Experimental study of minimum ignition energy of lean H2-N2O mixtures

Ignition energies for short duration (<50 ns) spark discharges were measured for undiluted and nitrogen-diluted H₂-N₂O mixtures of equivalence ratios ? = 0.15 and 0.2, dilution of 0% and 20% N₂, and initial pressures of 15–25 kPa. The ignition events were analyzed using statistical tools and the probability of ignition versus spark energy density (spark energy divided by the spark length) was obtained. The simple cylindrical ignition kernel model was compared against the results from the present study. Initial pressure has a significant effect on the width of the probability distribution, ranging from a broad (P = 15 kPa) to a narrow (P = 25 kPa) probability distribution indicating that the statistical variation of median spark energy density increases as initial pressure of the mixture decreases. A change in the equivalence ratio from 0.15 to 0.2 had a small effect on the median spark energy density. The addition of 20% N₂ dilution caused a significant increase in the median spark energy density when compared to no dilution. The extrapolation of the present results to atmospheric pressure, stoichiometric H₂-N₂O indicates that the electrostatic discharge ignition hazards are comparable to or greater than H₂-O₂ mixtures.     

Positive point-to-plane corona discharge forms in O2-N2 mixtures

An extensive survey has been made of positive point-to-plane corona discharges with 0–100% O₂ in O₂-N₂ mixtures in the pressure range 0.5–100 kPa. Selected results are presented in the form of pressure-voltage diagrams. They show the regions of existence for the different point-plane discharge forms observed: burst pulses, glow corona with oscillations, streamers, sparks, and a pulse- and oscillationless glow-spike corona. The latter positive corona discharge form has formerly only been observed in very pure nitrogen. The main aim of the paper is to give a short account of these different forms of positive coronas and their stability or oscillations, and to guide experimenters in establishing experimental conditions for future studies of these phenomena. Dedicated to Prof. Jan Janča on the occasion of his 60th birthday.     

Single-molecule conductance determinations on HS(CH2)4O(CH2)4SH and HS(CH2)2O(CH2)2O(CH2)2SH, and comparison with alkanedithiols of the same length

The acetyl-protected, thiol-terminated ethers AcS(CH(2))(4)O(CH(2))(4)SAc and AcS(CH(2))(2)O(CH(2))(2)O(CH(2))(2)SAc have been synthesised, and a range of related scanning tunnelling microscopy (STM)-based methods have been employed to fabricate and electrically characterise gold | single molecule | gold junctions involving these molecules. The single-molecule conductance values obtained are consistently found to be substantially higher (by a factor of 2-3) than the conductances of analogous alkanedithiols of similar length (HS(CH(2))(9)SH and HS(CH(2))(8)SH, respectively). A rationalisation of these findings is suggested, namely that the lone pair electrons on the oxygen atoms are substantially closer in energy to the Fermi energy of the gold leads than are the occupied and unoccupied states of methylene chains, so that the ether oxygens behave in a manner analogous to 'wells' in a double-tunnelling-barrier system. In agreement with this suggestion, the current-voltage behaviour of the monoether can be fitted using the Simmons approach, and the barrier height is found to be significantly lower than for alkanedithiols of approximately the same length.     

Fundamental burning velocities of meso-scale propagating spherical flames with H2, CH4 and C3H8 mixtures

This study is performed to experimentally examine the fundamental burning velocity characteristics of meso-scale outwardly propagating spherical laminar flames in the range of flame radius r_f approximately from 1 to 5 mm for hydrogen, methane and propane mixtures, in order to make clear a method for improving combustion of micro–meso scale flames. Macro-scale laminar flames with r_f > 7 mm are also examined for comparison. The mixtures have nearly the same laminar burning velocity (S_L0 = 25 cm/s) for unstretched flames and different equivalence ratios ?. The radius r_f and the burning velocity S_Ll of meso-scale flames are estimated by using sequential schlieren images recorded under appropriate ignition conditions. It is found that S_Ll of hydrogen and methane premixed meso-scale flames at the same r_f or the Karlovitz number Ka shows a tendency to increase with decreasing ?, whereas S_Ll of propane flames increases with ?. However, S_Ll tends to decrease with the Lewis number Le and the Markstein number Ma, irrespective of the type of fuel and ?. It also becomes clear that the optimum flame size and Ka to improve the burning velocity exist for some mixtures depending on Le and fuel types.     

EPR of Co+2 in Mg(CH3COO)2·4H2O

EPR spectra of the ⁵⁹Co⁺² ion in oriented crystals of Mg(CH₃COO)₂·4H₂O have been measured at 9.4 GHz and a temperature of 4.2 K. The data for each of the two metal ion sites per unit cell are well-described by a spin Hamiltonian for $\text{S =}\text{1}\text{2}\text{, I =}\text{7}\text{2}$ with g_x = 6.13, g_y = 389, g_z = 2.49 and A_x = 0.0193, A_y' = 0.075, A_z' = 0.0032 cm^?1. Only the x-axes of the g- and A-tensors coincide. Orientation of the principal directions relative to the crystal axes has been established for the g- and A-tensors at both sites. They are consistent with the weakly ferromagnetic canted antiferromagnetism found in Co(CH₃COO)₂·4H₂O below T_N ?85mK.     

CH4-O2-N2预混气体爆炸二维数值模拟

 利用CFD软件,基于ISAT算法和简化的甲烷氧化基元反应模型,建立了CH₄-O₂-N₂预混气体的二维湍流爆炸模型。数值计算结果表明,该模型较好地模拟了CH₄-O₂-N₂预混气体在高温气团作用下的点火、燃烧和爆炸过程。计算中,考察了高温点火气团的初始温度和混合气体初始组分对CH₄-O₂-N₂预混气体燃烧和爆炸过程的影响。结果表明：高温气团的初始温度对CH₄-O₂-N₂预混气体的点火延迟时间和燃烧初始阶段有重要影响,但对CJ爆燃没有影响;惰性气体N₂的加入,降低了混合气体的反应活性,导致点火延迟时间增大,燃烧反应速度、爆炸超压和重要自由基浓度都随之减小。相同条件下爆炸超压峰值的计算结果与实验测量结果符合较好。     

CH4/H2和CH4/He体系等离子体发射光谱分析

 在CH₄/H₂和CH₄/He和CH₄/He两种系统中,利用光学发射谱技术对螺旋波放电产生低压甲烷等离子体内活性粒子的光学发射特征进行了原位诊断。在实验中,两种体系下同时都测得的主要荷电粒子为CH,Ha,Hb,Hg以及H2等。研究了各实验参数对这些活性粒子CH, Ha,Hb以及Hg的发射光谱强度的影响。结果表明：在CH₄/H₂体系下,随着射频功率的增大,Ha,Hb,Hg以及CH基团的相对强度都随着增加,而当放电气压变化时它们都呈现先增大而后减小的趋势。在CH₄/He体系下,随射频功率的增加,Ha,Hb,Hg以及CH相对强度变化的总体趋势也都是先增加而后减小,当工作气压增加时,Ha,Hb以及Hg的相对强度变化也是呈现先增大而后减小,但CH基团的相对强度却是逐渐减小的;这些结果为等离子体沉积各种薄膜过程的理解及制备工艺参数的调整提供了参考。     

Experimental and kinetic modelling studies of flammability limits of partially dissociated NH3 and air mixtures

This paper presents a set of experimental and kinetic modelling studies of the flammability limits of partially dissociated NH₃ in air at 295 K and 1 atm. The experiments were carried out using a Hartmann bomb apparatus. The kinetic modelling was performed using Ansys Chemkin-Pro with opposed-flow premixed flame model employing three detailed reaction mechanisms, namely, the Mathieu and Petersen, Otomo et al., and Okafor et al. mechanisms. The degree of NH₃ dissociation was varied from 0 to 25% (0 to 20%v/v H₂ in the fuel mixture with a fixed H₂/N₂ ratio of 3). It was found that the lower (LFL) and upper (UFL) flammability limits of pure NH₃ in air were 15.0%v/v and 30.0%v/v, respectively, consistent with the literature data. The flammability limits of the mixture widened significantly with increasing the degree of NH₃ dissociation. At 25% NH₃ dissociation, LFL decreased to 10.1%v/v and UFL increased to 36.6%v/v. All tested mechanisms were able to predict the extinction characteristics exhibited by the lean and rich mixtures of partially dissociated NH₃ in air with non-unity Lewis numbers. While all three mechanisms predict well LFL, the Otomo et al. mechanism showed the best agreement with the experimental data of UFL. The rate of production of radicals, sensitivity, and reaction path analyses were performed to identify the key elementary reactions and radicals during combustion of partially dissociated NH₃. The production of key radicals including OH, H, O, and NH₂ was enhanced in the presence of H₂ and thus the conversion of NH to NO and then NO to N₂ near LFL and the conversion of NH₂ and NO to N₂ near UFL leading to wider flammability limits.     

一种新型钼磷酸盐杂多化合物(NH3CH2CH2NH3)7H2[NaMo12O30(PO4)2(HPO4)5(H2PO4)]·7H2O的合成和光谱研究

本文采用水热反应条件 ,合成得到一种新型的含五价钼原子的杂多化合物 :(NH3CH2 CH2 NH3) 7H2[NaMo1 2 O30 (PO4 ) 2 (HPO4 ) 5(H2 PO4 ) ]·7H2 O ,在晶体结构测定的基础上对其进行红外、拉曼和紫外 可见漫反射光谱研究。结果表明 :较长的Mo(Ⅴ )—O键键长和分子内大量的氢键造成化合物红外光谱特征的红移。     

水对800℃下CH4在Ar气中分解制备单壁碳纳米管的影响

利用透射电子显微镜和拉曼光谱方法研究了水对800℃下Ar气中催化分解CH4制备单壁碳纳米管（SWCNTs）的影响.结果表明，反应气中少量水（室温下反应气的湿度约5%）的引入提高了产物中SWCNTs的含量.初步分析认为，水在反应过程中起到了提纯作用，从而降低无定形碳生成率.此外，还发现水的引入缩小了产物中SWCNTs的管径分布. 关键词： 单壁碳纳米管 水 拉曼光谱     

乙醇与水协同修饰Bi_2WO_6纳米片形貌及改善其光致发光性

通过控制前驱体中乙醇与水的体积比,采用溶剂热法得到不同形貌的Bi2WO6纳米片,探讨了Bi2WO6形貌与光致发光性能之间的关系。Bi2WO6的结晶度、纳米片的棱长以及片层间距随醇水比增大而逐渐提高,当醇水比为1∶1时,Bi2WO6纳米片的棱长、片层间距和比表面积(BET)最大,表现出优异的光致发光性能;而在醇水比大于1∶1时,纳米片的棱长、片层间距和BET随之降低,光致发光性能继而减弱。以上表明醇水比为1∶1时,复配乙醇与水协同修饰Bi2WO6纳米片形貌的效果最佳,BET最大,从而促进其光致发光性能的改善。     

Effect of ozone addition and ozonolysis reaction on the detonation properties of C2H4/O2/Ar mixtures

Ozone is one of the strongest oxidizers and can be used to enhance detonation. Detonation enhancement by ozone addition is usually attributed to the ozone decomposition reaction which produces reactive atomic oxygen and thereby accelerates the chain branching reaction. Recently, ozonolysis reaction has been found to be another mechanism to enhance combustion for unsaturated hydrocarbons at low temperatures. In this study, the effects of ozone addition and ozonolysis reaction on steady detonation structure and transient detonation initiation and propagation processes in C₂H₄/O₂/O₃/Ar mixtures are examined through simulations considering detailed chemistry. Specifically, the homogeneous ignition process, the ZND detonation structure, the transient direct detonation initiation, and pulsating instability of one-dimensional detonation propagation are investigated. It is found that the homogenous ignition process consists of two stages and the first stage is caused by ozonolysis reactions which consume O₃ and produces CH₂O as well as H and OH radicals. The ozonolysis reaction and ozone decomposition reaction can both reduce the induction length though they have little influence on the Chapman–Jouguet (CJ) detonation speed. The supercritical, critical and subcritical regimes for direct detonation initiation are identified by continuously decreasing the initiation energy or changing the amount of ozone addition. It is found that direct detonation initiation becomes easier at larger amount of ozone addition and/or larger reaction progress variable. This is interpreted based on the change of the induction length of the ZND detonation structure. Furthermore, it is demonstrated that the ozonolysis reaction can reduce pulsating instability and make the one-dimensional detonation propagation more stable. This is mainly due to the reduction in activation energy caused by ozone addition and/or ozonolysis reaction. This work shows that both ozone decomposition reaction and ozonolysis reaction can enhance detonation for unsaturated hydrocarbon fuels.     

Resonant tunnelling of the magnetization in molecular crystal of [（Mn1-xCrx）12O12 （CH3COO）16（H2O）4]·2CH3COOH·4H2O（x=0,0.03,0.04,0.05）

The quantum tunnelling of magnetization （QTM） in single crystals of the single molecule magnet （Mn1-xCrx）12- Ac （x=0, 0.03, 0.04, 0.05） has been investigated. In comparison with its parent Mnl2-Ac, a greater rate of magnetization relaxation and a lower effective potential-energy barrier have been observed in Cr-doping samples. This modulation of QTM due to the Cr-doping could be attributed to the small change of Sz due to the smaller spin of Cr itself and additional intrinsic but distributed transverse and longitudinal anisotropy raised by a subtle change of the local environment in the magnetic Mn12 core.     

Effect of pyrolysis conditions on the char gasification with mixtures of CO2 and H2O

Two kinds of char were prepared from a lignite by fast pyrolysis using a drop tube furnace and by slow pyrolysis using a fixed-bed furnace at the temperature of 1273 K. Scanning electron microscopy, X-ray diffractometry and the BET method were employed to characterize char properties. The chars were gasified with CO₂, H₂O and their mixtures in a thermogravimetric analyzer (TGA) system to investigate gasification kinetics and derive the rate expression. To validate the gasification rate equation derived from TGA, a fluidized-bed gasification experiment was also carried out. The results showed that both fast-char and slow-char were mainly composed of dense char. The shrinking core model was applicable to predict both gasification of fast-char and slow-char. It was found that the char gasification rate in the mixtures of CO₂ and H₂O was obviously lower than the sum of the two rates of the char independently reacting with CO₂ and H₂O but higher than the rate of each independent reaction, for both the fast-char gasification and slow-char gasification. Both of the results from the TGA and the fluidized-bed reactor showed that char-H₂O reaction was independent on char-CO₂ reaction, while char-CO₂ reaction was inhibited by char-H₂O reaction.     

50-5000 eV电子被C4H8O, C5H10O2, C6H5CH3和C4H8O2散射的总截面

使用电子被C, H和O原子散射总截面的实验数据, 利用修正后的可加性规则计算了能量为50-5000 eV的电子被4个复杂大分子C4H8O, C5H10O2, C6H5CH3和C4H8O2散射的总截面, 并将计算结果与实验结果及其他理论计算结果进行了比较. 结果表明, 即使是在几十电子伏的入射能量下, 修正后的可加性规则计算出的总截面依然能与实验结果符合很好, 而使用未修正的可加性规则进行计算, 即使是在几百电子伏的入射能量下得到的总截面仍偏离实验值较远. 分析表明, 未修正的可加性规则计算得到的总截面在中低能区过大、随电子入射能量的增加而衰减太快的问题是由可加性规则本身引起的, 其实质是未考虑低能下分子内原子间的相互屏蔽对散射总截面的计算所带来的影响.     

800℃下在氩气中分解甲烷合成单壁碳纳米管反应中水的作用(英文)

pacc:8220,7125X TheeffectsofH2OtosynthesizingSWCNTs bydecomposingCH4inArat800℃werestudied byTEMandRamanscattering.Itisshownthat thewaterinducedintothereactinggas(therela tivehumidityisabout5%atroomtemperature)increasedthepercentagecompositionoftheSWC NTs…