V. S. Tiwari

Diffuse ferroelectric transition and relaxational dipolar freezing in (Ba,Sr)TiO3

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.

Effect of powder synthesis procedure on calcium site occupancies in barium calcium titanate: A Rietveld analysis

(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.

The influence of a powder processing technique on chemical homogeneity and the diffuse phase transition behaviour of Ba0.9Ca0.1TiO3 ceramics

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 substitution 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.

Preparation of nearly single phase 2223 in Bi1.6Pb0.4Sr2Ca2Cu3Oy composition by a semi-wet route using Pb0.2SrCa(CO3)2.2 precursor

Abstract 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.

Neutron diffraction study of ferroelectric Ba0.88Ca0.12TiO3

Abstract Profile analysis of neutron diffraction data on Ba 0.88 Ca 0.12 TiO 3 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.

Developments in ferroelectric ceramics for capacitor applications

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.

Diffuse phase transition in (Ba, Ca) TiO3 ceramics

Abstract Dielectric behaviour of (Ba, Ca) TiO3 ceramics 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 BaTiO3 matrix, 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.

The effect of semi-wet synthesis on the formation of the 2223 phase in Bi1.6Pb0.4Sr2Ca2Cu3Oy composition

Abstract The superconducting composition Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O y has been synthesized by a novel semi-wet route involving solid-state thermochemical reactions between two precursors obtained by first coprecipitating Bi, Pb, Cu and Ca, Sr separately. It is shown that this route can yield samples with very high volume fraction of the 2223 phase whereas under identical heat treatment conditions, the dry route yields samples which are essentially 2212.

On the formation of the 2223 phase in the BiPbSrCaCuO system using Pb0.2SrCa(CO3)2.2 precursors

Abstract Formation of 2223 phase showing T c (R=0) of 120 K in a nominal composition Bi 1.6 Pb 0.4 Sr 2 Ca 2 Cu 3 O y synthesized using a semi-wet route involving Pb 0.2 SrCa(CO 3 ) 2.2 precursor is reported. A comparison of this semi-wet route with the conventional dry route in the nucleation and growth of the 2223 phase is presented.

A semi-wet route to the synthesis of YBa2Cu3O7-y ceramics

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 mu 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.