Structural and stability characteristics of agglomerated clusters

We investigated the cohesion of agglomerates formed by sticking two fractal clusters, each cluster having been previously generated by particle aggregation on a 3D lattice. The degree of cohesion of an agglomerate of a given configuration was defined by the number of connections simultaneously established on the two stuck clusters. All the possible nonoverlapping configurations were investigated and the corresponding porosity and brittleness as well as the pore volume and connection frequencies were determined. The numerical study showed the greater internal cohesion of agglomerates issued from sticking of reaction-limited aggregation (RLA) clusters compared to that of diffusion-limited-aggregation (DLA) clusters. DLA and RLA agglomerates presented continuously decreasing pore volume frequency curves, the latter agglomerates being characterised by a greater frequency of large pores. Comparison with typical controlled fragmentation experiments showed the number of connections to be the prevailing factor in the cohesion of aggregates formed in aqueous suspensions under various conditions. Received: 25 January 2001 Accepted: 16 May 2001     

煤的缔合结构研究Ⅱ溶液黏度变化

采用Pal-Rhodes方程描述了一种煤可溶组分PI-1溶液黏度随时间的变化关系,并计算得到了缔合物的缔合维数。25 ℃下PI-1在NMP和CS2/NMP混合溶剂中缔合物的缔合维数分别为2.08和2.19,表明其在低温下缔合属于反应控制机理。随着温度的提高,缔合维数趋于减小,表明在较高温度下,PI-1具有较快的缔合动力学过程,在50 ℃时PI-1在NMP中的缔合维数大于在CS2/NMP混合溶剂中的缔合维数,表明PI-1在NMP的缔合速率相对低于CS2/NMP混合溶剂。     

Mobility of permeable fractal agglomerates in slip regime

Hydrodynamic drag and mobility of fractal aggregates in the slip creeping flow regime are calculated. A theoretical continuum model of the gas slip flow past and within agglomerates is developed. It accounts for effects of flow rarefaction and porous fractal structure upon the molecular mean free path, apparent viscosity, and effective permeability of agglomerates. It is shown that flow rarefaction significantly diminishes the aggregates' drag to an extent that cannot be predicted by the Cunningham's drag correction factor. The developed model allows calculation the agglomerates' transport properties in a wide range of fractal dimensions. For low D(f) agglomerates the drag force agrees with the Friedlander's expression based on the Epstein's single sphere drag in the free molecular regime.     

无机电解质及聚合物对Mg-Al混合金属氢氧化物—高岭土分散体系触变性的影响

类水滑石;流变学;无机电解质及聚合物对Mg-Al混合金属氢氧化物—高岭土分散体系触变性的影响     

Al-Mg MMH正电胶体粒子体系的流变学

研究了铝／镁混合金属氢氧化物（MMH）粒子含量,PH值,电解质等因素对MMH悬浮液流变学的影响,发现MMH粒子浓度的增加使悬浮体从牛顿型流体变为带有屈服值的假塑性流体;在相对很低的粒子浓度（2％）时,通过改变粒子表面化学状态和粒子周围的介质环境可以改变悬浮体的流动形态;MMH悬浮体触变性结构与高PH值时合成锂皂石悬浮体的结构极为相似,是粒子间的静电排斥作用所致。     

甲基肼/四氧化二氮反应化学动力学模型构建及分析

甲基肼(MMH)和四氧化二氮(NTO)是常用的液体火箭发动机推进剂,但目前对其反应机理的研究还十分有限.本文首先构建了一个包含23种组分和20个基元反应的MMH/NTO反应动力学模型;对MMH/NTO自燃着火过程进行的验证计算表明,该机理能够合理地描述MMH/NTO的自燃温升过程,准确预测反应物系统的着火延迟时间及平衡温度,并能合理地反映MMH/NTO反应物系统着火延迟时间对反应初始压力以及氧燃比的依赖关系;通过灵敏度分析方法指出了影响MMH/NTO着火过程的关键反应.模拟分析了在不同压力和氧燃比条件下MMH/NTO系统的自燃温升过程,结果表明,随着压力的升高,系统着火延迟时间变短,平衡温度升高;在一定范围内增大氧燃比,着火延迟时间变长,平衡温度先升高后减小.     

带相反电荷胶体颗粒混和悬浮体的流变学性能

研究了带相反电荷的粘土颗粒和MMH(铝、镁混合金属氢氧化物)颗粒形成的混和悬浮体的流变学性能,考察了盐对混合体系流变学性能的影响.结果表明,当粘土质量分数为1%时,悬浮体为牛顿型流体;当质量分数升至2%时,悬浮体表现出固体的弹性响应;特定粘土含量的悬浮体中,随着MMH量的增加,混合体系的屈服值和弹性模量亦增加,表明凝胶结构增强.向混合体系中加入NaCl,弹性模量、屈服值和粘度等流变参数均降低.与单组分粘土悬浮体或MMH悬浮体相比,双组分混合体系的结构恢复很快.     

Change in particle size distribution of fractal agglomerates during Brownian coagulation in the free-molecule regime

Time evolution of particle size distribution of fractal agglomerates undergoing Brownian coagulation in the free-molecule regime was investigated. A simple analytical solution for the size distribution change was obtained by using the assumption that the size distribution during the coagulation process can be represented by a time-dependent log-normal function. The derived solution consists of three parameters of the log-normal distribution function. This study is believed to provide the first analytical solution for all the parameters of the log-normal distribution of fractal agglomerates undergoing coagulation in the free-molecule regime. To validate the derived solution, numerical computations were performed. The results were compared with the analytical solution and good agreement was obtained.     

Brownian Coagulation of Fractal Agglomerates: Analytical Solution Using the Log-Normal Size Distribution Assumption

An analytical solution to Brownian coagulation of fractal agglomerates in the continuum regime that provides time evolution of the particle size distribution is presented. The theoretical analysis is based on representation of the size distribution of coagulating agglomerates with a time-dependent log-normal size distribution function and employs the method of moments together with suitable simplifications. The results are found in the form that extends the spherical particle solution previously obtained by K. W. Lee (J. Colloid Interface Sci. 92, 315-325 (1983)). The results show that the mass fractal dimension has a significant effect on the size distribution evolution during coagulation. When the obtained solution was compared with numerical results, good agreement was found. The self-preserving size distribution of nonspherical agglomerates is discussed. Copyright 2000 Academic Press.     

甲基肼自由基在高温燃烧分解反应下的非谐振效应

In this work, the harmonic and anharmonic rate constants of the decomposition reaction of monomethylhydrazine (MMH) radicals have been calculated by using transition state (TS) and Rice-Ramsperger-Kassel-Marcus (RRKM) theories with either MP2 or B3LYP method at 6-311++G (3df, 2p) basis set, respectively. The reaction mechanism and anharmonic effect of the MMH radicals are studied in detail and both of the harmonic and anharmonic rate constants increase sharply with increasing temperature in the canonical system. In the microcanonical system, these constants also show sharp increase with the energies. Overall, the anharmonic effect becomes more pronounced with the increasing temperature or energy in the canonical and microcanonical systems, respectively. These results indicate that the anharmonic effect of the decomposition reaction of MMH radicals is quite significant and cannot be ignored.     

电解质和阳离子表面活性剂对MMH/粘土体系胶体性能的影响

通过流变参数、电泳淌度和pH值的测定,研究了NaCl、MgCl₂、Na₂SO₄等电解质和阳离子表面活性剂(十六烷基三甲基溴化铵)对铝镁混合金属氢氧化物(MMH)/钠土(Mt)悬浮体胶体性能的影响.发现所研究的电解质和表面活性剂都能使MMH/Mt悬浮体的Bingham屈服值降低,但不同处理剂影响pH值和滤失量的变化趋势却不同.认为电解质中的阳离子以影响粘土颗粒间的缔合为主,而阴离子以影响MMH的荷电性能为主,前者使滤失量上升,pH值下降;后者使滤失量下降,pH值上升.电解质对MMH/Mt悬浮体滤失量和pH值的影响取决于二者相对能力的大小.阳离子表面活性剂由于在粘土上吸附后不仅影响颗粒之问的缔合,也能使其润湿性反转,导致MMH/Mt悬浮体的滤失量和Bingham屈服值的变化幅度明显高于无机电解质.     

Multiparticle sintering dynamics: from fractal-like aggregates to compact structures

Multiparticle sintering is encountered in almost all high temperature processes for material synthesis (titania, silica, and nickel) and energy generation (e.g., fly ash formation) resulting in aggregates of primary particles (hard- or sinter-bonded agglomerates). This mechanism of particle growth is investigated quantitatively by mass and energy balances during viscous sintering of amorphous aerosol materials (e.g., SiO(2) and polymers) that typically have a distribution of sizes and complex morphology. This model is validated at limited cases of sintering between two (equally or unequally sized) particles, and chains of particles. The evolution of morphology, surface area and radii of gyration of multiparticle aggregates are elucidated for various sizes and initial fractal dimension. For each of these structures that had been generated by diffusion limited (DLA), cluster-cluster (DLCA), and ballistic particle-cluster agglomeration (BPCA) the surface area evolution is monitored and found to scale differently than that of the radius of gyration (moment of inertia). Expressions are proposed for the evolution of fractal dimension and the surface area of aggregates undergoing viscous sintering. These expressions are important in design of aerosol processes with population balance equations (PBE) and/or fluid dynamic simulations for material synthesis or minimization and even suppression of particle formation.     

The effects of the pause to pulse ratio in the regime of pulsating overpotential on the formation of honeycomb-like structures

The formation of honeycomb-like structures in the pulsating overpotential (PO) regime was examined by scanning electron microscopy (SEM). The honeycomb-like structures obtained with a square-wave PO of different pause to pulse ratios were compared to the one obtained in the constant potentiostatic regime. Increasing the pause to pulse ratio led to a decrease of the diameter of the holes formed by attached hydrogen bubbles, as well as to an increase in the number of holes formed at the surface area of copper electrodes. The size of the agglomerates of copper grains, of which the walls of the holes were constructed, was reduced with increasing duration of the pause. Also, the uniformity of the honeycomb-like structures was increased by application of the PO regime. It was shown that the effects attained by application of this regime were comparable with those obtained by electrodeposition in a constant potential regime with the addition of specific additives.     

A study of the dioxane lignin of the stems of the cotton plant of variety S-4880

The dioxane lignin (DLA) has been isolated from the ripe stems of a cotton plant of the variety S-4880 B by a modification of Pepper's method. The yield was 16.9% on the Komarov lignin. The semiempirical formula of a phenylpropane structural unit has been calculated on the basis of elementary and functional analyses. It has been established that the amount of the main functional groups in the DLA and in the lignins isolated from the varieties of 108-F and Tashkent-2 basically coincide, but the DLA investigated was appreciably more highly methoxylated. The IR, UV, and PMR spectra of the DLA have been taken. The molecular-weight distribution of the DLA has been investigated and it has been shown that is is poly-disperse and has a weight-average molecular weight of 7400. Nitrobenzene oxidation and cleavage with sodium and liquid ammonia of the isolated DLA has shown that guaiacyl structural units predominate in it.Institute of the Chemistry of Plant Substances, Academy of Sciences of the Uzbek SSR, Tashkent. Translated from Khimiya Prirodnykh Soedinenii, No. 3, pp. 430–434, May–June, 1987.     

Quantitative analysis by resonant laser ablation with optical emission detection: Resonant laser-induced breakdown spectroscopy

Resonant laser ablation (RLA) is a solid sampling technique that makes use of radiation trapping, and desorption induced by electronic transitions (DIET), to produce enhanced numbers of analyte atoms in the laser-induced plasma (LIP). This is achieved by tuning the laser ablation wavelength to a gas-phase resonant transition of the analyte. In this paper, RLA was coupled with detection of optical emission in the LIP to perform resonant laser-induced breakdown spectroscopic (RLIBS) experiments with a miniature, portable spectrometer. The experiments were designed to continue an examination of the proposed mechanism of RLA, and to perform an initial assessment of the applicability of RLIBS for quantitative analysis. The study indicated that for a multi-component sample, such as steel, the signal of the wavelength-targeted atom was enhanced the most compared to other constituents, and this supported the hypothesis that the DIET phenomenon is involved. Also, RLIBS experiments indicated that ablation yields of other components were enhanced by resonant ablation of the major component, which supported the contribution of radiation trapping to the RLA phenomenon. A linear, positive slope of the RLA induced atom yield as a function of concentration suggested that the RLA mechanism allows for quantitative analysis of solid samples. Calibration graphs were created for tungsten in spectrographic steels using RLIBS. A limit of detection of 4% was calculated for tungsten in steel.     

2D and 3D quantum scattering calculations on the CH4 + OH → CH3 + H2O reaction

The effect on the thermal rate constant and the differential cross-sections of varying the dimensionality of quantum scattering calculations of a polyatomic reaction is investigated. The rotating bond approximation (RBA; 3D) and a rotating line approximation (RLA; 2D) are used for the CH₄ + OH → CH₃ + H₂O reaction. It is found that the RBA and RLA results are in close agreement when an adiabatic treatment is used for the degree of freedom which is treated explicitly in the RBA but not in the RLA.     

Thermal decomposition of monomethylhydrazine: Shock tube experiments and kinetic modeling

The thermal decomposition of gaseous monomethylhydrazine (MMH) was studied by recording MMH absorption at 220 nm of the reacting gas behind a reflected shock wave at temperatures of 900–1370 K, pressures of 140–450 kPa, and in mixtures containing 97.5–99 mol% argon. Based on previous work (Sun and Law; J Phys Chem A 2007, 111(19), 3748–3760), a kinetic mechanism was developed over extended temperature and pressure ranges to model these experimental data. Specifically, the temperature and pressure dependence of the unimolecular rate coefficients on the dissociation of MMH and the associated radicals were calculated by the QRRK/Master equation analysis at temperatures of 300–2000 K and pressures of 1–100 atm based on published thermochemical and kinetic parameters. They were then fitted using the Troe formalism and incorporated in the kinetic model. This unadjusted model was then used to predict the MMH decomposition profiles at different temperatures and pressures for seven groups of MMH/Ar mixtures and the half‐life decomposition times from shock tube experiments. Good agreement was observed below 940 K and above 1150 K for the diluted MMH/Ar mixtures. The model predictions further show that the overall MMH decomposition rate follows first‐order kinetics, and that the N–N bond scission is the most sensitive reaction path for the modeling of the homogeneous decomposition of MMH at elevated pressures. However, the model predictions deviate from the experimental data with the incubation period of ca. 100 μs observed in the 1030–1090 K temperature range, and it also predicts longer ignition delays for highly concentrated MMH/Ar mixtures. The discrepancy between the model predictions and experimental data at these special conditions of MMH decomposition was analyzed. © 2008 Wiley Periodicals, Inc. Int J Chem Kinet 41: 176–186, 2009     

铝/镁混合金属氢氧化物正电胶体粒子体系的触变性

采用恒定低剪切速率方法和动态实验方法研究了铝/镁混合金属氢氧化物(MMH)悬浮体的流变性,着重考察了剪切历史和恢复时间等因素对悬浮体触变性的影响,发现MMH粒子深度的增加使得悬浮体从粘性流体变为“类固体”;恒定低剪切速率方法和动态实验方法研究悬浮体的结构恢复过程给出了不同的结果,唯象地解释了MMH悬浮体触变性产生的原因,认为触变性结构是由于粒子间的静电排斥作用而形成的,与粘土悬浮体相比,两者的流变性具有诸多相似之处,只是所带电荷符号相反。     

A study of the lignin of the cotton plant of the subspeciesmexicanium

p-Hydroxybenzoic, vanillic, ferulic, and syringic acids, and also p-hydroxybenzalde-hyde and vanillin, have been found in an ethanol-benzene extract of the stems of cotton plants of the subspeciesmexicanium. The dioxane lignin (DLA) has been isolated by Pepper's method, the semiempirical formula of its phenylpropane structural unit and its molecular-mass distribution have been established, and its UV, IR, and PMR spectra have been recorded. The DLA has a comparatively low molecular mass and a lower degree of polydispersity than the dioxane lignin of the cotton plant of variety Tashkent-1, while the amounts of functional groups in them coincide, with the exception of a lower amount of carboxy groups in the DLA.     

铝镁混合金属氢氧化物溶胶阴离子交换性能研究

本文研究了电解质对铝镁混合金属氢氧化物（Ａｌ－ＭｇＭＭＨ）溶胶粒子中Ｃｌ－和ＯＨ－的阴离子交换性能。在所研究的电解质ＮａＮＯ３，ＨＣＯＯＮａ，ＣＨ３ＣＯＯＮａ，Ｎａ２ＣＯ３，Ｎａ２ＳＯ４和Ｎａ３ＰＯ４中，发现高价阴离子的交换能力大于低价阴离子的交换能力，无机阴离子的交换能力大于有机阴离子的交换能力。所研究Ａｌ－ＭｇＭＭＨ溶胶粒子的阴离子交换容量为２．８２ｍｍｏｌ／ｇ，比粘土粒子的阳离子交换容量大得多。