Micromechanics of piezocomposites

Over the past two decades, materials engineers have developed piezoelectric-polymer composite materials that enable effective electromechanical properties to be tailored for a specific application. These materials are made by combining conventional piezoelectric ceramics with piezoelectrically passive polymers in a variety of geometrical configurations. As with any composite material, the properties and behavior of piezocomposites are highly dependent on the properties of the constituent materials and the local arrangement of the different phases. In particular, the ceramic-polymer interface plays an important role in determining the electromechanical coupling in the piezocomposite. In this dissertation, the electromechanical behavior of 1-3 piezocomposites is investigated from both theoretical and experimental standpoints.Theoretical investigations centered on the development of a micromechanics model for predicting the local fields and effective behavior in piezocomposites with 1-3 connectivity. Since the presence of a thin interlayer or polymer coating around the ceramic rods can influence the local interaction between the piezoceramic and polymer matrix and change the overall performance of the composite, a finite composite cylinder model was developed to incorporate an interlayer with varying properties. Experimental studies focused on probing the surface displacements of 1-3 piezocomposites using a scanning heterodyne laser interferometer. Static surface displacements of 1-3 PZT rod-epoxy samples with different interphase regions were measured and correlated with the effective low-frequency performance of the composite. Several types of interphase region were considered. Coatings with elastic moduli lower than that of the epoxy matrix were applied to the rods. The influence of a silane coupling agent was also investigated. Experimental displacement profiles were compared with micromechanical predictions using the finite composite cylinder model. The results demonstrate that the presence of an interphase between the piezoceramic and the polymer matrix influences the local deformations and changes the overall performance of the composite. Thus, the interphase plays an important role in determining the electromechanical coupling in the piezocomposite.The study of electromechanical coupling in piezocomposites was further developed by investigating the hydrostatic performance of 1-3 piezocomposites, an important issue in design of piezocomposites for low-frequency applications. Emphasis was placed on determining the stress transferred between the passive matrix and the active piezoceramic rods and using this data to indicate the level of electromechanical coupling. The stress field in the piezoelectric ceramic under hydrostatic loading was predicted using the analytical micromechanical model developed and a finite-element model as well. Optimal electromechanical coupling was achieved when a certain favorable stress field was induced in the piezoceramic. The influence of such design parameters as the matrix stiffness, the interphase stiffness, the interphase thickness, the Poisson's ratio of the polymers, and piezoceramic rod aspect ratio on the hydrostatic performance of 1-3 piezocomposites was also investigated. Although the current work is focused on the electromechanical behavior of 1-3 piezocomposites at low frequency, the research results and conceptual understanding obtained have importance for optimizing the design of piezocomposites in other applications as well.U of I OnlyETDs are only available to UIUC Users without author permissio