The mixed alkali effect in sodium and potassium galliosilicate glasses II. DC electrical conductivity

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Journal of Non-Crystalline Solids 86 (1986) 350-360 North-Holland, Amsterdam

T H E MIXED ALKALI E F F E C T IN S O D I U M AND P O T A S S I U M GALLIOSILICATE GLASSES II. D C electrical conductivity * Josef C. LAPP and James E. SHELBY Institute of Glass Science and Engineering, NYS College of Ceramics at Alfred University, Alfred, N Y 14802, USA Received 18 December 1985

The DC electrical conductivities of several series of mixed alkali galliosilicate glasses have been measured. The appearance of a minimum in the electrical conductivity of these glasses, independent of the gallium content, suggests that the mixed alkali effect is independent of the non-bridging oxygen content. These results are discussed in terms of current theories proposed to explain this anomalous behaviour.

1. Introduction

When an alkali ion in a glass is systematically replaced by a different alkali ion, several of the physical properties of the glass change in a nonlinear manner. This effect is known as the mixed, or poly, alkali effect and has been found to occur in a large number of glass-forming systems, including silicates, borates and germanates [1,2]. Those properties involving the diffusion of the alkali ions are most noticeably affected. For example, the DC electrical conductivity of a mixed alkali glass is orders of magnitude less than that of a corresponding glass containing a single alkali oxide. Several explanations for this effect have focused on either the clustering [3] of the alkali ions in the glass or the presence of non-bridging oxygen ions [4]. Recently, Dietzel proposed [5] that the mixed alkali effect is a function of the difference between the modifying cation field strengths. According to this theory, the larger this difference, the more stable groups of the form: Si

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(where R 1 and R 2 a r e different alkali ions) are in the glass, and the more pronounced is the mixed alkali effect. This theory requires the presence of * This material is based upon work supported by the National Science Foundation under grant No. DMR-8304445. 0022-3093/86/$03.50 © Elsevier Science Publishers B.V.

J. C. Lapp, J.E. Shelby / Mixed alkali galliosilicate glasses

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non-bridging oxygen ions in the glass structure before a mixed alkali effect can occur. Hayward [6,7] measured the electrical conductivity of glasses in the mixed alkali aluminosilicate system. These glasses are unusual in that aluminium is proposed [8-10] to behave as both a network former and modifier depending on the aluminum to alkali ion ratio, A1/R. According to the more recent theories [11,12], at an A1/R of approximately 1.1 the glass is fully linked with the alkali ions individually associated with aluminium-containing tetrahedra. Hayward reported that no mixed alkali effect occurred in those glasses with an A 1 / R of 1.0 (a glass that is nearly fully linked), but that one developed as the (A104/2)- units were gradually replaced by non-bridging oxygen ions. Recent studies [13,14] in this laboratory on lithium silicate glasses containing both aluminium and gallium have shown that the physical properties vary in a linear manner as aluminium is progressively replaced by gallium. This suggests that gallium behaves in a manner directly analogous to aluminium in these glasses. That is, as either a network former, or modifier, depending upon the gallium to alkali ion ratio. In an earlier paper [15], the present authors reported that the glass transformation temperatures of glasses of the general formula 20(Na, K)20x G a 2 0 3 - ( 8 0 - x)SiO 2 demonstrated a mixed alkali effect irrespective of the gallium to alkali ion ratio. This paper extends that earlier work to include the DC electrical conductivity of these mixed alkali galliosilicate glasses.

2. Experimental procedure

2.1. Preparation of samples Five series of glasses of the general formula 20(Na, K ) 2 0 - x G a 2 0 3 (80 - x)SiO 2 were examined in this study, with both the soda to alkali oxide and the gallia to alkali oxide ratios varied. The glasses were prepared by conventional melting techniques using 99.999% Ga203 and reagent grade SiO2 and alkali carbonates. The glasses were melted in platinum crucibles at temperatures between 1500 and 1700°C. The details of the melting procedure are described elsewhere [16]. After cooling, the glass was removed from the crucible, annealed (at approximately 25°C above the glass transformation temperature), and slowly (1-2 K / m i n ) cooled to room temperature. The samples were cut using a low-speed diamond saw with distilled water as the coolant. As an indicator of possible compositional changes during melting, the weight loss on melting was monitored. In every instance, this loss was < 1 wt%, with more typical values < 0.5 wt%. Wet chemical analysis of representative sodium galliosilicate glasses [16] indicate excellent agreement between theoretical and actual compositions.

J. C, Lapp, J.E. Shelby / M i x e d alkali galliosilicate glasses

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2.2. Method of measurement The DC Electrical resistivity was measured isothermally in a standard series resistance circuit. Samples were approximately 1.5 mm thick. Gold-palladium alloy electrodes were sputtered onto the glass to a thickness of approximately 150 nm in a guard ring configuration. The DC electrical conductivity at 200°C and the activation energy for conduction were calculated from parameters derived from a least-squares exponential fit of the resistivity data. The conductivity apparatus was calibrated with a SRM 624 glass standard. Isoconductivity temperatures for several series of measurements for the standard were accurate and reproducible to within +_2°C of those reported by NBS.

3. Results

3.1. Influence of gallia substitution Fig. 1 shows the resistivity data as a function of reciprocal temperature. Within the range of this study, the data are well described by an Arrhenius equation of the form:

P = Oo e x p ( Q / R T ) , where p is the resistivity, 90 is a pre-exponential term, Q is the activation energy for conduction, R is the gas constant, and T is the absolute temperature. Fig. 2 shows the DC electrical conductivity at 200°C of three series of glasses of the general formula 2 0 R 2 0 - x G a 2 0 3 - ( 8 0 - x ) S i O 2 , where R is either Na, K, or an equimolar mixture of the two, as a function of G a / R . Similar trends are evident in the data irrespective of the alkali type or the presence of a second alkali. The electrical conductivity initially decreases with gallia substitution, passing through a minimum at a G a / R of approximately 0.3-0.5. The magnitude of this decrease is dependent upon the alkali type, increasing in the order K < N a : K < Na. At intermediate G a / R values (0.5-1.0), the electrical conductivity increases with gallia substitution, passing through a maximum at a G a / R of approximately 1.0-1.1. At higher gallium to alkali ion ratios, the conductivity again decreases with increasing G a / R . The conductivity of these glasses can be expressed by the formula: In o = In oo - Q / R T , where a is the conductivity, % is a temperature-independent term, and the other symbols are as defined above. The temperature-independent term, In oo, was found to be independent of the gallium to alkali ion ratio for these glasses. Therefore, the changes in the conductivity shown in fig. 2 are a result of changes in the activation energy for conduction (see fig. 3). The activation energy for conduction passes through a minimum at a G a / R of approximately 1.0-1.1.

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3.2. Trends in the mixed alkali glasses The electrical conductivity of glasses of the general formula 20(Na, K ) 2 0 - 8 0 S i O 2 have been previously studied [17,18]. In fig. 4, the published values of (A) the DC electrical conductivity at 200°C and (B) the activation energy for conduction are compared with the results of this study. The present results agree very well with those published by others. The conductivity passes through a minimum at a soda to total alkali oxide ratio of approximately 0.4, while a maximum in the activation energy occurs at the same ratio. Fig. 5 shows the DC electrical conductivity at 200°C versus the ratio of soda to total alkali oxide for all of the glasses of the present study. The gallia content of each glass is indicated on the figure. Although there are small systematic differences with G a / R , a single curve is shown for clarity. The conductivity for every glass series decreases with increasing soda to total alkali oxide ratio, passing through a minimum at a ratio of approximately 0.4. When plotted against the soda to total alkali oxide ratio, both the temperature-independent term, In o0, and the activation energy for conduction of these glasses exhibit maxima, as is shown in figs. 6(a) and (b), respectively. These two term affect the conductivity of the glass in opposite ways. However, the

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4. Discussion 4.1. Structure o f alkali galliosilicate glasses

The trends observed in the electrical conductivity and activation energy for conduction shown in figs. 2 and 3 have been explained [19] on the basis of a structural model proposed for these glasses. The prominent feature of this model is the dependance of the glass structure on the gallium to alkali ion ratio. Gallium can behave as either a network modifier or network former depending on the gallium to alkali ion ratio. At low gallium to alkali ion ratios, a significant portion of the gallium is a network modifier with the remainder a network former. These units influence

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the conductivity of the glass in opposite ways, resulting in the relatively constant conductivity exhibited by the potassium-containing glasses, and the initial decrease in the electrical conductivity exhibited by the sodium and mixed alkali glasses, As the gallium content of the glass is increased, a progressively larger percentage of the gallium enters the glass network as (GaO4/2) tetrahedra.

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This results in the localization of an alkali ion into one of the four equivalent sites surrounding these units, with the concurrent conversion of a non-bridging oxygen ion into a bridging oxygen. The alkali ions may move relatively easily among these four equivalent sites, resulting in a decrease in the activation energy for electrical conduction [14,19]. This trend continues until a (GaO4/z)to alkali ion ratio of one. At this point, the glass is fully linked with all alkali ions associated with (GaO4/2)- tetrahedra. Because some gallium still behaves as a network modifier, this point occurs at a G a / R somewhat greater than one. These gallium-oxygen tetrahedra are thought [20] to be uniformly distributed throughout the glass. Any gallium added in excess of that required for complete polymerization of the glass again enters the glass as a network modifier, increasing the activation energy for conduction [14,19]. 4.2. Mixed alkali effect The deviations from additivity in the DC electrical conductivity and the activation energy for conduction are shown in figs. 7 and 8, respectively. These figures show that a significant deviation from additivity, i.e. a mixed alkali effect, exists in these glasses regardless of the gallium content. In fact, the conductivity data show a slightly larger mixed alkali effect in the higher gallia containing glasses. Thus, a mixed alkali effect occurs whether the alkali ions are paired, as when they are associated with non-bridging oxygen ions ( G a / R of 0.0), or are randomly distributed in the glass, as for those ions associated with (GaO4/2)- tetrahedra ( G a / R of 1.0-1.1). This result casts serious doubt on the validity of any model for the mixed alkali effect (such as that of Dietzel [5]) which requires either the presence of non-bridging oxygen ions or the occurrence of pairs of alkali ions in the glass. The present results also contradict the earlier report of Hayward [7] that no mixed alkali effect occurs in the absence of non-bridging oxygen ions in mixed alkali aluminosilicate glasses. This finding is somewhat surprising in view of the structural similarity between the alkali gallio- and aluminosilicate glasses. A possible explanation for this conflict may well lie in the choice of glasses studied. At least 10 mol% total alkali is required [1,2] in simple mixed alkali silicate glasses before the mixed alkali effect becomes evident. Hayward worked with glasses containing relatively low (12 mol%) levels of alkali oxide, in which the mixed alkali effect should be small. The present study, on the other hand, dealt with glasses containing 20 mol% R20, where a much larger effect is expected. Additional problems result from Hayward's choice of the base binary sodium silicate glass composition. This glass is known [21] to be phase separated. Although the addition of either potash, alumina, or gallia to a sodium silicate glass is known [22,23] to result in the homogenization of the glass, the influence this has on the measured electrical conductivity remains uncertain. Several theories [24-26] for the mixed alkali effect have focused on a proposed increased energy binding an alkali ion to its site in the glass when a

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s e c o n d t y p e of alkali is i n t r o d u c e d . S u c h theories h a v e e x a m i n e d the s t r e n g t h of the b o n d b e t w e e n the alkali a n d the n o n - b r i d g i n g o x y g e n i o n s i n the glass. I n light of the p r e s e n t results, these theories m u s t b e e x p a n d e d to i n c l u d e the effects of d i f f e r e n t a n i o n types, s u c h as the g a l l i u m - o x y g e n t e t r a h e d r a p r o p o s e d to o c c u r i n the p r e s e n t glasses.

References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17]

[18] [19] [20] [21] [22] [23] [24] [25] [26]

J.O. Isard, J. Non-Cryst. Solids 1 (1969) 235-261. D.E. Day, J. Non-Cryst. Solids 21 (1976) 343-372. R. Terai, J. Non-Cryst. Solids 6 (1971) 121-135. O.V. Mazurin, Structure of glass, Vol. 4 (Consultants Bureau, new York, 1965) pp. 5-55. A.H. Dietzel, Phys. Chem. Glasses, 24[6] (1983) 172. P.J. Hayward, Phys. Chem. Glasses 1713] (1976) 54. P.J. Hayward, Phys. Chem. Glasses 1811] (1977) 1. D.E. Day and G.E. Rindone, J. Am. Ceram. Soc. 45110] (1962) 489. D.E. Day and G.E. Rindone, J. Am. Ceram. Soc. 45110] (1962) 496. D.E. Day and G.E. Rindone, J. Am. Ceram. Soc. 45112] (1962) 579. J.E. Shelby, J. Appl. Phys. 49112] (1979) 5885. V.K. Hunold and R. Bruckner, Glastechn. Ber. 53[6] (1980) 149. J.l. Piguet and J.E. Shelby, J. Am. Ceram. Soc. 68[9] (1985) C232. J.L. Piguet and J.E. Shelby, Advan. Ceram. Mat. 1 (1986) 192. J.C. Lapp and J.E. Shelby, Proc. Conf. on Physics and Chemistry of Glass and Glassmaking, Alfred Univ., Alfred, NY (July 1985) J. Non-Cryst. Solids 84 (1986) 463. J.C. Lapp and J.E. Shelby, J. Am. Ceram. Soc. 69 (1986) 126. K.S. Evstroyev, as cited in: Properties of glass and glass-forming melts: Vol. III, ON. Mazurin, M.V. Strel'tsina and T.P. Shvaiko-Shvaikovskaya (Izdaterstvo Nauka, Leningrad, 1973) p. 60. G.A. Pavlova, Ref. [17], p. 50. J.C. Lapp and J.E. Shelby, Advan. Ceram. Mat. 1 (1986) 174. M. Okuno, F. Marumo, T. Sakamaki, S. Hosoya and M. Miyake, Mineral. J. 1213] (1984) 101. W. Hailer, D.H. Blackburn and J.H. Simmons, J. Am. Ceram. Soc. 57[3] (1974) 120. y. Kawamoto and M. Tomozawa, Phys. Chem. Glasses 23[2] (1982) 72. J.A. Topping and M.K. Murthy, J. Am. Ceram. Soc. 56[5] (1973) 270. J.R. Hendrickson and P.J. Bray, Phys. Chem. Glasses 1312] (1972) 43. J.R. Hendrickson and P.J. Bray, Phys. Chem. Glasses 1314] (1972) 107. A. Klonkowski, J. Non-Cryst. Solids 57 (1983) 339.