Electron emission properties of detonation nanodiamonds

This paper summarizes the results of systematic studies of field electron emission from detonation nanodiamond coatings corresponding to nanodiamond powders of different modifications. The role of the chemical composition of the surface of detonation nanodiamond particles in field emission mechanisms is discussed. Field emission-related electronic properties of single diamond nanodots are studied using tight-binding calculations and continuum electrostatic simulations.     

Effects of pressure on the structure and the thermal decomposition kinetics of RDX

Abstract

High pressure and temperature structural changes for RDX were investigated to 7.0 GPa and 570 K in a diamond anvil cell apparatus using FTIR absorption, optical microscopy, and energy-dispersive powder x-ray diffraction techniques. Three distinct solid phases were observed. The effects of pressure on the thermal decomposition kinetics as a function of RDX pressure were investigated using an infrared absorption technique. Solid phase I was found to have a pressure enhanced reaction rate with an energy and volume of activation of 51 Kcal/mole and -5.6 cc/mole respectively. Solid II was not observed to react and the observed reaction rate of Solid III decreased with increasing pressure.     

Specific features of the structure of detonation nanodiamonds from results of electron microscopy investigations

The experimental results of a comprehensive investigation of the structure of detonation synthesis nanodiamonds by electron microscopy methods have been presented. The morphology of diamond nanoparticles has been investigated and the microdiffraction patterns have been analyzed. The method of characteristic fast electron energy loss spectroscopy in transmission electron microscopy has been used. The local density of structural components of a nanodiamond (diamond core and fullerene-like shell) has been obtained. The shape of the shell surrounding the nanocrystal has been determined using model calculations. A hypothesis explaining the charging of the nanodiamond surface has been proposed.     

Formation of a detonation wave in the thermal decomposition of acetylene

The formation of a condensation detonation wave has been experimentally observed in the shock-induced thermal decomposition of acetylene. The stable detonation wave in the 20% C₂H₂ + 80% Ar mixture has been obtained at an initial pressure behind the shock wave of no less than 30 atm. The main kinetic characteristics of the pyrolysis of acetylene—the period of the induction of condensation and the growth rate constant of condensed particles—have been determined. The correlation of various stages of the process with the heat release in the condensation has been analyzed. It has been shown that the period of the particle growth induction is not accompanied by noticeable heat release. The subsequent condensation stages characterized by significant heat release occur very rapidly (faster than 10⁻⁵ s) in the so-called explosive condensation. The analysis of the results indicates that the reactions leading to the growth of large polyhydrocarbon molecules, which precede the formation of condensed carbon particles, constitute the limiting stage of the process, which determines the possibility of the formation of the condensation detonation wave in acetylene. An increase in the pressure is accompanied by the sharp narrowing of the induction region and the transition of the process to the condensation detonation wave.     

Predictive kinetics for the thermal decomposition of RDX

The application of Variable Reaction Coordinate Transition State Theory for an energetic material is presented. The homolysis of the N–N bond in RDX is characterized using an embedding methodology in which key atoms in the bond-dissociation process are computed using CASPT2(10e,7o)/jun-cc-pVTZ, while the rest of the molecule is computed using M06-2X/jun-cc-pVTZ. Microcanonical rate theory is used to quantify the temperature and pressure dependent rate constants. The cleavage of the N–N bond is by far the dominant channel, with HONO elimination a distant second. The predicted rate constants are in excellent agreement with the experimental data. The computational approach can be used to provide accurate models for the combustion properties of novel energetic materials.     

Effect of thermal and chemical treatment on the structure and composition of nanodiamonds

The effect of chemical and thermal treatment on structural characteristics and chemical composition of detonation synthesis ultradispersed diamonds (UDDs) was studied. UDDs of five different producers were compared. Special attention was paid to structural characteristics of samples (size of coherent scattering regions, aggregation), which were determined by SEM, AFM, and X-ray diffraction methods. Impurities were analyzed by methods of mass spectrometry, atomic and absorption analysis, and X-ray spectral analysis.     

Features of the transformation of detonation nanodiamonds into onion-like carbon nanoparticles

On the basis of the results of investigations carried out by differential scanning calorimetry, X-ray diffraction, nuclear magnetic resonance, high-resolution transmission electron microscopy, and Raman spectroscopy, a scheme for the transformation of detonation nanodiamonds (which are agglomerates of smaller particles) into onion-like carbon nanoparticles during vacuum annealing is verified. At high temperatures, the transition of an individual nanodiamond occurs in a short period of time and may proceed via an amorphous state.     

Study on the kinetics properties of lithium hexafluorophosphate thermal decomposition reaction

The thermal decomposition of lithium hexafluorophosphate was studied by using C80 calorimeter, and the samples were heated at a 0.2 K·min^− 1 heating rate from ambient temperature to 573 K in pressure-sensitive transducer fitted and sealed vessel with argon atmosphere. It is found that LiPF₆ decomposes near 433 K, and the gas product PF₅ causes the pressure increasing. The LiPF₆ decomposition reaction order is calculated based on the pressure data by two methods, its average value is n = 1.5, then, the reaction is assumed to be dependent on the Arrhenius law and mass action law, and thus the activation energy and pre-exponential factor were calculated to be E = 104.2 kJ·mol^− 1, A = 1.12 × 10⁷ s^− 1, respectively.     

Model of formation of three-dimensional polyurethane films modified by detonation nanodiamonds

The possibility of increasing the strain resistance of three-dimensional polyurethane films modified by detonation nanodiamonds is analyzed. It is found that the physicomechanical characteristics of polyurethane foams modified by nanodiamonds at a content of several thousandths of a weight percent change significantly. Models of the hardening of polyurethane films are proposed.     

Structure of the dispersive medium and sedimentation resistance of suspensions of detonation nanodiamonds

The specific features of the stabilization of suspensions formed by detonation nanodiamonds in polar and nonpolar media are investigated. It is demonstrated that the polydispersity of nanodiamond particles in an aqueous medium periodically changes in an ultrasonic field. The conditions are determined under which the optimum dispersity can be maintained for a long time. A technique is devised for chemical modification of the nanodiamond surface through the grafting of organosilyl groups. This technique makes it possible to prepare finely dispersed suspensions of nanodiamonds in nonpolar organic media. A model is proposed for an aggregate that consists of detonation nanodiamond particles and is stabilized through hydrogen bonds formed by functional groups of different types.     

Comparative structural characterization of the water dispersions of detonation nanodiamonds by small-angle neutron scattering

The structure of nanodiamond water dispersions prepared under different conditions was investigated by small-angle neutron scattering at the scale of 1 to 100 nm. The study of diluted dispersions was regarded as of paramount importance. Similarly to previous studies, strong clustering of particles was revealed in the solutions. The typical size of clusters (40 nm and above) depends on the modification of the dispersions. A common property can be distinguished for different systems: the fractal dimension of the clusters is in the range of 2.3?C2.4, which indicates that there is a common clustering mechanism in such systems. Using contrast variation, the existence of a nondiamond component in the colloidal particles of the dispersions was confirmed; it correlates with the presence of a graphene shell on crystallite surfaces.     

Electron paramagnetic resonance detection of the giant concentration of nitrogen vacancy defects in sintered detonation nanodiamonds

A giant concentration of nitrogen vacancy defects has been revealed by the electron paramagnetic resonance (EPR) method in a detonation nanodiamond sintered at high pressure and temperature. A high coherence of the electron spins at room temperature has been observed and the angular dependences of the EPR spectra indicate the complete orientation of the diamond system.     

On the kinetics of spinodal decomposition

We discuss the kinetics of phase separation based on the mechanism of spinodal decomposition. The starting point is Boltzmann's transport equation. A perturbation theory will be developed leading to a hierarchy of differential equations for the local density. The first member of this hierarchy is a nonlinear wave-type equation, which linearized version will be solved in order to discuss the behaviour of the amplification factor. Additionally, we compare our results with those of standard diffusion-type approximations and molecular dynamics calculations.Research supported in part by National Science Foundation, Grant ENG-7 515 882-A01     

Mixing and detonation structure in a rotating detonation engine with an axial air inlet

High-fidelity simulations of an experimental rotating detonation engine with an axial air inlet were conducted. The system operated with hydrogen as fuel at globally stoichiometric conditions. Instantaneous data showed that the detonation front is highly corrugated, and is considerably weaker than an ideal Chapman–Jouguet wave. Regions of deflagration are present ahead of the wave, caused by mixing with product gases from the previous cycle, as well as the injector recovery process. It is found that as the post-detonation high pressure flow expands, the injectors recover unsteadily, leading to a transient mixing process ahead of the next cycle. The resulting flow structure not only promotes mixing between product and reactant gases, but also increases likelihood of autoignition. These results show that the detonation process is very sensitive to injector design and the transient behavior during the detonation cycle. Phase-averaged statistics and conditionally averaged data are used to understand the overall reaction structure. Comparisons with available experimental data on this configuration show remarkable good agreement of the predicted reacting flow structure.     

Dynamics of detonation waves in a channel with variable cross section and filled with bubbly fluid

The flow of bubbly fluid comprising a mixture of bubbles filled with explosive and inert gases, which is driven through a converging channel, was studied. Depending on the velocity of the hummer hitting the bubbly fluid boundary, the flow may be accompanied by the development of detonation waves which compress the bubbles with inert gas.     

Multi-dimensional mesoscale simulations of detonation initiation in energetic materials with density-based kinetics

In this work we present multi-dimensional mesoscale simulations of detonation initiation in energetic materials. We solve the reactive Euler equations, with the energy equation augmented by a power deposition term. The reaction rate at the mesoscale is modelled using density-based kinetics, while the deposition term is based on simulations of void collapse at the microscale, modelled at the mesoscale as hot spots. We carry out two- and three-dimensional mesoscale simulations of random packs of HMX crystals in a binder, and show that transition between no-detonation and detonation depends on the number density of the hot spots, the packing fraction, and the post-shock pressure of an imposed shock. In particular, we show that, for a fixed post-shock pressure, there exists a critical value of the number density of hot spots, such that when the number density is below this value a detonation wave will not develop. We highlight the importance of morphology to initiation by comparing with a homogeneous counterpart, and we compare relevant length scales by examining their corresponding power spectra. We also examine the effect of packing fraction and show that at low post-shock pressures there is significant variation in the initiation times, but that this variation disappears as the post-shock pressure is increased. Finally, we compare three-dimensional simulations with the experimental data, and show that the model is capable of qualitatively reproducing the trends shown in the data.     

Aerosol deposition of detonation nanodiamonds used as nucleation centers for the growth of nanocrystalline diamond films and isolated particles

The aerosol deposition of detonation nanodiamonds (DNDs) on a silicon substrate is comprehensively studied, and the possibility of subsequent growth of nanocrystalline diamond films and isolated particles on substrates coated with DNDs is demonstrated. It is shown that a change in the deposition time and the weight concentration of DNDs in a suspension in the range 0.001–1% results in a change in the shape of DND agglomerates and their number per unit substrate surface area N _s from 10⁸ to 10¹¹ cm⁻². Submicron isolated diamond particles are grown on a substrate coated with DND agglomerates at N _s ≈ 108 cm⁻² using microwave plasma-enhanced chemical vapor deposition. At N _s ≈ 10¹⁰ cm⁻², thin (∼100 nm) nanodiamond films with a root-mean-square surface roughness less than 15 nm are grown.     

Quantum effects in the kinetics of the initiation of detonation condensation waves

The features of the kinetics of the initiation of detonation condensation waves in carbon suboxide and acetylene have been experimentally studied at high pressures near the low-temperature limits. The role of quantum effects in the expansion of detonation limits has been analyzed. Quantum corrections to the endothermic reaction rates, which are caused by an increase in the high-energy tails of the momentum distribution functions at high pressures due to the manifestation of the uncertainty principle for the energy of colliding particles at a high collision frequency, have been quantitatively estimated. It has been shown that experimentally observed deviations in Arrhenius dependences of the induction periods of the initiation of detonation condensation waves are well described by the proposed quantum corrections.     

Statistical analysis of detonation wave structure

Hydrocarbon fueled detonations are imaged in a narrow channel with simultaneous schlieren and broadband chemiluminescence at 5 MHz. Mixtures of stoichiometric methane and oxygen are diluted with various levels of nitrogen and argon to alter the detonation stability. Ethane is added in controlled amounts to methane, oxygen, nitrogen mixtures to simulate the effects of high-order hydrocarbons present in natural gas. Sixteen unique mixtures are characterized by performing statistical analysis on data extracted from the images. The leading shock front of the schlieren images is detected and the normal velocity is calculated at all points along the front. Probability distribution functions of the lead shock speed are generated for all cases and the moments of distribution are computed. A strong correlation is found between mixture instability parameters and the variance and skewness of the probability distribution; mixtures with greater instability have larger skewness and variance. This suggests a quantitative alternative to soot foil analysis for experimentally characterizing the extent of detonation instability. The schlieren and chemiluminescence images are used to define an effective chemical length scale as the distance between the shock front and maximum intensity location along the chemiluminescence front. Joint probability distribution functions of shock speed and chemical length scale enable statistical characterization of coupling between the leading shock and following reaction zone. For more stable, argon dilute mixtures, it is found that the joint distributions follow the trend of the quasi-steady reaction zone. For unstable, nitrogen diluted mixtures, the distribution only follows the quasi-steady solution during high-speed portions of the front. The addition of ethane is shown to have a stabilizing effect on the detonation, consistent with computed instability parameters.     

Effect of thermal pressure in converging detonation waves

Propagation of converging detonation waves in various explosives is studied using the equation of state, which considers both the thermal and elastic pressures. It is seen that the rate of increase of thermal pressure is higher than that of the elastic pressure during convergence. The present equation of state is better since it also gives the variation of temperature, whereas the polytropic form of the equation of state is independent of temperature. It is seen that the total detonation pressure is slightly greater than the elastic pressure. Results are compared with those reported elsewhere.