Residual Ferroelectricity, Piezoelectricity, and Flexoelectricity in Barium Strontium Titanate Tunable Dielectrics

Open Access
Author:
Garten, Lauren M
Graduate Program:
Materials Science and Engineering
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
October 01, 2014
Committee Members:
  • Susan E Trolier Mckinstry, Dissertation Advisor
  • Clive A Randall, Committee Member
  • Michael T Lanagan, Committee Member
  • Thomas Nelson Jackson, Special Member
Keywords:
  • Piezoelectricity
  • Ferroelectricity
  • Flexoelectricity
  • Barium Strontium Titanate
  • Tunable Dielectrics
Abstract:
Loss reduction is critical to the development of Ba1-xSrxTiO3 (BST) thin film tunable microwave dielectrics. This work addresses mechanisms of loss and performance of Ba1-xSrxTiO3, such as residual ferroelectricity, enhanced flexocoupling, and dc electric field induced piezoelectricity. The presence of residual ferroelectricity –a persistent ferroelectric response above the global phase transition temperature, adds a contribution to dielectric loss from either motion of domain walls or the boundaries of micropolar regions, degrading the tunable performance over a wide frequency range. Rayleigh behavior as a function of temperature was used to track the ferroelectric behavior of BST materials through the ferroelectric to paraelectric transition temperature. The irreversible Rayleigh parameter serve as a metric for the presence of ferroelectricity because this response is dependent on the presence of domain walls, cluster boundaries or phase boundaries. Chemical solution deposited Ba0.7Sr0.3TiO3 films, with relative tunabilities of 86% over 250kV/cm at 100kHz, demonstrated residual ferroelectricity at least 65°C above the ostensible paraelectric transition temperature. The Rayleigh behavior was further corroborated with second harmonic generation, polarization-electric field hysteresis loops and the frequency dependence of the Rayleigh response. The temperature extent of residual ferroelectricity in sputtered and chemical solution deposited films and bulk ceramics was investigated as a function of chemical inhomogeneity on the A-site using electron energy loss spectroscopy. All samples showed some residual ferroelectricity, where the temperature extent was a function of the sample processing. The application of AC electric field for residual ferroelectric measurements of these samples lead to a 100% increase in loss for ac fields exceeding 10kV/cm at room temperature. The presence of residual ferroelectricity in BST also correlates to the increased flexoelectric response in these materials. Residual ferroelectricity is observed in barium strontium titanate ceramics 30°C above the global phase transition temperature, in the same temperature range in which anomalously large flexoelectric coefficients are reported. The application of a strain gradient in this temperature range was shown to lead to strain gradient-induced poling, or flexoelectric poling, enhancing the flexoelectric response. Flexoelectric poling was observed by the development of a remanent polarization in flexoelectric measurements upon the removal of the applied strain gradient. Additionally, an induced d33 piezoelectric response was observed in samples after the removal of the applied strain gradient, indicating that the polarization was realigned during flexoelectric measurements. Flexoelectric poling lead to the production of an internal bias of 9 kV/m. It is concluded that residual ferroelectric response considerably enhances the observed flexoelectric response. In order to investigate the effects of dc electric field induced piezoelectricity, metrology was designed, developed and calibrated for the measurement of the e31,f piezoelectric coefficient as a function of applied electric field and strain. This allowed for direct measurements of the field-induced piezoelectric response for Ba0.7Sr0.3TiO3 (70:30) and Ba0.6Sr0.4TiO3 (60:40) thin films on MgO and silicon. The relative dielectric tunabilities for the 70:30 and 60:40 composition on MgO were 83% and 70% respectively, with a dielectric loss of 0.011 and 0.004 at 100 kHz respectively. A linear increase in induced piezoelectricity with field to –3.0 C/m2 and –1.5 C/m2 at 110 kV/cm was observed in 60:40 BST on MgO and 70:30 BST on Si. Large and hysteretic piezoelectric and tuning responses were observed in the 70:30 BST thin films on MgO. This was consistent with the irreversible Rayleigh behavior, indicating a ferroelectric contribution to the piezoelectric and dielectric response 40°C above the global paraelectric transition temperature. This information should enable advancements in tunable dielectric components through the removal of piezoelectric resonance-based loss mechanisms.