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Zoubeida Ounaies, Mechanical Engineering, Virginia Commonwealth University
"Organic-Inorganic Nanocomposites: Enhanced Sensor and Actuator Response"
The first piezoelectric polymer was discovered more than forty years ago, but it is only in recent years that piezoelectric polymers have gained notoriety as a valuable class of smart materials. Polymers offer the advantage of processing flexibility because they are lightweight, durable, readily manufactured into large areas, and conformable to complex shapes. Other notable features of polymers include their low dielectric constant, low elastic stiffness, and low density, which result in a high voltage sensitivity (excellent sensor characteristics) and low acoustic and mechanical impedance (crucial for medical and underwater applications).
In order to meet sensors requirements in aerospace applications, our group focused on developing piezoelectric polymers that combine improved properties over available piezoelectric polymers, namely, temperature and chemical stability, good mechanical properties and controllable electrical properties. Accordingly, amorphous polyimides containing polar functional groups have been synthesized and investigated for potential use as high temperature piezoelectric sensors. The polyimides show distinct improvements over state-of-the-art piezoelectric film sensors, which include enhanced polarization, polarization stability at elevated temperatures, and improved processability. Structural changes, such as increase in dipolar concentration, and process variation, such as in-situ poling while curing, were investigated in previous studies to improve the piezoelectric behavior. In recent work, a different approach is chosen to enhance the piezoelectricity of the polyimides. As a first step, a polyimide is selected as a matrix to develop piezoelectric organic-inorganic nanocomposites. These composites could further improve the performance of micro- and nano-devices by expanding the temperature range and significantly enhancing the mechanical and piezoelectric properties. Further, the composites utilize carbon nanotubes (CNT) to enhance the dielectric, mechanical and thermal properties of the polymer matrix and nanoscale piezoelectric lead zirconate titanate inclusions (PZT) to strengthen the electroactive properties of the overall composite. The dielectric and electrical properties of the composites are investigated as a function of CNT volume content. The dynamic and static mechanical properties are also presented to assess the effect of the inclusions on the macro-scale properties of the composites. The degree of dispersion of the CNTs and PZT powder is demonstrated by HRSEM. It is observed that the CNTs increase the dielectric, piezoelectric, mechanical and thermal properties of the polyimide matrix. Addition of the CNT in the Lead Zirconate Titanate (PZT)/polyimide composites facilitates poling and results in an increase of the piezoelectric properties of the composite; a small amount of CNT (0.1 to 0.2 wt%) facilitated the simultaneous poling of the polyimide and PZT inclusions, resulting in a value of remanent polarization, Pr, one order of magnitude higher than that of the pristine polyimide. The PZT-CNT-polyimide composite remained flexible and was relatively easy to process, where a standard polymer casting technique was utilized.
THURSDAY, October 23, 2003