Nahum Masó

Electrical properties of Fe-doped BaTiO3

The polymorphism of BaTi1−xFexO3−δ solid solutions: 0 ∼1000 °C. With increasing x, the high temperature hexagonal polymorph is stabilised to increasingly lower temperatures. The temperature of the tetragonal to cubic transition also decreases with increasing x, as shown by a combination of permittivity measurements, DSC and X-ray powder diffraction. The electrical properties of ceramics of tetragonal/cubic solid solutions, determined by impedance spectroscopy, show a ferroelectric bulk phase (for T < Tc) and a Schottky barrier associated with the grain boundaries whose resistance is sensitive to a dc bias voltage. The bulk properties are sensitive to sample heat treatment and in particular, for samples reheated in air ≤700 °C, oxidation occurs, generating Fe4+ and an increase in the level of p-type conductivity.

Phase transition hysteresis and anomalous Curie–Weiss behavior of ferroelectric tetragonal tungsten bronzes Ba2RETi2Nb3O15:RE=Nd,Sm

The ferroelectric tetragonal tungsten bronze (TTB) phases, Ba2RETi2Nb3O15:RE=Nd,Sm, were prepared by low temperature solvothermal synthesis. The permittivity versus temperature data of sintered ceramics show two unusual features: first, a hysteresis of 50–100 °C between values of the Curie temperature Tc on heat-cool cycles and second: a huge depression in the Curie–Weiss temperature T0. Both effects are attributed to the complex nature of their TTB-related crystal structures with different superstructures above and below Tc and the difficulty in nucleating ferroelectric domains on cooling through Tc. Several factors may contribute to the latter difficulty: first, the structures contain two sets of crystallographic sites for the “active” Ti, Nb ions; second, the distribution of Ti and Nb over these two sets of sites is not random but partially ordered; and third, below Tc a weak commensurate superstructure perpendicular to the polar c axis is present, but above Tc a weak incommensurate superstructure in ...

Electrical properties of ferroelectric BaTi2O5 and dielectric Ba6Ti17O40 ceramics

Single phase powders of BaTi2O5 and Ba6Ti17O40 were made by a sol-gel method with final heating at 1100–1200°C. Ceramic samples were prepared and fired at temperatures in the range 1100–1300°C. Both sets were highly insulating, with conductivities of, e.g., 10−6–10−7Scm−1 at 600°C and activation energies for conduction in the range 1.75–1.86eV. BaTi2O5 compositions are ferroelectric with a permittivity maximum dependent on firing conditions: emax′=122 at 450°C on firing at 1100°C; and emax′=130 at 475°C on firing at 1225°C. Ba6Ti17O40 is a dielectric with e′∼57–55 over the range 40–400°C.

Synthesis and electrical properties of Nb-doped BaTiO3

Nb-doped BaTiO3 solid solutions of general formula, BaTi1−5xNb4xO3 prepared by two methods, solid state reaction at 1500 °C and sol–gel synthesis from high purity alkoxide precursors with final heat treatment at 900 °C, form over the range 0 ≤ x ≤ 0.055(5). Permittivity measurements demonstrated the difficulty in obtaining high quality, electrically-homogeneous samples by solid state reaction, even though samples were nominally phase-pure by X-ray powder diffraction. A partial phase diagram has been constructed showing the transitions between tetragonal, cubic and hexagonal polymorphs as a function of composition. Pellets derived from sol–gel samples and sintered at 900 °C were highly insulating, with an activation energy of ∼1.5 eV. Under these firing conditions, there was no evidence of electronic compensation, and semiconductivity, on doping BaTiO3 with Nb.

Environmental study of Cr2O3-Al2O3 green ceramic pigment synthesis

A green pigment was synthesised based on a Cr2� xAlxO3 solid solution,using a standard ceramic industry composition with and without different mineralisers and using raw materials industrial grade reagents. The resulting products were compared with a pigment made using chemically pure (CP grade) to establish the most appropriate reagents for achieving minimum Cr(VI) segregation during the pigment washing stage,and comparable chromatic qualities to those of a standard industrial pigment. Chromium sesquioxide was used as Cr(III) precursor instead of a Cr(VI) compound,and the chromium content was also optimised. The X-ray photoelectron spectroscopy (XPS) results indicated that a solid solution only forms when CP grade reagents are used. This is consistent with the scanning electron microscopy (SEM) and X-ray diffraction (XRD) data obtained in the study. Smaller Cr(VI) and B contents were found in the washing liquids when CP grade reagents were used,but the resulting chromatic quality was slightly lower than that of a standard pigment made and used in the ceramic industry. # 2003 Elsevier Ltd. All rights reserved.

Field enhanced bulk conductivity of acceptor-doped BaTi1−xCaxO3−x ceramics

The electrical properties of Ca-doped BaTiO3 are very different when Ca substitutes onto Ba or Ti sites. The p-type semiconductivity of Ti-substituted ceramics increases reversibly by one to two orders of magnitude under a dc-bias voltage of ≤100 V cm−1, whereas Ba-substituted ceramics show little sensitivity to a dc bias. This increase in BaTi1−xCaxO3−x, studied over the temperature range 150–600 °C, is independent of electrode material and atmosphere and is attributed to ionization of underbonded O2− ions adjacent to acceptor-doped Ca2+ ions.

Single phase, electrically insulating, multiferroic La-substituted BiFeO3 prepared by mechanosynthesis

Single phase, electrically insulating samples of Bi1−xLaxFeO3 solid solutions have been prepared by mechanosynthesis over the whole compositional range for the first time. Lanthanum substitution influenced the kinetics of the mechanochemical reaction and crystallite size of the products. For 0 ≤ x ≤ 0.15, an increase in the La content produced a significant decrease in the weight-normalized cumulative kinetic energy required to obtain the final product and an increase in the resulting crystallite size. Larger La contents did not affect either the reactivity or the crystallite size. The effect of x on the structure has been identified. Samples in the ranges x ≤ 0.15 and x ≥ 0.45 gave single phase solid solutions with R3c and Pnma space groups, respectively, while for the intermediate range, a non-centrosymetric Pn21a(00γ)s00 super structure was obtained. For 0 ≤ x ≤ 0.30, differential scanning calorimetry showed two endothermic effects corresponding to the Neel temperature (TN, antiferromagnetic–paramagnetic transition) and the Curie temperature (TC, ferroelectric–paraelectric transition), demonstrating their multiferroic character. Compositions with a larger La content only showed TN. Dilatometric and permittivity measurements confirmed the results obtained by DSC for the ferroelectric–paraelectric transition. The composition dependence of TN and TC showed that, at low x, TN < TC, but a cross-over, or isoferroic transition occurred at x ∼0.28, when TN = TC = 386 °C. Ceramics with 0 ≤ x ≤ 0.15 were highly insulating at room temperature with a resistivity, extrapolated from the Arrhenius plots, of ∼7 × 1016 to 8 × 1014 Ω cm and an activation energy ∼1.14–1.20 eV. Magnetization of the samples improved with La substitution.

Structural characterisation of ferroelectric Ag2Nb4O11 and dielectric Ag2Ta4O11

Structural characterisation of Ag2Nb4O11 and Ag2Ta4O11 powders has been carried out by Rietveld refinement of neutron powder diffraction data in the temperature range 10–573 K, symmetry-adapted mode analysis and second-harmonic generation. Ag2Nb4O11 ceramics appear to be ferroelectric with a maximum permittivity of ∼150 at TC ≈ 400 K, frequency-independent permittivity below this temperature and a shoulder in the e′ vs. temperature profile at ∼200 K. The space group changes from Rc to R3c as the temperature decreases through TC with a further transition from R3c to R3 below ∼200 K. The origin of the polar structure is related to displacement of Nb atoms in pentagonal bipyramids towards apical oxygens and in octahedra towards faces. This displacement towards octahedra faces is observed for some Ag atoms too. Ag2Ta4O11 ceramics show temperature-independent permittivity of ∼20, with a centrosymmetric crystal structure, Rc. Comments on possible phase transition(s) of several compounds similar to Ag2(Nb,Ta)4O11 are made.

Field enhanced bulk conductivity of BaTiO3 : Mg ceramics

The electrical conductivity of ceramics of composition Ba(Ti1−xMgx)O3−x: 0.00003 ≤ x ≤ 0.01 increases with time on application of a small dc bias voltage in the range 1 to 50 V cm−1 and at temperatures in the range 200 to 600 °C. The conductivity increase is reversible on removal of the dc bias. This low field effect is not observed in undoped BaTiO3, is intrinsic, is a property of the bulk material, is independent of atmosphere and electrode material, is not associated with charge injection into the BaTiO3 conduction band and differs from those of both varistors and memristors. The origin of the effect is attributed to a two-level electronic structure in a defect complex that contains Mg located on a Ti site, giving rise to underbonded oxide ions in the vicinity of the defect complex that are readily ionised on application of a dc bias. Conduction is predominantly p-type and therefore, holes located on oxygen, as O− ions, are more mobile than both electrons in Ti 3d levels and oxygen vacancies.

Synthesis, Structural Characterization, and Electrical Properties of New Oxygen-Deficient Tetragonal Tungsten Bronzes Ba2NdTi2+xNb3–xO15–x/2

Oxygen-deficient tetragonal tungsten bronzes ceramics with general formula Ba(2)NdTi(2+x)Nb(3-x)O(15-x/2) (0 ≤ x ≤ 1) have been prepared by low temperature solvothermal synthesis with final firing of ceramics at 1100-1300 °C in air. Rietveld refinement of X-ray powder diffraction (XRD) and neutron powder diffraction (ND) data at room temperature of Ba(2)NdTi(3)Nb(2)O(14.5) shows that Ba and Nd are ordered on the 15-coordinate and 12-coordinate sites, respectively, Ti and Nb are disordered nonrandomly over the two octahedral sites, and oxygen vacancies locate preferentially in the coordination sphere of Nd and Ti/Nb(2) atoms. Variable frequency impedance measurements show that samples are poor electronic conductors with activation energies ∼0.8-1.7 eV, conductivities ∼1 × 10(-5) S cm(-1) at ∼725 °C and with some evidence of oxide ion conduction at high x values. Composition dependence of the dielectric properties shows a transition from classic ferroelectric behavior with Ba(2)NdTi(2)Nb(3)O(15) to a relaxor-like behavior with Ba(2)NdTi(3)Nb(2)O(14.5). At intermediate compositions, both a first-order phase transition and relaxor-like behavior are observed.