High Thermal Conductivity Nanocomposite Encapsulants for Torpedo Nose Arrays and High Power/ High Duty Cycle Acoustic Sources
Within the U.S. Navy today, there is a growing need for acoustically clear polymers that exhibit high thermal conductivity but low electrical conductivity. These materials are needed to encapsulate "next-generation" high power/high duty cycle acoustic sources that generate considerable amounts of heat when operating. Polymers are typically poor heat conductors; they rarely exhibit thermal conductivities greater than 0.3 W/m K. The piezoelectric ceramics such as lead-zirconate-titanate (PZT) that frequently serve as the active elements in acoustic sources typically exhibit thermal conductivities of about 2.0 W/m K. This heat conduction mismatch between the active element and its encapsulant in high power/high duty cycle acoustic sources can result in a thermal runaway that may thermally degrade critical components and/or depole the ceramics. While the thermal conductivity of polymers can be improved by adding high thermal conductivity filler particles, the percolation threshold for heat conduction in conventional composites requires such a heavy loading of filler particles that the other desirable properties of the polymer (e.g., acoustic clarity, mechanical strength and toughness) are compromised. To avoid this problem, we synthesize nanocomposite encapsulants using highly thermally conductive boron nitride (BN) nanoparticles. Nannocomposites are attractive for solving the encapsulant heat management problem because very low loadings (typically 1% - 5%) can improve targeted physical properties by an order of magnitude or more. Thus, it may be possible to make a BN-based nanocomposite encapsulant that be both acoustically clear and much more thermally conductive. During this project, we investigate the effect of adding BN nanoparticles on both thermal and mechanical properties of the polyurethanes commonly used to encapsulate acoustic sources. This project is conducted in collaboration with Naval Undersea Warfare Center (NUWC), Newport, RI.