Burning Behaviors of Solid Propellants Using Graphene-Based Micro-Structures: Experiments and Simulations

Shourya Jain, Purdue University

Abstract

Enhancing the burn rates of solid propellants and energetics is a crucial step towards improving the performance of several solid propellant based micro-propulsion systems. In addition to increasing thrust, high burn rates also help simplify the propellant grain geometry and increase the volumetric loading of the rocket motor, which in turn reduces the overall size and weight. Thus, in this work, burn rate enhancement of solid propellants when coupled to highly conductive graphene-based micro-structures was studied using both experiments and molecular dynamic (MD) simulations. The experiments were performed using three different types of graphene-structures i.e. graphite sheet (GS), graphene nano-pellets (GNPs) and graphene foam (GF), with nitrocellulose (NC) as the solid propellant. For the NC-GS samples, propellant layers ranging from 25 µm to 170 µm were deposited on the top of a 20 µm thick graphite sheet. Self-propagating combustion waves were observed, with burn rate enhancements up to 3.3 times the bulk NC burn rate (0.7 cm/s). The burn rates were measured as a function of the ratio of fuel to graphite layer thickness and an optimum thickness ratio was found corresponding to the maximum enhancement. Moreover, the ratio of fuel to graphite layer thickness was also found to affect the period and amplitude of the combustion wave oscillations. Thus, to identify the important non-dimensional parameters that govern the burn rate enhancement and the oscillatory nature of the combustion waves, a numerical model using 1-D energy conservation equations along with simple first-order Arrhenius kinetics was also developed. For the GNP-doped NC films, propellant layers, 500 ± 30 µm thick, were deposited on the top of a thermally insulating glass slide with the doping concentrations of GNPs being varied from 1-5% by mass. An optimum doping concentration of 3% was obtained for which the burn rate enhancement was 2.7 times. In addition, the effective thermal conductivities of GNP-doped NC films were also measured experimentally using a steady state, controlled, heat flux method and a linear increase in the thermal conductivity value as a function of the doping concentration was obtained. The third type of graphene structure used was the GF - synthesized using a chemical vapor deposition (CVD) technique. The effects of both the fuel loading ratio and GF density were studied. Similar to the GNPs, there existed an optimum fuel loading ratio that maximized the burn rates. However, as a function of the GF density, a monotonic decreasing trend in the burn rate was obtained. Overall, burn rate enhancement up to 7.6 times was observed, which was attributed to the GF’s unique thermal properties resulting from its 3D interconnected network, high thermal conductivity, low thermal boundary resistance and low thermal mass. Moreover, the thermal conductivity of GF strut walls as a function of the GF density was also measured experimentally. Then as a next step, the GF structures were functionalized with a transition metal oxide (MnO2). The use of GF-supported catalyst combined the physical effect of enhanced thermal transport due to the GF structure with the chemical effect of increased chemical reactivity (decomposition) due to the MnO2 catalyst, and thus, resulted in even further burn rate enhancements (up to 9 times).

Degree

Ph.D.

Advisors

Qiao, Purdue University.

Subject Area

Thermodynamics

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