Abstract
Bismuth glasses containing ZnO and Li2O were prepared by conventional melt-quench technique. ZnO is gradually substituted by Li2O, and its effect on various physical, thermal, and electrical conduction properties was studied and analyzed. Raman studies revealed that these glasses are mainly made up of [BiO3] and [BiO6] units. The density decreases and molar volume increases with the incorporation of Li2O into these glasses. MDSC studies have been performed on these glasses to determine the glass transition temperature and other related thermal parameters such as change in the glass transition temperature () and specific heat capacity difference (). These studies revealed that the glasses possess high values, and decreases with Li2O content. The trend of these properties is attributed to the changes in the glass network structure. Dc electrical conductivity revealed that the conductivity increases and activation energy decreases with Li2O content.
1. Introduction
The glassy materials have become increasingly important in the field of solid state ionics. They are of technological interest because of their various applications in solid state batteries, cathode ray tube, flat panel display, thick-film hybrid integrated circuits, magnetic heads, and so forth. In recent years, bismuth borate glasses [1β6] attracted the attention of researchers due to their useful physical properties, their important applications in the field of glass ceramics, thermal and mechanical sensors, reflecting windows, and so forth. A survey of the literature shows that there are many reports available on ternary bismuth borate glasses [7β11]. Recently, attention has been given to the investigation of glasses containing transition metal cations. The structural investigation of such glasses is essential to obtain a better insight into their structure-property relations. Especially, zinc-oxide-based glasses/ceramics have special applications in the areas of varistors, dielectric layers, and transparent dielectric and barrier ribs in plasma display panels [12, 13]. Furthermore, ZnO can be made versatile material with broad applications through a proper doping process such as transparent-conducting electrodes (doped with group IIIB, fluorine, and aluminum) and piezoelectric as well as ferroelectric layers [14]. Very recently structural [15], thermal, and electronic polarizability [16] studies on Bi2O3-ZnO-based glasses were reported.
In the present paper, we report the spectral and physical properties of Bi2O3-B2O3-ZnO-Li2O glasses. the presence of Bi2O3, a conventional glass former, and B2O3, a classical glass former, increases the interest in the present study. The influence of ZnO and Li2O on various properties is discussed.
2. Experimental Details
2.1. Glass Preparation
Glass samples of composition 50Bi2O3-15B2O3-(35-x)ZnO-xLi2O were prepared by conventional melt-quench technique using reagent grade chemicals Bi2O3, B2O3, ZnO, and Li2O (with purity more than 99%). The mixture of powders taken in a porcelain crucible was heated at 450Β°C for one hour to evaporate the moisture if any. The mixture was melted at a temperature of 1100β1200Β°C depending on the composition. The melt was then rapidly quenched in air between two highly polished steel blocks maintained at 200Β°C. This gave the transparent glasses with shining surfaces. The glass samples were annealed at 200Β°C to remove thermal stress and strain (if any) during formation. The glass composition mentioned is the nominal composition (the starting mixture).
2.2. Glass Characterization
A small amount of the glass was powdered for the X-ray diffraction studies, and a PANalytical X-pert PRO model with Cu-K Alpha radiation (Ξ» = 1.54048βΓ ) was used. The glassy phase of the samples was confirmed from XRD pattern (Figure 1) as it showed a broad hump at 2ΞΈ ~ 30Β°.
The room temperature Raman measurements were performed in the range of 100β1700βcmβ1 on a micro-Raman system from Jobin-Yvon Horiba (LABRAM HR-800) spectrometer. The system is equipped with high-stability confocal Microscope for Micro-Raman 10x, 50x, 100x objective lens to focus the laser beam. Ar+ laser beam of 488βnm (E = 2.53βeV) was used for excitation. The incident laser power is focused in a diameter of ~1-2βΞΌm, and a notch filter is used to suppress Rayleigh light. In the present system, Raman shifts are measured with a precision of ~0.3βcmβ1.
The density () of the glass samples was determined using Archimedes principle, by using xylene (βg/cc) as the working fluid. The molar volume () has been determined as , where is the molar weight of the glass calculated by multiplying Γ times the molecular weights of the various constituents. Also, the values of the packing density number of oxygen atoms per formula unit were calculated. Li+ concentration () and interionic distance () were also evaluated.
The glass transition temperature, , was measured in all samples using a temperature-modulated differential scanning calorimeter (TA Instruments, DSC 2910). All samples were heated at the standard rate of 10Β°Cβminβ1 in aluminum pans.
The electrical measurements were made on pure samples by the usual technique of two electrodes method. Silver paste was painted on the polished circular disc surface of the samples with thickness of ~1βmm and ~10βmm diameter. With painted silver paste, good ohmic contacts were found. The electrical conductivity was measured as a function of temperature from 200Β°C up to below glass transition temperature of the respective glass sample. The sample was loaded in a cylindrical furnace using a spring. The dc electrical conductivity measurements were made using a Keithley electrometer model 614. The temperature of the specimen was recorded with a chromel-alumel thermocouple kept in close thermal contact with the specimen surface.
3. Results and Discussion
3.1. Raman Spectra
Figure 2 shows the effect of composition on the Raman spectra of the studied glasses consisting of broad peaks in the range of 100β1700βcmβ1. The broadening of peaks is due to the disorderness in the glass matrix. To find out the exact mode of vibrations and also the Raman shifts, the spectrum with superimposed broad peaks was deconvoluted into six peaks using a Gaussian distribution (dotted curves). The peak position and the peak width of the Raman spectra thus determined for all the glasses are presented in Table 1. Vibrational spectra of Bi2O3 and derivatives studied by Raman spectroscopy [17] indicated that bismuth does not form a simple structure. However, it is well known that bismuth ions can form [BiO3] pyramidal or [BiO6] octahedral units [18β20]. Pyramidal units have four fundamental vibrations, which are all Raman active, while octahedral units have six modes of vibrations, three of which are Raman active [21]. In the present investigation, the Raman spectra are dominated by the bands associated to the structural vibrations of the heaviest cation, Bi3+. The dominating peak around 130βcmβ1 in all the spectra is an evidence for the presence of [BiO6] octahedral and [BiO3] pyramidal units. The bands in the range of 300β600βcmβ1 can be assigned to symmetric stretching anion motion (~i.e., vibration of bridging oxygen) in an angularly constrained BiβOβBi configuration [22].
The broad but strong band occurring in the present Raman spectra around 390βcmβ1 can be attributed to the BiβOβBi vibrations of [BiO6] octahedral units. It can be observed that the peak position shifts towards lower wave number with Li2O content. This can be assumed due to the presence of lithium ions near the [BiO6] octahedra which in turn shortens the BiβO bonds and hence increases the energy for vibration [23]. The shoulder at 555βcmβ1 can be attributed to BiβOβ/BiβOβZn stretching vibrations [22]. The band at 555βcmβ1 increases in intensity with the increase in the ZnO content. In the present glass system, B2O3 is constant at 15βmol%. Only orthoborate and pyroborate units may exist [24]. Therefore, in the present glass system, the Raman band around 927βcmβ1 is due to orthoborate groups, and a very weak broad band around 1234βcmβ1 is due to pyroborate groups. The presence of Raman peak at 220βcmβ1 indicates the presence of ZnβO tetrahedral bending vibrations of ZnO4 units in the present glass system [20, 25]. The Raman band assignments are presented in Table 2.
3.2. Density and Related Parameters
The density is perhaps the most important measure of a glass. Its value is needed in manifold techniques such as neutron, electron, and X-ray scattering. Density also stands on its own as an intrinsic property capable of casting light on short-range structure. The density is generally affected by the structural softening/compactness, change in geometrical configuration, coordination number, crosslink density, and dimension of interstitial spaces of the glass [26]. Figure 3 shows the variation of density with Li2O content. It is observed that the density decreases linearly from 5.98βg/cc to 5.45βg/cc with the increase in Li2O content. Figure 4 shows the variation of as well as OPD with Li2O content. It is noted that the increases and OPD decreases with Li2O content. The decease in density with Li2O is obvious since it is being replaced by heavier ZnO.
3.3. MDSC
It is a known fact that the glass transition temperature is a measure of structural relaxation. If a material shows the phenomenon of structural relaxation, we speak of a glass. Structural relaxation is defined by the existence of a well-defined temperature interval, the βglass-transition region,β where, due to the onset of long-range atomic and molecular motion, mechanical and thermodynamic properties become time dependent. The onset of molecular motion manifests itself in a dramatic increase of the heat capacity, which is easily detected by differential scanning calorimetry.
The MDSC thermograms of the present glasses are shown in Figures 5(a) and 5(b). Figure 6 shows the heat capacity signals of the present glass samples. In DSC experiments, glass transition process shows up as an upward jump in heat capacity baseline, and the enthalpic relaxation process accompanying it exhibits an endothermic peak in the heat flow. Since in MDSC these two signals are independently obtained, heat capacity and nonreversible heat flow (NRHF) are the two raw signals of interest. Therefore, it enables the accurate determination of the point of inflexion, the point where attains a maximum value (Figure 7). We have used this definition to determine the value of . The errors in measurements in determination from the point of inflexion are found to be Β±1βK. The method of evaluation of various thermodynamical parameters such as glass transition temperature , specific heat capacity , change in the transition temperature Ξ, and specific heat capacity difference Ξ is reported earlier [6].
(a)
(b)
Figure 8 shows the variation of with glass composition. As can be seen from the figure, the range in in this study is from 454 to 491Β°C. That means that these glasses are stable at high temperatures up to 450Β°C and can be used for various high-temperature applications. Glass transition temperatures of bismuth-based glasses have been reported previously [27, 28] in which the values are much lower than the present glasses. The high glass transition temperature of the present glasses can be attributed due to the presence of ZnO. From the figure, it is also evident that the glass transition temperature decreases and shows a tendency of linear relationship with Li2O content. It is considered that depends on the strength of the chemical bond, crosslink density in the glass structure. In general, the incorporation of alkaline metals at lower concentration into a base glass network (borate, silicate, bismuthate, etc.) plays a role of network modifier due to which the number of nonbridging oxygen increases. This results in the decrease in the crosslink density, loosening the glass network. It is widely known that the bond between alkaline atom and oxygen is ionic bond which is in general weaker than the covalent bond. Hence, in the present study, decreases with the increase in Li2O content. This result is supported by the density measurements, where the molar volume increases with Li2O content.
The values of , Ξ,, and Ξ for the present glasses are given in Table 3. For all samples, the smaller values of Ξ are an indication of strong resistance to any structural change, showing that the glass-forming liquid has a number of configurational degrees of freedom associated with it.
3.4. DC Electrical Conductivity
The reciprocal temperature dependence of the dc conductivity for different compositions is shown in Figure 9. The figure shows that the plots are straight lines indicating that the dc conductivity obeys the Arrhenius relation
where is the thermal activation energy, is the Boltzmann constant, is the preexponential factor, and is the absolute temperature. It can be observed from the figure that the electrical conductivity increases with lithium content and also with temperature. The activation energy has been calculated for different compositions from least square straight line fits of Arrhenius plots. The variation of dc conductivity at a particular temperature and the activation energy with Li2O content are presented in Figure 10. It can be noted from the figure that the dc conductivity increases and the activation energy decreases with Li2O content.
As Li2O (which acts as a network modifier) is added, the base glass network is modified by the formation of more and more nonbridging oxygen (BiβOββLi+). These kinds of entities create a large number of charge carriers Li+ which can easily move through the glass during heating. This fact contributes to the increase of electrical conduction. On the other hand in the other way, when ZnO replaces Li2O, BiβOβZn bonds are formed along with a stronger covalent BiβO bonds which are stronger than Li+βO one. This is in accordance with the increase of glass transition temperature and decrease of the molar volume with ZnO. Similar results were reported previously [6, 29].
4. Conclusions
From the present investigation, it can be concluded that(i)these glasses are made up of [BiO3] and [BiO6] units. Formation of ZnO4 units was also observed;(ii)the density decreased and molar volume increased with Li2O content;(iii) these glasses possess in the range of 454β491Β°C. The glass transition temperature decreases with Li2O content, which was due to the decrease of crosslink density;(iv) the dc electrical conductivity studies revealed that the electrical conductivity increases with lithium and also with temperature and obeys Arrhenius law, whereas the activation energy decreases with Li2O content.