Study on Strain Gradient Sensing and Flexoelectric Micro/Nano Structures.

摘要

Flexoelectricity defines the electric polarization induced by mechanical strain gradient, or the mechanical strain generated under an electric field gradient. Compared with the more widely applied electromechanical coupling effect, piezoelectricity, flexoelectricity is free of the material symmetry limitation and exists in all dielectrics in principle. However, flexoelectricity has not attracted much of research interest until very recently, attributing to the experimental demonstration of large flexoelectricity in high permittivity ferroelectric materials.;To demonstrate the size independence of flexoelectricity in ferroelectric materials, we investigated the transverse flexoelectric coefficient in cantilever structures with the thicknesses ranging from millimeter to tens of micrometers. Consistence of the measurement results indicate that flexoelectric microstructures holds great potential for sensing/actuating applications due to the scale effect of the flexoelectricity. On the other hand, shear flexoelectric coefficient of ferroelectric ceramic was measured in the converse manner for the first time. These work lay the foundation of attaining the comprehensive flexoelectric tensors.;Strain gradient is regarded as a better indicator of the structural health than mechanical deformation or strain at some circumstances. A strain gradient sensor was designed and fabricated based on the barium strontium titanate (BST) micro-bar. Experimental charge output showed a good linearity with the average strain gradients with a sensitivity of 88 pC·m. The strain gradient sensor was further used to monitor the curvature of mechanical structures. The curvature sensor provides a sensitivity of 30.78 pC·m in comparison with 32.48 pC·m from theoretical prediction. In addition, the bending stiffness can be acquired from beam moment-curvature relationship that is derived from the curvature sensor reading and is verified by Euler- Bernoulli beam theory. In addition, the strain gradient sensor was applied to read out the curvature alteration and the moment of stiffness of the structure, which lacks a sensitive approach by conventional fiber optic or strain gauge methods. Furthermore, the strain gradient sensor was employed to measure the opening mode stress intensify factor. The effect of constant stress component in precisely determining the stress intensity factor KI can be eliminated by SGS assisted measurement, attributed to the nature of strain gradient. This helps to diminish the measurement error to less than 5%. Given the known intensity factor the crack, the strain gradient sensor could be used to monitor the crack initiation and propagation status.;Another favorable inherent property of flexoelectricity is the scale effect embedded in the gradient term of flexoelectric polarization. This allows the flexoelectric effect to be more prominent in miniaturized electromechanical systems. To overcome the fabrication difficulties, we developed a hybrid technique for flexoelectric micro structure fabrication by combining the mechanical precision dicing and wet etching. Micro pyramids with the size of tens micrometers were fabricated and the converse flexoelectric properties were measured using a laser vibrometer based system. The effective d33 were measured to be 39.8 pm/V and 85.8 pm/V for two pyramid arrays with different sizes, as expected by the scale effect. Preliminary study of the nanopyramid fabrication using focused ion beam and light interference lithography were present. Future work will be the characterization of flexoelectric properties of nanopyramid using a customized atomic force microscope system.