Hari S. Potdar

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).

Highly sensitive and fast responding CO sensor based on Co3O4 nanorods

Co(3)O(4) nanorods (diameters approximately 6-8 nm and lengths approximately 20-30 nm) were synthesized for the first time through a simple co-precipitation/digestion method by calcination of cobalt hydroxyl carbonate in air and their CO gas sensing properties were investigated. The Co(3)O(4) nanorods exhibited outstanding gas sensing characteristics such as, higher gas response (approximately 6.55-50 ppm CO gas at 250 degrees C), extremely rapid response (approximately 3-4s), fast recovery (approximately 5-6s), excellent repeatability, good selectivity and lower operating temperature (approximately 250 degrees C). Furthermore, the Co(3)O(4) nanorods are able to detect up to 5 ppm for CO with reasonable sensitivity (approximately 3.32) at an operating temperature 250 degrees C and they can be reliably used to monitor the concentration of CO over the range (5-50 ppm). The experimental results clearly demonstrate the potential of using the Co(3)O(4) nanorods as sensing material in the fabrication of CO sensors. Plausible CO sensing mechanism of the Co(3)O(4) nanorods is also discussed.

Origin of high room temperature ferromagnetic moment of nanocrystalline multiferroic BiFeO3

Single phase nanocrystalline BiFeO3 of average crystallite size ∼25 nm with very high magnetization at room temperature is synthesized by an autocombustion method. Magnetic measurements above room temperature show deviation between field cooled and zero field cooled magnetization below 645 K, the Neel temperature (TN) of the bulk material, indicating intrinsic nature of ferromagnetism. However, observation of a broad magnetic transition above TN of BiFeO3 and extended up to 800 K suggests the presence of Fe3O4 as a possible magnetic impurity phase. Evidence for the presence of Fe3O4 is obtained from detailed analysis of the powder x-ray diffraction pattern.

Synthesis of yttria stabilized cubic zirconia (YSZ) powders by microwave-hydrothermal route

Yttria stabilized cubic zirconia (YSZ) ceramic powders are prepared by microwave-hydrothermal (MH) treatment (200 ◦ C, 194 psi, 30 min) using ZrO(NO3)2·xH2O and YCl3 as starting precursors and KOH as mineralizer. Solutions of ZrO(NO3)2·xH2O (0.018 M) and YCl3 (0.002 M) were mixed in appropriate proportions and were added to 1.2 M KOH solution. Resulting mixture was treated in specially designed vessels, under high pressure using microwave accelerated reaction system (MARS-5). The resultant precipitates are characterized by XRD, IR, EDAX, BET and SEM techniques. The results showed that the as-precipitated particles consisted of YSZ having cubic fluorite structure and thermal stability up to 800 ◦ C. The surface area of the microwave-hydrothermally prepared powder at 200 ◦ C (194 psi), 30 min was found to be 125 m 2 g −1 . The SEM photograph shows agglomerated YSZ particles having varied size distribution (0.2–3m). The cubic phase is obtained at a temperature/pressure and time as low as 125 ◦ C/18 psi and 5 min, respectively.

Simple chemical route for the quantitative precipitation of barium–strontium titanyl oxalate precursor leading to Ba1−xSrxTiO3 powders

Abstract Barium–strontium titanyl oxalate (BSTO) [Ba 1− x Sr x TiO(C 2 O 4 ) 2 ·4H 2 O with x =0.25 precursor powders] were prepared successfully by using the simple chemical exchange reaction between oxalotitanic acid (HTO) [H 2 TiO(C 2 O 4 ) 2 ·2H 2 O], barium hydroxide and strontium nitrate at room temperature. Initially, oxalotitanic acid was prepared by using the reaction between 0.1 M titanium tetrabutoxide and 0.2 M oxalic acid in isopropanol (IPA). Barium hydroxide and strontium nitrate were added directly to this solution in stoichiometric amounts. The required solubility achieved only after addition of distilled water dropwise leads to the controlled precipitation of BSTO molecular precursor. To maintain the equilibrium (pH conditions), more and more Ba(OH) 2 and Sr(NO 3 ) 2 solids get dissolved and reaction continues. The kinetics of exchange reaction is governed by the rate of addition of water. The pyrolysis of BSTO at 750 °C/4 h in air produced the Ba 1− x Sr x TiO 3 (BST) powders. The characterization studies were carried on the as-dried and calcined powders by various physicochemical techniques viz. microanalysis, chemical analysis, DTA/TGA, XRD, IR, scanning electron microscopy (SEM), etc. It revealed that the BST powders are cubic, stoichiometric, highly pure with yield>99% and agglomerated nature (size: 0.5–2 μm).

LOW-TEMPERATURE SYNTHESIS OF ULTRAFINE BARIUM TITANATE (BATIO3) USING ORGANOMETALLIC BARIUM AND TITANIUM PRECURSORS

Nano size (< 200 nm) barium titanate was synthesized using barium acetate and potassium titanyl oxalate as precursors. Aqueous (0.01 M) potassium titanyl oxalate was added dropwise to a 0.01 M aqueous barium acetate solution with continuous stirring at room temperature to precipitate barium titanyl oxalate (BTO). The precipitate, after careful washing with distilled water several times, was dried in an oven at 70°C and calcined in air at 550°C/6 h to produce spherical, homogeneous, stoichiometric powder of BaTiO3 < 200 nm in size. The calcined powder was characterized using different techniques such as XRD, DTA/ TG/DTG, SEM/TEM and IR techniques.

Synthesis of nanosized Ce0.75Zr0.25O2 porous powders via an autoignition: glycine nitrate process

Abstract The GNP route has been successfully employed to generate nanosized, porous, stoichiometric, homogeneous Ce 0.75 Zr 0.25 O 2 powders. In this route, a precursor solution is prepared by mixing glycine with an aqueous solution of mixed (Ce–Zr) metal–nitrates in their stoichiometric ratios. Initially, the glycine mixed precursor solution was heated in a petri dish to evaporate the excess water to form a viscous semitransparent material. Then the petri dish was kept in a 5-l capacity beaker covered with a metallic mesh and then the temperature was increased slowly to 190–200 °C to autoignite the material. The combustion was self-sustaining and very rapid, producing yellowish coloured powders. The as-prepared powders were nanosized (20–35 nm), having a spherical shape, and were crystallized in cubic fluorite structure with lattice parameter a 0 =5.38 A. The powders showed a very wide pore size distribution in the range of 20–2500 A as is shown by mercury porosimetry measurements with surface area ≈40.0 m 2 /g. No change in crystalline phase was observed even after giving heat treatments to as-prepared powders up to 1000 °C for 4 h in air .The SEM/TEM studies on these powders confirmed their nanosized nature with porous structure.

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.

Simplified chemical route for the synthesis of barium titanyl oxalate (BTO)

A simple chemical route is developed to obtain an important molecular precursor (leading to barium titanate powders), namely barium titanyl oxalate (BTO) with nearly theoretical yield. In this route, 0.1 M alcoholic solution of titanium tetrabutoxide is reacted with 0.1 M alcoholic solution of oxalic acid to form titanyl oxalate (TiOC2O4). This was further converted to soluble ammonium titanyl oxalate (ATO); (NH4)2TiO(C2O4)2.H2O by reacting with an equimolar aqueous solution of ammonium oxalate (pH 4.25). Instead of using available barium salts (halide/nitrate/acetate) solution, a modified barium precursor solution with controlled pH (=4.2) is prepared for carrying out cation-exchange reaction with ATO. For this purpose the modified Ba-precursor solution is freshly prepared by partial neutralization of a 0.1 M aqueous solution of barium hydroxide with controlled addition of dilute HNO3 (1.1 M) solution. The pH of this solution is the same as that of the ATO solution. The exchange reaction between equimolar ATO and modified Ba-precursor solution precipitated barium titanyl oxalate (BTO) with a quantitative yield at room temperature. Submicron-sized, stoichiometric, pure BaTiO3 powders were obtained by the controlled pyrolysis of BTO in air. The present communication deals with the detailed analysis of this simplified method in producing BTO/BT powders and their characterization employing various physicochemical techniques.

A self-sustaining acid-base reaction in semi-aqueous media for synthesis of barium titanyl oxalate leading to BaTiO3 powders

A self-sustaining controlled acid–base reaction between oxalotitanic acid [H2TiO(C2O4)2] (HTO) and barium hydroxide [Ba(OH)2� 8H2O] at room temperature is utilized for the quantitative precipitation of barium titanyl oxalate (BTO). For this purpose, an intermediate soluble oxalotitanic acid precursor is generated by reacting 0.1 M solution of titanium tetrabutoxide in isopropanol (IPA) with 0.2 M solution of oxalic acid in IPA. Equimolar suspended Ba(OH)2 particles in IPAwere mixed with the above-mentioned oxalotitanic acid solution. The self-sustaining acid–base reaction between Ba(OH)2 and H2TiO(C2O4)2 was initiated only when necessary amount of water was added. This shift from non-aqueous to semi-aqueous condition allows a required solubility of Ba(OH)2, which makes Ba 2+ ions available for the exchange reaction. The solubility of Ba(OH)2 and its dissociation governs the kinetics of the reaction leading to the precipitation of BTO with yield z99%. The controlled pyrolysis of BTO at 750 jC/4 h in air-produced agglomerated cubic BaTiO3 powders having spherical particles with size c100 nm. These powders are sintered in the form of pellets at 1300 jC/4 h to obtain compacts with density c95%. These compacts showed dielectric constant eRT=1680 (tandV2%), emaxc7780 at TC=121 jC. D 2002 Elsevier Science B.V. All rights reserved.