Hydrophobic silica aerogel production at KEK

Abstract

We present herein a characterization of a standard method used at the High Energy Accelerator Research Organization (KEK) to produce hydrophobic silica aerogels and expand this method to obtain a wide range of refractive index (n = 1.006−1.14). We describe in detail the entire production process and explain the methods used to measure the characteristic parameters of aerogels, namely the refractive index, transmittance, and density. We use a small-angle X-ray scattering (SAXS) technique to relate the transparency to the fine structure of aerogels.

Progress in Development of Silica Aerogel for Particle- and Nuclear-Physics Experiments at J-PARC

“…The most basic solvent in the wet-gel synthesis process is methanol; instead, the KEK-Matsushita group used ethanol when they produced aerogels with n < 1.02 to obtain long transmission lengths [3]. Currently, DMF is persistently used as a solvent to fabricate aerogels with n > 1.04 (the modernized KEK method) [1]. In the above (modernized) conventional method, the index of refraction of the aerogel is controlled in the wet-gel synthesis process by adjusting the ratio of the chemical solutions.…”

“…When we use aerogels as radiators in Cherenkov counters, the aerogel transparency is an important parameter for detector performance. An aerogel's transparency is parameterized by the transmission length Λ T (λ) = −t/lnT (λ), where λ is the wavelength of the emitted light, t is the aerogel thickness, and T (λ) is the transmittance measured with a spectrophotometer [1]. Another characteristic of the aerogel is its index of refraction n, which is determined by the silica-air volume ratio and is tunable over a given range, as discussed below.…”

“…The bulk density is also a useful aerogel parameter for certain applications; e.g., as a medium to capture hypervelocity comic dusts intactly. An empirical relationship exists between the index of refraction and the density ρ: n(λ) − 1 = k(λ)ρ, where k(λ) is a constant that depends on the wavelength of light and on the fine structure of the aerogel (i.e., its production method) [1].…”

“…At that time, the classic method allowed us to produce aerogels with a maximum index of refraction of 1.10; however, for practical use, the transmission length had to be improved. In addition, it was impossible to produce aerogels with n > 1.14 with the sol-gel method [1]. For low indices of refraction, aerogels with n = 1.008 (density of 0.03 g/cm 3 ) were used for capturing cosmic dust at low earth orbit in the MicroParticles Capturer (MPAC) experiment implemented by the Japan Aerospace Exploration Agency aboard the International Space Station (e.g., Ref.…”

“…The program began by introducing a new solvent, N,N-dimethylformamide [DMF, HCON(CH 3 ) 2 ] in the sol-gel step. This resulted in improving the transmission length for the range n = 1.03 to 1.065 [1,8,9]. The study was also motivated by the Belle detector upgrade program at the KEK [10,11].…”