·Table of Contents
·Materials Characterization and testing
Elastic Properties of the Lead Containing Bismuth Tellurite Glasses - An Ultrasonic StudyV. Rajendran,
Department of Physics, Mepco Schlenk Engineering College.
Mepco Engineering College (PO) - 626 005
Virudhunagar (Dt), Tamilnadu, INDIA.
N. Palanivelu, B. K. Chaudhuri,
Solid State Physics Department,
Indian Association for the Cultivation of Science,
Calcutta - 700 032, INDIA.
The elastic properties of bismuth tellurite (85-XBi2O3 + 15TeO2 + XPbO) glasses with different compositions of lead (X=55 to 80 wt.% in steps of 2.5 wt.%) have been studied from 150 to 600 K. The precise transit time measurement has been made using a high power ultrasonic process control system with DSO employing cross correlation technique. The ultrasonic velocity and attenuation measurements have been made using a transducer having resonating frequency of 5 MHz (both longitudinal and shear). The density, ultrasonic velocity and attenuation show an interesting observation, which are used to explore the structural changes in the network. Elastic moduli, Debye temperature and acoustic impedance of the glasses have been determined using the experimental data. Anomalies observed in the elastic properties, Debye temperature and acoustic impedance exhibits similar trend of behaviour as that of density. The temperature dependence of the elastic properties explores useful information about the structural stability and physical properties of the bismuth tellurite glasses doped with lead. The observed results through ultrasonic non-destructive evaluation explore the structural stability, structural changes and mechanical properties of the glasses.
The application of tellurite glasses in industries [1,2] such as electric, optical, electronic and other fields are immense due to their good semiconducting properties, chemical durability, electrical conductivity, transmission capability, high dielectric constant, high refractive indices, and stable & low melting points. The application of fast ion conducting glasses  has shown distinct advantages over their crystalline counterparts as solid materials for solid state batteries. The Fe2O3 containing tellurite glasses exhibit higher electronic conductivity [4,5] than that of the equivalent Fe2O3 containing phosphate, silicate and borate glasses.
The availability of structural changes, structural stability and elastic properties over a wide range of temperatures on Bi2O3-TeO2-PbO glasses meet the industrial requirements. By considering the importance of the tellurite glasses, in the present investigation, Bi2O3-TeO2-PbO glass system has been prepared for different PbO content for ultrasonic NDE studies. Therefore, in the present paper the structural and physical properties of Bi2O3-TeO2-PbO glasses has been studied for the first time from 150 to 600 K in our laboratory.
Among the various non-destructive evaluation (NDE) techniques, ultrasonic technique [6-8] is a versatile tool for investigating the changes in microstructure, deformation process and mechanical properties of materials / components. It is due to the fact that, the ultrasonic waves are closely related with the elastic and inelastic properties of the materials. The increased utilisation of this technique is due to the availability of different frequency ranges and many modes of vibration of the ultrasonic waves to probe into the macro, micro and submicropic levels.
In the present paper
The starting materials used in the present work were of Analar grade having purity of more than 99.9 %. Lead containing bismuth tellurite [(85-X) Bi2O3 + 15TeO2 + X PbO) glasses with different PbO content such as X = 55, 57.5, 60, 62.5, 65, 67.5, 70, 72.5, 75, 77.5 & 80 wt. % (hereafter termed respectively as BTP55, BTP58, BTP60, BTP63, BTP65, BTP68, BTP70, BTP73, BTP75, BTP78 and BTP80) have been prepared. The required amount of chemicals was weighed and ground for about 2 h. The required homogeneity of the mixture was achieved by the repeated grinding. The mixtures were preheated at a temperature of 360 K for 5 h in ambient atmosphere in order to remove H2O, CO2, etc.
The preheated mixtures were again ground for 1 1/2 h to reduce the particle size and also to obtain a specific melting temperature. The mixture was then fired in a furnace at 860 to 960 K (depends on the PbO content) for about 90 min, till a bubble free liquid was formed. The melt was homogenized and poured into a preheated copper block and then quenched by using another thick copper plate. Employing similar procedure glasses with different compositions were prepared.
The prepared specimens were cut into required dimension for velocity and attenuation studies. Both the surfaces of the specimens were polished using a fine lapping paper to achieve a plane parallelism. The plane parallelism between the faces was of the order of ± 0.001 mm.
Ultrasonic Velocity Measurements
A specially designed and fabricated experimental set up in the author's laboratory was used for velocity and attenuation measurement in low (150 K to room temperature) and high (room temperature to 600 K) temperature studies. The salient feature of the cryostat is that the mechanical arrangement for specimen holder avoids damage of transducer due to heavy pressure during clamping. Similarly, the technique used in high temperature experimental set up  prevents the transducer from thermal effects, and also it requires only the normal transducers.
A high power ultrasonic process control system (Fallon Ultrasonics Inc. Ltd., Model-FUI1050, Canada) and a digital storage oscilloscope (Hewlett Packard, Model-54600B, US) were employed for recording r.f. signals. Precise transit time measurements were carried out by employing cross correlation technique for both longitudinal and transverse waves using 5 MHz transducers. The block diagram of the experimental set up used for velocity and attenuation measurement is shown in Fig.1. Various steps involved in the precise ultrasonic transit time measurements as given in  was followed to get an overall accuracy of ± 0.2 m sec. in transit time measurement. Knowing the specimen thickness in micron resolution, ultrasonic velocities were obtained. Overall accuracy obtained in velocity measurement is 5 m/s. This was possible by maintaining variation in the plane parallelism of specimen surfaces within 5 micron and with good surface finish.
|Fig 1: Block diagram of the experimental set up - velocity and attenuation measurement|
Ultrasonic Attenuation Measurements
A low frequency 5 MHz contact type transducer was used for attenuation measurements. A constant pressure was maintained between transducer and specimen during the contact measurements and care was taken to avoid the problem of near field effects. From the peak amplitude measurements of first and second back wall echo signals from the specimen, attenuation coefficient was obtained for each specimen.
XRD and SEM Measurements
The density of the specimens was measured using Archimedes principle. The overall accuracy in the density measurement is ± 0.005 g. The amorphous nature of BTP glasses was confirmed by the X-ray diffraction (JEOL, Model: JDX 8027) analysis and Scanning Electron Microscopic (Hitachi, Model:415A, Japan) studies. In addition, the elements present in the glass specimens with different contents were also analysed by Energy Dispersive Analysis X-rays (EDAX).
Elastic properties at room temperature
The elastic moduli such as longitudinal (L), shear (G), bulk (K) & Young's (Y) moduli, Debye temperature (q D) and acoustic impedance (Z) of all glasses were computed from the known formulae as earlier .
Density is an effective tool to explore the degree of structural compactness , modification of the geometrical configurations of the glass network, change in coordination and the variation of the dimensions of the interstitial holes. The addition of PbO in Bi2O3-TeO2-PbO glasses causes an increase in density which is not only due to the introduction of a high atomic weight element (atomic weight of PbO is 207.2) but also due to volume contraction. The progressive addition of PbO shows, a non linear variation in density with minima respectively at 70 wt.% and 77.5 wt.% , and a maxima at 70 wt.% and 75 wt.% of PbO. The anomalous behaviour showed in density [Fig.2.] is attributed to the rearrangement of structure with the addition of PbO into Bi2O3-TeO2-PbO glasses.
TeO2 and Bi2O3 are conditional glass formers [13,14] and will form glass only with the addition of another oxide like PbO. Normally, the presence of TeO2 in a glass, makes it as a colored one and hence in the present glass system all the glasses are pearl yellow in color (glasses which contains equal amount of TeO2). The existence of such coloring property in these glasses may influence over the insulation and optical transmission properties .
The addition of PbO into the Bi2O3-TeO2-PbO structure, increases the longitudinal and shear velocities upto 57.5 wt.% of PbO content. Then, the progressive addition of PbO content, shows a decrease in both longitudinal and shear velocities upto 67.5 wt.% of PbO which is followed by an another maximum at 70 wt.% and then a sudden decrease in velocity as shown in Fig.3. This is possible due to the addition of Pb2+ into the glass network as a network former  results in the structural rearrangement of the glasses due to the transfer of the bridging oxygens into non-bridging oxygens.
|Fig 2: Variation of density with PbO content||Fig 3: Variation of velocity with PbO (error band 5)|
The observed anomalies in the variation of velocity with the change in PbO content in the glass network can be discussed as follows: The addition of metal ions such as Pb2+ to an oxide glass shows the interaction of different alkali ions and oxygen atoms and also the
structural rearrangement in the glass system. In the present glass system, there are two possibilities of transformation of linkages which are:
The Pb2+ ions may enter the glass network interstitially and the breaking of Bi-O-Bi linkage in the network can take place. As Pb2+ ion is added as network former, the formation of bridging linkage Bi-O-Pb is possible. The formation of new linkages with the addition of PbO content contributes to a volume contraction. As a consequence, the density (increased packing density) and ultrasonic velocities increases. The structural compactness with increase of TeO2 content has been explained in other glass systems . Similarly, the Pb2+ ion may also occupy interstitial positions and create stable bonds (Te-O-). The formation of stable bonds (Te-O-) will also increase the compactness of the glass network prevailing in the glass. Further, the Te-O-Te bonds are broken and hence the formation of unstable bonds (Te..O-) may take place. The introduction of coordinated defect bonds known as dangling (broken) bonds results in softening of the glass network. Thus, an increase in the volume results in a decrease in density and both in longitudinal and shear velocities.
The above three linkages may be responsible for the observed anomalies in density, velocities, elastic moduli and Debye temperature with addition of Pb2+ into glass network. The presence of stable bonds (Te-O-) and unstable bonds (Te..O-) in ZnF2-PbO-TeO2 glasses were explored from elastic properties studies by Ravikumar et al. .
|Fig 4: Variation of Bulk and Young's moduli with PbO content||Fig 5: Variation of Debye temperature and acoustic impedance with PbO|
It is interesting to note that, a similar minimum and maximum in all the elastic moduli [Fig.4.] viz. L, G, K, Y, Debye temperature (qD) and acoustic impedance (Z) [Fig.5.] showing the five compositional region behaviour could be identified from density and velocity variations. The bulk modulus of the glass network depends on both the cross link density and bond stretching force constant . An increase in cross linking contributes to an increase in bulk modulus. The addition of Pb2+ ion to the glass transfers some double bonds into bridging bonds and as a consequence an increase in dimensionality of the glasses enhances the linkages (cross linkage) and rigidity of the glass network. This results in an increase in velocity and elastic moduli. Similarly, the decrease in dimensionality of the glasses reduces the linkages (cross linkage) and rigidity of the glass network. This results in a decrease in velocity and elastic moduli. The observed anomaly in elastic properties indicates the change in dimensionality  of the glasses.
Effect of temperature
The temperature dependence of both longitudinal and shear ultrasonic velocities for all the specimens are shown in Fig.6, from 150 to 600 K. The observed results indicate that both longitudinal and shear wave velocities, decrease slowly with increase in temperature in all the specimens. In the present measurement range (150 to 600 K), the variation of velocity with temperature does not show any minimum. The bulk and Young's modulus [Fig.7.] also follow the same trend as that of velocity with change in temperature. However, the percentage variation of velocity and bulk modulus with change in temperature is more at 70 wt.% of PbO content as observed at room temperature studies.
|Fig 6: Variation of velocity with temperature||Fig 7: Variation of moduli with temperature|
For a given temperature, the anomalies in the variation of velocity and elastic moduli exhibits a similar five compositional region behaviour with change in PbO content as obtained at room temperature studies.
The following are the conclusions drawn from the present study:
The authors (V.R & N.P) are grateful to Prof. G. Shanmugam, Principal and Thiru Yennarkay R. Ravindran, Correspondent, Mepco Schlenk Engineering College for their interest in this work and are also thankful to Council of Scientific & Industrial Research, New Delhi (Scheme no.: 03(0811)/97/EMR- II) for providing financial support. One of the authors (V.R) is thankful to the sponsoring agencies for providing support to present the paper in the conference.
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