IMPEDANCE/THERMALLY STIMULATED DEPOLARIZATION CURRENT AND MICROSTRUCTURAL RELATIONS AT INTERFACES IN DEGRADED PEROVSKITE DIELECTRICS
Open Access
- Author:
- Liu, Wei-En
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 18, 2008
- Committee Members:
- Prof Clive A Randall, Dissertation Advisor/Co-Advisor
Clive A Randall, Committee Chair/Co-Chair
Leslie Eric Cross, Committee Member
Susan E Trolier Mckinstry, Committee Member
Elizabeth C Dickey, Committee Member
Michael T Lanagan, Committee Member - Keywords:
- thermally stimulated depolarization current
Impedence
defect chemistry
electrical degradation - Abstract:
- This investigation address electrical degradation processes in dielectric materials. Specifically multiple characterization techniques have been used to develop a more comprehensive understanding of mechanisms that control transient change during electrical degradation. We utilize techniques such as impedance spectroscopy (IS), thermally stimulated depolarization current (TSDC), and modern analytical transmission electron microscopy methods including electron energy loss spectroscopy (EELS). In this work, a detailed investigation of electrical degradation has been performed on a model perovskite dielectric, Fe-doped SrTiO3 in both single and polycrystalline forms. In the single crystals, three different types of relaxation process were identified by TSDC, namely dipolar orientation of complexes, trap charges of , and ionic space charge with the mobile . The energetics and concentrations of these are monitored as a function of the degradation process. Furthermore, IS is used to model the mechanisms that are spatially redistributed owning to the migration of towards the cathodic region of the crystal. Through modeling all the complex impedance Z*, modulus M*, admittance Y* and capacitance C*, an equivalent circuit model can be developed and key contributors to the IS can be identified. From this it is considered that the cathodic region changes to a conduction mechanism that is both band electron and polaron controlled. The major change during the degradation is to the polaron conduction pathways. Due to the nature of low polaron hopping mobility in this model system, the conductivity from both conductions become comparable providing that the calculated polaron concentration is around 5 order greater than that of band electron. The spatial dimension of the distributed conduction mechanisms is also modeled through the I.S. analysis. Excellent agreement is obtained between the IS data and the EELS data, where ≈30 μm of conducting region is developed at the cathode, and a corresponding high oxygen vacancy concentration on the order of 1019/cm3 is obtained after degradation. Other than those relaxation mechanisms identified in the Fe-doped SrTiO3 single crystal system, an extra relaxation mechanism was found in the polycrystalline systems and was attributed to the relaxation of oxygen vacancies across grain boundaries. Using the initial rise method of TSDC, the activation energies estimated for the relaxation of defect dipoles, the in-grain oxygen vacancies pile up at grain boundaries, and relaxation of oxygen vacancies across grain boundaries are 0.73±0.03, 0.86±0.07, and 1.1±0.09 eV, respectively. An ionic demixing model is applied to account for the evolution of TSDC spectra and to explain changes to the leakage behavior of the degraded samples. In the case of the polycrystalline system, it is suggested that a strong degradation to the insulation resistance occurs when oxygen vacancies migrate across grain boundaries and start to pile up at the cathode region of metallic electrodes. Prior to that point, the vacancies accumulate at partial blocking grain boundaries in each of the crystallites. After gaining a comprehensive understanding of TSDC on Fe-doped SrTiO3 systems, the thermally stimulated depolarization current (TSDC) spectra conducted on real capacitive systems which include multilayer ceramic capacitors (MLCCs), COG capacitors and electrolytic capacitors are discussed based on the theoretical background that was developed from the Fe doped SrTiO3 single and polycrystalline model systems. For the TSDC studies in Ni-BaTiO3 MLCCs, besides two pyroelectric peaks released from the ferroelectric core and shell phase regions, an additional two peaks above the core Curie temperature were ascribed to the relaxation of two types of oxygen vacancy motions, in grain and across grain boundary oxygen vacancy transportation. Activation energies calculated for in grain and across grain boundary oxygen vacancy peaks are 1.06±0.05 and 1.24±0.08 eV, respectively. Another important multi-layer capacitive device is the so-called COG capacitor. In designing COG capacitors, high field break down properties are required at elevated temperatures above 85 oC. A source of the electrical breakdown could be the depopulation of trapped charge. Therefore the trapped charge energies and concentrations in COG capacitors were investigated. The capacitor’s MnO content was found to strongly influence the trapped charge concentration as measured by TSDC. TSDC to electrolytic capacitors was also demonstrated. It is shown that TSDC technique can be a powerful tool to understand underlying defect properties which are not manifested in traditional electrical measurements such as I-V measurement. Electrolytic capacitors based on tantalum oxide are often limited in their performance at high fields and high temperatures due to trapped charges. It is known that leakage is often controlled by Poole-Frankel conduction mechanisms in Ta2O5 electrolytic capacitors. It is determined through I-V measurements that the leakage current indeed follows the Poole-Frenkel conduction characteristic under high field. A parallel TSDC study also confirms at high field and high temperature trapped charge phenomenon. Through the use of TSDC, a new high voltage Poole–Frenkel mechanism at highest field range, >64V, in this study was discovered. It is concluded that TSDC is one of best techniques for capacitor characterization, and recommended other TSDC methods that could be extended to enhance our understanding of structure-property-processing relations in capacitor characterization.