Combustion of mixtures of ultrafine powders of aluminum and boron in air

Results of investigation of the combustion of mixtures of ultrafine aluminum and boron powders (the oxidizer is air) are presented. It is shown that the combustion proceeds in two steps, which differ in temperature. The addition of boron influences the concentrations of AlN, residual Al, and α-Al₂O₃ in the end products of combustion of mixtures of ultrafine powders of Al and B in air. For a fixed sample weight of 4 g, the maximum AlN content is observed in the combustion of an Al+20% B mixture of ultrafine powders, and the combustion temperature is also maximum in this case. When the sample weight is smaller than a certain critical value, the combustion proceeds in one step. Increasing the sample weight of the starting mixture of ultrafine powders of Al and B leads to an increase in the AlN content in the combustion products with simultaneous rise in the combustion temperature. A considerable part of the combustion products stabilizes as acicular polycrystals of micron and submicron sizes formed with participation of a gas phase during combustion. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 61–64, November–December 1999.     

Combustion of Mixtures of Commercial Aluminum Powders and Ultrafine Aluminum Powders and Aluminum Oxide in Air

The paper studies the combustion of mixtures of commercial aluminum powders (ASD-1 and ASD-4) and ultrafine powders of Al and -Al₂O₃ in air. It is shown that the combustion of coarsely dispersed commercial powders is accompanied by binding of air nitrogen with formation of AlN and AlON. The combustion of mixtures proceeds in two stages with the possible formation of intermediate gaseous and liquid products. The processes of sintering and incomplete combustion play an important role in the combustion of mixtures of commercial powders and ultrafine powders of aluminum.     

Vacuum freeze-drying assisted preparation of spherical AlB2 powders with ultrafine microstructure

In this paper, we report the preparation of the spherical AlB₂ powders with uniform particle size, high crystallinity and free of agglomeration. The spherical powders were fabricated by a facile solid state reaction involving the initial preparation of homogenously mixed elemental reactants via the vacuum freeze-drying method. The obtained AlB₂ powders with spherical structure and narrow size distribution showed enhanced sphericity and uniformity when being compared with the ones previously reported. The ultrafine microstructure contributed to agglomerate-free and good flowability, which could efficiently improve its combustion performance. The good antioxidative activity, stable combustion rate and high volumetric heat (53.31 kJ/g) made itself as great potential candidate as high-energy density fuel additives.     

Preparation and characterization of poly(isobutylene oxide)

High molecular weight poly(isobutylene oxide) has been synthesized and characterized. Two novel catalysts were found: (1) a Zn-S composition made by reacting ZnEt₂ and H₂S in presence of cyclohexylamine and (2) a ternary system consisting of ZnEt₂ a metal compound (such as ZnO, ZnS, or Zn titanate), plus cyclohexylamine. Products with reduced specific viscosities of 1.5 to 13 dl/g were easily made. Highest yields were obtained at 110°C, but these products had lower molecular weights than products synthesized at 70°C. The activity of the Nippon-Carbide catalyst was increased two to three times by heating it with a small amount of styrene oxide. High molecular weight poly(isobutylene oxide) appears to have an attractive balance of properties which are in the range of many engineering thermoplastics.     

Morphological and Elemental Classification of Freshly Emitted Soot Particles and Atmospheric Ultrafine Particles using the TEM/EDS

The Transmission Electron Microscopy (TEM) and Energy Dispersive Spectroscopy (EDS) were used to determine morphology and elemental composition of a variety of freshly emitted soot particles (acetylene flame, candle flame, kerosene flame, diesel exhaust, electric arc, plastic burning, styrofoam burning, wood burning [white oak and pine bark], and rice straw burning), which can be possible candidate soot in the ambient atmosphere, and ultrafine particles sampled in urban, industrial, and coastal sites during ultrafine particle formation events (combustion and photochemical events). By using mobility-classified non-refractory ((NH ₄ ) ₂ SO ₄ ) and refractory (Polystyrene latex (PSL) and salt (NaCl)) particles, limitation of the TEM was tested. Data showed that the TEM method can be used to examine shapes of both volatile particles such as (NH ₄ ) ₂ SO ₄ (100 nm) at low, but not high magnification (refer to low and high beam intensity, respectively), and non-volatile particles like NaCl (100 nm) and PSL (84 nm) at either low or high magnification. Distinct differences in morphological properties such as primary particle diameter, fractal dimension, and microstructure were observed among the different types of fresh soot particles. The atmospheric ultrafine particles were classified as agglomerates, sulfate mixtures (spherical), metallic oxides (spherical and polygonal), C-rich refractory (not agglomerated), C-rich non-refractory (not agglomerated), Si-rich (spherical), Na-rich (porous), or P-containing (non-spherical) particles. At the urban Gwangju site, a higher fraction of fresh and aged agglomerates was observed than at other sites. The C-rich non-refractory and sulfate mixtures were often observed in the photochemical event. The C-rich refractory particles were abundant at the Gwangju and Yeosu sites. The coastal Taean site had few agglomerates due to limited anthropogenic combustion source.     

Ultrafine SnO2 nanoparticles as a high performance anode material for lithium ion battery

In this paper, the ultrafine tin oxides (SnO₂) nanoparticles are fabricated by a facile microwave hydrothermal method with the mean size of only 14 nm. Phase compositions and microstructures of the as-prepared nanoparticles have been investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that the ultrafine SnO₂ nanoparticles are obtained to be the pure rutile-structural phase with the good dispersibility. Galvanostatic cycling and cyclic voltammetry results indicate that the first discharge capacity of the ultrafine SnO₂ electrode is 1196.63  mAh g⁻¹, and the reversible capacity could retain 272.63 mAh g⁻¹ at 100 mA g⁻¹ after 50 cycles for lithium ion batteries (LIBs). The excellent electrochemical performance of the SnO₂ anode for LIBs is attributed to its ultrafine nanostructure for providing active sites during lithium insertion/extraction processes. Pulverization and agglomeration of the active materials are effectively reduced by the microwave hydrothermal method.     

Catalysis and inhibition of the combustion of ammonium perchlorate based solid propellants

Results of studies of the combustion of a composite solid propellant based on ammonium perchlorate are presented. The effect of additives of metal oxides is studied for the high and low-temperature decompositions, linear pyrolysis, and combustion of ammonium perchlorate, the decomposition of HClO₄, isobutylene oxidation by oxygen and perchloric acid, and the combustion of propellants with various organic combustibles. It is shown that the efficiency of metal oxides in the reactions of oxidation of isobutylene and propellant combustion is related to the energy of the Me—O bond in the surface oxide layer or the enthalpy of formation of this bond. The extremal nature of the catalytic effect of metal oxides on the burning rate of the propellant is due to the small time of residence of the oxide particles in the zone of intense oxidation-reduction reactions. For this reason, the same additives of metal oxides has different effects on the combustion of the propellant with different organic, combustibles, and the most efficient catalyst can be chosen by a simplified algorithm. The potentials for affecting the composite solid propellant via gas-phase oxidation-reduction reactions is indicated by the effect of additives of organic sources of active species—amines and halides. Translated fromFizika Goreniya i Vzryva, Vol. 35, No. 6, pp. 76–90, November–December 1999.     

Simultaneous pulsed current activated combustion synthesis and densification of NbSi2–SiC composite

Dense ultrafine NbSi₂–SiC composite was synthesized by the pulsed current activated combustion synthesis (PCACS) method within 3 min in one step from mechanically activated powders of NbC and 3Si. Simultaneous combustion synthesis and densification were accomplished under the combined effects of a pulsed current and mechanical pressure. Highly dense NbSi₂–SiC composite with relative density of up to 97% was produced under simultaneous application of a 60 MPa pressure and the electric current. The average grain size and mechanical properties (hardness and fracture toughness) of the composite were investigated.     

Wet Atomisation of Gd‐doped CeO2 Electrolyte Slurries for Intermediate Temperatures' Microtubular SOFC Applications

A wet atomising system has been employed as a novel method to prepare ultrafine Gd‐doped CeO₂ (GDC) electrolyte slurries. By changing the fluid flow pressure and repeating the atomisation process several times for the same atomised slurries, we have obtained optimised ultrafine GDC slurry with high‐dispersed and homogeneous distribution. The sizes of the particles of GDC were in the range of tens of nanometres. A highly dense electrolyte layer (membrane) was prepared using the ultrafine GDC slurries for intermediate temperatures microtubular solid oxide fuel cell (SOFC) applications. The SOFC was fabricated by using supporting porous anode tubes of NiO and GDC, and the cathode consisted of La_0.6Sr_0.4Co_0.2Fe_0.8O_3–y and GDC. A dense 10 μm GDC electrolyte layer was obtained at a lower sintering temperature of 1,250 °C for 1 h. The SOFC was tested with humidified (3% H₂O) hydrogen as a fuel and the static air as an oxidant, and the tubular cell maintained its high performance even at 500 °C.     

Production of silicon containing particles by laser induced reaction of silane with methane,ethane and acetylene

Silicon containing ultrafine particles (Si and SiC) have been obtained by pulsed IR laser irradiation of gaseous SiH₄/hydrocarbon (CH₄, C₂H₆ and C₂H₂) mixtures. The chemical composition and structure of the powders formed were determined by scanning and transmission electron microscopy (SEM and TEM), X-ray diffraction (XRD) and I.R. and UV-v spectroscopy. The particles are small, uniform, loosely agglomerated spheres with a mean size of approximately 19 nm. To complete the characterisation of these ultrafine silicon-containing particles, the X-ray absorption spectra (XAS) were obtained using synchrotron radiation at the silicon absorption K-edge. We show that the use of pulsed laser radiation results in the generation of amorphous Si and crystalline SiC particles depending on the gas mixtue used, among other parameters such as composition of the mixture, laser fluence, pressure and number of laser pulses.     

Laser approaches for deposition of carbon nitride films — chemical vapour deposition and ablation

The deposition of carbon nitride films by laser-induced chemical vapour deposition (LCVD) from the system NH₃/CCl₄ and by laser ablation of a graphite target in nitrogen atmosphere has been studied. Two types of lasers were used: a copper bromide vapour laser for LCVD and an eximer KrF laser for the ablation process. The first laser source has a set of parameters (wavelengths, repetition rate, pulse duration) which makes it unique in material processing. During the ablation deposition an additional DC discharge (ignited by each shot) or RF discharge (permanent) was utilised to increase the reactivity of the nitrogen.The deposition rate for LCVD is almost one order of magnitude higher than for the laser ablation (0.050–0.350 and 0.025–0.040 nm/pulse, respectively) due to the direct laser impact on the substrate surface.The composition of the layers was studied by Auger electron spectroscopy (AES) and wavelength dispersive X-ray analysis (WDX). For the laser ablation the N/C ratio increases when an additional DC or RF discharge was used compared to the non-assisted process. Maximum values are reached at nitrogen pressures twice lower for RF discharge than for DC discharge assisted deposition. In the case of LCVD the composition depends mainly on the laser power.Chemical bonding was investigated by Fourier transform infrared (FTIR) spectroscopy. The FTIR spectra of the films deposited by LCVD show well defined bands which are superimposed to a broad peak for samples prepared by laser ablation. At higher RF plasma power a weak band around 2200 cm⁻¹ appears in the spectra which can be attributed to the stretching mode of triple bonded C–N. Such a peak is not observed in cases of DC discharge and LCVD.     

Preparation and characterization of carbon-coated LiFePO4 cathode materials for lithium-ion batteries with resorcinol-formaldehyde polymer as carbon precursor

LiFePO₄/C composites were synthesized by two methods using home-made amorphous nano-FePO₄ as the iron precursor and soluble starch, sucrose, citric acid, and resorcinol-formaldehyde (RF) polymer as four carbon precursors, respectively. The crystalline structures, morphologies, compositions, electrochemical performances of the prepared powders were investigated with XRD, TEM, Raman, and cyclic voltammogram method. The results showed that employing soluble starch and sucrose as the carbon precursors resulted in a deficient carbon coating on the surface of LiFePO₄ particle, but employing citric acid and RF polymer as the carbon precursors realized a uniform carbon coating on the surface of LiFePO₄ particle, and the corresponding thicknesses of the uniform carbon films are 2.5 nm and 4.5 nm, respectively. When RF polymer was used as the carbon precursor, the material showed the highest initial discharge capacity (138.4 mAh g^− 1 at 0.2 C at room temperature) and the best rate performance among the four materials.     

Design and fabrication of flexible mesoporous Si-doped Al2O3 ultrafine fibers by electro-blow spinning (EBS) technique

In this paper, flexible mesoporous Si-doped Al₂O₃ ultrafine fibers were successfully prepared by electro-blow spinning (EBS) technology followed by a calcination process. The effects of spinning parameters, including air pressure, voltage, and solution injection rate on the morphology and diameter of fibers were investigated. The results showed that the spinning parameters played a key role on determining the morphology and diameter of fibers, and the specific surface area of fibers significantly increased with the increasing content of pore forming agents (polytetrafluoroethylene, PTFE), at the same time the pore size distribution was mainly concentrated at ~5 nm. Most importantly, Si(OC₂H₅)₄ was introduced to act as the flexible regulator which could inhibit the growth of alumina crystal, decrease the crystal size, and thus the flexibility of obtained mesoporous Si-doped Al₂O₃ ultrafine fibers can be improved. Moreover, the fibers productivity by EBS method was higher than that of traditional electro-spinning. In the future, the mesoporous Si-doped Al₂O₃ ultrafine fibers may perform desired function and spread their applications in different areas.     

Desorption of isoamylene from loaded acid liquors

The desorption of 2-methyl-2-butene (isoamylene) from loaded sulphuric acid solutions, having an acid strength of about 60 per cent (w/w) and loading upto 0·45 g mole of isoamylene per g mole of H₂SO₄, was found to be preceded by a fast reaction, which occurred in the film adjacent to the interface. The specific rates of desorption of isoamylene into inert hydrocarbons—n-heptane and toluene and an inert gas—nitrogen, were found to be proportional to the isoamylene concentration j, expressed as g mole of isoamylene per g mole of H₂SO₄, and agreed among themselves at the same value of j.The technique of desorption preceded by a fast reaction was employed for the measurement of effective interfacial area in liquid—liquid and gas—liquid agitated contactors. The absorption of isobutylene into fresh and loaded solutions of sulphuric acid was also used for the measurement of effective interfacial area in gas—liquid agitated contactors for comparative purposes. The values of effective interfacial area for the gas—liquid system obtained by the desorption technique were found to be comparable with those obtained from the absorption of isobutylene in fresh and loaded solutions of sulphuric acid, under otherwise comparable conditions.     

New telechelic polymers and sequential copolymers by polyfunctional initiator-transfer agents (inifers)

Summary The possibility of undesirable intramolecular cycloalkylation exists in the polymerization of iso-butylene induced by the p-dicumyl chloride/BCl₃ inifer system. A scheme has been proposed which shows the sequence of reactions leading to indane skeletons. The structure of the indane derivatives has been characterized by ¹H NMR spectroscopy. Systematic experiments have been carried out using low isobutylene and high p-dicumyl chloride concentrations leading to polyisobutylene oligomers needed for accurate endgroup analysis. The effect of temperature, solvent composition (polar/nonpolar) , isobutylene and BCl₃ concentration on the extent of indane skeleton formation has been investigated. Indane skeleton formation can be completely suppressed by the use of relatively non-polar media, e.g., 11 mixture of CH₂Cl₂ :n-C₆H₁₄, at or below –40°C, and conditions under which symmetrical telechelic polyisobutylenes can be obtained have been defined.     

Investigation of 1,3,5-tris(2-metoxypropane)benzene/BCl3 initiated living isobutylene polymerization by C13 and B11 NMR spectroscopy

Summary The living carbocationic polymerization of isobutylene initiated by 1,3,5-tris(2-methoxypropane) benzene (TriCumOMe)/BCl₃ system was investigated by C¹³ and B¹¹ NMR spectroscopy. The reaction between the TriCumOMe and BCl₃ at-30°C in CH₂Cl₂ after 15 mins reaction time resulted in 1,3,5-tris(2-chloropropane) benzene (TriCumCl) and methyl-dichloroboronite (BCl₂OMe). The same system in the presence of isobutylene yielded three-arm star, chlorine terminated telechelic polyisobutylene and BCl₂OMe. No counterions, i.e., BCl₃OMe, BCl ₄ , or neutral boron complexes, e.g., TriCumOMe, 3BCl₃ could be detected. The simultaneous measurement of static permittivity (direct monitoring method) showed different reaction rate patterns in the case of AMI method, and when the TriCumOMe+BCl₃ mixture was aged and the polymerization was started by isobutylene.     

Effects of cobalt-ion doping on the electrochemical properties of spinel lithium manganese oxide prepared via a reverse-micelle route

Monophasic, ultrafine cobalt ion-doped lithium manganate powders have been prepared via a reverse-micelle process. LiMn_1.8Co_0.2O₄ powders were obtained after heating at 500 °C for 2 h. Both the required reaction temperature and duration for synthesizing this compound were greatly reduced, compared to those in the conventional technique involving solid-state reaction. Increasing the water to oil (W/O) volume ratio resulted in an increase in the particle size of LiMn_1.8Co_0.2O₄ as well as the formation of impure Li₂MnO₃ phase. Nanosized, monophasic powders were obtained when the W/O volume ratio was 1/5. Raising the heating temperature led to an increase in the crystallite size as well as the particle size of LiMn_1.8Co_0.2O₄. Based on the analytic results of electrochemical properties, the derived cobalt ion-doped spinel phase was confirmed to exhibit moderate discharge capacity and improved cycleability at ambient as well as elevated temperatures.     

Living cationic random copolymerization of β-pinene and isobutylene with 1-phenylethyl chloride/TiCl4/Ti(OiPr)4/nBu4NCl

The first example of living cationic random copolymerization of β-pinene and isobutylene was achieved with 1-phenylethyl chloride/TiCl₄/Ti(OiPr)₄/nBu₄NCl (TiCl₄/Ti(OiPr)₄ mole ratio: 3/1) initiating system in CH₂Cl₂ at −40 °C. β-Pinene and isobutylene was consumed at almost the same rate, suggesting that the two monomers exhibit almost equal reactivity. At any monomer feed ratio, the number-average molecular weight (M_n) of the copolymers increased in direct proportion to the total monomer conversion, and the molecular weight distribution was relatively narrow (M_w/M_n=1.1-1.2) throughout the reaction. The reactivity ratios determined by the Kelen-Tüdõs method were rβ-pinene=1.1 and r_isobutylene=0.89, which indicated that the composition of copolymer is approximately identical to the monomer feed ratio. The analysis of the structure and sequence distribution of the copolymers by ¹H NMR spectroscopy further confirmed that perfectly random copolymers were obtained by this living cationic polymerization system. The glass transition temperatures of the copolymers obtained with varying monomer compositions were also determined by DSC method.     

Rate‐Dependent Mechanical Behavior and Amorphization of Ultrafine‐Grained Boron Carbide

An investigation into mechanical properties and amorphization behavior of ultrafine‐grained (0.3 μm) boron carbide (B₄C) is conducted and compared to a baseline coarse‐grained (10 μm) boron carbide. Static and dynamic uniaxial compressive strength, and static and dynamic Vickers indentation hardness were determined, and Raman spectroscopy was then conducted on indented regions to quantify and compare the intensity of amorphization. In relation to coarse‐grained B₄C the ultrafine‐grained material exhibited, on average, a 33% higher static compressive strength, 20% higher dynamic compressive strength, 10% higher static Vickers hardness, and 23% higher dynamic Vickers hardness. In addition, there was an 18% reduction in indentation‐induced radial crack length in ultrafine‐grained B₄C, which corresponded to an increase in estimated fracture toughness. Although traditional coarse‐grained B₄C exhibits an 8.6% decrease in hardness from the static to dynamic regimes, ultrafine‐grained B₄C showed only negligible change under similar conditions, suggesting a reduced propensity for amorphization. Raman spectroscopic analysis confirmed this result by revealing significantly lower amorphization intensity in ultrafine‐B₄C compared to coarse‐grained B₄C. These results may have significant positive implications in the implementation of ultrafine‐grained boron carbide as a material for improved performance in impact and other high‐pressure applications.     

Reaction path in formation of Ti1−xWxC solid solution by combustion synthesis

Ti_1?xW_xC solid solution powders have been prepared by combustion synthesis in two reaction modes of self-propagating high-temperature synthesis (SHS) and thermal explosion (TE). The reaction mechanism in combustion synthesis is investigated and the effects of processing parameters are discussed. Two reaction paths are proposed for the formation of the Ti_1?xW_xC solid solution. In path 1, Ti_1?xW_xC crystals are directly precipitated from Ti-W-C melt, and in path 2 the Ti_1?xW_xC phase is formed by solid-state reaction between TiC and WC. The availability of the two reaction paths depends on the x value and reaction temperature. For smaller x values and higher temperatures, the reaction path 1 dominates and almost full conversion is achieved. For larger x values and lower temperatures, however, path 2 becomes dominant. In path 2, the rate-limiting step is the formation of WC by slow diffusion-controlled solid-state reaction between W and C, which is unable to be completed during the short reaction period and results in the presence of W₂C and unreacted W in the products. The reaction between W and C cannot be improved by the addition of excessive C, but can be promoted by using carbon black instead of graphite powder as C source or carrying out the synthesis in a high-pressure N₂ atmosphere instead of vacuum. In the fast combustion reaction with high heating/cooling rates, a non-equilibrium state is likely to be created because the mass transfer and energy exchange are limited. The non-equilibrium condition has a strong influence on the reaction kinetics, where the decomposition of W₂C is avoided, a localized compositional heterogeneity is caused, and ultrafine grains on a submicron scale are obtained in the products.