The topology of percolating porosity in woven fiber ceramic matrix composites

Three-dimensional x-ray microtomography has been used to visualize porosity in ceramic matrix composites during chemical vapor infiltration processing. The topology of percolating pores was determined in both 0°/90° and 0°/45° architectures. At densities greater than 75%, consolidation can be described with percolation theory.     

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文章首先利用先进的激光刻蚀技术，将岩石的真实裂缝结构刻蚀在光学玻璃板上，制作成可供摄像用的透明微观物理模型, 并加压让气水两相流过微裂缝，观察到气水两相在裂缝性地层中的流动分为三种现象，即水窜、绕流、和卡断现象。在此基础上，建立了气水两相裂缝渗流模型，将气水两相流动分为四个流动阶段，即静止、运动、卡住和卡断阶段。从微观渗流角度出发，利用能量守恒原理详细地分析了裂缝性地层中气水两相渗流机理，证明裂缝性地层中气水两相流动具有不连续性。该过程不能用Vogel方程进行分析计算，同时也说明裂缝性地层中的气水分布与压力分布无关，只与构造形态、水侵特征和产水历史有关。由此我们知道有水气井井底产水具有波动性，不能用井口产水来描述井底产水，从而提高了对裂缝性地层中气水两相渗流特征的认识，可以更好地指导裂缝性地层中有水气井的生产。     

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通过对凝析气井产能分析方法以及流体相态、组分变化、渗流特征的研究，结合状态方程理论及相平衡闪蒸计算方法，引入反映地层中凝析油气体系变化的两相拟压力函数，提出了考虑近井带反凝析液饱和度分布以及动态污染影响的凝析气井产能分析新方法。通过实例计算认为该产能计算方法能较好地反映近井带地层凝析油析出时造成的动态表皮伤害对产能的影响；当地层压力高于露点压力时，预测方法所得的流入动态曲线在井底压力为露点压力处有一拐点，这与实际情况符合，反映了凝析液析出对气井产能的影响。     

基于直线段特征单元提取的隧道砼衬砌接缝去除算法

由于部分隧道砼衬砌表面图像中固有衬砌接缝与裂缝灰度值相似且线性一致,衬砌接缝处易产生起砂、空鼓、掉块及渗漏水,已有的裂缝检测算法提取单一的裂缝存在缺陷。提出基于直线段特征单元提取的隧道砼衬砌表面衬砌接缝去除算法。在裂缝聚类特征粗检测的基础上,首先通过改进的累计概率霍夫变换检测出显著的直线特征;然后利用像素点的延伸搜索计算来提取衬砌接缝可处理的最小直线段特征单元线;最后根据单元线标记信息及定区域内单元线特征去除部分衬砌接缝,并运用渗流去噪算法得到隧道砼衬砌表面真实裂缝。实验结果表明,提出的算法弥补了已有隧道砼衬砌表面裂缝检测技术的不足,能够精确、快速、有效地去除相似线性特征对单一的真实裂缝检测的干扰,具有较强的鲁棒性。     

Effects of dopants on percolation behaviors and gas sensing characteristics of polyaniline film

Effects of dopants on the correlation between the polyaniline (PAn) film resistance (R) and the reduction charge (Q) injected to the PAn film was investigated in dry acetonitrile solutions during electrochemical reduction by using the double potential step method. The R-Q correlation behaves as S-type curves, leading to the determination of the critical reduction charge (Q_c). The latter represents the reduction charge required for the formation of a continuous partially reduced phase in the PAn film. It was observed that the PAn film doped with sodium dodecylbenzene sulphate (SDBS) yielded a smaller Q_c than that doped with perchlorate, when the PAn films were electrochemically reduced under given conditions. The resistance of the pre-doped PAn film will increase significantly when the film is injected with reduction charge more than Q_c. Hence, a smaller Q_c means that the film can respond to very light reduction (or dedoping), being indicative of better sensing ability toward alkaline and reducing gases. This was confirmed by the increased sensitivity of the PAn/SDBS sensor toward 100 ppm NH₃ vapor, compared with the PAn/ClO₄⁻ sensor.     

Percolation of capillary pores in hardening cement pastes

This paper presents a study of the pore structure and permeability of hardening cement pastes. Cement pastes with water/cement (w/c) ratio 0.4, 0.5 and 0.6 from 1 up to 28 days of age were tested with mercury intrusion porosimetry. In parallel, the water permeability was measured. With the help of a numerical simulation model, correlations between pore structure and permeability were found. Water permeability is related more to the pore size distribution and to the effective porosity than to the total porosity of the pastes. No depercolation of the capillary pores was evident neither in the experiments nor in the numerical simulations during the first 28 days after casting.     

FRAGMENTATION IN RANDOM PORE STRUCTURES

The problem of fragmentation of solid phase in reacting porous structures that consist of d (d = 1,2,3) mutually perpendicular bundles of parallel, randomly overlapping cylindrical capillaries is considered. The evolution of fragmentation with the porosity is studied using a Monte Carlo stimulation procedure which rests in using the trajectories of test particles randomly travelling in the solid phase to identify fragments and determine fragment size and fragment surface area distributions. Our results show that fragmentation occurs at about the same value of pore number density per bundle for all d values. This observation is used to develop a correlation that satisfactorily predicts the extent of fragmentation in 2- and 3-bundle capillary structures using the simulation results for pore structures consisting of a single bundle of overlapping capillaries.     

A multiscale analysis of elastic contacts and percolation threshold for numerically generated and real rough surfaces

In this paper, we present numerical investigation of the contact between an elastic solid and a randomly rough surface. In agreement with recent results, we find that the contact area vs load relation depends on the statistical parameters only through the root mean square slope of the heights distribution. Such result extends to contact pressure regimes where the area/load relation is non-linear.Moreover, we show that fractal self-affine surfaces give a good representation of real surfaces from both topographical and contact mechanics points of view.Finally, we investigate how the network of non-contact areas evolves as the real contact area is increased, finding that the percolation threshold is smaller than the one predicted by Bruggeman's theory.     

Manufacturing pharmaceutical granules: Is the granulation end-point a myth?

The moist agglomeration process by high-shear mixing/granulation, i.e. the wet massing, screening and subsequent drying is a wide spread but critical unit operation. Since for decades formulators are looking for the correct “end-point” of this important process, i.e. when do you need to stop massing or stop with the addition of granulating liquid? What is the correct amount of granulating liquid? A similar situation exists in case of the moist agglomeration in fluidized bed equipment. In the latter case the simultaneous drying of the still moist granules have to be taken into account. Recently a science-based virtual equipment simulator could be developed mimicking the granule size evolution in a fluidized bed granulator during the addition of granulating liquid. For the simultaneous drying the Mollier chart is used. With this virtual equipment simulator it is possible to simulate “crash situations”, i.e. by overwetting or by an incorrect use of the parameter setting. However the determination of the “end-point” depends only on the operator, who desires a certain granule size distribution and a well-defined final moisture content of the batch. Thus the existence of a process intrinsic “end-point” has to be questioned. The same situation can be reported in case of high-shear mixing/granulation based on many years of research. In fact nobody could clearly show the existence of an intrinsic “end-point”. However during the continuous addition of a granulating low viscous liquid a sudden increase in power consumption can be measured, which levels off. Such a measurement depends on the formulation and leads to an “early signal” not to the “end-point”. This signal can be used for a tight control of the granulation process and leads to a low batch to batch variability in the final granule size distribution. The latter is the goal of the PAT (Process Analytical Technology) Initiative emphasizing “Quality by Design”.