The Effect of Pyrolysis Temperature and Formulation on Pore Size Distribution and Surface Area of Carbon Aerogels
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THE EFFECT OF PYROLYSIS TEMPERATURE AND FORMULATION ON PORE SIZE DISTRIBUTION AND SURFACE AREA OF CARBON AEROGELS* S.S. HULSEY, C.T. ALVISO, F.M. KONG, AND R.W. PEKALA Chemistry & Materials Science Department, Lawrence Livermore National Laboratory, Livermore, CA. 94550
ABSTRACT Recently we reported the chemistry-structure-property relationships of organic aerogels, which are synthesized by the polycondensation of resorcinol and formaldehyde in a slightly basic medium, followed by supercritical drying. These materials can be pyrolyzed in an inert atmosphere to form vitreous carbon aerogels. As measured by gas adsorption techniques, the BET surface area and pore size distributions of micro and meso pores of the carbon aerogels are affected both by the pyrolysis temperature and the formulation. Definite trends are observed in our preliminary measurements; for example, the surface area decreases with increasing pyrolysis temperature until a plateau is reached at about 9000C. This paper explores the effects of pyrolysis temperature and aerogel density on the BET surface area and pore size distributions. INTRODUCTION Resorcinol formaldehyde (RF) aerogels are made by the aqueous polycondensation of 1 mole of resorcinol (1,3 dihydroxybenzene) with 2 moles of formaldehyde which proceeds through a sol-gel transition [1]. The substituted resorcinol rings condense with each other to form nanometer-sized clusters in solution, which crosslink through their surface groups to form a gel. The size and number of resorcinol-formaldehyde (RF) clusters generated during polymerization are controlled by the [Resorcinol]/[Catalyst] (R/C) ratio in a formulation. In this study, the R/C ratio was held constant at 200. After supercritical drying, the resultant aerogel is a very open structure of interconnected bead chains, with large open pores between the RF particles (see Fig. 1 in [1]); smaller pores are assumed to occur within the molecular structure itself. RF aerogels in this study were pyrolyzed in an inert atmosphere to form vitreous carbon aerogels, which are black and no longer transparent due to the visible absorption properties of the carbon matrix. This paper focuses on the structural properties of carbon aerogels based on gas adsorption data. We performed two studies: (1) a temperature study in which the pyrolysis temperature was varied from 610 0 C to 1050oC while keeping the percent solids and R/C ratio constant, and (2) a density study in which the density was varied from 0.155 to 0.803 g/cc while keeping the pyrolysis temperature and R/C ratio constant. Parameters measured include the BET surface area and micro- and meso- pore size distributions. MEASUREMENTS Gas adsorption data was determined with the ASAP 2000 Surface Area Analyzer (Micromeritics Instrument Corporation, Norcross, Georgia), using sample sizes of approximately 10 mg. Each sample was outgassed at 100oC for approximately 20 hours. The BET surface area was determined from nitrogen adsorption data. Five data points were taken from P/PO = 0.06 to 0.20 where P/P
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