Thermal reliability of piezoelectric materials and structures

压电材料是在机械应变时产生电并在电场下变形的材料。压电材料由于其优异的机电耦合效应，在许多领域得到了广泛的应用，如微声发射传感器、振动监测器、分子识别生物传感器、精密定位器、微型泵、直线步进电机、能量采集装置等，而功能器件的可靠性在很大程度上取决于压电智能元件的正常工作。智能材料的热疲劳导致功能失效如去极化或结构失效如开裂。压电材料在不同工作温度下的疲劳强度-疲劳寿命曲线，即所谓的S-N曲线，是智能器件设计的核心。电循环加载已被广泛应用于压电材料的疲劳性能测试，而机械循环加载，特别是在不同温度下的循环加载，很少用于压电材料的疲劳测试。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，例如突然暴露于高温或低温环境的情况。热效应会以一种随时间变化的波的形式被材料感受到，这是传统的傅里叶热传导理论所无法描述的。非傅立叶热传导理论引入了所谓的热弛豫时间，以考虑热通量和温度梯度响应相对于初始热扰动的时间滞后，从而得到波形双曲线热传导方程。理论结果表明，热机械响应远远超出了静态结果的基础上傅立叶热传导，导致一个重要的可靠性问题。然而，没有实验结果可用于热弛豫时间的精确值。目前的研究主要集中在材料热可靠性的两个主要问题，热疲劳和热松弛。特别地，我们将确定在不同温度下的S-N曲线，和热弛豫时间来解释智能结构中的非傅立叶，波状热扰动，并建立多物理框架来处理智能结构的热可靠性问题。目前的研究将对智能设备的可靠性产生重大影响。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。该项目的成功完成将极大地有利于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.***