Maria Teresa Buscaglia

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.

Functional properties of BaTiO3–Ni0.5Zn0.5Fe2O4 magnetoelectric ceramics prepared from powders with core-shell structure

In the present work, diphasic ceramic composites with core-shell nanostructures formed by Ni0.50Zn0.50Fe2O4 core and BaTiO3 shell were investigated. Their properties were compared with those of composites prepared by coprecipitation. The core-shell structure was confirmed by microstructural powder analysis. Homogeneous microstructures with a good phase mixing and percolated dielectric phase by the magnetic one were obtained from coprecipitated powders. Less homogeneous microstructures resulted in ceramics produced from the powder prepared by core-shell method, with isolated small ferrite grains besides large ferrite aggregates embedded into the BaTiO3 matrix. Both the ferroelectric and magnetic phases preserve their basic properties in bulk composite form. However, important differences in the dielectric relaxation and conduction mechanisms were found as result of the microstructural difference. Extrinsic contributions play important roles in modifying the electric properties in both ceramics, causing spa...

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.

Grain size effect on the nonlinear dielectric properties of barium titanate ceramics

The nonlinear dielectric properties of dense BaTiO3 ceramics with grain size of 1 μm–90 nm were investigated. In the finest ceramics, the permittivity reduces below 1000 and a remarkable nonhysteretic linear dc-tunability [e(E)] is obtained at high field, above 40 kV/cm. The observed behavior was explained by considering the nanostructured ceramic as a composite formed by ferroelectric grains, whose nonlinearity is reducing, and by low-permittivity nonferroelectric grain boundaries, whose volume fraction increases when decreasing the grain size. Reducing the grain size in ferroelectric dense materials is an alternative route to accomplish the application requirements: nonhysteretic tunability and permittivity below 1000.

Second-harmonic generation of single BaTiO3 nanoparticles down to 22 nm diameter.

We investigate the second-harmonic generation (SHG) signal from single BaTiO3 nanoparticles of diameters varying from 70 nm down to 22 nm with a far-field optical microscope coupled to an infrared femtosecond laser. An atomic force microscope is first used to localize the individual particles and to accurately determine their sizes. Power and polarization-dependent measurements on the individual nanoparticles reveal a diameter range between 30 and 20 nm, where deviations from bulk nonlinear optical properties occur. For 22 nm diameter particles, the tetragonal crystal structure is not applicable anymore and competing effects due to the surface to volume ratio or crystallographic modifications are taking place. The demonstration of SHG from such small nanoparticles opens up the possibilities of using them as bright coherent biomarkers. Moreover, our work shows that measuring the SHG of individual nanoparticles reveals critical material properties, opening up new possibilities to investigate ferroelectricity at the nanoscale.

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.

Oxygen deficiency and grain boundary-related giant relaxation in Ba(Zr,Ti)O3 ceramics

The influence of the oxygen vacancies on the dielectric response of BaZr0.10Ti0.90O3 ceramics prepared by solid-state reaction and sintered at 1400 °C for 2 h was investigated. The as-sintered ceramic exhibits a giant relaxation with a shift of the transition temperature from ∼85 °C to above 170 °C in the frequency ranges of 1 Hz–100 kHz, with high losses above unity and two components in the complex impedance plot. A complex dielectric relaxation response, with at least two thermally activated defect mechanisms with activation energies of ∼0.2 eV below the transition temperature and ∼0.7 eV for higher temperatures in the range of 85 °C–170 °C was determined. The observed giant relaxation is an extrinsic effect related to the oxygen deficiency, inhomogeneous distributed in the ceramic grain and not to the relaxor behavior of this system. After a post-annealing treatment at 1000 °C for 50 h, the dielectric response is completely changed: the permittivity vs. temperature dependences present maxima located a...

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.