Matthew C. Ferrarelli

Comment on the origin(s) of the giant permittivity effect in CaCu3Ti4O12 single crystals and ceramics

Impedance spectroscopy has been performed on CaCu3Ti4O12 (CCTO) single crystals and ceramics using Au and InGa alloy electrodes. The extrinsic effect responsible for the giant permittivity observed at radio-frequencies near room temperature is shown to be different for single crystals and ceramics. For single crystals it is attributed to a non-ohmic electrode contact to semiconducting CCTO. For ceramics it is attributed to an internal barrier layer capacitor mechanism associated with semiconducting CCTO grains and insulating grain boundaries.

High intrinsic permittivity in Na1/2Bi1/2Cu3Ti4O12

The electrical properties of Na1∕2Bi1∕2Cu3Ti4O12 (NBCTO) ceramics between ∼8 and 400K are electrically inhomogeneous, consisting of semiconducting grains and insulating grain boundaries, similar to isostructural CaCu3Ti4O12 (CCTO); however, the intrinsic permittivity of NBCTO of ∼250 at 10K is much higher compared to ∼110 for CCTO. Possible explanations for this high intrinsic permittivity are discussed.

Ferroelectric, electrical, and structural properties of Dy and Sc co-doped BaTiO3

Ba1−xDyxTi1−xScxO3 ceramics, 0 ≤ x ≤ 0.075, were prepared and the influence of Dy and Sc co-doping on the ferroelectric, electrical and structural properties of BaTiO3 (BT) investigated. A solid solution limit of x ≈ 0.05 was established using X-ray diffraction, and all compositions x ≤ 0.05 were tetragonal at room temperature. Fixed-frequency capacitance measurements showed the temperature of the cubic to tetragonal phase transition, TC, decreased non-linearly with increasing x and the associated permittivity peak broadened, with frequency-dependent ‘relaxor’-type behaviour observed for cubic x = 0.075. In addition, the magnitude of the relative permittivity at TC showed a negative dependence on x, decreasing from ∼11700 for x = 0 to ∼6950 for x = 0.05. In contrast to the non-linear decrease in TC, the tetragonal to orthorhombic and orthorhombic to rhombohedral phase transition temperatures decreased linearly at rates of ∼40 and ∼28 °C mol%−1 of DyScO3, respectively. The effect on the ferroelectric properties of BT was to quadruple the coercive field, from ∼0.9 kV cm−1 for x = 0 to ∼4.4 kV cm−1 for x = 0.025, without affecting the remnant polarisation, ∼10 to 11 µC cm−2 for x = 0 to 0.025. Finally, the non-linear decrease in TC is discussed in terms of the contributions of the mean size and statistical variance of the cations on the A and B sites of the perovskite, ABO3, lattice and the replacement of Ti4+ with Sc3+ cations.

Possible incipient ferroelectricity in Mn-doped Na1/2Bi1/2Cu3Ti4O12

The grain semiconductivity of Na1/2Bi1/2Cu3Ti4O12 ceramics is suppressed by several orders of magnitude by Mn-doping and Na1/2Bi1/2Cu2.82Mn0.18Ti4O12 ceramics exhibit high room temperature relative permittivity, er, ∼140. er data at 100 kHz fit the Barrett equation over the range ∼10–320 K and the magnitude and temperature dependence of er are consistent with incipient ferroelectricity in this compound.

Detection of heterogeneities in single-crystal CaCu3Ti4O12 using conductive atomic force microscopy

This paper reports on a conductive atomic force microscopy (C-AFM) investigation to provide local electrical characterization in a single crystal of CaCu3Ti4O12 (CCTO). The microstructure and dielectric properties were studied and provide evidence for an insulating secondary phase embedded within the semiconducting CCTO matrix. Such insulating electrical heterogeneities cannot be observed with macroscopic measurements such as conventional Impedance Spectroscopy and this study reveals C-AFM to be a powerful tool to assess the electrical homogeneity of semiconducting single crystals such as CCTO.

Microwave dielectric properties of Na1/2Bi1/2Cu2.82Mn0.18Ti4O12 ceramics

The microwave dielectric resonance properties of Na1/2Bi1/2Cu2.82Mn0.18Ti4O12 (NBCMTO) ceramics are reported. Single-phase NBCMTO powder were obtained from the mixed oxide route at 975°C and dense ceramics (~ 97% of the theoretical X-ray density) with a grain size distribution between ~ 2 and 10 μm by sintering in air at 1060°C. Ceramics exhibit room temperature microwave dielectric resonance with er ~ 130, Qf0 ~ 1750 GHz and a temperature co-efficient of resonance, τf, ~ +1000 ppm/K. The high permittivity and the large magnitude and sign of τf are attributed to incipient ferroelectricity.