Properties of flame synthesized germanium oxide nanoparticles

Germanium oxide (GeOx) nanoparticles in the size range from 1.5 to 10 nm were synthesized in a low-pressure premixed H2/O2/Ar flame in the pressure range 25-55 mbar. The flame was doped with different amounts of tetramethylgermanium (Ge(CH3)4) ranging from 500 to 2000 ppm. The influence of process parameters such as pressure, flame coordinate, and cold gas flow velocity with respect to growth of germanium oxide particles were investigated. The formed particles were analyzed in-situ according to their mass and charge by means of a particle mass spectrometer (PMS). The specific surface area was determined ex-situ by the BET method. The crystal structure and chemical composition of the produced nanopowder was characterized by EDX and XRD measurements. Additionally, the particles were analyzed by means of FT-IR spectroscopy.     

Formation of nanoparticles in flames; measurement by particle mass spectrometry and numerical simulation

The size distributions of nanoparticles in flames are measured using a novel particle mass spectrometer (PMS), which is developed for the size range between 0.3 and 50?nm and for number concentrations between 10(9) and 10(13). Using this instrument the particles are sampled without prior dilution from the flame into a molecular beam. The charged nanoparticles are then deflected by an electric field, to determine the mass according to the time-of-flight principle. The PMS is installed in a low pressure combustion chamber operated at 30?mbar. Measurements are made on primary soot particles and iron oxide particles in a laminar, premixed acetylene/oxygen flame. The soot particles increase in size as a function of the height above the burner and the C/O ratio from 2 up to 10?nm. Iron oxide particles of 3-5?nm are detected as a function of burner height. The soot particles form more rapidly than the iron oxide particles. A model calculation for the formation of silica and iron oxide in hydrogen/oxygen flames is developed, based on previously published reaction mechanisms. On adding a mono-disperse particle coagulation scheme, the time history of the particle number concentration and the particle size is calculated. In agreement with experimental data, the calculations show that iron oxide particles are formed more slowly than silica particles.     

Flamelet modeling of laminar pulverized coal combustion with different particle sizes

In this work, flamelet modeling of pulverized coal flame stabilized in a laminar counterflow is conducted. Particularly, the effects of particle size on the flamelet model performance are investigated. The a priori tabulated thermo-chemical quantities are compared with the corresponding values in the detailed chemistry solutions, which are obtained by solving the transport equations for all species mass fractions existing in the employed detailed chemical reaction mechanism. The detailed chemistry solutions show that the pulverized coal flame structure can be changed significantly by varying the particle size. While pulverized coal flame structure with small particles is similar to the pure gas flame, it becomes much more complex when the particle Stokes number is larger than unity. Large coal particles can directly cross the flame front to the opposite side and disperse over a large region, resulting a significantly wrinkled flame front and a considerably broadened reaction zone. Comparisons between the tabulated thermo-chemical quantities and the detailed chemistry solutions show that the CH₄, O₂ and H₂O species mass fractions and their RMS (root mean square) values for all cases can be well predicted by the classical flamelet model without any modification, while the peak values of OH mass fraction, gas temperature and heat release rate for the large particle case cannot be correctly captured. For all cases, the mass fractions of combustion-mode-sensitive species cannot be correctly predicted in the premixed flame reaction zone by the employed diffusion-flame-based flamelet model. Such discrepancies are even more pronounced at large particle conditions since the premixed flame reaction zone tends to be broadened.     

Synthesis, characterization and magnetic properties of Zn substituted Li-Ni nano ferrites

Nanosized Li-Ni-Zn ferrites with general chemical formula Li0.5 Ni(0.75-x/2) Zn(x/2) Fe2O4 (0 < x < 1) were synthesized from a simple polymer matrix based precursor solution. The solution was composed of metal nitrates, polymer (PVA) and disaccharide (sucrose). Thermolysis/flame pyrolysis of the precursor mass in an external temperature resulted in the oxide phase formation. X-ray diffraction studies confirm the formation of single phase ferrites. The microstructural analysis was carried out by scanning electron microscopy (SEM). The average grain diameter was estimated by the Scherrer method and grain diameter is found to vary from 16 nm to 33 nm. The saturation magnetization was studied as a function of composition.     

Laser-induced incandescence: excitation intensity

Assumptions of theoretical laser-induced incandescence (LII) models along with possible effects of high-intensity laser light on soot aggregates and the constituent primary particles are discussed in relation to selection of excitation laser fluence. Ex situ visualization of laser-heated soot by use of transmission electron microscopy reveals significant morphological changes (graphitization) induced by pulsed laser heating. Pulsed laser transmission measurements within a premixed laminar sooting flame suggest that soot vaporization occurs for laser fluences greater than 0.5 J/cm(2) at 1064 nm. Radial LII intensity profiles at different axial heights in a laminar ethylene gas jet diffusion flame reveal a wide range of signal levels depending on the laser fluence that is varied over an eight fold range. Results of double-pulse excitation experiments in which a second laser pulse heats in situ the same soot that was heated by a prior laser pulse are detailed. These two-pulse measurements suggest varying degrees of soot structural change for fluences below and above a vaporization threshold of 0.5 J/cm(2) at 1064 nm. Normalization of the radial-resolved LII signals based on integrated intensities, however, yields self-similar profiles. The self-similarity suggests robustness of LII for accurate relative measurement of soot volume fraction despite the morphological changes induced in the soot, variations in soot aggregate and primary particle size, and local gas temperature. Comparison of LII intensity profiles with soot volume fractions (f(v)) derived by light extinction validates LII for quantitative determination of f(v) upon calibration for laser fluences ranging from 0.09 to 0.73 J/cm(2).     

One-Dimensional, Time-Resolved Raman Measurements in a Sooting Flame made with 355-nm Excitation

Single-shot vibrational Raman measurements were performed along an 11-mm-long line crossing the reaction zone in a premixed, fuel-rich (phi = 10), laminar methane-air flame by use of a frequency-tripled Nd:YAG laser with a 355-nm emission wavelength. This laser source seems to have advantages relative to KrF excimer lasers as well as to Nd:YAG lasers at 532 nm for hydrocarbon combustion diagnostics. The Raman emissions of all major species (N(2), O(2), CH(4), H(2), CO(2), H(2)O) were detected simultaneously with a spatial resolution of 0.4 mm. By integration over selected spectral intervals, the mole fractions of all species and subsequently the local gas temperatures have been obtained. A comparison of the temperatures that were found with results from filtered Rayleigh experiments showed good agreement, indicating the success of what are to the best of our knowledge the first one-dimensional single-shot Raman measurements in a sooting hydrocarbon flame.     

Detector for liquid chromatography based on acoustic emissions from an oscillating flame

The acoustic flame detector (AFD) is examined as a novel detector for liquid chromatography (LC). It is based upon the acoustic emission frequency of an oscillating hydrogen/oxygen premixed flame and produces a universal response toward organic molecules. A stable frequency near 1000 Hz, which further depends on mobile-phase composition, is achieved for flow rates in the microliter per minute range. The mass flow sensitivity of the AFD demonstrates a linear response over 3 orders of magnitude and a detection limit (S/sigma = 3) of approximately 15 ng of C/s for a series of alcohols. For cyclopentanol, this amounts to an injected mass of approximately 77 ng based on a 0.5-microL injection of a 196 ppm solution in methanol (flow rate 20 microL/min methanol; peak width 30 s). Similar sensitivity is observed using a water mobile phase. Low-frequency (1/f ) noise contributions are dominant with or without mobile phase present. The AFD demonstrates a uniform molar sensitivity toward carbon compounds independent of their optical properties or volatility. Results suggest the device might serve as a simple, inexpensive universal LC detector.     

变截面管道结构对H2/Air预混气体燃爆特性的影响研究

实际工业中,管道截面突变可改变预混燃气火焰的燃烧模式和燃爆特性,影响燃气输送的安全性。为此,利用Fluent软件,数值模拟研究了4种变截面管道结构(突变扩张型管道、突变收缩型管道、渐变扩张型管道和连续突变扩张型管道)对H_2/Air预混气体燃爆过程火焰动态变化的影响,并分析了管内温度场、压力场的分布情况、气体流速和涡量等燃爆特征参数以及焓、熵值等热力学参数的变化规律。研究结果表明:扩张型管截面的突扩诱导作用和收缩型管截面的收缩阻挡作用改变了管内预混气体燃烧火焰传播的动态结构以及火焰前锋的稀松波和预混气流方向,使火焰出现了"杯口"型和"长颈花瓶"型前锋面;突变收缩型管截面的阻挡、反射和抑制作用加强了湍流强度和涡流运动强度,管内最高温度、峰值压力和平均流速等燃爆特征参数也相对较高,而涡团向上、下管壁的运动抑制了火焰尖端的反应强度和传播速度;渐变扩张型管道能降低突扩诱导作用的影响;而连续突变扩张型管内发生在层流向湍流火焰转变阶段的首次截面突扩产生的诱导作用对其燃爆特性的影响更为显著。     

N2 O-C2 H4预混气体火焰的传播特性

运用标准k-ε模型,对N₂O-C₂H₄预混气体在水平半封闭管道内火焰传播过程进行了数值模拟,得到了火焰锋面结构、传播速度、出口压力和燃烧区的气流速度随时间的变化规律。研究结果表明,管道内预混火焰传播过程分为3个阶段:点火初期的平面火焰传播阶段、Tulip火焰传播阶段和指形火焰传播阶段;火焰传播速度呈指数增长,管道出口处压力和气流速度均呈现出先增大后减小的趋势。同时,采用高速摄影系统、压力传感器、有机玻璃管等装置对预混气体的火焰加速进程和压力演变过程进行了验证,实验结果与数值模拟结果一致。     

Sodium and potassium released from burning particles of brown coal and pine wood in a laminar premixed methane flame using quantitative laser-induced breakdown spectroscopy

A quantitative point measurement of total sodium ([Na](total)) and potassium ([K](total)) in the plume of a burning particle of Australian Loy Yang brown coal (23 ± 3 mg) and of pine wood pellets (63 ± 3 mg) was performed using laser-induced breakdown spectroscopy (LIBS) in a laminar premixed methane flame at equivalence ratios ( U ) of 1.149 and 1.336. Calibration was performed using atomic sodium or potassium generated by evaporation of droplets of sodium sulfite (Na(2)SO(3)) or potassium sulfate (K(2)SO(4)) solutions seeded into the flame. The calibration compensated for the absorption by atomic alkalis in the seeded flame, which is significant at high concentrations of solution. This allowed quantitative measurements of sodium (Na) and potassium (K) released into the flame during the three phases of combustion, namely devolatilization, char, and ash cooking. The [Na](total) in the plume released from the combustion of pine wood pellets during the devolatilization was found to reach up to 13 ppm. The maximum concentration of total sodium ([Na](max)M(total)) and potassium ([K](max)(total)) released during the char phase of burning coal particles for φ = 1.149 was found to be 9.27 and 5.90 ppm, respectively. The [Na](max)(total) and [K](max)(total) released during the char phase of burning wood particles for φ = 1.149 was found to be 15.1 and 45.3 ppm, respectively. For the case of φ = 1.336, the [Na](max)(total) and [K](max)(total) were found to be 13.9 and 6.67 ppm during the char phase from burning coal particles, respectively, and 21.1 and 39.7 ppm, respectively, from burning wood particles. The concentration of alkali species was higher during the ash phase. The limit of detection (LOD) of sodium and potassium with LIBS in the present arrangement was estimated to be 29 and 72 ppb, respectively.     

Work function measurements for gas detection using tin oxide layers with a thickness between 1 and 200 nm

Tin oxide films, 1, 2, 20, 50, 80 and 200 nm thick, were deposited on platinum and titanium/tungsten surfaces by reactive sputter deposition. The structure and composition of tin oxide as well as the film homogeneity were investigated by scanning electron microscopy, Auger electron spectroscopy, Rutherford backscattering spectrometry and atomic force microscopy.

The 200 nm SnO₂ films are polycrystalline, with a grain size between 40 and 80 nm. For layers between 20 and 200 nm the composition is characteristic for stoichiometric SnO₂. The 1 and 2 nm layers show a homogeneous coverage of the metal. For all sensitive systems gas measurements were carried out with a Kelvin probe. Testing gases were humidity (40% r.h.), carbon monoxide (1000 ppm), hydrogen (100 ppm), nitrogen dioxide (8 ppm) and ammonia (10 ppm).

For thinner SnO₂ adlayers only surface-related processes contribute to the response, which is still material specific. It turns out that the thin layers are more stable.     

Modeling of nitrogen oxide formation in a turbulent diffuse flame

A mathematical model is represented for a turbulent diffuse methane flame in air with nitrogen oxide formation in the combustion products according to the Ya. B. Zel'dovich reaction mechanism taken into account. The model is tested on experimental data for diffusion and premixed flames.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 56, No. 6, pp. 885–894, June, 1989     

Surface-enhanced infrared spectroscopic studies of the catalytic behavior of silver nanoparticles on a germanium substrate

The catalytic activity of silver nanoparticles (AgNPs) on a germanium substrate is reported. Para-nitrothiophenol (pNTP) that had been adsorbed on this substrate is converted to p-aminothiophenol (pATP) under very mild reaction conditions, such as simply soaking in water. The AgNPs may be formed either by physical vapor deposition or by electroless deposition from a solution of silver nitrate. Analogous reactions were not observed on copper nanoparticles on germanium or AgNPs on silicon or zinc selenide even though very slow conversion of pNTP to pATP was observed with Au nanoparticles (AuNPs) on Ge under controlled reaction conditions. The effects of factors that could influence the catalytic reaction were examined; these included the particle size of the AgNPs, reaction temperature, concentration and chemical nature of other ions present in the solution, the pH of the water, and the nature of the substrate. The reaction rate was approximately independent of the particle size for AgNPs between 50 and 150 nm in diameter. Increasing the temperature accelerates the reaction significantly; at temperatures above 40 °C, the adsorbed pNTP is completely converted by water within five minutes. Not surprisingly, the reaction rate was increased as the pH of the solution was decreased, as the reduction of each nitro group to an amino group requires six protons. The presence of Br(-) and I(-) ions accelerated the reaction to the point that even at 4 °C, the conversion of the nitro group was still observable, while solutions containing chloride ions had to be heated to 40 °C before their effect became apparent. Apparently, Br(-) and I(-) ions remove the oxide layer from the surface of the germanium substrate, facilitating transfer of electrons from the germanium to the nitro group of the pNTP.     

Numerical investigation for prediction of pollutants formation type CO and NO in premixed turbulent flame using an extended coherent flame model

This study describes a numerical effort to predict pollutant species such as CO and NO formed in premixed turbulent flame using an extended coherent flame model. This model is used to predict pollutant emissions of methane-air combustion. A semi-global kinetic scheme is tested, using a two steps reaction mechanism of Westbrook and al. The model combines coherent flame model to estimate the CO formation and the well-stirred reactor concept for calculating the NO and all other species which are taken into account in this investigation. Essentially, we assume that CO is analysed near the flame zone because is formed with very fast intermediate reaction compared to the other characteristic times. Other species formed with longer time are supposed to take place in high temperature region where can be consider that corresponding kinetics are best represented by homogeneous reactor conditions. Numerical results for emission species and dynamic parameters in premixed flame of methane-air are given for a duct combustor where the flame is stabilised behind a rod flame holder with a blockage ratio is taken as 6% and a mass flow rate $\dot{m}$ _a=35 g/s.     

Use of laser-induced ionization to detect soot inception in premixed flames

Experimental measurements of laser-induced ionization were performed for ethene-air premixed flames operated near the soot inception point. Soot was ionized with a pulsed laser operated at 532 nm. The ionization signal was collected with a tungsten electrode located in the postflame region. Ionization signals were collected by use of both single-electrode and dual-electrode configurations. Earlier laser-induced-ionization studies focused on the use of a single biased electrode to generate the electric field, with the burner head serving as the path to ground. In many practical combustion systems, a path to ground is not readily available. To apply the laser-induced-ionization diagnostic to these geometries, a dual-electrode geometry must be employed. The influence of electrode configuration, flame equivalence ratio, and flame height on ionization signal detection was determined. The efficacy of the laser-induced-ionization diagnostic in detecting soot inception in the postflame region of a premixed flame by use of a dual-electrode configuration was investigated. Of the dual-electrode configurations tested, the dual-electrode geometry oriented parallel to the laser beam was observed to be most sensitive for detecting the soot inception point in a premixed flame.     

Laser-induced-fluorescence detection of nitric oxide in high-pressure flames with A-X(0, 2) excitation

Laser-induced fluorescence techniques have been used successfully for quantitative two-dimensional measurements of nitric oxide. The commonly applied D-X(0, 1) or A-X(0, 0) schemes are restricted to atmospheric-pressure flames and engines driven with gaseous fuels because of strong attenuation of the exciting laser beam by combustion intermediates. The properties of a detection scheme for which excitation in the nitric oxide A-X(0, 2) band was used were investigated. We discuss the advantages of the A-X(0, 2) system (excited at 247.95 nm) based on measurements in laminar premixed methane/air flames at 1-40 bars.     

磷酸铵锌/埃洛石纳米管/环氧树脂纳米复合材料的阻燃性能

采用吸附-化学沉淀法合成了磷酸铵锌/埃洛石纳米管(ZAP/HNT)复合材料,并将ZAP/HNT和Exolit○R OP 1230膨胀型阻燃剂一起用于阻燃环氧树脂(EP)。采用红外光谱(FT-IR)和扫描电子显微镜(SEM)对合成的ZAP/HNT及其环氧树脂复合物(EP/ZAP/HNT)进行了表征,ZAP/HNT中ZAP的粒径约为20~50 nm,表面的-OH与环氧基发生了作用,在EP中分散均匀。热分析(TG)表明,ZAP/HNT在加热过程中脱水、脱氨,600℃时总质量损失约11.34%。790℃时,EP/ZAP/HNT的残炭量为26%,具有良好的成炭性能。用微尺度燃烧量热法(MCC)和UL-94等方法分析了复合材料的阻燃性能,与EP相比,EP/ZAP/HNT(25%)的HRR下降幅度可达到58.82%。ZAP/HNT与OP 1230具有明显的协同阻燃性能。对炭渣用能谱仪(EDS)和SEM进行了分析,P、Al、Si、Zn主要分布在残渣外层,说明燃烧中阻燃剂在气体的作用下迁移到表面,形成由焦磷酸盐、碳、Al 2Si 2O 5等金属氧化物组成的稳定炭层。     

Graphene oxide effects on the properties of Al_2O_3-Cu/35W5Cr composite

Graphene oxide(GO) nanosheets were dispersed into premixed powders(Cu-0.4 wt% Al/35W5Cr) by wet grinding and vacuum freeze-drying process. The 0.3 wt% GO/Al_2O_3-Cu/35W5Cr and 0.5 wt%GO/Al_2O_3-Cu/35W5Cr composites, used for electrical contacts, were fabricated by vacuum hot-pressing sintering.The microstructure was analyzed by field emission scanning electron and transmission electron microscopy. In addition, the Raman spectroscopy and X-ray photoelectron spectroscopy were used to investigate the structural changes of GO before and after sintering. The arc erosion behavior was investigated by the JF04 C electrical contact testing apparatus. Consequently, the Al_2O_3 nanoparticles were evenly dispersed in the matrix, causing dislocation tangles. GO was converted to reduced graphene oxide after sintering. A group of carbon atoms combined with Cr forming Cr_3C_2 in situ during sintering, which enhanced the interface bonding. Compared with the Al_2O_3-Cu/35W5Cr composite, the tensile strength of the two contact materials containing 0.3 wt% GO and 0.5 wt% GO was increased by 45% and 34%, respectively. Finally, pips and craters were present on the anode and cathode surfaces, respectively.Tungsten has undergone re-sintering during arcing and formed needle-like structures. Compared with Al_2O_3-Cu/35W5Cr, the GO/Al_2O_3-Cu35W5Cr composites have better welding resistance. The final mass transfer direction of the two composites was from the cathode to anode.     

A novel chemical route for the preparation of nanosized oxides, phosphates, vanadates, molybdates and tungstates using polymer precursors

A variety of nanosized (particle diameter ranging between 10–90 nm) ceramic oxide powders have been prepared from a versatile, efficient and technically simple polymer matrix based precursor solution. The precursor solution constituted of the metal nitrates mixed with the polymer-PVA/PAA/carboxylated starch in presence of mono-/disaccharides or, poly hydroxy compound. Thermolysis/flame pyrolysis of the precursor mass at external temperatures of around 300–500°C resulted in the oxide phase.