Carbon Vol. 35, No. 2, pp. 259-266, 1991 Copyright 0 1997 Elsevier Science Ltd Printed in Great Britain. All rights reserved 000%6223/97 $17.00 + 0.00
CARBONIZATION OF WOOD FOR ADVANCED MATERIALS APPLICATIONS
The Johns Hopkins University,
and D. C.
of Materials U.S.A.
Science and Engineering,
(Received 19 August 1996; accepted 6 September 1996) Abstract-A unique process for conversion of monolithic wood structures to carbons that retain the cellular structure of the wood without the formation of cracks and other defects associated with charcoal materials is described. A variety of wood species are carbonized to produce the materials which are characterized using TGA, density, dimensional changes, acoustic velocity, SEM and mechanical testing. We demonstrate that through controlled pyrolysis monolithic carbonized wood can be produced without the macro-cracks normally associated with charcoal. A linear relationship is established between the bulk densities of wood and carbonized wood which spans the entire range of species. For the conditions presented, the carbonized wood had 82% of the bulk density of the precursor wood. Carbonized wood acoustic velocity ranged from 4.7 to 1.3 mm/ps for Tibia americana and Ochroma pyramidale, respectively. Carbonization resulted in decreased acoustic velocity in the axial principal direction and increases in the radial and tangential directions. Acoustic anisotropy was retained through carbonization, but reduced in magnitude. Mechanical testing showed the carbonized wood to be 28% stronger than the precursor.
The cellular morphology of the porous carbonized wood is described and compared to carbon foams. 0 1997 Elsevier Science Ltd. All rights reserved Charcoal. B. carbonization, D.-mechanical properties.
C. thermal analysis (TGA), scanning electron microscopy,
1 .l Wood carbonization and modern materials It has recently been demonstrated that net-shape polymer, ceramic and carbon composites can be produced using wood as a precursor. This was accomplished by controlled thermal decomposition to form a monolithic carbon template which retains the anatomical features of the precursor. The carbon template was shown to be easily machined to net shape prior to conversion to a carbon/polymer composite, carbon/carbon composite, a ceramic, or a ceramic composite [l-3]. This unique approach to materials manufacture demanded a study of the conditions necessary for carbonization of monolithic wood precursors. We will describe in this paper the method by which monolithic pieces of carbonized wood were produced without the formation of macro-cracks, along with the resulting yields, densities and mechanical properties. In a second paper we describe the properties of the monolithic carbonized woods in further detail. 1.2 Historical background Carbonization of wood for the manufacture of charcoal has been practiced since the beginning of history. The Industrial Revolution brought about a heavy demand for wood charcoal, and up until the late 1800s the largest volume was used by the iron industry. Today, coal derived cokes are used and *Corresponding author. 259
wood carbonization, and use of distillates, is practiced more in developing countries. Despite the decline of the traditional wood pyrolysis industries, a great deal of technical information regarding cellulose (and lignocellulosic) decomposition kinetics, mechanisms, thermodynamics and byproducts has been acquired using modern analytical techniques. Several different markets provided motivation for the pursuit of this information. Those markets include: activated carbons, textiles, composite materials, fire retardant wood products and energy from biomass and refuse. While much information is available from these works, there exists a gap in knowledge regarding monolithic carbonized wood.