Multifunctional and Piezoelectric Energetic Materials

Polyvinylidene fluoride (PVDF) and its copolymer poly(vinylidene fluoride-co-trifluoroethylene) P(VDF-TrFE) has attracted great interest due to its ability to be utilized as a matrix binder capable of producing modified ignition sensitivity in energetic systems. While there have been studies on the combustion of fluoropolymer/aluminum systems, there is still a shortage of knowledge on what roles the electromechanical properties of the piezoelectric P(VDF-TrFE) and nano-aluminum (nAl) composite films play in the processes leading to their ignition upon a mechanical impact. To help bridge this gap, we conducted experiments and computational simulations to elucidate the underlying electromechanical properties that the films exhibit and to quantify the time duration it takes to commence ignition (i.e., ignition time). Based on our systematic assessment, we conclude that both piezoelectricity and flexoelectricity of the P(VDF-TrFE) can influence the ignition sensitivity as measured by the ignition time by locally enhancing the electric field near the nAl particles (by a factor of ~6.0) beyond the binder’s breakdown strength, resulting in concentrated channels of heat dissipation and ultimately ignition reactions. This suggests that the piezoelectric effect can catalyze the ignition process. The effect of poling films was also investigated by comparing how the sensitivity of the poled films differs from that of the unpoled films, thereby offering a mechanism to tune the ignition sensitivity by varying the level of piezoelectricity in the films. Results indicate that poling the films can enhance sensitivity by decreasing the minimum ignition energy (MIE) by 8%.

Additionally, we employed a mini drop-weight and shaker setup to investigate the response of the films to pressure and vibrations. Our findings demonstrate that these piezo-energetic films can replicate the behavior of a commercial PVDF gauge at relatively low-pressure impacts, indicating their potential use as shock or pressure sensors in various fields, as well as an accelerometer gauge. Furthermore, aging studies of up to one year indicated minimal loss in the energetic content of the created films, enabling the use of energetic gauges for an extended period. Our findings support the effectiveness of piezo-energetic composite films as pressure sensors or accelerometers and highlight their potential for energetic applications.

Solid propellants are used in propulsion systems for their high performance, feasibility to manufacture, long shelf life, and ease of storage and handling; however, they are limited by the inability to actively control the burning rate. The overall burning rate of solid propellant is predetermined based on geometry (surface area) and initial conditions such as temperature and pressure. This work proposes two methods to tailor the effective burning rate of solid composite ammonium perchlorate/hydroxyl-terminated polybutadiene (AP/HTPB) propellants using embedded shape memory alloy (SMA) and Nichrome^TM wires through Ohmic heating. First, Nitinol^TM wires, coiled into a spring, are demonstrated to increase the burning surface area upon expanding inside of the solid propellant. The result of this geometry modification was shown to increase the rate of pressurization (dP/dt) by over 180%, thereby throttling the propellant sample. Second, Nichrome^TM wire was embedded in the samples to demonstrate varying the heat flux (between 24.4 kW/m2 and 153.6 kW/m2) also affects the rate of pressurization under heated conditions. Preheating the solid propellant in this manner increased the rate of pressurization from 69 MPa/s to 110 MPa/s. This concept is useful for many applications such as in electro-explosive devices (EEDs) or actuators where it is desired to have the resulting gas expansion rate tailored.