Results

1. Density

Densities were calculated using the physical dimensions and the mass of the samples, employing calipers and a microbalance, respectively. Therefore, the density measurements refer to the ‘bulk’ densities of the samples. Bulk densities ranged from 0.07 g.cm3 to 0.06 g/cm3. Densities quoted in the literature for aerogels fabricated by supercritical carbon dioxide extraction method are as low as 0.005 g/cm3 and as high as 0.03 g/cm3.

2. Surface Area and Pore Measurements

A Micromeritics Nitrogen Gas Adsorption System Model 2010 was used to perform surface area and porosity analysis on aerogel samples. Adsorption occurs when gas/liquid phase particles deposit on a solid substrate. Adsorption increases with decreasing temperature or increasing pressure. Isotherms are required to establish the structural characteristics. Isotherms represent the quantity of gas taken up/released at constant temperature as a function of gas pressure. For this system the analysis was done at liquid Nitrogen temperature of 77.35K. Generally, the surface area is calculated (using BET Theory) when a single molecular layer (monolayer) is formed on the surface, knowing the mean surface area occupied by each molecule. The pore characteristics are calculated (using BJH Theory) near the bulk condensation point from the fact that smaller pores will fill up quicker than the larger pores by the multiple layers being adsorbed. Pore distribution analysis showed that majority of the pores in the mesoporous range around 70nm. Our samples exhibited surface areas between 320m2/g and 440m2/g. Surface areas quoted in literature for silica-aerogel using supercritcal carbon dioxide extraction technique are as high as 900m2/g.

3. Thermal Conductivity

A Hot Disk thermal constant analyzer was used to measure the thermal conductivity of the samples. This instrument uses the transient plane source method to calculate the thermal conductivity and diffusivity of the sample. The sensor is placed between two halves of the sample and heat is generated at the sample surface by the sensor. The sensor acts both as a heat source and a ‘resistance thermometer’ for recording the time dependant temperature increase. The thermal conductivity is calculated from the time dependant temperature increase using an infinite slab model. The figure to the right shows the sensor sandwiched between two aerogel samples. Preliminary measurements of aerogel thermal conductivities were obtained. Thermal conductivities ranged from 0.032W/m/K to 0.035W/m/K.

4. Transmission

A Perkin-Elmer Lambda 900 spectrophotomer was used to study the transmission characteristics of the aerogel samples. This instrument probes the sample with light wavelengths ranging from 200nm to 2500nm. A detector on the other side collects the transmitted light and plots intensity as a function of transmitted wavelength. The plot on the right is a characteristic plot for silica-aerogels. The plot shows that our aerogels have transmission rates in the 80-90% range.