High Porosity Aerogel Research Articles

Observation of a superfluid He-3 A- B phase transition in silica aerogel

New NMR studies of 3He in high-porosity aerogel reveal a phase transition from an A-like to a B-like phase on cooling. The evidence includes frequency shift and magnetic susceptibility data, and similar behavior is found in two quite different aerogel samples. The A-like phase is stable only very near to T(c) but can be supercooled to below 0.8T(c). This behavior has been seen clearly at 32- and 24-bar pressures, and the presence of negative frequency shifts suggests that an A-like phase exists near T(c) at pressures as low as 12 bars in a magnetic field of 28.4 mT.

Cooper pairing in 3He in the presence of spin-polarized scattering centers

The influence of spin-exchange scattering centers on the triplet Cooper pairing is considered theoretically in an attempt to model the behavior of 3He in high-porosity aerogel with magnetically active silica stands. The homogeneously located and isotropically scattering system of spin-polarized centers is adopted as a simple model to explore the dependence of the superfluid critical temperature on the polarization S T of scattering centers. It is demonstrated that for the equal-spin-pairing case this dependence stems from an interference between the potential and exchange parts of the quasiparticle scattering amplitudes, and the main effects is linear in S T leading to the splitting of superfluid transition for ↑↑ and ↓↓ Cooper condensates.

Nuclear magnetic resonance of3He superfluid confined in high porosity aerogel

We report pulsed nuclear magnetic resonance (NMR) measurements of the transverse frequency and magnetization of3 He confined in aerogel with 98.2% open volume porosity. A homogeneous superfluid phase is observed at pressures, P > 12 bar, accompanied by a sharp onset of frequency shifts showing little or no textural broadening; a result quite unexpected for3He confined in random porous media. The transition temperatures and order parameter are both significantly suppressed from their bulk values and the magnetization data indicate this low temperature phase is an equal-spin pairing superfluid. For NMR tipping angles greater than 40 ° the resonance frequency drops abruptly to the normal state value. This tip angle dependence is not consistent with either the bulk3He-A or3He-B superfluid phases.

Processing and Characterization of High Porosity Aerogel Films

AbstractAerogels are highly porous solids having unique morphology among materials because both the pores and particles making up the material have sizes less than wavelengths of visible light. Such a unique morphology modifies the normal molecular transport mechanisms within the material, resulting in exceptional thermal, acoustical, mechanical, and electrical properties. For example, aerogels have the lowest measured thermal conductivity and dielectric constant for any solid material. Special methods are required to make aerogel films with high porosity. In this paper, we discuss the special conditions needed to fabricate aerogel films having porosities greater than 75% and we describe methods of processing inorganic aerogel films having controllable thicknesses in the range 0.5 to 200 micrometers. We report methods and results of characterizing the films including thickness, refractive index, density (porosity), and dielectric constant. We also discuss results of metallization and patterning on the aerogel films for applications involving microminiature electronics and thermal detectors.

Second and fourth sound modes for superfluid helium in aerogel

Because of the interest in superfluid onset in porous media (high porosity aerogel in particular) one would like to measure the superfluid fraction as well as the heat capacity near T c . However, standard techniques such as torsional oscillators and fourth sound have encountered difficulties because the compliant aerogel allows the viscous normal fluid to move. Using data we obtained for fourth sound propagation below 1.8 K, we develop a theory which fits the data with no adjustible parameters, and show that the theory predicts another second-sound-like mode which may not be damped near T c .

Second and fourth sound modes for superfluid helium in aerogel

Because of the interest in superfluid onset in porous media (high porosity aerogel in particular) one would like to measure the superfluid fraction as well as the heat capacity near T c . However, standard techniques such as torsional oscillators and fourth sound have encountered difficulties because the compliant aerogel allows the viscous normal fluid to move. Using data we obtained for fourth sound propagation below 1.8 K, we develop a theory which fits the data with no adjustible parameters, and show that the theory predicts another second-sound-like mode which may not be damped near T c .