Journal of Sol-Gel Science and Technology, 2, 9t7-920 (1994) (g) 1994 Kluwer Academic Publishers, Boston. Manufactured in The Netherlands.

Preparation a n d Characterization of SiO2 Two-Step Aerogels Code: HP15 A. BECK, G. POPP, A. EMMERLING AND J. FRICKE Physikalisches lnstitut der Universitiit Wiirzburg, Am Hubland, D-97074 Wiirzburg, Germany

Abstract. Aerogels are well suited as transparent insulation materials in solar architecture and collector systems. Their nanoporous structure provides a high solar transmittance and a low thermal conductivity, generally below 0.02 W m - 1 K - 1. Transparent aerogels with densities above 80 kgm -a can easily be prepared at room temperature via a one-step sol-gel process with subsequent supercritical drying. Separating hydrolysis and condensation via a two-step method allows the preparation of transparent ultra-low density SiO2-aerogels. To optimize the optical properties, characterized by the scattering coefficient of the gels, we have investigated the influence of preparation parameters, such as pH-value of the sol-gel starting solution and macroscopic density, on the gel structure. To determine the nanostructure we performed spectral light scattering as well as small angle X-ray scattering (SAXS) measurements.

Keywords: aerogels, SAXS, light scattering 1. Introduction

2. Specimens

Aerogels [1, 2, 3] are promising materials for transparent insulation systems for passive solar energy usage. Transparent insulation technology allows to reduce the energy consumption for room heating in dwellings and industrial buildings considerably [4]. An important component of this strategy, in connection with aerogels, is the improvement of windows and daylighting systems.

Preparation of aerogels via the two-step sol-gel process [6] divides the classical procedure, which involves hydrolysis and condensation of silicon alkoxides according to nSi(OR) 4 + 4nH20 ~- nSi(OH) 4 + 4nROH r~Si(OH) 4 ~ nSiO2 + 2nH20 into two steps. In the first step tetramethoxysilane (TMOS) is mixed with a sub-stoichiometric amount of 1.3 mol water per mol TMOS under acid conditions. After partial hydrolysis and slight precondensation of TMOS, the produced sol was distilled. To prevent esterification and to stabilize the sol, the partially condensed silica (CS) was diluted in acetonitrile.

Silica aerogels provide a high transmission in the solar spectral range and a small thermal conductivity, both of which are due to the highly porous nanostructured build-up. Today the maximal size of monolithic samples is in the range of 60 cm x 60 cm x 2 cm [5]. Heat transfer coefficients of 1 W m -2 K -1 in the nonevacuated and 0.5 W m -2 K -1 in the evacuated case are typical for monolithic layers of 20 mm thickness. Aerogels with a density below 80 kgm -a prepared by the classical single-step procedure show enhanced scattering of visible light and thus a drastic decrease in transmission. In order to produce transparent aerogels in this low-density regime we have employed a two-step process [6] depending on the pH-value of the preparation process.

In the second step CS was used as precursor. To complete the hydrolysis reaction the remaining 2.7 tool of water per mol TMOS were added to the precursor. To achieve various macroscopic densities a sufficient amount of acetonitrile as solvent was added. Ammonium hydroxide was used as catalyst to induce gelation. The resulting gels were transformed into aerogels via supercritical drying [5]. The concentration of NH4OH in the solution described above is given as a pseudo pH-value, i.e. pH = 14 - log [NH4OH].

918 3.

Beck,Popp, Emmerlingand Fricke Theory

To describe the structural build-up of the investigated aerogel samples we use the following hierarchical model. The basic constituents on the lowest structural level are monodisperse nanometer-sized spherical particles. Interconnection of these primary spheres results in clusterlike building blocks which are again cross-linked. The radiation intensity scattered at these inhomogeneities is given by [7].

_~ { DfF(Df-1) I(q) = IoV 62 1+. (qR)Ds

Integration of eq. 4 over the total solid angle yields the light scattering coefficient S [8]: 87r3 . Sbulk = ~ 4 ( E - - 1)2ff(1 - ~5)

47r x -~-r(D~ +

1)~DIR(3_DI).

(5)

If we take into account large micrometersized surface imperfections which cause additional forward peaked scattering [9], the total scattering coefficient Stotal becomes: Stotal : Sbulk -}- Ssurface,

(6)

where S~urface is wavelength independent.

× si~[(Ds - 1)~rct~n(q~)] 4.

x

v~

2 ,.2'~ -2 1 + cos 2 0

1 + --~-q n )

2

(I)

I0 is the incident intensity, q = (47r/),) sin(0/2) the momentum transfer, with A the wavelength of the incident radiation and 0 the scattering angle; V is the observed scattering volume and A the illuminated detector area, r the distance between detector and scattering volume, R the radius of the primary particles, the diameter and Df the fractal dimensionof the clusters. In the case of small angle X-ray scattering (SAXS) measurements the scattering length density 6 2 is related to the classical electron radius r~ and the fluctuation of the electron density Pe around the average electron density ~: 6~AXS = r e2 v1 fv (Pe-p)2d3x.

To determine the light scattering properties of the two-step aerogels we used a Perkin-Elmer doublebeam spectrophotometer. Measuring the directionaldirectional transmission tdd(A) allows to derive the scattering coeffÉcient Stota I via Beer's law 1 Stotal = ~ in tdd

(2)

For light scattering (LS) purposes 6 2 is given in the Rayleigh-Debye approximation [8] by

(3)

where e is the dielectric constant of the primary particles and ff is the volume fraction of the solid phase. If the inhomogeneities are small compared with the wavelength, i.e. qLS << 1/~, 1/R, eq. 1 reduces to:

I(q) c< _1~DIR (Dy-a) 1 + A4

cos 2 0 2

(7)

because aerogels show no absorption in the spectral range between 300 and 800 nm. d denotes the sample thickness. In order to analyse the nanostructure SAXSmeasurements were performed using a Kratky-camera with Cu-K~-radiation (A = 0.154 nm); the scattered intensity was recorded with an one-dimensional position-sensitive detector.

5.

7r2 6~s = ~-~(e - 1)2@(1 - @),

Measurements

(4)

Results

In Fig. 1 the SAXS distributions for the three differently catalysed aerogel specimens having nearly the same density of p -- (80 + 5) kgm -a are depicted. The use of eq. 1 allows to characterize the structure of the gel network. The q-value of the cross-overs in the scattering intensity is inversely proportional to the size of the structural entities. For the primary particle radius R only a slight increase occurs with increasing pH-value (see Table 1). On the other hand, the cluster size ~ is significantly influenced by the concentration of the catalyst in the second step. An increasing pHvalue leads to a decrease in the cluster size. The fractal dimension Df = 1.9 -t- 0.1 derived from a fit to eq. 1,

SiOg. Two-Step Aerogels

Table 1. SAXS-fitparameters of the investigated two-step aerogels. R/nm

(/nm

Df

1.2 -t- 0.1 0.7 -I- 0.2 0.7 4- 0.2

10 4- 1 15 -4- 2 > 50

1.8 4- 0.1 1.9 4- 0.1 1.9 + 0.1

pH-value 13 11 9 1000 ! ~ " ' ~

~ ~ f = - 1 . 9 100

~

1,

e:

1/R

-,

919

In Fig. 2 the spectral scattering coefficients for the three specimens are depicted as a function of wavelength. The optical transmission experiments show a strong decrease of the scattering coefficients with increasing pH-value (see Fig. 2). Using eqn. 5 and 6 the Rayleigh scattering behaviour of the volume scattering coefficient (Sbulk O ( / ~ - 4 ) carl be separated from the total scattering coefficient. Typical values for the bulk scattering coefficients ofpH = 13 catalyzed specimens at a wavelength of 600 nm are in the range of Sbulk,600 = (4.5 4- 0.5) m -1.

p.;11

p.:l

pH= 9 lO

1

6.

"%, +

I

I

0.01

,

,

I

0.02

,

J

,

,

t

0.1

(i05

q

0.2

/A 1

Fig. 1. SAXS intensity of three two-step aerogels reacted at pHvalues of 9 ( - - - ) , 11 ( )and 1 3 ( - - - ) . To illustrate the potantial law of the intensity in the fractal regime a straight line with a slope of - 1 . 9 is also shown. The arrows mark the cross-overs induced by the corresponding structural size. The macroscopic density of all samples is about p = (80 4- 5) kgm -3.

'4 D

100

10

1

i

400

i

,

,

I

500

,

,

~

,

I

600

,

,

,

,

I

700

. . . .

800

wavelength / nm

Discussion

The primary patricles, which are mainly formed during the first preparation step, are not influenced by the concentration of the catalyst except for pH -- 13 aerogel; the reason for the latter is yet unknown. On the other hand the pH-value during the polycondensation reaction in the second step determines the cluster size. The decreasing cluster size with increasing pH-value can be explained by a strongly increased polycondensation rate of the primary particles with increasing pH-value. Because the amount of primary particles is given, an accelerated polycondensation reaction leads to a higher cluster number with reduced size. Since the fractal dimension D f is constant the internal structure of the clusters is not influenced by the catalyst concentration. The dependence of the light scattering coefficients on the pH-value is due to the fact, that the scattering properties are directly correlated with the structural build-up. Df = 2 yields Sbulk O( ~ 2 R (see eq. 4), i.e. the scattering coefficient is mainly affected by the cluster size. This explains the strong dependence on the pH-value. At larger wavelengths the strongly forward peaked scattering of the external sample surface begins to dominate the scattering properties [9].

Fig. 2. Spectral scatteringcoefficientsStotaI depending on the pHvalue p H ----9 (A), p H = i I ([3)and p H = 13 (×). The fulllines (-)

are fits using eq. 6. Additionally the volume scattering coefficient Sbu[k (. . . . . ) is plotted. The densities of the samples are p = (80 4- 5) kgm - a .

Acknowledgment

describing the internal cluster structure, is not affected by the different preparation conditions. In the fractal region the scattering distribution corresponds to a potential law with DI as exponent.

We would like to thank L.W. Hrubesh and R.W. Pekala, Lawrence Livermore National Laboratory, Livermore USA, for generously providing condensed silica. This work was supported by the German Federal Ministry for Research and Technology (BMFT) in Bonn.

920

Beck,Popp, Emmerling and Fricke

References 1. Fricke, J., Aerogels,Springer Proc. in Physics 6, Springer Verlag Heidelberg (1986). 2. ISA2, Proceedings of the Second Int. Symp. on Aerogels, ed. R. Vacher, J. Phallipou, J. Pelous and T. Woignier, Rev. Phys. Appl. Colloq. 24-C4, (1989). 3. Aerogels 3, Proceedings of the Third Int. Symp. on Aerogels, edited by J. Frick, J. Non-Cryst. Solids 145 (1992). 4. Goetzberger, A. and Wittwer, V., in [2], p. 82-93. 5. Henning, S., in [1], p. 38.

6. Tillotson, T.M. and Hrubesh, L.W., in [3], p. 44-50. 7. Posselt, D., Pedersen, J.S., and Mortensen, K., in [3], p. 128-132. 8. Emmerling, A., Wang, E, Popp, G., Beck, A., and Fricke, J., "Nanostructure and Optical Transparency of Silica Aerogels," IX. Intern. Conf. on Small Angle Scattering, Saclay, France (1993), to be published in J. Phys. IV (France). 9. Wang, P., Ktirner, W., Emmerling, A., Beck, A., Kuhn, J., and Fricke, J., in [3], p. 141-145.