BaTi(1-2x)MnxNbxO3 ceramics with 0 ≤ x ≤ 0.10 were fabricated and the crystal structure, microstructure, dielectric, and ferroelectric properties were investigated. The samples with x ≥ 0.06 showed a pseudo-cubic symmetry in a wide temperature range of -150 oC to 200 oC but exhibited domain structures and a slightly slanted polarization-electric field response. The dielectric constant for these samples exhibited a comparatively flat response with temperature. The DC-bias dependence of the dielectric constant improved drastically from the samples with x = 0.06. A qualitative model of chemical grain size (CGS), defined by the distance between the co-dopant ions, was proposed which expressed the phonon propagation length in a wide temperature range and thus induced superparaelectricity in the ceramics. A critical CGS = 810 pm was determined geometrically for the sample with x = 0.06 such that samples with x ≥ 0.06 were superparaelectric in nature.
The grain size effect on the dielectric, ferroelectric, and piezoelectric properties of BaTiO 3 (BT) ceramics with different Ba/Ti ratios, prepared by solid-state reaction and conventional sintering, was investigated, and the corresponding electrical properties of these ceramics with similar grain size were compared. The BT ceramics with Ba/Ti ratios of 0.997, 1.000, and 1.003 and grain sizes of approximately 1 μm exhibited dielectric constants of 4265, 4275, and 4835 and converse piezoelectric coefficients, d 33 *, of 703, 645, and 466 pm V −1 , respectively. The remanent polarization, P r , was relatively higher for Ba-excess BT ceramics. These results indicated that, in Ti-excess BT ceramics, domain walls were strongly pinned due to oxygen vacancies under small electric fields and they were de-pinned at high electric fields, contributing to the higher d 33 *, while the orientation of domains in high electric fields did not retain upon electric field unloading, resulting in lower P r .
The leakage current mechanism in the as-sintered and quenched 0.75BiFeO3–0.25BaTiO3 (0.75BF–0.25BT) ceramics is evaluated by the space-charge-limited current (SCLC), Poole–Frenkel (P–F) emission, Schottky (ST) emission, and Fowler–Nordheim (F–N) tunneling mechanism. The discrepancy observed in the optical dielectric constant of BF and BT between the reported value and the values calculated from the P–F and ST emission plots suggests that the P–F emission and ST emission mechanisms do not contribute to the leakage current behavior of the 0.75BF–0.25BT ceramics. Subsequently, F–N tunneling is observed under a high electric field in the as-sintered 0.75BF–0.25BT ceramics, whereas the direct tunneling effect is exhibited throughout the measured electric field in the quenched ceramics. The SCLC mechanism is dominant in both the as-sintered and quenched 0.75BF–0.25BT ceramics. A change from Ohmic conduction to trap-filled-limit conduction is observed with an increase in the applied electric field in the as-sintered ceramics, whereas the quenched ceramics only revealed Ohmic conduction over the entire range of the measured electric field. The different behaviors exhibited in the SCLC mechanism between the as-sintered and the quenched ceramics imply that the conduction mechanism can be controlled by heat treatment.
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