Journal of Applied Sciences

Year: 2008 | Volume: 8 | Issue: 10 | Page No.: 1956-1961
DOI: 10.3923/jas.2008.1956.1961

Effect of Zinc on the Physical Properties of Tellurite Glass

S. Rosmawati, H.A.A. Sidek, A.T. Zainal and H. Mohd Zobir

Abstract: A series of binary tellurite glass samples containing zinc oxide, ranging from 10 to 40 mol%, were successfully prepared. Longitudinal and shear ultrasonic wave velocities were measured at room temperature and were taken at 5 MHz. Elastic properties, Poison`s ratio, microhardness, Debye temperature and fractal bond connectivity have been calculated from the measured density and ultrasonic wave velocities. Results from the studies show that the densities of all glasses increase from 5098 to 5283 kg m^-3, molar volumes decrease from 29.773 to 24.287 m³ mol^-1 and the transition temperature, T_g decreases from 381.17 to 360.52°C with the substitution of TeO₂ by ZnO content. Both the longitudinal and shear wave velocities decrease with increase of ZnO composition. The experimental results indicated that the elastic constants depend upon the composition of the glasses. The role of the ZnO inside the glass network is also discussed. Quantitative analysis based on bond compression model has been carried out, in order to obtain more information about the structure of these glasses.

Keywords: Tellurite glasses, bond compression model, cross-link density, elastic properties and ultrasonic velocity

INTRODUCTION

Vitreous tellurite with tellurium dioxide as the main glass-former, which under normal condition, does not have the ability to transform into the glassy state without a modifier (Bahgat et al., 1987). It has superior physical properties such as low melting temperature, high refractive index, high dielectric constant and good infrared transmission (Rajendran, 2000). The distinguish factor about the matrix of this glass is that the tellurium atoms have unshared pairs of electrons which do not take part in bonding. Their relatively low temperature of crystallization and melting makes these types of glass an active candidate for CD memory devices (Sabry and El-Samanoudy, 1995).

Elastic properties are important parameters for understanding the structural characteristics of glass network. The measurement of ultrasonic parameters such as velocity and attenuation as a function of composition, temperature and frequency is of great interest in glass. The variation of ultrasonic parameters, besides density and molar volumes, are closely related to the changes occurred in the structure of glass network (Mallawany et al., 1994; Sidkey et al., 1997). Such properties are associated with the inter-atomic forces and potentials in the lattice structure (Saddeek, 2004). In general, strength of amorphous materials increases with their elastic moduli; therefore, it is possible to asses the strength indirectly from their elastic properties (Saddeek, 2005).

The objectives of this research are to study the elastic moduli of zinc tellurite glasses and to investigate the structural changes and their physical properties of the glasses.

MATERIALS AND METHODS

This study was started at 2004 until now. It was conducted in Ultrasonic Research Laboratory at Universiti Putra Malaysia, Malaysia. The binary tellurite glasses, (TeO₂)_1-x (ZnO)_x were prepared by mixing together specific weights of tellurium dioxide (technical grade) and zinc oxide (purity 99%) in alumina crucible. The x percentage was ranging from 10 to 40 mol %. The samples were prepared by melt quenching method. The 20 g batches were then preheated in a first furnace to 400°C for 30 min to reduce tendency of volatilization. The crucible was then transferred to a second furnace at 750-800°C for 1 h, depending on the percentage of ZnO.

The melt was then removed from the second furnace before it was poured in a stainless steel cylindrical shaped split mould which had been preheated to 400°C followed by annealing in the first furnace at 350°C for 1 h, after which the furnace was switched off and the glass was allowed to cool in situ for 24 h. This was done to remove residual stresses before cutting procedures. The prepared samples were then cut into required dimension for density and ultrasonic measurement and a fine glass powder was used for transition temperature measurement. Detailed information on glass preparation can be found elsewhere (Sidek et al., 2004).

The glass transition temperatures (Tg) were determined by differential thermal analyzer SETARAM (DTA). The densities of the glasses were determined by the Archimedes method using acetone as immersion liquid. The X-ray diffraction pattern of the glass samples were examined using diffractometer (model X’Pert Pro Panalytical) by employing Cr-Kα radiation. For measurement of ultrasonic velocity in the glass samples, MATEC MBS 8000 was used. All measurements were taken at 5 MHz frequency and at room temperature.

RESULTS AND DISCUSSION

Density and molar volume: The X-ray diffraction patterns of the studied glass system reveal the absence of any discrete or continuous sharp crystalline peaks but show homogeneous glassy characters. The experimental values of the density and the molar volume of the studied glasses are shown in Table 1. The density of all glasses increase from 5098 to 5283 kg m^-3 with the substitution of TeO₂ by ZnO, while molar volume decrease from 29.773 to 24.287 m³ mol^-1. The similar behavior has been found by Mallawany (1993). However, the present values are somewhat lower than those found by Mallawany, perhaps due to different preparation technique.

Addition of ZnO to the tellurite network causes some type of structural rearrangement of the atoms as found by Hoppe et al. (2004). Therefore, there is a possibility for the modification of the glass microstructure upon substitution of TeO₂ by ZnO.

Apparently, while substituting TeO₂ by ZnO, the density should decrease because of the lower molecular weight of ZnO (M_zn = 65.37 g mol^-1) than that of TeO₂ (M_Tc = 127.6 g mol^-1). The increase of density indicates that the zinc ions reticule the network of the glass. This was confirmed by the results of the molar volume where it shows a decrease values.

As reported earlier by Hoppe et al. (2004), Lambson et al. (1984), Neov et al. (1979) and Khozukharov et al. (1986), the structure of the tellurite-rich glasses is a laminar network consisting of the structural units TeO₄ trigonal bipyramids (tbp) and TeO₃ trigonal pyramids (tp). There is 4-coordination of Te in the tetragonal form, the nearest-neighbours being arranged at four of the vertices of a trigonal bipyramid, suggesting considerable covalent character of the Te-O bonds.

An oxygen atom introduced into glasses with high TeO₂ contents opens a Te-O-Te bridge and changes two TeO₄ into TeO₃ units. Each of these TeO₃ units has two terminal Te-O bonds with partial double bond character (Hoppe et al., 2004). The increase in density is due to glass structural change of which influence of Zn²⁺ on breaking tellurium-oxygen networks. In this case, all oxygen atoms from ZnO are used for rupture of Te-O-Te bridges which accompanied with a change of nearly all participating TeO₄ to TeO₃ groups (Kozhukharov et al., 1986).

In general, the decrease in the molar volume indicates a decrease in the interatomic distances. Therefore, the compactness of the glass will increase and more bridging oxygen’ (BOs) will be created which increase the rigidity of the glass (Saddeek, 2005).

Ultrasonic velocity and elastic moduli
The determination of the experimental and theoretical derived parameters: The ultrasonic wave velocities measured in this work are found to be sensitive with the glass composition. The elastic properties of the binary zinc-tellurite glasses with different percentage of x, such as, the longitudinal modulus L, the shear modulus G, the bulk modulus K, the Young’ modulus E and the Poisson’ ratio σ will be calculated using the following equations:

The micro-hardness H and the Debye temperature θ_D were calculated using the following equations:

The composition dependence of longitudinal and shear wave velocities is shown in Fig. 1. Both the longitudinal and shear wave velocities (v_l and v_s, respectively) decrease gradually with the concentration of ZnO. The decrease in ultrasonic velocity of the studied glass reveal the fact that, adding ZnO content to the tellurite network causes a difficult movement from the ultrasonic wave inside the network of the glass structure and hence the tendency of the ultrasonic velocity to decrease as the mole percentage of ZnO increases.

It can be seen that the values of v_l are higher than v_s. For most materials, ultrasonic wave velocities will increase as the density increases. The similar behavior has been found by Mallawany (1993) for the same glass systems. But in this study, the unique is that it shows the opposite phenomena, where the ultrasonic velocities decrease as the density increases. The difference of this behavior may be due to the difference of preparation technique as mentioned earlier. The Zn²⁺ ions in ZnO give rise to the non-bridging oxygen bond (single bonded oxygen ion).

It was inferred from a study on binary zinc tellurite glasses performed by Hoppe et al. (2004) that the average coordination number of TeO₂ decreases from 4 to 3 as the ZnO content increases. Therefore, the structural unit TeO₄ tbps, will be converted into the structural unit TeO₃ tp, which in its turn is accompanied by the creation of non-bridging oxygens (NBOs).

Table 2 shows the variation of elastic moduli of different values of ZnO mol percentage content. The results indicate that the elastic moduli decrease with increasing of ZnO mole percentage content, while the values for bulk modulus, K increase. In comparison, the longitudinal modulus, L, is always higher than the shear modulus, G. This means that the glasses can withstand longitudinal stress better than shear stress. Young’ modulus is the ratio of the linear stress to the linear strain. The decreasing Young’ modulus implies the weakening of the overall bonding strength.

Comparing the bulk modulus to the Young’ modulus, the values of bulk modulus are relatively lower. This concludes that the glass samples can tolerate stress in one direction better than stress acting in all directions; they are tougher at certain axes (Sidek et al., 2004).

Rajendran et al. (2003) reported that Poisson’ ratio is affected by the changes in the cross-link density of the glass network. In the studied glass system, the value of Poison’ ratio is varied from 0.20 to 0.24 (Table 2). The increasing trend of Poisson’ ratio suggests that if the same amount of stress is applied over the whole range of the glass composition, the lateral strain will gradually level out (Sidek et al., 2004).

To predict the compositional dependence of elastic moduli of polycomponent glasses, the interpretation of the variation in the experimental elastic behavior observed in this study will be based on the bulk compression model proposed by Bridge and Higazy (1986). Formulae were developed to express the elastic constants as functions of number of network bonds per unit volume, bond length, first order stretching force constant and crosslink density per cation.

According to bond compression model, quantitatively, Poisson’ ratio is inversely proportional to the quantity named the average cross-link density, . The average number of cross-links per glass formula () is calculated according to Bridge and Higazy (1986),

The average cross-link density increases from 2.2 to 2.8 when ZnO content increases (Table 4). The increase in cross-link density is due to the increase in dimensionality, since tellurium atom (with coordination number 4) is replaced by zinc atom (with coordination number 6). To isolate another possible variable affecting the Poisson’ ratio of the glass, which is the relation of Poisson’ ratio with (E/G), applied to the three-chain network (Mallawany et al., 2006). From Fig. 2 and 3, it is clear that the behavior of the Poisson’ ratio is like the behavior of E/G ratio with increase of ZnO mol%.

For a structure consisting of one type of network bond, the bulk modulus calculated according to bond compression model can be expressed in the form:

For a polycomponent glass with n different types of network bonds, Eq. 10 can be rewritten as:

First stretching force constant is given by the equation:

Bridge et al. (1983) found that K_bc value is usually greater than the experimental value K_exp by a factor of 3-10. They concluded that the ratio K_bc/K_exp is a rough measure of the degree to which bonding processes are involved in isotropic elastic deformation.

It is clear from Table 5 that the bulk modulus K_bc, increases from 87.58 to 135.69 GPa with the increase of ZnO content from 0.10 to 0.40 mol%. This increase in K_bc is expected since K_bc depends on the number of network bonds per unit volume (N_b) and the first order stretching force constant F (Eq. 9).

Moreover, the number of network bonds per unit volume increases from 9.19x10²⁸ to 14.17x10²⁸ m^-3, the average stretching force constant increases from 216.43 to 217.50 N m^-1 with the increase of ZnO content. Furthermore, values of the ratio (K_bc/K_exp) range between 3.09 and 4.43 which indicate a relatively open three dimensional network.

Debye temperature (θ_D ) is a characteristic property of a solid lattice related to its acoustic phonon spectrum (Varshneya, 1994). It represents the temperature at which nearly all modes of vibration in a solid are excited (Sidkey et al., 1999). Also increase in the rigidity of the glass is associated with an increase in the lattice vibrations.

The Debye temperature (Fig. 3) decreases from 259 to 251 K as ZnO content increases. It decreases when the ultrasonic velocity decreases. The observed decrease in θ_D indicates a monotonic decrease in the total vibrational energy of the system. This is because any of the conceivable vibrational units resulting from the substitution will be of lower energy.

Micro-hardness expresses the stress required to eliminate the free volume (deformation of the network) of the glass. The free volume in the glass is the openness of the glasses over that of the corresponding crystal (Saddeek, 2004). Therefore, application of high hydrostatic pressure will reduce this free volume, i.e., the glasses will be compacted. Figure 4 shows that micro-hardness of the glass system decrease with the addition of ZnO. It is presumably because of the connectivity of the glass structure decreases (Varshneya, 1994).

DTA curves for the studied glass samples with different ZnO contents have been obtained to determine the glass transition temperature (T_g) values. The composition dependence of T_g in the present glass system reveals that T_g (Fig. 4) decreases from 381.17 to 360.52°C with an increase in ZnO content. The decrease in the glass transition temperature values implies that number of bridging oxygen group decreases.

The observed decrease in micro-hardness and T_g with the increase of ZnO content indicate a weakening in the glass structure which may be due to the introduction of weaker ionic bonds in the glass network. Tellurite glass with higher percentage of ZnO has a lower value of T_g that is ZnO oxides creates a weaker tellurite glass (Mallawany, 1999).

The fractal bond connectivity, (d) of these glasses ranges between 2.47 and 2.97 (Table 2). These suggest an intermediate connectivity between a two and a three-dimensional disordered network, which implies a marked degree of cross-links between TeO₂ chains. As the ZnO content increases, the value of fractal bond connectivity was found to decrease, indicating the weakening of structure in the glass.

CONCLUSIONS

The present study on the binary TeO₂-ZnO glass system revealed the following conclusions:

ACKNOWLEDGMENTS

The author, Rosmawati, wish to thank the Ministry of Education Malaysia for the financial support. This research project is funded by the Ministry of Science, Technology and Innovation Malaysia under the IRPA Research Programs (54275/54061).

REFERENCES