The substitution behavior and lattice parameter of barium titanate between solid_solubility with a dopant concentration in the range of 0.25 to 1.5 mol% are studied. The influences of dysprosium-doped fraction on the grain size and dielectric properties of barium titanate ceramic, including dielectric constant and breakdown electric field strength, are investigated via scanning electronic microscopy, X-ray diffraction and electric property tester. The results show that, at a dysprosium concentration of 0.75 mol%, the abnormal grain growth is inhibited and the lattice parameters of grain rise up to the maximum because of the lowest vacancy concentration. In addition, the finegrain and high density of barium titanate ceramic result in its excellent dielectric properties. The relative dielectric constant (25 °C) reaches to 4100. The temperature coefficient of the capacitance varies from -10 to 10% within the temperature range of -15 °C -100 °C, and the breakdown electric field strength (alternating current) achieves 3.2 kV/mm. These data suggest that our barium titanate could be used in the manufacture of high voltage ceramic capacitors.
BaTiO_(3) ceramics doped with different concentrations of lanthanum were prepared by sintering under a reducing atmosphere of H_(2) and Ar(1∶99,in volume),and subsequently reoxidized by annealing in the atmosphere of oxygen partial pressure p(O_(2))=260 Pa(mixture of Ar and O_(2)).The influences of oxygen adsorption quantity for grain and grain boundary on the resistivity of samples were investigated via oxygen calorimetry and impedance spectroscope.The results show that the critical donor concentration which causes blocking of the exaggerated grain growth is observed to be 10%(in mole) La in reducing atmosphere during sintering.The quantity of oxygen adsorption of samples during reoxidization process increases with the increase of reoxidation temperature(1 017—1 380 ℃).The potential barrier at grain boundary consists of two parts: the extrinsic grain boundary is related to acceptor created by the adsorption of oxygen atoms,and regarding the intrinsic grain boundary the ironized cation(Ti) vacancies during reoxidation serve as acceptor states.The high acceptor density leads to the increase of the positive temperature coefficient of the resistivity effect.
The question whether ferroelectricity (FE) may coexist with a metallic or highly conducting state, or rather it must be suppressed by the screening from the free charges, is the focus of a rapidly increasing number of theoretical studies and is finally receiving positive experimental responses. The issue is closely related to the thermoelectric and multiferroic (also magnetic) applications of FE materials, where the electrical conductivity is required or spurious. In these circumstances, the traditional methods for probing ferroelectricity are hampered or made totally ineffective by the free charges, which screen the polar response to an external electric field. This fact may explain why more than 40 years passed between the first proposals of FE metals and the present experimental and theoretical activity. The measurement of the elastic moduli, Young's modulus in the present case, versus temperature is an effective method for studying the influence of doping on a FE transition because the elastic properties are unaffected by electrical conductivity. In this manner, it is shown that the FE transitions of BaTiO3-d are not suppressed by electron doping through O vacancies; only the onset temperatures are depressed, but the magnitudes of the softenings, and hence of the piezoelectric activity, are initially even increased.
Monolithic ceramics with an uniaxial gradient of piezoelectric and/or dielectric coefficients (functionally graded materials—FGM) are suitable for bending actuators with strongly reduced internal mechanical stresses. We prepared BaTi1—xSnxO3 ceramics with a Sn-content of 7.5 mol% (BTS7.5) at the upper side and 15 mol% (BTS15) at the lower side. Two sheets of powder with different stoichiometry were pressed and sintered. The gradient of stoichiometry was modified by additional layers of powder with 10 and 12.5 mol% Sn, respectively. The Sn content after sintering across the thickness was investigated by Electron Probe Micro-Analysis (EPMA). The gradient of the chemical composition was transformed into a functionally gradient of the electromechanical properties by a poling process. An increasing of the bending deflection was obtained by cycling with an unipolar AC-voltage. In this work we studied two effects which can explain this behavior.
X-ray diffraction (XRD) patterns, electron paramagnetic resonance (EPR) powder spectra (9 and 34 GHz) and the magnetic susceptibility of BaTiO3 + 0.04 BaO + x/2 Co2O3 (0.001 ⩽ x ⩽ 0.02) ceramics were studied to investigate the incorporation of Co ions in the BaTiO3 lattice and their valence states as well as the development of the hexagonal phase (6H modification) in dependence on doping level x and sintering temperature Ts. At Ts = 1400 °C the 6H modification begins to occur at a nominal Co concentration x of about 0.001 and for x > 0.005 the samples are completely hexagonal at room temperature. Two different EPR spectra were observed in the 6H modification of BaTiO3, which were both assigned to paramagnetic Co(2+) ions located at the two crystallographically non-equivalent Ti sites in 6H-BaTiO3. The EPR g tensor values as well as the molar paramagnetic susceptibility, measured in the temperature range 5 K-300 K at a magnetic field of 9 T, were analyzed in the framework of the ligand field theory using the program CONCORD. The combination of EPR and magnetic measurements reveals that in air-sintered 6H BaTiO3, the incorporated Co occurs as a mixture of paramagnetic Co(2+) and diamagnetic Co(3+) ions, whereas in samples annealed in reducing atmosphere the majority of Co is in the divalent state. The occurrence of Co(4+) can be excluded for all investigated samples. The sample color caused by Co(2+) and Co(3+) ions is beige/light yellow and dark grey/black, respectively. The majority of the Co(2+) ions substitutes Ti in the exclusively corner-sharing oxygen octahedra possessing nearly cubic symmetry. The corresponding ligand field parameter [Formula: see text] amounts to about -28 000 cm(-1) (Wybourne notation, 10Dq ≈ 20 000 cm(-1)). In the reduced samples nearly 5% of the detected Co(2+) ions occupy the Ti site in the face-sharing oxygen octahedra, which are significantly trigonally distorted. The negative sign of the obtained ligand field parameter [Formula: see text] ≈ -7300 cm(-1) reflects a compression of this octahedron in direction of the hexagonal c-axis.
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