Electrical conductivity change with crystallization in lithium metasilicate glass

Solid State Ionics 37 (1989) 79-81 North-Holland, Amsterdam

ELECTRICAL CONDUCTIVITY C H A N G E WITH CRYSTALLIZATION IN L I T H I U M METASILICATE GLASS P. PERNICE, A. A R O N N E and A. MAROTTA Department of Materials and Production Engineering, Naples, Italy Received 14 November 1988; in revised form 12 May 1989; accepted for publication 15 June 1989

The effect of crystallization on the electrical conductivity of lithium metasilicate glass were studied. Conductivities were measured by a.c. complex impedance method. Lithium metasilicate glass devitrifies in two steps i.e. I) crystallization of a first phase, Li4SiO 4, with a composition different from the that of the initial glass and 2) at a higher temperature transformation of this phase into a second phase, Li2SiO 3. After each crystallization step a decrease of the electrical conductivity and an increase of the conduction activation energy was observed.

i. INTRODUCTION Increasing interest, (1,2) exists in the study of glassy solid electrolytes. They find application in electronic devices, batteries and electrochemical sensors. Glasses have many advantages over crystalline electrolytes as physical isotropy, the absence of grain boundary, contlnuosly variable composition and good workability. Much attention has been devoted to lithium ion conducting solid electrolytes for their possible application to batteries and for elucidating the ionic conductivity mechanism in solids. The aim of the present work was to evaluate the effect of heat treatements of crystallization on the electrical conductivity and on the electrical conduction activation energy in lithium metasilicate, Li2SiO3, glass. To select the temperatures of heat treatments the devitrification behaviour of the lithium metasilicate glass was studied by differential thermal analysis (DTA) and X-ray diffraction

(X'~).

2. EXPERIMENTAL The lithium metasilicate, Li SiO 3, glass was prepared by melting small quantities (I0 grams)

of analytical grade reagents in a Pt crucible in an electric oven. The melts were cast at high cooling rate between two brass plates. The as quenched glass was cut in order to obtain small bulk samples (suitable for the size of the sample holder of the DTA apparatus). Differential thermal analysis curves of 50 mg specimens in air were recorded. Powdered AI203 was added to improve the heat transfer between bulk samples and the sample holder. A Netzsch thermoanalyzer (Model 404M) was used for the analyses, with powdered AI203 as reference material. Phases crystallizing after the thermal treatment were identify by X-ray diffraction. A Guinier-de Wolff camera and CuKa radiation were used. The electrical conductivity measurements were carried out in the temperature range 25 to 250°C, keeping the samples in dry condition. The electrical conductivities were determined by means of a Solartron 1250 frequency response analyser (FRA) and a Solartron 1286 Electrochemical Interface, both controlled by an Hewlett-Packard 86-B desktop computer. Gold electrodes were made onto the specimen faces by vacuum sputtering. The signal applied across the sample was 30mV. The complex impedance measured in the frequency range 60 to 0.5 kHz allowed us to obtain the sample bulk d.c. conductivity, @ , by means of the usual impedence analysis (3,4).

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P. Pernice et al. / Lithium metasilicate glass crystallizing during the DTA run were identified by X-ray diffraction. The XRD pattern of a sample heated in the DTA furnace until the temperature of the first exo-peak shows few reflections that can be attributed to Li SiO 4 crystals. The founded reflections 4 correspond to the strongest lines of Li4SiO 4 X-ray pattern as reported by the ASTM card 34-1416. These reflections disappear completely on the XRD pattern of a sample heated until the temperature of the second exo-peak. The reflections of this pattern can be all assigned to Li2SiO crystals. These results suggest that 3 lithium metasillcate glass devitrify in two steps

I-

I 400

I 600

p T(°C)

Fig.l DTA curve. Heating rate: 10°C/min. Bulk sample (Li2SiO 3 glass).

3 RESULTS AND DISCUSSION

GLASS ~ L i 4 S i O

The DTA curve of the studied glass, fig. l, exhibit a slope change at 427°C that can be attributed to the glass transition. A first exothermic crystallization peak appears just above the glass trasition. A second exotermic crystallization peak appears about 100°C above the temperature of the first peak. The phases

The proposed crystallization mechanism is consistent with the glass structure. M.Tatsumlsago et al. (5) have found in lithium metasilicate glass SiO 4 tetrahedral units with different number of non-brldgin8 oxygen atoms per a silicon atom. In thls4glass there is an appreciable amount of SiO 4 ions. When the

4 + GLASS RESIDUE ~ L i 2 S i O

3

-2

-3

-4

-~'-5

Eu w

u

-7

-8 I 2~

Fig.2 Arrhenlus plots of electrical il) heated 550°C sample.

I 2.5

I 3,0

1000 T

(K-l)

conductivity data. g) quenched sample; i) heated 450°C sample;

P. Pernice et al. / Lithium metasilicate glass lithium metasilicate

81ass is heated in the DTA apparatus first the more simple Li SiO 4 4 crystals are formed. Then at higher temperature where the decrease of the glass viscosity allows a more complicated rearrangement of the network the Li4SiO 4 crystals are converted in Li2SiO crystals whose composition is the same 3 of the mother glass. Electrical conductivity data have been determined for:a) an as quenched glass sample, b) a glass sample heated for lh at 4 5 0 ~ (first DTA peak temperature) and c) a glass sample heated for lh at 5 5 0 ~ (second DTA peak temperature). Ionic conductivity of samples were calculated from the data taken in the frequency domain of widely adopted complex impedance plots. The intercept of low frequency arc with z'axis was assumed as bulk resistance of the sample, d.c. conductivity data coll~cted by this method for all the samples over a wide temperature range were analysed by an Arrhenius equation of the form O = ao exp (- E/RT) where ao and E represent the pre-exponential factor and activation energy respectively. R and T take the usual meaning. Linear plots of log versus 1000/T plotted for all compositions are shown in fig.2 in which solid lines represent the least square fit for data points. These lines indicate that experimental data agree well with the Arrhenius equation although the conductivity of the glass sample changes by nearly four order of magnitude in the studied temperature range. The structure of alkali silicate glasses has large holes which are available sites for ionic conduction. These holes are surrounded by non-bridging +oxygen atoms which act as trapping sites for Li ions. The ionic conductivity in lithium metasilicate

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+ glass can be attributed to Li ions which are dissociated by non-bridging oxygen atoms. The decrease of the ionic conductivity of three order of magnitude, as shown in fig.2, after each crystallization step suggests that the growth of Li4SiO 4 crystals and their transformation in Li SiO crystals involves @ 2 3 progressive decrease of the number of free Li ions. Moreover the activation energy rises from 0.52 eV in the as quenched glass to 0.71 eV after the first crystallization step and to 0.97 eV after the second crystallization step. + This suggests that mobile Li ions encounter increasingly difficult energy barrier for their migration when the glass structure is rearranged to allow the crystals growth.

4.CONCLUSIONS Lithium metasilicate glass devitrifies in two steps. First Li4SiO 4 crystals are formed that are then converted at a higher temperature into Li^SiO3z crystals. The electrical conductivity decreases and the electrical conduction activation energy increases after each crystallization step.

REFERENCES I. H.L.Tuller and M.W.Barsoum, J.Non-Cryst. Solids 73 (1985) 331 2. D.Ravaine,ibid. 38-39 (1980) 353 3. D.Ravaine,ibid. 49 (1982) 507 4. J.Maier, Z.Phys. Chem.Neue Folge 140 (1984) 191 5. M.Tatsumisago,T.Minami,N.Umesaki and N.lawamoto,Chem.Lett.8 (1986) 1371