahimina for this kind of application, especially when certain advantages of glass (e.g. ease of fabrication) are taken into consideration. This problem is under active consideration [29,30] at the present time. It is hoped that by the turn of the century, when oil supplies are less plentiful than now, sodium sulphur batteries will play an important role as energy storage devices either in the electrical supply industry or as automotive power sources. However, this will depend on improving the reliability of existing fl-alumina solid electrolytes, or else finding new vitreous electrolytes to take their place.
The author has presented here a personal assessment of the prospects for ionic conduction in glass in the next twenty years. He acknowledges with pleasure discussions on glass science with his co-workers and with Professor Kreidl, and also his indebtedness to the Science and Engineering Research Council (UK), and to Chloride Silent Power. Ltd, for providing financial support over a number of years.
M.D. l n g r a m / Ionic conduction in glass
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