Controlling Thermal Conductivity in Mesoporous Silica Films Using Pore Size and Nanoscale Architecture

Positron annihilation gamma energy distribution, lifetime spectroscopy and time-of-flight method were used to study surfactant-templated mesoporous silica films deposited on glass. The lifetime depth profiling was correlated to Doppler broadening and 3γ annihilation fraction measurements to determine the annihilation characteristics inside the films. A set of consistent fingerprints for positronium annihilation, o-Ps reemission into vacuum, and pore size was directly determined. The lifetime measurements were performed in reflection mode with a specially designed lifetime spectrometer mounted on a slow positron beam system. The intensity of the 142 ns vacuum lifetime component was recorded as a function of the energy of the positron beam. In a film with high porosity a reemission efficiency of as high as 40 % was found at low positron energy. Positron lifetime in samples capped by a thin silica layer was used to determine the pore size. The energy of the reemitted o-Ps fraction was measured by a time-of-flight detector, mounted on the same system, allowing determination of both o-Ps re-emission efficiency and energy in the same sample. We demonstrate the potential of the simultaneous use of different positron annihilation techniques in the study of thin porous films.

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Polyaniline nanowire arrays generated through oriented mesoporous silica films: effect of pore size and spectroelectrochemical response

Faraday Discussions ◽

10.1039/d1fd00034a ◽

2021 ◽

Author(s):

Wahid Ullah ◽

Gregoire Herzog ◽

neus vila ◽

Alain Walcarius

Keyword(s):

Mesoporous Silica ◽

Pore Size ◽

Tin Oxide ◽

Indium Tin Oxide ◽

Self Assembly ◽

Cationic Surfactants ◽

Nanowire Arrays ◽

Silica Films ◽

Oxide Electrodes ◽

Vertically Aligned

Indium-tin oxide electrodes modified with vertically aligned silica nanochannel membrane have been produced by electrochemically assisted self-assembly of cationic surfactants (cetyl- or octadecyl-trimethylammonium bromide) and concomitant polycondensation of the silica...

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Microstructural Analysis of Disordered and Ordered Mesoporous Silica Films

Microscopy and Microanalysis ◽

10.1017/s1431927600023758 ◽

1998 ◽

Vol 4 (S2) ◽

pp. 726-727

Author(s):

C. A. Drewien ◽

Y. Lu ◽

C. J. Brinker ◽

R. Ganguli ◽

M. T. Anderson

Keyword(s):

Mesoporous Silica ◽

Pore Size ◽

Size Structure ◽

Sol Gel ◽

Microstructural Analysis ◽

Dip Coating ◽

Inorganic Membranes ◽

Cross Sectional ◽

Silica Films ◽

Plan View

Processing can be controlled to produce a family of mesoporous silica films with either disordered, lamellar, hexagonal, or cubic pore distributions[l]. These films, formed by surfactant-templated synthesis and exhibiting a unimodal pore size, promise potential use as inorganic membranes, catalysts, and optically-based sensors[l,2]. The mesoporous films can be formed from initially homogeneous silica sols by a continuous, surfactant-templated process, which relies upon solvent evaporation during the sol-gel dip-coating process. Films of 100-500 nm thick are formed within seconds in a continuous coating operation. The microstructure of the films is dependent upon the cationic surfactant concentration CTAB (CH3(CH2)15N+(CH3)3Br-) and the dip-coating rate. Several films, processed under differing conditions, were investigated by TEM to characterize pore size, structure, and orientation.Figures 1 a & b show the plan view and cross-sectional microstructure of a 2-d hexagonal mesoporous silica film deposited on silicon; the sample was calcined at 400 °C for 3 hours in air. The images were obtained on a Philips CM30 TEM, operated at 300 kV.

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Estimating Density Reduction and Phonon Localization From Optical Thermal Conductivity Measurements of Porous Silica and Aerogel Thin Films

ASME/JSME 2011 8th Thermal Engineering Joint Conference ◽

10.1115/ajtec2011-44137 ◽

2011 ◽

Cited By ~ 2

Author(s):

Patrick E. Hopkins ◽

Bryan J. Kaehr ◽

Darren Dunphy ◽

C. Jeffrey Brinker

Keyword(s):

Thin Films ◽

Thermal Conductivity ◽

Mesoporous Silica ◽

Porous Silica ◽

Thermal Effusivity ◽

Solid Matrix ◽

Silica Films ◽

Density Reduction ◽

The Difference ◽

Phonon Localization

In this work, we measure the thermal conductivity of mesoporous silica and aerogel thin-films using a non-destructive optical technique: time domain thermoreflectance (TDTR). Due to the rough surfaces of the optically transparent silica-based films, we evaporate an Al film on a glass cover slide and fabricate the silica structures directly on the Al film, providing a “probe-through-the-glass” configuration for TDTR measurements. This allows the thermal conductivity of mesoporous silica and aerogel thin films to be measured with traditional TDTR analyses. As the thermoreflectance response is highly dependent on the thermal effusivity of the porous structures, we estimate the density of the films by varying the heat capacity in our analysis. This density determination assumes that the solid matrix in the silica structure has the thermal conductivity as bulk SiO2, which is valid if all the lattice vibrations are localized, consistent with the minimum thermal conductivity concept. We independently determine the density of the porous silica films with nitrogen sorption measurements of thin films using a surface acoustic wave (SAW) technique. The difference between the determined from the SAW technique and that estimated by the TDTR effusivity analysis lends insight into the relative contributions of localized and propagating modes to thermal transport.

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Thermal Conductivity of Sol-Gel Amorphous Mesoporous Silica Thin Films: Molecular Dynamics Simulations Versus Experiments

ASME/JSME 2011 8th Thermal Engineering Joint Conference ◽

10.1115/ajtec2011-44046 ◽

2011 ◽

Author(s):

Thomas Coquil ◽

Laurent Pilon

Keyword(s):

Experimental Data ◽

Thermal Conductivity ◽

Molecular Dynamics ◽

Mesoporous Silica ◽

Pore Size ◽

Amorphous Silica ◽

Sol Gel ◽

Md Simulations ◽

Silica Matrix ◽

Simulation Cell

This study reports non-equilibrium molecular dynamics (MD) simulations predicting the thermal conductivity of amorphous mesoporous silica. The heat flux was imposed using the Muller-Plathe method and interatomic interactions were modeled using the van Beest, Kramer and van Santen (BKS) potential. First, simulations were validated against results reported in the literature for dense quartz and amorphous silica. The BKS potential was found to significantly overestimate the thermal conductivity of dense amorphous silica and results depended on the length of the simulation cell. Then, highly ordered pores were introduced in an amorphous silica matrix by removing atoms within selected areas of the simulation cell. Effects of the simulation cell length, pore size, and porosity on the thermal conductivity were investigated at room temperature. Results were compared with predictions from commonly used effective medium approximations as well as with previously reported experimental data for films with porosity and pore diameter ranging from 20% to 48% and 30 to 180 Å, respectively. Predictions of MD simulations overestimated the experimental data and agreed with predictions from the coherent potential model. However, MD simulations confirmed that thermal conductivity in sol-gel amorphous mesoporous materials was independent of pore size and depended only on porosity.

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