摘要 :
Characterizing the combustion behaviors of energetic materials requires diagnostic tools that are often not readily or commercially available. For example, a jet of thermite spray provides a high temperature and pressure reaction ...
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Characterizing the combustion behaviors of energetic materials requires diagnostic tools that are often not readily or commercially available. For example, a jet of thermite spray provides a high temperature and pressure reaction that can also be highly corrosive and promote undesirable conditions for the survivability of any sensor. Developing a diagnostic to quantify heat flux from a thermite spray is the objective of this study. Quick response sensors such as thin film heat flux sensors cannot survive the harsh conditions of the spray, but more rugged sensors lack the response time for the resolution desired. A sensor that will allow for adequate response time while surviving the entire test duration was constructed. The sensor outputs interior temperatures of the probes at known locations and utilizes an inverse heat conduction code to calculate heat flux values. The details of this device are discussed and illustrated. Temperature and heat flux measurements of various thermite sprays are reported. Results indicate that this newly designed heat flux sensor provides quantitative data with good repeatability suitable for characterizing energetic material combustion.
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A ceramic layer was produced on a steel pipe inner surface by a centrifugal thermite process.A powdery mixture of ferric oxide and aluminum was used with the glass whose major compositions were SiO2,Na2O,CaO,and MgO.The ceramic la...
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A ceramic layer was produced on a steel pipe inner surface by a centrifugal thermite process.A powdery mixture of ferric oxide and aluminum was used with the glass whose major compositions were SiO2,Na2O,CaO,and MgO.The ceramic layer consisted of the crystalline structures of corundum(alpha-Al2O3)and hercynite(FeAl2O4).The amorphous phases of Ca3Al2(SiO4)3,MgFeAlO4 and NaAlSiO4 in the ceramic layer were found to be responsible for a significant improvement of the dense structure.The glass addition increased the density of the ceramic layer from 2.9 g/cm~3 to 3.6 g/cm~3 and the hardness from 1,450 to 1,800 Hv.
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Oxidizer and fuel particles are the ingredients of classical pyrotechnics. Particle concentration, size, melting, evaporation, and decomposition of the particles, heat and mass transfer, reaction kinetics, and heat of reaction con...
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Oxidizer and fuel particles are the ingredients of classical pyrotechnics. Particle concentration, size, melting, evaporation, and decomposition of the particles, heat and mass transfer, reaction kinetics, and heat of reaction control the burning behavior of these mixtures. A hot-spot approach models the reaction progress in three dimensions taking into consideration the particulate nature of pyrotechnic compositions. The governing reaction is assumed to be the oxidizer decomposition described by an Avrami-Erofeev model. Predominantly, the distribution of the oxi- dizer and fuel particles and their size for various concentrations influence the burning rate beneath the reaction kinetic parameters. The computational results were compared with experimental progression rates and temperatures measured for an example system composed of various Al/ CuO-thermite mixtures with aluminum contents from 8% to 70%. The particle sizes were fixed to micrometer-scale. The curve of progression rate calculations depending on the aluminium particle concentration and their distribution show the same shape as the experimental results.
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A gasless combustion wave in a cylindrical channel of near-critical diameter under conditions ofintense heat losses has a wide preheat zone with considerable excess enthalpy. Changes of the thermal waveunder the action of reaction...
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A gasless combustion wave in a cylindrical channel of near-critical diameter under conditions ofintense heat losses has a wide preheat zone with considerable excess enthalpy. Changes of the thermal waveunder the action of reaction retardation and acceleration waves propagating over the front surface resemblethe phenomena observed during the combustion of propellants and explosives.
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In this work, the Al/CNT/CuO nano-thermite samples are prepared by ultrasonic mixing with variable CNT content. The morphology of nano-thermites analysed by electron microscopy revealed that the CNTs are dispersed and there are in...
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In this work, the Al/CNT/CuO nano-thermite samples are prepared by ultrasonic mixing with variable CNT content. The morphology of nano-thermites analysed by electron microscopy revealed that the CNTs are dispersed and there are intimate contacts between fuels (Al and CNT) and oxidiser (CuO) constituents of the nano-thermite. Raman spectroscopy technique is used to analyse the structural integrity of the CNTs in the nano-thermite. The thermite reaction characteristics are evaluated by simultaneous thermogravimetric analysis/differential scanning calorimetry technique. The exothermic enthalpy of the Al/CNT/CuO nano-thermite samples increased with increasing CNT content. The effect of Al particle size and Al/Cu molar ratio variation on the thermite reaction enthalpy is also analysed. The ignition temperature of the thermite reaction is also lowered by 71 degrees C than that of Al/CuO nano-thermite. The activation energy for thermite reaction of Al/CNT/CuO nano-thermite is also lowered by 23% to that of pure Al/CuO. The residues of the nano-thermites after the thermite reaction at 1010 degrees C are collected and analysed by the X-ray diffraction.
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Aluminum is traditionally used as the primary fuel in nanocomposite energetic systems due to its abundance and high energy release. However, thermodynamically boron releases more energy on both a mass and volumetric basis. Kinetic...
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Aluminum is traditionally used as the primary fuel in nanocomposite energetic systems due to its abundance and high energy release. However, thermodynamically boron releases more energy on both a mass and volumetric basis. Kinetic limitations can explain why boron rarely achieves its full potential in practical combustion applications, and thus has not replaced aluminum as the primary fuel in energetic systems. In particular, the existence of the naturally formed boron oxide (B_2O_3) shell is believed to play a major role in retarding the reactivity by acting as a liquid barrier if it cannot be efficiently removed. In this paper we demonstrate from constant-volume combustion experiments that nanoboron can be used to enhance the reactivity of nanoaluminum-based Metastable intermolecular Composites (MICs) when the boron is < 50 mol% of the fuel. It was also observed that an enhancement was not achieved when micronboron (700 nm) was used. Thermodynamic calculations showed that the aluminum reaction with CuO was sufficient to raise the temperature above ～2350 K in those mixtures which showed an enhancement. This is above both the boiling point of B_2O_3 (2338 K) and the melting point of boron (2350 K). A heat transfer model investigates the heating time of boron for temperatures >2350 K (the region where the enhancement is achieved), and includes three heating times; sensible heating, evaporation of the B_2O_3 oxide shell, and the melting of pure boron. The model predicts the removal of the B_2O_3 oxide shell is fast for both the nano- and micronboron, and thus its removal alone cannot explain why nanoboron leads to enhancement while micronboron does not. The major difference in heating times between the nano- and micronboron is the melting time of the boron, with the micronboron taking a significantly longer time to melt than nanoboron. Since the oxide shell removal time is fast for both the nano- and micronboron, and since the enhancement is only achieved when the primary reaction (Al/CuO) can raise the temperature above 2350 K, we conclude that the melting of boron is also necessary for fast reaction in such formulations. Nanoboron can very quickly be heated relative to micronboron, and on a timescale consistent with the timescale of the Al/CuO reaction, thus allowing it to participate more efficiently in the combustion. The results indicate that sufficiently small boron can enhance the reactivity of a nanoaluminum-based MIC when added as the minor component (< 50% by mole) of the fuel.
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Aluminum (Al) is widely used in thermites, and iodine pentoxide (I2O5) is a strong oxidizer that can release gas phase iodine. The result is a potentially very powerful and effective energetic composite with biocidal properties. H...
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Aluminum (Al) is widely used in thermites, and iodine pentoxide (I2O5) is a strong oxidizer that can release gas phase iodine. The result is a potentially very powerful and effective energetic composite with biocidal properties. However, the alumina coating on the Al presents a diffusion barrier that increases the ignition temperature. In this paper, nano titanium powders (nTi) were added at various mass ratios to Al/I2O5 and a composite mesoparticle was assembled by electrospray. Combustion studies showed that nano Ti can enhance the reactivity of Al/I2O5 thermites significantly with increases in pressure, pressurization rate and dramatic decreases in burn time. Part of this behavior can likely be attributed to less sintering enhancing the completeness of reaction in the Ti added cases. Addition of titanium to aluminum was found to have a significant impact on ignition and when Al is replaced with Ti, the ignition temperature was similar to 300 degrees C below that of the corresponding aluminum thermite. These results show that titanium can be used as a fuel to tune energetic behavior and iodine release temperature and in some ways superior to aluminum. (C) 2019 Published by Elsevier Inc. on behalf of The Combustion Institute.
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In nano-thermites, due to improper contacts between the reactants in solid-state reaction, the complete energy release during redox-reaction is still challenging. Here, Fe2O3/Al nano-thermite is synthesized with different weight f...
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In nano-thermites, due to improper contacts between the reactants in solid-state reaction, the complete energy release during redox-reaction is still challenging. Here, Fe2O3/Al nano-thermite is synthesized with different weight fraction of multi-layered graphene (MLG) nanoplatelets by physical mixing technique. X-ray diffraction reveals that the synthesis method does not change the phase of reactants. The morphology of nano-thermite is examined by scanning electron microscopy and transmission electron microscopy which reveals that MLG nanoplatelets are homogeneously dispersed in the samples. Structural stability of MLG nanoplatelets present in nano-thermite is observed by Raman spectroscopy. The nano-thermite reaction is analyzed by differential scanning calorimetry. The results show that an increase in MLG content in nano-thermite sample Al/Fe2O3, from 0 wt% to 12 wt% is responsible for increase in the corresponding exothermic enthalpy from 71 J/g to 1537 J/g. On the contrary, ignition temperature and activation energy of these samples get reduced with increasing MLG.
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Thermite and related processes have attracted considerable attentions as highly useful technologies for material and energy supplies in extreme environments such as geothermal and disaster-stricken areas as well as in space explor...
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Thermite and related processes have attracted considerable attentions as highly useful technologies for material and energy supplies in extreme environments such as geothermal and disaster-stricken areas as well as in space exploration. As part of the development of casing pipes used in geothermal power plants started from the 1970s, long ceramic-lined pipes have been obtained via thermite reaction under effective centrifugal force without any additional heat treatment [Centrifugal-Thermite (C-T) process]. The potentials of the ceramic-lined pipes in deeper geothermal utilizations are revalidated in details by reviewing the results obtained through the erosion-corrosion tests carried out under the conditions of high velocities (up to 100 m/s) and various acidities (from pH 2.0 to pH 4.0) of two-phase flows provided by the geothermal field test apparatus at the Onikobe geothermal power plant in Japan. By reducing the FeO content of the ceramic layer processed under proper additives and reactant amounts, the tested ceramic-lined pipes were significantly improved compared to highly corrosion-resistant stainless steels. The technological advantages obtained through the development of the C-T process can show a useful model to promote thermite-related technologies onward.
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A recently developed model for low-temperature exothermic reactions in nanocomposite Al-CuO thermites described the evolution of an alumina layer growing between Al and CuO and changes in its diffusion resistance as critically aff...
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A recently developed model for low-temperature exothermic reactions in nanocomposite Al-CuO thermites described the evolution of an alumina layer growing between Al and CuO and changes in its diffusion resistance as critically affecting ignition of the composite reactive material. The model was successful in describing ignition of individual composite particles in a CO_2 laser beam. However, it was unable to conclusively predict ignition of the same powder particles coated onto an electrically heated filament. In this work, ignition of fully-dense 2Al·3CuO nanocomposite powder prepared by arrested reactive milling was studied using a modified electrically heated filament experiment, located in a miniature vacuum chamber. Thin layers of the powders coated on a nickel-chromium filament were ignited at heating rates between 200 and 16,000 K/s. The ignition was accompanied by both optical emission and pressure signals. The pressure signals occurred before emission, with increasing delay at higher heating rates. Ignition temperatures were only slightly affected by the heating rate. The results are interpreted proposing that the low-temperature redox reaction produces a metastable CuO_(1-x) phase with 0 < x ≤1 which releases oxygen upon heating. It is shown that despite a relatively small heat release, the low-temperature reactions in nanocomposite thermites are important as producing destabilized, partially reduced oxides that decompose with gas release upon heating. In the present experiments, the gas release changed thermal properties of the powder coating, reducing the efficiency of heat exchange with the supporting filament and thus enabling its thermal runaway and ignition.
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