Paolo Nanni

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.

Reaction sintering of aluminium titanate: I—Effect of MgO addition

Abstract The effect of 2 wt% MgO addition on the reactive sintering of Al 2 TiO 5 was studied starting from commercially available alumina and rutile powders. The formation of titanate, the successive sintering process and the corresponding microstructure evolution have been investigated using differential dilatometry, X-ray powder diffraction and scanning electron microscopy. For pure Al 2 TiO 5 the reaction occurs initially by a nucleation and rapid growth process and, afterwards, by the slow conversion of unreacted oxide particles by solid-state diffusion. The addition of MgO changes the mechanism of titanate formation, probably by making easy the nucleation of the new phase through the intermediate formation of MgAl 2 O 4 , followed by the growth of a Mgrich solid solution. The main result is a strong reduction of the average grain size and remarkable increase of the density of the final material.

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.

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.

XPS investigation of surface properties of Ba(1-x)SrxTiO3 powders prepared by low temperature aqueous synthesis

Abstract Barium titanate powders prepared by wet chemical routes are known to show large deviation from stoichiometry and contamination at surface, due to chemical instability in solution at low pH values ( (1− x ) Sr x TiO 3 ( x ≤0·1), have been investigated on pressed pellets of as-prepared powders by X-ray photoelectron spectroscopy. BaCO 3 content has been evaluated from Ba3d and Ba4d spectra. The total relative amount of carbonate has been measured by X-ray diffraction and compared to that present on surface. From these data, the formation of BaCO 3 by surface reaction with the atmosphere or by bulk precipitation is discussed.

Reaction diffusion in the Y2O3-Fe2O3 system

Abstract Solid state reactions in the Y2O3-Fe2O3 system were studied by means of different types of diffusion couples. Simultaneous growth of the YFeO3 and Y3Fe5O12 ternary phases was observed on Y2O3-Fe2O3 couples, whereas only the YFeO3 phase grows on Y2O3-Y3Fe5O12 and Y2O3-gas-Fe2O3 couples. The growth rates were determined by layer thickness measurements on couples reacted at 1220, 1300 and 1352°C in air and at 1403°C in oxygen for times ranging from 8 to 256 h. The growth kinetics is parabolic in all cases. Based on a model for growth of multiple layers, the intrinsic parabolic growth constants were calculated from the apparent experimental rate constants. In the case of Y3Fe5O12 growth, the intrinsic rate constants were compared with those predicted by Wagners theory. The probable mechanism is the equidirectional coupled diffusion of O2− and Y3+ ions. Gas phase transport of iron, in the form of Fe(OH)2, becomes important above 1300°C.

Magnetic properties of PbFe2/3W1/3O3-PbTiO3 solid solutions

The magnetic properties of the ferroic (1−x)PbFe2/3W1/3O3–xPbTiO3 system with various compositions in the range x∈(0,0.50) were investigated. A magnetodielectric coupling is proved by an anomaly of the magnetic hysteresis loop found in the Curie range of temperatures for the relaxor PbFe2/3W1/3O3 ceramic. Two types of superexchange magnetic interactions and a succession of magnetic transitions with Neel temperatures dependent on the composition were found in the system. The present results confirm the hypothesis of a relationship between the magnetic properties and the nanopolar ordering in the relaxor–ferroelectric system.