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Elastic properties of sodium phosphate glasses at hypersonic frequencies by brillouin scattering
Journal of Non-Crystalline Solids 30 (1979) 385-388 © North-Holland Publishing Company
ELASTIC PROPERTIES OF SODIUM PHOSPHATE GLASSES AT HYPERSONIC FREQUENCIES BY BRILLOUIN SCATTERING
J. PELOUS, R. VACHER Laboratoire de Spectrom~trie Rayleigh Brillouin, ERA 460, Universitd des Sciences et Techniques du Languedoc, Place E. Bataillon, 34060 Montpellier Cedex, France
and J. PHALIPPOU Laboratoire des Verres du CNRS, U.S. T.L. , Montpellier, France
Received 3 November 1978
Brillouin scattering is a tool for the study of the elastic and viscoelastic properties (EVEP) of solids in the hypersonic frequency range [1]. We report here measurements of EVEP at about 28 GHz for temperatures ranging from 4 to 300 K. The chemical composition of the glasses investigated is given in table 1. These glasses have been prepared from Na2H2PO4, 2 H20 and Na2HPO4, 12 H20 (analytic grade). Products are dehydrated in an oven, and melting at 1000°C is obtained in a platinum crucible. The glasses are cast in a steel mould and annealed for four hours at 260°C. The temperature of the furnace is then reduced to room temperature in twelve hours. Finally, samples of 10 × 5 × 5 mm 3 are optically polished. Some impurities (mainly platinum atoms introduced during the preparation) make the samples slightly absorbent for visible light. In such conditions, temperatures lower than 4 K cannot be obtained with our 4He cryostat. When the Na20 molar content of the glass is more than 50%, paper chromatography studies indicate the constitution of the glass. The structural elements are either cyclic or variable length linear polyatomic chains [2]. When the molar conTable 1 Chemical composition of the glasses investigated No.
Composition (in moles)
1 2 3
Na20 56 54 50
P20 s 44 46 50
385
386
J. Pelous et aL / Elastic properties of sodium phosphate glasses
tent of the two components, Na20 and P2Os, tends to be equal, the length of the chains can be expected to be infinite. Thus, even if the structural unit (PO4 tetrahedron) is similar to that of silicate glasses, phosphate glasses present some analogies with organic glasses. As their structure reflects both that of polymers and that of tetrahedral coordinated glasses, it seems interesting to observe the possible effects of this structural particularity on the EVEP of these glasses. The Brillouin spectrometer, which has been described previously [3], is a tandem arrangement using a double-pass plane Fabry-P6rot as a monochromator and a confocal Fabry-P6rot as a resolving unit. The thickness was 2 mm and 50 mm for the plane and spherical interferometers respectively, leading to a resolving power of about 107 and a contrast higher than 108. Due to the optical imperfections of the sample, the absolute values of the frequency shift (i.e. the frequency of the hypersonic wave responsible for the scattering) and of the linewidth of the
29 ÷ glass 3 • glass 2 I E r r o r limit
\.
a glass 1
~28
W
\.+ d"
27
lbO
2bo
TEMPERATURE ( K )
3bo
Fig. l. Variation with temperature of the Brillouin frequency shift in phosphate glasses.
J. Pelous et al. / Elastic properties of sodium phosphate glasses
387
+ 91ass 3 • 91ass 2 91Qssl
150
i10[
Error
limit
//~. .i. °
) laJ Z ...1
5C
2bo
3bo
TEMPERATURE ( K )
Fig. 2. Variation with temperature of the half-width at half-maximum of the Bnllouin lines in phosphate glasses.
Brillouin components were obtained with a relative accuracy of about 1% and 10% respectively. As the temperature variation of the refractive index is unknown for these sodium phosphate glasses the velocity and attenuation of hypersonic waves cannot be deduced from our measurements. However, this variation is known to be less than 1% in the range 0 to 300 K for usual glasses, which is small compared with the strong temperature dependence of the frequency shift and linewidth. Therefore, the velocity and attenuation of hypersounds can be assumed proportional to the frequency shift and linewidth of Brillouin lines, respectively. The results are given in figs. 1 and 2. For the three samples, a remarkable linearity is observed for the variation of the frequency shift (i.e. the sound velocity) in a very large temperature range (from 20 to 300 K). Assuming that the refractive index is 1.5, the three curves give A u / A T = - 1 . 6 ms -1 K -1. This slope has a negative value similar to that obtained for the amorphous polymer polymethyl methacrylate (PMMA) [4]. The sound absorption decreases continuously as the temperature is lowered. This behaviour is also very similar to that of PMMA. The absorption peak observed in
388
J. Pelous et al. / Elastic properties o f sodium phosphate glasses
several oxide glasses [5], does not appear on the curves on fig. 2. This result, together with the linear variation of sound velocity, shows that there is no dominant relaxational process in these glasses, at least for hypersonic frequencies - i.e. with relaxation times in the range from 10 -11 to 10 -9 s. It should be noted that the sound velocity is larger for the sample no. 2 than for the other two glasses. Such an anomalous behaviour has already been observed in other physical properties for sodium phosphate glasses with a molar content of about 53% Na20 [6]. A similar effect has also been noted for the EVEP of other phosphate glasses [7]. A first conclusion can be drawn from the above result. It is known [2] that increasing the Na20 content of sodium phosphate glasses leads firstly to an increase of the rigidity of the network and then to a decrease for larger Na20 contents. Our measurements agree with this observation. Furthermore they show that a sharp rigidity maximum is reached when the chemical composition is that of sample no. 2. On the other hand, the temperature dependence of sound velocity and absorption shows that, even if these glasses have a tetrahedral structural unit, the EVEP are not dominated by the coupling with relaxational motions. This may be taken as one more indication that these relaxational motions in glasses are not related to vibrations of individual atoms or tetrahedra.
References [1] R. Vacher and L. Boyer, Phys. Rev. B6 (1972) 639. [2] A.E.R. Westman, Modern aspects of the vitreous state, ed. J.D. Mackenzie (Butterworths, London, 1960). [3] R. Vacher and J. Pelous, Phys. Rev. B14 (1976) 823. [4] R. Vacher and J. Pelous, Phys. Rev. 58A (1976) 139. [5] R.E. Strakna and H.T. Savage, J. Appl. Phys. 35 (1964) 557. [6] M. Fanderlik and M. Paleeek, Silikat J. 5 (1966) 127. [7] C.A. Elyard,P.L. Baynton and H. Rawson, Glastechn. Ber. 32K, V (1959) 36.
ELASTIC PROPERTIES OF SODIUM PHOSPHATE GLASSES AT HYPERSONIC FREQUENCIES BY BRILLOUIN SCATTERING
J. PELOUS, R. VACHER Laboratoire de Spectrom~trie Rayleigh Brillouin, ERA 460, Universitd des Sciences et Techniques du Languedoc, Place E. Bataillon, 34060 Montpellier Cedex, France
and J. PHALIPPOU Laboratoire des Verres du CNRS, U.S. T.L. , Montpellier, France
Received 3 November 1978
Brillouin scattering is a tool for the study of the elastic and viscoelastic properties (EVEP) of solids in the hypersonic frequency range [1]. We report here measurements of EVEP at about 28 GHz for temperatures ranging from 4 to 300 K. The chemical composition of the glasses investigated is given in table 1. These glasses have been prepared from Na2H2PO4, 2 H20 and Na2HPO4, 12 H20 (analytic grade). Products are dehydrated in an oven, and melting at 1000°C is obtained in a platinum crucible. The glasses are cast in a steel mould and annealed for four hours at 260°C. The temperature of the furnace is then reduced to room temperature in twelve hours. Finally, samples of 10 × 5 × 5 mm 3 are optically polished. Some impurities (mainly platinum atoms introduced during the preparation) make the samples slightly absorbent for visible light. In such conditions, temperatures lower than 4 K cannot be obtained with our 4He cryostat. When the Na20 molar content of the glass is more than 50%, paper chromatography studies indicate the constitution of the glass. The structural elements are either cyclic or variable length linear polyatomic chains [2]. When the molar conTable 1 Chemical composition of the glasses investigated No.
Composition (in moles)
1 2 3
Na20 56 54 50
P20 s 44 46 50
385
386
J. Pelous et aL / Elastic properties of sodium phosphate glasses
tent of the two components, Na20 and P2Os, tends to be equal, the length of the chains can be expected to be infinite. Thus, even if the structural unit (PO4 tetrahedron) is similar to that of silicate glasses, phosphate glasses present some analogies with organic glasses. As their structure reflects both that of polymers and that of tetrahedral coordinated glasses, it seems interesting to observe the possible effects of this structural particularity on the EVEP of these glasses. The Brillouin spectrometer, which has been described previously [3], is a tandem arrangement using a double-pass plane Fabry-P6rot as a monochromator and a confocal Fabry-P6rot as a resolving unit. The thickness was 2 mm and 50 mm for the plane and spherical interferometers respectively, leading to a resolving power of about 107 and a contrast higher than 108. Due to the optical imperfections of the sample, the absolute values of the frequency shift (i.e. the frequency of the hypersonic wave responsible for the scattering) and of the linewidth of the
29 ÷ glass 3 • glass 2 I E r r o r limit
\.
a glass 1
~28
W
\.+ d"
27
lbO
2bo
TEMPERATURE ( K )
3bo
Fig. l. Variation with temperature of the Brillouin frequency shift in phosphate glasses.
J. Pelous et al. / Elastic properties of sodium phosphate glasses
387
+ 91ass 3 • 91ass 2 91Qssl
150
i10[
Error
limit
//~. .i. °
) laJ Z ...1
5C
2bo
3bo
TEMPERATURE ( K )
Fig. 2. Variation with temperature of the half-width at half-maximum of the Bnllouin lines in phosphate glasses.
Brillouin components were obtained with a relative accuracy of about 1% and 10% respectively. As the temperature variation of the refractive index is unknown for these sodium phosphate glasses the velocity and attenuation of hypersonic waves cannot be deduced from our measurements. However, this variation is known to be less than 1% in the range 0 to 300 K for usual glasses, which is small compared with the strong temperature dependence of the frequency shift and linewidth. Therefore, the velocity and attenuation of hypersounds can be assumed proportional to the frequency shift and linewidth of Brillouin lines, respectively. The results are given in figs. 1 and 2. For the three samples, a remarkable linearity is observed for the variation of the frequency shift (i.e. the sound velocity) in a very large temperature range (from 20 to 300 K). Assuming that the refractive index is 1.5, the three curves give A u / A T = - 1 . 6 ms -1 K -1. This slope has a negative value similar to that obtained for the amorphous polymer polymethyl methacrylate (PMMA) [4]. The sound absorption decreases continuously as the temperature is lowered. This behaviour is also very similar to that of PMMA. The absorption peak observed in
388
J. Pelous et al. / Elastic properties o f sodium phosphate glasses
several oxide glasses [5], does not appear on the curves on fig. 2. This result, together with the linear variation of sound velocity, shows that there is no dominant relaxational process in these glasses, at least for hypersonic frequencies - i.e. with relaxation times in the range from 10 -11 to 10 -9 s. It should be noted that the sound velocity is larger for the sample no. 2 than for the other two glasses. Such an anomalous behaviour has already been observed in other physical properties for sodium phosphate glasses with a molar content of about 53% Na20 [6]. A similar effect has also been noted for the EVEP of other phosphate glasses [7]. A first conclusion can be drawn from the above result. It is known [2] that increasing the Na20 content of sodium phosphate glasses leads firstly to an increase of the rigidity of the network and then to a decrease for larger Na20 contents. Our measurements agree with this observation. Furthermore they show that a sharp rigidity maximum is reached when the chemical composition is that of sample no. 2. On the other hand, the temperature dependence of sound velocity and absorption shows that, even if these glasses have a tetrahedral structural unit, the EVEP are not dominated by the coupling with relaxational motions. This may be taken as one more indication that these relaxational motions in glasses are not related to vibrations of individual atoms or tetrahedra.
References [1] R. Vacher and L. Boyer, Phys. Rev. B6 (1972) 639. [2] A.E.R. Westman, Modern aspects of the vitreous state, ed. J.D. Mackenzie (Butterworths, London, 1960). [3] R. Vacher and J. Pelous, Phys. Rev. B14 (1976) 823. [4] R. Vacher and J. Pelous, Phys. Rev. 58A (1976) 139. [5] R.E. Strakna and H.T. Savage, J. Appl. Phys. 35 (1964) 557. [6] M. Fanderlik and M. Paleeek, Silikat J. 5 (1966) 127. [7] C.A. Elyard,P.L. Baynton and H. Rawson, Glastechn. Ber. 32K, V (1959) 36.