Niall J. Donnelly

Mixed conduction and chemical diffusion in a Pb(Zr0.53,Ti0.47)O3 buried capacitor structure

Impedance spectroscopy is performed on a buried capacitor structure composed of a PZT-0.75% Nb ceramic with platinum electrodes. The ionic and electronic conductivities (σion,σelec) are extracted from the impedance spectra using an equivalent circuit based on the premise of mixed conduction. In the temperature range 500–700 °C, a change in local pO2 mainly affects σelec, suggesting that the samples are ionically compensated, i.e., [VO••]=[VPb″]. The chemical diffusion coefficient, D, is obtained by a conductivity relaxation technique assuming two-dimensional diffusion geometry. In comparison to BaTiO3, or SrTiO3, the chemical diffusivity is found to be relatively high, D=2.0×10−4 cm2 s−1 (700 °C, in air).

Time dependent dc resistance degradation in lead-based perovskites: 0.7 Pb(Mg1/3Nb2/3)O3−0.3 PbTiO3

Highly accelerated lifetime tests (HALTs), thermally stimulated depolarization current (TSDC), and impedance spectroscopy (IS) measurements were performed on 0.7 Pb(Mg1/3Nb2/3)O3−0.3 PbTiO3 (PMN-PT) single crystal to investigate time dependent dc resistance degradation under a dc bias. A low activation energy of 0.61±0.04 eV which controls the degradation process in PMN-PT single crystal is determined from the characteristic degradation time tC in HALT. Meanwhile, in a complementary TSDC investigation, a broad depolarization peak with an activation energy of 0.6±0.03 eV is observed in virgin PMN-PT single crystal having the characteristics of ionic space charge. Finally, impedance spectra of degraded PMN-PT single crystal exhibited three relaxations in contrast to two relaxations in virgin PMN-PT single crystal. In terms of equivalent circuit, an element combination R-ZCPE corresponding to ionic conduction is common to both virgin and degraded single crystals, and an activation energy about 0.64 eV, attri...

Pb loss in Pb(Zr,Ti)O3 ceramics observed by in situ ionic conductivity measurements

Analysis of the impedance spectra of Nb-doped Pb(Zr,Ti)O3 (PZT) embedded capacitors revealed that the ionic conductivity increased monotonically during annealing at 700 °C. Furthermore, the rate of increase was lowered by a reduction in the ambient pO2. The results could be explained by a model in which oxygen vacancies are generated as a consequence of Pb evaporation from the PZT. At 700 °C, this process is most likely limited by surface kinetics rather than Pb bulk diffusion. It was shown that the Pb loss could be completely recovered by annealing in a high activity Pb source with a commensurate reduction in oxygen vacancy concentration. The electronic conductivity was predominantly p-type and was relatively unaffected by the Pb loss throughout the course of the experiment.

Impedance spectroscopy of PZT ceramics- measuring diffusion coefficients, mixed conduction, and Pb loss

Sintering of PZT at high temperatures results in loss of Pb unless an ambient Pb activity is maintained. The tell-tale sign of Pb loss is an increased conductivity, usually manifested in unacceptably high values of tan δ. The conductivity is caused by oxygen vacancies and/or electron holes which are a by-product of Pb evaporation.

Piezoelectric actuators with Cu inner electrode

The piezoelectric material composition is PZT-SKN with a Li2CO3 flux. This system has been developed successfully for air-fired actuators in PSU with Ag-Pd alloyed inner electrodes. We tried to control the lead activity by adding higher excess PbO in the formulation. Samples were all sintered at 900°C for 6 hours in PO2 = 5.38×10¿12 atm. With 3.0, 4.0wt% PbO excess addition, we note that density, micro-structure and permittivity are similar and there is a high electro-mechanical performance for example, the high field piezoelectric d33 was ~480pm/V. However, there is some decrease in d33 after the sintering in low PO2 and Rayleigh coefficients shows PO2 to slightly suppressing both the intrinsic and extrinsic contribution to piezoelectric properties. Multilayer actuators with copper inner electrode were successfully fabricated with the PZT-SKN after processing with techniques and materials that have low residual carbon.

Impedance spectroscopy of PZT ceramics - measuring diffusion coefficients, mixed conduction and Pb loss

Sintering of lead zirconate titanate (PZT) at high temperatures results in loss of Pb unless an ambient Pb activity is maintained. The tell-tale sign of Pb loss is an increased conductivity, usually manifested in unacceptably high values of tan ¿. The conductivity is caused by oxygen vacancies and/or electron holes which are a byproduct of Pb evaporation. In the first part of this paper, it is shown how impedance spectroscopy can be used to separate ionic and electronic conductivity in a properly designed sample by selection of appropriate boundary conditions. Subsequently, impedance is used to probe defect concentrations in PZT during prolonged annealing at 700°C. It is found that oxygen vacancies are generated during annealing in air but the rate of generation actually decreases upon lowering the ambient pO2. These results are explained by a model of Pb evaporation which, in this case, leads predominantly to oxygen vacancy generation. In principle, this effect could be used to generate a specific vacancy concentration in similar Pb-based oxides.

Kinetics of Oxygen Diffusion into Multilayer Ceramic Capacitors during the Re-Oxidation Process and its Implications on Dielectric Properties

Re-oxidation is an important thermal process to minimize oxygen vacancies and produce high reliable Ni-MLCCs. The re-oxidation of these devices is then investigated with a series of “in-situ” impedance measurements between 400 and 500 °C in air. From the relative impedance change, chemical diffusion coefficients, associated activation energy and effective equivalent circuit model are determined. Those values were found to be reasonable compared with previous researchers’ data. Moreover, the proposed effective equivalent circuit model successfully represents the real Ni-MLCC morphology. From transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS), it is found that the electrical properties and reliabilities of the Ni-MLCCs re-oxidized under different conditions are identical.