Stressing the Limits of Piezoelectricity

压电材料具有在机械应变和电场之间进行可逆转换的能力：这一行为与我们的日常生活息息相关，使得加速计、安全气囊传感器和麦克风成为航空航天、汽车和消费电子行业的关键，这些行业都是加拿大经济的重要分支。通常通过改变成分或化学成分来控制这些材料的行为，但收效甚微。相反，我们探索了一种互补的方法：如何利用施加的机械应力来增强压电材料的性能？例如，压电性能会随着温度的升高而衰减，一旦达到居里温度，通常会在25-250°C的范围内消失。我们将使用应力工程来使铌酸锂(LiNbO_3)最终克服这一限制，从而使压电传感器最终能够在超过700°C的温度下工作，这将使关键高温系统的实时健康监测能够预测(和防止)灾难性故障，如在飞机涡轮发动机、核反应堆或石化工厂中。绝大多数压电器件采用正常的机械应变(即平行或垂直于电场的变形)。这种应变模式允许工程师开发广泛的重要应用(吉他拾音器、声纳、新生儿超声波等)。然而，如果存在一种材料，当暴露在电场中时，它可以以纯扭转的形式发生扭曲，那该怎么办？二氧化碲(Ti02)被预测是一种如此特殊的压电材料。我们将进行必要的实验验证，并研究机械应力对这一有趣行为的影响。通过这种独特的几何结构实现的革命性的新颖应用将包括用于飞机、卫星和手机的纳米级陀螺式加速计。一般来说，大多数压电材料是刚性的、脆性的陶瓷。由于聚合物的压电响应相对较小，比陶瓷低几个数量级，因此压电聚合物的应用非常有限。然而，一些应用要求机械灵活性、生物兼容性、低成本、外形因素和可制造性，这些都是只有聚合物才能提供的。因此，我们将利用机械应力工程来建造第一个真正的聚偏氟乙烯(PVDF)聚合物纳米纤维压电器件。实现这样一种比人类头发还细一千倍的智能聚合物纳米纤维的后果怎么说都不为过：想象一下，人造皮肤可以“感觉”温度和压力，一件T恤可以监测你的心跳，或者飞机机翼可以测量自己在飞行中的变形。机械应力工程可以帮助克服这些有前途的压电材料的温度、几何和机械限制。

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.