Comparison of some physical properties of silica aerogel monoliths synthesized by different precursors

doi:10.1016/S0254-0584(98)00217-X

Materials Chemistry and Physics

Volume 57, Issue 3, 25 January 1999, Pages 214-218

https://doi.org/10.1016/S0254-0584(98)00217-X Get rights and content

Abstract

The experimental results on the physical properties such as monolithicity, optical transmission, thermal conductivity and porosity of silica aerogels processed by three different precursors: (a) tetraethoxysilane (TEOS) (b) polyethoxydisiloxane (PEDS) (c) tetramethoxysilane (TMOS) are reported. The aerogels have been prepared by sol–gel polymerization of the parent solvent diluted precursor in the presence of a catalyst, followed by supercritical CO₂ solvent exchange and drying. It has been found that the monolithicity of the aerogels is strongly dependent on the type of catalyst used for each precursor. For TEOS and PEDS precursors, acid catalysts and for TMOS precursor base catalysts resulted in monolithic aerogels. It has been found that the optical transmission at 900 nm for 1 cm thick sample of the TMOS and PEDS precursor aerogels are the highest (>92%) and far lower (∼70%) for the TEOS precursor aerogels. The thermal conductivities of the PEDS and TMOS aerogels have been found to be lower (0.015 and 0.020 W m⁻¹ K⁻¹, respectively) compared to the TEOS (0.060 W m⁻¹ K⁻¹) aerogels. The pore sizes obtained from the N₂ adsorption measurements varied from 30 to 180 nm, 60 to 190 nm and 80 to 200 nm in the TEOS, TMOS and PEDS precursor aerogels, respectively. The scanning electron microscopy studies of the aerogels indicated that the PEDS and TMOS aerogels show narrow and uniform pores while particles of the SiO₂ network are very small. On the other hand, TEOS aerogels show non-uniform pores such that the number of smaller size pores are less compared to the pores of larger size while SiO₂ particles of the network are larger compared to both the PEDS and TMOS aerogels. Hence, the surface area of the aerogels prepared using TEOS precursor has been found to be the lowest (∼800 m² g⁻¹) compared to the PEDS (∼1100 m² g⁻¹) and TMOS (∼1000 m² g⁻¹) aerogels.

Introduction

It is a well known fact that the atomic density of condensed matter is, roughly speaking, a universal constant of the order of 10²⁹ m⁻³. This statement, however, does not hold true for aerogel materials which can be considered as solids with atomic densities as low as 10²⁶ m⁻³. These interesting characteristics make the aerogels to have striking properties so as to allow them to be considered as a new state of matter.

Silica aerogels are unique porous transparent materials, consisting of less than 2% silicon dioxide and more than 98% air. Due to their high porosity (up to 99%) and their large inner surface area (up to 1000 m² g⁻¹), they serve as especially active catalysts or as catalytic substrates 1, 2, 3, 4, 5, 6, filters, reinforcement agents, pigments and gellifying agents [7]. Silica aerogels as translucent or transparent superinsulating sheets in double walled window systems could help considerably to reduce thermal losses in windows and to improve the efficiency of all solar thermal energy storage devices 8, 9, 10, 11, 12. Owing to their ultra low densities (<50 kg m³) [13], low refractive indices (<1.03) [14], small pore and particle size (∼100 nm) and transparency in the visible range [15], much attention has been given to silica aerogels in recent years for their use in several technological applications including: Cerenkov radiation detectors in nuclear reactors [16], ICF targets for thermonuclear fusion reactions [17], radioluminiscent light and power systems [18], precursors for gel derived glasses and optical fibers [19].

Even though most of the published results on silica aerogels deal with TMOS precursor 14, 15, 16, 20, 21, 22, 23, 24 and TEOS precursor 25, 26, 27, 28, there is a little information in the literature regarding the comparison of physical properties like monolithicity, density, optical transmission, surface area, porosity, pore size distribution and thermal conductivity of silica aerogel monoliths synthesized by different precursors. Therefore, we have taken up systematic and detailed studies, and we present and discuss our results on the effect of various precursors such as TEOS, TMOS, and PEDS on the physical properties of silica aerogels.

Section snippets

Sample preparation

Silica alcogels were prepared by hydrolysis and polycondensation of solvent (alcohol) diluted alkoxide in the presence of a catalyst. The hydrolysis and polycondensation reaction mechanism for tetraethoxysilane (TEOS) precursor is as given below:

Hydrolysis: $Si(OC_{2} H_{5})_{4} +4H_{2} O → K Si(OH)_{4} +4C_{2} H_{5} OH$ where K is a catalyst (Critic acid).

Condensation: $Si(OH)_{4} +Si(OH)_{4} →≡Si−O−Si≡+4H_{2} O$ $Si(OH)_{4} +Si(OC_{2} H_{5})_{4} →≡Si−O−Si≡+4C_{2} H_{5} OH$

Similarly, tetramethoxysilane (TMOS) and polyethoxydisiloxane (PEDS) precursors follow the

Results and discussion

From the present studies, it has been found that the precursors: TEOS, TMOS and PEDS strongly affect the physical properties like bulk density, percentage of porosity, pore size distribution (PSDs), optical transmission, surface area, thermal conductivity and microstructure of silica aerogels. Table 1 shows the effect of various precursors on some physical properties of silica aerogels. The pore size distributions (PSDs) for silica aerogels prepared using different precursors is shown in Fig. 2

Conclusions

Silica aerogels of different physical properties have been obtained using three different precursors namely TEOS, TMOS and PEDS. So far as the monolithicity is concerned, the reproducibility of the TMOS and PEDS aerogels is 100%. On the other hand, reproducibility of the TEOS aerogels is only 80%. It has been found that the optical transmission at 900 nm (for 1 cm thick monolith) of the TMOS and PEDS precursor aerogels are the highest (>92%) and far lower (∼70%) for the TEOS precursor aerogels.

Acknowledgements

The grant received from the Department of Science and Technology (DST Project No. III 5 (21)/92-ET), Government of India, for the research work on `Silica Aerogels' is gratefully acknowledged. The authors (P.B. Wagh and D. Haranath) are highly thankful to the DST for the Senior Research Fellowships. The authors are grateful to Dr. Navneet, Director, and S.V. Rao, Incharge of SEM, Regional Sophisticated Instrumentation Centre (RSIC), Nagpur University, India, for help in taking the SEM

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¹: Present address: ASPEN SYSTEMS INC., 184 Cedar Hill, Marlborough, MA 01752, USA.

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Comparison of some physical properties of silica aerogel monoliths synthesized by different precursors

Abstract

Introduction

Section snippets

Sample preparation

Results and discussion

Conclusions

Acknowledgements

J. Mul. Catal.

Appl. Catal.

Appl. Catal.

Appl. Catal.

Sol. Energy Mater.

Sol. Energy Mater.

Nucl. Instrum. Methods

J. Non-Cryst. Solids

Adv. Coll. Interface Sci.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Mater. Chem. Phys.

Appl. Catal.