Resorcinol-formaldehyde Aerogels Research Articles

Reactant Concentration and Carbonization to the Controllable Fabrication of Carbon Aerogels

To improve the controllability of the fabrication of carbon aerogels, the effects of the concentration of the reactant (RF%) on the structural properties of organic resorcinol formaldehyde (RF) gel and the effects of the carbonization temperature on nanostructure of carbon aerogels were discussed. The concentration of the reaction was turned from 5%,10%,20%,30%,40%,50%,55% and 60% to prepare the samples. The RF aerogels were carbonized at temperature of 700, 900 and 1050. The shrinkage and nitrogen gas adsorption were measured. Experimental results showed that the structural stability of the organic RF aerogel can be improved by decreasing the shrink in drying process and increasing the condensation of reactant in the starting solution to a certain value, such as 55%. The ordered pore size distribution of carbon aerogels with less structure defects is able to be produced through the effective particle fusing at the carbonization temperature as high as 1050.

TiO2-doped resorcinol–formaldehyde (RF) polymer and carbon gels with photocatalytic activity

Resorcinol-formaldehyde (RF) polymer gels offer a relatively easy and versatile route for incorporating metals into a carbon aerogel matrix. The hybrid materials thus obtained are ideal candidates for applications involving enhanced adsorption or catalysis. This paper presents a detailed study of Ti-doped RF and carbon aerogels. The metal was introduced into the system at three different stages of the preparation process: during polymerization, by impregnation of the RF gel, or by impregnation of the carbon gel. The structure and morphology of the samples are compared using low temperature N2 adsorption, SEM, and small and wide angle X-Ray scattering (SAXS/WAXS) methods. The TiO2-doped carbon aerogels display photocatalytic activity in breaking down aromatic compounds.

In situ SAXS investigation of structural changes in soft resorcinol–formaldehyde polymer gels during CO2-drying

Drying is one of the key steps in the preparation of resorcinol–formaldehyde (RF) gels. Supercritical drying has long been known to be a method by which the structure of RF aerogels has the greatest chance of being perfectly preserved. Although several investigations have been conducted into the effects of the applied drying parameters on the final gels, the structural evolution of the gels during the drying process is nevertheless poorly known. In this work we present an in situ small angle X-ray scattering study of (1) the changes that occur in the structure of RF networks during supercritical drying under various conditions, and (2) the kinetics of the drying process.During the pressurization stage the overall structure of the network remains unaffected by the rate of increase of the CO2 pressure. By contrast, depressurization with supercritical CO2 at rates in excess of 4bar/min, or with liquid CO2 alone, causes bubbles to form and yields a shrunken state of the final network. This densification preferentially shrinks the larger pores, the smaller pores being much less affected. We find that liquid CO2 is also an efficient drying medium that can preserve the structure, but the depressurization stage must always start from the supercritical state. The incomparable advantage of supercritical CO2, however, is that its higher rate of molecular diffusion makes solvent exchange substantially faster than with liquid CO2.

Flexibilisation of resorcinol–formaldehyde aerogels

Ambient pressure dried resorcinol–formaldehyde (RF) organic aerogels are usually hard and brittle with Young's moduli in the range of 1–2 MPa, strengths of about 100 kPa and densities in the range of 0.2 to 0.4 g cm−3. Modifications of the classical sol–gel synthesis route transform these brittle materials into rubber- or cork-like flexible aerogels. We observed that in a small window of process parameters, the aerogel density decreases by an order of magnitude as well as the Young's moduli and the compression strengths. These new types of RF aerogels are elastically deformable by more than 40% in an almost reversible manner. In this paper we describe the effects of various sol–gel parameters on the flexibility, such as resorcinol to water and catalyst molar ratio and the pH of the initial solution. The aerogels are characterized with respect to the envelope density, stress–strain behavior and microstructure as observed from SEM fractographs. The chemical structure and structural differences arising between brittle and flexible RF aerogels were studied by recording 13C-NMR spectra.

Nanoconfined 2LiBH4–MgH2 for reversible hydrogen storages: Reaction mechanisms, kinetics and thermodynamics

Samples of nanoconfined Reactive Hydride Composites in resorcinol–formaldehyde aerogel scaffolds (RF–CAS) are prepared by (i) direct melt infiltration of bulk 2LiBH4–MgH2; and (ii) MgH2 impregnation and LiBH4 melt infiltration. The reaction mechanisms, kinetics and thermodynamics of the systems are concluded. Activation energy (EA) and dehydrogenation enthalpies of LiBH4 and MgH2(ΔHdes,MgH2+ΔHdes,LiBH4) of nanoconfined 2LiBH4–MgH2 are in this work of interest. The hydrogen sorption reactions in both nanoconfined samples are reversible as shown by the recovering of LiBH4 and MgH2 after rehydrogenation. The titration results show the remarkable improvement in desorption kinetics of nanoconfined samples over the bulk material, such as more than 90% of overall hydrogen storage capacity is obtained within 2 h from the nanoconfined samples during the 1st dehydrogenation, while that of bulk material needs more than 16 h. The activation energy of the composites decreases by 27–170 kJ/mol (ΔEA) due to nanoconfinement. For thermodynamics, (ΔHdes,MgH2+ΔHdes,LiBH4) calculated from DSC results of the nanoconfined samples are in the range of 41–46 kJ/mol H2.

Carbon Aerogel Composites Prepared by Ambient Drying and Using Oxidized Polyacrylonitrile Fibers as Reinforcements

Carbon fiber-reinforced carbon aerogel composites (C/CAs) for thermal insulators were prepared by copyrolysis of resorcinol-formaldehyde (RF) aerogels reinforced by oxidized polyacrylonitrile (PAN) fiber felts. The RF aerogel composites were obtained by impregnating PAN fiber felts with RF sols, then aging, ethanol exchanging, and drying at ambient pressure. Upon carbonization, the PAN fibers shrink with the RF aerogels, thus reducing the difference of shrinkage rates between the fiber reinforcements and the aerogel matrices, and resulting in C/CAs without any obvious cracks. The three point bend strength of the C/CAs is 7.1 ± 1.7 MPa, and the thermal conductivity is 0.328 W m(-1) K(-1) at 300 °C in air. These composites can be used as high-temperature thermal insulators (in inert atmospheres or vacuum) or supports for phase change materials in thermal protection system.

Pyrolysis of N-doped organic aerogels with relation to sorption properties

Resorcinol–formaldehyde aerogels (five samples, three of them with addition of nitrogen containing precursors—3-hydroxypyridine, 3-aminophenol and melamine) have been prepared by sol–gel polycondensation, subcritical drying and pyrolysis. The pyrolysis of prepared organic aerogels has been studied by non-isothermal TG at constant heating rate. The process of pyrolysis has been found to consist of three steps with the total mass loss 40.2–61.7% (room temperature—1,000 °C). The resulted carbon aerogels have been tested as sorbents of Ni(II), Pb(II) and Cu(II) ions from aqueous solutions. Various relations have been found among the results obtained from the pyrolysis experiments and properties affecting adsorption. Besides the expected correlation between the mass loss gained from TG (isothermal step at 500 °C was applied) and from heating in the laboratory oven, the relationship between the mass loss during pyrolysis and sorption capacities for all three metal ions has been found. Other relations among pyrolysis behaviour, surface area and content of nitrogen have been also examined. Batch adsorption experiments show (with an exception of one sample) that N-doped samples have higher adsorption capacity for metal ions. In addition, changing of nitrogen functionalities during the pyrolysis has been considered and pyridinic-N (N-6) functionality has been contemplated as a suitable structure for the adsorption process.

Carbon fiber reinforced carbon aerogel composites for thermal insulation prepared by soft reinforcement

To overcome the brittleness and the pyrolysis shrinkage of carbon aerogels, carbon fiber reinforced composites were prepared by copyrolysis of polyacrylonitrile fiber reinforced resorcinol-formaldehyde aerogel composites (PAN/RFs). The PAN/RFs were obtained by impregnating the PAN fiber felt with RF sol and then supercritical drying. Upon carbonization the PAN fiber shrinks with the RF aerogel, thus reducing the shrinkage differences between the fiber and the aerogel, and results in crack-free carbon fiber reinforced carbon aerogel composites, with a thermal conductivity of 0.073 W/m K at 25 °C in air. Our new method may greatly expand the usage of carbon aerogels in general applications.

Synthesis of Resorcinol Formaldehyde Aerogel Using Photo-Acid Generators for Inertial Confinement Fusion Experiments

ABSTRACTTraditionally, the synthesis of resorcinol formaldehyde (R/F) aerogels consists of a 2-step (base/acid catalysis) polycondensation reaction. Since the acid catalyst in the reaction controls the gelation time, the replacement of the acid catalyst with a non-ionic photo-acid generator decreased the gelation time from hours, down to a few minutes at room temperature using a UV light source. The reaction rate was not only fast, but the liquid precursor was stable for several hours prior to UV exposure. After drying, the resulting aerogel porosity was characterized by scanning electron microscopy (SEM) and confirmed the internal structure of the aerogel was similar to the original R/F pore structures. This paper will discuss the modifications made to the traditional R/F formulation, as well as the benefits of a fast gelation time for aerogel casting applications such as thin films, cylinders, and solid and hollow microspheres.

Hierarchically Nanostructured Zeolites of Tunable Porosities with Aerogel Templating

ABSTRACTWe present the synthesis of resorcinol-formaldehyde aerogels and carbon aerogels of different nanoporosities, emphasizing on the recent developments in fabrication pathways of lower cost. Recent results showed a simple way to the production of highly nanoporous carbon xerogels. While using an approach combined colloidal silica nanocasting and carbon dioxide supercritical drying, hydrophilicity-controlled carbon aerogels with high mesoporosity were synthesized. Then, we demonstrate the functions of these aerogels for template synthesis of hierarchically nanostructured zeolites having micropores and mesopores.

Adsorption of Pt(cod)me 2 onto organic aerogels from supercritical solutions for the synthesis of supported platinum nanoparticles

The thermodynamics and kinetics of adsorption of Pt(cod)me 2 onto resorcinol–formaldehyde aerogel (RFA) from supercritical carbon dioxide (scCO 2) was investigated by using high performance liquid chromatography (HPLC) to measure Pt(cod)me 2 concentrations in the fluid phase. It was found that the adsorption isotherms of Pt(cod)me 2 at 35 °C for different CO 2 pressures could be represented by modified Langmuir isotherms. The kinetics of adsorption was determined by following the Pt(cod)me 2 uptake of the RFA spheres; these data correspond closely to the behavior from a mass transfer model based on diffusion within the pore volume with the assumption of local equilibrium at the solid–fluid interface. The adsorbed Pt(cod)me 2 molecules were reduced at atmospheric pressure under flowing hydrogen at 200 °C. The resultant Pt nanoparticles were distributed uniformly on the surface and had narrow size distributions. The average particle size of the nanoparticles increased with platinum loading from 2.0 nm at 10 wt.% to 3.3 nm at 34 wt.%. The Pt nanoparticles in an RFA pellet had a uniform radial size distribution, even though the pellet was impregnated with Pt(cod)me 2 for too short a short period of time for the system to reach adsorption equilibrium. The high mobility of the atomic Pt evolved during the reduction process is believed to be responsible for this phenomenon. Performing the adsorption of Pt(cod)me 2 onto RFA at 80 °C led to concurrent reduction and Pt nanoparticle growth.

Compressive behaviors and morphological changes of resorcinol–formaldehyde aerogel at high strain rates

The dynamic compressive properties of resorcinol–formaldehyde (RF) aerogel were investigated using a spilt Hopkinson pressure bar. The effects of strain rate, water absorption and sample basal area on the dynamic behaviors of RF aerogel were investigated through a series of dynamic experiments. Morphological changes of RF aerogel under compression were studied by SEM, TEM, BET and BJH methods. Results show that the compressive behaviors of RF aerogel display a remarkable strain rate strengthening effect. The water-saturated RF aerogel shows stiffened behavior at high strain rates in comparison with the dry RF aerogel, but the dynamic failure strain is small. The dynamic compressive behaviors of RF aerogel display a remarkable size effect. The stress increases with the sample basal area at the same strain. At high strain rates, the pores shrink rapidly; RF particles fuse together to form larger particles and surface area reduces rapidly. It is the fusing of gel particles that allows RF aerogel to be much more ductile than silica aerogels and not to break into fragments at high strain rates.

Sol–gel synthesis of nanocomposite crystalline HMX/AP coated by resorcinol–formaldehyde

This work aims to improve the performance of composite explosive by using the sol–gel method to mix high explosive and oxidizer in nanoscale. Nanocomposite materials of HMX and AP were prepared by using resorcinol–formaldehyde (RF) as binder. Its structure was characterized by scanning electron microscopy (SEM), BET method, X-ray powder diffraction (XRD), and DSC. SEM images indicate that HMX/AP/RF aerogel has a laminate-like structure with uniform pores. The XRD results show that the mean crystal size of HMX is less than 100 nm; HMX and AP are mixed uniformly in nanoscale. The specific surface area of HMX/AP/RF is 27 m 2/g and much less than that of RF aerogel. The mesopores and micropores of HMX/AP/RF aerogel mainly focus in the range of 2–20 and 0.6–1.6 nm, respectively. DSC analysis indicates that the thermal decomposition temperature of HMX/AP/RF is reduced compared to that of original HMX.

Fabrication of Hollow Carbon Spheres with Porous Shells by Interfacial Sol–Gel Polymerization

Abstract Hollow resorcinol–formaldehyde (RF) aerogel spheres were synthesized by interfacial polymerization of resorcinol and formaldehyde monomers with NaOH as a catalyst. Commercial hollow polystyrene spheres were used as catalyst storage materials and templates in the synthesis. Hollow carbon aerogel spheres were prepared by carbonization of the RF aerogel spheres at a high temperature under a nitrogen atmosphere. The processing procedures and the conditions of the structures of hollow carbon spheres were studied.

Optimization of Phase Transfer Catalysis for In Situ Coating of Resorcinol Formaldehyde Targets

To produce foam capsule fusion energy targets with smoother surfaces, we have combined two previously reported resorcinol formaldehyde (RF) aerogel synthesis techniques. Using this new hybrid process, we have successfully produced aerogel spheres that are coated in situ with a smooth submicron-thick skin. The surface roughness of these spheres is compared to the conventionally synthesized RF capsules. We also illustrate the microscopic surface morphology of the new and traditional techniques. We propose that this new approach to capsule synthesis be investigated further as a target candidate; both the enhanced smoothness and the altered surface morphology make for a more desirable coating substrate.

Smooth Membrane Formation on Resorcinol-Formaldehyde Aerogel Balls Gelated Using a Basic Phase-Transfer Catalyst

Low-density foam balls with a diameter of ~1 mm were produced from a density-matched emulsion consisting of a resorcinol-formaldehyde (RF) aqueous solution (W) and an exterior oil of carbontetrachloride/(mineral oil) (O). Phase-transfer catalysts such as an alkyl amine were dissolved in the exterior oil, following which the catalyst moved into the RF solution from the exterior oil. A gelation process was monitored by a complete gelation test. When the basic catalysts were used at room temperature as a phase-transfer catalyst, gelation occurred within 30 to 120 min, whereas when the acidic catalyst was used, gelation occurred within 20 to 30 min at room temperature. When ~0.39 wt% of triethylamine and tri(n-butyl)amine in the oil phase were used, complete gelation took place. A basic catalyst with a long alkyl chain such as dimethyl(n-hexyl)amine did not induce gelation. The gelated balls obtained using the basic catalyst with a short alkyl chain were dried by extraction using supercritical fluid CO2 and the solvent was replaced with 2-propanol to produce the foam structure. Except 0.39 wt% tri(n-butyl)amine, the basic catalysts yielded foam balls with higher densities of 173 to 184 mg/cm3 as compared to those obtained from a benzoic acid catalyst, namely, 158 mg/cm3. The density difference can be attributed to the inclusion of the basic catalyst in the RF solution. Scanning electron microscopy images revealed a surface membrane formation, which can be explained by local concentration at the W/O interface. The cell size of the bulk foam was observed to depend on the catalysts, and it was surmised that the cell sizes varied because of the different gelation rates. A smooth surface membrane tri(n-butyl)amine was used as a catalyst. The membrane obtained on using a basic phase-transfer catalyst was smoother than that obtained on using an acid catalyst. Such a smooth membrane is useful for coating the ablation layer of foam capsule targets.

Tin-Doped Resorcinol-Formaldehyde Aerogel with Decanano-Cell Structure

Tin-doped resorcinol-formaldehyde (RF) aerogel was synthesized through immersion of tin (IV) alkoxide in RF gel and drying with supercritical carbon dioxide. The obtained density was 150-280 mg/cm3 , depending on the synthesis conditions. Despite the tin doping, the density is similar to that of undoped aerogel, implying that the shrinkage was suppressed. The cells were ∼50 nm in size, much finer than undoped RF. Such characteristics were discussed as they relate to chelate formation between tin (IV) and RF ligands.

Macroporous Electrically Conducting Carbon Networks by Pyrolysis of Isocyanate-Cross-Linked Resorcinol-Formaldehyde Aerogels

Carbon (C) aerogels are made by pyrolysis of resorcinol-formaldehyde (RF) aerogels under Ar, and they combine electrical conductivity with a high open mesoporosity. However, because macropores are known to facilitate mass transfer, macroporous C-aerogels could be useful for application in separations or as fuel cell and battery electrodes. Macropores are typically incorporated in C-aerogels during gelation of the RF precursors by using either “hard” templating with silica or polystyrene beads, or “soft” templating with surfactants. Here, we report an alternative method, where open macroporosity is introduced by pyrolyzing RF aerogels whose skeletal nanoparticles have been cross-linked covalently with an isocyanate-derived polymer that coats conformally the entire RF framework. The structural, physical, and chemical evolution of the X-RF network was monitored at various stages during pyrolysis by DSC, TGA, SEM, N2 adsorption porosimetry, and 13C NMR. The accumulated evidence shows that the cross-linker fir...

Structure development of resorcinol-formaldehyde gels: Microphase separation or colloid aggregation

Time-resolved small-angle x-ray scattering (SAXS) is used to follow the formation of resorcinol-formaldehyde (RF) gels. An existing morphological model based on Gaussian random fields, and validated on RF aerogels, is generalized to analyze the data. The generalization is done in two different ways, one being relevant to colloid aggregation and the other to microphase separation. The SAXS data do not enable discrimination between the two mechanisms of gel formation, which shows that aggregation and microphase separation can generate very similar morphologies at the length scales explored by SAXS. Furthermore, physical arguments suggest that, in the case of RF gels, aggregation and microphase separation can be regarded as two idealizations of the same complex physical process.

Modifying the Pore Size of Resorcinol Formaldehyde Aerogels for Fabrication of Hollow Spheres for Direct Drive ICF Experiments

AbstractThis work investigates an alternative way to modify the pore size of a 100 mg/cc resorcinol formaldehyde (R/F) aerogel without any significant change to the aerogel target density. This was successfully accomplished by an addition of hydrophilic polymer additive [Poly Vinyl Alcohol (PVA) or Poly Acrylic Acid (PAA)] to the R/F precursor solution which acts as an impurity in the reaction. The polymer can modify the cross linking or aggregation of the primary particles which can change the structure formation of the aerogel, thus changing the pore size. This paper will discuss this process modification and the fabrication of hollow, large pore R/F aerogel spheres that are used for direct drive inertial confinement fusion (ICF) cryogenic ice layering experiments at the University of Rochester Laboratory for Laser Energetics (LLE). The aerogels were characterized using scanning electron microscopy (SEM), nitrogen gas adsorption, and ultra small angle x-ray scattering (USAXS).