Characterizing the Pyroelectric Effects in Macro-fiber Composites

Abstract

Pyroelectricity occurs in polar materials, which causes an electric potential to be generated when there is a temporal temperature change. This dissertation seeks to investigate this pyroelectric effect in Macro-fiber Composites (MFCs), a composite material that has mainly been utilized in piezoelectric applications. The pyroelectric coefficient is a parameter for measuring the efficiency of the pyroelectric effect within a material. It has contributions from the piezoelectric effect due to the coefficient of thermal expansion (CTE), temperature gradients within the material, and any applied external electric field. It is useful in infrared detection and energy harvesting using waste thermal energy.

This work takes strides in understanding the thermal micromechanical interactions that MFCs experience due to this effect and leads to a better understanding of how MFCs behave in thermal environments. The pyroelectric coefficient is estimated for P1 and P2 MFCs using micromechanical modeling and experimental techniques. The CTE is estimated using micromechanical theory for both MFC types, which is consequently used in the modeling of the pyroelectric coefficient. Secant-based CTEs and Elastic Modulus are used to approximate the properties of the pyroelectric components of MFCs in the regime of linear elasticity.

The total pyroelectric coefficient was experimentally measured using two separate thermal chambers via the linear temperature ramping and thermal cycling methods. These results were compared with measurements taken with potential electromagnetic interference and used in a figure of merit to determine how well the material functions as an energy harvester using this effect. Experiments were conducted to measure the pyroelectric coefficient under different boundary conditions and compared to the modeled pyroelectric coefficient.

Applications using the pyroelectric effect were explored. Waste energy harvesting is a method of generating small amounts of energy, usually through vibrations or thermal energy. The energy generated through the pyroelectric effect in MFCs was modeled through numerical temperature data, along with two analytical temperature functions, and compared to experimental tests. The maximum specific power was also estimated analytically, numerically, and experimentally. A resistor sweep was performed using the numerical model to calculate the optimal resistance that would provide the most energy. This was validated with experiments conducted at varying resistances. Depending on the size of the MFC, type, resistor used, and the temperature rate, the amount of energy harvested will change and can be optimized for a specified application.

Two proof of concept applications were considered combining pyroelectric energy harvesting with either piezoelectric energy harvesting or SHM in MFCs. Three experiments were conducted, one with just thermal variations, one with just vibrations, and one with both thermal changes and vibrations. The energy was calculated in each case and compared. The second proof of concept application involved using a relay circuit to switch between SHM, which uses impedance measurements as a damage detector, and pyroelectric energy harvesting.

Because this is energy generated from the ambient environment, pyroelectric harvesting can be used to power devices without cost to the source. This form of harvesting can be used in any place where there is a natural thermal cycle (i.e., due to weather or machine giving off thermal energy). Since MFCs are used in a multitude of piezoelectric applications, they can utilize both effects in places where there is a temperature variation with time. This dissertation furthers understanding the pyroelectric effect with the hopes of advancing potential novel applications using MFCs.