Formation of a detonation wave in the process of chemical condensation of carbon nanoparticles

A new physical phenomenon — the formation of a detonation wave as a result of the condensation of substances — was investigated in detail. A detonation wave was formed under the action of the energy released in a process of chemical condensation of carbon nanoparticles behind a shock wave in the mixture initially containing 10–30% of the carbon suboxide C₃O₂ or acetylene C₂H₂ in argon. The propagation of the shock wave in this mixture led to the rapid thermal disintegration of its initial molecules and the subsequent formation of a condensed carbon with a significant energy release. The increase in the temperature and in the pressure of the reacting mixture leads to a strengthening of the shock wave and to its transformation into a detonation wave. The main kinetic characteristics of the reaction of thermal disintegration of the indicated molecules and their subsequent chemical condensation as well as the interrelation of these characteristics with the heat-release processes forming the detonation wave were determined.     

Investigation of the kinetics of carbon nanoparticle charging in shock-heated plasma

The experimental results and a computer simulation of charging of carbon nanoparticles formed in the process of pyrolysis of the carbon suboxide C₃O₂ behind shock waves are presented. It is shown that the time necessary for the onset of an equilibrium distribution of charges in the gas-nanoparticle system at pressures of 15–30 bar decreases from 400 to 40 μs with a temperature rise from 2000 to 3600 K. The equilibrium concentration of free electrons in the presence of nanoparticles in the mixture is much lower than that in the same gas without particles. Step ionization of sodium and a subsequent recombination of free electrons and sodium ions on the nanoparticle surface are considered in order to analyze the kinetics of nanoparticle charging.     

Preparation and mechanism of nanometer Al5O6N via shock wave plasma technique

Cubic Al₅O₆N nanocrystals were successfully synthesized via a novel strategy called shock wave plasma technique, using trinitrotoluene (TNT) and aluminum powder as raw materials and water as protection medium. The precursor including carbon and Al₅O₆N was engendered firstly during the detonation of compound dynamite, and then the pure Al₅O₆N nanoparticles were obtained when the carbon was removed through calcining at high temperature. The precursor and the final as-synthesized Al₅O₆N powder were characterized by X-ray diffraction (XRD), Raman spectrum and high-resolution transmission electron microscope (HRTEM), respectively. The calcining temperature schedule of the precursor was decided through DTA/TG analysis. The results indicate that the precursor consists of 37.7% carbon and 62.3 °C Al₅O₆N. After calcining at 600 °C for 1 h, the average diameter of the as-synthesized Al₅O₆N nanocrystal is 30-40 nm and the morphology micrograph takes on uniform spherical shape. The lattice parameters are consistent completely with the standard cubic Al₅O₆N (JCPDS 48-0686). The well-dispersed Al₅O₆N nanocrystals synthesized by shock wave plasma technique can be attributed to the covering of carbon and fast cooling of water medium. A possible reaction mechanism was also proposed preliminarily based on the experimental results.     

Electronic properties of calcined materials from a scandium-O-phenylene-O-yttrium-O-phenylene hybrid copolymer

Calcination of a scandium-O-phenylene-O-yttrium-O-phenylene hybrid copolymer under an argon atmosphere at 400–800 °C was performed. The calcined materials were found to be composed of nano-sized Sc₂O₃ and Y₂O₃ particles in the matrix of carbon clusters. ESR spectral examinations of the calcined materials suggest that a two-step electron transfer of Y₂O₃ → carbon clusters → Sc₂O₃ → carbon clusters took place to provide a photo-responsive oxidation–reduction function with an oxidation site at Y₂O₃ particles and a reduction site at carbon clusters.     

Reaction in the Al-ZrO<Subscript>2</Subscript>-C system

The reaction in ball milled Al-ZrO₂-C powders with different ZrO₂/C molar ratios was studied by using DSC, XRD and EPMA. During heating fully milled powders, the Al first reacts with ZrO₂ and carbon simultaneously to form ZrAl₃ and Al₄C₃ phases, which further react with each other at about 896^∘C, causing the formation of Zr₂Al₃C₅. Then at 1140^∘C, Zr₂Al₃C₅ reacts with ZrAl₃, producing ZrC and ZrAlC₂ phases. In the un-fully milled powder, only an exothermic reaction between the three starting phases occurs at a higher temperature of about 956^∘C. Heating Al-ZrO₂-C powders with different ZrO₂/C ratio (r) to 1250∘C produces different phases in the final aluminum matrix: Al-Al₂O₃-Zr₂Al₃C₅-Al₄C₃ (r < 2:5), Al-Al₂O₃-Zr₂Al₃C₅ (r = 2:5), Al-Al₂O₃-Zr₂Al₃C₅-ZrC-ZrAlC₂ (2:5 < r < 3:5), Al-Al₂O₃-ZrC-ZrAlC₂ (3:5 ≤ r ≤ 3:4), Al-ZrAl₃-Al₂O₃-ZrC-ZrAlC₂ (r > 3:4). Zr₂Al₃C₅ presents fine platelet morphology, both ZrAlC₂ and ZrC have polygonal shape, ZrAl₃ exhibits flaky morphology at lower temperature, but blocky form at higher temperatures.     

Crystal Structure of a Double Oxalate of Np(V) and Co(NH3)

The crystal structure of a dioxalate complex Co(NH₃)₆NpO₂(C₂O₄)₂·1.5H₂O was studied. The structure consists of centrosymmetrical dimeric anions [NpO₂(C₂O₄)₂] ₂ ⁶⁻ , [Co(NH₃)₆]³⁺ cations, and water molecules of crystallization. The NpO ₂ ⁺ dioxocations have close Np-O bond lengths (average 1.824 Å) and are noticeably bent (ONpO angle 175.6°). Each dioxocation in the [NpO₂(C₂O₄)₂] ₂ ⁶⁻ anion is surrounded by five oxygen atoms of three C₂O ₄ ²⁻ anions; the Np coordination polyhedra are pentagonal bipyramids sharing a common edge. __________ Translated from Radiokhimiya, Vol. 47, No. 6, 2005, pp. 495–499. Original Russian Text Copyright ? 2005 by Charushnikova, Krot, Polyakova.     

The unstructured two-dimensional grid-based computation of selective thermal radiation in CO<Subscript>2</Subscript>-N<Subscript>2</Subscript> mixture flows

The possibility of applying the P ₁ approximation of the spherical harmonics method to the computation of the radiant heat transfer in heterogeneous volumes of complicated geometry is investigated. This approximation is used to evaluate the radiant heating of the surface of a spacecraft descending in the Martian atmosphere. The chemical composition of the gas heated behind the shock wave is calculated by using a kinetic model including 79 chemical reactions and ten components, such as CO₂, CO, C, O, O₂, C₂, N, N₂, CN, and NO. The optical properties are set by a spectral multigroup model computed with the help of the ASTEROID computer code with averaging over the rotational molecular spectrum structure in each group. The mechanisms of the radiant heating of the surface of descent space vehicles in the Martian atmosphere are studied.     

Investigation of chlorination process in aluminum production by carbothermic-chlorination reduction of Al2O3 under vacuum

The chlorination process in aluminum production by carbothermic-chlorination reduction of Al₂O₃ under vacuum has been investigated by XRD, SEM and thermodynamic analysis. The results of thermodynamic calculations indicated that the chlorination of Al₄C₃ or Al₄O₄C possibly proceeded at temperature 1300 °C under 5-50 Pa. The experiment results showed that aluminum could be produced by the chlorination of Al₄C₃ or Al₄O₄C and the decomposition of AlCl(g). According to the analysis of experiment results, Al₄C₃ and Al₄O₄C were the main reactants participated in chlorination process in aluminum production by carbothermic-chlorination reduction of Al₂O₃ under vacuum. AlCl(g) generated in chlorination of Al₄C₃ or Al₄O₄C would decompose into Al and AlCl₃(g), and then aluminum product would condense in condensation tower.     

Effects of physical growth conditions on the structural and optical properties of sputtered grown thin HfO2 films

HfO₂ thin films were prepared by reactive DC magnetron sputtering technique on (100) p-Si substrate. The effects of O₂/Ar ratio, substrate temperature, sputtering power on the structural properties of HfO₂ grown films were studied by Spectroscopic Ellipsometer (SE), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectrum, and X-ray photoelectron spectroscopy (XPS) depth profiling techniques. The results show that the formation of a SiO_x suboxide layer at the HfO₂/Si interface is unavoidable. The HfO₂ thickness and suboxide formation are highly affected by the growth parameters such as sputtering power, O₂/Ar gas ratio during sputtering, and substrate temperature. XRD spectra show that the deposited films have (111) monoclinic phase of HfO₂, which is also supported by FTIR spectra. XPS depth profiling spectra shows that highly reactive sputtered Hf atoms consume some of the oxygen atoms from the underlying SiO₂ to form HfO₂, leaving Si-Si bonds behind.     

Two Electrically Different Molecular Conductors Based on Unsymmetrical Au(III)-Dithiolene Complexes with Similar Crystal Structures

We report the first molecular charge transfer salt containing layers of Na⁺ within its lattice. From the crystal structure and from C=C and C-S bond lengths and Raman stretching frequencies we obtain a formula a′"-(BEDT-TTF)₉[Fe(C₂O₄)₃]₈Na₁₈(H₂O)₂₄. The structure consists of alternating layers of BEDT-TTF, and layers comprising [Fe(C₂O₄)₃]³⁻, Na⁺ and H₂O. The packing of the layers follows the sequence ABCDABCDA as previously seen in the compound β″-(BEDT-TTF)₄[Fe(C₂O₄)₃]_2.C₁₂O₆H₂₄.9H_xO¹.     

Effect of annealing on the microstructure, room-temperature strength, and fracture toughness of Al2O3−ZrO2−TiN ceramics

The microstructure, phase composition, room-temperature flexural strength, and fracture toughness of Al₂O₃−ZrO₂−TiN (AZT) ceramics were studied on specimens annealed in air at 1000, 1200, and 1400°C. The strength of the ceramics decreased with annealing temperature. The degradation in strength was caused by defects formed on or near the surface of the ceramics during oxidation of TiN which started at 600–700°C. The surface defects after annealing are influenced by the formation of rutile (TiO₂) at 1000 and 1200°C, aluminum titanate (Al₂TiO₅), and titanium suboxide Ti₅O₉ at 1400°C as well as by diffusion processes associated with ZrO₂. If the annealing of smooth AZT specimens in air resulted in lower strength, specimens in the form of single-edge notched beam (SENB) exhibited a considerable increase in fracture toughness (K _Ic) with annealing temperature. Such behavior was caused by the formation of an oxide layer which hindered the propagation of the main crack from the notch base. Thermal treatment of the smooth AZT specimens and further edge notching and testing did not result in a change of K _Ic values. The Al₂O₃ and Al₂O₃−ZrO₂ ceramics were also tested for comparison. Translated from Problemy Prochnosti, No. 1, pp. 132–138, January–February, 1999.     

Excitation and quenching of detonation in gases

The results of investigations on the problems of initiation, propagation, and stabilization of detonation waves and flowing combustible gaseous mixtures are presented. To describe the flows, we used ideal perfect gas equations and two models of the detonation wave: the classical infinitely thin model and a model in which behind the shock wave chemical reactions described by the single-stage kinetics for propane– and methane–air combustible mixtures proceed. Investigations were carried out by both analytical and numerical methods based on the S. K. Godunov scheme on stationary and movable computational meshes with explicit resolution of the bow shock and the surfaces separating gases with different properties.     

Gas-liquid detonation synthesis of CNTs@Fe/Fe3C composites and their application as electrode materials for double-layer capacitors

Abstract

The carbon nanotubes doping with Fe and Fe₃C nanocrystal (CNTs@Fe/Fe₃C) are successfully synthesized by the gas-liquid detonation (GLD) decomposition of CH₄, O₂, C₁₀H₁₀Fe and C₁₀H₈. The composition and structural properties of the as-obtained composites were investigated by XRD, TEM, XPS and Raman spectroscopy. The obtained composites were also applied to the electric double-layer capacitor. The results showed that the specific capacitance of CNTs@Fe/Fe₃C can reach 125?F·g^?1 at the current density of 100?mA·g^?1 and after 10000 cycles the capacitance retention is 93.1% at a current density of 2?A·g^?1.     

Room-temperature deposition of carbon nanomaterials by excimer laser ablation

Numerous investigators have reported on pulsed laser deposition of carbon nanotubes, mostly using the Nd:YAG laser for ablation. In all cases the depositions have been conducted at high-temperatures and high pressures. Here we report on the deposition of carbon nanostructures at room temperature using a 248 nm excimer laser nm to ablate mixed graphite-nickel/cobalt targets. We find that the formation of the carbon nanomaterials is dependent on the particular ambient gas employed. In O₂ gas, carbon nanotubes and nano-onions are produced. The nanotubes have notably large channel diameters of 100-200 nm and the nano-onion structures are 100-200 nm in diameter, also much larger than previously observed. High-resolution, in-situ, time-resolved emission spectroscopy has been used to follow the production of molecular carbon species such as C₂ and C₃, as well as metals such as Ni or Co in the different ambients employed. Spectral modeling reveals significant differences in the vibrational-rotational temperatures of C₂ spectra in O₂ versus Ar. Mechanistic details of the formation of carbon nanotubes and nano-onions, and in-situ optical emission spectroscopy are described.     

Thermal and dielectric properties of the LTCC composites based on the eutectic system BaO–Al<Subscript>2</Subscript>O<Subscript>3</Subscript>–SiO<Subscript>2</Subscript>–B<Subscript>2</Subscript>O<Subscript>3</Subscript>

The low-temperature co-fired ceramic (LTCC) composites containing quartz based on the eutectic system BaO–Al₂O₃–SiO₂–B₂O₃ are fabricated at the sintering temperature below 980 °C. Preparation process and sintering mechanism were described and discussed, respectively. The results indicated that the addition of quartz to the eutectic system can availably improve dielectric properties of the LTCC composites. In addition, The LTCC composites with optimum compositions, which were obtained by the regulation of an Al₂O₃ content in the composite, can express excellent dielectric properties (permittivity: 5.94, 5.48; loss: 7 × 10⁻⁴, 5 × 10⁻⁴), considerable CTE values (11.7 ppm. °C⁻¹, 10.6 ppm. °C⁻¹) and good mechanical properties (128 MPa,133 MPa).     

The formation of vortex shocks in a 3D subsonic flow behind a weak shock wave emerging from a channel

The structure of a 3D subsonic flow behind a diffracted shock wave was studied by experimental and numerical methods for the incident shock wave Mach numbers M ₀ close to unity. It is established that vortex shocks appear in the flow behind the diffracted shock wave even when M ₀ decreases to 1.04, which is much lower than the threshold Mach number obtained analytically for a 2D automodel case. The time interval from the outflow start to the local supersonic zone formation, as well as the experimentally measured time of appearance of the first vortex shock, increase with decreasing M ₀.     

A comparison study of the agglomeration mechanism of nano- and micrometer aluminum particles

The agglomeration mechanism of micro- and nanosize aluminum particles with a primary mean particle diameter of 4.5 μm and 75 nm, respectively, was comparatively investigated under an incident shock wave. The morphology, particle size, and agglomeration process of micro- and nanometer alumina particles were comprehensibly compared by X-ray diffraction, transmission electron microscopy, scanning electron microscopy and X-ray photoelectron spectroscopy. Images of X-ray diffraction reveal that a varied of phases of alumina (γ-, δ-, ε-, and α-Al₂O₃) were simultaneously found in the nanosize alumina products, which may give some detail information of the wide variety of reacting temperature of aluminum nanoparticles, while Al₄C₃ was detected in micrometer alumina products, which also gives some dynamic information of aluminum to alumina, i.e., aluminas have actually reacted with the free active carbon atoms to produce their intermediates. The microstructure of aluminas induced by the incident shock waves was detected and analyzed by using transmission electron microscopy combined with X-ray photoelectron spectroscopy spectrum. These results are an additive evidence to support that the initial stage sintering of the alumina nanosize powders is dominated by grain boundary diffusion, while the volume diffusion is the main character for the initial stage sintering of the micrometer alumina powders.     

Preparation of SrAl2O4: Eu, Dy nanometer phosphors by detonation method

SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer phosphors were synthesized by detonation method. The particle morphology and optical properties of detonation soot that was heated at different temperatures (600–1100 °C) had been studied systematically by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Results indicated SrAl₂O₄: Eu²⁺, Dy³⁺ nanometer powders in monoclinic system (a = 8.442, b = 8.822, c = 5.160, β = 93.415) can be synthesized by detonation method, when detonation soot was heated at 600–800 °C. The particle size of SrAl₂O₄: Eu²⁺, Dy³⁺ is 35 ± 15 nm. Compared with the solid-state reaction and sol-gel method, synthesis temperature of the detonation method is lower about 500 and 200 °C respectively. After being excited under UN lights, detonation soot and that heated at 600–1100 °C can emit a green light.     

Mn<Subscript>3</Subscript>O<Subscript>4</Subscript>/worm-like mesoporous carbon synthesized via a microwave method for supercapacitors

A quick and facile microwave method has been employed to prepare Mn₃O₄/worm-like mesoporous carbon (Mn₃O₄–MC) composites. Structural and morphological characterizations of worm-like mesoporous carbon and Mn₃O₄–MC composites have been carried out using X-ray diffraction, transmission electron microscopy, N₂ adsorption–desorption, and electrochemical measurement. Cyclic voltammograms demonstrate that the Mn₃O₄–MC composites perform improved capacitive behavior at the range of −0.8~0.2 V (vs. Hg/HgO electrode) with reversibility. The Mn₃O₄–MC composite electrode possesses an enhanced specific capacitance of 266 F g⁻¹ at a sweep rate of 1 mV s⁻¹.     

Microstructural characteristics of alumina-based composite prepared by in situ reaction of alumina-silicon carbide-nickel system

 In situ reaction of nickel and silicon carbide has been attempted to prepare alumina-based composites containing some kinds of dispersed phases. The composites were fabricated by reducing and sintering of Al₂O₃/NiO/SiC mixtures. Reaction products (Ni₃Si and C) and metallic Ni were found to disperse at the matrix grain boundaries, while Ni was partly trapped into Al₂O₃ grains. In addition, carbon nanoballs encapsulating Ni₃Si were produced and dispersed in the composites. The carbon cages were approximately 80–100 nm in diameter with polyhedral shape, and had lattice spacing of 0.35 nm that was typical for the graphite. Encapsulated Ni₃Si had facet planes which were parallel to the carbon layers surrounding. Production of metal encapsulated carbon nanoball within ceramic materials is the first successive result that might promote researches on such novel ceramic composites. Received: 2 January 1997 / Accepted: 15 March 1997