Transparent ferroelectric transducers for flexible haptic feedback touch devices

• Main Author: Fook, Tony Hiu Tung
• Other Authors: Lee Pooi See
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2021
• Subjects: Engineering::Materials::Compositional materials science; Engineering::Materials::Nanostructured materials
• Online Access: https://hdl.handle.net/10356/146727
• Institution: Nanyang Technological University

Relaxor ferroelectric polymers (RFPs), a class of electroactive polymers (EAPs), have shown immense potential for haptic feedback and pressure sensing surfaces. RFPs can be applied in flexible electronics due to its excellent electromechanical properties, flexibility, light weight, mechanical toughness and ease of processing. A major advantage of RFPs such as PVDF-based terpolymers over other EAPs is its excellent optical properties, allowing for integration into devices where transparency is required, such as a haptic film directly mounted on a touch screen. However, a key drawback of RFP actuators is the high voltage needed for actuation compared to other actuator technologies. Common methods to overcome this issue such as adding nanofillers into the EAP matrix for dielectric property enhancement results in a degradation in the electric breakdown strength, dielectric loss and mechanical properties. Achieving enhanced dielectric properties while maintaining a good electric breakdown strength proves to be a challenge. This research presents a novel approach to enhance of the dielectric properties of P(VDF-TrFE-CTFE) while maintaining its electric breakdown strength and mechanical properties via incorporation of an interlayer. The interlayer is formed by either purely nanoparticles or nanoparticles added in EAP to form composites. Insulating ceramic nanoparticles, TiO2, ZrO2 and BaTiO3 and conductive metal Ag nanoparticles in EAP matrix are utilized as the interlayer and are compared in terms of their effects on dielectric and electromechanical properties. When an external electric field is applied, the nanocomposite interlayer induces local intensification of the electric field at the filler/matrix and the interlayer/neat layer interfaces due to trapped charges. This leads to enhanced polarization of the RFP matrix at the interfaces and results in a higher electromechanical performance of the fabricated actuators. Furthermore, by using a flat network of AgNWs with high aspect ratio as the interlayer, similar enhancement in output force can be achieved while attaining excellent optical transparency. This thesis proposes new figure of merits for electrostrictive polymer actuator force and strain that take into account the non-linear dependence of the EAP dielectric properties to the applied electric field strength. The proposed figures of merit hypothesize that the output force is heavily dependent on the polarizability of the active material while strain is dependent on the ratio of polarizability to Young’s modulus. While the overall trend of the measured output force and strain of the EAP actuators is well predicted by the new figures of merit, the model can be further improved by considering the activation field phenomenon observed in the fabricated actuators. In the EAP actuators tested in the study, an activation field was observed where the output force and strain increased drastically when the applied electric field is stronger than the activation field strength. In the interlayer modified EAP actuators, the activation field strength was reduced by 60%, a major advantage for low power haptic devices. By utilizing the new figures of merit for electrostrictive polymer actuator output force and strain, new EAP materials can be realized for next generation actuator devices.