Stressing the Limits of Piezoelectricity

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Piezoelectric materials have the reversible ability to convert between a mechanical strain and an electric field: this behavior touches our daily lives, enabling accelerometers, air-bag sensors, and microphones that are essential for the aerospace, automotive, and consumer electronics industries - all important branches of the Canadian economy. Controlling the behavior of these materials is generally attempted by varying the composition or chemistry, with limited success. Instead, we explore a complementary approach: how can an applied mechanical stress be harnessed to enhance the properties of piezoelectric materials? For example, piezoelectric properties decay with increasing temperature, and disappear once the Curie Temperature is reached, typically in the range of 25-250 °C. We will use stress engineering to enable lithium niobate (LiNbO3) to finally overcome this limitation, thereby allowing piezoelectric sensors to finally operate at temperatures exceeding 700 °C. This will allow the real-time health monitoring of critical high temperature systems to predict (and prevent) catastrophic failure, such as in aircraft turbine engines, nuclear reactors, or petrochemical plants. The vast majority of piezoelectric devices employ normal mechanical strains (i.e. deformation parallel or perpendicular to the electric field). This strain mode has allowed engineers to develop a wide range of important applications (guitar pick-ups, sonar, neonatal ultrasounds, etc.). However, what if a material existed that could instead twist in pure torsion when exposed to an electric field? Tellurium dioxide (TiO2) is predicted to be such an exceptional piezoelectric material. We will perform the necessary experimental confirmation and study the effect of mechanical stress on this interesting behavior. The revolutionary novel applications enabled by this unique geometry would include nanoscale gyroscopic accelerometers for use in airplanes, satellites, and cell phones. In general, most piezoelectric materials are rigid, brittle ceramics. Piezoelectric polymers have only very limited applications due to a polymer's relatively small piezoresponse, orders of magnitude lower than ceramics. However, some applications require the mechanical flexibility, biocompatibility, low cost, form factor, and manufacturability that only polymers can provide. We will therefore use mechanical stress engineering to build the first true polyvinylidene fluoride (PVDF) polymer nanofiber piezoelectric device. The consequences of realizing such a smart polymeric nanofiber, one thousand times thinner than a human hair, cannot be overstated: imagine artificial skin that can "feel" temperature and pressure, a T-shirt that can monitor your heartbeat, or an aircraft wing that measures its own deformation in-flight. Mechanical stress engineering can help overcome the temperature, geometry, and mechanical limitations of these promising piezoelectric materials.