Abstract
Solid solution of perovskite Pb,SrTiO3 in Pb-rich composition can be crystallized in borosilicate glassy matrix. The addition of rare earth and transition metal oxides is known to influence the crystallization behavior and surface morphology of perovskite crystallites in glassy matrix. In the present paper, the glasses in the lead-rich system 64[(PbxSr1-x)Β·TiO3]-25[2SiO2Β·B2O3]-5[K2O]-5[BaO] () with the addition of 1βmol % La2O3 were prepared to study its effect on their crystallization behavior. Differential thermal analysis (DTA) patterns show one or more exothermic crystallization sharp peaks, which shift towards higher temperature with increasing concentration of SrO. The glasses were subjected to various heat-treatment schedules for crystallization. X-ray diffraction analysis of these glass ceramic samples shows that major crystalline phase of the entire glass ceramic sample with was found to have tetragonal structure similar to PbTiO3 ceramic, and addition of La2O3 enhances the crystallization of the perovskite phase and retards the crystallization of minor phases.
1. Introduction
Glass ceramics are an important class of materials that have been commercially quite successful. The pore-free polycrystalline materials are produced by the controlled crystallization of glass and composed of randomly oriented crystals with some residual glass [1]. Crystallization is accomplished by subjecting the glasses to a carefully regulated heat treatment schedule, which results in the nucleation and growth of crystal phases within the glass samples [2]. Extensive studies have been reported on the crystallization and dielectric behavior of ferroelectric glass ceramics, specifically PbTiO3 and NaNbO3 [3]. These studies show that both the parent glass composition and heat treatment schedule determine the crystalline phase constitution, microstructure, and dielectric properties of respective glass ceramics. Bergeron and Russell investigated the growth of PbTiO3 from PbO-B2O3-TiO2 glasses and found that the crystallization proceeded mainly from the surface [4]. The crystallization and microstructural behavior of glass ceramic with perovskite titanate phases, such as PbTiO3 [5β11] and SrTiO3 [12β15], have been investigated. Limited work has been carried on the lead strontium titanate borosilicate glass-ceramic system, despite its wide applications. Thakur et al. [16] investigated the crystallization, microstructure, and dielectric behavior of SrTiO3 glass ceramic with different oxide additions. They could crystallize SrTiO3 as a major phase in borosilicate glass ceramic system with addition of proper concentration of alkali oxide K2O and selected heat treatment schedules. It was also reported that the addition of La2O3 enhances the crystallization of strontium titanate [17]. Later on Sahu et al. [18, 19] explored the possibility of substitution of strontium for lead in the system [(Pb1-xSrx)OTiO2]-[2SiO2B2O3]-[K2O]-[BaO] for crystallization of solid solution perovskite phase. The phase development, microstructural analysis, and dielectric behavior of the glass ceramics indicated that both the glass composition and heat-treatment schedules determine the crystalline phase constitution. Solid solution of PbTiO3 and SrTiO3 phases could be crystallized in borosilicate glasses [20β23].
A brief report on crystallization and dielectric behavior of lead strontium titanate borosilicate glass ceramics with addition of La2O3 was reported [21]. In the present paper, a detailed study has been made to understand the crystallization behavior and microstructural morphology in the lead-rich glass ceramic samples in the system 64[(PbxSr1-x)TiO3]-25[2SiO2B2O3]-5[K2O]-5[BaO] with addition of 1% La2O3. The crystallization and surface morphology of strontium-rich compositions are given in the second paper βCrystallization behavior and surface morphology of strontium rich (PbxSr1-x) TiO3 glass ceramics in presence of La2O3-II.β
2. Experimental Procedure
Glasses in the system 64[(PbxSr1-x)TiO3]-25[2SiO2B2O3]-5[K2O]-5[BaO]-1[La2O3] with varying lead to strontium ratio () have been prepared by melt-quench method. The highly pure chemicals powder of PbO, SrCO3, TiO2, SiO2, H3BO3, BaCO3, K2CO3, and La2O3 was mixed in a mortar using acetone as a grinding medium. The dry powders were melted in the temperature range 1120β1240Β°C in an electrically heated furnace. The melt was poured into an aluminium mould, pressed by a thick aluminium plate, and immediately annealed at temperature 400Β°C for three hours in another furnace. All the glasses were characterized by differential thermal analysis (DTA) to determine glass transition and the crystallization temperatures (Table 1). Differential thermal analysis was done on the powdered glass samples at a heating rate of 10Β°C/min. On the basis of DTA results, various glass ceramic samples were prepared by heat treating the glasses in the temperature range 600β806Β°C. The different heat-treatment schedules and nomenclature of glass ceramic samples are listed in the Table 2. The glasses were heat treated by heating them at a heating rate of 5Β°C/min to the desired temperature and holding them for 3 or 6 hours. The samples were then cooled to room temperature at a cooling rate of 10Β°C/min. Three-hour heat treatment was given to all glasses at their respective DTA peaks, whereas six-hour heat treatment was given to all glasses at their DTA corresponding to major crystalline perovskite phase. The nomenclature of glass ceramic samples includes the code of their parent glass composition followed by heat-treatment temperature and followed by a letter ββTββ and ββSββ for 3 and 6 hours heat treatment, respectively. X-ray diffraction patterns were recorded using a Rigaku X-ray diffractometer using Cu KΞ± radiation. X-ray diffraction patterns were compared with standard d-values from JCPDS files for different constituting phases. Gold coatings were applied by the sputtering method to the etched surfaces of various glass ceramic samples intended for scanning electron microscopy (SEM) in order to study the morphology of different crystalline phases.
3. Results
3.1. Differential Thermal Analysis (DTA)
DTA curves for these glasses with different (Pb) lead to strontium (Sr) ratio (x = 1.0 to 0.5) are shown in Figures 1 and 2. DTA patterns of different glasses show one or more exothermic peaks. These exothermic peaks represent the temperature at which the rate of crystallization of different phases is maximum. All glasses show a shift in the base line at a temperature, depending on the composition, in the temperature range of 510β600Β°C. This shift in the base line shows a change in the specific heat of the glass, which is attributed to the glass transition temperature, . Glass transition temperatures of different glass samples are given in Table 1. The glass transition temperature has been found to increase with the increasing concentration of SrO. This may be due to increase in the viscosity of the melt.
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DTA pattern of the glass PTL5B with no strontium shows two exothermic peaks at 620 and 700Β°C (Figure 1(a)). The peak at 700Β°C represents the crystallization of the major perovskite lead titanate, (P), PbTiO3 phase in the glass ceramic. The peak at 620Β°C represents the crystallization of the minor PbTi3O7 (PT). This is confirmed by powder X-ray diffraction (XRD) studies. The peak corresponding to crystallization of major perovskite phase is present in DTA patterns of all the glasses. Two DTA peaks are also observed for the glass 7PL5B.
Three DTA peaks are observed for the glass 9PL5B. All other glasses show a single exothermic peak in their DTA patterns. The temperature of peaks Tc1, Tc2, and Tc3 for different glasses in this system with PbO to SrO ratio and 1% La2O3 is given in Table 1.
3.2. X-Ray Diffraction Analysis and Crystallization Behavior
X-ray diffraction (XRD) patterns for various glass ceramic samples crystallized at different temperatures for 3 hours are shown in Figure 3. All the peaks in respective XRD patterns were matched with JCPDS of various compounds data for constituent oxides. X-ray diffraction pattern for the glass ceramic samples PTL5B700T (x = 1.00) is shown in Table 2.
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In Figure 3(a), it is observed from the XRD pattern that PbTiO3 (P) is the major crystalline phase and PbTi3O7 (PT) is the secondary phase in this glass ceramic sample. Figures 3(b) and 3(c) show the XRD patterns of glass ceramic samples 9PL5B695T and 8PL5B726T with heat treatment at different temperatures. XRD patterns of glass ceramic sample 9PL5B695T show the presence of tetragonal perovskite phase as major phase and PbTi3O7 as a minor phase. When the glass is heat treated at lower temperature, the amount of minor PbTi3O7 phase is more in comparison to the glass ceramic sample obtained by heat treating at 695Β°C. This is indicated by the relative intensity of XRD lines of P and PT phases. This shows that the DTA peaks at lower temperatures correspond to crystallization of minor phase, while the peak Tc3 at 695Β°C corresponds to crystallization of perovskite phase.
The XRD data of the major phase of these glass ceramic samples were indexed on the basis of tetragonal unit cell similar to lead titanate. The lattice parameters (s) for the major crystalline phases in the glass ceramic system were obtained by using software CEL. The structure, lattice parameters (c, a), tetragonality, c/a, for the crystalline phase are given in Table 3. The glass ceramic samples obtained by heat-treating glasses with x = 1.0 and 0.9 were found to show similar crystallization behavior (Figure 3). They differ only in terms of minor phase and value of lattice parameters. XRD patterns for glass ceramic samples 9PL5B597T; 9PL5B635T; 9PL5B695T are shown in Figure 4. The XRD patterns for these glass ceramic samples show the formation of perovskite as a major phase. Large amount of PT and small amount of rutile (R) are also present. The amount of secondary phase of PbTi3O7 for x = 0.9 obtained by heat treatment at higher temperature is found to be less in comparison to the composition with x = 1.0. The glass with x = 0.7 was heat treated for 3 and 6 hours at 686Β°C and 739Β°C to study the effect of the crystallization temperature and soaking time on the microstructure and dielectric behavior of the resulting glass ceramic samples. Perovskite titanate was found to be major phase with secondary PbTi3O7 and trace amount of rutile phase. But for glass ceramic obtained by heat-treating at 739Β°C for 3 hours, the presence of secondary phases of PbTi3O7 and rutile (TiO2) was not observed. Figure 5 shows the XRD patterns for the glass ceramic samples 6PL5B730T and 5PL5B806T. Perovskite titanate (P) was found as a major phase and rutile (R) as a minor phase. They contain the same phases but only differ in the tetragonality.
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XRD patterns for glass ceramic samples PTL5B700S, 9PL5B700S, and 8PL5B726S crystallized at 700Β°C and 726Β°C for 6 hours, respectively, are shown in Figure 6. Glass ceramic sample PTL5B700S has the phase constitution similar to PTL5B700T that is obtained by heat treating the glass for 3 hours.
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For the glass ceramic sample 9PL5B700S, a change is observed in the crystalline phase in comparison to 9PL5B700T. This change is in the form of secondary phases, PbTi3O7 (PT) and Pb2O4 (Pb). For sample heat treated at 700Β°C for 3 hours, the secondary phase is PbTi3O7 whereas for 6βhrs of heat treatment, the secondary phase is Pb2O4. Figure 6(c) depicts the XRD pattern of glass ceramic sample 8PL5B726S. This shows the presence of perovskite titanate as the major phase. A little amount of TiO2 (rutile) phase is also present. This glass ceramic sample shows better amount of perovskite phase in comparison to 3-hour heat treatment schedules. Figure 7(a) shows the XRD pattern for the glass ceramic sample 7PL5B739S. This sample contains PbTi3O7 as minor phase along with perovskite as major phase.
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Figures 7(b) and 7(c) show XRD patterns for the glass ceramic samples 6PL5B730S and 5PL5B806S. Perovskite lead strontium titanate is crystallized as the major phase. It is also observed that the secondary phase of rutile (trace amount) is also present in these glass ceramic samples except for the glass ceramic sample 6PL5B730S. In 6PL5B700S glass ceramic sample, an unidentified phase is observed in small amount in place of rutile phase.
Comparison of XRD data of these glass ceramic samples with standard data from JCPDS files for different possible phases of constituent oxides indicates that although the major phase is lead titanate or lead strontium titanate, solid solution perovskite and many minor phases form in significant proportion. The parent glasses for lead rich glass ceramic samples also show many exothermic peaks in their respective DTA plots. Since these glass ceramic samples are rich in lead, minor phases PbTi3O7 and Pb borate form. As the strontium content increases in the glass ceramic sample, the exothermic peaks corresponding to the crystallization of minor phases are suppressed. It results in the crystallization of lead strontium titanate perovskite phase predominantly. XRD patterns of these glass ceramic samples indicate the presence of the other minor phases only in trace amount. In strontium-rich compositions, rutile is mostly present in trace amount. Glass ceramic samples for all glasses were also prepared with 6 hours holding time at their respective crystallization temperatures for the major phase. XRD patterns of these glass ceramic samples with the XRD patterns of the respective glass ceramic samples, which were crystallized for 3 hours, were compared. It is found that XRD peaks of the major phase are well developed and sharp for samples obtained by heating for 6 hours. The peak intensity of minor phase/s decreases. In a few cases, there is a change in the nature of the minor phase. In case of glass ceramic samples 9PL5B695T, the minor phase is PbTi3O7 (PT), whereas in glass ceramic samples, the minor phase present is PbB2O4. The crystallization rate of PbB2O4 may be slower than that of PbTi3O7 and hence PbB2O4 minor appears on longer heat treatment.
3.3. Surface Morphology
The surface morphology of all glass ceramic samples shows fine crystallites of perovskite major phase of lead titanate and lead strontium titanate. Qualitative inspection of all these micrographs revealed that the relative content of residual glass phase was little or not significant. The coexistence of coarse and ο¬ne perovskite particles has also been observed and reported in similar lead titanate glass ceramics [18, 24]. Scanning electron micrographs of the various glass ceramic samples heat treated for 3 and 6 hours are shown in Figures 8 and 9. Figure 8 shows scanning electron micrographs of glass ceramic samples PTL5B700T, 9PL5B695T, 8PL5B726T, 7PL5B739T, and 5PL5B806T. The glass ceramic sample PTL5B700T is found to be composed of interconnected fine crystallites of lead titanate (PbTiO3), which are dispersed in the glassy matrix (Figure 8(a)). XRD studies confirm that the fine crystallites are of PbTiO3, which is the major crystalline phase. In general, the white region in the microstructure represents the major crystalline phase/secondary phase, while the black region depicts the residual glass in all scanning electron micrographs. For the glass ceramic sample PTL5B700S, there is a change in the morphology of the crystallites of the major phase, PbTiO3, (Figure 9(a)). These crystallites are found to have round shape and are agglomerated. The size of the crystallites is higher in comparison to that for 3-hour of heat-treated glass ceramic sample. Figure 8(b) shows the scanning electron micrograph of the glass ceramic sample 9PL5B700T. The crystallites size is in the submicron range. The volume fraction of the residual glass is small. Some smaller whitish grains represent the secondary phase of PbTi3O7. Figures 8(c) and 9(b) are the scanning electron micrographs for the glass ceramic samples 8PL5B726T and 8PL5B726S, respectively. The glass ceramic sample 8PL5B726T shows dispersion of the major perovskite phase in the glassy matrix. As the crystallization time is increased from 3 to 6 hours, well-separated crystallites of major phase are formed as shown in Figure 9(b).
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It is confirmed from the study of XRD and SEM that the 6-hour heat-treatment schedule is not suitable for the crystallization of glass sample 7PL5B (x = 0.7). The scanning electron micrograph of glass ceramic sample 6PL5B730S shown in Figure 9(d) shows crystallization of submicrometer grains of perovskite phase. The shiny white region represents the trace amount of the unidentified phase.
Figures 8(e) andββ9(e) show the scanning electron micrographs of chemically etched surfaces of glass ceramic samples 5PL5B806T and 5PL5B806S, which are obtained by the crystallization of the glass 5PL5B at 806Β°C for 3 and 6 hours of heat treatment schedule. The large amounts of rutile (TiO2) are crystallized on the upper side of the perovskite titanate major phase, while for the 6-hour heat-treated glass ceramic sample 5PL5B806S, the trace amount of rutile is distributed inside the glassy matrix.
4. Discussion
There is a shift in different XRD peaks positions of major perovskite phase with changing lead to strontium ratio in the compositions of the base glasses. The position of XRD peaks for various glass ceramic samples shifts systematically with composition, x or Pb/Sr ratio. The lattice parameters ββcββ and ββaββ and the axial ratio (c/a) of the perovskite phase continuously decrease as the concentration of SrO increases in the glass. In general, XRD patterns of the glass ceramic samples rich in lead with x = 1.0 to 0.5 indicate the formation of tetragonal crystals similar to lead titanate. The shift in the XRD peak positions and hence the resulting changes in the lattice parameters from that of undoped PbTiO3 ceramics could be due to two factors: (i) formation of PbTiO3 solid solution with SrTiO3 (ii) and strain due to crystal clamping. Since both of these effects may have been present, the crystal phase developed in these compositions cannot be identified unambiguously through room temperature XRD techniques. Some other characterization techniques have to be adopted to confirm the composition of crystallites.
5. Conclusions
Differential thermal analysis (DTA) patterns show more than one peak in the lead-rich glass compositions. These peaks are sharp. Doping of La2O3 affects the crystallization behavior and dielectric properties of the glass ceramic samples. The addition of La2O3 promotes the crystallization of major phase and retards the crystallization of minor phases. X-rays diffraction patterns of lead-rich glass ceramic samples show that the major phase of PbTiO3 or perovskite (Pb,Sr)TiO3 and a trace amount of pyrochlore phase of PbTi3O7. Crystalline phase of all the glass ceramic sample of glasses with x β€ 0.5 was found to have tetragonal structure. Surface morphology of the fined crystalline phase is observed uniform and well interconnected in residual glassy matrix.
Acknowledgments
The authors are highly grateful to Dr. Chandra Prakash and Dr. O. P. Thakur of the Electroceramic Division, Solid State Physics Laboratory, Delhi (India) for providing access to the gold coating facility for the glass ceramic samples. Financial support from Defence Research and Development Organization (India) is gratefully acknowledged.