Structural and thermal study of highly porous nanocomposite SiO2-based aerogels

doi:10.1016/j.jnoncrysol.2010.01.003

Journal of Non-Crystalline Solids

Volume 356, Issues 18–19, 15 April 2010, Pages 879-883

https://doi.org/10.1016/j.jnoncrysol.2010.01.003 Get rights and content

Abstract

Nano-porous silica-based aerogels with additive of TiO₂ were successfully synthesized by an ambient pressure drying method. After aging and washing of wet gel, surface modification with cyclohexane was followed to replace the solvent in the pores of wet gel to reduce the surface tension during ambient drying. The microstructure and thermal properties of the SiO₂–TiO₂ aerogels were characterized by scanning electron microscopy (SEM), transmitting electron microscopy (TEM), high resolution electron microscope (HREM), Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD). The results showed that silica aerogels exhibit a sponge-like microstructure. The spherical SiO₂ particles with a size of a few tens of nm form a 3-dimensional network containing homogeneous pores. Nano-TiO₂ powders are physically embedded by SiO₂ aerogel, and there is an obvious Ti–O–Ti and Si–O–Si bonding group based on structural analysis. The optical transmittance of the heat-treated composite aerogels was obviously decreased with increasing nano-sized TiO₂ additive.

Introduction

Nano-porous silica aerogels are unique materials often having a high specific surface area, a high porosity (75–99%), a low thermal conductivity (0.01–0.03 W/mK), and a low index of refraction [1], [2], [3], [4]. Because of their unique properties, SiO₂ aerogels have been extensively studied, not only for use as transparent thermal insulators but also as inter-metal dielectric materials, optical and acoustic applications, and the space industry [5], [6], [7], [8].

Often SiO₂ aerogels are fabricated by supercritical drying of wet gels and usually using expensive TEOS (tetraethylorthosilicate) as silica source. Supercritical drying process can avoid capillary stress and associated drying shrinkage, which are usually prerequisite of obtaining aerogels structure. However, supercritical drying process is so energy intensive and dangerous that real practice and commercialization are difficult [9]. So it is very necessary to prepare SiO₂ matrix aerogels by ambient pressure drying technique at a reasonable cost.

Generally monolithic SiO₂ aerogels provide a whole low thermal conductivity due to its extremely high porosity [10], [11]. However, at ambient and higher temperatures, radiative absorption/emission becomes the dominant heat transfer mechanism. Aerogel is poor thermal insulator because it is highly transparent in the 3–8 μm wavelength regions. The heat transfer through monolithic aerogel generally treats the conduction and radiation as additive contributions by defining an effective radiative conductivity [12]. To improve its thermal insulation capacity, approaches such as doping aerogel with carbon have been applied to minimize infrared radiation heat transfer. The opacifier also suppresses radiation transport by eliminating the infrared transparent window of aerogel and strengthens the brittle monolithic aerogel when it incorporated into the aerogel matrix. Therefore, an appropriate selection of opacifier type and concentration is critical to optimize the thermal insulation capacity of the material, especially at high temperatures.

TiO₂ is an efficient opacifier due to its high reflection index, thermal stability and strong broad-band absorber. The specific extinction of the opacified aerogel is drastically increased, especially in the range from 2 to 8 μm. Mineral powder integrated silica aerogels have total thermal conductivities of about 0.025 W/mK at 300 K and 0.038 W/mK at 800 K in air [7], [13]. The objective of this study was to prepare a series of nanostructured SiO₂ aerogels doped with TiO₂ powder. The effect of TiO₂ powder additive on the microstructure and physiochemical properties of SiO₂–TiO₂ aerogels was characterized and discussed.

Section snippets

Sample preparation

SiO₂ composite aerogels doped TiO₂ powder (1–8 wt% TiO₂ in the final aerogel) were prepared according to the acid (HCl)-catalyzed sol–gel procedure. Briefly, tetraethoxysilane (Si(OC₂H₅)₄, Aldrich 98.5%, TEOS), EtOH and H₂O with molar ratios of 1:7:4 were used as precursors for the silica. The pH value of above mixture was adjusted to 2 by HCl (0.2 mol/L). The TiO₂ powder (diameter of 10–30 nm, 95% purity, containing anatase and rutile phase) was ultrasonically dispersed for 60 min in the

Structural properties

Fig. 1 shows the microstructure of silica matrix aerogel composite observed by SEM. TiO₂ particles were dispersed within silica aerogel matrices and most TiO₂ particles were adhered to silica network.

As the arrows in Fig. 1 indicated, there are small irregularly localized white particles with uniform diameters of approximately 50 nm. These nanoparticles are attributed by EDS analysis (not shown here) to titania oxide (TiO₂) incorporated into silica glass.

TEM micrographs of the obtained aerogels

Structural properties

Based on IUPAC classification for porous materials, the pores less than 2 nm in diameter are termed “micropores”; those with diameters between 2 and 50 nm are term “mesopores”, and those greater than 50 nm in diameter are termed “macropores”. The majorority of the pores in this study fall in the mesopore range (10–15 nm from Fig. 2(a)), with relative few macropores due to the additive of TiO₂ powder (as shown in Fig. 1).

The TiO₂ crystal peaks were found during heat treated at 500–1000 °C and all of

Conclusions

Silica composite aerogels with TiO₂ additive were successfully synthesized by sol–gel and ambient pressure drying method. The composite silica aerogels exhibit a sponge-like microstructure. The spherical SiO₂ particles with a size of a few tens of nm form a 3-dimensional network containing homogeneous pores. There is no chemical reaction during synthesis process and heat-treatment between SiO₂ aerogel and TiO₂ powder. The additive of TiO₂ can greatly decrease the transmittance both in room

Acknowledgement

This work was funded by the National Natural Science Foundation of China (No. 50902026)

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Structural and thermal study of highly porous nanocomposite SiO2-based aerogels

Abstract

Introduction

Section snippets

Sample preparation

Structural properties

Structural properties

Conclusions

Acknowledgement

J. Non-Cryst. Solids

Int. J. Heat. Mass Transfer

Mater. Lett.

J. Non-Cryst. Solids

J. Non-Cryst. Solids

J. Phys. Chem. Solids

J. Non-Cryst. Solids

Structural and thermal study of highly porous nanocomposite SiO₂-based aerogels