Sanjay R. Dhage

Microwave hydrothermal preparation of submicron-sized spherical magnetite (Fe3O4) powders

Submicron-sized (0.15–0.2 μm) spherical agglomerates of magnetite (Fe3O4) powders have been prepared successfully by microwave hydrothermal (MH) reaction of ferrous sulphate and sodium hydroxide in the temperature range of 90–200 °C. X-ray powder diffraction patterns of all these powders indicated that the product is single-phase magnetite with cubic spinel structure having lattice parameter, a0=8.392 A. The Mossbauer spectra of these powders indicated that stoichiometric Fe3O4 particles are obtained only when molar ratio of Fe/NaOH≥0.133 is maintained in the solution. It is observed that Fe/NaOH ratio is an important parameter for the controlled oxidation of ferrous salts in alkaline media under MH condition to produce stoichiometric Fe3O4. Further, the kinetics of MH synthesis is one order faster than the reported conventional hydrothermal (CH) synthesis. The value of saturation magnetization M=70 emu/g is obtained in case of stoichiometric Fe3O4. However, when ferric salt is treated in alkaline medium single-phase α-Fe2O3 is obtained under the MH conditions of 200 °C (194 psi).

Synthesis of ultrafine TiO2 by citrate gel method

A gel was formed when a mixture of TiOCl{sub 2} and citric acid is heated on a water bath. An ultrafine powder of TiO{sub 2} in the anatase phase is formed, when the gel was decomposed at 623 K. The anatase phase is converted into rutile phase on annealing at higher temperatures >773 K. These powders are characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface area measurements. The average particle size obtained for anatase phase is 3.5 nm whereas it is 45 nm for rutile phase.

Influence of various donors on nonlinear I-V characteristics of tin dioxide ceramics

The influence of different donors such as Nb, Ta, Sb, and V on nonlinear current (I)-voltage (V) relations of tin dioxide ceramics is investigated. The room-temperature resistivity (ρ), nonlinear coefficient (α), breakdown electric field (EB) and barrier height (ΦB) are calculated as a function of donor content. The present system contains fewer additives as compared to ZnO varistor wherein small quantities of variety of metal oxides are added. Optimum doping of antimony-doped samples shows promising behavior for low-voltage varistor application.

Effect of variation of molar ratio (pH) on the crystallization of iron oxide phases in microwave hydrothermal synthesis

Abstract Microwave hydrothermal (MH) route was employed to synthesize various phases of iron oxide powders by using ferrous sulphate and sodium hydroxide as starting chemicals. All the MH reactions were carried out under the identical MH conditions of 190 °C, 154 psi, 30 min by varying molar ratio (MR) of FeSO 4 /NaOH (i.e. pH variation) from 0.133 to 4.000 in the solution. It was found that the variation of MR of FeSO 4 /NaOH has a profound effect on the crystallization of various phases of iron oxides under identical MH processing conditions. The stoichiometric, submicron-sized (0.15–0.2 μm), spherical agglomerates of Fe 3 O 4 powders were obtained if MR of FeSO 4 /NaOH equal to 0.133 (pH≥10) was maintained. On the other hand, non-stoichiometric Fe 3 O 4 powders were obtained for all the higher MR of FeSO 4 /NaOH between 0.133 and 4.00 (6.6 4 /NaOH was equal to 4.00 (pH≈6.6), a varied distribution of shapes and sizes (1–5 μm) of agglomerates of α-Fe 2 O 3 powders were produced. Various analytical tools such as XRD, Mossbauer spectroscopy, scanning electron microscopy (SEM) and EDAX were used to characterize the MH-derived powders.

Co-precipitation technique for the preparation of nanocrystalline ferroelectric SrBi2Ta2O9

A simple co-precipitation technique had been successfully applied for the preparation of pure ultrafine single phase SrBi{sub 2}Ta{sub 2}O{sub 9}. Ammonium hydroxide was used to precipitate Sr{sup 2+}, Bi{sup 3+} and Ta{sup 5+} cations simultaneously. No pyrochlore phase was found while heating powder at 800 deg. C and pure SrBi{sub 2}Ta{sub 2}O{sub 9} phase was found to be formed by X-ray diffraction. Particle size and morphology was studied by scanning electron spectroscopy. Ferroelectric hysteresis loop parameters of these samples were also studied.

Influence of lanthanum on the nonlinear I–V characteristics of SnO2: Co, Nb

Abstract Small quantities of rare earth element such as La or Y were incorporated into the SnO 2 -based primitive varistor compositions. This leads to increase in nonlinear coefficient, α from 9 to 30 and the breakdown voltage increases from 120 to 220 V/mm. The enhancement in α was expected to arise from the possible segregation of lanthanide ion due to its larger ionic radius to the grain boundaries, thereby modifying its electrical characteristics. Furthermore, the dopants such as La may help in adsorption of O′ and O″ at the grain boundaries [J. Appl. Phys. 84 (1998) 3700], thereby improving grain boundary barrier characteristics.

SYNTHESIS OF NANOCRYSTALLINE TIO2 BY TARTARATE GEL METHOD

A gel was formed when a mixture of TiOCl2 and tartaric acid was heated on a water bath. Ultrafine powders of TiO2 in the anatase phase were formed, when the gel was decomposed at 623 K and the mole ratio of tartaric acid to titanium was 2. The anatase phase was converted into rutile phase on annealing at higher temperatures, > 773 K. When initial ratio of titanium to tartaric acid was < 2, the decomposition of gel leads to the formation of mixed phases of rutile and anatase. However, pure rutile phase was not formed by the decomposition of gel for any ratio of tartaric acid and titanium. These powders were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM) and surface area measurements. The average particle size obtained for anatase phase was 3 nm whereas it was 30 nm for rutile phase. Raman scattering experiments were also performed to confirm both anatase and rutile phases.

Synthesis of nanocrystalline SnO2 powder at 100°C

A simple gel to crystal conversion route has been followed for the preparation of nanocrystalline SnO2 at 80–100°C under refluxing conditions. Freshly prepared stannic hydroxide gel is allowed to crystallize under refluxing and stirring conditions for 4–6 h. Formation of nano crystallites of SnO2 is confirmed by X-ray diffraction (XRD) study. Transmission electron microscopic (TEM) investigations revealed that the average particle size is 30 nm for these powders.

Chemical co-precipitation of mixed (Pb+Ti) oxalates precursor for the synthesis of PbTiO3 powders

Abstract A chemical co-precipitation route is developed to precipitate mixed oxalates (PbC 2 O 4 +TiOC 2 O 4 .2H 2 O) at room temperature. In this synthesis route, an anionic TiO(C 2 O 4 ) 2 −2 species obtained by dissolving potassium titanyl oxalate (KTO) (0.01 M) stable over narrow pH range are destabilized with the addition of 0.01 M solution of lead chloride (PbCl 2 ) to produce insoluble TiOC 2 O 4 .2H 2 O and oxalic acid. The in situ generated oxalic acid during destabilization process reacts with Pb 2+ ions to precipitate lead oxalate (PbC 2 O 4 ). The mixed oxalates on pyrolysis in air at 750 °C/4 h produced submicron sized (0.13–0.15 μm) stoichiometric PbTiO 3 (PT) powders having tetragonal symmetry (with c / a ratio ∼1.063) which form agglomerates of size 0.5–5 μm. Various physico-chemical techniques were used to characterize the as-dried mixed oxalates precursor and PbTiO 3 powders . All these experimental results related to synthesis and characterization of these powders are presented in this paper.

Photoluminescence properties of thermally stable highly crystalline CdS nanoparticles

Thermally stable and highly crystalline CdS nanoparticles were obtained via chemical bath method. The optical properties of CdS nanocrystals were characterized by ultraviolet-vis and photoluminescence spectroscopy. Improvement in the photoluminescence properties of the synthesized CdS nanocrystals was observed. This improvement is believed to be due to highly crystalline CdS nanoparticles which may reduce the local surface-trap states. The CdS nanoparticles were characterized by x-ray powder diffraction (XRD), thermo gravimetric analysis (TGA/DTA) and transmission electron microscopy (TEM).