Combustion characterization and modeling of novel energetic materials: Si/PTFE/Viton and Al/PTFE/Viton

The energetic materials Si/Polytetrafluoroethylene/Viton (SiTV) and Al/Poly\-tetra\-fluoro\-ethylene/Viton (AlTV) have drawn interest recently due to the increased availability and decreased cost of nano materials. The reactivity of aluminum based energetic materials is greatly enhanced with the use of nanometric reactants. Silicon sees order of magnitude increases in burning rate when nano silicon is used in place of micron sized silicon. A broad characterization of the combustion of these composites has been undertaken. Theoretical equilibrium calculations have been performed, as well as characterization of combustion in the instrumented burn tube at various mixture ratios, and pressed pellet burns at different mixture ratios and applied pressures. A comparison of two morphologically different Al materials was also carried out using the instrumented burn tube. Spectroscopic measurements were made of deflagrating SiTV and AlTV pellets. High-speed images were also recorded, and the synchronization of these with the emission spectra provided details the macro scale combustion behavior. Time dependent emission intensity was caused by growing product layers on the surface of the pellets. Temperatures for each mixture ratio were measured by emission spectroscopy and used as input to three simple combustion models. The Ward, Son, and Brewster model (WSB), the Koch model, and the Williams model were applied to AlTV combustion. The modeling results provided further support of the non constant pressure exponent theory, i.e., that AlTV burns in a transition region between a coupled flame and a condensed phase controlled reaction, resulting in pressure exponents that increase with pressure. Micron sized doped silicon powders were manufactured by ball milling commercially obtained doped silicon wafers. Four powders using different dopants and dopant concentrations were manufactured. These powders were characterized to determine their average particle size, specific surface area, crystalline content, and electrical and thermal properties. The effect of the doping on combustion was investigated by way of pressed pellet burning rate measurements. The burning rate was found to correlate with dopant concentration, regardless of the dopant type. It is proposed that the doping effect can be explained by increased electron mobility or increased lattice defects, both of which are proportional to dopant concentration

\u3ci\u3eDrosophila\u3c/i\u3e Muller F Elements Maintain a Distinct Set of Genomic Properties Over 40 Million Years of Evolution

The Muller F element (4.2 Mb, ~80 protein-coding genes) is an unusual autosome of Drosophila melanogaster; it is mostly heterochromatic with a low recombination rate. To investigate how these properties impact the evolution of repeats and genes, we manually improved the sequence and annotated the genes on the D. erecta, D. mojavensis, and D. grimshawi F elements and euchromatic domains from the Muller D element. We find that F elements have greater transposon density (25–50%) than euchromatic reference regions (3–11%). Among the F elements, D. grimshawi has the lowest transposon density (particularly DINE-1: 2% vs. 11–27%). F element genes have larger coding spans, more coding exons, larger introns, and lower codon bias. Comparison of the Effective Number of Codons with the Codon Adaptation Index shows that, in contrast to the other species, codon bias in D. grimshawi F element genes can be attributed primarily to selection instead of mutational biases, suggesting that density and types of transposons affect the degree of local heterochromatin formation. F element genes have lower estimated DNA melting temperatures than D element genes, potentially facilitating transcription through heterochromatin. Most F element genes (~90%) have remained on that element, but the F element has smaller syntenic blocks than genome averages (3.4–3.6 vs. 8.4–8.8 genes per block), indicating greater rates of inversion despite lower rates of recombination. Overall, the F element has maintained characteristics that are distinct from other autosomes in the Drosophila lineage, illuminating the constraints imposed by a heterochromatic milieu