Physical characteristics of carbon materials derived from pyrolysed vascular plants

The purpose of this study was to develop new monolithic porous carbon materials from vascular plants using highly controlled pyrolysis. Perennial plants belonging to the grass family Poaceae such as bamboo (Bambusa vulgaris) and to the family Agavaceae such as yucca (Yucca flaccida) characterized by a homogeneous profile and homogenous vessel distribution were selected for the study. They were heat-treated at temperatures 550 and 950 °C in a nitrogen atmosphere to produce a crack-free monolithic porous carbon materials for which physical characteristics such as density, porosity, yield and dimensional changes were determined. The EPR spectroscopy, ultrasonic technique and optical microscopy were applied for further characterization. All samples studied demonstrated a reduction in apparent density and dimensions due to carbonisation. It was found that similarly as in the case of hardwoods, the higher the carbonisation temperature, the greater the dimensional shrinkage. The greatest changes were observed in “transverse” to plant fibres directions, i.e., for radial and tangential. It was found that the dimensional changes under heat-treatment exhibited transverse isotropy. Carbonised plants were characterised by elastic moduli almost independent of apparent density in contrast to elasticity of precursors. Elastic moduli of samples carbonised to 950 °C were higher than those heat-treated to 550 °C. Results showed that materials carbonised at higher temperature were more stiff—more ordered in structure. Microscopic observations showed that during heat-treatment of yucca and bamboo, their tissue structure remained unaltered. There was the increase in order of aromatic layers in the walls of fibres expressed by the increase of optical reflectance values through the carbonisation process. It was found that heating plants to 950 °C quenched paramagnetic centres in carbonised samples. This effect resulted from an increase of multi-ring aromatic units in the samples. The observed lack of saturation of the EPR spectra evidenced that during slow pyrolysis defects were not created. Carbonised woody stems of perennials studied were found as very porous, but stiff materials, which can be excellent precursors (as skeleton) for new eco-materials, e.g., for wood-ceramics.