Low-temperature Exothermic Reaction Research Articles

The ignition characteristics of dual-fuel spray at different ambient methane concentrations under engine-like conditions

In the present study, dual-fuel ignition process of pilot diesel spray injection into a natural gas-air mixture under engine-like conditions is numerically investigated via large-eddy simulation (LES). Methane and n-dodecane are considered as surrogates for natural gas and diesel fuels, respectively. This study aims to provide understandings of kinetic interactions for dual-fuel spray during the ignition process. The effects of ambient methane concentration on the ignition delay times (IDTs), product distributions, and heat release rates (HRRs) during dual-fuel ignition process are emphasized. The results show that methane prolongs the IDT of n-dodecane, especially for the first-stage IDT. Kinetic analyses reveal that the competition for OH between the dehydrogenation reactions of methane (CH4 + OH <=> CH3 + H2O) and n-dodecane (RH + OH <=> R + H2O) results in the increasing first-stage IDT of pilot spray. Moreover, it is found that the retarding effect positively correlates with the methane concentration. For product distributions, it is observed that the transition of CH2O from fuel-lean to fuel-rich regions is inhibited in the first-stage ignition. In the second-stage ignition, the concentration of CO is decreased as the methane concentration increases and the area with high CO concentration is gradually shifted to the downstream of the spray. In addition, HRRs of low-temperature exothermic reactions appear at the radial periphery as well as the head of the spray front; HRRs of high-temperature exothermic reactions are gradually shifted from the head to the center of the spray with the increase of ambient methane concentration. The HRRs of low- and high-temperature reactions are decreased with increasing ambient methane concentration, and the major reactions for heat release at various ambient methane concentrations are different.

Low-Temperature Exothermic Reactions in Al/CuO Nanothermites Producing Copper Nanodots and Accelerating Combustion

Several Al/CuO nanocomposite reactive materials, prepared recently by arrested reactive milling, were found to exhibit a distinct low-temperature exothermic peak around 600 K seen by differential scanning calorimetry. This work is an experimental study aimed to establish whether this low-temperature reaction affects combustion and why it is observed in some but not all materials with identical compositions. The peak is only observed when aluminum is initially separately milled in acetonitrile. Electron microscopy showed resulting composite powders to be porous, unlike fully dense powders obtained without premilling aluminum. The as-prepared and partially reacted powders recovered after heating just past the peak to 650 K were examined using high-angle annular dark-field scanning transmission electron microscopy and electron energy loss spectroscopy. Additionally, X-ray diffraction measurements were performed for all materials. Finally, the reactive powders were ignited using plasma and shock generated by an electrostatic discharge. Results show that the low-temperature exothermic peak is associated with a redox reaction pathway involving release of oxygen by CuO that is not in direct contact with Al and free-molecular transport of that oxygen to the nearby surface of Al. This reaction pathway inhibits the formation of Al/Cu intermetallic phases. Instead, nanometer-scale metallic Cu particles are formed due to CuO reduction. It is also found that this reaction pathway accelerates ignition of reactive powders, which in turn leads to a higher burn rate of aerosolized powder.

Metal-rich aluminum–polytetrafluoroethylene reactive composite powders prepared by mechanical milling at different temperatures

Aluminum–polytetrafluoroethylene (Al·PTFE) composite materials with 90 wt% Al are prepared by mechanical milling at both room and cryogenic temperatures. Distribution of PTFE in the material is more homogeneous in the cryogenically milled materials. Thermal analysis in inert atmosphere shows at least three distinct exothermic reaction steps below Al melting as well as evolution of AlF3 at temperatures above 800 °C. The heat of low-temperature exothermic reactions first increases and then decreases as a function of the milling time. In an oxidizing environment, all materials oxidize qualitatively similar to pure Al, and a composite milled cryogenically for 6 h oxidizes as fast as or faster than nano-sized aluminum powder. When heated at several thousand degrees per second, all composites ignite around 730 °C. Only the powders prepared by cryogenic milling could be ignited by electrostatic discharge (ESD) with energies of 720 mJ, while materials milled at room temperatures could not be ignited with energies as high as 2 J. In the ESD ignition experiments, the optical emission pulse is delayed compared to the pressure pulse, suggesting that a gas-generating PTFE decomposition triggers the ignition. The material cryogenically milled for 6 h is the most attractive, based on the magnitude of both pressure and emission pulses generated upon its ESD ignition, and based on the rate and extent of its oxidation in thermo-analytical experiments.

Low-temperature exothermic reactions in fully-dense Al/MoO3 nanocomposite powders

Low-temperature redox reactions in nano-thermites prepared by arrested reactive milling (ARM) were described by Cabrera–Mott (CM) mechanism, in which the growth of very thin oxide layers is accelerated by an electric field induced across such layers. A reaction mechanism combining the initial CM step with following oxidation steps identified earlier for oxidation of Al and representing growth of and phase transitions in various polymorphs of alumina was proposed and shown to be valid for different Al/CuO nanocomposite powders prepared by ARM. This work explores whether a similar multi-step reaction mechanism can be applied to describe thermal initiation in another ARM-prepared thermite system, Al/MoO3. The powder particles comprise a fully dense Al matrix with nano-sized MoO3 inclusions. The structure, morphology, and compositions of the prepared powder with composition 8Al·MoO3 were characterized using X-ray diffraction, scanning electron microscopy and energy dispersive X-ray spectroscopy. Differential scanning calorimetry (DSC) and thermogravimetry (TG) data were collected at varied heating rates, in addition to micro-calorimetry data collected both isothermally and at several very low heating rates. The shapes of the recorded DSC and TG traces at low temperatures were substantially different compared to those obtained earlier for Al/CuO. A weight loss representing dehydration of the composite material was observed at low temperature TG measurements. The effect of this endothermic process was quantified to account for it in the micro-calorimetry and DSC experiments. Kinetics of the initial exothermic redox reaction observed in both DSC and micro-calorimetry experiments could be successfully described by the CM mechanism. Reactions at elevated temperatures were also well described by the previously identified sequence of steps associated with phase changes and oxidation of various alumina polymorphs. A modification to the CM kinetics was necessary to describe an intermediate range of temperatures, between approximately 400 and 600K. This modification is suggested to represent a temperature induced change in the structure of a very thin precursor aluminum oxide layer separating Al and MoO3.

Effect of substrate doping on microstructure and reactivity of porous silicon

The effect of substrate doping and the choice of oxidizer on the reactivity of energetic porous silicon-oxidizer composites were examined in this study. Porous silicon samples were prepared from N and P-type substrates with widely different dopant concentrations, and impregnated with perchlorate salts or sulfur to form energetic composites, which were characterized using differential scanning calorimetry. While the low temperature exothermic reactions with perchlorate salts were found to be unaffected by the substrate doping properties, low temperature reactions with sulfur were found to be affected by the substrate doping. This is attributed to the effect of substrate doping on the microstructure of the porous silicon formed.

A Study of the Reactivity of De-Intercalated NaNi0.5Mn0.5O2 with Non-Aqueous Solvent and Electrolyte by Accelerating Rate Calorimetry

O3-type layered structure NaNi0.5Mn0.5O2 was successfully synthesized by solid state reaction. NaNi0.5Mn0.5O2 was tested at room temperature using two-electrode Na/NaNi0.5Mn0.5O2 coin cells with NaClO4/Propylene Carbonate electrolyte. NaNi0.5Mn0.5O2 delivered a reversible capacity of 120 mAh/g during the first charge to 3.8 V, which corresponds to the de-intercalation of about 0.5 mol Na per mole transition metal. The reactivity of de-intercalated NaNi0.5Mn0.5O2 (∼Na0.5Ni0.5Mn0.5O2) with ethylene carbonate/diethyl carbonate (EC:DEC) solvent and with NaPF6/EC:DEC electrolyte was studied using Accelerating Rate Calorimetry (ARC). X-ray diffraction was used to study the products of the reactions of Na0.5Ni0.5Mn0.5O2 with solvent or electrolyte after the ARC experiments. Na0.5Ni0.5Mn0.5O2 shows high reactivity in both solvent and electrolyte. In solvent, Na0.5Ni0.5Mn0.5O2 decomposes at elevated temperature to form nickel manganese oxide and Ni metal, and the released oxygen reacts with the solvent. In NaPF6-based electrolyte, the reaction mechanism changes completely as new reaction products, Na3(Ni,Mn)F6 and Na(Ni,Mn)F3, are formed in a new low temperature exothermic reaction. These results imply important consequences regarding safety for those researchers selecting electrolytes and electrode materials for Na-ion batteries. In both cases, NaF was found as a product of Na0.5Ni0.5Mn0.5O2 reacting with PVDF binder.

ON GAS RELEASE BY THERMALLY-INITIATED FULLY-DENSE 2Al·3CuO NANOCOMPOSITE POWDER

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 CO2 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 CuOl−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.

Low-temperature exothermic reactions in fully dense Al–CuO nanocomposite powders

A hypothesis stating that the Cabrera–Mott kinetics describes low temperature redox reactions in fully dense nanocomposite thermite powders is explored. Earlier experiments involving differential scanning calorimetry (DSC) and new isothermal microcalorimetry measurements were interpreted using a reaction model employing the Cabrera–Mott mechanism. Following earlier work, the effect of Mott potential on the mass transport rate is considered to be temperature dependent. Simultaneous processing of both experimental data sets enabled us to determine parameters for the Cabrera–Mott mechanism describing the redox reaction in a nanocomposite Al–CuO thermite prepared by arrested reactive milling. The reaction model developed enables one to describe the initial portions of the DSC curves (up to 600 K) measured at different heating rates as well as microcalorimetry traces recorded in the temperature range of 303–413 K. The model is expected to be useful for describing both aging and ignition of fully dense, nanocomposite reactive materials.

IMPACT OF THE LOW-TEMPERATURE REACTIVITY OF REILLEX™ HPQ ON ACTINIDE PROCESSING

ABSTRACT Reactive System Screening Tool™ data and a computational model were used to predict the impact of pressurization on a typical process-scale ion exchange column due to gases generated by the low temperature exothermic reaction (LTE). The LTE results from a reaction between nitric acid and the ethylbenzene pendant groups of the Reillex™ HPQ resin. This reaction would occur if the resin bed were inadvertently heated above 70˚C. Without accounting for heat transfer, activation of the LTE at 70˚C would result in a calculated maximum temperature of 106˚C, which is below the boiling point of 8 M nitric acid (112˚C), and well below the temperatures leading to runaway reaction (∼145˚C). A pressure drop calculation shows that if a typical process-scale ion exchange column of Reillex™ HPQ is inadvertently heated to activate the LTE, the resulting pressurization (ΔP=0.37 psi) could be dissipated safely via a 0.5-inch ever-open vent without challenging a 60-psi g rupture disk.

LOW TEMPERATURE EXOTHERMIC REACTION OF REILLEXTM HPQ IN NITRIC ACID

ABSTRACT Despite reports of the relative inertness of ReillexTM HPQ to chemical and radiolytic degradation, thermal stability studies in nitric acid solutions identified a low temperature exothermic reaction using the Reactive System Screening Tool (RSST). Comparative data for ReillexTM HPQ and IonacTM A-641 both show runaway behavior in nitric acid above 130˚C, but only ReillexTM HPQ had a low temperature exotherm that initiated at about 69˚C. While this exotherm is small in magnitude, gaseous products were identified, leading to concerns of pressurization within the ion exchange column. This work identifies and characterizes the low temperature exothermic reaction of ReillexTM HPQ in nitric acid, identifies ethylbenzene pendant groups as the origin of this reaction, and proposes a chemical pretreatment to eliminate it.

Kinetics of Alder-ene reaction of Tris(2-allylphenoxy)triphenoxycyclotriphosphazene and bismaleimides — a DSC study

Tris(2-allylphenoxy)triphenoxycyclotriphosphazene was reacted with three bismaleimides (BMI), viz. bis(4-maleimido phenyl)methane (BMM), bis(4-maleimido phenyl)ether (BME) and bis(4-maleimido phenyl)sulphone (BMS) via the Alder-ene reaction. The differential scanning calorimetric analysis of the blend manifested two distinct exotherms. The low temperature exothermic reaction was attributed to the Wagner-Jauregg reaction following the ene reaction and the strong exotherms at around 250–270°C to the cross-linking Diels–Alder reactions of the initially formed adducts. The Kinetic parameters, viz. activation energy ( E) and pre-exponential factor ( A) of the reactions were evaluated by Kissinger and Ozawa methods using the variable heating rate method. The kinetic data revealed that the Wagner-Jauregg reaction was disfavoured by electron-withdrawing nature of the BMI. The Diels–Alder reaction was facilitated by the electron-withdrawing nature of the bismaleimide. The activation energy for the first exothermic stage decreased and for the second major step increased on enhancing the stoichiometry of BMI in the blend for a given pair. The activation parameters served to predict the isothermal cure profiles of the blends and deduce the possible network structure under the given conditions of cure temperature and stoichiometry.

Precipitation of icosahedral quasicrystalline phase in Hf69.5Al7.5Ni11Cu12 metallic glass

A Hf69.5Al7.5Ni11Cu12 metallic glass was prepared by a single roller melt-spinning method, and the crystallization process was studied by x-ray diffraction, differential scanning calorimetry, and transmission electron microscopy. The metallic glass crystallizes through three exothermic reactions. The low-temperature exothermic reaction corresponded to the precipitation of an icosahedral quasicrystalline phase. Further annealing at higher temperature led to the decomposition of the icosahedral quasicrystalline phase to other stable crystalline phases, indicating that the precipitated icosahedral quasicrystalline phase was in a metastable state. The crystallization process of the present alloy was compared with that of other Hf–Al–Ni–Cu alloys, and the reason for the precipitation of the icosahedral quasicrystalline phase was discussed.

Ignition of AP-based composite solid propellants containing nitramines exposed to CO 2 laser radiation at subatmospheric pressures

Ignition characteristics of AP-based composite solid propellants that contained either HMX or RDX, were investigated using a CO 2 laser at relatively low heat flux and subatmospheric pressures, in order to simulate space environments and also simplify our understanding of the nature of ignition phenomena. The results are presented graphically as a function of surface heat flux and ambient pressure. The ignition process was found to be roughly divided into two regions of self-sustaining ignition (S.S.I.) and non-S.S.I., depending on whether the ignition reaction is self-sustained or extinguished when the external radiation is interrupted after ignition. Furthermore, the non-S.S.I. was composed of two regions of radiation-assisted combustion (R.A.C.) and radiation-assisted unstable combustion (R.A.U.). The addition of nitramines, in particular, HMX to AP-based composite propellants resulted in a marked reduction in the ignition times and made the self-sustaining ignition region shift to lower heat fluxes at each specific pressure. It was found, from the results of differential thermal analysis and thermogravimetry, that nitramine-containing propellants exhibit an exothermic decomposition at relatively low temperatures that is independent of the presence of other ingredients. This triggers the ignition reaction of the AP-based propellants and lowers temperature of the main exotherm. Consequently, it appears that the improvement of ignitability by the addition of nitramines is due to the intense low-temperature exothermic reaction of nitramines. It was found from the time-temperature histories measured with fine thermocouples on the surface during the ignition process that ignition temperatures also are lowered by the addition of nitramines. Howevver, when a propellant sample containing nitramines was exposed to higher heat flux levels, the shorter ignition times reduced self-sustaining ignitability slightly, presumably because these propellants could not form a sufficiently thick thermal layer for steady-state burning.