M. Leoni

Barium perovskites as humidity sensing materials

The response to humidity of porous ceramics with composition BaMO3, with M=Ti, Zr, Hf or Sn, was studied. Samples were obtained by cold isostatic pressing and sintering of fine powders prepared by wet chemical synthesis. Sensitivity and response time have been obtained by electrical impedance measurements realised in different conditions of relative humidity (R.H.) at 25 � C. An increase of both capacitance and conductivity with humidity was observed in all samples with open porosity. Sensitivity increased with R.H. and decreased with frequency, indicating a major contribution from the surface of crystals exposed to water vapour, particularly in BaTiO3 where 4000% change in permittivity was registered over the range 20–80% R.H. Time response was in the typical range of capacitive humidity sensors, with fast (10–100 s) equilibration time for intermediate values of R.H. # 2001 Elsevier Science Ltd. All rights reserved.

Low-temperature aqueous preparation of barium metatitanate powders

Abstract Submicron BaTiO 3 powders have been prepared at 85°C under atmospheric pressure starting from TiCl 4 and Ba(OH) 2 in NaOH aqueous solution. Characterisation by BET, thermal analysis, differential dilatometry, X-ray diffraction and SEM/TEM microscopy has been performed. The powders, with high specific surface area, show a cubic lattice and a quasi-spherical morphology. A slight deviation from stoichiometry and a low level of residual carbonate have also been observed. Sintering tests on isostatically pressed samples without binders have been carried out. A 15 h thermal treatment at 950°C upon firing results in improved final density (≈ 96% of the theoretical density) and in very fine grain size (1·1 μm). Washing with dilute (0·1 m ) acetic or hydrochloric acid to eliminate BaCO 3 is detrimental to a high final density and gives rise to polytitanate formation.

Decomposition of Al2TiO5-MgTi2O5 solid solutions: a thermodynamic approach

The decomposition of Al1−xMgxTi1+xO5 solid solutions with x=0.0, 0.1, 0.2, 0.4, 0.5 and 0.6 was studied in the temperature range 900–1175 °C using a 250 h annealing test. As x increases from 0–0.2 there is a strong stabilizing effect and the decomposition temperature decreases from 1280 °C (Al2TiO5) down to ≈ 1125 °C. For 0.2⩽x⩽0.5 the decomposition temperature does not decrease further. For x=0.6 no decomposition was observed. For x⩽0.5 decomposition is complete or almost complete at 1000 °C; at 900 °C transformation is kinetically hindered and solid solutions with x=0.2 and 0.4 are unaffected by the thermal treatment. A relationship between the decomposition temperature and the parameter x has been derived using the regular solution model to describe the Al2(1−x)MgxTi(1+x)O5 solid solution.

Aqueous synthesis and sintering of zirconium titanate powders for microwave components

Zirconium titanate (ZT–ZrTiO4) is a well-known ferroelectric material commonly used as a dielectric in microwave devices due to its high permittivity at microwave frequencies. Reactive precursors of ZT in form of nanopowders have been prepared in a very simple way by hydrolysis of an aqueous solution of TiCl4 and ZrOCl2 using ammonia. ZT single-phase powder has been obtained by high temperature reaction during calcination at 1200 � C. a-PbO2-type structure was stabilised by air quenching to room temperature. Sinterability was investigated by constant heating-rate and isothermal sintering. High-density ceramics were obtained by firing performed at different temperatures and prototypes of substrates for microwave devices were prepared. # 2001 Elsevier Science Ltd. All rights reserved.

The effect of MgAl2O4 on the formation kinetics of Al2TiO5 from Al2O3 and TiO2 fine powders

The formation of Al2(1−x)MgxTi(1+x)O5 solid solutions from Al2O3-TiO2-MgAl2O4 powder mixtures of ≈1 μm particle size and moderate purity has been studied at 1300°C for different final composition values: x=0 (“pure” Al2TiO5), 10−3, 10−2 and 10−1. Analysis of the kinetic data and microstructural observation indicates that MgAl2O4 affects the mechanism of Al2TiO5 formation by providing active nuclei for the growth of the new phase. These nuclei are probably constituted by Mg0.5AlTi1.5O5, i.e. the equimolar Al2TiO5-MgTi2O5 solid solution, and are formed by reaction between MgAl2O4 and TiO2 at temperatures above ≈ 1150 °C. As the value of x increases, the number of titanate particles per unit volume accordingly increases and the conversion of the original oxides is faster. At values of x⩽10−2, the prevailing mechanism is the nucleation and growth of Al2TiO5 nodules for fractional conversion up to ≈ 0.8. Further conversion of the residual Al2O3 and TiO2 particles dispersed into the titanate nodules is slower and controlled by solid-state diffusion through Al2TiO5. At x=0.1, a large number of nucleation sites is present, and solid-state diffusion through Al2TiO5 becomes important even in the initial stage of reaction, as the diffusion distances are strongly reduced. The study of Al2TiO5 formation under non-isothermal conditions in the temperature range 1250–1550°C shows that reaction proceeds between 1300 and 1350 °C for x=0.01 and between 1250 and 1300 °C for x=0.1. Densification of the titanate becomes important at temperatures above 1300°C for x=0.1, but only above 1450 °C for x=0.01.

Synthesis and characterization of BaSn(OH)6 and BaSnO3 acicular particles

The synthesis of BaSn(OH) 6 acicular crystals by precipitation at 100 °C from aqueous solutions and their transformation in the perovskitelike compound BaSnO 3 was investigated. Single acicular crystals 100–200 μm in length were obtained from a 0.05M solution, whereas bundlelike aggregates of 20–40 μm were precipitated from 0.2–0.6 M solutions. The x-ray diffraction pattern of barium hexahydroxostannate was indexed according to monoclinic symmetry with cell parameters a = 11.029 ± 0.002 A, b = 6.340 ± 0.001 A, c = 10.563 ± 0.001 A = 128.51 ± 0.01°, α = γ = 90°. The BaSn(OH) 6 particles decomposed to BaSnO 3 and water at approximately 270 °C and the original morphology was retained. The resulting product had specific surface area up to 30–40 m 2 /g and consisted of 10–20 nm crystallites. The larger unit cell edge in comparison to the reference value and the continuous weight loss up to 1200 °C indicate that water is not completely released during decomposition and a substantial amount of proton defects (up to 0.4 mol per mole of BaSnO 3 ) is incorporated in the perovskite lattice as OH − groups. Normal crystallographic properties of BaSnO 3 are restored only after calcination at 1300 °C.

Impedance spectroscopy of n-doped (Ba,Sr)TiO3 ceramics prepared by modified low temperature aqueous synthesis

Abstract Low temperature aqueous synthesis (LTAS) has been applied to the preparation of La or Nb-doped (Ba,Sr)TiO 3 powders. Doping elements have been mixed to precursors, in order to directly obtain crystalline powders with modified composition. Similar powders, doped by conventional wet ball-milling process, have been also prepared for comparison. Ceramic samples have been obtained by isothermal sintering in air at different temperatures. Microstructural characterisation showed the presence of a secondary phase, identified as a Ti-rich barium titanate compound. Impedance spectroscopy (IS) has been used to study the PTCR behaviour of such materials. IS results are analysed, taking into account effect of the secondary phase, to discuss bulk and grain boundary characteristics.

Slip casting of submicron BaTiO3 produced by low-temperature aqueous synthesis

Abstract Two submicron BaTiO 3 powders of different stoichiometries, prepared by an aqueous method have been characterized and submitted to chemical, thermal and mechanical treatments in order to optimize the processing conditions. Acid cleaning at different pH values, calcinations at different temperatures and ultrasonication were performed. The resulting powders were characterized by TGA, SEM, specific surface area, XPS and particle size analysis. Fired samples were obtained via slip casting and isothermal or non-isothermal sintering. Densification was studied by dilatometric measurements. Acid cleaning treatments caused desintering phenomena and low fired density, while high density samples with an homogeneous fine grained microstructure were obtained from powders calcined at 950 °C.

Electrical characterization of BaTiO3 made by hydrothermal methods

The manufacture of multilayer capacitors based on barium titanate requires improved precursor powders. In this paper the dielectric properties of barium titanate ceramics made from hydrothermally precipitated material are compared with those powders obtained commercially and prepared by calcination. The greater porosity of hydrothermal powders tends to reduce their effective dielectric constant but preheating them to 950 ‡C before sintering produced high density ceramic with higher values of ɛr. The dielectric losses of these materials are higher than those produced by traditional methods. Their low Curie temperatures (106–8 ‡C) are related to the high strontium concentration (4.7 mol%) present in the starting materials. The grain sizes of these ceramics are less sensitive to sintering temperature than those made using calcined powders, with no anomalous grain growth.