Analysis of a Rapid Supercritical Extraction Aerogel Fabrication Process: Prediction of Thermodynamic Conditions during Processing.


Aerogels have unusual mechanical and thermal properties that render them useful in a range of applications from thermal insulators to chemical sensors. However, aerogel fabrication can be difficult, typically requiring the use of supercritical extraction of solvent from the sol–gel matrix. We employ a rapid supercritical extraction (RSCE) technique for aerogel fabrication that is faster and simpler than the standard methods. This technique relies on a hydraulic hot press to heat the chemical precursors and provide the required temperature and pressure increase needed to reach a supercritical state within a contained mold. Experimental results show that the pressure–temperature curve is characterized by a ‘take-off point’ and a ‘leak point’. The take-off point occurs at the start of a rapid increase in pressure and the leak point occurs where the pressure increase subsides. This paper presents an analytical model that predicts the pressure–temperature relationship of the aerogel precursors during the RSCE fabrication process and shows that the model can be extended to other RSCE systems. Using pure methanol, water and aerogel precursors in separate tests, the effects of initial liquid volume and press force on the pressure and temperature during processing were studied. We find that the take-off point can be estimated using a maximum specific volume model, which is a function of the initial percent volume fill of liquid used in the test. We are also able to predict the rapid pressure increase region observed during the process using a constant volume model. Leaking is determined to be a function of the mechanical forces acting on the system and occurs when the pressure in the contained mold nears the value of the pressure imparted onto the gasket by the hot press. The leak point pressure is found to be independent of the initial percent volume of liquid used in the test; however the leak point temperature decreases with increasing initial percent volume fill.


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