Effects of fictive temperature and water content on electrical conductivity of silica glasses

J O U R N A L OF

ELSEVIER

Journal of Non-CrystallineSolids 203 (1996) 262-267

Effects of fictive temperature and water content on electrical conductivity of silica glasses Dong Wook Shin, Minoru Tomozawa Materials Science and Engineering Dept., Rensselear Polytechnic Institute, Troy, NY 12180, USA

Abstract

Electrical conductivities of type I and IV silica glasses were measured as a function of fictive temperature and water content. Silica glasses with different fictive temperatures were produced by a heat treatment in ambient air and subsequent quenching. The water content in silica glasses was varied by a high temperature hydrothermal treatment. For type IV silica glass, the electrical conductivity decreased and the activation energy increased with increasing fictive temperature. These changes appear to be related to the decrease in specific volume of the silica glass with increasing fictive temperature. For type I glass, the changes were much smaller. Changes in water content produced different effects on electrical conductivities of different types of silica glasses; the conductivity of type I silica glass increased slightly while that of type IV silica glass decreased with increasing water content. These opposite trend in these two types of glasses can be attributed to the different content of aluminum, which interacts with protons introduced by the hydrothermal treatment.

1. Introduction

The water-related species influences the optical and electrical properties of silica glasses but its exact role is poorly understood. Garino-Canina and Priqueler [1] reported a change in hydroxyl distribution in silica glasses containing alumina when a high electric field was applied at high temperature ( ~ 1050°C), and they proposed the movement of protons as a cause of inhomogeneous hydroxyl distribution. Recently, Yamamoto and Namikawa [2] showed that the electrical conductivity of some silica glasses changed with heat treatment. They attributed the phenomenon to a shift in the equilibrium of the chemical reaction between silica and water.

Corresponding author, Tel.: +1-518 276 6451; fax: + 1-518 276 8554; e-mail: [email protected].

Several research groups have reported second harmonic generation (SHG) in electrically poled silica glasses [3-6]. Interestingly, the water content in silica glass is reported to affect the SHG; glasses of greater water content showed larger SHG signal intensity [5]. These observations indicate a need to understand the role of water in determining the electrical properties of silica glasses. One way to investigate the effect of water on electrical behavior of glass would be to measure the electrical conductivity cr as a function of water content. Samples of high water content can be prepared by hydrothermal treatment. However, because the glass samples treated hydrothermally can undergo structural relaxation and a change of fictive temperature, it is necessary to distinguish the effect of changes in fictive temperature [7] from changes in water content. Changes in the fictive temperature of a glass affects various properties, including the elec-

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D. W. Shin, M. Tomozawa / Journal of Non-Co'stalline Solids 203 (1996) 262-267

trical conductivity [8,9]. In the present work, a systematic study was carried out to determine the effect of water content and fictive temperature on the electrical conductivity of silica glasses.

2. Experimental method Samples used in this experiment were high purity silica glasses, Suprasil W2 and Infrasil, (HeraeusAmersil). In addition, Furukawa (Furukawa Electric) glass was used to investigate the effect of chlorine. Table 1 shows selected impurity contents of these glasses reported by the manufacturer. Since Suprasil W2 (type IV) and Furukawa are produced by the CVD method using flame in a chlorine-rich atmosphere, these specimens have very low water content and a high chlorine content compared to other silicas. The sodium content of Infrasil (type I) is greater, ~ 1 ppm. One difference between type I (fused silica glasses) and type IV (synthetic glasses) is the amount of A1; fused silica glasses contain 10 to 50 ppm A1 by weight, while synthetic glasses contain less than l ppm AI by weight. Samples were cut into approximately 1 cm × 1 cm plates with thickness ~ 0.5 mm, polished with 1000 grit SiC powder in oil, and subsequently finished to a mirror surface with a CeO 2 slurry in water. Polished samples were cleaned with acetone for 3 min in an ultrasonic cleaner. Samples were etched in a 10%HF-2.5%H2SO 4 solution for 3 min to remove the surface damaged layer formed during polishing [10]. Samples of different fictive temperature were prepared by heat treatment in the temperature range

Table 1 Silica glass types and impurity contents (ppm by weight) of samples used in the present study. Type IV glass is produced from SiCla by plasma fusion process; type I glass is produced by electric fusion of natural quartz. Furukawa glass is produced by CVD process called 'axial vapor deposition' Glass

Glass Na type

Suprasil W2 IV Furukawa Infrasil I

K

Li

CI

AI

OH

0.04 < 0.01 < 0.05 ~ 240 0.1 5 0.04 ~ 0.001 ~ 1000 < 0.04 ~ 0.1 1 ~ 0.8 1 - 10-50 < 8

263

1000 to 1300°C in ambient air. Heat treatment of silica glasses in ambient air is always accompanied by water diffusion into the sample. Agarwal [l l] found that the structural relaxation during heat treatment arises both from the thermally induced relaxation, which is uniform throughout the whole sample, and from water diffusion-assisted structural relaxation, which occurs at the glass surface. In the present work a sufficient heat treatment time was employed to achieve a uniform equilibrium structure in this temperature range. After the heat treatment for a specified period of time, samples were quenched in water to freeze the structure at the heat treatment temperature. Subsequently, the surface layers affected by the water diffusion were removed from these heat-treated samples by the polishing and etching procedure described earlier. The final specimen thickness for electrical measurement was 30 to 120 ~m.

Infrared spectra measured with a Fourier transform infrared spectrometer (FTIR) (Model 1800, Perkin Elmer, Norwalk) were used to monitor the structural changes in the silica glasses. There is quantitative relationship between the fictive temperature of silica glass and the peak position of IR bands due to the SiO 2 network (1122 cm ~ fundamental reflection band, its overtone absorption band at 2260 c m - l, or the overtone/combination absorption band at 1873 cm -~) determined previously [l 1-13]. In the present work the 1873 cm -L absorption band position was used to monitor the fictive temperature of the glasses. Silica glass specimens with various water contents were produced by hydrothermal treatment for 450 to 900 h at > 950°C using a tube furnace into which water vapor at various partial pressures was introduced from a water bath. After the treatment, the samples were quenched into water to preserve the equilibrium structure that resulted from the hydrothermal treatment. FTIR was used to determine the water content and fictive temperature of the samples after hydrothermal treatments. In commercially available silica glasses, the OH stretching band of the silanol group (=SiOH) is located at 3673 + 3 c m - I [12,14]. Since hydroxyl is the predominant water-related species produced by high temperature hydrothermal treatment, this band was used to determine the water content in silica

264

D. W. Shin, M. Tomozawa / Journal of Non-Crystalline Solids 203 (1996) 262-267

gl,asses using the extinction coefficient Con = 155 L / m o l . cm [12]. The thin surface layer of hydrothermally treated samples was sliced off the specimen and used for the electrical measurements to obtain the maximum water concentration with approximately uniform distribution across the sample. The experimental method for measurement of electrical conductivity o- was identical to that in a previous work by the authors [15].

T (°C) le-10

400 i

300

200

150

100

i

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~

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3. Results

le-16

The position of the 1873 c m - ~ combination peak for silica glasses showed a linear relationship with the reciprocal of the heat treatment temperature, as shown in Fig. 1. The data at the highest temperature ( > 1350°C) deviates slightly from the correlation line, probably because the quenching rate was not fast enough to completely freeze in the structure at this temperature. The variation of 1873 cm-~ band

le-17 1.6

1316°C i

1.8

2.0

i

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2.2

2.4

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1000/T (K11 Fig. 2. Arrhenius plots of the electrical conductivity of Suprasil W2 silica glasses with various fictive temperatures in °C. The error bar is smaller than symbol size.

position, b'1873 ( c m - I ) , with heat treatment temperature, THT (K), can be represented by 1000 v1873 = (1859.42 + 0.24) + (17.61 + 0.33) - -

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!000/TnT (K"1) Fig. 1. 1873 c m - J infrared absorption peak position of silica glasses as a function of inverse of heat treatment temperature THT. Figure includes data by Agarwal [15] for Suprasil W2 and data measured in the present work for 8uprasil W2 (SW2, Type IV), Infrasil (Type I), and Suprasil 2 (82, Type III) from HerauesAmersil. The solid line represents linear fit to the data. The dotted lines are 95% confidence limits.

It is assumed that the heat treatment temperature, Tr~v, is the same as the fictive temperature, TF, since the heat treatment time used were long enough to achieve the equilibrium structure for this glass [11]. The electrical conductivity of Suprasil W2 decreases with increasing fictive temperature as shown in Fig. 2. Correspondingly, the activation energy for electrical conduction increases with increasing tictive temperature. The activation energy of electrical conductivity for the Suprasil W2 silica glass, A Q (kJ/mol), appears to be a linear function of fictive temperature in the range T F = 1300 ~ 1600 K: AQ = 59.4954 + 0.0711T F ( k J / m o l ) .

(2)

The unit of Tr is K. Similar results for Infrasil glasses (type I) are shown in Fig. 3. In this case, the changes of conductivity and activation energy with the fictive temperature are much smaller compared with those of Suprasil W2.

D. W. Shin, M. Tomozawa / Journal of Non-Crystalline Solids 203 (1996) 262-267 I

T (°C) 400 I

le-10

300 i

200 i

150 i

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T~

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Water Content (wt ppm H20)

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~ 1.8

1.7

Fig. 5. Electrical conductivity activation energy as a function of water content in Suprasil W2 silica glass.

I 1.9

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I 2.2

i 2.3

i 2.4

i 2.5

2.6

1000/T (K-1) Fig. 3. Arrhenius plot of the electrical conductivity of Infrasil silica glasses with various fictive temperaturein °C. The error bar is smaller than symbol size. For Suprasil W2, the electrical conductivity decreased and the activation energy increased with increasing water content as shown in Figs. 4 and 5. Water content is in ppm H 2 0 by weight. The open symbols in Fig. 5 are activation energies calculated from Arrhenius plots of the measured conductivities. T (°C) 400 i

le-10

le-ll

300 I

200 i

150 I

100 i

~

SW2

Since the hydrothermal treatments were carried out at the different temperatures and the glass underwent structural relaxation during the hydrothermal treatment, the fictive temperatures of the samples were different for different hydrothermal treatment temperatures. The activation energy data were corrected for difference in fictive temperature using Eq. (2), assuming that the effects of fictive temperature and water content on the activation energy are additive. The reference fictive temperature for the correction was that of the as-received sample, 1069°C, for Suprasil W2. The filled symbols in Fig. 5 represent the corrected data. The corrected data, show an increase in the activation energy of electrical conductivity with increasing water content. In contrast, Infrasil showed the reverse trend. With increasing water content the electrical conductivity increased slightly, the activation energy decreased.

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4. Discussion

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2.6

2.8

1000/T (K-1) Fig. 4. Arrhenius plot of the electrical conductivityof Suprasil W2 silica glass with various water content in ppm H20 by weight.

The increase of the electrical conductivity of alkali silicate glasses with increasing fictive tempera'ture can be correlated with an increase in the specific volume of glass [8,9]. Although atfirst sight Suprasil W2 appears to behave in the opposite manner, with the higher fictive temperature giving the lower conductivity and the higher activation energy, silica glasses have an inversion region where the ,specific volume of the glass decreases with the increasing annealing temperature [16] or fictive temperature. Thus the change of electrical conductivity of Suprasil

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D.W. Shin, M. Tomozawa / Journal of Non-Co,stalline Solids 203 (1996) 262-267

W2 silica glass with changing fictive temperature can also be correlated with changes in specific volume in the same way as alkali silicate glasses. On the other hand, the cause of the much smaller change in conductivity of Infrasil with changing fictive temperature is not known, since an equivalent or greater volume change with changing fictive temperature is expected for this glass [16]. The effect of the volume change on electrical conductivity may be equivalent to the effect of pressure on conductivity. It is known that the pressure effect on conductivity is much less or even in opposite direction in mixed alkali glass than in single alkali glass [17]. Furthermore, it is also known that the mixed alkali effect becomes less in the glass with lower total alkali concentration [18]. While both Infrasil and Suprasil W2 are mixed alkali glasses, Infrasil contains higher total alkali concentration than Suprasil W2 glass and it is expected to exhibit the mixed alkali effect more. Thus Infrasil is expected to be less sensitive to pressure or volume change caused by the fictive temperature change. With increasing water content the conductivity of Suprasil W2 decreased, while that of Infrasil increased slightly. The behavior of Suprasil W2 is similar to that of alkali silicate glasses [19] in which the conductivity decreases initially with increasing water content and in which the conduction was due entirely to alkali ionic motion. Therefore it is reasonable to assume that the alteration of conductivity by water in silica glasses is also due to structural modifications a n d / o r changes in interionic interactions which affect the diffusion of the major charge cartier, alkali ions (in particular the sodium ions). The different behaviors of two types of silica glasses studied here may be attributable to the differences in alumina content. Bihuniak et al. [20] found that the composition dependence of the isothermal viscosity of silica glasses with various impurities can be represented in terms of the Al203/alkali oxide ratio. Jain et al. [21 ] found that the electrical conductivity of silica glasses also follows a similar trend. Rajaram and Mukhopadhyay [22] extended this analysis and found that the viscosities of silica glasses with different water contents can be correlated with the ratio R = A I / ( M + H), where M is total number of alkali ions, H is the number of protons, and AI is the number of aluminum ions. For R = A I / ( M + H) < 1, the viscosity of glass rapidly increases with

increasing R, while for R = A I / ( M + H) > 1 the viscosity of glass is nearly constant or decreases with increasing R. This behavior can be explained in terms of the number of non-bridging oxygens, which is zero at R = 1. The activation energy for electrical conduction in glass is believed to include the coulombic interaction between alkali ions (sodium ions, in the present case) and the adjacent anion sites. The magnitude of Coulombic interaction is inversely proportional to the cation-anion distance. For alumina containing glasses, since the (AIO4)--Na + distance is much larger than the non-bridging oxygen-alkali distance, the binding energy of (AIO4)--Na + is expected to be lower. Hence one expects that sodium ions can move through the glass network by hopping between (A104)- sites more easily than by hopping between non-bridging oxygen sites, since the activation energy for hopping between the (AlO4)- sites is less. When water is introduced in the silica glass, some of the alkali ions adjacent to (A104) are displaced by protons, Therefore, instead of AI/Na, A I / ( N a + H) becomes an appropriate measure of the number of the (AIO 4) - N a + pairs. For R = A I / ( N a + H) < I, the probability for the conduction through the (A104)- sites increases with increasing R (i.e., decreasing water content), and the activation energy for conduction decreases, and the conductivity increases. For R = A I / ( N a + H ) > 1, the conductivity reaches a maximum and nearly levels off or decreases with increasing R (i.e., decreasing water content). Fig. 6(a) and (b) show respectively the conductivity at 200°C and activation energy as a function of A I / ( N a + H), where the amount of H was estimated from the OH content. The variation of the conductivity and the activation energy with increasing R follows expected trend, although the A1/(Na + H) value at which the conductivity is a maximum and the activation energy is minimum is not unity, but instead around 10 -3 . This discrepancy may be due to the fact that only a small fraction of the OH in glass provides protons which replace sodium ions adjacent to (AIO a) group. Another major difference in composition of the glasses investigated is chlorine content. Although chlorine is known to influences glass properties such as viscosity [23], the effect on conductivity appears to be small. The activation energy for conduction of

D.W. Shin, M. Tomozawa / Journal of Non- Co'stalline Solids 203 (1996) 262-267 a)

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the glasses appear to be responsible for the different behavior.

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Acknowledgements 10-~5 10-'6

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b)

Careful reading of the manuscript by Dr Steven Crichton in RPI is greatly appreciated,

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References

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"~

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//" ///[]

140


120

100

ON

// I

104

10-3

I

10-2

L

I

10-1

10o

101

A1 Na+H Fig. 6. (a) Electrical conductivity at 200°C and (b) activation energy of two silica glasses, Suprasil W2 and Infrasil, as a function of the ratio (AI/(Na + H)). Lines are drawn as guides for the eye.

Furukawa glass showed a similar variation to that of Infrasil glass when fictive temperature was varied (AQ = 25.034 + 0.0799T v) even though two glasses have different chlorine content, ~ 1000 ppm by weight for Furukawa versus ~ 240 ppm by weight for Suprasil W2. Thus it is unlikely that the different response of Suprasil W2 and Infrasil glass to the variation of fictive temperature and water content are due to the different chlorine content in glass.

5. Conclusion The electrical conductivity of Suprasil W2 (type IV) silica glass decreases with increasing fictive temperature. The effect of water content on electrical conductivity of silica glasses depends upon the impurity content. Difference in aluminum contents of

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