Porous Silica Aerogel Research Articles

Modification of the superfluid 3He phase diagram by impurity scattering.

The effect of impurity scattering on the phase diagram of pure superfluid 3He has been investigated by acoustic techniques near the bulk polycritical pressure. Impurities were introduced with a 98% porous silica aerogel. In zero applied field, the equilibrium phase is a B phase, consistent with predictions from isotropic scattering. The superfluid transition in a magnetic field at 25 bars is from normal to A phase and is independent of the magnetic field up to 2.9 kG. A first-order phase transition between A and B superfluids is observed with surprisingly strong supercooling in both zero and nonzero applied field.

Silica aerogel vibration testing

As has been shown in previous experiments (Dolesi R et al 2000 J. Low Temp. Phys. 118 219),a porous silica aerogel network can adequately constrain on-orbit motion of superfluid helium, and subsequently remove the possibility of a `helium tide' gravitational disturbance appearing in the Satellite Test of the Equivalence Principle (STEP) signal bandwidth. Silica aerogel is used to decouple the superfluid helium from the Earth (STEP source field). This paper addresses two key issues for flight implementation. The first is to see whether the litre size samples of aerogel needed to fill the flight Dewar behave significantly differently to the millilitre size samples that have been extensively studied in the literature. We have found no significant difference. The second issue is to see whether silica aerogel filled with superfluid helium will survive launch.Based on the tests reported here, we have found no intrinsic property of the silica aerogel/superfluid helium system that would cause it to fail in a launch environment.

Acoustics of a Dirty Fermi liquid : 3He in aerogel

An acoustic cavity containing 3He in 98% porous silica aerogel was used to investigate the effects of impurity scattering in a Fermi liquid. The pressure and temperature dependence of the sound attenuation in the normal Fermi liquid was extracted from the cavity response. The attenuation of sound displays behavior very different from the bulk owing to strong elastic scattering of quasi-particles by the silica strands. Using a visco-elastic model of the Fermi liquid, we find a mean free path restricted to ∼340 nm. Information on the sound velocity is inferred from the pressure dependence of the oscillation period of the cavity response. The data can be accounted for by a Biot model of the 3He liquid in the porous aerogel.

High-frequency acoustics of 3He in aerogel

High-frequency ( approximately 15 MHz) acoustics were performed on 3He in 98% porous silica aerogel using an acoustic cavity technique. Measurements of the sound attenuation in the normal Fermi liquid and superfluid display behavior quite different from the bulk owing to strong elastic scattering of quasiparticles. The transition from first-to-zero sound is completely obscured with a quasiparticle mean-free path estimated to be in the range of 200-300 nm. No collective mode attenuation peak was observed at or below the superfluid transition.

NMR of hyperpolarised 3He gas in aerogel

We report preliminary low-field (2 mT and 0.1 T) NMR measurements on room temperature laser polarised 3He gas imbibed into a 98% porous silica aerogel. The hyperpolarised helium is introduced into the cell in which the aerogel had been grown, and large NMR signals can be recorded. Long polarisation lifetimes ( T 1≃50 s) have been observed, revealing a very weak specific wall relaxation in the cell. NMR of hyperpolarised 3He gas is thus a potential tool to measure physical parameters inside the aerogels which are used to study quantum fluids in confined geometries. This has been demonstrated using a multi-echo technique to characterize spin diffusion in an applied magnetic field gradient. Spin diffusion was found to be isotropic and its pressure dependence was studied. An estimate of the mean free path for gas atoms in the aerogel is deduced from this behaviour.

Probing silica aerogel structure with spin-polarised helium-3 gas

We report on systematic low-field NMR measurements on room temperature 3 He gas imbibed into a 98% porous silica aerogel. Laser polarised helium is used to obtain large NMR signals at 0.1 T even for small amounts of gas. We use a multi-echo technique in a controlled magnetic field gradient to characterize spin diffusion. Measurements are carried out for pressures P ranging from 10 mbar to 1 bar (mean free path of free 3 He ranging from 19 to 0.19 μm respectively). In a free gas, the damping rate 1/T 2 of the amplitude of the echoes is checked to be proportional to the known diffusion coefficient (T 2 P). Damping rates 2-5 times smaller than those in a free gas are measured within the aerogel. This reduction is quite significant at 1 bar, suggesting that 3 He- 3 He and 3 He-silica collisions occur with comparable frequencies at this density. However, a stong dependence of T 2 on P is still observed at low pressure, revealing a significant reduction of diffusion due to binary atomic collisions even for the lowest gas density. Measurements over such a large range of gas densities indirectely probe collisional effects over a wide range of sizes up to tens of μm. In our sample, results are not consistent with a single geometrical mean free path for 3 He-silica collisions.

Ordering of Helium Mixtures in Porous Gold

Torsional oscillator measurements show that the phase diagram of ${}^{3}\mathrm{He}{\ensuremath{-}}^{4}\mathrm{He}$ mixtures in porous gold is fundamentally different from that of bulk mixtures but similar to that found for mixtures in $98%$ porous silica aerogel. The similarities found in the mixture phase diagram and in the vapor pressure isotherms in these two media clarify our understanding of the nature of the ordering and the phase transitions in these systems.

Dynamic Light Scattering in Nematic Liquid Crystals in Confined Geometries

With photon correlation spectroscopy we have studied dynamical behavior in several systems of nematic liquid crystals embedded in host matrices, i.e., polymer dispersions of nematic droplets, Nuclepore membranes with cylindrical cavities, and porous silica aerogel. In all cases the dynamics of orientational fluctuations is substantially changed compared to the bulk nematic phase and some new phenomena appear. While due to the confinement no long wavelength modes of the director orientational fluctuations are present in the nematic droplets and cylinders, in the nematic liquid crystal in the aerogel a mode with the correlation length of 1–2 orders of magnitude larger than the average pore size was observed. An additional dynamic process with a nearly logarithmic autocorrelation function extending from millisecond to more than ten seconds is more or less pronounced in all three systems and we attribute it to reorientational diffusion in random potential.

Superfluidity ofH3ein Aerogel Covered with a ThickH4eFilm

We have examined the superfluidity of ${}^{3}\mathrm{He}$ in 98.2% porous silica aerogel with up to 34% ${}^{4}\mathrm{He}$ at 21.6 bars. The mixture is phase separated for ${}^{4}\mathrm{He}$ fractions between $\ensuremath{\sim}11%$ and 34%, and the ${}^{4}\mathrm{He}$-rich phase preferentially occupies the regions of highest silica density in the aerogel, thus modifying the distribution of the correlated disorder seen by superfluid ${}^{3}\mathrm{He}$. The ${}^{3}\mathrm{He}$ ${T}_{c}$ increases slightly with ${}^{4}\mathrm{He}$ content while the superfluid fraction decreases rapidly. This behavior is inconsistent with that of ${}^{3}\mathrm{He}$ in a homogeneously scattering medium and is analogous to that of a granular superconductor.

Reorientation in Random Potential: A Model for Glasslike Dynamics in Confined Liquid Crystals

Liquid crystals confined to structures with characteristic lengths below 1 mm, for example, in droplets dispersed in polymer (PDLC) or in silica aerogel, exhibit many new static and dynamic phenomena connected with the mesoscopic size, large surface to volume ratio, and strong elastic deformations imposed on the liquid crystal by the embedding matrix [1,2]. Most of the research focused on the structure and the effect of confinement on the phase transitions [3]. Some dynamic light scattering experiments, giving the fluctuations spectrum, were also performed, mainly on liquid crystals confined to porous glasses and silica aerogels [2,4]. The observed behavior shows generally a very wide distribution of the relaxation rates, different from the well defined exponentially relaxing modes in the bulk liquid crystals. The understanding of these dynamical processes is rather incomplete. In particular, little is known how much of the changes are due to the effects of the confinement on the viscoelastic properties of the liquid crystals and how much to the surface dynamical processes at the interfaces of the liquid crystal with the solid matrix, which dominate the dynamics due to the very large surface to volume ratio

3He in Aerogel – an Inhomogeneously Disordered Unconventional Superfluid

We have examined the superfluidity of3He in 98.2% porous silica aerogel with up to 34% 4He at 21.6 bar. The mixture is phase-separated for4He fractions between ∼ 11% and 34%. The4He-rich, phase preferentially occupies the regions of highest silica density in the aerogel, thus modifying the distribution of the correlated disorder seen by superftuid3He. The3He Tcincreases slightly with4He content while the superfluid fraction decreases rapidly. This behavior is inconsistent with that of3He in a homogeneously scattering medium and is analogous to that of a granular superconductor.

Thermal properties of porous silicon layers

Porous silicon layers of different morphologies and porosities have been produced and characterized by means of a photoacoustic technique, showing in some cases a very low thermal conductivity with respect to crystalline bulk silicon. The results confirm other experimental evidence and suggest in particular strong similarities between highly porous silicon layers and silica aerogel systems.

STUDY ON FRACTAL STRUCTURE OF POROUS SILICA:AEROGELS AND XEROGELS

Porous materials silica aerogels and xerogels were obtained through a sol-gel process from TEOS. Samples derived from different preparation circumstances were investigated via small-angle X-ray scattering. The power-law dependence of the scattered intensity, a characteristic of fractal structures, was observed for all the one-step aerogels. While for the xerogels and aerogels derived from a two-step sol-gel process, the power-law dependence of the scattered intensity was not observed. The xerogels are found to consist of a number of large spherical particles which were densely packed together.

Geometric Structure and Thermal Conductivity of Porous Medium Silica Aerogel

In this paper, three periodic structures are used to derive the apparent conductivity of a porous medium and a specific application is made to aerogel. The characteristics of the structure are determined using results for the porosity and specific surface area obtained from nitrogen adsorption-desorption measurement. 10 refs., 2 figs., 2 tabs.

Structural and optical characteristics of fullerenes incorporated inside porous silica aerogel

The successful synthesis of fullerene-doped silica aerogel by the sol-gel process followed by the supercritical drying technique is reported. The fullerenes (C 60/C 70) can be embedded inside micropores linked by the silica skeleton and the fullerene structures were retained in the fabrication process as seen using mass spectroscopy, Fourier transform infrared spectroscopy and transmission electron microscopy. The fulleren-doped composites were found to be able to emit intense visible light (2.26 eV) under Ar + laser (488 nm) excitation at room temperature. The luminescent intensity increased strongly with low doping of fullerenes (0.05 mol%), then decreased while the peak was red-shifted and finally quenched upon heavy doping (2.5 mol%). This phenomenon is explained as a result of the quantum confinement effect for fullerenes which produces emissions far above the inter-band gap of C 60/C 70.

Deuteron NMR study of liquid crystals confined in aerogel matrices.

The temperature dependence of the deuteron NMR line shape was studied in pentylcyanobiphenyl (5CB) and octylcyanobiphenyl (8CB) liquid crystals confined in porous silica aerogel matrices with two different pore size distributions. The average pore sizes 〈${\mathit{R}}_{\mathit{c}}$〉 of the two aerogel matrices were 0.8 and 0.2 \ensuremath{\mu}m, respectively. The deuteron NMR lines of 5CB and 8CB confined liquid crystals are in the isotropic regime broader than in the corresponding bulk phases. This observation shows that the interaction with the aerogel internal surface induces nematic order far above the bulk isotropic-nematic transition temperature. The nematic-isotropic (N-I) transition for both confined liquid crystals is continuous. No hysteresis was observed. A simple Landau-type model is used to describe these observations.

Properties of water adsorbed in porous silica aerogels

Silica aerogels have been prepared by hydrolysis and condensation of sols of composition TMOS-methanol-H2O with molar ratio TMOS:H2O=4:1 and volume ratio TMOS:methanol=0.2; 0.3; 0.4 and 0.6. Their true and apparent densities, BET surface area, total pore volume and pore size distribution have been systematically investigated as a function of densification heat treatment. The relaxation properties of water adsorbed on the highly reactive surface have been studied by dielectric and nuclear magnetic resonance techniques as a function of the H2O coverage (0 < ϑH2O < 6) and temperature.

Structural modifications of highly porous silica aerogels upon densification

Structural changes during isothermal sintering were studied for two base-catalyzed SiO2 aerogels with initial densities ρ1 = 122 and 256 kg m−3 by means of small-angle X-ray scattering (SAXS). The relevant structural parameters are the specific surface area, which is determined by the size of the compact primary particles, the mean diameter of the secondary particles, which represent the building blocks of the gel network, and finally the size of the macropores within the tenuous tertiary structure. The major finding of our investigations is the observation of two sintering processes with different time scales: a fast reduction of the size of the macropores and a slow increase in the diameter of the secondary particles as well as a relatively small decrease in the specific surface area. For the two samples investigated the changes of these parameters can be unequivocally correlated with the achieved density upon sintering – independent of the initial density.

The densification of mass-fractal aerogels to fused silica: A Raman study of vibrational evolution

The low frequency Raman spectrum has been investigated for porous silica aerogels progressively densified, starting from a mass-fractal sample to obtain finally fused silica. The spectra show a broad peak due to modes characteristic of the constituent particles of mean dimension a. In the first stage of densification the peak shifts to lower frequencies corresponding to an increase of a. The disappearance of the peak below the instrumental resolution is accompanied with the appearance of the usual fused silica spectral features.

Optical Characterization of Silica Aerogel

ABSTRACTTwo optical methods are described for mapping the local variations of refractive index within monoliths of porous silica aerogel. One is an interferometrie measurement that produces “iso-index” fringes in a two dimensional image; an orthogonal view gives the third dimension information. The other method uses the deflection of a He-Ne laser beam to map the gradient index within a sample. The quantification of the measurements is described and the accuracy of the results is discussed.