Experimentally Obtaining Diffusion Constants of Catalytic Aerogels

Aerogels’ have characteristically large surface area to volume ratios as well as thermophysical properties whicy allow them the potential to be used as catalytic substrates. In order to use them in this way, we must first understand how gas flows through them. This project sets out to design an experimental apparatus and methodology that can be used to obtain "diffusivity" values of various kinds of aerogels. The flow of gases through a porous material can be characterized by the material’s diffusivity which relates the flux of gas through the material to the gradient driving the flow which, in this case, is pressure. By obtaining diffusivity values for a certain type of aerogel over a range of pressures, we should be able to identify the theoretically linear relationship between pressure and diffusivity when in the viscous flow regime. This would allow us to predict the flow rate of gas through the aerogel at most pressures. Below a certain pressure, the diffusivity stays constant; this is called the Knudsen flow regime. Adapting the method outlined in Stumpf et. al 1992, a device was designed and built to measure the diffusivity of aerogels over a range of pressures from 10 to 100 kPa absolute. This device utilizes a control mass system which allows us to avoid experimental difficulties, such as small flow rates and significant gas leaks, associated with the more common control volume approach. The device is split into two volumes, each monitored via one Baratron® Type 122A absolute pressure transducer. By allowing a known amount of gas to diffuse into the aerogel of interest, we can obtain a pressure curve which can then be used to solve for the diffusivity. We obtain diffusivity values for gas flow in both the Knudsen and viscous flow regimes. This allows us to obtain a complete profile describing how the diffusivity of the aerogel of interest will respond to any given pressure. Proof of concept as well as preliminary test results will be presented.