Mark C. Thies, Ph.D., P.E. -- Research Activities

Dense-Gas Extraction for the Molecular Design of Advanced Carbon Materials

Carbonaceous pitches serve as precursors for advanced carbon materials, such as high modulus (HM) carbon fibers, high thermal conductivity (HTC) carbon fibers, and the matrix phase of carbon-carbon (C-C) composites. Depending on the final product application, pitches with different average molecular weights (MW) and properties are desired.  Thus, it would be desirable to both measure and control the MW distribution (MWD) of the starting pitch, much in the same way that is done for the manufacture of articles from polymers.  Unfortunately, there are two significant barriers to “molecularly designing” carbonaceous pitches for a given product application:  First, we know surprisingly little about the molecular composition of pitches, including both isotropic and liquid crystalline types.  Second, conventional separation technologies for processing pitches allow us to control their MW only in a qualitative sense.

The empirical approach that industry now follows to produce advanced carbon materials from pitch is no longer acceptable, as their high cost is impeding use in both structural and thermal applications. For example, HTC applications are becoming increasingly critical in both commercial and military applications as energy densities in avionics, computer guidance, and electronics applications continue to increase. Yet, as long as ultra HTC carbon fiber costs over $1500/lb, little will change.  A new approach is needed to realize the long-awaited potential of inexpensive pitches for producing low-cost, high-performance carbon products.

The goal of Prof. Thies and his research team is to tailor the molecular composition of carbonaceous pitches for a desired end-product application.  A novel separation process known as dense-gas extraction (DGE; see Fig. 1) has been developed and is being investigated for fractionating pitches into oligomers and oligomeric mixtures of well-defined MWD.  Compared to conventional separation techniques, DGE has several advantages, including multistage capability, the use of pressure as an operating variable, and the use of a retrograde temperature gradient to induce reflux for higher product purity.  The dramatic improvements in the control of product MWD with DGE as compared to conventional solvent extraction are shown in Fig. 2.  Furthermore, the process can be operated in a continuous manner in order to produce the relatively large cuts of pitch required for manufacturing prototype advanced carbon materials.

Fig. 1.  Schematic of our dense-gas extraction apparatus for fractionating pitches into their constituent oligomers.

Fig. 2.  Left: Broad MWD of pitch obtained by conventional solvent extraction.  Right: DGE yields a dimer cut at greater purity than can be obtained by liquid chromatography.

Cervo, E. G.; Thies, M. C. Control of the Molecular Weight Distribution of Petroleum Pitches via Dense-Gas Extraction. Chem. Eng. Tech. 2007, 30, 742-748.

Brunner, E.; Thies, M. C.; Schneider, G. M. Fluid mixtures at high pressures: phase behavior and critical phenomena for binary mixtures of water with aromatic hydrocarbons. J. Supercrit. Fluids 2006, 39, 160-173.

Edwards, W. F.; Thies, M. C. Fractionation of Pitches by Molecular Weight using Continuous and Semibatch Dense-Gas Extraction. Carbon 2006, 44,243-252.

Thies, M. C.; Burgess, W. A. SAFT–LC:  Predicting Mesophase Formation from a Statistical Mechanics-Based Equation of State. PPEPPD 2007 [CD-ROM]; Hersonissos, Crete, Greece, May 2007, paper MOD27.

Burgess, W.A.; Thies, M.C. Predicting Mesophase Formation from a Statistical-Mechanics-Based Equation of State. Presented at Carbon 2006. An International Conference on Carbon [CD-ROM]; The Robert Gordon University: Aberdeen, Scotland, July 2006; Paper SA-494.

Thies, M.C.; Cervo, E.G.; Edwards, W.F. Isolating Pitch Oligomers by Dense-Gas Fractionation. Presented at Carbon 2006. An International Conference on Carbon [CD-ROM]; The Robert Gordon University: Aberdeen, Scotland, July 2006; Paper SA-493.

Burgess, W.A.; Thies, M.C. Prediction of Mesophase Formation in Carbonaceous Pitches. 231st ACS National Meeting, Chemistry of Carbon Materials and Nanomaterials Symposium, Atlanta, GA, March 2006; Paper 119.

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