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This work presents an experimental investigation of the combustion characteristics of micron scale aluminum and aluminum-magnesium alloy powders coated with a thin layer of fluoropolymer. Burn times of the coated powders ignited by CO 2 laser were estimated from the time resolved emission signals recorded by photomultiplier tubes. Both fluoropolymer coated powders recorded reduced burn times. This result is likely associated with the lowered diffusion barrier in the fluoropolymer coated particles due to the gasification of oxide shell in the presence of fluorinated species from the decomposing fluoropolymer. Combustion temperatures determined using two-color pyrometry and optical spectroscopy were consistently higher for the fluoropolymer coated powders in comparison with that of the pristine. The reactivity of Al and Al-Mg alloy powders as assessed by constant volume explosion experiments was improved due to the fluoropolymer coating. Dust clouds of fluoropolymer-coated samples could achieve higher burning velocity as estimated from the experimental pressure traces using a semi-empirical correlation for dust explosions. A plausible mechanism responsible for the improvement in metal combustion due to the incorporation of fluoropolymer was proposed. [ABSTRACT FROM AUTHOR]

To improve the ignition and combustion behaviors of Al, the strategy of interfacial modification using transition metals (TMs) for the Al/AP composites to produce the multi-layered core-shell structured Al@TMs@AP composite fuels is provided. The relevant Al-based composite fuels are fabricated using spray granulation method and their morphologies are characterized by SEM and TEM techniques. The ignition and combustion of the solid propellants prepared with the core-shell structured composite fuels are evaluated and compared with that of reference propellant formulated with typical compositions using laser ignition setup and combustion diagnostic system. Results indicate that the heats of reaction measured for the Al@TMs@AP contained propellants are increased by as high as 13%, suggesting the completeness in combustion of Al is greatly improved by construction of Al composites with the multi-layered core-shell structure. The ignition delay times are apparently shortened from 421 ms to 373 ms for the propellants with Al@TMs@AP involved, which proves that the promoted ignition of Al is achieved by surface modification with transition metals. Additionally, the significantly increased burning velocity is monitored for the Al@TMs@AP contained propellants at low-pressure regime leading to the corresponding pressure exponents to be reduced by more than 20%, which is beneficial to the stability in operation of solid rocket motors. At last, the higher combustion wave temperature and finer particle size in CCPs are observed for the propellants containing surface modified Al, and the plausible combustion mechanism is proposed based on the phase composition analysis for the CCPs of studied propellants. [ABSTRACT FROM AUTHOR]

Four bimetal Al–Ni nano-powders with compositions varied from 5 to 45 at% of nickel were synthesized by explosion of electrically heated twisted pure Al and Ni wires in argon. The nano-powders were characterized using electron microscopy, x-ray diffraction, and thermal analysis. Materials were ignited using an electrically heated filament coated with powder and electrostatic discharge (ESD). The results were compared to those for pure nano-aluminum powder (n-Al) prepared using the same wire explosion technique. The nano-powders with high nickel concentrations contain fully reacted intermetallic phases, which are difficult to oxidize making them unattractive for energetic formulations. Nano-powders with lower nickel concentrations do not contain significant amounts of the intermetallic phases. No intermetallics were detected in the powder with 5 at% Ni, which oxidized qualitatively similar to n-Al. The overall mass gain during oxidation for the bimetal powder was nearly identical to that of n-Al, suggesting the same heat release anticipated from their combustion. Oxidation kinetics assessed for this material accounting directly for the measured particle size distribution was compared to that of n-Al. The bimetal powder oxidized slower than n-Al, indicating its greater stability during handling and storage. The bimetal powder was less ESD-ignition sensitive than n-Al, but generated a stronger emission signal when ignited. Therefore, the bimetal powder with 5 at% Ni is an attractive replacement of n-Al for advanced energetics with lower ESD sensitivity, better stability, and improved combustion performance. [ABSTRACT FROM AUTHOR]

• Al/oxidizer composites integrated with energetic coordination polymers as catalyst are prepared. • The ignition and combustion of propellants are enhanced under the synergistic effects of Al/oxidizer interfacial control and the catalysts. • The burning rates of propellants can be well controlled in a wide range and lowered pressure exponent is achieving. The control of burning rate pressure sensitivity is essential for stable and reliable combustion of solid propellants. It has been proved that the combustion behavior of propellants can be effectively tuned by interfacial control of Al/oxidizers. In this work, core-shell Al-based composites, using oxidizers such as ammonium perchlorate (AP) and cyclotetramethylene tetranitramine (HMX) as the shell layer, have been prepared with a well-controlled location of graphene-based carbohydrazide complexes (GO-CHZ-M, M = Co2+ or Ni2+) as the catalysts. The catalysts can be located inside HMX or embedded on the surface of AP particles. The decomposition of AP and HMX could be greatly promoted with a trace amount of catalyst. Under the synergistic effects of Al/oxidizer interfacial control and GO-CHZ-M, the ignition delay time is shortened by >48 % with increased in flame radiation. More importantly, the burning rate (r) and pressure exponent (n) of the solid propellants are effectively regulated in a wide range. In particular, the use of Al@HMX/Co results in a lowered n of 0.29 within 1∼20 MPa compared to 0.79 for the blank reference sample. The agglomeration of Al particles during combustion was also inhibited due to the micro-explosion effect of Al@HMX composites. The innovative approaches of integrating Al/HMX/AP composites and precise catalysis of graphene-based energetic catalysts in solid propellants have successfully achieved, regarding the modulation of the burning rates and pressure sensitivity. Moreover, the effect of catalyst location on the ignition delay and burning rates has also been studied thoroughly and clarified. This paper provides new insight into the regulation of combustion performance of solid propellants, with improved reaction efficiency and maintained energy density. [ABSTRACT FROM AUTHOR]

Abstract: Micron-sized metal powders carried by a nitrogen flow were fed along the axis of a cylindrical hydrogen/oxygen diffusion flame. The particles ignited and burned in the water vapor at approximately 2500K. Experiments were performed at atmospheric pressure. The environment in which particles burned was characterized in detail using computational fluid dynamics. The computations confirmed that the metal powders burned in water while the effect of oxygen and other oxidizing species could be neglected. Combustion was characterized experimentally for micron-sized powders of both aluminum and magnesium. Particle size distributions were measured using low-angle laser light scattering. Optical emission of the burning particles was recorded using filtered photomultiplier tubes. Measured durations of individual particle emission pulses were assumed to represent their burn times; these data were classified into logarithmically spaced time bins. The distribution of the particle burn times was correlated with their size distributions assuming that larger size particles burned longer. It was observed that correlation between the burn times, t, and particle diameters, D, can be approximately described as t ∼ D 0.64 and t ∼ D 0.68 for aluminum and magnesium powders, respectively. The results were compared to previous reports and possible reasons for discrepancies between the present and earlier results were discussed. [Copyright &y& Elsevier]

Recently fluoropolymers are of great interest as an oxidative binder and modifier in Al-based metastable intermixed composites (MICs) due to the high heat of reaction between Al-F and gas production characteristics. In this paper, Polytetrafluoroethylene (PTFE) and Polyvinylidene Fluoride (PVDF) have been incorporated into three typical Al-based MICs independently using CuO, Fe 2 O 3 , and Bi 2 O 3 as oxidizers, where the contents of Al to metal oxide are maintained optimum stoichiometric ratios. Heats of reaction have been measured by a bomb calorimeter for mechanically mixed MICs samples, from which the optimum fluoropolymer content has been determined using the maximum calorific values as the criteria. After that, the optimized formulations have been prepared by a spray drying method. The thermal behavior, combustion characteristics and pressurization features of the studied Al-based MICs were evaluated and compared with those without fluoropolymers. The experimental results showed that with a minor inclusion of fluoropolymers, the combustion heat was improved by 3.8% to 8.6% depending on the type of oxidizers, and the onset temperature of the exothermic reaction was observed to be decreased by 250 °C for Al/CuO, 200 °C for Al/Fe 2 O 3 and 16 °C for Al/Bi 2 O 3 with a higher reaction heat release, showing a higher reactivity. Besides, the maximum pressure and pressurization rate were both increased for the fluorine-containing MICs, whereas the burn rates under confinement were found to drop significantly by 72% to 91%. The AlF 3 , Cu-Al alloy, and Fe-Al alloy are found in the condensed combustion products (CCPs), and it tends to have smaller and more uniform particles. It probably results from the large difference in the energy and mass transfer process on the combustion surface when the gaseous phase products are generated. [ABSTRACT FROM AUTHOR]

In this paper, three nanocomposite fuels (Al/TMs@AP) including Al/Co@AP, Al/Ni@AP and Al/Ti@AP have been fabricated by a spray-drying technique. Their morphology, heat of reaction, ignition and combustion performances, as well as their condensed combustion products (CCPs) have been comprehensively investigated. The results suggested that the core-shell structured Al/TMs@AP has lower ignition delay and better combustion performances comparing to the corresponding mechanically mixed ones. The promoted thermal reactivity could be attributed to resulted intimate reactants contacts with shortened heat and mass transfer distance. Furthermore, the reactivity of Ti is higher than that of Co and Ni, so that a much more violent chemical reaction occurred for the Ti-containing composites, and thereby the combustion wave temperature was increased to 2136.6 °C and the particle size of the corresponding CCPs was reduced to as low as 170 nm. Moreover, the compositions of CCPs for Al/Ti/AP and Al/Ti@AP are quite different. The typical CCPs phases for Al/TMs@AP are dominated with Al x TM y O z and Al 5 O 6 N, whereas the CCPs of the other samples are dominated with transition metals oxides such as Co 3 O 4 , NiO and TiO 2 depending on the type of transition metals involved. [ABSTRACT FROM AUTHOR]