气体吸附法测定二氧化硅干凝胶的分形维数

提出了一种方便、科学有效的利用气体吸附法测定二氧化硅干凝胶等多孔材料分形维数（表面分形维数和孔分布分形维数）的方法，不需要进行一系列的吸附/脱附实验，只需要利用单一气体的一次吸附/脱附实验得出的样品孔分布、比表面数据，与不同的标尺进行关联，即可同时获得表面分形维数和孔分布分形维数.通过误差分析和校正，保证了结果的可靠性.用上述方法测定了二氧化硅干凝胶的分形维数，以FHH法和SAXS法对所得结果进行了比较和验证，并对吸附/脱附过程所得结果的差异进行了初步分析. 关键词： 分形维数 气体吸附 二氧化硅 干凝胶     

溅射过程中粒子能量对钛薄膜表面形貌影响

借助于原子力显微镜研究了离子束溅射沉积工艺中入射离子能量对制备的Ti薄膜表面形貌的影响。对薄膜表面高度数据进行相关运算，发现在此工艺条件下制备的薄膜具有典型的分形特征，利用分形表面高度—高度相关函数的唯象表达形式对不同能量下制备Ti薄膜表面的高度相关函数进行拟合。得到了薄膜表面的分形维数、水平相关长度、标准偏差粗糙度等参量。研究发现，入射Ar离子能量在300—700eV之间薄膜表面的粗糙度随着沉积粒子的能量增加而增大，分形维数随着入射离子能量的增加而减少。另外，在得到的分形维数基础上对不同溅射电压下Ti薄膜的生长机制进行了初步研究。     

KDP晶体单点金刚石车削表面形貌分形分析

 分别使用2维和3维分形方法对单点金刚石车削加工的KDP晶体表面形貌进行了分析，并对表面的3维分形维数和3维粗糙度表征参数进行了比较，分析了二者对表面形貌表征的差异。使用2维轮廓分形方法计算了KDP晶体表面圆周各方向上的分形维数。通过分析得出：3维分形维数与表面粗糙度值成反比关系；使用单点金刚石车削方法加工KDP晶体会形成各向异性特征明显的已加工表面，在一定程度上容易形成小尺度波纹；已加工表面是否具有明显的小尺度波纹特征与表面粗糙度值并无直接关系，但与其表面轮廓分形状态分布密切相关；KDP晶体表面2维功率谱密度与其分形状态具有相近的方向性特征。     

利用分形理论研究气液界面特性

利用分形理论对Lennard-Jones12-6流体气液界面性质进行了研究。得到了密度分布及温度分布,将表面张力 与气液界面的分形维数进行关联。分形维数是界面层所固有的,它与界面的其它特性有着本质的联系,因此,可以通过测 量界面层的分形维数,再利用此分形维数确定界面的其它特性。这种全新的测量界面特性的方法有望由此产生。     

空温式翅片管气化器结霜模型及数值模拟

以分形理论的DLA模型为基础,建立了空温式翅片管气化器深冷表面上霜晶生长的二维模型,模拟了深冷表面上的霜晶生长过程.采用计盒维数法对模拟出的霜晶生长图像进行了分形维数的计算,结果表明深冷表面上霜晶的分形维数较一般冷表面上的分形维数大,从而说明深冷表面上的霜晶具有更加复杂的结构,充满空间的能力更大.这对进一步理解空温式深...     

基于分形维数表征的跨尺度拼接方法

针对跨尺度数据之间尺度差异导致拼接困难、精度低等问题,提出了一种使用分形维数表征尺度的跨尺度数据拼接方法。利用离散小波变换对原始数据进行多尺度分解,获取其在多个尺度上的近似数据,并采用分形维数对数据之间的尺度差异进行表征与衡量,找出跨尺度数据在相近尺度上的小波变换数据;而后通过迭代最近点算法对获取的尺度近似数据进行拼接。最终将所获得到的转换关系应用于原始数据,完成跨尺度数据的拼接。对获取的不同尺度下的三维表面形貌数据进行分形维数的计算,结果表明,分形维数能有效表征以及衡量尺度参数。跨尺度拼接结果表明,所提出的方法能有效地实现跨尺度的数据拼接。     

粗糙表面形貌对湿润性的影响

采用自仿射分形对粗糙表面形貌进行了定量描述。基于分形粗糙表面形貌,建立了粗糙表面润湿性的理论模型并进行了数值计算,分析讨论了均方根粗糙度和表面分形维数对接触角的影响。研究结果表明,均方根粗糙度对接触角的影响较大,而粗糙表面的分形维数对润湿性的影响则并不明显。     

科赫分形基底上受限固-固模型动力学标度行为的数值研究

为分析基底结构对离散生长模型动力学性质的影响, 本文在随机游走指数十分接近而分形维数和谱维数均不相同的科赫格子和科赫曲线分形基底上对受限固-固(restricted solid-on-solid)模型的生长过程进行数值模拟研究. 通过分析表面宽度和饱和表面极值高度的统计行为发现: 随机游走的动力学指数能够对饱和粗化表面的动力学行为起主要贡献. 尽管分形基底具有不同的分形维数和谱维数, 但是在两种分形基底上得到了在误差范围内相同的粗造度指数. 两种分形基底上饱和表面相对生长高度极大(小)值分布分别可以很好的塌缩在一起, 且很好的满足Asym2Sig函数分布.     

基于分形特征的复杂环境目标检测方法研究

针对复杂的自然背景下的运动目标检测,提出了一种基于分形特征的运动目标检测算法;该算法利用目标的分形维数与自然背景分形维数的差异将目标从背景检测出来;首先应用改进的地毯覆盖法快速得到图像的分形维数,然后通过比较邻域之间分形维数的相互关系进行目标检测;实验结果表明,该方法能对复杂背景下的运动目标进行检测,由于采用分块求分形特征的方法,能有效地减少搜索目标所带来的计算量,算法过程简单、检测速度快、检测结果精确,目标与背景对比度的变化对检测结果几乎没有影响, 且噪声对该算法的检测结果影响较小;在运动目标实时检测问题上有着很好的实用价值和应用前景。     

纳米流体对流换热机理分析

考虑在纳米流体中纳米颗粒做布朗运动引起的对流换热, 基于纳米颗粒在纳米流体中遵循分形分布, 本文得到纳米流体对流换热的机理模型. 本解析模型没有增加新的经验常数, 从该模型发现纳米流体池沸腾热流密度是温度、纳米颗粒的平均直径、 纳米颗粒的浓度、纳米颗粒的分形维数、沸腾表面活化穴的分形维数、基本液体的物理特性的函数. 对不同的纳米颗粒浓度和不同的纳米颗粒平均直径与不同的实验数据进行了比较, 模型预测的结果与实验结果相吻合. 所得的解析模型可以更深刻地揭示纳米流体对流换热的物理机理.     

Adsorption mechanisms of translationally-energetic O2 and N2: direct dissociation versus direct molecular chemisorption

A direct dissociation mechanism has been traditionally assigned to molecular beam data that exhibit an increase in the initial adsorption probability with increasing kinetic energy. Yet, recent experiments of nitrogen and oxygen adsorption provide support for an alternative high kinetic energy pathway in which incident energy assists in surmounting barriers to molecular chemisorption on a surface as the first step to dissociation. Moreover, systems for which the experimental evidence supports such a mechanism also demonstrate that molecularly chemisorbed intermediates can be spectroscopically observed at low temperatures and coverages from exposure to a gas in thermal equilibrium at room temperature. Likewise, such observations have not been measured for systems which are consistent with direct dissociation. A consideration of this trend regarding the existence of molecularly chemisorbed states and the implications for the dominant, dissociative chemisorption pathway at high kinetic energy is presented for a number of gas surface systems.     

Simulation study of argon adsorption on (0 0 1) faces of phyllosilicates

Grand Canonical Monte Carlo molecular simulations have been performed for argon and nitrogen adsorption on the basal surfaces of phyllosilicates without surface cations. The results have been compared with derivative isotherms analysis of experimental data. An optimization of the surface-Ar interaction has been performed by varying the oxygen atom LJ ?/k_B parameter and the optimized value was used to perform the nitrogen adsorption simulations. The analysis of the argon adsorption simulation indicates that adsorption mechanisms are more complex than may be suggested by experimental results obtained by low-pressure adsorption. The structure of the adsorbed film has a marked dynamic behaviour and the monolayer capacity strongly depends on the equilibrium relative pressure. For nitrogen adsorption, while high pressure behaviour is simulated adequately, some deviation is observed in low-pressure region of the isotherms suggesting that additional simulation and perhaps the use of a more sophisticated potential to model the nitrogen molecule can be necessary to understand fully the behavior of this gas on clay minerals.     

Porosity of microporous zeolites A, X and ZSM-5 studied by small angle X-ray scattering and nitrogen adsorption

Small-angle X-ray scattering (SAXS) using synchrotron radiation and nitrogen adsorption have been applied to characterizations of porosities and microporous structures for the zeolites of NaA, KA, CaA, NaX and ZSM-5. Besides the information on the external morphology of the particles of the zeolites, the complementation of the two techniques has revealed rich and consistent structural and surface information on the molecular scale crystalline pores of these zeolites. Analyses of the data suggest that the determined sizes of the micropores imply the pore spaces occupied by the probe molecules of water in the SAXS and nitrogen in adsorption techniques, respectively. The microporous information of NaA and KA are difficult to obtain from nitrogen adsorption, due to the blocking of nitrogen by their narrow channels, but have been satisfactorily measured by SAXS. The factors causing variations of the measured values of the parameters in different analysis methods have been discussed.     

Diffusion hysteresis in mesoporous materials

The pulsed field gradient NMR method has been used to study molecular diffusion of adsorbate molecules in mesoporous materials with different pore morphologies as a function of the external gas pressure. The obtained experimental results reveal that in line with the adsorption hysteresis, the measured diffusivities as well exhibit a history-dependent behavior, i.e. diffusivities on the adsorption branch do not follow those measured on the desorption branch. Particular details of the diffusion hysteresis loop depend on the structure of the used porous material. New insight into the mechanisms of adsorption hysteresis, namely indication of two-step relaxation process in the hysteresis region, is gained by comparing the dynamical properties of the adsorbate obtained under equilibrium and non-equilibrium conditions.     

二元致冷剂气体水合物相平衡计算

本文在测定 ＨＦＣ１５２ａ／ＨＣＦＣ１４１ｂ和 ＨＦＣ１３４ａ／ＨＣＦＣ１４１ｂ 二元气体水合物相平衡数据的基础上［３］,建立二元混合气体水合物计算模型和方法。利用 Ｗｉｌｓｏｎ活度系数理论和Ｌａｎｇｍｕｉｒ等温吸附理论建立了水合物相各组成的逸度计算模型,并依据水合物中客体分子的逸度平衡条件进行了混合水合物的相平衡计算。计算结果正确地表征混合气体水合物相平衡特性,并与实验数据较精确吻合。     

Nanostructured Silica-Gel Doped with TiO2 for Mercury Vapor Control

A novel high surface area SiO₂–TiO₂ composite has been developed for elemental mercury vapor removal from combustion sources. The composite exhibits synergistic adsorption and photocatalytic oxidation. Mercury vapor in the gas stream is adsorbed, oxidized and stays on the composite. The composite has demonstrated a high mercury capacity (1512ug/g) although in its current 3-mm pellet form only the outer layer is effectively utilized. The loading of 13% TiO₂ shows the best removal, both with and without UV irradiation. Increasing TiO₂ loading beyond this level does not enhance the removal further. It has also been observed that the composite after being 'activated' by photocatalytic oxidation has better performance, probably due to the change of surface functional groups. The examination of the effects of flow velocity reveals that mass transfer is the rate limiting step. Relative humidity has been found to impede adsorption therefore decreasing the overall removal efficiency. By rinsing with acid, both the deposited mercury and composite can be regenerated.     

Characterization of partially sintered ceramic powder compacts using fluorinated gas NMR imaging.

We use nuclear magnetic resonance (NMR) imaging of C2F6 gas to characterize porosity, mean pore size, and permeability of partially sintered ceramic (Y-TZP Yttria-stabilized tetragonal-zirconia polycrystal) samples. Conventional measurements of these parameters gave porosity values from 0.18 to 0.4, mean pore sizes from 10 nm to 40 nm, and permeability from 4 nm(2) to 25 nm(2). The NMR methods are based on relaxation time measurements (T(1)) and the time dependent diffusion coefficient D(Delta). The relaxation time of C2F6 gas is longer in pores than in bulk gas and it increases as the pore sizes decrease. NMR yielded accurate porosity values after correcting for surface adsorption effects. A model for T(1) dependence on pore size that accounts for collisions between gas molecules and walls as well as surface adsorption effects is proposed. The model fits the experimental data well. Finally, the long time limit of D(Delta)/D(o), where D(o) is the bulk gas diffusion coefficient is useful for measuring tortuosity, while the short time limit was not achieved experimentally and could not be used for calculating surface-area to volume (S/V) ratios.     

Studies on ordered mesoporous materials for potential environmental and clean energy applications

Two series of ordered mesoporous materials, SBA-15 silica and CMK-3 carbon were synthesized. The ordered nanostructure of these materials was confirmed by TEM and XRD analysis. Structural parameters including the specific surface area, pore volume and pore size distribution were determined on the basis of nitrogen adsorption data at 77 K. Potential applications of these materials were explored in relation to the CO₂ sequestering, methane storage and fuel desulfurization. Initial studies of both materials showed their usefulness for environmental and clean energy applications. SBA-15 modified with triethanolamine showed a very good adsorption selectivity for CO₂ while its adsorption reversibility was retained. Also, this material after CuCl deposition was useful for removal of fuel thiophenes. However, CMK-3 was shown to be promising material for storage of natural gas. As high as 41 wt.% of methane was stored in this material in the presence of appropriate amount of water.     

Co-extraction of water vapor and helium from natural gas

A study of the sorption properties of a composite sorbent prepared from pseudoboehmite and synthetic sodium-borosilicate glass microspheres was performed with the aim of using the sorbent in membrane-sorption processes of helium extraction from natural gas with its simultaneous drying. Experimentally, permeability of the composite sorbent under study with respect to helium and its impermeability to air and methane has been demonstrated. Under experimental conditions, the absolute moisture content of the gas mixture having passed through the sorbent has reduced from 21.1 to 0.013 g/m³. The rate of helium adsorption by the composite sorbent has increased nearly by two orders of magnitude in comparison with the initial microspheres. It was found that the degree of saturation of the sorbent with water vapor had almost no influence on the rate of helium adsorption. A possibility of optimal use of the composite sorbent by combining the process of natural-gas drying from water vapor and the process of helium extraction from natural gas is shown. This possibility permits shortening of the process sequence for natural gas pre-conditioning prior to helium extraction.