Densification study of ITO films during high temperature annealing by GISAXS

Three thermal routes were treated on the sol-gel ITO films, i.e. conventional thermal annealing (CTA), rapid thermal annealing (RTA) and thermal cycle annealing (TCA). The near surface and internal structures of films were characterized by grazing incidence small angle X-ray scattering (GISAXS) technique. It is found that slit-like pores show fractal structures laterally and the near surface is sparser with bigger pores. Ordered pore structure normal to the film appears when films are annealed at high heating rate. The shrinkage of pores is mainly owing to structural relaxation and diffusion during the superheating process. However, the supercooling process has no significant effect on the structures. Furthermore, CTA samples have the greatest porosity and surface roughness due to the prevailing crystallization as well as the coarsening procedure. However small pores inside the films are eliminated at low temperature.     

Phonon scattering from the boundaries of small crystals embedded in a dielectric porous-glass matrix

The thermal conductivity of porous glass with randomly distributed connecting pores ～70 Å in size (glass porosity ～25%), as well as of a porous glass + NaCl composite, was measured in the temperature range 5–300 K. NaCl filled one fourth of the pores in the composite. The experimental results on the composite thermal conductivity can be accounted for only by assuming that phonons scatter from the boundaries of NaCl nanocrystals embedded in channels of the porous glass.     

Anomalous surface diffusion of water compared to aprotic liquids in nanopores

1H nuclear magnetic relaxation dispersion experiments show remarkable differences between water and acetone in contact with microporous glass surfaces containing trace paramagnetic impurities. Analyzed with surface relaxation theory on a model porous system, the data obtained for water show that proton surface diffusion limited by chemical exchange with the bulk phase permits long-range effectively one-dimensional exploration along the pores. This magnetic-field dependence coupled with the anomalous temperature dependence of the relaxation rates permits a direct interpretation in terms of the proton translational diffusion coefficient at the surface of the pores. A universal rescaling applied to these data collected for different pore sizes and on a large variety of frequencies and temperatures, supports this interpretation. The analysis demonstrates that acetone diffuses more slowly, which increases the apparent confinement and results in a two-dimensional model for the molecular dynamics close to surface relaxation sinks. Surface-enhanced water proton diffusion, however, permits the proton to explore a greater spatial extent of the pore, which results in an apparent one-dimensional model for the diffusive motions of the water that dominate nuclear spin relaxation.     

Fractal dimension of cluster boundaries in porous polycrystalline HTSC materials

The fractal dimension of the boundaries of clusters formed by pores and granules in polycrystalline materials is shown to be determined by the sample density and crystallite sizes. The dependence of the fractal dimension on the density has a maximum. It is shown that the maximum diamagnetic response can be obtained in a porous high-temperature superconductor with a porosity of 50?C60% and small crystallite sizes.     

基于微结构参数建模的多孔硅绝热层热导率研究

多孔硅由于具有较低的热导率,因而可以将其作为半导体器件中的绝热层.与其他从边界散射等复杂微观热传导机制出发建模研究多孔硅的热导率不同,将多孔硅热导率影响机制更表观地归结到孔洞的存在和分布等结构因素上,把整个多孔硅视为由硅连续材料介质和孔洞连续介质通过串联和并联组合成的复合微结构,给予其低热导率一个更为易于理解和简化的解释.进一步把孔隙率对等效热导率的影响分解为两个不同的部分,即纵向部分和横向部分,半定量地给出不同的孔洞结构和分布下孔隙率与等效热导率的关系.与实验数据进行对比后验证了模型的有效性.继而从结构的角度说明了多孔硅热导率较低的原因.     

Theoretical analysis of thermal rectification in a bulk Si/nanoporous Si device

We present a theoretical analysis of thermal rectification in a porous Si/bulk Si device, taking into account ballistic effects in phonon-pore collisions when phonon mean free path is much longer than the radius of the pores. Starting from an approximate analytical expression for the effective thermal conductivity of porous Si, we obtain the thermal rectifying coefficient of the device as a function of porosity, pore size, temperature interval, and relative lengths of porous and bulk samples.     

Enhanced anomalous diffusion of sputtered atoms in nanosized pores

Plasma sputtering deposition of platinum catalysts in porous anodic aluminum oxide (AAO) templates is shown to generate an anomalous superdiffusion concentration profile. The growth of an overlayer between the hexagonal array of pores is shown to enhance the diffusion into the pores, leading to a diffusion coefficient having superlinear time dependence. The Pt clusters in the pores have a mean size of 10 nm and almost concentrate on the inner pore surface, and are present up to a depth of 6 μm.     

碳/碳化硅复合材料静止环境下氧化行为模拟

碳/碳化硅(C/SiC)复合材料是应用于临近空间高超声速飞行器热防护的一种新型防热材料.国内外通过性能测试较多地研究了材料不同制备工艺对抗烧蚀性的影响,提出的抗烧蚀分析理论模型均基于液态氧化膜.而近期开展的C/SiC复合材料管式炉加热实验和试样微观形貌电镜表征显示:常压下,当温度低于1696 K时,C/SiC复合材料氧...     

Surface diffusion and entrapment of simple molecules on porous silica

Interactions of simple molecules with the surface of porous silica have been investigated using time-of-flight secondary ion mass spectrometry and temperature programmed desorption. A monolayer of water diffuses into pores at temperatures higher than 110 K. Multilayers of water are also incorporated in pores via sequential surface diffusion. In contrast, a methanol monolayer tends to stay on the surface up to 150 K, and carbon dioxide diffuses into pores rather gradually. Results can be explained as the contribution of hydrogen bonds between the adsorbate–substrate and adsorbate–adsorbate interactions. The predominance of the former (latter) might be responsible for single-molecule migration of methanol and carbon-dioxide (collective diffusion of water molecules) on the surface. These molecules are entrapped at higher coordination sites in pores, as revealed from thermal desorption peaks appearing at higher temperatures than those from non-porous silica. However, no significant difference is observed in desorption kinetics of CF₂Cl₂, Kr, CH₄, and N₂ molecules between the porous and non-porous silica substrates.     

An Insight into the Diffusion of Unsaturated Polyester (UP) Resin into Expanded Graphite (EG) to Improve the Mechanical Properties of the UP/EG Composites

Abstract

Polymer/expanded graphite (EG) nanocomposites have great importance in many industrial applications mainly due to their high electrical/thermal conductivity or flame retardancy. However, to fully employ the benefits of polymer/EG nanocomposites one must consider the high degree of porosity of EG. The high degree of porosity of EG can deteriorate the composites’ mechanical properties if the polymer chains cannot diffuse completely into the EG pores. In this article, an insight is given into the diffusion of unsaturated isophthalic polyester (UP) resin, consisting of a combination of maleic anhydride and isophthalic anhydride in the resin backbone, with two viscosities, into the pores of the EG particles of various degrees of porosity. The diffusion experiments were carried out on compressed EG tablets with the same density but different porosity due to the different porosity of the EG particles. The results showed that the diffusion rate of the UP resin with higher viscosity slightly decreased when the EG porosity decreased but, in the opposite way, it strongly increased for the low viscosity UP resin. The EG nanocomposites samples were molded at varying pressures. The micrographs of the fractured surfaces of the EG nanocomposites showed that the EG pores were not filled with resin, thus the EG nanocomposites had residual pores. It was found that composites containing EGs with higher expansion ratio and larger particles and pores showed larger residual pores. Furthermore, the composites prepared with the more viscous UP resin showed more residual pores. By applying a pressure of 10?bar instead of 1?bar, a reduction of 7–20% in the residual pores of the nanocomposites was observed which led to improved mechanical properties by up to 20% in flexural strength for the EG with the highest expansion ratio.     

Correlation effects during liquid infiltration into hydrophobic nanoporous media

To explain the thermal effects observed during the infiltration of a nonwetting liquid into a disordered nanoporous medium, we have constructed a model that includes correlation effects in a disordered medium. It is based on analytical methods of the percolation theory. The infiltration of a porous medium is considered as the infiltration of pores in an infinite cluster of interconnected pores. Using the model of randomly situated spheres (RSS), we have been able to take into account the correlation effect of the spatial arrangement and connectivity of pores in the medium. The other correlation effect of the mutual arrangement of filled and empty pores on the shell of an infinite percolation cluster of filled pores determines the infiltration fluctuation probability. This probability has been calculated analytically. Allowance for these correlation effects during infiltration and defiltration makes it possible to suggest a physical mechanism of the contact angle hysteresis and to calculate the dependences of the contact angles on the degree of infiltration, porosity of the medium, and temperature. Based on the suggested model, we have managed to describe the temperature dependences of the infiltration and defiltration pressures and the thermal effects that accompany the absorption of energy by disordered porous medium-nonwetting liquid systems with various porosities in a unified way.     

Simulation of porous silicon formation and silicon epitaxy on its surface

Processes of porous silicon formation and silicon epitaxy on its surface are studied using the Monte Carlo method. The model for porous silicon formation under anode etching allows for non-uniformity of charge distribution over the silicon-electrolyte interface. Processes of diffusion, generation and recombination of holes, as well as dimensional quantization, are also considered. Gilmer's model, extended to the case of a rough surface, is used to study epitaxy. The structures obtained by simulations at different levels of doping of the crystal substrate and for various parameters (temperature, HF concentration, and anode current density) are presented. Analysis of nanoporous structures showed that the porosity changes with depth, and fractal dimensionality exists below 10 nm. It has been shown that epitaxy, developing by formation of metastable nuclei at the edges of pores, by their subsequent growth along the perimenter and by formation of a thin continuous overhanging layer, may be described within the framework of this model. Three-dimensional images of near-surface layers formed at different stages of epitaxy have been obtained. The dependence of the epitaxy kinetics on the amount of deposited silicon for different structure porosities has been revealed. Institute of Physics of Semiconductors, Siberian Branch, Russian Academy of Science. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 3, pp. 49–56, March, 1999.     

Surface terracing on ferritic stainless-steel fibres and potential relevance to in vitro cell growth

Since many natural and biological materials are cellular, often with relatively high porosity levels (although sometimes these pores are partly filled with a fluid or a soft and compliant solid in the natural state), various types of porous materials are of interest for biological applications. A key feature for such applications is the space afforded for invasion, first by cells and ultimately by osseous tissue and vasculature. The surface should be chemically and topologically suitable for cells to penetrate and interlock. There is evidence that fine scale topographic features can affect both the adhesion and ingrowth of cells. One way of creating topographic features, such as terraces, is to employ suitable heat treatments so as to expose preferentially the low surface energy crystallographic planes via surface diffusion. The topography and crystallography of surface terraces, generated on solid-state-sintered ferritic stainless-steel fibre networks, have been characterised by electron back-scattered diffraction, atomic force microscopy and scanning electron microscopy. Initial work on the effect of these fine scale topographic features on cell proliferation shows encouraging results.     

On the porosity of coldly condensed sers active Ag films: II. Comparison of adsorption and Raman scattering of pyridine

The adsorption of pyridine on coldly deposited Ag films annealed at temperatures ranging from 58 to 330 K, the porous surface topography of which has been investigated in part I of this work, has been studied by means of UPS, work function change and thermal desorption measurements. Pyridine induced work function changes have been employed to follow the surface diffusion of pyridine molecles into the pores of these Ag films. The surface diffusion is very slow below 60 K, but readily takes place at 130 K with an estimated activation energy of surface migration of E_m ≈ 4 kcal/mol. Preadsorption of Xe into the pores of the films causes inhibition of pyridine diffusion into the pores. The onset of pyridine desorption from porous films is detected at ≈ 200 K while from flat films the desorption begins already at 150 K. The careful analysis of our data on the structure of the coldly deposited Ag films and the adsorption behavior of pyridine on these films as well as a survey of published SERS data lead us to conclude that the SERS active sites of coldly deposited Ag films are within the pores. This conclusion is in agreement with recent theoretical calculations.     

Effect of porosity on the thermoelectric efficiency of PbTe

The effect of electron and phonon scattering on nanometer-sized pores on the thermoelectric properties of lead telluride has been studied theoretically. Estimations show that the thermoelectric efficiency can increase by 20–25% at room temperature and by 5–10% at 600 K at the optimal pore size of several nanometers and the porosity of ~10%. An analysis shows that the increase in the thermoelectric efficiency due to additional scattering in the porous material is related to the decrease in the thermal conductivity of the lattice and the increase in the thermoelectric coefficient due to the change in the energy dependence of the relaxation time. To estimate the multiple scattering at high pore concentration, the lattice thermal conductivity by the molecular-dynamics method and the electron free paths in a coherent potential approximation were calculated. It is shown that the inclusion of the multiple scattering slightly influences the thermoelectric properties at noted sizes and pore concentrations.     

Fast magic-angle spinning proton NMR studies of polymers at surfaces and interfaces

Solid-state proton NMR with fast magic-angle sample spinning has been used to study the structure and dynamics of polymers and the water interface in porous glass composites. The composites were prepared by photopolymerization of poly(ethyl acrylate) and other acrylate formulations in a high surface-area rigid glass matrix with 40-A interconnected pores. High resolution solid-state proton spectra were obtained for polymer films and composites with 15 kHz magic-angle sample spinning at temperatures above the polymer glass transition temperature. The solid-state proton spectra can be detected with high sensitivity and used to determine the composition of polymer and water filling the pores. These results and spin diffusion studies using 1H-29Si 2D heteronuclear correlation and wideline separation NMR show that the polymer fills the central 30 A of the pore, and that the remaining volume is filled with surface hydroxyl groups and water.     

多孔硅光致发光的温度效应研究

通过对多孔硅光致发光峰随测量温度变化的研究，发现随测量温度的下降，光致发光峰位有两种截然不同的移动方向：发光峰中心波长较长的样品，主峰向低能方向移动(即红移)；而发光峰位波长较短的样品则向高能方向移动(即蓝移).根据多孔硅光致发光峰的温度效应，定性地给出了发光效率随波长变化的模拟曲线，并由此能较好地解释多孔硅光致发光峰位随温度变化而移动的实验现象. 关键词：     

Prediction of effective stagnant thermal conductivities of porous materials at high temperature by the generalized self-consistent method

The generalized self-consistent method is developed to deal with porous materials at high temperature, accounting for thermal radiation. An exact closed form formula of the local effective thermal conductivity is obtained by solving Laplace's equation, and a good approximate formula with uncoupled conductive and radiative effects is given. A comparison with available experimental data and theoretical predictions demonstrates the accuracy and efficiency of the present formula. Numerical examples provide a better understanding of interesting interaction phenomena of pores in heat transfer. It is found that the local effective thermal conductivity divides into two parts. One, attributed to conduction, is independent of pore radius for a fixed porosity and, furthermore, is independent of temperature (actually, it is approximately independent of the temperature) if it is non-dimensionalized by the thermal conductivity of the matrix. The other is due to thermal radiation in pores and strongly depends on the temperature and pore radius. The radiation effect can not be neglected at high temperature and in the case of relatively large pores.     

Restricted diffusion and exchange of water in porous media: average structure determination and size distribution resolved from the effect of local field gradients on the proton NMR spectrum

NMR Pulsed field gradient measurements of the restrained diffusion of confined fluids constitute an efficient method to probe the local geometry in porous media. In most practical cases, the diffusion decay, when limited to its principal part, can be considered as Gaussian leading to an apparent diffusion coefficient. The evolution of the latter as a function of the diffusion interval yields average information on the surface/volume ratio of porosities and on the tortuosity of the network. In this paper, we investigate porous model systems of packed spheres (polystyrene and glass) with known mean diameter and polydispersity, and, in addition, a real porous polystyrene material. Applying an Inverse Laplace Transformation in the second dimension reveals an evolution of the apparent diffusion coefficient as a function of the resonance frequency. This evolution is related to a similar evolution of the transverse relaxation time T2. These results clearly show that each resonance frequency in the water proton spectrum corresponds to a particular magnetic environment produced by a given pore geometry in the porous media. This is due to the presence of local field gradients induced by magnetic susceptibility differences at the liquid/solid interface and to slow exchange rates between different pores as compared to the frequency differences in the spectrum. This interpretation is nicely confirmed by a series of two-dimensional exchange experiments.