Massimo Viviani

Influence of foreign ions on the crystal structure of BaTiO3

Abstract The influence of dopants (Cr, Co, Fe, Ni, Y, Er, Tb, Gd, Pr and La) on the crystal structure of barium titanate was studied at room temperature and at 250°C using X-ray diffraction. Fine BaTiO3 powders (Ba/Ti=1.02, size ≈30 nm) have been doped with 1 at% of the foreign element and annealed for 14 and 62 h at two different temperatures: 950 and 1350°C. The room temperature structure of doped BaTiO3 was in any case tetragonal with c/a ratio lower than in the undoped perovskite, but dependent on dopant nature and particle size. For powders calcined at 1350°C, the particle size was in the range 1–5 μm and the decrease in tetragonality was mainly determined by dopant incorporation. Powders treated at 950°C had particles more than one order of magnitude finer (0.1–0.2 μm) and a systematic lowering of the c/a ratio in comparison to the samples annealed at higher temperature was observed. Comparison of the experimental variations of the unit cell edge of cubic BaTiO3 (250°C) with the results of atomistic computer simulations gives indication on the preferential incorporation site of the dopant. In particular, La3+ and Pr3+ prefer to substitute at the Ba site, whereas Tb3+ and Gd3+ give partial substitution at the Ti site. For Er3+ and Y3+ preferential substitution at the Ti site is predicted. For transition metal ions, substitution at the Ti site with oxygen vacancy compensation is confirmed, although their behaviour is less accurately reproduced.

Ferroelectric properties of dense nanocrystalline BaTiO3 ceramics

Dense BaTiO3 ceramics with 50?nm average grain size obtained by spark plasma sintering were investigated. The dielectric data show a broad ferro?para phase transition with a maximum permittivity of at 390?K and 1?kHz. The local ferroelectric switching behaviour was investigated by piezoresponse force microscopy. Typical piezoelectric hysteresis loops were recorded at different positions of the sample. The present results provide experimental evidence for polarization switching at the local scale, indicating that the critical grain size for the disappearance of ferroelectric behaviour in dense, bulk BaTiO3 nanocrystalline ceramics is below 50?nm.

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.

Local switching properties of dense nanocrystalline BaTiO3 ceramics

The switching properties of dense BaTiO3 ceramics with 50 nm average grain size were investigated at local scale by piezoresponse force microscopy. Large areas with low piezoelectrical activity beside islands with strong piezoresponse were found. The application of electric fields induces stable domain structures and changes in the polarization state far away from the probing area, probably via trans-granular dipole interactions. Piezoelectric hysteresis loops were recorded on various positions, even in regions with initial zero piezoresponse, which possibly showed a superparaelectric behavior. The results are incontestable proof that 50 nm BaTiO3 ceramics retain ferroelectricity at a local scale.

Low temperature aqueous synthesis (LTAS) of BaTiO3: a statistical design of experiment approach

Abstract The effect of a selected number of parameters of the Low-Temperature Aqueous Synthesis (LTAS) on BaTiO 3 particle size was investigated by the statistical design of experiment approach. Different syntheses were carried out changing the value of six physico-chemical parameters involved in the process, following the scheme of a 2 6-2 fractional factorial design. The characterisation of the submicron powders produced shows variations of the average size in the range 30–55 nm, deduced from specific surface area measurements. The statistical analysis of experimental results, performed by the ANOVA method, allows to ascribe such variations to the effect of solutions concentration, stirring energy and precursors mixing conditions. The significance of such parameters, together with the apparent ineffectiveness of the ageing time, is discussed. Also the XRD measurement of barium carbonate amount is presented and its dependence on experimental conditions is discussed.

Giant permittivity and Maxwell?Wagner relaxation in Yb?:?CaTiO3 ceramics

Yb-modified CaTiO3 ceramics sintered for 24 h at 1450 °C exhibit a giant apparent permittivity of about 104 with a remarkable temperature (30–300 K) and frequency (101–106 Hz) stability (maximum variation: ±20%). After a post-annealing treatment of 48 h at 1100 °C, the dielectric response is strongly modified, with a high frequency, low temperature permittivity corresponding to the intrinsic dielectric behaviour of CaTiO3. A step-like rise in the apparent permittivity and a complex dielectric behaviour is observed with increasing temperature. The overall dielectric response of the ceramics can be interpreted in terms of the Maxwell–Wagner interfacial relaxation and modelled using equivalent circuits. The analysis demonstrates that the as-sintered ceramic consists of semiconducting grains (resistivity <104 Ω cm at 300 K) and insulating grain boundaries (resistivity ≈107 Ω cm at 300 K). Partial reoxidation takes place during post-annealing with the formation of oxygen gradients and increased electrical heterogeneity. In general, the dielectric response will be determined by the extent of the reoxidation reaction during cooling or post-annealing treatment below the sintering temperature.

Structural study of Pb(Fe2/3W1/3)O3–PbTiO3 system

Abstract The crystalline structure of the solid solution (1− x )Pb(Fe 2/3 W 1/3 )O 3 – x PbTiO 3 (PFW–PT) with x =0, 0.2, 0.25, 0.3, 0.35, 0.4 and 1 was analysed using X-ray diffraction (XRD) with comparative cell-refinement methods, in order to detect the change in symmetry with composition. Pure PFW shows pseudo-cubic symmetry for temperatures in the range 85–423 K. For the temperatures T =85 K and T =300 K, the solid solutions have cubic or tetragonal structures. The cell constants are functions of PbTiO 3 amount. A temperature-dependent Morphotropic Phase Boundary (MPB) separating the pseudo-cubic (relaxor) and tetragonal (ferroelectric) phases might be present. The ranges of compositions for this boundary are: x T =85 K and x ∈(0.3,0.35) for T =300 K.

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.

Growth of ternary oxides in the Gd2O3-Fe2O3 system. A diffusion couple study

Abstract The simultaneous, diffusion-controlled growth of GdFeO 3 (perovskite) and Gd 3 Fe 5 O 12 (garnet) was studied at 1200–1400 °C in Gd 2 O 3 –Fe 2 O 3 diffusion couples. Both compounds were found to grow as parallel layers according to the parabolic rate law. The parabolic rate constants of the second kind for the exclusive growth of each compound ( k 1 II for GdFeO 3 , k 2 II for Gd 3 Fe 5 O 12 ) were calculated from the experimentally determined rate constants of the first kind assuming coupling between diffusion fluxes and chemical reactions at phase boundaries. In the case of GdFeO 3 , the calculated values of k 1 II are in good agreement with the experimental values measured on Gd 3 Fe 5 O 12 –Fe 2 O 3 couples, where exclusive growth of GdFeO 3 is observed. Growth of the perovskite phase, probably related to formation of gaseous Fe(OH) 2 , is also observed on Gd 2 O 3 –gas–Fe 2 O 3 couples. The values of k 2 II for of Gd 3 Fe 5 O 12 are very close to those found for the growth of the isostructural compound Y 3 Fe 5 O 12 . The most likely reaction mechanism is the coupled diffusion of Gd 3+ and O 2− for garnet growth and the coupled diffusion of Fe 3+ and O 2− for perovskite growth. The activation energy is ≈550 kJ mol −1 for Gd 3 Fe 5 O 12 and ≈400 kJ mol −1 for GdFeO 3 .

Synthesis, sintering and expansion of Al0.8Mg0.6Ti1.6O5 : A low-thermal-expansion material resistant to thermal decomposition

A low-thermal-expansion ceramic with the composition Al0.8Mg0.6Ti1.6O5 that is isostructural to aluminium titanate Al2TiO5, was prepared by solid-state reaction of a mixture of α-Al2O3, MgO and TiO2-rutile. The synthesized material does not decompose after annealing for 250 h in the temperature range of 900–1175°C. The ceramic sintered at 1350°C has a 97% relative density, a grain size of ≈5 μm and an average thermal expansion coefficient between 80 and 1000°C of ≈2×10−6 K−1. Materials with the same composition, but obtained by reaction sintering, have a higher thermal expansion coefficient (4×10−6 K−1) and showed the presence of secondary phases.