Determination of the effective diffusion coefficient in porous media including Knudsen effects

A 3-D bond pore network model is presented and used to evaluate the effect of pore size and connectivity on the effective diffusion coefficient in random porous media. The control equations of the system are set up and the simulation method is discussed. The simulation results show that when the average pore size d _m < 1 μm, the effective diffusion coefficient is strongly dependent on pore size. The analysis shows that Knudsen and bulk diffusion effects can be decoupled, and for any given diffusion conditions, the effect coefficient accounting for Knudsen diffusion can be obtained. Thus, the effect of pore size can been readily accounted for by correcting the bulk effective diffusivity with the Knudsen effect coefficient. The simulations also show that the percolation threshold in random porous materials decreases with increased pore network connectivity.     

The effect of different GNPs addition on the electrical conductivities and percolation thresholds of the SiAlON matrix composites

Insulating SiAlON ceramics may become electrically conductive with the addition of a conductive phase such as GNPs and can be used more widely. However, the differences in the properties of the used GNPs significantly affect the amount of electrical conductivity that they provide to the matrix. In this study, four different GNPs with different properties such as lateral dimension, thickness and aspect ratio were added to SiAlON in the amount of 1.5, 2, 3 and 4 wt. % and the effects of different properties on the conductivity of composites were investigated. The thinnest GNPs with largest dimension and aspect ratio among the used GNPs provided the highest electrical conductivity and lowest percolation thresholds to SiAlON. The decrease in dimension, aspect ratios and the increase in thickness decreased the electrical conductivity of GNPs. Composites exhibited anisotropic behavior with better conductivity and percolation threshold values in the in-plane direction than through-plane direction.     

聚丙烯/炭黑复合材料的导电逾渗行为和PTC特性

以聚丙烯(PP)为基体、炭(黑CB)为填料制备复合材料,研究了不同CB种类的PP/CB复合材料的导电逾渗行为和电阻正温度系数效(应PTC)特性,同时利用扫描电镜对复合材料的微观形态进行了分析。结果表明:CB的结构性越高,比表面积越大,则其填充的复合材料逾渗阈值越低,PP/VulcanXC-72体系的逾渗阈值约为2.5%,而PP/40B2体系约为3.5%;CB的比表面积越小,结构性越低,则相同质量分数时其填充的复合体系的PTC强度越大P,P/40B2体系的PTC强度大于PP/VulcanXC-72体系。     

Transparent,conductive polymer blend coatings from latex-based dispersions

Flexible, transparent and conductive polymer blend coatings were prepared from aqueous dispersions of poly(3,4-ethylenedixoythiophene)/poly(styrenesulfonate) [PEDOT/PSS] gel particles (∼80 nm) and latex (∼300 nm). The stable dispersions were deposited as wet coatings onto poly(ethylene terephthalate) substrates and dried at 80 °C. Microstructure studies using tapping mode atomic force microscopy (TMAFM) indicate that a network-like microstructure formed during drying at 0.03 volume fraction PEDOT/PSS loading. In this network-like structure, the PEDOT/PSS phase was forced into the boundary regions between latex. In addition, migration of the PEDOT/PSS particles towards coating surface is likely during drying of the aqueous dispersions. The addition of a small amount of dimethyl sulfoxide (DMSO) in dispersions altered the distribution of the PEDOT/PSS phase. As PEDOT/PSS concentration increases to 0.15 volume fraction, the coating surface is dominated by the PEDOT/PSS phase. The effect of DMSO on microstructure becomes less apparent as PEDOT/PSS concentration increases. The conductivity of the polymer blend coatings increases in a percolation-like fashion with a threshold of ∼0.02 volume fraction PEDOT/PSS. The addition of DMSO in dispersions enhanced the coating conductivity beyond the threshold by more than two orders of magnitude. The highest conductivity, ∼3 S/cm, occurs at 0.20 volume fraction PEDOT/PSS concentration. The polymer blend coatings have good transparency with only a weak dependence of transparency on wavelength due to the small refractive index difference between filler and matrix.     

永久型抗静电剂的研究进展

介绍了高分子永久型抗静电剂的特点,作用机理和目前的应用概况。高分子永久型抗静电剂对空气的相对湿度依赖性小,抗静电效果持久,无诱导期,不受擦拭和洗涤等条件影响。高分子抗静电剂在基体树脂中形成网络结构,树脂中聚集的电荷通过形成的导电通路得以释放。     

Charge Order versus Superconductivity in Inhomogeneous Systems

We examine the disordered system with defined attractive centers of carries attraction. By changing concentration of such centers one can model the appearance of a superconducting order. To study electron correlations and to describe disorder effects, the Negative U Hubbard model and the Coherent Potential Approximation (CPA) have been applied. Our approach includes the charge fluctuations on the equal foot.     

Non-stoichiometry and problem of heavy doping in semiconductor phases

The experimental results on the influence of non-stoichiometry on defect structure, band structure and physical properties of the IV–VI and I–III–VI₂ semiconductor compounds with wide homogeneity regions were analyzed. It was shown that compositions at which peculiarities in the concentration dependences of properties in the homogeneity region are observed, correspond to the maximum in the melting curves. Using the approach of percolation theory, the transition from weak to heavy self-doping within the homogeneity region was considered, and the percolation threshold under the introduction of non-stoichiometric defects was estimated. Compositions optimal for long-range ordering were determined.     

Patch microstructure in cement-based materials: Fact or artefact?

The appearance of patch microstructure, i.e. broad dense and porous regions separated by sharp and distinct boundaries and occurring randomly in bulk and interfacial transition zones, has been reported previously in various site- and laboratory-mixed concretes and mortars. In this paper, evidence is presented to show that patch microstructure is an artefact of sample preparation and does not reflect the true nature of the hydrated cement paste. The appearance of dense patches comes from paste areas that have been ground and polished beyond the epoxy resin intrusion depth. In a backscattered electron image, pores not filled with epoxy are not visible because the signal is generated from the base or side walls of the pores. A modified method for epoxy impregnation, which can achieve a much deeper epoxy penetration than conventional vacuum impregnation, is presented.     

Comparison studies on the percolation thresholds of binary mixture tablets containing excipients of plastic/brittle and plastic/plastic deformation properties

Percolation theory has been used with great interest in understanding the design and characterization of dosage forms. In this study, work has been carried out to investigate the behavior of binary mixture tablets containing excipients of similar and different deformation properties. The binary mixture tablets were prepared by direct compression using lactose, polyvinyl chloride (PVC), Eudragit RS 100, and microcrystalline cellulose (MCC). The application of percolation theory on the relationships between compactibility, P_max, or compression susceptibility (compressibility), γ, and mixture compositions reveals the presence of percolation thresholds even for mixtures of similar deformation properties. The results showed that all mixture compositions exhibited at least one discreet change in the slope, which was referred to as the percolation threshold. The PVC/Eudragit RS100 mixture compositions showed significant percolation threshold at 80% (w/w) PVC loading. Two percolation thresholds were observed from a series of binary mixtures containing similar plastic deformation materials (PVC/MCC). The percolation thresholds were determined at 20% (w/w) and 80% (w/w) PVC loading. These are areas where one of the components percolates throughout the system and the properties of the tablets are expected to experience a sudden change. Experimental results, however, showed that total disruption of the tablet physical properties at the specified percolation thresholds can be observed for PVC/lactose mixtures at 20-30% (w/w) loading while only minor changes in the tablets' strength for PVC/MCC or PVC/Eudragit RS 100 mixtures were observed.