Structural Changes During Fatigue Of Thin Film Ferroelectric Lead Zirconate Titanate

Author : Travis Peters
Publisher :
ISBN 13 :
Total Pages : 0 pages
Book Rating : 4.:/5 (138 download)

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Download or read book Extrinsic Contributions in Lead Zirconate Titanate Films written by Travis Peters and published by . This book was released on 2023 with total page 0 pages. Available in PDF, EPUB and Kindle. Book excerpt: This thesis probes how extrinsic contributions affect the dielectric, piezoelectric, and ferroelectric properties of morphotropic phase boundary lead zirconate titanate (PZT) films. Secondly, the influence of grain and grain boundary microstructure on domain behavior under an electric field was investigated. Domain wall mobility via the Rayleigh Law was locally probed to investigate avalanche characteristics and the width of influence of individual grain boundaries on the nonlinear piezoelectric response. This was coupled with macroscopic characterization showing the dependence of the domain structure on the thermal stress induced from substrate clamping effects. The results guided an attempt to fabricate a self-powered, wireless PZT thin film insole sensor for applications involving balance detection to assist the elderly population. A novel lead-free flexoelectric array was also prototyped for eventual use in a self-powered force sensing device, that can harvest energy from a heel-strike via the direct flexoelectric effect. In undoped lead zirconate-titanate (PZT) films 1-2 [mu]m thick, domain walls move in clusters with a correlation length of ~ 0.5--2 [mu]m. Mapping of the piezoelectric nonlinearity via band excitation piezoresponse force microscopy (BE-PFM) showed that doping with niobium (Nb) increases the average concentration or mobility of domain walls without changing the cluster area of correlated domain wall motion. In contrast, manganese (Mn) doping reduces the contribution of mobile domain walls to the dielectric and piezoelectric responses without changing the cluster area for correlated motion. In both Nb and Mn doped films, cluster area increases as film thicknesses rise from 250 to 1250 nm while cluster density drops; this can be seen in spatial maps generated from the analysis of irreversible to reversible ratios of the Rayleigh coefficients. Next, the effect of microstructural features such as grain boundaries and triple points on the pinning of domain wall motion in perovskite PZT films was investigated. Spatial variability in the collective domain wall dynamics was assessed using non-linearity mapping via BE-PFM. Collocating the non-linearity maps with triple point locations (visualized by electron back scatter diffraction) allowed for exploration of the effect that local microstructure (e.g., grain boundary) has on domain wall motion. It was found that the extrinsic behavior varied with both the misorientation angle and the proximity to the grain boundary. The width of influence of individual grain boundaries on the motion of domain walls was a function of the character of the grain boundary; random grain boundaries exhibit deeper minima in [alpha]d/d33,initial and larger widths of influence (up to 905 nm) compared to coincident site lattice (CSL) boundaries (up to 572 nm). Additionally, triple points containing larger numbers of random boundaries exhibited non-Rayleigh behavior to greater distances, suggesting that the triple point provides either a deep potential minimum or a region where domain wall motion is unfavorable. Piezoelectric thin films were dip coated onto flexible metal substrates to investigate the dependence of macroscopic dielectric and ferroelectric properties on the coefficient of thermal expansion mismatch and substrate thickness. The bending stiffness was controlled by the thickness of the substrate. Grazing incidence x-ray diffraction displayed distinct peak splitting for Nb-doped PZT on flexible Pt, Ni, Ag, and stiff Ni substrates, where the out-of-plane d-spacing and integrated peak area for c-domains was highest with the largest film compressive stress. As expected, PZT films on stiff Si were under tensile stress and contained more in-plane domains. The dielectric permittivity was highest in PZT on stiff Si and lowest for PZT on thick Ni, while remanent polarization displayed the opposite trend, commensurate with the residual stress state as well as the resistance to bending in thick substrates as a strain-relief mechanism. The irreversible Rayleigh coefficient decreased dramatically upon poling for PZT on flexible substrates compared to PZT on stiff substrates; the [alpha][epsilon]/[epsilon]initial ratio was 56% higher in PZT on a flexible Ni substrate relative to a stiff Ni substrate at 100 Hz prior to electrical poling. This investigation distinguishes the impact of substrate flexibility from thermal expansion on ferroelectric domain mobility and provides dip coating conditions for high quality piezoelectric films on any substrate. The resulting PZT films on metal foils were employed in the fabrication of a low power insole embedded force sensor array attempting to monitor a patient's balance and weight distribution while standing, walking, or running. Flexible piezoelectric films as force sensors eliminate the need for standby energy, providing high sensitivity and flexibility in sensor array design. Lead zirconate-titanate piezoelectric films 1 [mu]m thick were dip coated onto a 25 [mu]m thick stainless steel flexible metal foil. The film displayed a 47% Lotgering factor for the 100 crystallographic direction and exhibited a high-density granular perovskite structure with little pyrochlore near the middle and bottom of the dip cast film. The films showed high remanent polarization values of +28.2 [mu]C/cm2 and -24.3 [mu]C/cm2 and typical coercive fields of 59.4 kV/cm and -56.7 kV/cm. This piezoelectric sensing array with 24 photolithographically-defined electrodes enabled the simulation of a single toe response, the ball of the foot rolling during a step response, and a heel-strike emulation response. Voltage measurements extracted from cyclic applied forces from 0 to 30 N showed a linear response with a sensitivity of -9.76 mV/N between 0 to 12 N and a nonlinear response between 12 to 30 N. The roll test provided ~100 mV responses when expected during a perpendicular and diagonal roll on four individual sensors, each with fast response times and some mixture of bending and compressive stresses. The heel-strike emulation above a single electrode exhibited a response of ~300 mV with 60 N compressive force, ~100 mV from a nearby electrode, and minimal response from electrodes further from the applied force. A discrete circuit was designed and tested on a printed circuit board for multi-channel sensing, digitization, amplification, and wireless transmission of the activation signal. Finally, a lead-free flexoelectric device was fabricated in an attempt to provide a power-source for the electronics associated with the PZT film insole sensor. Flexoelectric polarization output scales with dielectric permittivity and strain gradient; thus, it is proposed that a barrier layer capacitor with doped silicon as the conducting medium will enhance the flexoelectric coefficient via space charge polarizability. A cantilever beam was fabricated as proof of concept, which displayed a flexoelectric coefficient of 4.9 ± 0.4 [mu]C/m. Furthermore, a centrosymmetric 100 silicon wafer was processed with an anisotropic wet etchant into truncated pyramid arrays varying in size from 100s of microns to tens of microns. A dielectric passivation layer acted as the insulating region within the asymmetric barrier layer capacitor, and interfacial space charge polarizability generated effective permittivities that exceed those possible with paraelectrics. The novel centrosymmetric flexoelectric fabrication procedure exhibited here generated the capability to decrease the structure size by orders of magnitude as well, thereby increasing the flexoelectric polarization response in proportion. A scanning probe-based methodology was developed to directly measure the local converse flexoelectric response of a single pyramid with a height of 70 [mu]m. The feasibility of ferroelectric material-free flexoelectricity was analyzed via both direct and converse flexoelectric measurements at the macro-scale and nano-scale.