The Effect of Nanopowder Attributes on Reaction Mechanism and Ignition Sensitivity of Nanothermites (original) (raw)

Combustion Characteristics of Reactive Nanomaterials

JOURNAL OF FACULTY OF ENGINEERING & TECHNOLOGY, 2014

The combustion ofmetallic particlesis analogous to the combustion of hydrocarbon particles andthe particleburn time can be related to its diametric length. The relationship is called ‘d’ law and represented ast b =d 2 . From the physics aspect, many deviations from the established laws at the bulk scale have been reported. As the ignition temperature of energetic nanomaterials is more sensitive to the passivation layer and the external heating conditions, and the burning time of nanomaterials is deviated from the conventional d 2 law. Due to the variation of certain parameters such as the particles size distribution, agglomeration, morphology, level of contamination and initial particle size,thecorrect and precise value of the exponentisdifficult to find.Consequently, there’s no universal law for the burn time and a variant of the d n law is always proposed whose exponent is less than 2 (~ 1.3-1.7).In this research, combustion experiments are performed using a Bunsen burner in a par...

Comparison of engineered nanocoatings on the combustion of aluminum and copper oxide nanothermites

Surface and Coatings Technology, 2013

Water-repellent nano-coatings for submerged combustion of nano-energetic composite materials were developed. These coatings may have applications for oceanic power generation, underwater ordnance, propulsion, metal cutting, and torch technologies. Nano-coatings were deposited on thermite pellets by a vapor-phase technique. Two types of deposition techniques studied were chemical vapor deposition (CVD) and atomic layer deposition (ALD). A total of six types of nano-coatings were applied on the thermite pellets. Various process parameters to produce the coatings were explored. Characterization of the nano-coatings was carried out using Fourier Transform Infrared Spectroscopy (FTIR), Scanning Electron Microscopy (SEM), Atomic Force Microscopy (AFM), and contact angle goniometry. Submerged combustion tests of the nano-coated thermite pellets were performed as a function of submerged time. The pellets were submerged in de-ionized water for 3, 5, and 10 days. The energy released by the thermite reaction was analyzed and compared to other types of nano-coated pellets. Initial results of a fluorocarbon self-assembled monolayer (FSAM) coating were compared with an ALD coating composed of Al 2 O 3 . Results show that with increasing submerged time, there was a decrease in the ratio of bubble energy to total energy of combustion (K c =K bubble /K combustion ) for all coatings tested. The initial bubble energy of the pellets coated with FSAM and ALD with Al 2 O 3 was 133.3 and 142.2 (KJ/Kg), respectively. After submersion for 10 days, the bubble energy reduced to 10.4 and 15.6 (KJ/Kg), respectively. The value of K c for the FSAM coating decreased by a factor of 12.8 whereas the ALD with Al 2 O 3 coating decreased by a factor of 9.1. The hydrophobic coating is critical for energy generation because without it, the pellets do not ignite, resulting in 100% loss of energy.

The effect of pre-heating on flame propagation in nanocomposite thermites

Flame propagation in a confined tube configuration was evaluated for aluminum (Al) and molybdenum trioxide (MoO 3 ) thermites starting at room temperature and pre-heated up to 170°C. Flame propagation was analyzed via high speed imaging diagnostics and temperatures were monitored with thermocouples. Experiments were performed in a semi-confined flame tube apparatus housed in a reaction chamber initially at standard atmospheric pressure. The flame propagation behavior for the nano-particle thermite was compared to micron particle thermite of the same composition. Results indicate that increasing the initial temperature of the reactants results in dramatically increased flame speeds for nanocomposite thermite (i.e., from 627 to 1002 m/s for ambient and 105°C pre-heat temperature, respectively) and for micron composite thermite (i.e., from 205 to 347 m/s for ambient and 170°C pre-heat temperature, respectively) samples. Experimental studies were extended giving a cooling time for the heated thermites prior to ignition and flame propagation. It is shown that when 105°C and 170°C pre-heated thermites are cooled at a rate of 0.06 K/s, almost the same flame speeds are obtained as thermites at ambient temperature. However, when the cooling rate is increased to 0.13 K/s, the measured flame speeds approach the flame speeds of pre-heated samples.

Assembly and reactive properties of Al/CuO based nanothermite microparticles

Combustion and Flame, 2014

It is generally agreed that a key parameter to high reactivity in nanothermites is intimate interfacial contact between fuel and oxidizer. Various approaches have been employed to combine fuel and oxidizer together in close proximity, including sputter deposition [1], and arrested milling methods [2]. In this paper, we demonstrate an electrospray route to assemble Al and CuO nanoparticles into micron composites with a small percentage of energetic binder, which shows higher reactivity than nanothermite made by conventional physical mixing. The electrospray approach offers the ability to generate microscale particles with a narrow size distribution, which incorporates an internal surface area roughly equivalent to the specific surface area of a nanoparticle. The size of the micron scale composites could be easily tuned by changing the nitrocellulose content which is used as the binder. The composites were burned in a confined pressure cell, and on a thin rapidly heated wire to observe burning behavior. The sample of 5 wt.% nitrocellulose showed the best response relative to the physical mixing case, with a 3Â higher pressure and pressurization rate. The ignition characteristics for these micron particles are essentially equivalent to the nanothermite despite their significantly larger physical size. It appears that electrospray assembly process offers to potential advantages. 1. Enhanced mixing between fuel and oxidizer; 2. Internal gas release from nitrocellulose that separates the particles rapidly to prevent sintering. The later point was shown by comparing the product particle size distribution after combustion. Published by Elsevier Inc. on behalf of The Combustion Institute.

Combustion characteristics of novel hybrid nanoenergetic formulations

Combustion and Flame, 2011

This paper presents the combustion characteristics of various copper oxide (CuO) nanorods/aluminum (Al) nanothermite compositions and hybrid nanoenergetic mixtures formed by combining nanothermites with either ammonium nitrate (NH 4 NO 3) or secondary explosives such as RDX and CL-20 in different weight proportions. The different types of nanorods prepared in this study are referred to as CuO-VD (dried under vacuum at 25°C for 24 h), CuO-100 (at 100°C for 16 h) and CuO-400 (short time (1 min) calcination at 400°C). The physical and chemical characteristics of these different kinds of CuO nanorods were determined using a variety of analytical tools such as X-ray diffractometer, transmission electron microscope (TEM), Fourier transform infrared spectrometer (FTIR), surface area analyzer and simultaneous differential scanning calorimeter (DSC)/thermogravimetric analyzer (TGA). These measured characteristics were correlated with the combustion behavior of the nanoenergetic compositions synthesized in this work. The use of different drying and calcination parameters produced the synthesis of CuO nanorods with varying amount of hydroxyl (OH) and CH n (n = 2, 3) functional groups. The experimental observations confirm that the presence of these functional groups on the surface of CuO nanorods enabled the formation of assembled nanoenergetic composite, upon mixed with Al nanoparticles. A facile one-step synthesis of assembled composite through surface functionalization is reported and it can be extended to large-scale preparation of assembled nanoenergetic mixtures. The combustion behavior was studied by measuring both combustion wave speed and pressure-time characteristics. Pressurization rate was determined by monitoring the pressure-time characteristics during the combustion reaction initiated by a hot wire in a fully-confined geometry. Different amounts of nanothermite powder were packed in the same volume of combustion chamber by applying different packing pressures and the pressure-time characteristics were measured as a function of varying percent theoretical maximum density (% TMD). The experimental setup used in this work enabled us to study the functional behavior of initiating explosives such as NH 4 NO 3 nanoparticles, RDX and CL-20 using nanothermites under fully-confined test geometry. The dent tests performed on lead witness plates support the experimental observations obtained from pressure-time and combustion wave speed measurements of hybrid mixtures.

Burn Rate Measurements of Nanocomposite Thermites

41st Aerospace Sciences Meeting and Exhibit, 2003

Combustion velocities were experimentally determined for nanocomposite thermite powders composed of aluminum ͑Al͒ fuel and molybdenum trioxide ͑MoO 3 ͒ oxidizer under well-confined conditions. Pressures were also measured to provide detailed information about the reaction mechanism. Samples of three different fuel particle sizes ͑44, 80, and 121 nm͒ were analyzed to determine the influence of particle size on combustion velocity. Bulk powder density was varied from approximately 5% to 10% of the theoretical maximum density ͑TMD͒. The combustion velocities ranged from approximately 600 to 1000 m / s. Results indicate that combustion velocities increase with decreasing particle size. Pressure measurements indicate that strong convective mechanisms are integral in flame propagation.

Reactive characterization of nanothermites

Journal of Thermal Analysis and Calorimetry, 2012

Conventional thermal analysis techniques (TG and DSC) give valuable information on the activation energy and the reactivity of energetic materials such as organic explosives. Here, we discuss the use of these methods for characterizing nanothermites, energetic compositions made of metallic oxides and a fuel (often a reducing metal). The experimental limitations of these analysis techniques are identified. It is difficult to ignite nanothermites with slow heating rates as those used in DSC. This is due to the inorganic nature of the thermite components and because the reaction involves interparticular heat and matter transfers. In addition, during the progressive decomposition of nanothermites, there is no change in mass, so it cannot be observed by thermogravimetric analysis. The use of laser ignition to prime the abrupt combustion of nanothermite pellets allows determining the ignition energy and analyzing the propagation of the combustion front. It also provides qualitative data that can be used to understand the combustion mechanism and to correlate it to the microstructure of the nanothermites. By analyzing several examples, we will show that the coupling of high speed video to existing thermal analysis techniques could significantly extend their utilization range for the characterization of new energetic materials.

Afterburn modeling of nanothermite composites

SHOCK COMPRESSION OF CONDENSED MATTER - 2019: Proceedings of the Conference of the American Physical Society Topical Group on Shock Compression of Condensed Matter

The Jones-Wilkins-Lee (JWL) EOS is widely used to capture detonation energy release but not the energy release from secondary combustion, or afterburn. To account for the burn mechanism of detonation products with that of combustion, the JWL EOS had been extended such as in the work of Miller[1], whose reactive flow model for highly nonideal metallised explosives displays characteristics of both fast detonation and slow metal combustion chemistry. In this work, the focus is on non-detonating aluminum-containing nanothermite composites, in contact with and ignited by a detonating explosive. A small scale test is set up to study the enhancement in burn front propagation in the presence of nanothermites. Miller's original Moby Dick test had been modified wherein the slow burning component is in contact with the explosive. Thermochemical calculations of the afterburn phase were made with a modified version of the thermochemical code EXPLO5 to determine the afterburn release energy. Analysis of the experimental findings and subsequent calibration work will allow determination of the system-specific EOS. This reported methodology can be applied to study and calibrate the afterburning of other energetic materials.