Browsing by Author "Ch. Durga Prasad"
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PublicationArticle Dielectric behaviour of glasses and glass ceramics in the system BaO-PbO-TiO2-B2O3-SiO2(Springer India, 1987) R.K. Mandal; Ch. Durga Prasad; O.M. Parkash; Devendra KumarGlasses with varying molar ratios of PbO/BaO in the system BaO-PbO-TiO2-B2O3-SiO2 were prepared keeping (BaO + PbO)/TiO2 ratio equal to one. The glasses were ceramized by two-stage heat treatment. X-ray diffraction indicates that PbTiO3 crystallizes in lead-rich glasses while BaTiO3 precipitates in barium-rich compositions. Solid solution (Ba, Pb)TiO3 does not seem to crystallize over the entire range of compositions. Simultaneous presence of PbO and BaO in the initial glass composition reduces the yield of ferroelectric phase. Dielectric properties have been interpreted in terms of microstructural features. © 1987 Indian Academy of Sciences.PublicationArticle Diffuse ferroelectric transition and relaxational dipolar freezing in (Ba, Sr)TiO3: III. Role of order parameter fluctuations(1996) Neelam Singh; Anirudh P. Singh; Ch. Durga Prasad; Dhanajai PandeyResults of a high-temperature x-ray powder diffraction study of the phase transition between cubic and tetragonal phases of (Ba1-xSrx)TiOs are presented for x = 0.08 and 0.12 which exhibit sharp and diffuse dielectric anomalies, respectively. Experimental evidence and arguments are advanced to show that the cubic and tetragonal phases coexist over a wide range of temperatures and that this coexistence is due to the fluctuation of the order parameter which is coupled electrostrictively to strains. For x = 0.08, the phase coexistence disappears below the dielectric anomaly temperature T′m, indicating the critical nature of the fluctuations. For x = 0.12, these fluctuations are non-critical since the phase coexistence persists even below T′m. The complete conversion to the tetragonal phase occurs at Ts ≈ 75 °C which is nearly 25 °C lower than T′m(= 102 ± 1 °C). It is shown that the spontaneous polarization also increases gradually below T′m and levels off at Tp ≈ 75 °C. The existence of structural and polarization anomalies well below the frequency-independent dielectric anomaly temperature T′m cannot be rationalized either in terms of the Landau-like theories for ferroelectric transitions which predict Ts = Tp = T′m or in terms of dipole glass transitions and/or relaxor ferroelectric transitions for which T′m should be frequency dependent.PublicationArticle Electrical properties of the system lanthanum lead cobalt titanium oxide(Springer India, 1987) Devendra Kumar; Ch. Durga Prasad; Om ParkashMeasurements of DC electrical resistivity and Seebeck coefficient on the perovskite system La1-x Pb x-δ Co1-x Ti x O3-δ for 0·2≤x≤0·9 have been made in the temperature range 300-800 K. AC conductivity, σ a.c, of all the samples were measured as a function of temperature (300-573 K) and frequency (1 kHz-1 MHz). DC resistivity behaviour of all the samples is similar. However, the resistivity value, which varies over 5-6 orders of magnitude, depends on both the composition x and the structure of the samples. All the samples exhibit p-type electronic conduction. The value of Seebeck coefficient, α, for samples with x≤0·5 initially increases with temperature up to a particular temperature. Above this temperature, the behaviour of α for samples with x≤0·5 and for samples with x>0·5 over the entire temperature range is similar to that of La CoO3. All the samples exhibit frequency-dependent a.c. conductivity at low temperatures. © 1987 the Indian Academy of Sciences.PublicationArticle Preparation and characterization of the system Ba1-xLaxTi1-xCoxO3 (x≤0.50)(Kluwer Academic Publishers, 1989) O.M. Parkash; Ch. Durga Prasad; Devendra Kumar[No abstract available]