Microstructure development in furfuryl resin-derived microporous glassy carbons
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Microstructure development in furfuryl resin-derived microporous glassy carbons Kristen Persels Constant,a) Jonq-Ren Lee, and Yet-Ming Chiang Department of Materials Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 (Received 28 November 1995; accepted 28 May 1996)
The processing of microporous glassy carbon derived from furfuryl alcohol and ethylene glycol mixtures has been studied, with emphasis on understanding and controlling microstructure development. It is shown that this system exhibits a polymerization-dependent miscibility gap, and that the carbon microstructure is determined by phase separation in the liquid state. Variations in carbon microstructure with composition and thermal history can be understood in terms of the time-dependent immiscibility and resulting phase separation.
I. INTRODUCTION
Glassy carbon has been used for a number of applications, including filters, electrodes, molecular sieves, and catalyst supports, since its discovery in the early 1960’s.1,2 More recently a microporous form of glassy carbon3 has found use as a preform for reaction forming of structural ceramics.4–7 In this application, the uniformity, density, pore size, and pore interconnectivity of the carbon preform play an important role in determining the rate of infiltration and reaction and the resulting microstructure. Control of the carbon microstructure is therefore crucial for control of the reaction-forming process. The liquid-based system for producing microporous carbon discussed here was first described by Hucke in 1975.3 The carbon-yielding furfuryl resin is first mixed with a pore-forming alcohol and optional inert ingredients. Thermal polymerization of the furfuryl resin, catalyzed by acid, results in a rigid solid from which the fluid pore former can be removed. The body is then pyrolyzed in inert atmosphere to form porous glassy carbon. The microstructure of the resulting body appears to be defined by a number of parameters including the ratio and specific composition of the components, as well as the thermal history of the mixture. The mechanisms responsible for the evolution of microstructure, and the methods of controlling the microstructure, have not previously been investigated in detail. However, the polymerization of furfuryl alcohol is known to occur by the following condensation reaction8 :
O
a)
C
OH
O
C
O
C
OH 1 H2O
Now at Iowa State University, Materials Science and Engineering, Ames, Iowa 50010.
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J. Mater. Res., Vol. 11, No. 9, Sep 1996
Significant crosslinking occurs with a maximum of four linkage points per monomer unit. It has not been determined whether this crosslinking occurs during growth of the polymer chains or afterward. Regardless, as the polymer chains lengthen, the viscosity increases significantly, and mobility decreases.8 In this study, we explore the effects of various processing parameters and show that the microstructure of these glassy carbons is predominantly determined
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