Nanoporous Foams Research Articles

Application of Notched Long-Period Fiber Grating Based Sensor for CO2 Gas Sensing

An inductively coupled plasma etching process to fabricate notched long-period fiber gratings for CO2 gas sensing is proposed in this article. In the gas sensing test, the 15% mixed CO2 gas was used for characterization of CO2 adsorption by the amine-modified nanoporous silica foams of the notched long-period fiber grating sensor. The results shows the spectra were changed with the CO2 gas flow within 13 min. During the absorption process, the transmission of the resonant dip was decreased by 2.884 dB. Therefore, the proposed notched long-period fiber grating gas sensor shows good performance and is suitable as a gas sensor for monitoring the CO2 adsorption process.

Characterization and Functional Applications of Nanoporous Ag Foams Prepared by Chemical Dealloying

In this study, the pure Ag nanoporous foams, with open cell pore volume fractions 55 to 70 pct and pore sizes 100 to 400 nm, have been prepared by chemical dealloying. The Ag nanoporous foams possess favorable modulus (~0.7 GPa) and strength (~14 MPa), much higher than most of the polymers or ceramic foams. It is found that the Ag nanoporous foams are appropriate candidates for the catalytic and electrode applications. However, they do not exhibit efficient anti-bacterial effect, unless much smaller bacteria with cell sizes of 100 nm or less in the neighborhood are encountered.

Transition Metal Foam Electrocatalysts for Hydrogen Evolution Reaction

Nickel (Ni) alloys are low-cost materials with good electrocatalytic activity for the hydrogen evolution reaction (HER) in alkaline water electrolysis. The electrode cost can be further reduced by using alloys with a porous structure, providing high surface area with low catalyst amounts. Herein, transition metal 3D nanoporous foams based on Ni, copper (Cu), and cobalt (Co) alloys are produced over stainless steel substrates. The metallic foams are obtained by galvanostatic electrodeposition using hydrogen bubble evolution as a dynamic template. Their morphology and chemical composition is analyzed by SEM and EDS, respectively. Linear scan voltammetry in 8 M KOH solutions are used to study the HER at the Ni-Cu/Co electrodes at room temperature. Additional chronoamperometry measurements show good stability of the tested foam electrodes. The results suggest that the 3D electrocatalysts can attain high HER efficiencies while using low metal amounts. Moreover, their low-cost, one-step fabrication method presents several advantages over conventional procedures

Raspberry-like morphology of polyvinyl chloride/zinc oxide nanoparticles induced by surface interaction and formation of nanoporous foam

P(VC-co-AAEM)/ZnO nanoparticles are prepared by a nano-coating method, and the morphology of the raspberry-like particles is adjusted by hydrophilicity and NaOH concentration.

Transition Metal Foams as Electrocatalysts for Hydrogen Evolution Reaction

Alkaline water electrolysis is a promising method for large-scale hydrogen production. In order to improve the energy efficiency of current alkaline electrolyzers, the development of new electrocatalytic materials for the hydrogen electrode is of paramount importance. Noble metal alloys are known to possess good catalytic activity towards the hydrogen evolution reaction (HER) but their high price and low availability prevents them to be considered for practical applications.Nickel (Ni) and its alloys are relatively low-cost materials and have been shown to present good electrocatalytic activity for the HER, making these Ni-based electrodes the most usual choice for industrial applications in alkaline media [1]. The amount of the electrocatalytic material can be further reduced by using Ni alloys with a porous structure, which provide high surface areas while lowering the cost.In this study, transition metal 3D nanoporous foams based on Ni, copper (Cu), and cobalt (Co) alloys are produced on stainless steel substrates by galvanostatic electrodeposition using hydrogen bubble evolution as a dynamic template [2]. The metallic foams are obtained from acidic sulfate-based solutions by applying current densities of 1 or 1.8 Acm-2 and deposition times between 60 and 180 s.The surface morphology of the metal foams was analyzed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) was used for the determination of their chemical composition. X-ray diffraction (XRD) was used to examine the crystallographic structure of the deposited metallic foams.SEM analysis revealed that they have a nanostructured morphology consisting of micrometric pores with dendritic walls. The morphology (pore and dendrite size) and chemical composition were related to the deposition parameters.The produced transition metal foams were evaluated as electrodes for HER in alkaline media. Chronoamperometry (CA) and linear scan voltammetry (LSV) were used to study the HER at the Ni/Cu/Co electrocatalysts in 8 M KOH solutions for temperatures ranging from 25 to 85 ºC.The analysis of the obtained polarization curves enabled the determination of the main kinetic parameters, namely Tafel coefficients, charge transfer coefficients, exchange current densities and activation energies, allowing a direct comparison of the studied electrodes performances for the HER. The obtained parameters were also compared to that of single Ni and other Ni-based alloys.The results suggest that these transition metal foams can be used to attain reasonably high HER efficiencies while using very low metal amounts. Moreover, these 3D electrocatalysts are produced using a low cost, one-step fabrication method that presents several advantages over conventional procedures.ReferencesD. Pletcher, X. Li, Prospects for alkaline zero gap water electrolysers for hydrogen production, Int. J. Hydrogen Energy 36 (2011) 15089.H.C. Shin, J. Dong, M. Liu, Nanoporous structures prepared by an electrochemical deposition process. Adv. Mater. 15 (2003) 1610.

Compressive Behavior and Deformation Mechanism of Nanoporous Open-Cell Foam with Ultrathin Ligaments

AbstractThe compressive response of nanoporous (np) Au with different porosities and ultrathin ligaments of widths 0.5–16 nm is investigated through molecular dynamics (MD) simulations. From the results of the uniaxial compressive loading, it was found that these materials behave in a ductile manner and possess characteristic high yield strength, suggesting that these unique materials may even be stronger than bulk Au and also have the advantage of being highly porous. Their deformation behavior shows three characteristic stages, namely: (1) the linear elastic region, (2) the work hardening region, and (3) the densification region. Surprisingly, even with extremely small ligament widths, where surface stress becomes significant, scaling equations can predict the relative yield strength given the relative density of the nanoporous foam. Through examination of the crystallographic defects at different strain levels, the strain hardening behavior has been attributed to defects in the crystal structure that a...

Strength scale behavior of nanoporous Ag, Pd and Cu foams

A comprehensive study of the deformation mode for nanoporous Au, Pd, Cu and Ag foams is presented. Several nanoporous models that include surface, nodes and ligament size effects are investigated within the scope of elastic buckling or plastic collapse foam models. The predictions from the deformation models are then compared to experimental foams hardness values for a wide range of ligament sizes. Results show that plastic collapse appears to be the dominant failure mode for all the nanoporous foams. Small deviations from the plastic collapse models are attributed to irregular cell size and relative density estimations.

A relationship between the geometrical structure of a nanoporous metal foam and its modulus

One of the crucial characteristics of a porous medium is its relative density ρ*, i.e. the volume fraction of the solid. For example, in low density foams (ρ*<0.1) the macroscopic material properties depend on the foam geometry only through the relative density. The relative density of nanoporous (NP) metal foams synthesized by dealloying is typically in the range ρ*⩾0.4. This places NP metal foams in a previously unexplored parameter regime and necessitates reexamination of scaling laws derived for low density foams. In this work we demonstrate that the modulus of NP metal foams does not exhibit a clear correlation with the relative density. We trace this apparent disagreement with existing scaling laws to the agglomeration of mass in the junctions. We then derive and verify a new scaling relationship for the dependence of relative density and relative modulus on the geometric parameters such as strut thickness, length and junction size. We find good agreement between our model and experimental measurements for single crystal gold NP foams with large junction sizes. In contrast, polycrystalline NP platinum with nanosized grains within the struts shows an enhancement in the modulus that cannot be attributed to the foam geometry only. Based on this analysis, we infer a more than an order of magnitude enhancement of the modulus of the nanograined Pt struts compared to bulk platinum.

Cellulose as a novel amphiphilic coating for oil-in-water and water-in-oil dispersions

The amphiphilic character of cellulose molecules provides the opportunity to use it as a novel eco-friendly emulsifying agent for formation of stable oil-in-water or water-in-oil dispersions. This may be done by mixing water, oil and cellulose solution in an ionic liquid. A more practical alternative is to form first a hydrogel from the cellulose/ionic liquid solution by coagulation with water and applying it into the sonicated water/oil or oil/water mixtures. The dissolution/regeneration process affords higher mobility to the cellulose molecules so an encapsulating coating can be formed at the water-oil interface. A solid-state dispersion was obtained by drying liquid dispersions, which can be repeatedly dissolved in excess water reforming a sustainable dispersion. The damp dispersion can be blown under reduced pressure, yielding a nanoporous foam (“aerocellulose”). The n-eicosane based solid dispersion as well as the aqueous dispersion possess a very high effective heat-absorption capacity. X-ray diffraction patterns indicate that the encapsulating cellulose shell is indeed in the amorphous state. Small-angle diffraction patterns of n-eicosane dispersions exhibit two sharp reflections. One is due to the n-eicosane triclinic crystal bulk phase and the other at somewhat smaller angles is interpreted as due to less ordered phase, possibly due to interactions with the encapsulating cellulose.

Functional Nanoporous Graphene Foams with Controlled Pore Sizes

A simple hydrophobic-affinity-derived assembly approach to pack graphene sheets into a nanoporous foam structure has been developed. Nanoporous graphene foams with the highest pore volume and large surface area are obtained. The pore diameter of the graphene foams can be finely adjusted from the mesopore to the macropore range by employing spherical templates with different sizes.

Morphology, Oxidation, and Mechanical Behavior of Nanoporous Cu Foams

AbstractNanoporous copper foams (65–80% porosity) are synthesized by dealloying different copper alloys, including Cu20Zn80, Cu35Zn65, and Cu30Al70. Ligament sizes, porosity, and oxide content are examined by scanning electron microscopy, Raman spectroscopy, and X‐ray powder diffraction to determine the nanoporous foam's structure and thermal stability. The pores and ligaments of the copper foams can range in size from 35 to 220 nm. Both parameters can be controlled by either the dealloying process or a subsequent heat treatment. The results show no copper oxide peaks for the as‐prepared samples; however, any heat treatments above 200 °C lead to the formation of Cu oxide (CuO and Cu2O). These foams are shown to retain their structural integrity even after oxidation. A novel method is thus developed for synthesizing nanoporous Cu oxide foams by heat treating nanoporous copper.

Poroelastic PEM fuel cell catalyst layer and its implication in predicting the effect of mechanical load on flow and transport properties

This study experimentally and numerically investigates the polymer electrolyte fuel cell (PEFC) catalyst layers (CLs) to analyze the coupled pore-fluid diffusion and resultant stress distribution. In the study, effect of nanoporosity on mechanical strength is explored by analyzing the CL structure based on the elastic modulus and the yield strength scaling laws of open-cell foams. A finite element analysis of the CL is performed by adopting the biphasic consolidation theory. The biphasic theory in combination with the transient consolidation principle of a porous body is formulated to account for the variation in the external loading conditions as well as the internal pore pressure. The CLs are observed to have the elastoplastic ionomer matrix and anisotropic nanoporosity, which are responsible for the localized plastic densification on indentation. The indentation behavior of the CLs appears to respond similar to the conventional low-density nanoporous foams leading to the localized nonlinear response of contact stiffness. The mechanical properties were found to be insensitive to the constituents’ (Pt and Carbon) concentration gradation over the CL thickness. In the numerical results, effect of porosity loss on the transport properties is discussed to highlight the importance of estimating the stress levels. It is outlined from the present study that under critical loading conditions, the yield limits of the CL play a crucial role in estimating the extent of transport losses. The effective proton conductivity and oxygen diffusivity losses are dependent on the macroscopic strength of the ionomer and the effective electronic conductivity loss is a function of intrinsic strength of the CL, which is also responsible for the overall durability of the CL.

Strain gradients and the strength of nanoporous gold

The very high strengths that have been reported for nanoporous gold may be related to strain gradients within the deforming porous microstructure. We present a mechanism-based model for the strength of nanoporous foams that is derived from conventional models for the deformation of macroscopic foams and now includes the influence of strain gradients. This model predicts that the strength of the ligaments within the nanoporous gold is proportional to the ligament diameter raised to the power −0.5. We have used the model to analyze experimental data for the strength of nanoporous gold and find excellent agreement with published data.

Controlling cellular morphology by supercritical carbon dioxide

AbstractA series of high‐performance porous materials, including open‐cell and closed‐cell foams, were prepared by a two‐stage batch foaming process from fluorinated poly(arylene ether)s, and the porous morphology was characterized with scanning electron microscopy. The effects of saturation pressures, saturation temperatures, transfer times, molecular structures, and solvent traces on the cell sizes, cell densities, and bulk densities of porous materials were examined. According to the conclusions, the porous structures could be controlled with cellular sizes between the nanoscale and microscale. All nanoporous materials have dielectric constants in the ultralow‐dielectric‐constant range (1.4–2.0). The open structures can be prepared by two methods, and there are two kinds of morphologies from the two methods, including foams with small, spotlike openings (diameters between 10 and 100 nm) in the cell walls and bicontinuous nanoporous foams. Another special method for controlling the porous morphology was used here. Films with closed‐cell microcellular foams were placed in supercritical carbon dioxide again. The original morphology of the microcellular foam collapsed. The collapsed morphology was also characterized with scanning electron microscopy, and three kinds of cellular morphology were observed. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009

Modeling and limits of strength of nanoporous foams

To reveal the size effect of the strength of nanoporous foams, power functions were suggested to model the tested ligament strength. The expressions of the strength of the nanoporous foam were built on the basis of the power function and well fitted the experiment data. Two kinds of critical relative density were deduced to direct the foam design. The upper one was obtained to manufacture nanoporous foams with greater strength or specific strength compared with the bulk material. The lower limit was deduced to confirm the critical relative density below which the nanoporous foam would be controlled by far smaller elastic buckling collapse strength.

New Generation of Gold Catalysts: Nanoporous Foams and Tubes—Is Unsupported Gold Catalytically Active?

New Generation of Gold Catalysts: Nanoporous Foams and Tubes—Is Unsupported Gold Catalytically Active?

Observations of nanoporous foam arising from impact and rapid solidification of molten Ni droplets

The authors report highly nanoporous surfaces on the underside of impacted, rapidly quenched Ni droplets (splats) removed from substrates. The formation mechanism is interpreted as nucleation of gas bubbles during rapid postimpact depressurization, and subsequent freezing to a porous solid. Good agreement is found between experimental observations and predictions from bubble nucleation theory. Results from experimental variations suggest the potential for control of pore size and distribution.

Scaling equation for yield strength of nanoporous open-cell foams

A comprehensive study on the relationship between yield strength, relative density and ligament sizes is presented for nanoporous Au foams. Depth-sensing nanoindentation tests were performed on nanoporous foams ranging from 20% to 42% relative density with ligament sizes ranging from 10 to 900 nm. The Gibson and Ashby yield strength equation for open-cell macrocellular foams is modified in order to incorporate ligament size effects. This study demonstrates that, at the nanoscale, foam strength is governed by ligament size, in addition to relative density. Furthermore, we present the ligament length scale as a new parameter to tailor foam properties and achieve high strength at low densities.

Characterization and Mechanical Behavior of Nanoporous Gold

AbstractHere we present current issues in understanding the mechanical behavior of nanoporous foams as a new class of high yield strength / low density materials. Gold nanoporous foams are used for a systematic study of mechanical properties since they can be synthesized with a wide range of ligaments sizes and densities. Preliminary tests demonstrate that a) Au foams have a fracture behavior dictated by the ligament size, and b) nanoporous Au is a high yield strength material.

Low-dielectric, nanoporous polyimide films prepared from PEO–POSS nanoparticles

Dielectric insulator materials that have low dielectric constants ( k<2.5) are required as inter-level dielectrics to replace silicon dioxide (SiO 2) in future semiconductor devices. In this paper, we describe a novel method for preparing nanoporous polyimide films through the use of a hybrid PEO–POSS template. We generated these nanoporous foams are generated by blending polyimide as the major phase with a minor phase consisting of the thermally labile PEO–POSS nanoparticles. The labile PEO–POSS nanoparticles would undergoes oxidative thermolysis to releases small molecules as byproducts that diffuse out of the matrix to leave voids into the polymer matrix. We achieved significant reductions in dielectric constant (from k=3.25 to 2.25) for the porous PI hybrid films, which had pore sizes in the range of 10–40 nm.