SiO2 Xerogel Research Articles

The effect of sol viscosity on the sol–gel derived low density SiO2 xerogel film for intermetal dielectric application

SiO2 sol was prepared using a typical two-step acid/base catalyst procedure to investigate the effect of sol viscosity on the microstructure of SiO2 xerogel film. The viscosity of coating sol was varied in the range of 4 to 50 cP. With coating sols in the viscosity range of 10–40 cP, stable SiO2 xerogel films of 73.5–47.5% porosity could be prepared. During the spin coating of sol, high evaporation of solvent was inevitable and this induced the accelerated and abnormal gelation. The microstructure of SiO2 xerogel was found to be largely dependent on the viscosity of coating sol. Dielectric constant and leakage current density were obtained in metal-insulator-semiconductor structure. The measured dielectric constants were ranged from 1.99 to 2.45 according to the porous nature of xerogel films. Leakage current density measurements showed that at low electric field strength the conduction behavior of low density xerogel film was ohmic conduction, but at high field strength, Poole–Frenkel electron emission was the conduction mode.

Silylated Coumarin Dyes in Sol−Gel Hosts. 2. Photostability and Sol−Gel Processing

The photostability characteristics of numerous silylated coumarin dyes within SiO2 xerogels and SiO2:PDMS Polyceram films, and of neat silylated coumarin dye films have been determined and related to the sol−gel processing conditions and host composition. FTIR spectroscopy was used to monitor the hydrolysis reaction rates of the silylated dyes and other Si−alkoxide precursors. The silylated dyes had varying reaction rates depending on the degree of functionality and the linkage between the alkoxide and the dye. Matching the reaction rates between TMOS and the silylated dye was accomplished by prehydrolysis of the silylated dye. The photostability of the dyes in the xerogels was measured by monitoring the drop in fluorescence intensity upon pumping with a N2 laser and also by monitoring the drop in dye absorption upon irradiating the films with a UV lamp. At optimized prehydrolysis times, a silylated coumarin dye (derCoum) within a SiO2 xerogel host demonstrated a 3-fold improvement in long-term photostabi...

Synthesis and study of quantum size effect, XRD and IR spectral properties of PbS nanocrystals doped in SiO2 xerogel matrix

Abstract PbS nanocrystallites with a diameter smaller than 15 nm have been prepared in a pure silica matrix by sol-gel route. Silica xerogels containing 0.01–3 wt% of PbS were prepared by hydrolysis and polycondensation of methanolic tetramethyl orthosilicate [TMOS, Si(OCH 3 ) 4 ] in the presence of ammonium hydroxide (NH 4 OH) catalyst and lead nitrate Pb(NO 3 ) 2 and subsequent reaction with Na 2 S to form PbS nanocrystals. These PbS-nanocrystals-doped xerogels were characterised by optical absorption, X-ray diffraction (XRD) and infra-red spectroscopy (IR). Blue shift in optical absorption threshold indicate that the PbS crystallite size reduced from 16 to 5 nm with a decrease in the PbS wt% from 3 to 0.01. It was found from the XRD studies that weak and broad peaks are obtained because of nanocrystalline state of PbS. The PbS crystallite structure was found to be cubic in the SiO 2 matrix by observing the peak positions in the XRD pattern. In the IR spectra, the peaks related to PbS bonds were obtained and they are weak. It was observed that there was no change in peak position related to the SiO 2 matrix in the presence of PbS in between the SiO 2 network.

Structural characterization of SiO2 aerogels

Abstract SiO2 aerogels with densities between 0.131 and 0.182 g/cm3 were prepared by CO2 supercritical drying of wet gels formed from differently concentrated TMOS solutions by hydrolytic polycondensation under basic conditions (pH ≈ 8). The formation and growth of SiO2 particles in the originally homogeneous starting solutions has been followed in situ by SAXS (small-angle X-ray scattering). A chemical limited particle growth takes place until a reduced time of t/tg ≈ 0.3, which slowed to the point of gelation and produced, on the average, ⋍ 11 nm sized particles for all alcogels. These partices form branched and interconnected chains determining the fractal structure of the aerogels. The internal structure of these particles is disturbed by incorporated alcoholic groups caused by the basic hydrolysis and the relatively soft regime of CO2 supercritical drying. In particular, the reduced height of the first sharp diffraction peak suggests that the middle-range order is less pronounced than in the corresponding SiO2 xerogel.