Freeze-dried solid foams prepared from carbon nanotube aqueous suspension: Application to gas diffusion layers of a proton exchange membrane fuel cell

Freeze-dried macroporous solid foams were prepared from the multi-walled carbon nanotube (MWCNT) aqueous suspensions dispersed by chitosan. Thin film shaped CNT solid foams were prepared, and applied to the gas diffusion layers (GDLs) of a laboratory scale proton exchange membrane fuel cell (PEMFC). It was demonstrated that the prepared carbon foams in this study were useful to a fuel cell GDL material. The prepared cell performances were fairly comparable to the cell prepared with conventional carbon paper for GDL material. The microstructures of the prepared carbon foams were found to affect on the PEMFC performances. It was suggested that the interconnected carbon networks formed during the freezing step closely link to the cell performances. Hence, the defection of the interconnected microstructure lead degradation of the GDL quality. The impedance measurement made clear that the prepared foam materials were also advantageous for reducing the ohmic resistance in PEMFC assembly. The kinetic resistance values and the thermal conductive characteristics suggested that the freezing process would also control the degree of overlaps among single CNTs in a freeze-dried bulk that influenced on the electrochemical properties.     

Freeze-dried macroporous foam prepared from chitosan/xanthan gum/montmorillonite nanocomposites

Freeze-dried macroporous foams were prepared from an aqueous colloidal suspension of chitosan/xanthan gum/Na⁺-montmorillonite nanoclay (MMT). The suspension formed gel structure as a consequence of freezing, named cryogel. Cryogel is defined as a gel formed due to the concentration increase of the substrates caused by the ice formation during freezing. This obtained cryogel was subsequently dried under vacuum condition to produce porous foam materials. Two freezing methods were employed in the present work in order to investigate the influence of the processing on sample characteristics, namely; contact freezing with a heat exchanger and immersion freezing in a cryo-bath. Based on the SEM observation, in the case of contact freezing; rapid freezing (−2 °C/min) resulted in randomly aligned pores as compared to the pore alignment obtained in the case of slow freezing (−0.25 °C/min); the mean pore size for rapid freezing and slowing freezing were 40 μm and 68 μm, respectively. However, in immersion freezing samples, aligned and bamboo-like straight structures with pore layer spacing of 22 μm were observed. The different microstructures significantly influenced the mechanical hardness of the prepared foams nanocomposites. The MMT dispersion within the bionanocomposites was found to be characteristically exfoliated from X-ray diffraction and electron microscopy analysis. Small angle X-ray diffraction data indicated that the polymeric networks were modified by the exfoliated MMT and the MMT also improved the hardness of the prepared foams.     

Alumina aerogels with unidirectional channels under different freezing temperatures during freeze casting—Part I: Control and analysis of pore channels

Unidirectional alumina aerogels (AAs) were prepared by simultaneously using boehmite hydrosols and alumina nano-powder as solid solutes at different freezing temperatures. In order to understand the porous structural characteristics under different freezing parameters, the real-time frozen parameters were recorded at the designated positions under different freezing temperatures. Fourier transform infrared (FT-IR) spectra, X-ray diffraction (XRD), mercury intrusion porosimetry (MIP) and scanning electron microscopy (SEM) were used to investigate the functional groups, phase composites, microstructures and porous characteristics. The regularities and influence factors of unidirectional porous structures of AAs were detailedly explored under different frozen parameters. The results show that the AAs are characterized with unidirectional porous structures at the direction of ice-crystal growth. The cooling rate and ice front rate (v_inf) increase and the structure wavelengths (λs) decrease with the decrease of the freezing temperature. The relationship between λ and v_inf approximately satisfies a power law, which perhaps is related to the uniform dilute solution containing nanoparticle.     

Preparation and properties of polyimide/silver foams using a direct ion exchange method

A novel polyimide/silver (PI/Ag) foam was prepared by isocyanate foaming and direct ion exchange between Ag⁺ ions from silver nitrate and the H⁺ ions in the carboxyl (–COOH) groups of the polyamide molecular chain. The microstructures, chemical and phase composition of the PI/Ag foams were tested by SEM, FT-IR and XRD. The properties were tested by TG, DSC and UV spectrophotometer. The effects of different Ag contents on the microstructures and properties were analysized. SEM results showed that the PI/Ag foams had present the opened-cells type. With the increase in the Ag⁺ ions contents, both the amount of silver particles and the UV reflectivity of the PI/Ag foams increased, as the size of silver particles decreased. FT-IR test results show that the introduction of the silver nano-particles have no significant effect on the chemical structure of the PI foams. XRD results indicate that the silver nano-particles are face-centered cubic structure and have good crystalline properties. Thermal tests results show that the PI/Ag foams maintain the excellent thermal stability. Based on the above experimental results, the reaction mechanism during the foaming and curing process were also analysized.     

Poly(vinyl alcohol) foams reinforced with carbon nanotubes for stapedial annular ligament applications

This study aims to develop carbon nanotubes (CNTs) reinforced poly(vinyl alcohol) (PVA) foams as a possible material for stapedial annular ligament (SAL) application. As-grown (AG) and purified CNTs are used as reinforcing fillers for PVA foams. Uniaxial and cyclic compression tests reveal that specific modulus and energy dissipation behavior improve after reinforcing foam with CNTs. A relatively higher improvement in specific modulus is recorded for purified CNTs as they tend to produce stiffer cell walls. Thermogravimetric analysis shows thermal stability improves after addition of CNTs in PVA foams. The 50 wt % degradation temperature is higher for PVA_AG foam in comparison to neat PVA foam. Under dynamic loading storage, modulus is found to be higher for CNT doped foams with higher relative improvement with purified CNTs than AG CNTs. It is shown that reinforcing PVA foams with purified CNTs is a feasible strategy to improve their average mechanical properties and microstructure for SAL application. While the specific elastic modulus of neat PVA foam found to be in range of 0.05–0.06 MPa gcc⁻¹ with almost zero porosity. The addition of CNTs provides a wide range of specific elastic modulus 0.1–1.3 MPa gcc⁻¹ with an average pores size of about 300 μm. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48736.     

Preparation of cellular SiBOC foams by precipitating methylvinylborosiloxane oligomers within melamine formaldehyde foam scaffold

SiBOC cellular ceramic foams were realized by precipitating methylvinylborosiloxane (MVBS) oligomers within reticulated poly(melamine-formaldehyde) (PMF) foam followed by drying and ceramization. The conventional method of impregnation-squeezing-drying exhibited MVBS starvation at the foam-core as the MVBS oligomers migrate to the surface due to its high solubility in ethanol and small cell size (200 ppi) of the PMF foam. In contrast, the precipitation of the MVBS oligomers on the web of the PMF foam prevents their migration to the surface. Ceramization by heat treatment at 1500 °C in inert atmosphere resulted in amorphous SiBOC cellular foams with cells in the size range of 20–400 μm. The density, compressive strength and thermal conductivity of the SiBOC foams were modulated in the ranges of 0.18?0.39 gcm^?3, 0.3 to 0.87 MPa and 0.08?0.13 Wm^-1 K^-1, respectively, by using MVBS solution of concentrations in the range of 37–73 wt.%. The SiBOC foams remain amorphous up to 1600 °C beyond which extensive crystallization and phase separation occurs. Exposure of the SiBOC foam bodies at 1300 °C in air atmosphere showed negligible (～ 0.2 wt.%) weight gain due to oxidation, indicating its potential as thermal protection material in oxidizing atmosphere.     

Effect of freezing velocity and platelet size on structural parameters and morphology of freeze-cast porous alumina scaffolds

In this article, we examine the effects of two different platelet sizes (4 and 8 μm respectively) on the architecture of freeze-cast sintered alumina scaffolds as a function of a wide range of freezing velocities, 5–57 μm s^?1. The microstructural evolution along the freezing direction has been studied a-priori, explained on the basis of ice physics and the interaction of ceramic platelets with the advancing freezing front. An array of microstructures was produced to delve into the role of platelet sizes and freezing velocities on various structural parameters, viz. wavelength (λ), lamella thickness (δ), and bridge density (ρ_b). Regarding the pore morphology, transitions from lamellar to dendritic or isotropic structures were identified for the scaffolds containing smaller and bigger platelets, triggered by an increase in freezing velocity as well as platelet size. The different microstructures are quantified with a specific dimensionless parameter m. We identify the microstructure to be lamellar with low bridge density and m > 4. The wavelengths and bridge spacing were comparable for 2 < m < 4 and led to dendritic structure. For the morphologies characterized by m < 2, the spacing among the numerous interlamellar bridges was smaller than the structural wavelength and hence, the scaffolds revealed usually isotropic structure. Finally, the specific processing conditions that yield different morphologies and the parameter m are presented together in the form of a ‘morphology map’ to establish the different microstructural regimes.     

Generation and characterization of carbon nano-fiber-poly(arylene ether sulfone) nanocomposite foams

In this study, carbon nano-fibers (CNFs) were used to increase the compressive properties of poly(arylene ether sulfone) (PAES) foams. The polymer composite pellets were produced by melt blending the PAES resin with CNFs in a single screw extruder. The pellets were saturated and foamed with water and CO₂ in a one-step batch process method. Dynamic mechanical thermal analysis (DMTA) was used to determine the reduced glass transition temperature (T_g) of the CNF-PAES as a result of plasticization with water and CO₂. Sharp transitions were observed as peaks in the tan δ leading to accurate quantitative values for the T_g. By accurately determining the reduced T_g, the foaming temperature could be chosen to control the foam morphology. Foams were produced which ranged in density from 290 to 1100 kg/m³. The foams had cell nucleation densities between 10⁹ and 10¹⁰ cells/cm³, two orders of magnitude higher than unreinforced PAES foam, suggesting that the CNFs acted as heterogeneous nucleating agents. The CNF-PAES foam exhibited improved compressive properties compared to unreinforced PAES foam produced from a similar method. Both the specific compressive modulus and strength increased by over 1.5 times that of unreinforced PAES foam. The specific compressive strength of 59 MPa for the CNF-PAES foam is similar to that of commonly used high performance structural foam, poly(methacrylimide foam).     

A Direct Characterization Method of the Ice Morphology. Relationship Between Mean Crystals Size and Primary Drying Times of Freeze-Drying Processes

Abstract

In the freeze-drying process, the freezing step is one of the most important steps which determines the texture of the frozen material and, consequently, the final morphological characteristics of the freeze-dried material and its biological activity and its stability. As a matter of fact, the parameters of the freezing protocol have a direct effect on the pore size distribution and on the pore connectivity of the porous network of the freeze-dried matrix. Thus, the ice crystal morphology determines indirectly the mass and the heat transfer rates through the dry layer and, consequently, the freezing parameters have a strong influence on the total duration of the primary and secondary sublimation steps. The main objective of this study was to adapt and to develop a new optical direct microscopy method, based on the reflected flux differences, with episcopic axial lighting to characterize the structure of the different phases of a standard pharmaceutical matrix used for pharmaceutical proteins freeze-drying. First, the results obtained have been validated by another independent method, the scanning electron microscopy, carried out with freeze-dried samples. Finally, this technique has been principally used to investigate the effects of the freezing conditions on the ice crystal structure characterized by the distribution of the ice crystals mean sizes. Moreover, the influence of annealing treatment on ice crystal mean diameter and primary drying times has been also investigated.     

Graphene nanoribbons (GNRs) by unzipping MWCNTs for the improvement of PMMA microcellular foams

This work presents the cellular microstructures and properties of PMMA/graphene nanoribbons (GNRs) microcellular foams. GNRs were obtained by oxidative unzipping multiwalled carbon nanotubes and solvent thermal reduction in dimethylformamide (DMF), then they were mixed with PMMA to fabricate PMMA/GNRs nanocomposites by solution blending. Subsequently, supercritical carbon dioxide (scCO₂) as a friendly foaming agent was applied to fabricate PMMA/GNRs microcellular foam by a batch foaming in a special mold. The morphology of cell structure was analyzed by scanning electron microscopy and image software, showing that the addition of a smaller content of GNRs caused a fine cellular structure with a higher cell density (～3 × 10¹¹ cells/cm³) and smaller cell sizes (～1 μm) due to their remarkable heterogeneous nucleation effect. The mechanical testing of PMMA/GNRs microcellular foams demonstrated that the obtained GNRs also could be used as a reinforcing filler to increase the mechanical properties of PMMA foams. An improvement in the compressive strength of ～80% (about 39% increase standardized by specific compressive strength) was achieved by 1.5 wt % GNRs addition, and the thermal stability of PMMA/GNRs foams was enhanced too. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45182.     

Thermal transport phenomena and limitations in heterogeneous polymer composites containing carbon nanotubes and inorganic nanoparticles

An Off-Lattice Monte Carlo model was developed to investigate effective thermal conductivities (K_eff) and thermal transport limitations of polymer composites containing carbon nanotubes (CNTs) and inorganic nanoparticles. The simulation results agree with experimental data for poly(ether ether ketone) (PEEK) with inclusions of CNTs and tungsten disulfide (WS₂) nanoparticles. The developed model can predict the thermal conductivities of multiphase composite systems more accurately than previous models by taking into account interfacial thermal resistance (R_bd) between the nanofillers and the polymer matrix, and the nanofiller orientation and morphology. The effects of (i) R_bd of CNT–PEEK and WS₂–PEEK (0.0232–115.8 × 10⁻⁸ m²K/W), (ii) CNT concentration (0.1–0.5 wt%), (iii) CNT morphology (aspect ratio of 50–450, and diameter of 2–8 nm), and (iv) CNT orientation (parallel, random and perpendicular to the heat flux) on K_eff of a multi-phase composite are quantified. The simulation results show that K_eff of multiphase composites increases when the CNT concentration increases, and when the R_bd of CNT–PEEK and WS₂–PEEK interfaces decrease. The thermal conductivity of composites with CNTs parallel to the heat flux can be enhanced ∼2.7 times relative to that of composites with randomly-dispersed CNTs. CNTs with larger aspect ratio and smaller diameter can significantly improve the thermal conductivity of a multiphase polymer composite.     

Microstructure and properties of epoxy foams prepared by microwave

In this article, epoxy foams comprised of diglycidyl ether of bisphenol‐A (DGEBA) based epoxy resin E₃₁ and E₅₁, polyamide resin, and water were prepared by microwave irradiation method. The structure and properties of epoxy foams were analyzed by FTIR, TGA, SEM, and DMA methods. The density and compressive performance of epoxy foams was also determined. The results indicated that the epoxy foams had excellent compressive performance and the preparation of epoxy foam by microwave irradiation was high efficiency and convenient. The composition has great effect on density, foam structure, dynamical mechanic performance, and thermal degradation behavior of epoxy foams. The epoxy foam with density from 0.08 g cm^?3 to 1.05 g cm^?3 can be obtained by varying ratio of E₅₁ and E₃₁ to control the viscosity of mixtures. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Fracture properties of a rigid polyurethane foam over a range of densities

Single edge notch fracture tests have been carried out on rigid polyurethane foam in the density range 32 to 360 kg/m³. Fracture properties were characterized in terms of the fracture toughness parameter (K_Ic), the critical strain energy release rate (G_Ic) and crack opening displacement (c.o.d.). The values of crack opening displacement lead to a proposed mechanism of crack propagation in foams of density greater than about 140 kg/m³.     

Fabrication and properties of anisotropic polyimide aerogels with aligned tube-like pore structure

Polyimide (PI) aerogels with highly aligned tube-like pores were fabricated by unidirectional ice crystal-induced self-assembly method. During this process, the mold bottom contacted with the freezing medium, the aqueous solution of poly(amic acid) (PAA) ammonium salt in the mold was unidirectionally frozen, the ice crystals grew from the bottom to top of PAA ammonium salt (PAS) solution along the freezing direction, which endowed PI aerogels with aligned tube-like pores after sublimation of ice crystals and thermal imidization of PAS. The obtained aerogels had low densities (0.077–0.222 g cm⁻³) and high porosities (83.8–94.2%) and exhibited anisotropic morphology and properties. Their compression strength in vertical direction (parallel to freezing direction) was higher than that in horizontal direction (perpendicular to freezing direction). Their heat transport in horizontal direction was much slower than that in vertical direction; the aerogels had better thermal insulating property in horizontal direction. This facile approach contributed to prepare new type of PI aerogel materials. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48769.     

Synergistic Effects between Anionic and Sulfobetaine Surfactants for Stabilization of Foams Tolerant to Crude Oil in Foam Flooding

In foam flooding, foams stabilized by conventional surfactants are usually unstable in contacting with crude oil, which behaves as a strong defoaming agent. In this article, synergistic effects between different surfactants were utilized to improve foam stability against crude oil. Targeted to reservoir conditions of Daqing crude oil field, China (45 °C, salinity of 6778 mg L⁻¹, pH = 8–9), foams stabilized by typical anionic surfactants fatty alcohol polyoxyethylene ether sulfate (AES) and sodium dodecyl sulfate (SDS) show low composite foam index (200–500 L s) and low oil tolerance index (0.1–0.2). However, the foam stability can be significantly improved by mixing the anionic surfactant with a sulfobetaine surfactant, which behaves as a foam stabilizer increasing the half-life of foams, and those with longer alkyl chain behave better. As an example, by mixing AES and SDS with hexadecyl dimethyl hydroxypropyl sulfobetaine (C₁₆HSB) at a molar fraction of 0.2 (referring to total surfactant, not including water), the maximum composite foaming index and oil tolerance index can be increased to 3000/5000 L s and 1.0/4.0, respectively, at a total concentration between 3 and 5 mM. The attractive interaction between the different surfactants in a mixed monolayer as reflected by the negative β^s parameter is responsible for the enhancement of the foam stabilization, which resulted in lower interfacial tensions and therefore negative enter (E), spreading (S), and bridging (B) coefficients of the oil. The oil is then emulsified as tiny droplets dispersed in lamellae, giving very stable pseudoemulsion films inhibiting rupture of the bubble films. This made it possible to utilize typical conventional anionic surfactants as foaming agents in foam flooding.     

Facile fabrication of oriented polyethylene terephthalate foams as novel solar interfacial vapor generator from waste bottles

The massive arbitrarily discarded waste of polyethylene terephthalate (PET) bottles become a major environmental pollution source for soils and oceans. Using these waste products to acquire purified water has great significance. We developed a novel strategy of inducing crystallization coupled with solvent exchanging to obtain oriented PET/carbon nanotubes (CNT) foams from waste PET bottles and to fabricate solar interfacial vapor generators (SIVGs). The obtained PET/CNT foams had low densities of 0.18 g/cm³ and high specific strength with compressive stress of 1.8 MPa. The PET/CNT foams generated excellent capillary action that the water delivering rate reached 93.60 kg/m² h. In addition, the oriented PET foams demonstrated a promising thermal concentration effect that its surface temperature reached 132.8°C. It could evaporate up to 0.82 kg/m² h under the illumination of 1 sun. Meanwhile, the foams with CNT clusters dispersion also exhibited excellent electrical resistance of 3.57 × 10³ Ω m with a low percolation threshold of 0.5 wt.% CNT. This work provides a novel strategy of manufacturing PET SIVG. The key aspect of this work is the significant increase in the surface temperature and the conductivity of PET SIVG compared with other polymer-based SIVGs, highlighting its novelty and importance.     

Low density,three-dimensionally interconnected carbon nanotube/silicon carbide nanocomposites for thermal protection applications

Synthesis of silicon carbide (SiC) nanostructures and their composites has been a topic of interest for the scientific community due to the unique properties that can be obtained with nanoscale features. Herein, we report the scalable fabrication of anisotropic and low density, carbon nanotube/SiC (CNT/SiC) core-shell structures synthesized via chemical vapor infiltration (CVI) of silicon on aligned CNT foams followed by heat treatment at 1350 °C. Structures made of CNT/SiC nanotube networks with a thickness of 1 cm and length of 9 cm were prepared in the present work. Upon the removal of the CNT foam via calcination of the hybrid nanocomposite in air, a free-standing mechanically robust three-dimensional network of pure SiC nanotubes was left behind. The density of the synthesized CNT/SiC is the lowest reported for any C/SiC structure. Furthermore, the CNT/SiC hybrid nano-architecture demonstrated superb heat resistance and stability in ultrahigh temperature environment.     

Microstructure and thermophysical properties of graphite foam/glass composites

Mesophase pitch based graphite foams with different thermal properties and cell structures were infiltrated with glass by pressureless infiltration to prepare potential alternative composites for cooling electronics. Microstructure, thermal diffusivity and coefficient of thermal expansion (CTE) of the obtained composites were investigated. It was demonstrated that there was excellent wettability of the graphite foam by molten glass, and the foam framework was retained well after infiltration, which could facilitate good heat transfer throughout the composites. The highest thermal diffusivity of the composites reached 202.80 mm²/s with a density of 3.81 g/cm³. And its CTE value was 4.53 ppm/K, much lower than the corresponding calculated result (7.46 ppm/K) based on a simple “rule of mixtures” without considering the space limitations of the graphite foams. Thus, the mechanical interlocking within the space limitations of the graphite network played a crucial role in limiting the thermal expansion of the glass. The CTEs of the graphite foam/glass composites varied from 4.53 to 7.40 ppm/K depending on the graphite foam density which varied from 0.82 to 0.48 g/cm³. The CTEs were a good match to those of semiconductor chips and packaging materials.     

Development of mesophase pitch derived high thermal conductivity graphite foam using a template method

A simple and inexpensive method is described for preparing high thermal conductivity graphite foam by impregnating a coal tar pitch based mesophase pitch into a substrate polyurethane foam template. Mesophase pitch impregnated polyurethane foam was converted into graphite foam by several heat treatments in air as well as in an inert atmosphere. Scanning electron microscope images show the retention of an excellent open pore structure despite volume shrinkage of over 50%. The graphite foam prepared by this sacrificial template method is found to possess a thermal conductivity of 60 W/m K with a compressive strength in the range of 3.0–5.0 MPa. The X-ray diffraction pattern shows an interlayer spacing (d₀₀₂) of 0.3388 nm at a heat treatment temperature of 2400 °C. Different concentrations of slurries of mesophase pitch in water were used in combination with substrate foams of different densities to prepare graphite foams of density in the range 0.23–0.58 g cm⁻³. The specific thermal conductivity of the carbon foam with a low density of 0.58 g cm⁻³ is found to be higher than that of copper metal traditionally used in thermal management applications.