Browsing by Author "V.S. Tiwari"

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A semi-wet route to the synthesis of YBa2Cu3O7-y ceramics
(1989) Dhananjai Pandey; V.S. Tiwari; A.K. Singh
Advantages of a semi-wet route to the synthesis of YBa2Cu3O7-y employing coprecipitation of yttrium and barium as precursor carbonates are presented. It is shown that the precursor carbonate is an intimate mixture of fine barium carbonate and amorphous yttrium carbonate particles. YBa2Cu3O7-y powders prepared under identical conditions by the conventional dry route and the present semi-wet route are analysed for compositional inhomogeneities and coherently scattering domain size (D), using principles of X-ray line profile analysis. It is shown that unlike the powders prepared by the conventional method, which contain compositional inhomogeneities and possess domains of about 2200 AA, powders obtained under identical conditions by the semi-wet route are free from compositional inhomogeneities and possess larger domain size (around 4000 AA). Since the actual particle size in both the cases is several μ m, the loss of coherency in each particle is attributed to the presence of twin interfaces. Better compositional homogeneity and lower density of twin interfaces in specimens prepared by the semi-wet route are believed to be responsible for the improved superconducting behaviour of ceramics prepared by the present method. © 1989 IOP Publishing Ltd.
Characterization of ferroelectric and superconducting ceramics prepared from precursor carbonates
(Springer India, 1989) D. Pandey; V.S. Tiwari; A.K. Singh; Sangeeta Chaudhry
The advantages of a new semi-wet method for the synthesis of (Ba, Ca)TiO3 and YBa2Cu3O7 - y powders from precursor carbonates are described. The precursor carbonates are prepared using chemical coprecipitation technique under controlled pH conditions to ensure uniform distribution of Ba+2/Ca+2 and Y+3/Ba+2 ions. It is shown that the powders synthesized by this route possess better chemical homogeneity as compared to those obtained by the conventional ceramic technique involving calcination of a mixture of BaCO3, CaCO3, TiO2 and Y2O3, BaCO3, CuO powders. The behaviour of the ferroelectric (Ba, Ca)TiO3 and the superconducting YBa2Cu3O7 - y ceramics prepared from powders obtained by our method are found to be markedly different from those known for the conventionally prepared ceramics. © 1989 Indian Academy of Sciences.
Crystallographic and Microstructural Aspects of the Superconductor YBa2Cu3O7-y —An Overview
(1990) Dhananjai Pandey; V.S. Tiwari; A.K. Singh; V.K. Wadhawan; M.S. Somayazulu
Crystal structures and structural inhomogeneities observed in YBa 2 Cu 3 0 7 _, are reviewed. It is brought out that a proper understanding of the nature of the structural inhomogeneities is essential for exploiting the technological potential of this material. The need for an adequate characterization of specimens used for experiments is emphasized. Practical implications of the ferroelastic nature of the material are discussed. © 1990, Taylor & Francis Group, LLC. All rights reserved.
Developments in ferroelectric ceramics for capacitor applications
(Springer India, 1992) Dhananjai Pandey; A.P. Singh; V.S. Tiwari
A brief overview of the materials and processes for making ceramic capacitors based on BaTiO3 and relaxor ferroelectric compositions is presented with special emphasis on more recent developments. © 1992 Indian Academy of Sciences.
Diffuse ferroelectric transition and relaxational dipolar freezing in (Ba,Sr)TiO3
(1995) V.S. Tiwari; N. Singh; D. Pandey
The effect of Sr2+ substitution on the structure and dielectric behaviour of BaTiO3 is reported. It is shown that the smearing of the variation of dielectric constant with temperature in such samples is not due to compositional inhomogeneities. Observation of relaxational dipolar freezing below the hump temperature indicates the similarity of diffuse ferroelectric transition and orientational glass behaviour.
Diffuse Phase Transition in (Ba, Ca) TiO3Ceramics
(1987) Dhananjai Pandey; V.S. Tiwari; T.B. Singh; Pandey Lakshman; Om Parkash; P. Ramachandrarao
Dielectric behaviour of (Ba, Ca) TiO3ceramics prepared by conventional and a novel technique has been studied as function of temperature and frequency. It is shown that by ensuring uniform distribution of Ca2+ions in the BaTiO3matrix, the transition from the ferroelectric to the paraelectric phase becomes more diffuse and is free from any relaxational effects at least up to 10 KHz. It is suggested that the relaxor behaviour of ceramics exhibiting diffuse phase transitions is due to the limitations of the processing techniques and is not the intrinsic property of the material. © 1987, Taylor & Francis Group, LLC. All rights reserved.
Effect of heavy atom disorder on the intensities of powder xrd lines of YBa2Cu3O7-y
(1990) V.S. Tiwari; Sangeeta Chaudhry; Dhananjai Pandey
It is shown that heavy atom disorder resulting from Y↔ Ba, Ba ↔ Cu and Y↔Cu interchanges due to local stoichiometric constraints can cause significant changes in the intensities of X-ray powder diffraction lines of YBa 2 Cu 37 O 7-y. A comparison of the theoretically predicted intensities with published patterns for specimens prepared by the conventional dry route reveals that Ba↔Cu interchanges involving copper atoms in the CuO 2 planes can occur quite frequently and may be mistaken for (00l) texture. © 1990, Taylor & Francis Group, LLC. All rights reserved.
Effect of powder synthesis procedure on calcium site occupancies in barium calcium titanate: A Rietveld analysis
(1993) P.S.R. Krishna; Dhananjai Pandey; V.S. Tiwari; R. Chakravarthy; B.A. Dasannacharya
(Ba,Ca)TiO3 samples with the ratio of (Ba+Ca) to Ti as unity but prepared by (i) a semiwet route involving (Ba,Ca)CO3 solid solution precursors and TiO2 and (ii) the conventional dry route involving BaCO3, CaCO3, and TiO2 powders, exhibit diffuse and BaTiO3-type sharp ferroelectric phase transitions, respectively. Rietveld analysis of neutron powder diffraction data from the two types of samples shows that, whereas in the samples prepared by the semiwet route, Ca2+ exclusively occupies the Ba2+ site, nearly half of the substituted Ca2+ occupies the Ti4+ site in the case of samples prepared by the dry route. We show that Ca replacement at the Ba site leads to diffuseness whereas Ca introduction at the Ti site leads to sharpening of the transition. To our knowledge, this is the first clear demonstration of the powder synthesis procedure affecting the substitutional sites in multicomponent ceramics and this, in turn, influencing their physical behavior drastically.
Kinetics of formation of the pyrochlore and perovskite phases in sol-gel derived lead zirconate titanate powder
(Materials Research Society, 1998) V.S. Tiwari; Arun Kumar; V.K. Wadhawan; Dhananjai Pandey
Lead zirconate titanate (PZT) powder is prepared by the sol-gel method. The formation of pyrochlore and perovskite phases is investigated by high temperature x-ray diffraction (XRD) and thermal analysis techniques. The pyrochlore phase first appears in x-ray amorphous form, and then gets converted to crystalline state on annealing in air. We show that vacuum annealing of the pyrolyzed amorphous PZT gel suppresses the formation of the crystalline pyrochlore phase. This, in turn, enhances the kinetics of conversion of pyrochlore to perovskite, such that a pyrochlore-free perovskite phase can be obtained by annealing at about 500°C. On the other hand, if annealing is carried out in air, a crystalline pyrochlore phase is formed, which requires annealing temperatures higher than 600°C for transformation to the perovskite phase. These observations are explained tentatively in terms of the oxygen stoichiometry of the two phases.
Neutron diffraction study of ferroelectric Ba0.88Ca0.12TiO3
(1991) V.S. Tiwari; Dhananjai Pandey; P.S.R. Krishna; R. Chakravarthy; B.A. Dasannacharya
Profile analysis of neutron diffraction data on Ba0.88Ca0.12TiO3 samples shows that Ca can enter either Ba site exclusively or both Ba and Ti sites depending on the method of preparation. Nature of structural phase transition has been examined in samples where Ca occupies Ba site only. © 1991.
Preparation of nearly single phase 2223 in Bi1.6Pb0.4Sr2Ca2Cu3Oy composition by a semi-wet route using Pb0.2SrCa(CO3)2.2 precursor
(1991) Dhananjai Pandey; R. Mahesh; A.K. Singh; V.S. Tiwari; S.K. Kak
A new method for the synthesis of nearly single phase 2223 in Bi1.6Pb0.4Sr2Ca2Cu3Oy composition within 72 h of total heat treatment, using coprecipitates of (1) 0.2 Pb, Ca, Sr as carbonates and (2) 1.6 Bi, 3 Cu as oxalate, is reported. Samples prepared have been characterized by their crystal structure using XRD, microstructure using SEM and electrical resistivity. © 1991.
The influence of a powder processing technique on chemical homogeneity and the diffuse phase transition behaviour of Ba0 9Ca0 1TiO3 ceramics
(1989) V.S. Tiwari; D. Pandey; P. Groves
Ba0.9Ca0.1TiO3 ceramics have been fabricated using powders obtained by calcining mixtures of (i) BaCO3, CaCO3 and TiO2 and (ii) Ba0.9Ca0.1CO3 and TiO2. The precursor Ba0.9Ca0.1CO3 was prepared by a wet technique to ensure homogeneous mixing of Ba and Ca ions at the atomic level. DTA analysis of solid state thermochemical reactions in the two types of mixtures shows that the temperature of reaction completion is 140 degrees C lower for the mixture containing the precursor carbonate solid solution. XRD results indicate better chemical homogeneity for powders obtained by the new route. A powder diffraction pattern for Ba0.9Ca0.1TiO3 has been theoretically computed assuming partial infstitution of Ba2+ by Ca2+. The theoretically predicted intensities are in good agreement with those experimentally observed for powders prepared by the precursor route only. Better chemical homogeneity with respect to the distribution of Ca2+ ions leads to more pronounced diffuse phase transition in Ba0.9Ca0.1TiO3 ceramics. © 1989 IOP Publishing Ltd.
Zero resistivity at 122 K in Bi-Pb-Sr-Ca-Cu-O system
(1990) Dhananjai Pandey; R. Mahesh; V.S. Tiwari; A.K. Singh; S.K. Kak
Zero resistivity at 122 K in samples of nominal composition Bi1.6Pb0.535Sr2.675Su3Oy prepared via a semi-wet route using Pb0.2SrCa(CO3)2.2 precursors is reported. Possible reasons for the 10 K enhancement of Tc(R=0) over the commonly reported values are suggested. © 1990.