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(Fig. 9), its activation energy is higher and close to that of the sodium-enriched crystals (Fig. 10). This tendency becomes apparent at a 0.30. Thus, the comparison between the values of activation energy of partially crystallized samples, glasses, and crystals corroborate the above assumption that the electrical conductivity of partially crystallized glasses, beginning with a 0.54, is dominated by the crystals. This conclusion is reinforced by a weak inflection in the Arrhenius plot of a sample with a 0.54 (Fig. 6), which is attributed to the reversible polymorphic transition in the crystalline phase. The decrease in activation energy for conduction with the increasing of alkaline crystal lo g (

1000/T, K-1


cryst. glass α = 0.045

Fig. 6. Arrhenius plot of electrical conductivity of glass, partially crystallized glass, and poly-crystal.

Fig. 7. SEM of the glass cross-sections treated at T = 690 C for 17 (a) and 50 (b) min. Fractions of the crystallized volume are about 0.1 (a) and 0.54 (b). The bar has a length of 50 lm. The photo’s height is equal to the thickness of the plates used for electrical conductivity measurement.

log (


NaO 9.82 mol%

Fig. 8. Arrhenius plot of electrical conductivity of glasses with different sodium oxide content.

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content is an expected behavior for a glass system. Similar effect in the sodium calcium silicate glasses was already analyzed by Anderson and Stuart [8].

The measured and calculated compositions shown in

Fig. 4 are averages. The real situation is more complicated due to the diffusion zones that exist around the growing crystals. These zones become visible with a second heat treatment at a temperature Tn g, where the nucleation and growth rates are both important, since the nucleation

and growth rates decrease with decreasing sodium oxide in the melt [1,9]. Indeed, as demonstrated by Fig. 1,i n the areas close to the large crystals previously formed at T = 690 C, and exhausted in sodium, no new crystals are formed during a long heat treatment at Tn g = 590 C, which is close to the Tmax (see Fig. 2). Thus, Fig. 1 presents a ‘map’ of sodium distribution in the glassy matrix of a sample subjected to a heat treatment at T = 690 C. The areas that are distant from the large crystals form tracks enriched by sodium, which are responsible for the glassy matrix contribution to the overall conductivity of the sample. It should be noted here that crystallized volume fraction resulting from the heat treatment at T = 690 Ci s only about 0.15. At temperatures lower than the glass transition temperature (when the crystallization process becomes too slow), sodium diffusion still occurs and the gradient of sodium concentration (see Fig. 1) must diminish resulting in a reduction of the sodium content in the above mentioned tracks, which will be gradually dissolved, thus increasing the electrical resistance of sample. It seems that this effect is the origin of the slight, but well pro-

Eσ , kJ/mol

Na, at %

Fig. 10. Activation energy of electrical conductivity in glasses (1), partially crystallized samples (2) and crystals (3) versus sodium content. In the case of the partially crystallized glasses, sodium content corresponds to glassy matrix. The numbers close to points denote crystallized volume fractions.

Fig. 1. Optical micrograph of glass sample subjected to the following fully crystallized glass log (

Fig. 9. Electrical conductivity at T = 394 C of glasses (1), fully crystallized glasses (3) versus sodium content, and partly crystallized glass of composition close to N1C2S3 (2) versus sodium content in the glassy matrix. The numbers close to the opened stars denote crystallized volume fraction.

-5.4 1 2 log ( t, min

2242 A.C.M. Rodrigues et al. / Journal of Non-Crystalline Solids 353 (2007) 2237–2243

nounced decrease in the electrical conductivity of the partially crystallized sample observed during a long heat treatment at T = 400 C, as compared with the behavior of the pure glass (see Fig. 12).

5. Conclusions

Electrical conductivity measurements of partially crystallized samples corroborated EDS evidence for variations in the composition of both glassy phase and crystals during crystallization of a glass with composition close to the stoi- chiometric 1Na2O Æ 2CaO Æ 3SiO2. In this particular case, electrical conductivity was dominated by the glassy phase up to a 0.30, and by the crystals from a 0.54. These findings were confirmed by an analysis of the electrical conductivity of glasses and crystals having the same sodium contents as the partially crystallized samples, and were further substantiated by an analysis of activation energy. For samples with higher crystallinities, Arrhenius plots showed a kink due to a crystal phase transition, confirming that the crystals dominate the electrical conductivity.

Thus, electrical conductivity is a very sensitive property that can indicate changes in the glassy matrix and crystals compositions, and in the spatial distribution of chemical elements. Hence, electrical conductivity measurements could contribute to investigations of complex heterogeneous systems, such as partially crystallized glasses.


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