Percolative charge transport in a co-evaporated organic molecular mixture

Understanding the charge transport in molecular semiconductor mixtures remains challenging, largely due to the lack of a universal dependence of carrier mobility upon doping concentration. Here we demonstrate that it is feasible to use the percolation theory to explain the change of charge mobility in a model system of 4,4′-bis(carbazol-9-yl)-biphenyl (CBP) and tris-(8-hydroxyquinoline) aluminum (Alq₃) with various doping concentrations. As the fraction of CBP within the mixtures increases, the charge mobility firstly shows a reduction at low CBP fraction due to the scattering effect, and then increases well following a percolation model. Electron microscopy and atomic force microscopy analysis suggest that CBP and Alq₃ are homogeneously mixed in their co-evaporated amorphous films, which meets the precondition for using percolation theory. We describe the possible microcosmic percolating mechanism with a model combining bond percolation with charge transfer integral calculation. Based on this model, the percolation threshold in molecular semiconductor mixtures can be predicated. For the hole and electron transport in our system, the predicated percolation thresholds are very close to the experimental values.     

Combination of Plasma Functionalization and Phase Inversion Process Techniques for Efficient Dispersion of MWCNTs in Polyamide 6: Assessment through Morphological,Electrical, Rheological and Thermal Properties

Different contents of multiwalled carbon nanotube (MWCNTs) functionalized by He-dielectric barrier discharge (DBD) plasma followed by an exposure to NH₃ were incorporated into PA6 matrix via a phase inversion based solution method. Optical and electron microscopic results were indicative of the excellent dispersion state of the MWCNTs. The differential scanning calorimetry and thermogravimetric analysis measurements revealed that even addition of a slight amount of the MWCNTs significantly increased the thermal stability and crystallization temperature. Moreover, the low electrical percolation threshold of the PA 6/functionalized MWCNTs nanocomposites was another confirmation for achieving a good dispersion state of MWCNTs using this approach.     

Flow induced orientation of multiwalled carbon nanotubes in polycarbonate nanocomposites: Rheology, conductivity and mechanical properties

We investigated the effect of flow field and deformation rate on the nanotube alignment and on the properties of PC/multiwalled carbon nanotube nanocomposites. Samples of various MWCNT loadings were prepared by diluting a commercial masterbatch containing 15 wt% nanotubes using optimized melt mixing conditions. Different processing conditions were then used to systematically change the degree of nanotube alignment, from random orientation to highly aligned. Morphological studies and Raman spectroscopy analysis revealed that the nanotubes are preferentially aligned in the flow direction, particularly at large injection or compression rates. Rheological measurements corresponding to high shear rate conditions showed drastic changes in the viscoelastic behavior. The complex viscosity significantly decreased and percolation threshold notably rose. High degrees of nanotube alignment also resulted in a significant increase in the electrical percolation threshold. The mechanical properties of the nanocomposites for different nanotube loadings were also shown to depend on the processing conditions, and somehow improved when the material was processed at higher rates. Finally, we used a power-law type equation to correlate the percolation behavior and the nanotube alignment. The estimated percolation threshold and the power index, q, significantly increase with the degree of nanotube alignment as determined by Raman analysis.     

Polypyrrole nanocoatings of poly(styrene-co-methacrylic acid) particles

In this study the properties of polypyrrole (PPy) nanocoating over poly(styrene-co-methacrylic acid) (PS-MAA) particles were investigated. Monodisperse PS-MAA templates were obtained by free surfactant emulsion polymerization. The addition of methacrylic acid into the monomer feed mixture reduced particle size, ionic charge and hydrophobicity of the template surface. Correlations between template sizes and compositions, PPy confinement (granularity, shell size, etc.) and electrical conductivity, σ, are discussed. After dissolving the PPy/PS-MAA composites in tetrahydrofurane, PPy void spheres are obtained proving the core-shell nature of the coated particles. Bare styrene templates show densely packed PPy coatings and electrical conductivities near 7 S cm⁻¹ at high PPy loadings; on the contrary, at the same PPy mass percentage, the richer methacrylic acid particles show low packed PPy grains and conductivities that fall to 0.8 S cm⁻¹. In core-shell particles the PPy mass per unit area, Γ, is the key parameter which determines the insulator-conductor transition for any particle size. The conductivity values of PPy/PS-MAA composites follow a percolation law: σ∝t(Γ−Γc), with a critical threshold Γ_c=(0.262 ± 0.002)×10⁻⁶ g cm⁻². The critical exponent obtained t = 1.98 ± 0.07 agrees with the predicted value t = 2.0 for three-dimensional lattices of random resistors.     

Improved mechanical properties of carbon nanotube/polymer composites through the use of carboxyl-epoxide functional group linkages

Single-walled and multi-walled carbon nanotubes (CNTs) were functionalized with carboxyl groups and dispersed in a polymer containing an epoxide group. We have then observed experimentally that mutual chemical reaction between the functional groups on the CNTs with the polymer epoxide group can enhance, two-fold, both the tensile strength and elastic modulus, E, of single walled CNT/polymer composites. A simple model was formulated to understand the variation of E with CNT volume fraction, considering agglomeration effects as well. An increase in the work of fracture, obtained from the experimental stress-strain curves, was seen at low nanotube filling fractions and is presumably due to crack bridging of the polymer matrix by CNTs. The influence of CNT length and geometry on mechanical properties, along with the influences of electrical and mechanical percolation thresholds was considered.     

A new approach for morphology control of poly(butylene adipate-co-terephthalate) and soy protein blends

In contrast to most studies in the literature where soy protein was used as a particulate filler in hydrophobic thermoplastic polymers, in this study, soy protein concentrate (SPC) was processed as a plastic to blend with poly(butylene adipate-co-terephthalate) (PBAT). By adjusting water content in the formulated SPC from low to high prior to compounding, SPC exhibited behaviors ranging from rigid filler to deformable filler to plastic during blending. Detailed phase morphology of the blends was revealed by transmission electron microscopy and field emission scanning electron microscopy. Evidences showed that SPC formed percolated thread structures when additional water was added to SPC prior to compounding. Study of dynamic rheology also confirmed the formation of interconnected network structure. Accordingly, tensile mechanical properties were greatly improved for those blends with percolated SPC thread structures. The molecular weight change of PBAT in blends and influence of processing on soy protein solubility were also examined.     

Mechanism of calcium uptake in corn kernels during the traditional nixtamalization process: Diffusion,accumulation and percolation

In the present work the results of the mechanism of calcium uptake in nixtamalized corn kernel were studied. The results showed that at a critical time, the calcium uptake in the pericarp suffers a drastic change; this fact cannot be associated only to the diffusive mechanism. The SEM analysis of processed corn kernels, taken at 1 h of steeping time illustrates a calcium cluster formation. It means that for short steeping times the calcium uptake in the total kernel is governed by an accumulation process in the most external layers of the pericarp and the diffusion process is not relevant. The apparition of micro-holes for critical time in dependence on the temperature indicates that another mechanism is contributing to the calcium uptake. These holes can pass thought the whole pericarp or form an interconnected lattice to develop a percolation path. The holes formations allow the entrance of water and calcium directly to the outermost layers of the endosperm and now the accumulation of calcium is not the predominant mechanism. It could be implied that, at this critical time the percolation of water and calcium thought these holes is the main mechanism to calcium uptake in the corn kernel and the diffusion process is not relevant.     

高含硫气藏气体渗流规律研究

在高含硫气藏开发过程中，随着高含硫气体从地层中采出，地层压力不断下降，使元素硫从含硫饱和态气相中析出，并沉积到地层孔隙或喉道中，造成地层渗流通道的严重堵塞，增大气体渗流阻力。文章从物质平衡原理出发，推导了由于受元素硫沉积影响的地层物性参数随生产时间等的变化关系，并建立了高含硫气藏气体渗流模型。通过实例模拟分析了该类气藏气体渗流过程中孔隙度和渗透率的变化规律以及地层压力等的变化特征。研究结果表明，元素硫在地层中的沉积对储层的伤害沿径向呈不均匀变化；气体的渗流阻力增加，地层能量更多地消耗在近井地带，且气体渗流表现为强烈的流固耦合特征。该认识为高含硫气田的合理高效开发提供了参考依据。     

Factors Influencing Microimage Quality

During the past ten years, the applications of microimages have been extended from mere security records to records in active use. As a result, it is desirable to use the highest-quality microimage system that is available. It has become necessary, therefore, to acquire a better understanding of the factors which control this quality. The purpose of this paper is to discuss the manner in which the nature of the original, the properties of the lens and the properties of the film influence the quality of the image. The application of the concept of modulation transfer function to the study is discussed.     

聚乙烯/碳黑复合导电材料的正电子谱学研究

在高密度聚乙烯（HDPE）中掺入碳黑（CB）可以显著提高其导电性能,而且碳黑含量的变化对导电性有很在影响。本文用正电子湮没技术研究了这种复合体系中自由体积随碳黑含量的变化规律,证实了碳黑颗粒处于非晶区的观点,并结合电阻率测量和差热分析的实验结果探讨了渗流阈值以及结晶度等问题。