Nanothermite composites containing metallic fuel and inorganic oxidizer are gaining importance due to their outstanding combustion characteristics. In this paper, the combustion behaviors of copper oxide/aluminum nanothermites are discussed. CuO nanorods were synthesized using the surfactant-templating method, then mixed or self-assembled with Al nanoparticles. This nanoscale mixing resulted in a large interfacial contact area between fuel and oxidizer. As a result, the reaction of the low density nanothermite composite leads to a fast propagating combustion, generating shock waves with Mach numbers up to 3. © 2007 American Institute of Physics. ͓DOI: 10.1063/1.2787972͔ Nanothermite materials are comprised of a physical mixture of inorganic fuel and oxidizer nanoparticles. Nonhomogenous distribution of fuel and oxidizer has been observed in the microstructures. 1 This produces random hot spot density distribution and decreases the propagation speed of the combustion wave front. It is, therefore, important to achieve homogenous mixing of the oxidizer and fuel components for faster reaction kinetics. This can be achieved by selfassembly of fuel around the solid oxidizer. Enhancement in the combustion wave speed has already been reported for composites containing porous oxidizers and fuel nanoparticles, 2,3 and also for electrostatically charged selfassembled composites. 4 Recently, we reported that higher combustion wave speeds were achieved for the composites of ordered porous Fe 2 O 3 oxidizer and Al nanoparticles 5 as compared with the one containing porous oxidizer with no ordering of the pores and Al nanoparticles. We have also reported the composite of CuO nanorods and Al nanoparticles exhibiting a combustion wave speed of 1500Ϯ 100 m / s, which enhances to 2200 m / s for the self-assembled composites. 6-8 Interestingly, these higher combustion wave speeds are comparable to the lower end values of the detonation velocities ͑e.g., 2000 m / s for hydrocarbon/alkylene-air mixtures, 9 1500-2700 m / s for metallic azides and fulminates, 10 and about 3000 m / s for ammonium nitrate fuel oil͒ for explosives. 11 In conventional explosives, the gases produced during the chemical reaction develop turbulence due to a combined effect of high pressure and rapid shearing of molecular layers generating a shock wave. In a process called deflagration-todetonation transition ͑DDT͒, the wave propagates in the reactive medium creating localized high pressure at the hot spots and, after a certain run-up distance, rapid deflagration can transition to full detonation. 9 This distance depends on the dimensions of the shock tube and also the level of confinement. 9 In the case of low density superthermites, as the adiabatic reaction temperatures are several thousand degrees, the reaction products can volatilize rapidly 12 resulting in an increased level of turbulence and high localized pressures. Because of the low density and multiphase nature of reaction materials, the corresponding Chapman-Jouguet ͑CJ͒ pressure can be much lower ...
Crystallization of amorphous silicon ͑a-Si͒ thin film occurred by the self-propagation of copper oxide/aluminum thermite nanocomposites. Amorphous Si films were prepared on glass at a temperature of 250°C by plasma enhanced chemical vapor deposition. The platinum heater was patterned on the edge of the substrate and the CuO / Al nanoengineered thermite was spin coated on the substrate that connects the heater and the a-Si film. A voltage source was used to ignite the thermites followed by a piranha solution ͑4 Unlike most other techniques, laser induced crystallization does not require high temperatures ͑Ͼ180°C͒ and long processing times to produce good quality poly-Si films. The major disadvantage of laser crystallization is its low throughput due to small laser spot size, which is not suitable for large area such as solar cells. Thus, high temperature and long processing times for various crystallization methods, and small spot size of the laser are not suitable for producing poly-Si film on a large surface area of glass or flexible plastic substrates. Low cost plastic substrates such as polyethersulphone can be used for flexible electronics if the device is fabricated under 180°C. This study investigates the crystallization of a-Si layer achieved by the ignition of nanoengineered thermite materials such as CuO / Al. Explosives have been utilized previously to crystallize amorphous materials; 5,6 however, there is no information currently available on thin film crystallization using nanoengineered thermites. Our approach utilizes thermite reaction to induce crystallization of a-Si thin film; such energetic reactions are self-propagating exothermic reactions, which produce localized heating effects. We discovered that thermites, nanoengineered by the self-assembly approach, produced a self-propagating chemical reaction over a period of microseconds. 7,8 The exothermic reaction propagates at a rate of 1500-2000 m / s resulting in heat release. This heat can be used advantageously to crystallize a-Si. High quality poly-Si films can be prepared on large substrates utilizing this released heat. Nanoengineered thermites were prepared by sonicating a mixture of CuO nanorods ͑10 nm diameter and 70 nm long͒ and aluminum nanopowder ͑80 nm diameter͒. The details of the nanorod preparation and the characterization of the thermites are presented in Ref. 9. The thermites displayed the following chemical reaction:where ⌬H is the released heat. For CuO / Al exothermic reaction, the released heat is 604 kJ/ mol and the adiabatic reaction temperature is 3794 K. 10The a-Si samples were prepared by plasma enhanced chemical vapor deposition on glass substrates. The thickness of the a-Si layer was 300 nm. A thin layer of platinum ͑90 nm thick and 2.5 mm wide͒ was deposited on the edge of the substrates, which functioned as a heater for initiating the self-propagating reaction for the energetic materials. The substrates with a-Si and the platinum heater were spin coated with the thermites and dried at 105°C in an oven for 10 min. Th...
Current approaches of mixing fuel and oxidizer nanoparticles or adding fuel nanoparticles to oxidizer gel lead to an overall reduced interfacial area of contact between them and thus, limit their burn rates severely. We have developed an approach of self-assembling fuel nanoparticles around an oxidizer matrix using a monofunctional polymer, poly(4)-vinyl pyridine (P4VP). The polymer has been used to accomplish binding of fuel and oxidizer in a molecularly engineered manner. We use composite of Al-nanoparticles and CuO nanorods for executing this self-assembly. TEM images of this composite confirms the self-assembly of Al-nanoparticles around the oxidizer nanorods. The burn rate of self-assembled composite has been found significantly higher than that of the composite prepared by simple mixing.
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