Improving the Morphological Parameters of Aluminum Foam for Maximum Sound Absorption Coefficient using Genetic Algorithm

Fabricating of metal foams with desired morphological parameters including pore size, porosity and pore opening is possible now using sintering technology. Thus, if it is possible to determine the morphology of metal foam to absorb sound at a given frequency, and then fabricate it through sintering, it is expected to have optimized metal foams for the best sound absorption. Theoretical sound absorption models such as Lu model describe the relationship between morphological parameters and the sound absorption coefficient. In this study, the Lu model was used to optimize the morphological parameters of Aluminum metal foam for the best sound absorption coefficient. For this purpose, the Lu model was numerically solved using written codes in MATLAB software. After validating the proposed codes with benchmark data, the genetic algorithm (GA) was applied to optimize the affecting morphological parameters on the sound absorption coefficient. The optimization was carried out for the thicknesses of 5 mm to 40 mm at the sound frequency range of 250 Hz–8000 Hz. The optimized parameters ranged from 50% to 95% for porosity, 0.1 mm to 4.5 mm for pore size, and 0.07 mm to 0.6 mm for pore opening size. The result of this study was applied to fabricate the desired Aluminum metal foams for the best sound absorption. The novel approach applied in this study, is expected to be successfully applied in for best sound absorption in desired frequencies.     

单层元胞结构金属泡沫的透射特性

本文构建了开孔泡沫铝的简化几何模型,并利用有限积分法模拟了具有单层元胞结构的金属泡沫材料在线性极化平面波垂直入射情况下的透射率。基于菲涅耳-基尔霍夫衍射理论分析了孔隙率、孔径尺寸、材料厚度和骨架结构对金属泡沫材料辐射特性的影响。当尺度参数较大时,金属泡沫材料的固相支架结构满足良导体条件,宏观电磁屏蔽效应显著,金属泡沫材料呈现"非透明"性质。随着入射电磁波波长逐渐接近于孔径尺寸,散射效应越来越显著,金属泡沫材料的"半透明"性质开始显现,不同孔隙率的金属泡沫材料的透射率以相近的规律随波长变化。随着波长的进一步减小,衍射效应对于金属泡沫材料透射特性的影响逐渐占据主导地位,采用菲涅耳-基尔霍夫衍射理论可以较好地描述透射能流在孔隙结构内的分布。当衍射效应占据主导地位时,对于相同孔隙率金属泡沫材料,孔径尺寸对衍射光学行为影响不大,而材料厚度、孔隙率和骨架结构会显著影响金属泡沫材料的透射率。     

Morphology and Pore Size Distribution of Biocompatible Interconnected Porous Poly(L-lactic acid) Foams with Nanofibrous Structure Prepared by Thermally Induced Liquid–Liquid Phase Separation

Biocompatible, highly interconnected microporous poly(L-lactic acid) (PLLA) foams with nanofibrous structure, containing pores with average diameter below 1 μm and fibers with diameters of 10² nm scale, were prepared through the thermally induced liquid–liquid phase separation (TIPS) method consisting of quenching of the PLLA solution, freeze extraction with ethanol, and vacuum drying. Diverse foam morphologies were obtained by systematically changing parameters involved in the TIPS process, such as polymer concentration, solvent composition, and quenching temperatures. The morphology of different foams was examined by scanning electron microscopy to characterize the pore size and the pore size distribution. The results showed that most porous foams had a nanofibrous structure with interconnected open pores. In the case of using tetrahydrofuran (THF) as solvent, the higher the PLLA concentration, the smaller the average pore diameter and the narrower the pore size distribution. In the case of using the mixed solvents of THF/DOX (1,4-dioxane) with higher than 6/4 volume ratio, there appeared a maximum value of average pore diameter and a widest pore size distribution at 0.09 g/mL PLLA concentration. The average pore diameter of the foams increased with increasing DOX content in the mixed solvent and ranged from 0.2 to 0.9 μm depending on the process parameters. When the DOX content reached 60% by volume, the morphology of the foams contained some large closed pores with diameter ranging from 1 to 10 μm. By decreasing the quenching temperature, the average pore diameter of foams decreased and the pore size distribution became narrower. All the pore size distribution fit F-distribution equations.     

The influence of the micro-topology on the phononic band gaps in 2D porous phononic crystals

The phononic band structures of two-dimensional metal porous phononic crystals consisting of different lattices (the lattice structures transformed from square to triangle), and pores of various shapes (circle, square, and triangle) and sizes are studied numerically by using Finite Difference Time Domain (FDTD) scheme. It is found that for x-y mode waves, the absolute phononic band gaps (PBGs) rely more on the pore shapes. For triangular pores, the PBG is opening in the whole process of the lattice transformation, and for circular ones, the PBG is closed after a certain lattice structure. No PBG forms in the crystals with square pores. The PBG can be varied by adjusting the size of the pores. But a critical porosity exists for the opening of the PBG.     

A stochastic theory for deep bed filtration accounting for dispersion and size distributions

We develop a stochastic theory for filtration of suspensions in porous media. The theory takes into account particle and pore size distributions, as well as the random character of the particle motion, which is described in the framework of the theory of continuous-time random walks (CTRW). In the limit of the infinitely many small walk steps we derive a system of governing equations for the evolution of the particle and pore size distributions. We consider the case of concentrated suspensions, where plugging the pores by particles may change porosity and other parameters of the porous medium. A procedure for averaging of the derived system of equations is developed for polydisperse suspensions with several distinctive particle sizes. A numerical method for solution of the flow equations is proposed. Sample calculations are applied to compare the roles of the particle size distribution and of the particle flight dispersion on the deposition profiles. It is demonstrated that the temporal flight dispersion is the most likely mechanism forming the experimentally observed hyperexponential character of the deposition profiles.     

Heat transfer intensification in fixed bed adsorbers

The low thermal conductivity of the solid sorbents is one of the major drawbacks of the sorption heat pumps development. A better thermal transfer in the adsorber fixed beds is required to obtain a decreased time of the processing cycles and thus a reduced adsorber size per unit of power. Small improvements in the parameters of thermal transfer were obtained with unconsolidated porous mixtures such as bimodal mixtures and metallic foams. New consolidated materials made of metallic foam and zeolite were developed. The measurements of the thermal conductivity and of the wall heat transfer coefficient show a great improvement in the thermal transfer quality. Resistances to mass transfer appear but they are consistent with the adsorption heat pump process. With this type of composite material it seems possible to reduce the adsorber size by a factor from 5 to 10.     

Acoustic diagnosis for porous medium with circular cylindrical pores

Acoustic transmission coefficient and phase velocity of a Lucite slab with circular cylindrical pores with a nonrigid pore frame were experimentally and theoretically investigated. For theoretical investigation a new phenomenological model, the modified Biot-Attenborough (MBA) model, was proposed. The MBA model takes into account both the first kind and the second kind of waves introduced by Biot. It also separately considers viscous and thermal effects with three new phenomenological parameters: boundary, phase velocity, and impedance parameters. The theoretical estimation with three phenomenological parameters shows reasonably good agreement with the experimental data. The physical characteristics of porous medium such as porosity and pore size can be inversely analyzed in terms of the acoustic data such as the transmission coefficient and phase velocity as the functions of porosity and frequency. This makes acoustic diagnosis possible for noninvasively investigating physical characteristics of porous media such as bones and ocean sediments.     

Magnetic resonance studies of dissolving particulate solids

Magnetic resonance methods have been used to elucidate the internal pore structure of particulate solids, in particular detergent tablets. Such information is essential to a comprehensive understanding of the dissolution characteristics of these materials and how this property is related to processing conditions during tablet formation. In particular 3-D images of porosity are produced and 2-D self-diffusion maps are acquired as a function of observation time, which enables pore size to be quantified as a function of position via the extracted surface-to-volume ratio of the pore space. These properties are determined as a function of processing parameters, in particular the compression force used in tablet formation.     

Effect of the internal microstructure in rapid-prototyped polycaprolactone scaffolds on physical and cellular properties for bone tissue regeneration

Biomedical scaffolds should be designed to optimize their inter-microstructure to enable cell infiltration and nutrient/waste transport. To acquire these properties, several structural parameters, such as pore size, pore shape, porosity, pore interconnectivity, permeability, and tortuosity are required. In this study, we explored the effect of tortuosity on the viable cell proliferation and mineralization of osteoblast-like-cells (MG63) in polycaprolactone scaffolds. For analysis, we designed four different scaffolds of various tortuosities ranging from 1.0 to 1.3 under the same porosity (56?%) and 100?% pore interconnectivity. The pore size of the scaffolds was set as 150 and 300?μm, and a mixture of these sizes. We found that despite the porosity being same, the elastic modulus was dependent on the pore size of the scaffolds due to the distributed stress concentration. In addition, the relative water movement within scaffolds was also related to the internal microstructure. Cell viability and Ca²⁺ deposition of the cell-seeded scaffolds showed that the proliferation of viable cells and mineralization in the scaffolds with appropriate tortuosity (1.2) was relatively high compared to those of the scaffolds displaying low (1.05 and 1.1) or high (1.3) tortuosity. Our findings indicated that the internal microstructure of the scaffolds may influence not only the physical properties, but in addition the cellular behavior.     

The significance of passivation treatments on AISI 314 foam pieces to be used as substrates for catalytic applications

Several properties of metallic foams such as their low density, high mechanical strength and good coefficients of heat and mass transfer make them attractive for applications in catalysis. Important modifications in the composition and morphology of the metallic foam surfaces can take place when they are submitted to treatments at high temperatures. These surface changes are due to the migration of some elements from the metallic core to the pore surface, thus inducing a passivation via an oxide layer formation. This new layer avoids further metallic segregation and generates a surface roughness, both effects having a significant impact on the catalytic coating quality. This work analyzes the effects of calcination temperature and time on the chemistry and morphology of the metallic surface corresponding to the AISI 314 stainless steel foams of 50 and 60 ppi. The chemical and morphological surface changes were characterized by X-Ray Diffraction (XRD), Scanning Electron Microscopy (SEM), Energy-Dispersive X-Ray (EDX) analysis and Laser Raman Spectroscopy (LRS). The application of high temperature treatments on AISI 314 foams promotes the formation of a surface layer containing chromium oxide and spinel-type compounds of chromium, iron and manganese. The optimum treatment temperature for this type of structures seems to be 900 °C because both the adhesion and thickness of the layer formed are adequate. For the sample with smaller pores (60 ppi) the optimal treatment time is close to 2 h and for that with larger pores (50 ppi) the recommended time is 20 h. Under these conditions, a compromise is found between adhesion, thickness and surface roughness, suitable for the subsequent deposition of catalytic material.     

Stochastic metallic-glass cellular structures exhibiting benchmark strength

By identifying the key characteristic "structural scales" that dictate the resistance of a porous metallic glass against buckling and fracture, stochastic highly porous metallic-glass structures are designed capable of yielding plastically and inheriting the high plastic yield strength of the amorphous metal. The strengths attainable by the present foams appear to equal or exceed those by highly engineered metal foams such as Ti-6Al-4V or ferrous-metal foams at comparable levels of porosity, placing the present metallic-glass foams among the strongest foams known to date.     

Computational study on cell structure evolution of random liquid and metal-melt foams

A method for geometrical and topological modeling the evolution of close-cell metallic foams based on the Voronoi tessellation in three-dimensional space is presented. Numerical computations were carried out to examine the evolution of the bubble size distribution and topological and geometric properties of aluminum foams in the liquid state, which were implemented by using McPherson’s new theory on coarsening of microstructures as well as the topological transition rules (T1 and T2 processes) in 3D foams, accounting for remarkable effects of both the gas diffusion and surface tension. Computational results show that the bubble size distributions of metallic foams are strongly coupled to the evolution of the cellular structure and dependent on the gas diffusivity and surface tension. The way of foam coarsening can be expressed as RR ₃₂=−mt ²+1 approximately, and gas diffusion between bubbles dominates the evolution of bubble sizes and foam structures. It is found that the average number of faces per bubble is 〈f〉=13.8, which is in good agreement with the values reported in the literature.     

A facile method to determine pore size distribution in porous scaffold by using image processing

Image processing permits scientists to investigate morphological properties of three-dimensional structures starting from their bi-dimensional gray-scale representation. In many cases porous structure with complex architecture has to be designed in order to attempt specific properties such in the case of scaffold for tissue engineering.Traditional morphological characterization, like scanning electron microscopy, should be coupled with quantitative information such as pore size distribution (PSD) in order to get a deeper understanding of the influence of the porous structure on tissue regeneration processes and on other related applications, it is remarkable to study a quantitative analysis of porosity and of pores dimension.In this work it was developed as a software able to accomplish the segmentation of images containing pores of any geometry in a semi-automatic way with the aim to measure the PSD. Case study constituted by PLA porous scaffolds with different pore size was adopted. Results indicate that image processing methods well fit the pore size features of PLA scaffolds, overcoming the limits of the more invasive porosimetry techniques.     

Microstructure based model for sound absorption predictions of perforated closed-cell metallic foams

Closed-cell metallic foams are known for their rigidity, lightness, thermal conductivity as well as their low production cost compared to open-cell metallic foams. However, they are also poor sound absorbers. Similarly to a rigid solid, a method to enhance their sound absorption is to perforate them. This method has shown good preliminary results but has not yet been analyzed from a microstructure point of view. The objective of this work is to better understand how perforations interact with closed-cell foam microstructure and how it modifies the sound absorption of the foam. A simple two-dimensional microstructural model of the perforated closed-cell metallic foam is presented and numerically solved. A rough three-dimensional conversion of the two-dimensional results is proposed. The results obtained with the calculation method show that the perforated closed-cell foam behaves similarly to a perforated solid; however, its sound absorption is modulated by the foam microstructure, and most particularly by the diameters of both perforation and pore. A comparison with measurements demonstrates that the proposed calculation method yields realistic trends. Some design guides are also proposed.     

Nonlinearity of solids with micro- and nanodefects and characteristic features of its macroscopic manifestations

The meaning of the experimentally measured nonlinear parameters of a medium is discussed. The difference in meaning between the local nonlinearity, which is measured in the vicinity of a single defect and depends on the size of the region of averaging, and the effective volume nonlinearity of the medium containing numerous defects is emphasized. The local nonlinearity arising at the tip of a crack is calculated; this non-linearity decreases with an increase in the region of second harmonic generation. The volume nonlinearity is calculated for a solid containing spherical cavities. The volume nonlinearity is also calculated for a medium containing infinitely thin cracks in the form of circular disks, which assume the shape of ellipsoids in the course of the crack opening. The nonlinear acoustic parameter is calculated with the use of the exact classical results of the theory of cracks.     

A note on a circular panel sound absorber with an elastic boundary condition

Panel sound absorbers are typically used to absorb low-frequency noise in concert halls, auditoriums, recording studios, and other architectural applications. These systems are composed of flexible panels mounted over an air space that can be either partly or completely filled with a porous material. In this paper, a theoretical model is derived for predicting the sound-absorption coefficient of a cylindrical low-frequency absorber made of a circular plate. The theory takes into account the mass, bending stiffness, damping loss and the elastic boundary condition of the circular plate. The effects of the stiffness of an air-back cavity and of partially adding a porous material into the cavity are also considered. It is observed that the low-frequency resonances of such a system are dependent upon the clamping condition, the width of the air-back cavity, and mechanical properties of the plate. There is good agreement between theoretical and experimental results.     

低密度聚亚胺酮泡沫的制备

确定了以1,4-二氧六环为溶剂体系,经热致相分离和冷冻干燥技术制备了低密度聚亚胺酮泡沫,分析了其质量浓度对泡沫密度的影响,结果显示:泡沫实验密度与聚合物浓度有较好的线性关系,可实现对泡沫密度的有效控制。差示扫描量热法、热重法等热性能测试结果表明:聚亚胺酮泡沫材料的热分解行为与本体材料一致,但玻璃化温度较本体材料玻璃化温度高。泡沫孔结构测试结果表明:随着密度的增加,平均孔径有降低的趋势,孔径分布趋于单一化。对其力学性能进行分析可知:所制备的泡沫硬而强,具有较高的模量和抗压强度,断裂压缩随密度增加而增加。随着泡沫密度的增加,其破坏形变随之增加。     

A novel noncontacting waveguide backshort for submillimeter wave frequencies

A novel noncontacting waveguide backshort has been developed for millimeter wave and submillimeter wave frequencies. It employs a metallic bar with rectangular or circular holes. The size and spacing of the holes are adjusted to provide a periodic variation of the guide impedance on the correct length scale to give a large reflection of rf power. This design is mechanically rugged and can be readily fabricated for high submillimeter wave frequencies where conventional backshorts are difficult or impossible to fabricate. Model experiments have been performed at 4 GHz – 6 GHz to empirically optimize the design parameters. Values of reflected power greater than 95% over a 30% bandwidth have been achieved. A specific design is presented which has also been successfully scaled to WR-10 band (75 GHz – 110 GHz). A theoretical analysis is compared to the experiments and found to agree well with the measured data.     

Features of acoustic waves in media with large porosity values in the framework of the Biot theory

The Biot theory is used to study the properties of acoustic waves in fluid-saturated porous media with large porosity values. Materials are considered for which a large elasticity modulus of the matrix (skeleton) is comparable in size to the large fluid pore saturation modulus.     

Hybrid Al + Al3Ni metallic foams synthesized in situ via laser engineered net shaping

A hybrid, Al?+?Al₃Ni metallic foam was synthesized in situ via laser engineered net shaping (LENS®) of Ni-coated 6061 Al powder in the absence of a foaming agent. During LENS® processing, the Ni coating reacted with the Al matrix, resulting in the simultaneous formation of a fine dispersion of Al₃Ni, and a high volume fraction of porosity. As a reinforcement phase, the intermetallic compound formed particles with a size range of 1–5?µm and a volume fraction of 63%, with accompanying 35–300?µm pores with a 60% volume fraction. The microstructure of the as-deposited Al?+?Al₃Ni composite foams was characterized using SEM, EDS, XRD and TEM/HRTEM techniques. The evolution of the microstructure was analyzed on the basis of the thermal field present during deposition, paying particular attention to the thermodynamics of the Al₃Ni intermetallic compound formation as well as discussing the mechanisms that may be responsible for the observed porosity. The mechanical behavior of the as-deposited material was characterized using compression and microhardness testing, indicating that the yield strength and hardness are 190?MPa and 320 HV, respectively, which represents an increase of over three times higher than that of annealed Al6061, or similar to heat-treated Al6061 fully dense matrix, and much higher than those of traditional Al alloy foams, and with a low density of 1.64?g/m³.