Screen-printed low temperature piezoelectric thick films for energy harvesting on fabrics

Abstract

This thesis details the improvement in the dielectric, piezoelectric and mechanical (such as flexibility) properties of screen-printed flexible low-temperature PZT-polymer composite films on woven-fabrics. These improvements have been achieved by optimising the composite formulation and the post processing procedures. The polymeric binders were evaluated to find the optimum PZT-binder composite. The optimum PZT-binder composite with weight ratio 2.57:1 (denoted ECS-PolyPZT 6a) provided a relative dielectric constant of 146 and a d33 value of 22.8 pC/N at initial poling conditions measured on alumina. The PZT phase was a mixture of PZT particle sizes 2 and 0.8 μm with weight ratio of 4:1. Applying optimum poling conditions improved the d33 values to 36, 40 and 70 pC/N when the material screen-printed on alumina, Kapton and Polyester-cotton. The difference in the d33 coefficients on the different substrates is due to the clamping effect and in each case the free-standing d33fs value was calculated to be 80 pC/N. Applying cold isostatic pressing (CIP) at 250 MPa for 2 minutes improved the d33 values to 76.6 pC/N on Polyester-cotton. Adding 0.2% silver-nano particles by weight to the material improved the d33 to 76 pC/N on Polyester-cotton with no CIP. Combining these two measures yields a d33 value of 83 pC/N. The final optimised d33fs was 98 pC/N compared with the original value of 49 pC/N.

When evaluating these films for use in energy harvesting applications, it was found that when applying compressive and tensile forces, the output mainly depends on the compliance of the substrate. The optimum PZT-binder composite printed on 1 × 1 cm2 woven-fabric Kermel, which has a higher compliance than Polyester-cotton and Cotton fabrics, provided output energy of 0.2 and 0.018 μJ/cycle when connected to a 30 and 1 MΩ resistive load in compression and tension, respectively. However, when applying a bending force to the sample, the output voltage increases with reducing compliance and increasing thicknesses of the substrate. Cotton substrates with lower compliances and higher thickness gave the highest energy output of 0.192 μJ per bending cycle into 70 MΩ when applying a bending curvature of 5 mm radius. Therefore, careful selection of the substrates is important to maximise the performance in sensing or energy harvesting application.

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ORCID for Stephen Beeby: orcid.org/0000-0002-0800-1759

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