Characterisation of semiconducting V2O5–Bi2O3–TeO2 glasses through ultrasonic measurements

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Abstract

Tellurite containing vanadate (50−x)V2O5xBi2O3–50TeO2 glasses with different bismuth (x=0, 5, 10, 15, 20 and 25 wt%) contents have been prepared by rapid quenching method. Ultrasonic velocities (both longitudinal and shear) and attenuation (for longitudinal waves only) measurements have been made using a transducer operated at the fundamental frequency of 5 MHz in the temperature range from 150 to 480 K. The elastic moduli, Debye temperature, and Poisson’s ratio have been obtained both as a function of temperature and Bi2O3 content. The room temperature study on ultrasonic velocities, attenuation, elastic moduli, Poisson’s ratio, Debye temperature and glass transition temperature show the absence of any anomalies with addition of Bi2O3 content. The observed results confirm that the addition of Bi2O3 modifier changes the rigid formula character of TeO2 to a matrix of regular TeO3 and ionic behaviour bonds (NBOs). A monotonic decrease in velocities and elastic moduli, and an increase in attenuation and acoustic loss as a function of temperature in all the glass samples reveal the loose packing structure, which is attributed to the instability of TeO4 trigonal bipyramid units in the network as temperature increases. It is also inferred that the glasses with low Bi2O3 content are more stable than with high Bi2O3 content.

Introduction

Semiconducting oxide glasses containing transition metal ions are more important due to their increasing applications [1] in different fields. Similarly, tellurite glasses, a distinct type of non-crystalline materials, having unique structure and properties are of special interest in view of their applications [2] in IR domes, laser windows and multifunctional optical components. Tellurium oxide (TeO2) is a conditional glass former [3] and forms glass only with a modifier such as alkali, alkaline earth and transitional metal oxides (TMO) or other glass formers. In a binary tellurite glasses, the basic structural unit of TeO4 is trigonal bipyramid (tbp) with lone pair of electrons and the structural units take the Te–O–Te bond for glass formation [4].

The structure and physical properties of non-crystalline tellurite glasses with semiconducting oxides has attracted many researchers [5], [6]. This is mainly due to the numerous applications of these glasses in electronics and other industries, and also to the fundamental interest in understanding the microscopic mechanism involved. For the same amount of charge carrier, vanadium tellurite glasses are highly conductive than the vanadate phosphate glasses or other transitional metal oxide glasses [7]. The semiconducting properties of vanadium tellurite glasses containing oxides like SnO, ZnO and Sb2O3 has been analysed through conductivity measurements [8], [9], [10]. The change in structure and physical properties of binary and ternary tellurite glasses such as V2O5–TeO2, Li2O–TeO2, TeO2–BaO–TiO2 and V2O5–ZnCl2–TeO2 have been investigated using ESR spectra [11], thermal studies [12], Raman spectra [13] and IR spectra [14]. The electrical conductivity and transport behaviour of vanadium tellurite glasses with the addition of iron oxide [15] and lead fluoride [16] have been reported extensively.

Ultrasonic non-destructive characterisation of materials is a versatile tool for investigating the change in microstructure, deformation process and mechanical properties of materials over a wide range of temperatures [17]. This is possible due to the close association of the ultrasonic waves with elastic and inelastic properties of the materials. It is also due to the availability of different frequency range and many modes of vibration of the ultrasonic waves to probe into the macro, micro and submicroscopic levels. Recent ultrasonic studies on BaTiO3 doped vanadate lead glasses [18], bismuth lead tellurite [19], vanadium tellurite [20] and copper tellurite glasses [21] show interesting features with addition of second/third component. The structural stability of the glasses with change in temperature is also an interesting observation made in the above studies.

In view of the electronic and industrial application of vanadium tellurite glasses, the availability of the structural changes, structural stability and elastic properties over a wide range of temperature will suit the requirements. Although, the glass forming region and electrical conductivity of vanadate bismuth [22] glasses (V2O5–Bi2O3–TeO2) with addition of TeO2 have been studied well, while the ultrasonic non-destructive characterisation of this glasses both at room temperature and the effect of temperature has been found to be not reported. Therefore, in the present study, we have made an attempt to explore the structural changes, structural stability and physical properties of vanadium tellurite glasses with different Bi2O3 contents using ultrasonic measurements. We have measured the ultrasonic velocities, elastic moduli, attenuation (longitudinal waves only), Poisson’s ratio and Debye temperature as a function of temperature from 150 to 480 K in vanadium tellurite glasses with different Bi2O3 contents.

Section snippets

Preparation of glasses

Tellurite containing vanadium (50−x)V2O5xBi2O3–50TeO2 oxide glasses with different bismuth contents x=0, 5, 10, 15, 20 and 25 wt% (hereafter, termed as VBT0, VBT5, VBT10, VBT15, VBT20 and VBT25, respectively) have been prepared by rapid quenching method. The analytical grade materials of purity more than 99.9% of V2O5, Bi2O3 and TeO2 chemicals were used to prepare glass samples. The required amount (approximately 10 g) in wt% of chemicals in powder form was weighed using a digital balance

Determination of elastic constants

Elastic (longitudinal, shear, bulk and Young’s) moduli, Debye temperature and Poisson’s ratio of (50−x)V2O5xBi2O3–50TeO2 glasses with different Bi2O3 contents have been determined from the measured ultrasonic velocities and density using the standard relations [18]:

Bulk modulusK=L−(4/3)G;Young’s modulusE=(1+σ)2G,where L is the longitudinal modulus (L=UL2ρ) and G the Shear modulus (G=US2ρ). σ is the Poisson’s ratio and is given byσ=(L−2G)2(L−G);Debye temperature [18] of the glass samples was

X-ray diffraction and scanning electron microscopy

XRD pattern of VBT0, VBT5, VBT10, VBT15, VBT20 and VBT25 glass samples show the absence of any characteristic peaks confirms the glassy nature in all the glass samples. A typical XRD pattern for VBT5 and VBT25 glass samples are shown in Fig. 2. The absence of any microstructure on scanning electron micrograph in all the glass samples confirms the glassy nature. Fig. 3 shows the SEM micrograph for a typical VBT5 and VBT25 glass samples.

Thermal analysis

DTA and TG curves for VBT0, VBT5, VBT10, VBT15, VBT20 and

Thermal behaviour

The glass transition temperature Tg is useful in exploring the difference in glass structure. The thermal stability of the glass is a result of the glass structure. The thermally stable glasses will have close packed structure, while the unstable glasses will have loose packed structure [25]. In the case of open glass structure [1], [25], the value of Tg declines. The values of Tg decreases from 690 to 220 °C with addition of V2O5 from 20–80 mol% of V2O5 in P2O5 containing vanadate glasses [26]

Conclusions

In the present study, tellurite containing vanadium (50−x)V2O5xBi2O3–50TeO2 oxide glasses for different Bi2O3 contents x=0, 5, 10, 15, 20 and 25 wt% have been prepared by the rapid quenching method. The ultrasonic velocity both longitudinal and shear, attenuation, elastic moduli, Debye temperature and Poisson’s ratio in all the glass samples have been measured as a function of temperature from 150 to 480 K. The amorphous nature of all the prepared glass samples was confirmed using XRD and SEM

Acknowledgements

The authors are grateful to Professor G. Shanmugam, Principal and Thiru. Yennarkay R. Ravindran, Correspondent, Mepco Schlenk Engineering College for their interest in this work and for the support given by Council of Scientific and Industrial Research, New Delhi (Scheme no: 03(0811)/97/EMR-II) to carry out this work.

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