T.R.N. Kutty

Direct precipitation of lead zirconate titanate by the hydrothermal method

Direct precipitation of fine powders of lead zirconate titanate (PZT) in the complete range of solid solution, is investigated under hydrothermal conditions, starting from lead oxide and titania/zirconia mixed gels. The perovskite phase is formed in the temperature range of 165 – 340°C. Sequence of the hydrothermal reactions is studied by identifying the intermediate phases. The initial formation of PbO: TiO2 solid solution is followed by the reaction of the same with the remaining mixed gels giving rise to X-ray amorphous PZT phase. Further, through crystallite growth, the X-ray crystalline PZT is formed. This method can be extended for the preparation of PLZT powder as well. The resulting powders are sinterable to high density ceramics.

Hydrothermal preparation of Ba(Ti,Zr)O3 fine powders

Abstract Fine powders consisting of aggregated submicron crystallites of Ba(Ti,Zr)O 3 in the complete range of Ti/Zr ratios are prepared at 85–130°C by hydrothermal method, starting from TiO 2 + ZrO 2 · xH 2 O mixed gel and Ba(OH) 2 solution. The products obtained below 110°C incorporate considerable amounts of H 2 O and OH − within the lattice. As-prepared BaTiO 3 is cubic and converts to tetragonal phase after the heat treatment at 1200°C, accompanied by the loss of residual hydroxyl ions. TEM investgations of the growth features show a transformation of the gel to the crystallite. Ba 2+ ions entering the gel produce chemical changes within the gel, followed by dehydration, resulting in a cubic perovskite phase irrespective of Ti/Zr. The sintering properties of these powders to fine-grained, high density ceramics and their dielectric properties are presented.

Ultrafine powders of SrTiO3 from the hydrothermal preparation and their catalytic activity in the photolysis of water

Ultrafine powders of SrTiO3 are prepared at 100–150°C by the hydrothermal method, starting from TiO2·xH2O gel and Sr(OH)2 and H2O-isopropanol mixed solvent as the medium, The X-ray diffractograms of the powder show line broadening. The minimum crystallite size obtained ranges from 5 to 20nm with 20% H2O-80% C3H7OH as the reaction medium, as estimated from X-ray half-peak widths and TEM studies. The electron diffraction results indicate high concentration of lattice defects in these crystallites. The optical spectra of the particle suspensions in water show that the absorption around the band gap is considerably broadened, together with the appearance of maxima in the far ultraviolet. Aqueous suspensions of SrTiO3 powders, as such, do not produce H2 or O2 on UV irradiation. After coating with rhodium, H2 and O2 are evolved on illumination. However, the turn over number of O2 is lower than the stoichiometrically expected values from the corresponding values of H2. No correlation of the photocatalytic activity with surface area is observed. The activity of Rh-SrTiO3 slowly deteriorates with extended period of irradiation.

EPR evidence for activation of trap centers in PTCR BaTiO3 ceramics

Donor-doped BaTiO/sub 3/ ceramics, exhibiting PTCR, showed an EPR signal with g=1.997 which acquires high intensity above T /SUB C/ . This is indicative of activation of the corresponding defect centers, possibly V /SUB Ba./ . The intensity of g=1.997 varies with Ba/Ti ratio, rate of cooling from the sintering temperature and the addition of TiO/sub 2/ as sintering agent. The signal intensity decreases with increase in grain size and is weak in La-doped BaTiO/sub 3/ single crystals. Therefore the concentration of V /SUB Ba/ is more around the grain boundaries than that in the bulk of grains. Activated trap centers around T /SUB C/ also arises from background impurities; those with change of electronic state: M /SUP n+1/ +e = M /SUP n+/ (eg:-Mn) and those associated with oxygen vacancies: M /SUP p+/ -V /SUB O/ (e) (eg:-Fe/sup 3 +/). Activation of these trap centers are not dependent on the disappearance of spontaneous polarization but to the structural changes during phase transformation. The trapped charge carriers are not available for conduction and hence the increase in electrical resistivity.

BaSnO3 fine powders from hydrothermal preparations

Attempts to prepare BaSnO3 by the hydrothermal method starting from SnO2·xH2O gel and Ba (OH)2 solution in teflonlined autoclaves at 150–260°C invariably lead to the formation of a hydrated phase, BaSn(OH)6·3H2O. On heating in air or on releasing the pressure in situ at ≈260°C, BaSN (OH)6·3H2O converts to BaSnO3 fine powder which involves the formation of an intermediate oxyhydroxide, BaSnO(OH)4. TEM studies show that particle size of the resulting BaSnO3 ranges from 0.2–0.6 μm. Solid solutions of Ba(Ti, Sn) O3 were prepared from (TiO2+SnO2)·xH2O mixed gel and Ba(OH)2 solutions. Single-phase perovskite Ba(Ti, Sn)O3 was obtained up to 35 atom % Sn. Above this composition, the hydrothermal products are mixtures of BaTiO3 (cubic) and BaSn(OH)6·3H2O which on heating at ≈260°C give rise to BaTiO3+BaSnO3. Annealing at 1000°C results in monophasic Ba(Ti, Sn)O3, in the complete range of Sn/Ti. Formation of the hydrated phase is attributed to the amphoteric nature of SnO2·xH2O gel which stabilises Sn(OH)62− anions under higher H2O-pressures and elevated temperatures. The sintering characteristics and dielectric properties of ceramics prepared from these fine powders are presented.

Luminescence of Eu2+ in strontium aluminates prepared by the hydrothermal method

Abstract Preparation of Sr-aluminates in fine powder form, with Al/Sr ratio varying from 2 to 12 has been attempted by the hydrothermal method, starting from Al 2 O 3 .xH 2 O gel containing europium and Sr(OH) 2 at 240 – 250°C. The products are annealed at 850 – 1150°C in N 2 + H 2 atmosphere to yield, efficient blue to green luminescent phosphors. The predominant phase isolated belongs to the family of ‘stuffed’ tridymite-nepheline structures, having the general formula, Sr n Al 2 O 3+n where n ⩽ 1. With decreasing n-value, the relative intensity of the 405 nm emission band increases in preference to that of the 510 nm emission. The excitation spectra as well as the EPR results indicate the presence of Eu 2+ in the vacancy-associated nonequivalent sites. In presence of H 3 BO 3 in the hydrothermal medium, aluminoborates such as SrAl 5 BO 10 , Sr 2 Al 2 B 2 O 8 and Sr 3 Al 6 B 8 O 24 are produced, whose only emission maximum around 400 nm remains as a band even at LN 2 temperature.

Wet chemical formation of nanoparticles of binary perovskites through isothermal gel to crystallite conversion

Coarse BO2·xH2O (2 < x < 80) gels, free of anion contaminants react with A(OH)2 under refluxing conditions at 70i?½100°C giving rise to crystallites of single phased, nanometer size powders of ABO3 perovskites (A = Ba, Sr, Ca, Mg, Pb; B = Zr, Ti, Sn). Solid solutions of perovskites could be prepared from compositionally modified gels or mixtures of A(OH)2. Donor doped perovskites could also be prepared from the same method so that the products after processing are often semiconducting. Faster interfacial diffusion of A2+ ions into the gel generates the crystalline regions whose composition is controllable by the A/B ratio as well as the A(OH)2 concentration.

Activation of trap centres in PTC BaTiO3

Abstract EPR spectra of lanthanide-doped high-purity BaTiO 3 ceramics, exhibiting a PTC effect, showed two signals at room temperature with g = 1.963 of medium and g = 1.997 of very low intensity. Above T c , the g = 1.997 signal acqured high intensity indicating activation of the corresponding defect centres, possibly VBa, causing diminution of charge carriers and hence the PTC effect. The signal g = 1.963 arises from shallow donor-type centres, possibly trapped electrons, resonating between an oxygen vacancy and two of the adjoining titanium ions.

Low voltage varistors based on SrTiO3 ceramics

Varistors exhibiting nonlinear resistance at low voltages of 4.5 to

Effect of valence fluctuations in a sites on the transport properties of La1-xRxMnO3 (R = Ce, Pr)

30 V mm^{-1}