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Materials Science at Oregon State University

Graduate Student Seminars - Austin Fox and Nitish Kumar

Date: Thursday, Oct 17th
Presenter: Austin Fox and Nitish Kumar, OSU Materials Science

Abstract


Nitish Kumar abstract:
Absence of Fatigue and Fatigue Mechanisms in Pb-free Bi(Mg1/2Ti1/2)O3-(Bi1/2K1/2)TiO3-(Bi1/2Na1/2)TiO3 Ceramics for Actuator Applications

Lead-free ceramics of composition Bi(Mg1/2Ti1/2)O3-(Bi1/2K1/2)TiO3-(Bi1/2Na1/2)TiO3 were prepared using solid state synthesis techniques. The dielectric spectra showed a Tmax of more than 320 °C for all compositions and the transitions became increasingly diffuse as the Bi(Mg1/2Ti1/2)O3 content increased. A lower temperature transition, indicating a transformation from an ergodic to a non-ergodic relaxor state, was also seen for all compositions and this transition temperature decreased as the mole fraction of Bi(Mg1/2Ti1/2)O3 increased. The composition with 1% Bi(Mg1/2Ti1/2)O3 showed characteristic ferroelectric-like polarization and strain hysteresis. However, compositions with increased Bi(Mg1/2Ti1/2)O3 content became increasingly ergodic at room temperature with pinched polarization loops and no negative strain. Among these compositions, the magnitude of d33* increased with Bi(Mg1/2Ti1/2)O3 content and the composition with 10% Bi(Mg1/2Ti1/2)O3 exhibited a d33* of 422 pm/V . Fatigue measurements were conducted on all compositions and while the 1% Bi(Mg1/2Ti1/2)O3 composition exhibited a measurable, but small loss in maximum strain after a million cycles; all the other compositions from 2.5% to 10% Bi(Mg1/2Ti1/2)O3 were essentially fatigue-free. Optical and AC impedance measurements were employed to identify intrinsic conduction as the dominant conduction mechanism. These compositions were also highly insulating with high resistivities (107 ohm-cm) at high temperatures (440 °C). To investigate the role of point defects on the fatigue characteristics, the composition 5BMT-40BKT-55BNT was doped to incorporate acceptor and donor defects on the A and B sites by adjusting the Bi/Na and Mg/Ti stoichiometries. The presence of defects did not strongly influence the fatigue behavior in donor-doped compositions owing to the nature of their reversible field-induced phase transformation. Even for the acceptor-doped compositions which had stable domains in absence of electric field at room temperature, there was minimal to no degradation in maximum strain due to fatigue.

Austin Fox abstract:
Crystallographic Texture and Surface Morphology of Pt/TiO2 Templates for Enhanced PZT Thin Film Performance

Optimized processing conditions for Pt/TiO2/SiO2/Si templating electrodes were investigated. These electrodes are used for [111]-textured lead zirconate titanate (Pb[ZrxTi1-x]O3) (PZT) thin films, or [001] PZT when used in combination with a PbTiO3 seed layer. Titanium deposited by dc magnetron sputtering yields [0001] texture on a thermally oxidized Si wafer. When oxidized, the titanium converts to [100]-oriented rutile. This matches well with the (111) plane of Pt, and as a result it is possible to achieve strongly [111]-oriented Pt. The quality of the orientation and surface roughness of the TiO2 (and the preceding Ti) directly affect the achievable Pt texture and surface morphology. A transition between crystallographic texture control and substrate surface smoothness control occurs at approximately 30 nm Ti thickness (which converts to 45 nm of TiO2) corresponding to 0.5 nm (2 nm for TiO2) RMS roughness and 5.5° (3.1° for TiO2) full-width-half-maxima for the 0002 Ti and 002 TiO2 peaks, respectively.