Modification of the Glass Structure

MODIFICATION OF THE GLASS STRUCTURE
PRODUCED BY THERMAL PREHISTORY

N.A. Bokov1, V.L. Stolyarova2

1 Institute of Silicate Chemistry of the RussianAcademy of Sciences, Sankt-Petersburg, Russia

2 Sankt-PetersburgStateUniversity, Sankt-Petersburg, Russia

Abstract. It was shown the possibility of the modification the structure by the thermal prehistory of the oxide glass. The experimental results of the changes of the visible light scattered intensity after temperature jumps in the glass transition region of oxide glass were discussed. The influence of the thermal prehistory of the sample on the scattered intensity was studied. The effect of the voltage and the mechanical load on the process of the modification of the glass structure was observed. The universal character of the nonlinear coupling of the laser irradiation with the glass structure was found. The glass samples with modification structure have been prepared by the quenching technique.

Introduction

Recently, it was found that the temperature dependence intensity of the scattering of visible light by oxide glasses showed a peak in the glass transition region [1-3]. As it was shown that the height and location of the peak observed were very strongly depended on the thermal prehistory of the glass and the sample size. Taking into account these features it is assumed that the appearance of the peak is associated with the arising of the non-equilibrium fluctuations produced by coupling between the hydrodynamic modes [4, 5].

The striking feature of this phenomenon consists in that the development of the scattered intensity peak is attended by considerable change in the halo of the primary light beam passed through the sample. The detail investigation of this phenomenon had shown that the development of the peak was accompanied by appearance of the diffraction pattern in vicinity of the light beam passing through the sample [6, 7]. At present time the reasons of the appearance of this effect are not clear. Nevertheless it is clear that this phenomenon connected with the effect of nonlinear interaction of the laser irradiation with the structure of glass. The most important feature of this phenomenon is the application of the small power laser irradiation. It can be assumed that the glass structure is the highly unstable state at the moment corresponding to the peak of the scattered intensity.

The additional arguments in favor of this assumption were obtained by the study of the influence of voltage and the mechanical load on the behavior of the scattered intensity.

Experimental procedure

The scattered light intensity was measured at the angle 900 for the wavelength of incident laser radiation λ = 4880 Å. The laser beam was focused on the sample by a long-focus lens. The power density of radiation at the focus was approximately equal to 500 mW/mm2.

The primary light beam passed through the center of the sample along the largest edge. The magnitude obtained experimentally was the absolute value of polarized Vv component of scattered light intensity, where the index denotes the polarization status of the incident beam and capital letter stands for the orientation of the polarizer before the detector. For the observation the speckle, the light beam passed through the sample was projected on a screen and was recorded with the digital camera.

The usual experimental procedure for the study of the behavior of material characteristics depending on thermal prehistory of a glass in the glass transition region is the method of temperature jump, when the temperature of a sample is rapid changed and this is followed by isothermal relaxation. Namely this technique was used for the study of the scattered intensity and observation of the speckle [2-7]. According to this scheme, the sample under study was preliminary stabilized at the temperature Tst for the time tst and then the temperature was abruptly increased to Tobs at which the required parameters were measured as a function of the time t.

The objects under investigation were the phosphate glass contained 9.4Na2O*57.7ZnO *32.9P2O5 (mol%) and silicate glass STK-3 commercial glass containing silica, boron, and barium oxide. Aluminium, zinc, and lanthanum oxides were also the additives in STK-3. The phosphate glass was synthesized under laboratory conditions. Silicate glass was prepared industrially. The glass transition temperature Tg of phosphate glass and silicate glass were equal to 370 and 6400, respectively.

The glass samples were polished in the form of rectangular prism. The sample sizes of phosphate glass and silicate glass were 12.214.511.4 and 121714 mm, respectively.

Results and discussion

The data for the phosphate glass were obtained in two experiments carried out under the following conditions: (1) Tst = 3710, tst = 72 h, and Tobs = 4000 and (2) Tst = 3560, tst = 120 h, and Tobs = 4020.

The results of the first experiment are presented on Fig. 1. As seen from the upper of Fig. 1, after the stabilization of the sample at the temperature 3710 and the subsequent temperature jump ΔT = 390 the component Vv reaches a maximum value of 1510-6 cm-1 within 7 min and then recovers its initial value of 510-6 cm-1 within 11 min. A series of photographs that demonstrate the behavior of the light passed through the sample are shown in the lover part of Fig. 1. The laser beam spreading and the formation of set of diffraction ringers are clearly seen in fig. 1b and 1c. A comparison of the upper and lower parts of Fig. 1 shows that both effects are most pronounced when the Vv component reaches a maximum (i.e., at  7 min).

The second experiment was performed to elucidate the possibility of using the quenched technique for fixing the specific glass structure responsible for the above diffraction effects. For this purpose, the experimental conditions were changed in such way as to retard the processes proceeding after the temperature jump. As a result, we succeeded in increasing the lifetime of the Vv maximum by a factor of approximately 2.

Over period of 12 min, the experiment was performed according to previous scheme. Three typical images (photographs) of the light beam on the screen for this time interval are displayed in Fig. 2a, 2b, and 2c. It is evident that they are qualitatively similar to the photographs shown in Fig. 1. However, in the second experiment, within 12 min, which corresponds to the maximum value of Vv component, the power supply of the heater was turned off. This resulted in cooling of the sample to the room temperature at a mean rate of 50/min. As can be seen from Fig. 2d, the passage of the laser beam through the sample quenched in such a manner leads to typical diffraction pattern.

The examination of the quenched sample at room temperature revealed that this sample possesses an increased Vv component as compared to that of initial sample. At the same time, diffraction pattern displayed in Fig. 2b is observed only when the laser beam passed through the bulk of the sample subjected to irradiation at high temperature. It can be assumed that one of the possible reasons for the formation of a specific region in the sample is a nonlinear interaction of laser radiation with the glass structure formed at the Tobs.

The similar effect takes place for the silicate glass STK-3. The experiment carried out under the following conditions: Tst = 6320, tst = 119 h, and Tobs = 6980.

Fig. 3 shows the time dependence of the intensity of Vv component of the scattered light at an observation temperature 6980. The photographs of the laser beam passing through the sample are also displayed in Fig. 3. It can be seen from Fig. 3c and 3d (corresponding to the fifth and sixth minutes of the observation time) that an increasing of the scattered intensity leads to an increase of the diameter of sport and the formation of diffraction fringe. However, for silicate glass, the diffraction pattern turned out to be less pronounced as compared to the phosphate glass.

As in the case of the phosphate glass, the sample characterized by an increased scattered intensity, as compared to the initial sample, was prepared by the quenched method. The investigations of the quenched sample at a room temperature revealed that the diffraction pattern is observed only in the case when the laser beam passes through the sample volume subjected to irradiation at the observation temperature Tobs.

It is necessary to mention that the analogous phenomenon was observed for the sodium germinate glass, contained 12.5 mol% Na2O. The results obtained point out on the universal character of the effect observed for oxide glasses.

It seems likely that one of possible mechanisms responsible for the appearance of the diffraction pattern may be produced by the formation of the specific region in the sample volume, which characterized by the other the refractive index. In that case this phenomenon is similar the diffraction of the light on a small hole or screen. Since this effect related with the nonlinear interaction of the of the small power laser irradiation with the glass structure formed at the development of the maximum intensity, it can be assumed that this glass structure is the highly unstable state.

The results confirmed this assumption were obtained in the experiments connected with the study of the influence of electric voltage and mechanical load on the kinetics of the changes of scattered light intensity after a temperature jump in the glass transition region of oxide glasses.

The influence of electric voltage on the time dependence of the scattered light intensity were conducted with the phosphate glass contained 14.8Na2O*53.1ZnO *32.1P2O5 (mol%) synthesized under laboratory conditions. The glass transition temperature Tg of phosphate glass was equal to 3500. The glass sample was polished in the form of rectangular disk. The sample diameter was 26.3 mm and its height was 14 mm. The upper and lower sample surfaces were covered with the thin nickel foil, which were used to lead the electrical voltage. The experiment carried out under the following conditions: Tst = 3730, tst = 23 h, and Tobs = 4130.

The time dependences of the Vv component of scattered light measured for the different voltage values were presented on the Fig. 4. As it follows from Fig. 4 that the insignificant increase of the voltage applied to the sample to 200 – 300 v produce the increase of the maximum height and the displacement of its position on time scale. At the further expansion of the voltage to 700 v the maximum height raises approximately on the 20 % at that maximum position shifts from 14 to 8 min.

The influence of mechanical load on the time dependence of scattered light intensity were conducted with the phosphate glass contained 9.4Na2O*57.7ZnO *32.9P2O5 (mol%). The glass sample was polished in the form of rectangular prism. The sample sizes were 121531 mm. The laser beam passed through the sample into the sample side 1231 mm approximately in the middle of the sample height, i.e. 31/2 = 15.5 mm. The scattered light was registered at angle 900 from the sample side 1531 mm. The sample was subjected to the stress by means of the loads, which realized by the using the different weights. The weights were placed on the upper rod end. The underside end of the rod set on the quartz plate, which located on the upper side of the sample defined by the following edges 1215 mm. The weight mass were 3 and 5 kg. The experiment carried out under the following conditions: Tst = 3530, tst = 47 h, and Tobs = 4000. The weights were placed on the upper rod end at the moment, when the temperature ran up to T obs = 4000 corresponding at the beginning of the measurement of the scattered light intensity.

Figure 5 demonstrates the results obtained for the three conditions: (1) the rod without the weight, (2) the rod with the weight 3 kg, and (3) the rod with the weight 5 kg. As it follows from Fig. 5 that the increase of the load applied to the sample produce the decrease of the maximum height and the displacement of its position on time scale to the larger time.

Taking into account of the insignificant values of the enclosed impacts the results obtained confirm the assumption that the glass structure is the highly unstable state at the moment, which corresponded to the peak of the scattered intensity. It should be mentioned that in general the obtained results agree qualitatively with the analysis of the influence the pressure, tangential stress, and electric fields on the kinetic of the glass transition process [8, 9].

The analysis of the present results shows that the development of the diffraction pattern was greater than the scattered intensity maximum was higher. The obtained results shown that the maximum height was determined by the thermal prehistory of the glass, which depended on the stabilization Tst and observation Tobs temperatures, and the time stabilization tst. The present results indicate that on the maximum height expose the using not only the thermal influence. Namely, as it was shown above, the increase of the voltage applied to the sample produce the increase of the maximum height. This circumstance may be of use for the intensification of the process of nonlinear interaction of the light radiation with the glass structure.

Conclusion

The obtained results show that the universal phenomenon of the evolution of scattered intensity in the glass transition of oxide glasses is attended by the formation of diffraction pattern. The observed effect is associated with the nonlinear interaction of laser radiation with the glass structure. The kind of this phenomenon consists in the application of the small power laser irradiation, which has property to modify the refractive index in the glass volume. The observation carried out in the present work revealed that the glass structure is the highly unstable state when the scattered intensity amounted to the maximum values. It was shown that the process of nonlinear interaction of laser radiation with the glass structure may be amplifies by the electric voltage applied to the sample under investigation. For the practical purposes, it may be useful to modify the refractive index in the glass volume.

Acknowledgement

This study was carried out based on the financial support by the Russian Foundation for Basic Research according to the project N 04-03-32886.

REFERENCES

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