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Iron foams are potential materials for the production, purification, and recuperation of hydrogen through redox systems. They are inexpensive, recyclable, and environmentally friendly. Nevertheless, iron foams cannot be employed repeatedly for redox cycling at high temperatures because the structure suffers morphological changes and a decrease in the effective porosity. In this work, two different pore structures of Fe-foams fabricated by freeze-casting have been produced: constant (CP) and gradient (GP) pore size. CP Fe-foams were obtained by employing a double-sided cooling technique to minimize gradients in pore width that result when using one-sided, constant cooling solidification techniques. GP Fe-foams were manufactured using a fixed-temperature cold plate. Optical microscopy and X-ray tomography were employed to characterize the pore structure and, for GP Fe-foams, to investigate the effect of redox cycling. After redox cycling, GP Fe-foams exhibited significant pore degradation.
This paper presents shape-memory foams that can be temporarily fixed in their compressed state and be expanded on demand. Highly porous, nanocomposite foams were prepared from a solution of polyetherurethane with suspended nanoparticles (mean aggregate size 90 nm) which have an iron(III) oxide core with a silica shell. The polymer solution with suspended nanoparticles was cooled down to -20 °C in a two-stage process, which was followed by freeze-drying. The average pore size increases with decreasing concentration of nanoparticles from 158 µm to 230 µm while the foam porosity remained constant. After fixation of a temporary form of the nanocomposite foams, shape recovery can be triggered either by heat or by exposure to an alternating magnetic field. Compressed foams showed a recovery rate of up to 76 ± 4% in a thermochamber at 80 °C, and a slightly lower recovery rate of up to 65 ± 4% in a magnetic field.
A design and manufacturing method is described for creating a motor tendon–actuated soft foam robot. The method uses a castable, light, and easily compressible open-cell polyurethane foam, producing a structure capable of large (~70% strain) deformations while requiring low torques to operate (<0.2 N·m). The soft robot can change shape, by compressing and folding, allowing for complex locomotion with only two actuators. Achievable motions include forward locomotion at 13 mm/s (4.3% of body length per second), turning at 9◦/s, and end-over-end flipping. Hard components, such as motors, are loosely sutured into cavities after molding. This reduces unwanted stiffening of the soft body. This work is the first demonstration of a soft open-cell foam robot locomoting with motor tendon actuators. The manufacturing method is rapid (~30 min per mold), inexpensive (under $3 per robot for the structural foam), and flexible, and will allow a variety of soft foam robotic devices to be produced.
Utilizing the experimental and modeling approaches, the Gamma radiation effects on stress responses of the silicon rubber foam under quasistatic compression are investigated. In the experimental work, the samples of the silicon rubber and the silicon rubber foams are quasistatically compressed before and after the Gamma radiation (a dose of 500 kGy and a dose rate of 100 Gy/min). The data reveal that the Gamma radiation obviously increases the material hardness, e.g., the compressive stresses of the silicon rubber and the silicon rubber foams both increase over 5 times as the strain is 20%. In the simulation work, a multiscale method combined with finite element method is developed to numerically predict the compressive stress of the silicon rubber foams. The microscale models are first constructed based on the real microstructures of the silicon rubber foams. The compressive stress and strain relation before and after the Gamma radiation is then simulated and obtained utilizing the phenomenological constitutive models based on the testing data of the silicon rubber. The simulation reveals that the Gamma radiation strongly affects the compressive response of the microscale models. The stress responses of the microscale models are then transferred into the macroscale models. The results also prove that the Gamma radiation obviously increases the hardness of the macroscale models. Data comparison shows that the numerical results agree with the testing data well, which verifies the developed method. The present work develops a new method to predict the radiation effects on mechanical properties of rubber foams.
Among the unsolved mysteries of modern biology is the nature of a lining of blood vessels called the ‘endothelial surface layer’ or ESL. In venous micro-vessels, it is half a micron in thickness. The ESL is 10 times thicker than the endothelial glycocalyx (eGC) at its base, has been presumed to be comprised mainly of water, yet is rigid enough to exclude red blood cells. How is this possible? Developments in physical chemistry suggest that the venous ESL is actually comprised of nanobubbles of CO2, generated from tissue metabolism, in a foam nucleated in the eGC. For arteries, the ESL is dominated by nanobubbles of O2 and N2 from inspired air. The bubbles of the foam are separated and stabilized by thin layers of serum electrolyte and proteins, and a palisade of charged polymer strands of the eGC. The ESL seems to be a respiratory organ contiguous with the flowing blood, an extension of, and a ‘lung’ in miniature. This interpretation may have far-reaching consequences for physiology.
Honeycomb structures, owing to their microstructural periodicity, exhibit unique and complex acoustic properties. Tuning their acoustic properties typically involves either changing their topology or porosity. The former route can lead to topologies that may not be readily amenable for large-scale production, while the latter could negatively affect the honeycombs’ weight. An ideal approach for tailoring the acoustic behavior of honeycombs should neither affect their porosity nor should they require customized and expensive fabrication methods. In this work, a novel honeycomb design that alters the microstructural topological features in a relatively simple way, while preserving the porosity of the honeycombs, to tune the acoustic properties of the honeycombs is proposed. The proposed honeycomb can be fabricated using the traditional approach employed to mass produce honeycomb structures; that is by bonding identical corrugated sheets with two periodic thicknesses. The acoustic behavior of the proposed honeycomb in terms of dispersion and phase velocities is analyzed using the finite element method. Simulation results demonstrate the potential of the designed honeycomb to exhibit tailored acoustic behavior at a constant porosity or mass. For example, it is demonstrated that the phase velocities of asymmetric and symmetric waves traversing the proposed honeycomb of aluminum with 90% porosity can be tuned by 30% and 17%, respectively.
A method for the fabrication of interconnected ceramic sponges was used in the present work, designed by using a combination of two different, aqueous gel casting and sacrificial template, using aluminum nitride powder (99.97%) with a mean size of 2.4 micrometers. Two types of sponges were made by using two different monomers, acrylamide and methacrylamide, the resultants sponges have 60% of porosity after being sintered and pyrolyzed at temperature of 1673 K using an inert atmosphere of argon for 1 h. The hydrolysis evolution of this ceramic powder during the gelcasting process was studied by measuring the pH during the stirring time, the microstructure changes during the time of exposure were observed in a SEM. XRD were made to study the present phases after the gel was eliminated by thermal treatment at 873 K using an oxidizing atmosphere, observing a formation of up to 4 %wt. of cubic alumina phase which was made after the hydrolysis products. Infrared spectroscopy was used to study the changes in the ceramic powder.
Pure metallic nanofoams in the form of interconnected networks have shown strong potentials over the past few years in areas such as catalysts, batteries and plasmonics. However, they are often fragile and difficult to integrate in engineering applications. In order to better understand their deformation mechanisms, a multiscale approach is required to simulate the mechanical behavior of the nanofoams, although these materials will operate at the macroscale, they will still be maintaining an atomistic ordering. Hence, in this work we combine molecular dynamics (MD) and finite element analysis (FEA) to study the mechanical behavior of copper (Cu) nanofoams. Molecular dynamics simulations were performed to study the yield surface of a representative cell structure. The nanofoam structure has been generated by spinodal decomposition of binary alloy using an atomistic approach. Then, the information obtained from the molecular dynamics simulations in the form of yield function is transferred to the finite element model to study the macroscopic behavior of the Cu nanofoams. The simulated mechanical behavior of Cu nanofoams is in good agreement of the real experiment results.
We present classical potential molecular dynamics simulations of nanoporous gold (np-Au) impacted by a spherical indenter. The atomic structure was generated using a phase field model as a template. In agreement with previous experiments, we observe densification in the region under the indenter. The hardness values obtained from our simulations exhibit a transition from an initially perfect-plastic plateau to hardening behavior in the later stages of indentation. This transition occurs when the relative density beneath the indenter exceeds ∼0.9. Hardness values obtained from the nanoindentation simulations reach 0.6 GPa, due to the densification of the material under the indenter. Elevated dislocation densities are observed in the densified region. The mechanism of pore collapse in the densified layer under the indenter is seen to switch from uniaxial to triaxial, consistent with a change in deformation mechanism from one based on shearing of individual ligaments in np-Au to one involving dislocation-mediated plasticity around voids in a Au single crystal undergoing uniaxial compression.
3D microarchitected metamaterials exhibit unique, desirable properties influenced by their small length scales and architected layout, unachievable by their solid counterparts and random cellular configurations. However, few of them can be used in high-temperature applications, which could benefit significantly from their ultra-lightweight, ultrastiff properties. Existing high-temperature ceramic materials are often heavy and difficult to process into complex, microscale features. Inspired by this limitation, we fabricated polymer-derived ceramic metamaterials with controlled solid strut size varying from 10-µm scale to a few millimeters with relative densities ranging from as low as 1 to 22%. We found that these high-temperature architected ceramics of identical 3D topologies exhibit size-dependent strength influenced by both strut diameter and strut length. Weibull theory is utilized to map this dependency with varying single strut volumes. These observations demonstrate the structural benefits of increasing feature resolution in additive manufacturing of ceramic materials. Through capitalizing upon the reduction of unit strut volumes within the architecture, high-temperature ceramics could achieve high specific strength with only fraction of the weight of their solid counterparts.
Nanoporous metals made by dealloying are macroscopic network architectures that can contain ∼1015 nanoscale struts or ligaments per sample. Their mechanical performance is critical to their applications as functional or lightweight high-strength materials. Testing nanoporous metals at the macroscopic scale offers opportunities for unraveling the properties of nanoscale solids in general. The central questions in this area include whether the macroscopic strength and elastic modulus of nanoporous metals can be correlated with the properties of nanoscale ligaments by the classical Gibson–Ashby equations, whether the dealloying-made network structure differs from the conventional foam metals, how network connectivity influences mechanical response, and how ligament size and surface properties affect the elastic and plastic response of nanoscale solids and that of nanoporous metals, particularly the tension–compression asymmetry in strength. This article reviews the fundamental observations of the mechanical response of nanoporous metals with a focus on gold and the emerging understanding of the aforementioned issues.
Low-density, highly porous graphene/graphene oxide (GO) based-foams have shown high performance in energy absorption applications, even under high compressive deformations. In general, foams are very effective as energy dissipative materials and have been widely used in many areas such as automotive, aerospace and biomedical industries. In the case of graphene-based foams, the good mechanical properties are mainly attributed to the intrinsic graphene and/or GO electronic and mechanical properties. Despite the attractive physical properties of graphene/GO based-foams, their structural and thermal stabilities are still a problem for some applications. For instance, they are easily degraded when placed in flowing solutions, either by the collapsing of their layers or just by structural disintegration into small pieces. Recently, a new and scalable synthetic approach to produce low-density 3D macroscopic GO structure interconnected with polydimethylsiloxane (PDMS) polymeric chains (pGO) was proposed. A controlled amount of PDMS is infused into the freeze-dried foam resulting into a very rigid structure with improved mechanical properties, such as tensile plasticity and toughness. The PDMS wets the graphene oxide sheets and acts like a glue bonding PDMS and GO sheets. In order to obtain further insights on mechanisms behind the enhanced mechanical pGO response we carried out fully atomistic molecular dynamics (MD) simulations. Based on MD results, we build up a structural model that can explain the experimentally observed mechanical behavior.
This paper presents the results of a study on the impact of a precipitation hardening treatment on the mechanical properties of 17-4PH stainless steel open-cell foams produced using a powder-metallurgy-based process patented by the National Research Council Canada (NRC). Pre-alloyed powder was used to manufacture stainless steel (SS) foams with either medium or high porosity by changing the nature of the organic binder used to process the porous materials. Some of these were kept in the as-sintered state, while others were submitted to the H900 precipitation hardening treatment frequently prescribed for 17-4PH stainless steels.
Metallurgical and physical characterization was carried out on the resulting materials, along with mechanical testing at the micro (indentation testing) and macro (compressive testing) scales. It was found that the Medium-Porosity Foams (MPF) and High-Porosity Foams (HPF) had very different morphologies, the HPFs having a delicate porous structure featuring thin sintered walls with many openings (a.k.a. windows) between the main cells, while the MPFs exhibited much thicker walls with few windows connecting the larger pores. As expected from these foam morphologies, the mechanical properties of MPFs were much higher than those of the more porous and delicate HPF materials. For both foam types, the average mechanical properties were improved by the H900 treatment. A comparison with compressive properties of 17-4PH foams taken from the literature resulted in reasonable agreement. However, the large scatter observed on the average compressive properties of the NRC foams and the slightly different structure/composition of the literature materials mean that any comparison between these porous alloys must be interpreted with caution.
Alumina gel-cast foams were manufactured in a broad total porosity range (43–86%) by using agarose or ovalbumin as gelling agents. The foams were examined in terms of microstructural, permeability, and mechanical properties. For the achieved open porosity level (19–85%), the mean cell size (19–375 µm), and mean window size (8–77 µm) of the alumina foams manufactured using ovalbumin were slightly wider than those obtained using agarose (34–262 µm and 18–33 µm, respectively). By using different contents of agarose (0.3–1 wt%) or albumin (5 wt%) and solids (30–45.9 wt%), it was possible to vary the foaming yield from 1.6 to 4.4 and produce bodies with a very wide permeability level that included several classes of porous ceramics. Darcian (k1) and non-Darcian (k2) permeability coefficients displayed values in the range 3.2 × 10−18 to 4.3 × 10−9 m2 and 1.8 × 10−18 to 6.5 × 10−5 m respectively. Compressive strength of bodies was dependent upon the porosity level, with a variation of 8.5–149.7 MPa.
High-porosity metakaolin-based geopolymer foams (GFs) were fabricated by a gelcasting technique using hydrogen peroxide (foaming agent) in combination with Tween 80 (surfactant). Slurries processed in optimized conditions enabled to fabricate potassium based GFs with a total porosity in the range of ∼67 to ∼86 vol% (∼62 to ∼84 vol% open), thermal conductivity from ∼0.289 to ∼0.091 W/mK, and possessing a compressive strength from ∼0.3 to ∼9.4 MPa. Moreover, factors that influence the compressive strength, the porosity, the thermal conductivity, and the cell size distribution were investigated. The results showed that the cell size and size distribution can be controlled by adding different content of surfactant and foaming agent. The foamed geopolymer can also be used as adsorbents for the removal of copper and ammonium ions from wastewater. The foams, due to their low thermal conductivity, could also be used for thermal insulation. It was also possible to produce geopolymer formulations that could be printed using additive manufacturing technology (Direct Ink writing), which enabled to produce components with nonstochastic porosity.
Composite nanostructured foams consisting of a metallic shell deposited on a polymeric core were formed by plating copper via electroless deposition on electrospun polycaprolactone (PCL) fiber mats. The final structure consisted of 1000-nm scale PCL fibers coated with 100s of nm of copper, leading to final core-shell thicknesses on the order of 1000-3000 nm. The resulting open cell, core-shell foams had relative densities between 4 and 15 %. By controlling the composition of the adjuncts in the plating bath, particularly the composition of formaldehyde, the relative thickness of copper coating as the fiber diameter could be controlled. As-spun PCL mats had a nominal compressive modulus on the order of 0.1 MPa; adding a uniform metallic shell increased the modulus up to 2 MPa for sub-10 % relative density foams. A computational materials science analysis using density functional theory was used to explore the effects pre-treatment with Pd may have on the density of nuclei formed during electroless plating.
Types of melter feed materials affect glass production rates. This study focuses on the effects of alumina sources on melting behavior of high-alumina high-level-waste melter feeds containing different alumina sources, namely, gibbsite, boehmite, and corundum. The heat flow from the glass melt to the cold cap, a floating layer of the reacting feed, is partially hindered by a foam layer at the bottom of the cold cap. Volume expansion tests and thermoanalytical methods revealed that a slow-melting feed with corundum foamed extensively, whereas a fast-melting feed with boehmite had a low reaction heat and produced less stable foam. The foam thickness, a critical factor for the rate of melting, estimated using the relationship between the heat conductivity and foam porosity was in reasonable agreement with experimental observation.
The effect of magnesium nitrate on the thermo-foaming of powder dispersions in molten sucrose for the preparation of alumina foams has been studied. The magnesium nitrate decreases the melting point of sucrose from 180 to 160 °C and acts as a blowing and setting agent. The foaming time and setting time decreases with an increase in foaming temperature as well as an increase in magnesium nitrate concentration. The rapid foaming followed by foam collapse is observed beyond 140 °C. The accelerated foaming and setting is due to an increase in the rate of –OH condensation because of the catalytic effect of H+ generated by the hydrolysis of magnesium nitrate. The porosity of alumina foam increases while cell size and grain size decrease with an increase in magnesium nitrate concentration. A change in foam structure, from partially interconnected cellular to completely interconnected reticulate-like, occurs when the magnesium nitrate concentration increase from 4 to 8 wt%.
This paper describes the direct-foaming of plaster of Paris (CaSO4·½H2O) with up to 1.7 wt% of a nonionic surface-active agent to obtain highly porous, lightweight gypsum (CaSO4·2H2O) with 65–70% total porosity and a homogeneous, bimodal pore size distribution. Based on viscosity and temperature changes in the setting plaster of Paris slurry, the nonionic surfactant is seen to retard gypsum crystallization and thus extend the working time at higher water to solid ratios. The increase in viscosity during gypsum crystallization stabilizes the macropores formed during foaming. Gypsum foams of 32% density with a submicron matrix pore size, and relatively uniform macropore size of ∼100 µm were obtained. Seeding the process with 0.5 wt% 100 µm diameter gypsum particles accelerates gypsum crystallization in the presence of the nonionic surfactant and results in a more uniform, finer (92 µm) macropore structure due to less time for bubble coalescence.