Aerogel Surface Research Articles

Morphology and Electrochemistry of Ruthenium/Carbon Aerogel Nanostructures

The structure−property relationships of nanostructured Ru/carbon aerogel composite materials have been evaluated. These new materials were prepared via a novel two-step metal vapor impregnation method which enables one to control the Ru loading through repetition of the process. The resulting microstructure is characterized by highly dispersed Ru particles (≈20−30 Å in diameter) attached to the carbon aerogel surface and distributed homogeneously throughout the material; the open pore structure of the carbon aerogels remains largely unaffected. Electrochemical characterization indicates that it is possible to increase the specific capacitance of the carbon aerogels in 1.0 M H2SO4 from ≈100 F/g to greater than 250 F/g for samples with >50 wt % Ru due to the pseudocapacitive properties of hydrous RuO2. A volumetric capacitance greater than 140 F/cm3 was observed for some samples. In addition, these materials show good cycling characteristics. Electrochemical impedance spectroscopy was unable to distinguish between double-layer capacitance and pseudocapacitance in these materials.

Effect of Aging on Alumina Gels Rheology and Aerogels Surface Area

Alumina gels were synthesized by catalyzed gelation of aluminum sec-butoxide (ASB) via the Yoldas process. The gels were aged for up to 6 months and then supercritically dried (SCD) with CO2. The molar ratio of acid to ASB was in the range of 0.01–0.6. Viscosity measurements of the gels showed a shear thinning and plastic behavior with no response up to a limiting yield stress. The gel rheology obeys the Casson model. Analysis of the viscosity as a function of the acid to alkoxide molar ratio, showed that the average molecular weight of the gels is inversely proportional to the acid to alkoxide molar ratio.

Determination of elastic properties of single aerogel powder particles with the AFM

Atomic force microscopy (AFM) allows direct measurement of local elastic sample properties by force spectroscopy. The AFM tip indents into a soft sample and the resulting relation between loading force and indentation is used to determine the elastic properties of the sample. In order to calculate the indentation an analytical expression for the sensor response vs. z-piezo displacement based on the Hertz model of mechanical contact is derived. This model is fitted to data obtained on a silica aerogel sample using a least-squares method. The aerogel powder particles were analysed individually for their surface structure and elastic behaviour. Prior to the indentation experiments, the aerogel surface was characterised by AFM in the tapping mode. The results were validated by the comparison of data obtained by using two types of cantilevers of very different stiffnesses (spring constant k=0.2 and 54 N/m) and by assessing the reversibility of the indentation process. Tip indentations smaller than 200 nm were usually reversible, whereas indentations of 2000 nm caused irreversibilities.

Aerogels as a tool to study the electrical properties of ruthenium dioxide

RuO 2 is a well-known oxidation catalyst for electrolytic generation of Cl 2. Although bulk RuO 2 is a metallic conductor, studies of mixed-oxide aerogels containing RuO 2 confirm that the surface of RuO 2 is a hydrous oxide and a mixed conductor. Our previous work showed that at room temperature, protonic and electronic conductivity occurs in mixed phase aerogels of RuO 2+TiO 2 (designated as (Ru–Ti)O x ). Impedance measurements up to 340°C demonstrate that electron hopping and/or semiconduction also occurs in (Ru–Ti)O x aerogels. Electron hopping may occur between Ru 3+ and Ru 4+ ions in the ruthenia, while n-type semiconduction may be introduced into the titania by Ru 3+ defects. The multiplicity of charge-transfer mechanisms measured on the surfaces of RuO 2-based aerogels is indicative of the complexity of the catalytic and electrocatalytic chemistry performed on RuO 2.

A Comparative Study of Surface Fractality between Polymeric and Particulate Titania Aerogels

The surface fractality was studied for polymeric and particulate titania aerogels prepared via CO2supercritical drying of titania lyogels. The surface fractal dimensions of these mesoporous aerogels were determined from their nitrogen adsorption isotherms by means of the Frenkel–Halsey–Hill method and the thermodynamic method. The results from these distinctly different methods have been found to be similar; the fractal surfaces of particulate aerogels are slightly more irregular than those of polymeric aerogels. The difference in the surface morphology between the two types of aerogels, observed with scanning electron microscopy, can be attributed to the difference between the proposed mechanisms for formation of polymeric and particulate lyogels.

Deposition of Ruthenium Nanoparticles on Carbon Aerogels for High Energy Density Supercapacitor Electrodes

The preparation and characterization of high surface area ruthenium/carbon aerogel composite electrodes for use in electrochemical capacitors is reported. These new materials have been prepared by the chemical vapor impregnation of ruthenium into carbon aerogels to produce a uniform distribution of adherent ≈20 Å nanoparticles on the aerogel surface. The electrochemically oxidized ruthenium particles contribute a pseudocapacitance to the electrode and dramatically improve the energy storage characteristics of the aerogel. These composites have demonstrated specific capacitances in excess of 200 F/g, in comparison to 95 F/g for the untreated aerogel.

Fractal Analysis of Polymeric and Particulate Titania Aerogels by Adsorption

Polymeric and particulate titania (TiO2) aerogels with high surface areas were prepared from sol−gel derived TiO2 lyogels via freeze-drying as well as CO2 supercritical drying. The surface fractal dimensions of these aerogels were evaluated with two methods based on physisorption, one by Avnir and Pfeifer, and the other, the modified form of the Frenkel−Halsey−Hill (FHH) equation. The two approaches yielded almost identical results. The surface fractal dimensions measured are approximately 2.7 and 2.8 for polymeric and particulate TiO2 aerogels, respectively, thereby indicating that the fractal surfaces of particulate aerogels are slightly more irregular than those of polymeric aerogels.

Effect of Magnetic Scattering on the 3He Superfluid State in Aerogel.

Pure {sup 3}He in highly porous aerogel forms an equal-spin pairing superfluid with transition temperatures suppressed from bulk values. We have measured the magnetic field dependence of the transition temperature from which it can be inferred that both magnetic and nonmagnetic quasiparticle scattering from solid {sup 3}He at the aerogel surface is important to the superfluid state. Replacement of the solid {sup 3}He on the surface of the aerogel with {sup 4}He gives rise to a unequal spin pairing superfluid phase. {copyright} {ital 1996 The American Physical Society.}

NMR on superfluid3He in aerogel

The transition to superfluidity of3He in high porosity (98.2%) acrogel has been observed by nuclear magnetic resonance techniques. The onset of the transition at all pressures above 13 bar is marked by a sharp increase in NMR frequency similar to that observed in bulk3He-A. This suggests that the aerogel/superfluid phase is highly homogeneous although both the transition temperature, Tc, and the amplitude of the order parameter are substantially suppressed. The acrogel strands are ≈ 50A in diameter, much smaller than the superfluid coherence length. Consequently, we have attempted to interpret our observations as an impurity scattering problem. Based on our measurements of the magnetic field dependence of Tc it appears that both magnetic and potential scattering play important roles where the magnetic scattering can be associated with solid3He on the aerogel surface. This is determined by isotopic exchange with4He, a process which appears to stabilize a new superfluid state similar to the bulk B-phase.

Low-density, hydrophobic aerogels

Low-density, hydrophobic aerogels synthesis consisting of a two-step synthesis, supercritical drying with liquid carbon dioxide and surface modification by vapor-phase methoxylation is described. No alcohol was added in the first stage of oligomer synthesis. Wet gels were produced from tetramethoxysilane or tetraethoxysilane under basic conditions by adding water and NH 4OH. Non-alcoholic diluents were mixed to control the density of aerogels. The gelation time decreased with decreasing amount of diluents. Wet gels were extracted with liquid CO 2 followed by supercritical drying at 40°C and 100 atm. Aerogel densities of 0.034–0.38 g/cm 3 were produced without cracks. Hydrophilic surfaces of aerogels were modified with methanol vapor. Methoxylation was carried out at 240°C for 10 h. The density of the hydrophobic aerogel was unchanged for 40 days under a humid, open atmosphere. The Brunauer, Emmett and Teller surface area was in the range of 750–870 m 2/g for aerogel densities of 0.05–0.2 g/cm 3.

Bulk and surface light scattering from transparent silica aerogel

Abstract Elastic light scattering has been used to study structural properties of different transparent aerogels, which may be used as filling materials in super-windows. With a goniometer having an angular resolution better than 0.6° and a He-Ne laser as the light sourcewe investigated the angular distribution of scattered intensity from transparent silica aerogels and one xerogel. The densities ranged between 0.11 and 0.60 g cm -3 . An exponential correlation function for the density fluctuations of a random porous medium has been utilised to analyse the large-angle scattering, which is dominated by bulk scattering, for different polarisation of the incident light. The determination of correlation lengths in the nanometre range was possible, because the absolute scattering intensities were determined. For relative angular dependence measurements, this range would have been accessible only to small angle X-ray scattering (SAXS). The resulting mean pore sizes between 8 nm and 50 nm and specific surface areas between 500 and 700 m 2 /g agree well with nitrogen-porosimetry data from the literature. The data compare quite well with correlation lengths calculated from specular transmittance data from an ordinary spectrophotometer. This method, which is not sensitive to the angular distribution of superposed forward scattering with large correlation lengths, has also been applied to a series of base-catalysed TMOS aerogels with different catalystconcentrations. The forward scattering peak of the signal may be attributed to correlation lengths in the micrometre range. Experimental results for aerogel surfaces with evaporated aluminium indicate that this might be due to the surface properties. A quantitative analysis, however, is not possible yet.

The Aerocapacitor: An Electrochemical Double‐Layer Energy‐Storage Device

We have applied unique types of carbon foams developed at Lawrence Livermore National Laboratory to make an “Aerocapacitor.” The aerocapacitor is a high power‐density, high energy‐density, electrochemical double‐layer capacitor which uses carbon aerogels as electrodes. These electrodes possess very high surface area per unit volume and are electrically continuous in both the carbon and electrolyte phase on a 10 nm scale. Aerogel surface areas range from 100 to 700 m2/cm3 (as measured by BET analysis), with bulk densities of 0.3–1.0 g/cm3. This morphology permits stored energy to be released rapidly, resulting in high power densities (7.5 kW/kg). Materials parameterization has been performed, and device capacitances of several tens of Farads per gram and per cm3 of aerogel have been achieved.

Thermal muonium in vacuo from silica aerogels

Muonium atoms have been observed emerging into vacuum from silica aerogel surfaces with a thermal velocity distribution corresponding to a temperature of 298(10) K after stopping positive muons from a subsurface muon beam. Yields up to 2.3(3)% could be achieved for aerogel samples of 160 mg/cm3 density. They represent an interesting alternative to the SiO2 powders previously used for muonium production in vacuum.

Nuclear relaxation of deuterium–tritium adsorbed onto silica aerogel

The longitudinal nuclear relaxation times of tritons in equilibrium deuterium–tritium (eD–T, actually D2–DT–T2 ) adsorbed onto amorphous silica aerogel has been measured from 4.2 to 23 K from 0.5 to 6.2 monolayers of coverage. Below 7 K, the relaxation time dramatically increases with decreasing temperature, especially for low coverages. A value of 16 s for 0.5 monolayer at 4.2 K may be compared with the usual electric quadrupole–quadrupole (EQQ) determined relaxation time of 0.1 s. It is shown that absorption of 10% of the tritium beta particle energy occurs in aerogel fully loaded with D–T. This decreases the concentration of J=1 T2 and increases the nuclear relaxation time. Similar energy absorption calculations in the thin layers shows that long relaxation times are expected, and that the measured values must be caused by spin–lattice relaxation with the aerogel surface. Both (temperature)−7 and exponential mechanisms are considered with no final decision being possible. Electric field gradients from the aerogel walls are considered the likely relaxation mechanism. These findings suggest that a long triton relaxation time may be achievable in ultrapure DT in the silica aerogel. Such a result would be of considerable importance to nuclear polarized inertial confinement fusion targets.

Non-hermetic packaging for hybrid microcircuits : Dr. Jerry E. Sergent and Dennis R. Kling. Electron. Packaging Prodn, 170 (May 1982)

RuO2 is a well-known oxidation catalyst for electrolytic generation of Cl2. Although bulk RuO2 is a metallic conductor, studies of mixed-oxide aerogels containing RuO2 confirm that the surface of RuO2 is a hydrous oxide and a mixed conductor. Our previous work showed that at room temperature, protonic and electronic conductivity occurs in mixed phase aerogels of RuO2+TiO2 (designated as (Ru–Ti)Ox). Impedance measurements up to 340°C demonstrate that electron hopping and/or semiconduction also occurs in (Ru–Ti)Ox aerogels. Electron hopping may occur between Ru3+ and Ru4+ ions in the ruthenia, while n-type semiconduction may be introduced into the titania by Ru3+ defects. The multiplicity of charge-transfer mechanisms measured on the surfaces of RuO2-based aerogels is indicative of the complexity of the catalytic and electrocatalytic chemistry performed on RuO2.

Stability and deterioration mechanism of thick film resistors : Yoshiaki Taketa and Miyoshi Haradome. Microelectron. & Reliab.13, 281 (1974)

RuO2 is a well-known oxidation catalyst for electrolytic generation of Cl2. Although bulk RuO2 is a metallic conductor, studies of mixed-oxide aerogels containing RuO2 confirm that the surface of RuO2 is a hydrous oxide and a mixed conductor. Our previous work showed that at room temperature, protonic and electronic conductivity occurs in mixed phase aerogels of RuO2+TiO2 (designated as (Ru–Ti)Ox). Impedance measurements up to 340°C demonstrate that electron hopping and/or semiconduction also occurs in (Ru–Ti)Ox aerogels. Electron hopping may occur between Ru3+ and Ru4+ ions in the ruthenia, while n-type semiconduction may be introduced into the titania by Ru3+ defects. The multiplicity of charge-transfer mechanisms measured on the surfaces of RuO2-based aerogels is indicative of the complexity of the catalytic and electrocatalytic chemistry performed on RuO2.

The methanol-silica gel system. III. Kinetics of the methoxylation process

The methoxylation process of a silica gel (Aerogel) surface pretreated at 110°C in vacuum occurs between 150 and 190°C simultaneously through the esterification of the silanol group ( k 2) and through the opening of siloxane bridges ( k 1) in such a way that ( k 1 k 2 )=0.331 at 150°C and ( k 1 k 2 )=0.644 at 190°C At lower temperature the esterification reaction is thus favored. The maximum number of −CH 3 grafted on the surface is of the order of the monolayer content of physically adsorbed methanol. It seems that the protonated CH 3OH 2 + species is an important intermediate in the methoxylation reaction since the order of magnitude of k 1 and k 2 may be derived from the ratio of the lifetime of a molecule on an adsorption site to the lifetime of the protonmethanol association.The rate process observed by infrared spectroscopy using the development of the asymetric C-H stretching band is limited by diffusion, as opposed to what has been observed for the powder. Aerogel seems to be very sensitive to an “internal sintering process” observed under mild drying conditions and which effects the narrow pores.

Critical concentrations of polymers in solutions according to measurements of the viscosity and specific surface area of aerogels resulting after sublimation of the solvent

Critical concentrations of polystyrene and cyclolinear polyphenylsiloxane solutions corresponding to rising of structural networks have been determined by viscosity measurements and by measurements of specific surface of aerogels obtained by solvent sublimation. Theoretical and experimental values of critical concentrations coincide. Conditions for preparation of aerogels with developed surface by sublimation drying have been determined. Dependence of aerogel specific surface on polymer concentration in solutions shows distinct maximum at critical concentration found by viscosimetric method.

Critical concentrations of polymers in solutions according to measurements of the viscosity and specific surface area of aerogels resulting after sublimation of the solvent

Critical concentrations of polystyrene and cyclolinear polyphenylsiloxane solutions corresponding to rising of structural networks have been determined by viscosity measurements and by measurements of specific surface of aerogels obtained by solvent sublimation. Theoretical and experimental values of critical concentrations coincide. Conditions for preparation of aerogels with developed surface by sublimation drying have been determined. Dependence of aerogel specific surface on polymer concentration in solutions shows distinct maximum at critical concentration found by viscosimetric method.

Infrared Study of OH and NH2 Groups on the Surface of a Dry Silica Aerogel

ADVERTISEMENT RETURN TO ISSUEPREVArticleNEXTInfrared Study of OH and NH2 Groups on the Surface of a Dry Silica AerogelJ. B. PeriCite this: J. Phys. Chem. 1966, 70, 9, 2937–2945Publication Date (Print):September 1, 1966Publication History Published online1 May 2002Published inissue 1 September 1966https://doi.org/10.1021/j100881a037RIGHTS & PERMISSIONSArticle Views1507Altmetric-Citations255LEARN ABOUT THESE METRICSArticle Views are the COUNTER-compliant sum of full text article downloads since November 2008 (both PDF and HTML) across all institutions and individuals. These metrics are regularly updated to reflect usage leading up to the last few days.Citations are the number of other articles citing this article, calculated by Crossref and updated daily. Find more information about Crossref citation counts.The Altmetric Attention Score is a quantitative measure of the attention that a research article has received online. Clicking on the donut icon will load a page at altmetric.com with additional details about the score and the social media presence for the given article. Find more information on the Altmetric Attention Score and how the score is calculated. Share Add toView InAdd Full Text with ReferenceAdd Description ExportRISCitationCitation and abstractCitation and referencesMore Options Share onFacebookTwitterWechatLinked InReddit PDF (2 MB) Get e-Alerts