Structure-Property-Performance Relationships of New High Temperature Relaxors for Capacitor Applications
Open Access
- Author:
- Stringer, Craig Joseph
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- October 27, 2006
- Committee Members:
- Clive A Randall, Committee Chair/Co-Chair
Thomas R Shrout, Committee Chair/Co-Chair
Susan E Trolier Mckinstry, Committee Member
Leslie Eric Cross, Committee Member - Keywords:
- Relaxor ferroelectrics
High temperature capacitors
Vogel-Fulcher
Perovskite - Abstract:
- This thesis extends the investigations on perovskite solid solutions based on PbTiO3–Bi(Me',Me")O3 (Me'= Sc3+, Zn2+, Mg2+, Ni2+, In3+, Fe3+, etc and Me"= Ti4+, Nb5+, W6+) systems. The ferroelectric transition temperature (TC) behavior was considered in the tetragonal phase region of the PbTiO3–Bi(Me',Me")O3 systems. Trends in the TC compositional dependence exhibited three main cases: case 1, a continued increase in transition temperature above the end–member PbTiO3 (495 oC); case 2, an increase and then decrease of the transition temperature; and case 3, a continuous decrease in the transition temperature with Bi(Me',Me")O3 additions. New relaxor materials were developed from the PbTiO3–Bi(Me',Me")O3 solid solutions; specifically, the Bi(Mg3/4W1/4)O3–PbTiO3 (BMW–PT) binary solid solution and BiScO3–Pb(Mg1/3Nb2/3¬)O3–PbTiO3 (BS–PMN–PT) ternary solid solution were investigated. Permittivity, polarization and pyroelectric measurements were performed on BMW–PT and BS–PMN–PT compositions with respect to temperature with characteristic relaxor behavior observed. The complex solid solution BMW–PT exhibited a morphotropic phase boundary at ~ 48 mol% PbTiO3 with a corresponding TC of 205 oC. On further structural analysis with diffraction contrast transmission electron microscopy along with x–ray diffraction, evidence of B–site ordering was observed. The BS–PMN–PT proved to be a model system with high temperature relaxor properties of Tmax ~ 250 oC to 300 oC and max ~ 14,000 to 17,000 at 1 kHz. The deviation temperature, TD, or temperature of the onset of local spontaneous polarization, was determined by thermal strain measurement and high temperature dielectric measurement to be approximately 600 oC; up to 250 oC higher than any reported value for relaxor ferroelectrics. The frequency dependence of the temperature of the permittivity maximum was found to follow the Vögel–Fulcher relationship, with an activation energy (EA) of ~0.1 eV, and a freezing temperature (Tf) of ~ 150 oC. Static and in–situ transmission electron microscopy investigations of the BS–PMN–PT compositions demonstrated a frustrated microstructure of nanometer scale regions and were used to establish structure–property relationships with different electric field and thermal histories. A comparative study of the key relaxor parameters, EA, Tf, and TD was tabulated with previously investigated relaxor ferroelectrics. These parameters were found to scale relative to other lead–based perovskite relaxor ferroelectric compounds and solid solutions, with the BS–PMN–PT ternary system exhibiting the highest temperature behavior. Finally, to demonstrate one possible application area for these materials, multilayer ceramic capacitor devices were designed for operation at 300 oC and up to 10 kHz. The voltage saturation was found to be extremely encouraging at 300 oC with observed changes in capacitance (~3%) on the application of 10 kV/cm. The insulation resistivity followed an Arrhenius behavior and at 300 oC was ~ 1010 –cm. Weibull statistics were used to estimate a characteristic breakdown field at 300 oC for the BS–PMN–PT multilayer capacitors of ~40 kV/cm. Current–voltage measurements were performed to voltages up to breakdown and exhibited Ohmic behavior, indicating intrinsically controlled conduction. Highly accelerated life time tests were performed on BS–PMN–PT capacitors. It was observed that silver migration from termination electrodes caused premature failure at elevated temperature.