Fractal Dimensions of Model Particle Packings Having Multiple Generations of Agglomerates

We examine a simple model for a powder having multiple generations of agglomerates. Over the size range over which the generations span, this structure is shown to be fractal. An equation expressing the fractal dimension for any Euclidean dimension is derived.     

Coagulation Coefficient of Agglomerates with Different Fractal Dimensions

A coagulation coefficient of agglomerates with different fractal dimensions has not been considered in the past, even though there is a possibility of variations in the fractal dimension of agglomerates at any instant. In this study, a Brownian dynamics simulation was performed with simultaneous collision and sintering, and variations in the fractal dimension of agglomerates were observed. A coagulation coefficient expression for agglomerates with two different fractal dimensions was proposed. The coagulation coefficient based on the different fractal dimensions was at most 140% higher than that based on the average fractal dimension. To determine an accurate coagulation coefficient of agglomerates, the fractal dimension of each agglomerate has to be considered.     

Simulation of Condensation Aerosol Growth by Condensation and Evaporation

A new numerical method is reported for the solution of the condensation—evaporation equation, a first-order hyperbolic equation. The solution and properties of the nonlinear integrodifferential equation arising when the mass of the condensing vapor is conserved are discussed. For aerosol evolution in the conserved case it is shown that there develops an asymptotic regime analogous to the asymptotic behavior found for the coagulation process.     

Distortion of Size Distributions by Condensation and Evaporation in Aerosol Instruments

Aerosols that contain volatile species or condensable vapors may be altered by changes in temperature, pressure, and vapor concentration. When such changes occur within aerosol sampling instruments, the measured size distribution can be distorted significantly. The distortion of particle size distributions in a number of commonly used aerosol instruments, including cascade impactors, both conventional and low pressure instruments, and optical particle counters, is explored both theoretically and experimentally in this paper. Ammonium sulfate aerosols in humid atmospheres have been used to test the instruments. In a low pressure impactor in which the pressure is intentionally reduced to facilitate the collection of small particles, a water containing particle may shrink due to evaporation as the pressure is reduced. However, if the sample flow is also accelerated to high velocities, aerodynamic cooling can lead to condensation of water vapor and particle growth. Either of these competing effects may lead to erroneous estimates of the particle size distribution. Optical particle counters generally use a recirculated sheath airflow. Pumps and electrical dissipation heat this air, leading to a temperature increase that shifts the vapor equilibrium, causing a decrease in particle size due to evaporation. Modifications have been made to avoid this distortion in measured size distributions.     

Synthesis and Characterization of Oligo-azomethine by Thermal Condensation Using New Type of Monomers

Summary The synthesis and characterization of a variety of aromatic oligo-azomethine, by thermal polymerization of news monomer catalyzed by concentrated HCl, is reported. The new oligo-azomethine has been characterized using Uv-visible, infrared, ¹H-NMR and ¹³C-NMR spectroscopy. Other techniques such as elemental analysis and thermo-gravimetric analysis are also presented. The oligomeric material was synthesized by condensation between the ester group and the amine group. Surprisingly the oligomeric material retains the OCH₂CH₃ groups and it becomes bound to the C=N groups. The new type of monomers (aminophenyl ester) allow, at the first time, to introduce substituent covalent bonds to carbon atom of imine groups.     

Fractal Dimension of Alumina Aggregates Grown in Two Dimensions

The concepts of fractal geometry are applied to the analysis of 0.4-μm alumina constrained to agglomerate in two dimensions. Particles were trapped at the bottom surface of a drop of a dilute suspension, and the agglomeration process was directly observed, using an inverted optical microscope. Photographs were digitized and analyzed, using three distinct approaches. The results indicate that the agglomerates are fractal, having a dimension of approximately 1.5, which agrees well with the predictions of the diffusion-limited cluster–cluster aggregation model.     

Strengthening of Porous Mullite and Zirconia CMC Matrices by Evaporation/Condensation

A processing method using evaporation/condensation sintering in an HCl atmosphere was developed for strengthening porous materials without shrinkage. Strengthening without shrinkage is useful in preventing voids and cracks that might be formed during constrained densification, e.g., a porous matrix in a continuous fiber reinforced ceramic composite. Mixtures of mullite and zirconia (monoclinic, tetragonal (3 mol% Y₂O₃), and cubic (8 mol% Y₂O₃)) were studied and exposed to HCl vapor at temperatures up to 1300°C. It was observed that the evaporation–condensation mass transport process produced a porous material with minimal shrinkage. As the crystal structure of the starting tetragonal and cubic zirconia powders did not change after extensive coarsening, it appeared that zirconium and yttrium were transported in the same proportion via evaporation/condensation. The process produced significant coarsening of the zirconia grains, which made the material resistant to densification when heated to 1200°C in air. Because the sintering produced coarsening without shrinkage, the pores also coarsened and a porous microstructure was retained. Mixtures of mullite and zirconia were used because mullite does not densify under the processing conditions used here, namely, heat treatments up to 1300°C. The mullite particles acted as a non-densifying second phase to further inhibit shrinkage when the mullite/zirconia composite was heated up to 1200°C in air. The coarsened cubic zirconia plus mullite mixture had the least densification after heat treatments in air of 100 h at 1200°C.     

分形结构团聚体在剪切流场中分散行为的三维数值模拟

为深入了解颗粒团聚体的分散机理及其动态细节,在经典离散元理论模型中引入范德华力和流体拖曳力,开发了三维颗粒离散元程序DEMix3D,对具有分形结构的颗粒团聚体在纯剪切流场中的分散过程进行数值模拟。结果表明:团聚体在流场中先发生变形然后再分散,分散过程中存在剥蚀和破碎两种分散方式;相同剪切流场下,初始团聚体的分形维数越小其分散效率越高。     

Fractal Dimensions of Simulated and Real Fat Crystal Networks in 3D Space

The microstructure of fat crystal networks is closely related to rheological properties of fat products, and thus is of particular interest to food scientists. The fractal dimensions of fat crystal networks calculated by microscopy methods such as box-counting, D _b, particle-counting, D _f, and mass fractal dimension, D _m, have been extensively employed to quantify the microstructure of fats. This work revealed the microstructural basis of D _b, D _f, and D _m in 3D space through both computer simulation and experiments on the high melting fraction of milk fat crystal networks. Similar to our previous simulation study on the fractal dimensions of fat crystal networks in 2D space, D _b is sensitive to crystal size, area fraction, and not sensitive to distribution orderliness of crystals, which is the percentage of evenly distributed fat crystals within the simulated fat crystal networks. D _f and D _m were not affected by any of the microstructural factors studied in the simulation.     

考虑水蒸汽蒸发和冷凝的超声空化特性研究

超声空化泡在溃灭时能够产生局部高温高压等极端物理化学效应。今在现有超声空化理论的基础上,考虑了水蒸汽的蒸发和冷凝效应,建立了超声空化动力学方程。文中采用数值方法,模拟了超声空化气泡运动状态参量变换规律。结果表明,不考虑水蒸汽的蒸发冷凝时,气泡在溃灭时最高温度和最大压力分别为27944 K和9.9072 GPa;而考虑水蒸汽的蒸发冷凝时,其最高温度和最大压力分别为3303.7 K和2.2342 GPa。因此水蒸汽的蒸发和冷凝对超声空化过程具有不可忽略的影响。     

硝铵蒸发冷凝液废水治理回收技术研究

同时含NH3-N ,NO3- -N的硝酸铵装置生产废水的治理是氮肥行业尚未有效解决的环保“老大难”治理问题。本文详细地比较了目前国内外对NH3-N ,NO3- -N废水的各种治理技术的优缺点后 ,提出采用多流通旋转分配阀的连续离子交换分离法是治理回收高浓度、小水量NH3-N ,NO3- -N废水的较适宜技术。通过实验室动态摸拟试验表明 ,多流通旋转分配阀连续离子交换分离法不但能有效治理含NH3-N ,NO3- -N的硝铵废水使废水达标排放 ,而且还能回收、提浓再生的硝铵产品返回硝铵生产工艺而产生经济效益。     

高强度HPVC材板及其断裂分形的研究

对经硅烷偶联剂处理的碳纤维填充增强HPVc板材的力学性能进行研究,并对拉伸断裂断面的分形维数进行测量分析,详细论述了数盒子法计算分形维数的原理和计算机的模拟过程。结果表明,板材拉伸强度达到100MPa以上,冲击强度为25kJ／m^2,材料拉伸强度与断面视图的分维数呈正线性关系。     

蒸发式冷凝器的经济性及应用

本文通过两个方案的比较。阐述了蒸发式冷凝器在特定的环境下具有较好的经济性,特别是能节省运行费用,是一种合理的选择。     

大孔树脂对阿维菌素吸附性能的分形研究

介孔材料由于其特殊的结构而具有很高的活性和极大的吸附容量,一般的表征方法不能反映其中孔的真实情况。采用FHH法和NK法计算了大孔苯乙烯树脂的分维,分析了大孔树脂吸附阿维菌素的分形模型;探讨了分形维数对树脂吸附性能的影响规律。结果表明,NK法更能适用于对苯乙烯树脂孔道结构的表征,树脂对阿维菌素的吸附能力也随着NK分维的增大而增大。因而,用分形理论研究大孔树脂结构与性能的关系是可行的。     

分形吸附理论及其对粘土性质的表征方法

粘土被广泛地应用于环境污染物的吸附处理。概述了分形几何的基础及其对粘土吸附和性质表征的方法。归纳了常见的分形吸附模型以及分维数测定方法。     

Formation and Control of Agglomerates in Alumina Powder

High-purity alumina powder with submicrometer particle size was colloidally dispersed and classified in either water or ethanol to remove agglomerates from as-received powder. After the slurries were dried, fine alumina particles cohered to form agglomerates, which were "hard" when formed in a water slurry, and "soft" in an ethanol slurry. Firing of the powder compacts with "hard" agglomerates resulted in inhomogeneous microstructures, and homogeneous microstructures were formed with "soft" agglomerates. The reasons for their formation are discussed and experimentally confirmed. In the case of water slurry, a small amount of transition alumina reacted with water at low pH. After drying, alumina gels were formed and acted as a strong binder between α-alumina particles to form "hard" agglomerates.     

用于羟醛缩合反应的固体碱催化剂的制备及表征

采用溶胶凝胶法制备载体二氧化锆,并通过浸渍法负载铯制备固体碱催化剂,将其应用于以醋酸甲酯、甲醛为原料合成丙烯酸甲酯的反应。考察了不同温度,负载不同含量的硝酸铷、硝酸铯对催化剂性能的影响。利用微反色谱实验装置测试合成丙烯酸甲酯固体碱催化剂的催化活性,结果表明,负载铯比铷催化效果要好;催化剂最优条件：氧化锆负载20%铯,反应温度为380℃时,可使收率达到29.2%。     

Relative Changes of Porous Glass Dimensions in Humid Atmosphere

An interferometric technique for measurements of porous glass geometrical properties in humid ambiance is proposed. The developed technique is non-destructive and can be used for qualitative analyses of the residual gel of secondary silica and pore sizes evaluation in porous silica glasses.     

烯醛一步法合成异戊二烯固体催化剂研究进展

烯醛法是化学合成异戊二烯中最具前景的方法,本文简述了三种类型的固体催化剂在一步合成异戊二烯中的研究进展,通过比较,指出分子筛是最具应用前景的催化剂。     

重结晶碳化硅烧成中碳化硅的分解现象、热力学条件及对蒸发凝聚的影响

重结晶碳化硅陶瓷烧成试验采用国内碳化硅微粉,对其烧成后的现象与结果进行分析检测,在热力学分析和试验的基础上得出含有较多杂质的国内碳化硅微粉在重结晶烧成中有显著分解挥发,而且影响碳化硅的蒸发凝聚过程。