M. Allahverdi

Piezoelectric composites for sensor and actuator applications

In the last 25 years, piezoelectric ceramic-polymer composites have been conceptualized, prototyped, fabricated, and implemented in an array of applications encompassing medical imaging and military missions, among others. A detailed snapshot of the materials used, and a detailed account of the major innovative methods developed in making various piezoelectric ceramic-polymer composites are presented. The salient aspects of processing of such composites are summarized, and structure-processing-property relations are described using connectivity as the unifying central concept. Computer-aided design (CAD)-based fabrication methods, which result in composites whose structural complexity surpass that of composites obtained with traditional methods, are described to introduce the reader to novel concepts in processing of piezocomposites. A brief survey of some recent advances made in modeling of (0-3), (1-3), and (2-2) composites also is provided.

Processing of advanced electroceramic components by fused deposition technique

Abstract A variety of advanced ceramic components were fabricated using the fused deposition of ceramics (FDC) process. In FDC, ceramic loaded polymer filaments are used to build parts in a layer-by-layer fashion. A process map, based on the compressive strength and modulus of the FDC feedstock, was developed to predict the feasibility of deposition with a variety of FDC filaments. Alumina structures with photonic bandgap properties were deposited for high frequency (GHz) applications. Net shape bismuth titanate components with oriented grains were fabricated by pre-alignment of small volume of seeds in green FDC parts, followed by grain growth treatment. Piezoelectric actuators with novel structures such as spiral and bellows were manufactured and studied.

Solid freeform fabrication of piezoelectric sensors and actuators

The last two decades have witnessed the proliferation piezoelectric composite transducers for an array of sensor and actuator applications. In this article, a concise summary of the major methods used in composite making, with special emphasis on Solid Freeform Fabrication (SFF), is provided. Fused Deposition of Ceramics (FDC) and Sanders Prototyping (SP) are two SFF techniques that have been utilized to make a variety of novel piezocomposites with connectivity patterns including (1-3), (3-2), (3-1), (2-2) and (3-3). The FDC technique has also been used to prototype a number of actuators such as tube arrays, spiral, oval, telescoping, and monomorph multi-material bending actuators. It has been demonstrated that SFF technology is a viable option for fabricating piezocomposite sensors and actuators with intricate geometry, unorthodox internal architecture, and complex symmetry. The salient aspects of processing of such composite sensors and actuators are summarized, and structure-processing-property relations are elaborated on.

An Overview of Rapidly Prototyped Piezoelectric Actuators and Grain-Oriented Ceramics

Rapid prototyping (RP) has been used to fabricate a series of piezoelectric actuators, including spiral and tube actuators, to study the actuation mechanism in these geometries, and to obtain enhanced properties. PZT spiral actuators showed large displacement in mm range, and moderate blocking force. Unimorph spirals (PZT/metal shim) and dual-material (piezoelectric/electrostrictive) PMN-PT spirals were also prototyped and characterized. Tube actuators with inward and outward wall curvature showed slight improvement in axial and radial displacements compared to conventional straight-walled tube actuators. In order to improve the performance of ceramic actuators with polycrystalline microstructures, grain-oriented ceramics of bismuth titanate, lead metaniobate, and PMN-PT were investigated. Texturing was achieved by incorporating anisometric seeds into RP feedstock, aligning them during fabrication, and growing the seeds (templates) at elevated temperatures. Synthesis of anisometric seeds and pertinent processing conditions of the textured ceramics are presented. The feasibility of making net shape single crystal components was also explored. Single crystals of 0.65PMN-0.35PT were grown in FDC components using embedded (111) and (110) SrTiO3 seeds at 1250°C.

3‐D photonic bandgap structures in the microwave regime by fused deposition of multimaterials

Three‐dimensional photonic bandgap (PBG) structures using alumina (Al2O3) as the high permittivity material were modeled and then the structures were fabricated by Fused Deposition of Multi‐materials (FDMM) technology. A finite element method and a real‐time electromagnetic wave propagation software were used to simulate and design the layered PBG structures for applications in the microwave frequency range. The modeling predicted a 3‐D photonic bandgap in the 16.5–23.5 GHz range. FDMM provides a computer‐controlled process to generate 3‐D structures, allowing high fabrication flexibility and efficiency. Electromagnetic measurements displayed the presence of a bandgap between 17.1–23.3 GHz, showing a good agreement with the predicted values. These PBG structures are potential candidates for applications in advanced communication systems.

Processing of Piezoelectric Fiber/Polymer Composites with 3-3 Connectivity

The processing-property relationship of dielectric and piezoelectric properties of 3-3 fabric composites was studied. Fine PZT green fibers were woven into plane fabric, cut in 3 × 3 cm2 pieces, and stacked 8 plies high. The stacks were heat-treated with and without applied pressure and then embedded in soft and hard polymers. Increasing the applied pressure increased the density and electromechanical properties of composites due to the improvement of the ply-to-ply sintering. At an optimum uniaxial pressure of 588 Pa, the piezoelectric and dielectric properties of composites with ≈41 vol% PZT and soft matrix were K = 230, d33 = 220 pC/N, g33 = 108 mV/m, and d33g33 = 23778 × 10−15 m2/N. The porosity-permittivity relationship was used to determine the depolarizing factor of the composites, which represented the degree of ceramic continuity between electrodes. In general, composites with soft matrix had higher electromechanical properties than those with hard matrix. The effect of poling direction on the piezoelectric and dielectric properties was also investigated. The composites poled along the PZT fibers had higher electromechanical properties than those poled perpendicular to the PZT fibers.

Processing and properties of piezoelectric actuators developed by fused deposition technique

Fused Deposition of Ceramic (FDC) has been developed for rapid prototyping of electro-ceramic components via hot extrusion of ceramic-loaded polymer filaments. Several FDC filaments including PZT, PMN-PT ceramics, and electrode material (silver-palladium) have been developed and used successfully in the fabrication of a wide range of components. A new fused deposition facility capable of depositing up to four different materials in a single deposition step has also been designed and developed. Processing of single and multimaterial components and net-shape grain oriented parts is now under investigation. In this paper, processing and electromechanical properties of a few novel designs such as bellows, helical tubes, spiral and other multimaterial actuators are discussed.

Fabrication of bismuth titanate components with oriented microstructures via FDC and TGG

Fine Bismuth Titanate (Bi/sub 4/Ti/sub 3/O/sub 12/, BiT) powder and platelet seeds have been synthesized using sol-gel and molten slat techniques, respectively. BiT filaments of 48 vol.% solids loading (including 5 vol.% seeds) have been developed for fused deposition of net shape components. The survival of the seeds during filament fabrication (compounding and extrusion) has been monitored and necessary changes have been made to minimize seed damage during processing. The orientation of the seeds within the filament and the fabricated green components was studied to find the optimum deposition parameters and the conditions that yield maximum seeds alignment. The green FDC components were heated to remove the binder at 550/spl deg/C. During sintering and/or Templated Grain Growth (TGG) process at a temperature range of 1000 to 1100/spl deg/C, the seeds grew and induced an oriented microstructure by consumption of the fine grain matrix. X-ray technique and SEM were used to characterize the degree of orientation.

Direct-write of PZT thick films

Lead zirconate titanate (PZT) thick films have been prepared on alumina substrates using a direct-write technology. Thick films of 50 to 200 /spl mu/m were deposited, dried and then sintered at 1000/spl deg/ to 1200/spl deg/C for 30 minutes in PbO-rich atmosphere. Comparison of the thick film properties revealed that the dielectric constant and remnant polarization of the films sintered at 1100/spl deg/C were maximum, likely due to a balance between densification and lead loss. The effects of three sintering aids, i.e., lead oxide, lithium bismuth oxide, and a borosilicate glass, have been investigated on the microstructure and electrical properties of the PZT thick films. It was observed that a 2 wt% lithium bismuth oxide additive has a positive effect on the dielectric constant and remnant polarization of the PZT thick films sintered at 1100/spl deg/C. The microstructures of the films revealed that 3 wt% additives would result in excessive shrinkage and formation of large pores and cracks at a sintering temperature of 1100/spl deg/C and above. PZT films with 2 wt.% lithium bismuth oxide showed improved dielectric constant (/spl sim/40-50%) and remanent polarization (/spl sim/65%) compared to additive-free PZT thick films. It was also found that the properties are strongly thickness dependent.

Development of tube actuators with helical electrodes using fused deposition of ceramics

Fused Deposition of Ceramics (FDC), a Solid Freeform Fabrication technique, has been used to manufacture tube actuators with helical electrodes. It is expected that the special electroding along the length of the tubes results in higher axial displacements since d/sub 33/ coefficient is applicable as opposed to d/sub 31/ in radially poled tubes. A silver-palladium electrode filament with solids loading of 55 vol.% was developed. Two different methods of deposition were used to fabricate the tubes. with helical electrodes either within the tube wall or on the outer surface of the tubes. Electrodes with different thickness and width were deposited on the tubes. Modified heating procedures for binder burnout and sintering, as well as special electrode geometries were used to minimize the damage, occurring due to the oxidation and subsequent reduction of palladium at temperatures < 800/spl deg/C.