Effects of La Doping on the Structural and Dielectric Properties of Barium Titanate Ceramics

Polycrystalline samples of Ba1-xLaxTiO3 (x = 0.0, 0.002, 0.004) were prepared by a standard high-temperature solid-state reaction method. XRD studies confirmed the formation of a polycrystalline compound with a tetragonal crystal structure. SEM images suggested the presence of a polycrystalline microstructure with certain degree of porosity, and the grains appeared to be distributed inhomogeneously throughout the pallet samples. Dielectric studies indicated a ferroelectric–paraelectric phase transition with a clear shift in the Curie temperature (Tc) of BaTiO3 towards a lower temperature upon doping. The brick-layer model was used to study the potential barrier and the structure of the grain-boundary region of the Ba9.998La0.002TiO3 and Ba9.996La0.004TiO3 ceramics. These ceramics exhibited good density and a homogeneous distribution of the grains. The thickness of the grain-boundary region was calculated to be approximately 200 nm.

Grain Boundary Resistivity of Yttria-Stabilized Zirconia at 1400°C

The grain size dependence of the bulk resistivity of 3 mol% yttria-stabilized zirconia at 1400°C was determined from the effect of a dc electric field Ea=18.1 V/cm on grain growth and the corresponding electric current during isothermal annealing tests. Employing the brick layer model, the present annealing test results were in accordance with extrapolations of the values obtained at lower temperature employing impedance spectroscopy and 4-point-probe dc. The combined values give that the magnitude of the grain boundary resistivity ρb=133 ohm-cm. The electric field across the grain boundary width was 28–43 times the applied field for the grain size and current ranges in the present annealing test.

A Study of Phase Transition Behaviors of Chalcogenide Layers Using In-situ AC Impedance Spectroscopy

ABSTRACTThe electrical properties of chalcogenide thin films, both pristine Ge2Sb2Te5 (GST) and cerium-doped GST (Ce-GST), were investigated by in-situ AC impedance spectroscopy. In conjunction with the brick layer model, the contributions of both the grain and the grain boundary to the phase-transition behaviors of chalcogenide samples could be distinguished; the results illustrated the dominance of the grain boundary in the phase transition process. Moreover, impedance analysis applied to characterize the effects of doping on the phase-transition kinetics yielded results similar to those obtained by conventional methods. Therefore, in-situ AC impedance spectroscopy is a feasible tool for analyzing the phase transitions of chalcogenides.

Impedance/Dielectric Spectroscopy of Electroceramics in the Nanograin Regime

ABSTRACTIn the microcrystalline regime, the behavior of grain boundary-controlled electroceramics is well described by the “brick layer model” (BLM). In the nanocrystalline regime, however, grain boundary layers can represent a significant volume fraction of the overall microstructure and simple layer models are no longer valid. This work describes the development of a pixel-based finite-difference approach to treat a “nested cube model” (NCM), which more accurately calculates the current distribution in polycrystalline ceramics when grain core and grain boundary dimensions become comparable. Furthermore, the NCM approaches layer model behavior as the volume fraction of grain cores approaches unity (thin boundary layers) and it matches standard effective medium treatments as the volume fraction of grain cores approaches zero. Therefore, the NCM can model electroceramic behavior at all grain sizes, from nanoscale to microscale. It can also be modified to handle multi-layer grain boundaries and property gradient effects (e.g., due to space charge regions).

Space-charge concepts on grain boundary impedance of a high-purity yttria-stabilized tetragonal zirconia polycrystal

A detailed impedance analysis using the brick-layer model is performed on a high-purity yttria-stabilized tetragonal zirconia polycrystal (Y-TZP). Space-charge impedance is generally formulated and expressions for the respective space-charge models are therefrom derived depending on whether dopant ions are mobile or immobile. Pronounced yttrium segregation in Y-TZP is also considered in the analysis in that the dopant profile is assumed to be frozen from a high-temperature equilibrium distribution. Comparison with experimental observations shows that the electrically measured grain-boundary thickness corresponds to the Schottky-barrier width, slightly modified by the dopant segregation. The grain-boundary resistance is not consistent with any space-charge models and the strong defect interaction due to the yttrium enrichment is suggested to be mainly responsible.

AC Conductivity Based Microstructural Characterization of Yttria Stabilized Zirconia

AC conductivity experiments, or Impedance Spectroscopy, at temperatures between 100 and 400°C, made on 8 mole % Yttria Stabilized Zirconia (8YSZ) with different grain sizes, are presented. The results are analyzed using Effective Media Theories, the Brick Layer Model and percolation theory. The results obtained are satisfactory, only if the measured frequency dependant complex conductivity parameters of the grains are inserted in the equations.