Numerically evaluating energetic composite flame propagation with thermally conductive, high aspect ratio fillers

Carbon Fibers Enhance the Propagation of High Loading Nanothermites: In Situ Observation of Microscopic Combustion

“…In principle, the addition of carbon fibers might also affect the heat transfer in the composites. − The thermal conductivity of carbon fiber can vary widely from ∼10 to ∼1000 W·m –1 ·K –1 , while the value of carbon fibers used in this study is estimated to be ∼10–20 W·m –1 ·K –1 . , A ∼2.5 wt % addition of the carbon fibers, however, should not appreciably increase the calculated overall thermal conductivity/diffusivity, until a fiber conductivity of at least ∼250 W·m –1 ·K –1 (∼aluminum’s thermal conductivity), as shown in Figure S3 and Table S4. Although the addition of carbon fibers might not increase the overall thermal conductivity of the composites, conductive heat transfer from hot burned/burning particles to the unburnt materials via these fibers might be quite efficient, especially in the case where the carbon fibers themselves create a connective network (Figure d) . However, while the average conductivity might not be substantially impacted (because of its small mass fraction in the composite), the fact remains that the fibers have a much higher thermal conductivity (>10 W·m –1 ·K –1 ) than the polymer matrix (∼0.2 W·m –1 ·K –1 ) and the surrounding gas (∼0.02 W·m –1 ·K –1 ), ,− which provides 10× higher overall thermal conductivity (see detailed calculations in Figure S4 and Table S5) through the gas/carbon fiber combination route (0.17 W·m –1 ·K –1 , Table S5), compared to those via just gas heat transfer (0.017 W·m –1 ·K –1 , Table S5).…”

“…As illustrated in Figure 2b, since the carbon fibers are longer than the inner diameter of the needle (>1.2 mm), they are aligned when passing through the needle during the printing process (Figure 2b). [45][46][47][48]51 The morphology of the Al/CuO prints with embedded carbon fibers are depicted in an illustration, and an optical microscopy image is shown in Figure 2c. A cross-sectional SEM image (Figure 2d) shows that the carbon fibers are generally embedded parallel to the direction of writing.…”

“…Although the addition of carbon fibers might not increase the overall thermal conductivity of the composites, conductive heat transfer from hot burned/burning particles to the unburnt materials via these fibers might be quite efficient, especially in the case where the carbon fibers themselves create a connective network (Figure 4d). 47 However, while the average conductivity might not be substantially impacted (because of its small mass fraction in the composite), the fact remains that the fibers have a much higher thermal conductivity (>10 W•m −1 •K −1 ) than the polymer matrix (∼0.2 W•m −1 •K −1 ) and the surrounding gas (∼0.02 W•m −1 • K −1 ), 50,61−64 which provides 10× higher overall thermal conductivity (see detailed calculations in Figure S4 and Table S5) through the gas/carbon fiber combination route (0.17 W•m −1 •K −1 , Table S5), compared to those via just gas heat transfer (0.017 W•m −1 •K −1 , Table S5). It is also reasonable to expect that the fibers provide a local path for hot spots perhaps penetrating deep into the preheat zone of the composite.…”

“…For example, replacing conventional binders with energetic ones ,− or applying the catalyst embedded with ammonium perchlorate. , Embedding reactive or metal wires is known to promote combustion. − New techniques and formulations have been introduced to increase the energetic material content , and control combustion behavior via alterations to the chemical content. , However, since these materials largely rely on nanothermites as their primary energetic component to maximize energy release, it is possible that energy is being inefficiently coupled back into the system to promote propagation, given their longer reaction time . Considering that energy release rates in propellants are determined by how fast reactions occur and how fast energy can be transferred to unreacted material, one could feasibly use thermally conductive materials as a method to improve the heat transfer and energy release rate . Additionally, mechanical integrity may be bolstered by incorporating fibers that have been widely employed to reinforce polymer-based 3D structures. − Should the thermally conductive additives be fibrous, they could serve the dual purpose of increasing both the mechanical integrity and the energy release rate. − In this study, we choose carbon fibers as the additives as their high melting point (∼3700 °C) should maintain their shape during the combustion and should have no catalytic effects (e.g., polymer decomposition or Al/CuO reaction).…”

“…46 Considering that energy release rates in propellants are determined by how fast reactions occur and how fast energy can be transferred to unreacted material, one could feasibly use thermally con- ductive materials as a method to improve the heat transfer and energy release rate. 47 Additionally, mechanical integrity may be bolstered by incorporating fibers that have been widely employed to reinforce polymer-based 3D structures. 48−51 Should the thermally conductive additives be fibrous, they could serve the dual purpose of increasing both the mechanical integrity and the energy release rate.…”