Fabrication of Porous Alumina Structures by SPS and Carbon Sacrificial Template for Bone Regeneration

In this work, a procedure for fabricating porous alumina with the use of a carbon sacrificial template has been tested in order to optimize the fabrication of porous structures mimicking the porosity and mechanical properties of the human cortical bone. Two different sources of sacrificial carbon were used and compared, and different sintering and calcination routes were considered. The porosity of the alumina structures studied by Hg porosimetry revealed that the amount of porosity and the size and shape of the pores are still below the required values, although some acicular pores were clearly observed by SEM. Moreover, measured mechanical properties (Young’s modulus) remained below that of the bone, suggesting the need for further consolidation treatments. In summary, these encouraging results drive the optimization of future fabrication routes.     

Influence of Alkali-Activators on Acid Rain Resistance of Geopolymer-Recycled Pervious Concrete with Optimal Pore Size

Geopolymer-recycled pervious concrete (GRPC) is a novel concrete that can effectively inhibit the corrosion of acid rain and alleviate urban waterlog. The goal of this study is to ascertain the optimal pore size of GRPC and study its acid rain resistance activated by different alkali-activators. Three different sizes (0.8, 1.0, and 1.2 mm) were separately chosen as the pore diameters of GRPC. The alkali-activator solution adopted sodium hydroxide (NaOH), sodium silicate (Na₂SiO₃), and a mixture of the two. The mechanical properties and permeability coefficient were tested to determine the optimal pore size of GRPC. After that, specimens with the optimal pore size were immersed in a simulative acid rain solution (sulfuric acid solution with pH = 4.0) for 6 d and were dried 1 d until 56 d. The effects of different alkali activators on acid rain resistance of GRPC were analyzed by compressive strength, neutralization depth, and mass loss. The results manifested that the mechanical properties of GRPC were excellent, the compressive strength of GRPC_H+N reached more than 60.1 MPa, and their splitting tensile strength attained more than 5.9 MPa, meeting the strength requirement of the road for heavy traffic load. Considering the mechanical properties and the acid rain purification effect of alkaline GRPC required a relatively small permeability coefficient; the optimal pore size was 1 mm. When specimens with optimal pore size were exposed to acid solution, the corrosion products (gypsums) would block the pores of GRPC to inhibit further corrosion, keeping the stability of the compressive strength. GRPC activated by the mixture of NaOH and Na₂SiO₃ generated a more stable amorphous three-dimensional network structure, endowing GRPC_H+N with better mechanical properties and acid corrosion resistance.     

Characterization of Inclusion Size Distributions in Steel Wire Rods

The control of inclusions in steel components is essential to guarantee strong performance. The reliable characterization of inclusion populations is essential not only to evaluate the quality of the components but also to allow the use of analytical procedures for the comparison and discrimination of inclusion populations. In this work, inclusion size distributions in wire rod specimens from six plant-scale heats were measured and analyzed. For the measurements, the metallographic procedure specified in the ASTM E2283 standard was used. The population density function (PDF) approach and the extreme value statistical procedure specified in the ASTM E2283 standard were used to analyze the whole size distribution and the upper tail of the size distribution, respectively. The PDF approach allowed us to identify differences among inclusion size distributions and showed that new inclusions were not formed after the liquid steel treatment process. The extreme value statistical procedure led to the prediction of the maximum inclusion length for each heat, which was used for the statistical discrimination of heats. Furthermore, the estimation of the probability of finding an inclusion larger than a given inclusion size using the extreme value theory allowed us to order the heats for different critical inclusion sizes.     

The Impacts of Surface Microchannels on the Transport Properties of Porous Fibrous Media Using Stochastic Pore Network Modeling

A stochastic pore network modeling method with tailored structures is proposed to investigate the impacts of surface microchannels on the transport properties of porous fibrous media. Firstly, we simplify the original pore network extracted from the 3D images. Secondly, a repeat sampling strategy is applied during the stochastic modeling of the porous structure at the macroscale while honoring the structural property of the original network. Thirdly, the microchannel is added as a spherical chain and replaces the overlapped elements of the original network. Finally, we verify our model via a comparison of the structure and flow properties. The results show that the microchannel increases the permeability of flow both in the directions parallel and vertical to the microchannel direction. The microchannel plays as the highway for the pass of reactants while the rest of the smaller pore size provides higher resistance for better catalyst support, and the propagation path in the network with microchannels is more even and predictable. This work indicates that our modeling framework is a promising methodology for the design optimization of cross-scale porous structures.     

Ion Capture and Release Ability of Glass Ionomer Cement Containing Nanoporous Silica Particles with Different Pore and Particle Size

This study prepared glass ionomer cement (GIC) containing nanoporous silica (NPS) (GIC–NPS) at 5 wt% concentrations using 3 types of NPS with different pore and particle sizes and evaluated the differences in their cationic ion capture/release abilities and mechanical properties. The cationic water-soluble dye was used as cationic ion. The test GIC–NPS complexes captured dyes by immersion in 1 wt% dye solutions. All the GIC–NPS complexes released dyes for 28 d, and the amount of dye released from the complexes increased with decreasing pore size; however, the particle size of NPS did not affect the amount of dye released. Additionally, GIC–NPS was able to recharge the dye, and the amount of released the dye by the complexes after recharge was almost identical to the amount released on the first charge. Although not significantly different, the compressive strength of GIC–NPS was slightly greater than that of GIC without NPS regardless of the type of NPS. These results suggest that the degree of capture and release of cationic molecules, such as drugs, can be controlled by optimizing the pore size of NPS without sacrificing its mechanical strength when its content is 5 wt%.     

Effective Diffusion in Fibrous Porous Media: A Comparison Study between Lattice Boltzmann and Pore Network Modeling Methods

The understanding of the correlation between a pore-scale structure and its coupled diffusion transport property is crucial in the virtual design and performance optimization of porous fibrous material for various energy applications. Two most common and widely employed pore-scale modeling techniques are the lattice Boltzmann method (LBM) and the pore network modeling (PNM). However, little attention has been paid to the direct comparison between these two methods. To this end, stochastic porous fibrous structures are reconstructed reflecting the structural properties of the fibrous porous material on a statistical level with structural properties obtained from X-ray computed microtomography. Diffusion simulation through the porous phase was subsequently conducted using LBM of D3Q7 lattice and topological equivalent PNM derived from the watershed method, respectively. It is detected that the effective diffusion coefficients between these two methods are in good agreement when the throat radius in the pore network is estimated using the cross-section area equivalent radius. Like most literature, the diffusivity in the in-plane (IP) direction is larger than in the through-plane (TP) direction due to the laid fiber arrangement, but the values are very close. Besides, tortuosity was evaluated from both geometry and transport measurements. Tortuosity values deduced from both methods are in line with the anisotropy of the diffusion coefficients.     

Preparation,Surface and Pore Structure of High Surface Area Activated Carbon Fibers from Bamboo by Steam Activation

High surface area activated carbon fibers (ACF) have been prepared from bamboo by steam activation after liquefaction and curing. The influences of activation temperature on the microstructure, surface area and porosity were investigated. The results showed that ACF from bamboo at 850 °C have the maximum iodine and methylene blue adsorption values. Aside from the graphitic carbon, phenolic and carbonyl groups were the predominant functions on the surface of activated carbon fiber from bamboo. The prepared ACF from bamboo were found to be mainly type I of isotherm, but the mesoporosity presented an increasing trend after 700 °C. The surface area and micropore volume of samples, which were determined by application of the Brunauer-Emmett-Teller (BET) and t-plot methods, were as high as 2024 m²/g and 0.569 cm³/g, respectively. It was also found that the higher activation temperature produced the more ordered microcrystalline structure of ACF from bamboo.     

Effect of Particle Size Distributions and Shapes on the Failure Behavior of Dry Coke Aggregates

Carbon anodes participate in chemical reactions to reduce alumina in the Hall–Héroult process, of which coke aggregates make up a major part. The failure analysis of coke aggregates not only leads to a better understanding of the deformation mechanisms of anode paste under compressive loading but also can identify potential causes of structural defects in carbon anodes, such as horizontal cracks. The coke aggregates are composed of particles with different size distributions and shapes, which may strongly affect the failure behavior of the anode during compaction. In this paper, the effects of particle size distributions and shapes on the mechanical behavior and the failure of coke aggregates are investigated using the discrete element method modeling technique. The numerical results reveal that, although the mechanical behavior of coke mixtures is generally dependent on larger particles, the presence of fine particles in the coke aggregates reduces fluctuations in the stress–strain diagram. In addition, the rolling resistance model is employed as a parameter representing the effect of particle shape. It is shown that the rolling resistance model can be an alternative to the overlapped spheres model, which has a higher computational cost than the rolling resistance model. The second-order work criterion is used to evaluate the stability of the coke aggregates, the results of which indicate that the addition of fine particles as well as increasing the rolling resistance between the particles increases the stability range of the coke aggregates. Moreover, by using the analysis of micro-strain contour evaluations during the compaction process, it is shown that, both by adding fine particles to the coke mixture and by increasing the rolling resistance between the particles, the possibility of creating a compression band in the coke aggregates is reduced. Since the presence of the compaction bands in the anode paste creates an area prone to horizontal crack generations, the results of this study could lead to the production of carbon anodes with fewer structural defects.     

The Pore Microstructure Evolution and Porous Properties of Large Capillary Pressure Wicks Sintered with Carbonyl Nickel Powder

We investigated the effect of different sintering temperatures ranging from 200°C to 600°C on the porous properties and pore microstructure of large capillary pressure wicks made of carbonyl nickel powder. The evolution model of hydraulic diameter was established and verified by the maximum pore diameter. Hydraulic diameter changed as the roughness of particle surfaces decreased and sintering necks grew large during sintering. In the contact-formation stage and the initial sintering stage (200–500°C), the decrease in the roughness of particle surfaces played a decisive role, contributing to an increase in hydraulic diameter. In the intermediate sintering stage (600°C), the growth of sintering necks dominated the process, however the hydraulic diameter was reduced. These results show that the maximum pore diameter first increased and then decreased in the same way as our evolution model. Permeability and capillary performance of the wicks first increased and then declined with increasing sintering temperature. We found the optimal sintering temperature to be 400°C, at which point the wicks achieved the maximum pore diameter of 1.21 μm, a permeability of 1.77 × 10⁻¹⁴ m², and their highest capillary performance of 1.46 × 10⁻⁸ m.     

Pore Structural Features of Granite under Different Temperatures

To explore the effects of thermal actions on the pore structural features of granite, scanning electron microscope (SEM) and mercury injection experiments were carried out on granite after thermal treatment (25 °C to 400 °C). The pore structure was investigated from various perspectives, including the capillary pressure curve, the pore–throat ratio, the median saturation pressure, the median pore–throat radius, the porosity, the pore volume, and the pore size distribution. Based on mercury intrusion test data, the Winland model of permeability prediction was modified for a high-temperature tight granite reservoir. The results showed that: (1) As the temperature rose, the mercury injection curve was gradually flattened, and the mercury ejection efficiency gradually increased. Meanwhile, the pore–throat ratio and the median saturation pressure decreased exponentially, and the pore connectivity was enhanced. (2) The median pore–throat radius and the porosity of granite increased exponentially as the temperature increased. Above 200 °C, the median pore–throat radius and the porosity increased substantially. (3) The pore volumes of the transitional pores, mesopores and macropores, and the total pore volume inside the granite, increased as the temperature rose. Especially above 200 °C, the transitional pores and the mesopores were prominently developed, and the pore volumes of the transitional pores and the mesopores took up a significantly greater proportion of the total pore volume. (4) As the temperature rose, the pore size distribution of granite became more extensive, the pore–throat structure was obviously developed, and the pore–throat connectivity was enhanced. (5) The relationship between the micropores’ characteristic parameters and the macro-permeability in engineering was established though a modified Winland model, and the modified Winland model had a better prediction effect. The findings provide a solid basis for rock geothermal mining projects and related geotechnical engineering.     

In Situ Determination of Droplet and Nanoparticle Size Distributions in Spray Flame Synthesis by Wide-Angle Light Scattering (WALS)

The investigation of droplet and nanoparticle formation in spray flame synthesis requires sophisticated measurement techniques, as often both are present simultaneously. Here, wide-angle light scattering (WALS) was applied to determine droplet and nanoparticle size distributions in spray flames from a standardized liquid-fed burner setup. Solvents of pure ethanol and a mixture of ethanol and titanium isopropoxide, incepting nanoparticle synthesis, were investigated. A novel method for the evaluation of scattering data from droplets between 2 µm and 50 µm was successfully implemented. Applying this, we could reveal the development of a bimodal droplet size distribution for the solvent/precursor system, probably induced by droplet micro-explosions. To determine nanoparticle size distributions, an appropriate filter and the averaging of single-shot data were applied to ensure scattering from a significant amount of nanoparticles homogeneously distributed in the measurement volume. From the multivariate analysis of the scattering data, the presence of spherical particles and fractal aggregates was derived, which was confirmed by analysis of transmission electron microscopy images. Monte Carlo simulations allowed determining the distribution parameters for both morphological fractions in three heights above the burner. The results showed relatively wide size distributions, especially for the spherical fraction, and indicated an ongoing sintering, from fractal to spherical particles.     

Effects of Particle Size Distribution with Efficient Packing on Powder Flowability and Selective Laser Melting Process

The powder bed-based additive manufacturing (AM) process contains uncertainties in the powder spreading process and powder bed quality, leading to problems in repeatability and quality of the additively manufactured parts. This work focuses on identifying the uncertainty induced by particle size distribution (PSD) on powder flowability and the laser melting process, using Ti6Al4V as a model material. The flowability test results show that the effect of PSDs on flowability is not linear, rather the PSDs near dense packing ratios cause significant reductions in flowability (indicated by the increase in the avalanche angle and break energy of the powders measured by a revolution powder analyzer). The effects of PSDs on the selective laser melting (SLM) process are identified by using in-situ high-speed X-ray imaging to observe the melt pool dynamics during the melting process. The results show that the powder beds made of powders with dense packing ratios exhibit larger build height during laser melting. The effects of PSD with efficient packing on powder flowability and selective laser melting process revealed in this work are important for understanding process uncertainties induced by feedstock powders and for designing mitigation approaches.     

Size Effect of a Piezoelectric Patch on a Rectangular Plate with the Neural Network Model

Artificial neural networks have been widely used in many studies, such as the prediction of the piezoelectric effect of the plate of engineering structures in vibration and noise reduction. In this paper, an artificial neural network (ANN) model was employed to explore the piezoelectric patch size and thickness’s effect on the first order natural frequency and displacement amplitude of a plate. With the finite element method (FEM), a rectangular plate actuated by a piezoelectric patch was analyzed with various patch sizes. The FEM data was later used to build an ANN model. The dynamic response of the plate was predicted by the ANN model and validated with FEM in terms of 1st order natural frequency and displacement amplitude. Results from case studies showed that with the input of patch length, width and thickness, ANN model can accurately predict both natural frequency and displacement amplitude. When the input of ANN model was simplified to patch size and thickness or the volume of the patch, the accuracy became worse and worse. The influence of the patch size and thickness on the first order natural frequency was coupled and the maximal and minimal values were predicted based on the ANN model.     

Simulation and Experimental Substantiation of the Thermal Properties of Non-Autoclaved Aerated Concrete with Recycled Concrete Powder

Non-autoclaved aerated concrete (NAAC) is a two-phase material with a concrete matrix and air, exhibits good thermal insulation performance and shows good potential in the insulating construction industry. In this study, recycled concrete fine powder was used as an auxiliary cementing material, and the NAAC with different porosity and distribution was fabricated by the non-autoclaved method at different curing temperatures. The effect of porosity on the thermal conductivity and mechanical strength of NAAC is analyzed by experimental tests. A prediction method of thermal conductivity combining pore structure reconstruction and numerical simulation was proposed, which is established by two steps. Firstly, the pore size distributions of NAAC with different porosities were characterized by stereology image analyses. Secondly, the thermal conductivity prediction model based on the pore structure information was established by a COMSOL steady-state heat transfer module. The thermal conductivity results of COMSOL simulations were compared with the experiments and other theoretical models to verify the reliability of the model. The model was used to evaluate the effect of porosity, pore size distribution and the concrete matrix’s thermal conductivity on the thermal conductivity of NAAC; these are hard to measure when only using laboratory experiments. The results show that with the increase in curing temperature, the porosity of NAAC increases, and the number and volume proportion of macropores increase. The numerical results suggest that the error between the COMSOL simulations and the experiments was less than 10% under different porosities, which is smaller than other models and has strong reliability. The prediction accuracy of this model increases with the increase in NAAC porosity. The steady thermal conductivity of NAAC is less sensitive to the distribution and dispersion of pore size in a given porosity. With the increase in porosity, the thermal conductivity of NAAC is linearly negatively correlated with that of the concrete matrix, and the correlation is close to 1.     

Nitriding and Denitriding of Nanocrystalline Iron System with Bimodal Crystallite Size Distribution

An artificially prepared nanocrystalline iron sample with bimodal crystallite size distribution was nitrided and denitrided in the NH₃/H₂ atmosphere at 350 °C and 400 °C. The sample was a 1:1 mass ratio mixture of two iron samples with mean crystallite sizes of 48 nm and 21 nm. Phase transformations between α-Fe, γ’-Fe₄N and ε-Fe_3-2N were observed by the in situ X-ray powder diffraction method. At selected steps of nitriding or denitriding, phase transformations paused at 50% of mass conversion and resumed after prominent variation of the nitriding atmosphere. This effect was attributed to the separation of phase transformations occurring between sets of iron crystallites of 48 nm and 21 nm, respectively. This was due to the Gibbs–Thomson effect, which establishes the dependence of phase transformation conditions on crystallite sizes.