Multi-Material Three-Dimensional Printing of Piezoelectric Devices for Sensing and Energy Harvesting Applications

Piezoelectric materials serve as transducers to linearly convert mechanical energy to electrical energy and vice versa. Based on this capability, a variety of piezoelectric devices with desired geometry, performance and function have been fabricated for sensing, energy harvesting and actuating applications. Among them, three-dimensional (3D) flexible and conformal piezoelectric composite devices (high piezoelectric ceramic particles as filler; flexible polymers as matrix) are in great demand in many fields such as smart artificial skins, battery-free wearable sensors and energy harvesters, and soft robotics. Conventional manufacturing methods for making the piezoelectric devices such as solution casting, spin-coating, melt-extrusion, electrospinning, limit the complexity of the fabricated structures. Thus, 3D printing techniques such as direct-ink writing (DIW), stereolithography (SLA), digital light processing (DLP) and fused filament fabrication (FFF) have been increasingly used for this purpose. Additionally, to make the electrical connection between the piezoelectric structures (fabricated using conventional or 3D printing techniques) and the interfaces, there is usually an additional step to add electrodes on the piezoelectric structures by painting, sputtering, or attaching conductive tapes or fabrics. The emergence of multi-material 3D printing technique provides an efficient way to combine the piezoelectric structure and electrodes fabrication processes. The purpose of this dissertation is to develop 3D flexible piezoelectric composite devices with electrodes for piezoelectric sensing and energy harvesting applications based on multi-material 3D printing technique. First, to find out the most suitable piezoelectric material for the multi-material 3D printing process, we printed polyvinylidene fluoride (PVDF) films from the commercially available PVDF filament using a FFF 3D printer. We found that the as-printed PVDF films have a low electroactive β phase content (Fβ) of 39%, thus it requires post stretching process to increase the Fβ. The stretched and polarized PVDF films with a dimension of 40 × 20 × 0.06 mm3 (length × width × thickness) have a piezoelectric coefficient (d33) of 7.29 pC/N and a Fβ of 65 % at a stretching ratio (R) of 4 after being polarized under an electric field of 30 V/μm. Although the resulting d33 of the fabricated PVDF films has been substantially enhanced by ~10-100 times higher than the related reported values, the stretching process limits the possibilities of making complex geometries with PVDF. Subsequently, we incorporated high piezoelectric lead zirconate titanate (PZT) nanoparticles into flexible thermoplastic polyurethane (TPU) matrix and extruded the composite material into an FFF-printable filament (~1.75 mm). The developed TPU/30 vol% PZT piezoelectric composite material has a large elongation at breakage of ~56% and a d33 value of 6.8 pC/N after being polarized by the appropriate poling parameters. A multi-material FFF 3D printing technique was then utilized to fabricate 3D flexible piezoelectric composite lattices with electrodes with our developed piezoelectric composite filament and a commercially available conductive composite (TPU/carbon black) filament. Four types of 3D piezoelectric composite lattices: simple cubic (SC), body-centered cubic (BCC), cuboctahedron (CH) and octet truss (OT) (and one fully dense solid (FD) used as a benchmark) were designed and fabricated. The piezoelectric performance of the lattices was compared by means of experimental and finite element analysis (FEA) methods. At the same maximum compressive force, OT lattice can generate ~two times superior voltage output than FD sample. As a demonstrator, an original multi-material OT piezoelectric composite shoe sole can generate a high peak-to-peak voltage (Vpp) ~ 20 V by human stomping. Further, to fabricate freeform or conformal flexible piezoelectric structures, we conducted a multi-material DIW 3D printing process in a supporting fluid. The developed mineral oil/6% (w/v) fumed silica supporting fluid with appropriate yield stress behaves as a Herschel-Bulkley Fluid. During the printing process, the crosslinked fumed silica network temporarily flows under the stress generated by the moving dispensing needle, and then rapidly recover to a solid state to hold the deposited ink. The compositions of the inks are: polydimethylsiloxane (PDMS)/30 vol% PZT as the piezoelectric composite ink and PDMS/25 vol% silver (Ag) as the conductive composite ink. A comprehensive characterization of the inks' properties (i.e., piezoelectric or conductive behaviors, rheological and mechanical properties) and the rheological behavior of the supporting fluid with different compositions were performed. Three demonstrators: a multi-layer planar film, a conformal non-planar hemisphere and a 3D structure composed of six spirals printed vertically between two hexagon layers (freeform spiral-hexagon) were fabricated and tested under tension or compression tests, respectively. For example, by cyclically compressing the freeform spiral-hexagon structure at 8 Hz for more than 800 cycles, a Vpp of 86.39 ± 1.145 mV was obtained, which shows the good durability of the developed piezoelectric spiral-hexagon sensor and the potential of this technique to be used for fabricating compact freeform piezoelectric structures with electrodes.