Structural and Mechanical Properties of Nanoporous Silica

Nanoporous silica systems with porosity between 30% and 70% were developed using two Molecular Dynamics (MD) simulation protocols to obtain structures with dissimilar pore morphologies. Short‐ and medium‐range structural characteristics including bond angle distributions and pair distribution functions were analyzed and found to be consistent with experimental results. Surface area to volume ratio and pore microstructures were characterized and compared with experimental observations. Mechanical properties including elastic, shear, and bulk moduli of these nanoporous silica systems were calculated and their change as a function of porosity was compared with experimental data and theoretical models. It was found that the elastic modulus of porous silica with 50% porosity is 5–14 GPa which is consistent with experimental results. The elastic moduli–porosity relationship was fitted by exponential and power functions, and analysis of coefficients was performed to obtain microstructure characteristics of the simulated nanoporous silica structures. This works confirms that two distinct nanoporous silica microstructures are generated with MD simulations which result in variations in mechanical properties and highlight the importance of selecting a nanoporous silica simulation method which approximates experimental systems.     

Dynamic Young's modulus of open-porosity titanium measured by the electromagnetic acoustic resonance method

As biological implants, porous titanium with adjustable mechanical properties can solve the stress-shielding effect. In this paper, porous titanium was prepared by the powder metallurgy method, where urea powders as the second phase were removed by heat treatment. Pore morphology (such as pore size and character) was controlled by the character of urea powders. The dynamic Young's moduli of such porous titanium with different morphology was measured by the electromagnetic acoustic resonance method. From the semi-log plots of Young's modulus versus the porosity, it was found that with increased porosity this modulus firstly decreases linearly, then decreases rapidly and goes to zero at certain porosity. However, the Young's modulus was independent of pore size. The relationship between Young's modulus and the porosity was explained by a parallel model based on the Minimum Solid Area method. The value of linear slop `b' and the percolation limit `P_C' were used for predicting the trend of Young's modulus varied with the porosity and pore size. So porous titanium with appropriate Young's modulus can be chosen as a candidate for bone substitutes.     

Young's modulus of isotropic porous materials with spheroidal pores

Based on the Eshelby solution for the single-inclusion problem and Wu's specification of this solution to spheroidal pores, we show that the Eshelby–Wu coefficients for Young's modulus, in contrast to their counterparts for the bulk and shear moduli, are quite insensitive to changes of the Poisson ratio. Therefore the Eshelby–Wu coefficients of Young's modulus can be described (to a very good approximation) by a unique function of the aspect ratio, which is calculated in this paper and for which a master curve is obtained by segment-wise fitting. Also the implementation of the Eshelby–Wu coefficients into the well-known effective medium approximations (Maxwell, self-consistent, differential) and our exponential relation is discussed. Irrespective of the model into which the Eshelby–Wu coefficients are implemented, prolate pore shape affects the porosity dependence of Young's modulus only very weakly, whereas oblate pore shape can lead to an arbitrary reduction of Young's modulus.     

Numerical study of pore structure effect on SO2-CaO reactions☆

The effect of varying pore structures on the kinetics of SO2–CaO reactions is not fully understood in the previous studies. Combining fractal pore model, gas molecular movement model and two-stage reaction model, a new desulfurization model is established in this paper. Fractal pore model is used to simulate CaO particle and gas molecular movement model is used to simulate gas diffusion in pores. Fractal dimension is used to characterize complexity of pore structure instead of tortuosity factor. It is found that the reaction is significantly affected by pore structures. A modulus?is introduced to characterize the relationship between varying pore structures and apparent reaction parameters. And this relationship is verified by thermo-gravimetric analysis (TGA) data. Comparing to the previous models, the effect of varying pore structure on the kinetics of the reaction is described more accurately by the desulfurization model.     

玄武岩纤维混凝土应力应变关系及孔结构分析

为了探究纤维掺量对玄武岩纤维混凝土宏、微观性能的影响规律,采用数字图像相关方法与微观气孔结构分析仪对不同纤维掺量的混凝土试件进行试验研究,通过数据回归建立了纤维掺量与平均峰值应力之间关系,给出了玄武岩纤维混凝土单轴受压应力-应变曲线上升段表达式;结合微观孔结构,建立了平均峰值应力与含气量、平均气泡弦长、气孔间距、比表面积相关公式,同时采用分形维数定量描述微观孔结构与玄武岩纤维混凝土抗压强度、弹性模量的关系.结果表明:玄武岩纤维的加入能够提高混凝土平均峰值应力和峰值应变,对试件应力-应变关系曲线上升段改变不明显;玄武岩纤维的加入能够减少混凝土内部缺陷,降低混凝土含气量、减小平均气泡弦长,使混凝土内部孔结构得到优化,孔隙分布较为均匀,抗压强度提高,割线模量有所下降.     

Influence of pore former on porosity and mechanical properties of Ce0.9Gd0.1O1.95 electrolytes for flue gas purification

Single layered porous Ce_0.9Gd_0.1O_1.95 electrolytes were fabricated by tape casting using different types, shapes and sizes of pore formers and their respective strength and stiffness were compared. The sintered bodies were characterized by scanning electron microscopy, mercury porosimetry, impulse excitation technique (Young modulus) and flexural strength measurements, to investigate the role of the different pore formers on the properties of the compounds. The compared techniques used to evaluate porosity give consistent results. The ratio between open and total porosities, evaluated from mercury porosimetry, varies depending on the used pore formers. The stiffness and strength of the compounds show an exponential dependency to the total porosity. By considering the open porosity instead (functional porosity), we observe that samples with platelets shaped pore formers have higher in-plane strength than spherical pore formers. An optimum can be found in term of Weibull strength and strain of samples obtained with the various pore formers by considering the dependency on the functional open porosity instead of the total porosity.     

Influence of Processing Methods on Mechanical and Structural Characteristics of Vacuum Microwave Dried Biopolymer Foams

Control of physical and mechanical properties of biopolymer (derived from food hydrocolloid) porous solids in terms of stress strain relationship during compression, porosity and pore size would enable their use for a wider range of purposes. Different types of dried cellular biopolymer foams were produced using different food hydrocolloids such as locust bean and alginate gums, gelatin, low and high methoxy pectin, methyl cellulose and starches (corn and tapioca) at various proportions. First different types of wet hydrogels were prepared by varying gel processing methods. Then they dried using microwave energy under vacuum called vacuum microwave drying. Before performing the drying process the initial Young's modulus of the hydrogels was measured. Pore size analysis and distribution percentage were done using mercury pore size analyser after drying. Relationship between the pore size distribution after drying and the initial Young's modulus was developed. Compressive test was performed for dried porous solids and true stress strain relationship curves were developed to classify nature of dried foams obtained from various gel types. Scanning Electron Microscopic study of individual samples was performed to view the internal structure of dried porous biopolymers.     

Influence of random pore defects on failure mode and mechanical properties of SiC ceramics under uniaxial compression using discrete element method

To investigate the relationship between micro-defects in ceramic materials and macro mechanical properties and behaviours, a computational model of SiC ceramics with randomly oriented elliptical pores was established using the discrete element method (DEM). The effects of pore defect content and its aspect ratio on the failure mode, stress-strain curve and mechanical properties of specimen were investigated under uniaxial compression. The effective Young's modulus which was obtained from DEM simulations was compared with the predictions of Mori-Tanaka scheme (MTS) and Self-Consistent scheme (SCS) at various pore defect densities. The results showed that the compressive strength and crack initiation stress decrease nonlinearly as the pore defect content increases. Furthermore, the smaller the aspect ratio of the elliptical pore defects was, the more obvious the weakening trend was. As the pore defect content increases, the failure mode of the specimen changed from brittle fracture to tensile-shear mixing and then to axial splitting. The stress-strain curves showed a certain “softening” period during the loading process. The effective Young's modulus obtained from the DEM simulations coincides with the approximations of MTS and SCS at low pore densities. However, when the pore defect density became larger, the DEM simulation results were slightly lower than the theoretical results of the Mori-Tanaka scheme, which only considers the weak interaction between defects.     

Strength and Stiffness of Porous Materials

A contribution is given to the theoretical research on nondestructive testing of porous materials like ceramics, for example. Relations (so-called modulus of elasticity-modulus of rupture relations) are developed between stiffness and strength considering the material-specific pore geometry. The results presented on strength can be given a form similar to the well-known empirical Ryshkewitch formula. This means that the "constant" in this expression can be related to porosity and pore geometry such that consistency is respected between strength and elasticity.     

High-Temperature Young's Modulus of Alumina During Sintering

High-temperature Young's modulus of a partially sintered alumina ceramic has been studied dynamically during the sintering process. Comparative, room-temperature Young's modulus data were obtained for a suite of partially sintered alumina compacts with different porosities. The dynamic Young's modulus of a 1200°C partially sintered material was observed to decrease linearly with temperature, but then above 1200°C it increased sharply as sintering and densification of the alumina became dominant. The evolution of the Young's modulus due purely to sintering exhibited an exponential relationship with porosity in excellent agreement with room-temperature measurements of equivalent porous alumina ceramics.     

填料粒径分布及其对填充PP性能的影响

本文用分形理论分析填充料粒径分布,揭示其分形特性,把一定总量下,小于某一当量直径r的填料质量份数W(r)与孔径r之间的指数关系定义为粒径分布的分形维数—分数维,即InW(r)=D_fInr K,D_f即为粒径分布的分数维。分数维值适中时,复合材料的力学性能最佳。     

Elasticity and Anelasticity of Uranium Oxides at Room Temperature: I, Stoichiometric Oxide

The elastic modulus and internal friction of stoichiometric uranium oxide at room temperature were studied using a dynamic method. The elastic modulus of stoichiometric urania at room temperature increases with increasing density. When the volume fraction porosity is less than 0.1, either linear or exponential equations can he used to calculate the elastic modulus as a function of density. When the volume fraction porosity is more than 0.1, a linear equation seems to be more suitable. The elastic modulus of stoichiometric nonporous uranium oxide at room temperature was found, by extrapolation, to be 2243.56 ± 22.1 kbars when the exponential equation was used, and 2233.85 ± 22.05 kbars when the linear expression was used. The internal friction of stoichiometric urania decreases sharply as the grains become larger. The number, size, and position of pores may also affect the internal friction values.     

Effect of pore clustering on the mechanical properties of ceramics

The reduction in strength and, to a lesser extent, Young's modulus with increased amounts of discrete pores is frequently greater than that predicted by models based on a homogenous pore distribution. The effect of pore distribution has been examined in the present work by producing samples containing a non-homogenous distribution of pores and comparing the results with data reported for samples containing homogenously distributed pores. Young's modulus and, to a greater extent, strength were shown to have stronger dependencies on the porosity content than would be predicted for homogeneous samples. By considering the material as a composite consisting of a pore-rich continuous phase containing a dispersion of pore-free material, various models were used to predict behaviour. It was found that the strength of the material is likely to be governed by the properties of the continuous phase, while the Young's modulus is a function of the properties of the two phases, with the porous phase being described by the Spriggs equation. The implications of the different dependencies of strength and Young's modulus in terms of the resistance to crack propagation following a thermal shock were then considered. Predictions of retained strength were in good agreement with those observed after water quenching.     

Revisiting the concepts of competition between reaction and diffusion in poisoned catalysts

The competition between diffusion and first‐order irreversible reaction in poisoned catalysts is revisited. Two cases are considered for isothermal slab catalysts: uniform and shell‐progressive (or pore‐mouth) poisoning. Analytical concentration profiles are derived, and the implications on catalyst performance are evaluated in different regimes (chemical‐ or diffusion‐controlled) for different levels of poisoning. It was found that depending on the poisoning mechanism, the activity decay can be more or less pronounced. Being of particular concern is the pore‐mouth poisoning at high Thiele modulus, conditions under which catalyst performance is drastically affected. The reagent concentration profiles allowed the explanation of the phenomena occurring at the particle scale, in particular the effectiveness factors, observed reaction rates, and poisoning factors' dependence on the Thiele modulus and fraction of the poisoned catalyst; it was found that such relationships are dependent on the mechanism of deactivation. © 2012 Canadian Society for Chemical Engineering     

Characterization of oxidation film on SiC ceramic substrate based on indentation method

An indentation method is proposed to characterize the properties of oxidation film on a SiC ceramic substrate. In this method, a series of indentation tests on the oxidation film with different maximum indentation depths were performed. The relationship between the inverse of contact depth and the inverse of reduced modulus was fitted by an exponential function. The moduli of the oxidation film and substrate as well as the thickness of the oxidation film were estimated by analyzing the fitting parameters. In order to validate the method, indentation tests were conducted on SiC substrate to determine the reference modulus of the substrate. Microstructure observation was conducted to measure the reference thickness of the oxidation film. The estimated values agreed well with the reference values. Finite element analysis was also employed to simulate the indentation tests on the oxidation film. The simulation results agreed well with the experimental results.     

Benchmark polynomials for the porosity dependence of elastic moduli and conductivity of partially sintered ceramics

It is well known that no useful analytical models are available for predicting the porosity dependence of effective properties for partially sintered ceramics with an open (interconnected) pore space where concave pore surfaces are dominating. This does not mean, however, that that the problem of determining the effective properties of partially sintered ceramics does not have a unique solution for a given microstructure. It just means that this solution can be obtained only numerically. In this contribution we report the polynomial fits of our numerical results for the effective elastic moduli (Young’s modulus, shear modulus, bulk modulus) and conductivity (thermal or electrical) for model materials based on the random close packing of monosized spheres. These relations are recommended as benchmark relations for the porosity dependence of the effective properties of partially sintered ceramics.     

Effect of Surface Dilatational Modulus on Bubble Generation in Visualized Pore‐Throat Models

In order to illustrate the effect of surface dilatational modulus on bubble break‐up, different pore‐throat visualized models were constructed using oil–wet poly(methyl methacrylate). Bubble generation was studied by comparing the snap‐off between lauroamide propyl betaine (LAB) with low surface dilatational modulus and LAB‐myristic acid with high surface dilatational modulus. In the homogeneous pore‐throat model, when bubbles of LAB flowed through the throat, only deformation occurred and no snap‐off was observed, while for bubbles of LAB‐myristic acid, repeated snap‐offs occurred. Experiments with isolated bubbles showed that a high surface dilatational modulus facilitates liquid flowing into the throat, which causes the snap‐off. Besides, the generated small bubbles contributed to the snap‐off of other following bubbles. In the pore‐throat model with large pores, the pores supplied enough space for bubbles passing through throat side by side, which facilitated the snap‐off.     

Effect of temperature and age on the relationship between dynamic and static elastic modulus of concrete

This study investigates the effects of cement type, curing temperature, and age on the relationships between dynamic and static elastic moduli or compressive strength. Based on the investigation, new relationship equations are proposed. The impact-echo method is used to measure the resonant frequency of specimens from which the dynamic elastic modulus is calculated. Types I and V cement concrete specimens with water-cement ratios of 0.40 and 0.50 are cured isothermally at 10, 23, and 50 °C and tested at 1, 3, 7, and 28 days.Cement type and age do not have a significant influence on the relationship between dynamic and static elastic moduli, but the ratio of static to dynamic elastic modulus approaches 1 as temperature increases. The initial chord elastic modulus, which is measured at low strain level, is similar to the dynamic elastic modulus. The relationship between dynamic elastic modulus and compressive strength has the same tendency as the relationship between dynamic and static elastic moduli for various cement types, temperatures, and ages.