Wood-derived carbon holds promise for devices
The hidden architecture of wood could deliver strong biochar materials.
Biochar is a carbon-rich material produced by heating biomass in the absence of oxygen and has long been valued for soil improvement and pollution removal.
Yet its mechanical properties have been largely overlooked. The new research, published in Biochar X, explores how monolithic biochar, solid blocks of carbonised wood, can achieve strength and durability.
The study from the University of Toronto. Canada, reveals that the direction in which biochar is measured can make its hardness vary by more than 28 times, opening the door to a new generation of sustainable carbon materials for use in energy devices, filters, and structural applications.
The team, led by Professor Charles Jia and his colleagues at the Green Technology Laboratory, analysed biochar made from seven wood species including maple, pine, bamboo, and African ironwood.
By heating the samples between 600 and 1,000oC, they found that temperature and wood type both had major effects on hardness. African ironwood biochar reached an axial hardness of 2.25GPa, comparable to mild steel, while hemlock showed extreme directional differences.
Using micro- and nano-indentation techniques, the researchers were able to measure hardness at both structural and microscopic scales. They discovered that the striking anisotropy, the difference between axial and transverse hardness, originates from the wood’s hierarchical pore network rather than the carbon itself.
At the nanoscale, the hardness of all samples was remarkably uniform, suggesting that intrinsic cell-wall properties remain consistent regardless of species or direction.
The study also reveals strong correlations between hardness, bulk density and carbon content.
Denser biochar with higher-carbon fractions resists deformation more effectively. This relationship provides a quantitative foundation for tailoring biochar’s performance by adjusting the choice of feedstock and pyrolysis conditions.
Potential uses include high-strength electrodes for energy storage, lightweight carbon composites and flow-directional filters for environmental systems.
The ability to fine-tune mechanical anisotropy could allow matching of material design to performance needs.
