The rate of the resistance degradation of doped SrTiO3 ceramics is investigated as a function of various external and material parameters. The effects of the mutually interrelated parameters dc voltage, dc electric field, and thickness of the dielectric are described by power laws. Electron microscopic potential contrast studies show a Maxwell-Wagner polarization leading to a concentration of the electric field at the grain boundaries during the degradation. Based on this finding, the voltage step per grain boundary, ΔΘgb, is introduced as a rate-determining parameter which allows an explanation of the influence of the grain size on the degradation rate as well as the difference in the power laws for ceramic and single-crystal samples.

dc Electrical Degradation of Perovskite-Type Titanates: I, Ceramics

The grain boundary space charge depletion layers in acceptor-doped SrTiO3 and BaTiO3 ceramics were investigated by impedance spectroscopy in the time and frequency domain. Based on the layer width and its dependence on the acceptor concentration, the temperature, and the oxygen partial pressure during annealing, a suggestion for a refined Schottky model is proposed. The local distribution of the donor-type grain boundary states causing the depletion layer and the resulting band bending are discussed.

Grain Boundary Defect Chemistry of Acceptor‐Doped Titanates: Space Charge Layer Width

Grain boundaries in dielectric and mixed-conducting ceramics

Thin films of (Ba,Sr)TiO3 on Pt/SiO2/Si substrates were deposited using rf magnetron sputtering at various substrate temperatures and O2/(Ar+O2) mixing ratios (OMR). The crystallinity of the films improved significantly as the OMR increased. The dielectric constant increased with increasing OMR and reached a maximum value at 50% OMR. The leakage current density decreased with increasing oxygen flow, but had a minimum value at 40% OMR. The results for the dielectric constant and the leakage current were interpreted in terms of polarization effect and loss theory. The film deposited at 450 °C and 50% OMR exhibited good surface morphology and had a dielectric constant of 375, a tangent loss of 0.074 at 100 kHz, a leakage current density of 7.35×10−9 A/cm2 at an electric field of 100 kV/cm with a delay time of 30 s, and a charge storage density of 49 fC/μm2 at an applied field of 150 kV/cm. The 10 yr lifetime of time-dependent dielectric breakdown studies indicate that a 50% OMR sample has a longer lifetime than the 0% OMR sample.

Effect of oxygen to argon ratio on properties of (Ba,Sr)TiO3 thin films prepared by radio-frequency magnetron sputtering

Finite difference quasielectrostatic modeling is used to predict the complex dielectric permittivity of barium titanium oxide (BTO)—polymer composites of interest for capacitor applications. The simulations explore the effects of the microstructural arrangement of spherical ceramic particles, the volume filling fraction of ceramic, and the type of polymer on the composite permittivity. For composites with randomly positioned ceramic particles, a soft percolation regime is found between volume filling fractions of 0.35 and 0.5 that leads to a more gradual growth in permittivity compared to ordered arrangements of particles. For BTO dispersed in a representative relaxor ferroelectric polymer, dielectric constants as high as 300 are predicted at a filling fraction of 0.45. Electric field statistics inside the composites are also computed, and localized intensification factors in the range of three to eight times the applied field are predicted, with an incrementally linear growth in high-field probability wi...

Finite difference simulations of permittivity and electric field statistics in ceramic-polymer composites for capacitor applications

Leakage currents in new and degraded (typically at twice rated voltage, 125°C) ZSU and X7R types Of multilayer ceramic (MLC) capacitors show both ohmic and space charge limited current behavior. The near 3/2 power voltage characteristic (la V3/2) of new devices can be attributed to electron emission from electrode points. The quadratic behavior (I \alpha V2) seen for moderately degraded devices represents space charge limited emission from near planar electrodes. This emission may evolve from the point emission due to resistivity decreases that occur in the emission region as a result of ion movement. For these currents, electrons are believed to be the dominant charge carriers. Neither Schottky or Poole-Frenkel currents were identified. Thermal activation energies decrease from ~1.3 eV for new X7R devices, to zero for degraded ones, corresponding to resistivity decreases from ~ 1013 \Omega -cm to 105 \Omega -cm or less (at 125°C). Carrier concentrations and mobilities have been estimated from thermoelectric measurements on new and reduced nonelectroded X7R chips, which exhibit n-type conduction. These parameters both increase substantially with reduction, with mobility dominating the thermal activation energy. These measurements are consistent with a small polaron hopping mode for electron transport in the ceramic.

Leakage Currents in Multilayer Ceramic Capacitors

The following items are treated for BaTiO 3 -based ceramic and multilayer ceramic (MLC) capacitors: type of charge carrier, mode of transport, and time dependence of conductivity during degradation. For a specific X7R-type ceramic, the charge carrier is verified to be the electron by several independent measurements. Seebeck voltages are negative; Galvanic voltages are zero; leakage currents are constant over long time periods; space charge limited currents are evident. The shapes of complex impedance plots depend on ceramic composition, indicating that grain boundary impedance may be present in some cases and not others. A quantitative model based on ionic movement is presented to account for leakage currents that increase exponentially with time. Evidence for ionic movement for specific Z5U capacitors is reported. Degradation can be reversed by reversing bias polarity or by storage under zero bias.

Hee Lee

Papers

Leakage Currents in Multilayer Ceramic Capacitors

Charge Carriers and Time Dependent Currents in BaTiO 3 -Based Ceramic