Thermal reliability of piezoelectric materials and structures

压电材料是一种在机械应变时产生电能，并在电场作用下变形的材料。由于其优异的机电耦合效应，压电材料被广泛应用于声发射微传感器、振动监测器、分子识别生物传感器、精密定位器、微泵、直线步进电机、能量采集装置等领域。功能器件的可靠性在很大程度上依赖于压电智能元件的正常功能。*智能材料的热疲劳会导致功能失效，如去极化或结构失效，如破裂。作为压电材料在不同使用温度下的疲劳强度-疲劳寿命曲线的重要组成部分，所谓的S-N曲线是智能器件设计的核心。循环电载荷已被广泛用于检测压电材料的疲劳行为，但机械循环载荷，特别是在不同温度下的机械循环载荷，很少被用于材料的疲劳试验。因此，没有可靠的S-N曲线可用。另一个问题是结构在热冲击下的可靠性，例如突然暴露在高或低温度环境中的情况。材料感受到的热效应是一种随时间变化的波，这是传统的傅立叶热传导理论所不能描述的。非傅立叶热传导理论引入了所谓的热松弛时间来考虑热流密度和温度梯度响应相对于初始热扰动的时间滞后，从而得到了一个波形双曲型热传导方程。理论结果表明，热机械响应远远超出了基于傅立叶热传导的静态结果，这导致了一个重要的可靠性问题。然而，对于热弛豫时间的精确值，目前还没有实验结果。*本研究集中在材料的热可靠性的两个主要问题上，即热疲劳和热弛豫。特别是，我们将确定不同温度下的S-N曲线，以及考虑智能结构中非傅立叶、波状热扰动的热弛豫时间，并建立处理智能结构热可靠性问题的多物理框架。本文的研究将对智能设备的可靠性带来很大的影响。研究结果将满足理论界和应用界对智能材料热可靠性的高要求。拟议项目的成功完成将极大地惠及加拿大的先进材料和制造业。该计划将产生许多高质量的论文，这将提高加拿大在全球科学界的形象。

Piezoelectric materials are materials which generate electricity when mechanically strained, and deform under an electric field. Due to their distinguished electromechanical coupling effect, piezoelectric materials have widely been used in many applications, such as acoustic emission microsensors, vibration monitors, molecular recognition biosensors, precision positioners, micropumps, and linear stepper motors, energy harvesting devices, and so on. Reliability of functional devices heavily depends on the proper functioning of piezoelectric, smart components.****Thermal fatigue of smart materials results in functional failure like depolarization or structural failure like cracking. As an essential piece, a fatigue strength versus fatigue life curve, the so called S-N curve for piezoelectric materials at various service temperatures lies in the core of design of smart devices. Cyclic electric loading has been widely used to examine the fatigue behavior of piezoelectric materials, however, mechanical cyclic loading, particularly at various temperatures, has rarely been used in the fatigue test of the material. As such, no reliable S-N curve is available. Another issue is the reliability of structures under thermal shock, such as the case of sudden exposure to a high or low temperature environment. Thermal effect will be felt by the material as a time-dependent wave, which is unable to be described by the traditional Fourier heat conduction theory. Non-Fourier heat conduction theories introduced so-called, thermal relaxation times to account for the time lags of heat flux and temperature gradient response with respect to the initial thermal disturbance, leading to a wave-form, hyperbolic heat conduction equation. Theoretical results show that the thermomechanical response is well beyond the static results based on the Fourier heat conduction, leading to a vital reliability issue. However, no experimental results are available for the exact values of thermal relaxation times.****The present research focuses on the two major issues of thermal reliability of the material, thermal fatigue and thermal relaxation. In particular, we will determine the S-N curves at various temperatures, and the thermal relaxation times to account for the non-Fourier, wave-like thermal disturbances in smart structures, and build the multiphysical framework to deal with thermal reliability issues of smart structures. The present research will bring a strong impact on the reliability of smart devices. The results will meet the high demand of both the theoretical and application communities on thermal reliability of smart materials. The successful completion of the proposed program will greatly benefit the advanced materials and manufacturing sectors of Canada. The program will yield many high quality papers which will enhance the profile of Canada in the global scientific community.***