Stress and Interface Engineering of Functional Materials

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Arguably the most important class of functional materials is piezoelectrics. Piezoelectricity, the coupling of mechanical deformation and electrical charge, is the enabling material property for many advanced devices. Current applications, including gyroscopes, accelerometers, and microphones, are essential for the consumer electronics, automotive, and aerospace industries – all important branches of the Canadian economy. In addition, innovative microelectromechanical systems (MEMS), biosensors, and actuators will be needed to develop tomorrow’s economy with the “internet of things”. The newest and most innovative devices often require functional materials in highly restricted geometries, such as thin-films and nanofibers. This miniaturization causes materials to behave very differently from the “bulk” materials that have been so well characterized. In addition to interfaces, one of the primary mechanisms by which geometry can affect properties is by locally altering the stress state experienced by the material. This is not only an engineering challenge, but also a design opportunity: how can stress and interface engineering be employed to control the properties of functional materials in these devices? Efforts to optimize a unique property like piezoelectricity typically involve varying the chemical composition or microstructure. Residual stresses and interfaces are often seen as liabilities rather than powerful tools that can be harnessed to control the underlying physical mechanisms. However, realizing the full potential of functional materials, including piezoelectrics, requires employing all possible methods of manipulation by developing an adequate understanding of governing mechanisms at the restricted geometries applicable to modern devices. This research program develops a novel method for analyzing these complex stresses and interfaces, and uses it to explore their effect on the fundamental mechanisms in piezoelectric materials. Further, this research program investigates stress and interface engineering as a tool that can go beyond chemical composition and microstructure to realize greatly enhanced material properties that have so far been elusive. In particular, the objectives include enabling high-temperature stable piezoelectric ceramic sensors, as well as powerful piezoelectric polymer nanofibers that can act as flexible biosensors. A thermally stable sensor would be invaluable in the energy and aerospace industries, while a useful piezoelectric polymer would be a boon to the consumer electronics and biomedical fields.