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The effect of aluminum size on confined flame propagation velocities in thermite composites was investigated between 108 μm and 80 nm, and in all cases using nanometric copper oxide as the oxidizer. It was found that the velocity exhibited two distinct regimes; between 108 and 3.5 μm the velocity scaled as the particle diameter to the − 0.56 power, and becomes invariant of size below this. One explanation for the invariance is that the pressure‐driven flow reaches some peak velocity, controlled by the pressure gradient, pore size, and fluid viscosity. Another explanation is that the system becomes limited by the internal gas heating rate, defined by the intrinsic kinetic time scale, and which can significantly impact the effective particle heating time. The particle heating time was calculated as a function of particle size, and as a function of gas heating rates ranging from 105 K s−1 to infinity. It was found that at any finite gas heating rate, there exists a critical particle diameter below which all sizes take the same amount of time to heat. This is a direct artifact of the characteristic thermal relaxation time scale; if the heating rate is not sufficiently fast, then the particle will rapidly equilibrate with the gas at each time step. The inverse of thermal relaxation time was used to calculate a critical heating rate defining a transition point, and which exhibits a dp2 scaling. This scaling sets a constraint on the kinetics, which must at least scale with dp2 to remain in the size‐dependent regime.
The effect of aluminum size on confined flame propagation velocities in thermite composites was investigated between 108 μm and 80 nm, and in all cases using nanometric copper oxide as the oxidizer. It was found that the velocity exhibited two distinct regimes; between 108 and 3.5 μm the velocity scaled as the particle diameter to the − 0.56 power, and becomes invariant of size below this. One explanation for the invariance is that the pressure‐driven flow reaches some peak velocity, controlled by the pressure gradient, pore size, and fluid viscosity. Another explanation is that the system becomes limited by the internal gas heating rate, defined by the intrinsic kinetic time scale, and which can significantly impact the effective particle heating time. The particle heating time was calculated as a function of particle size, and as a function of gas heating rates ranging from 105 K s−1 to infinity. It was found that at any finite gas heating rate, there exists a critical particle diameter below which all sizes take the same amount of time to heat. This is a direct artifact of the characteristic thermal relaxation time scale; if the heating rate is not sufficiently fast, then the particle will rapidly equilibrate with the gas at each time step. The inverse of thermal relaxation time was used to calculate a critical heating rate defining a transition point, and which exhibits a dp2 scaling. This scaling sets a constraint on the kinetics, which must at least scale with dp2 to remain in the size‐dependent regime.
One of the groups of pyrotechnic compositions is thermite compositions, so-called thermites, which consist of an oxidant, usually in the form of a metal oxide or salt, and a free metal, which is the fuel. A characteristic feature of termite combustion reactions, apart from their extremely high exothermicity, is that they proceed, for the most part, in liquid and solid phases. Nanothermites are compositions, which include at least one component whose particles size is on the order of nanometers. The properties of nanothermites, such as high linear burning velocities, high reaction heats, high sensitivity to stimuli, low ignition temperature, ability to create hybrid compositions with other high-energy materials allow for a wide range of applications. Among the applications of nanothermites, one should mention igniters, detonators, microdetonators, micromotors, detectors, elements of detonation chain or elements allowing self-destruction of systems (e.g., microchips). The aim of this work is to discuss the preparation methods, research methods, direction of the future development, eventual challenges or problems and to highlight the applications and emerging novel avenues of use of these compositions.
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