This study introduces a multiscale hardness analysis of crack-free monolithic biochar derived from seven wood species pyrolyzed at temperatures ranging from 600 to 1,000 °C.
Biochar, a carbon-based material derived from sustainable biomass, has been increasingly explored for applications in energy storage, water purification, and structural composites. While biochar’s environmental and chemical properties have been widely studied, existing research predominantly focuses on powdered biochar, neglecting the role of its inherent hierarchical architecture. This oversight limits the material’s application in next-generation technologies that demand directional mechanical performance, such as structural composites and flow-through systems. Monolithic biochar, which preserves the natural wood structure, holds unique potential due to its anisotropic mechanical properties—characteristics that arise from the alignment and compaction of carbonized cell walls during pyrolysis. Yet, the understanding of how these properties vary with different wood species and pyrolysis conditions has been underexplored, especially in terms of their nanoscale and macroscale behavior.
A study (DOI:10.48130/bchax-0025-0007) published in Biochar X on 21 October 2025 by Charles Q. Jia’s team, University of Toronto, represents a significant step forward in the design and application of biochar as a versatile material for structural, energy, and environmental technologies.
This study investigates the temperature-dependent hardness anisotropy of biochar derived from various wood species, subjected to pyrolysis at temperatures of 600, 800, and 1,000 °C. The researchers employed micro- and nano-indentation techniques to measure hardness in both axial and transverse directions across biochar samples from maple, pine, hemlock, bamboo, redwood, African ironwood, and yew. The results show that the hardness of both maple and pine biochar increases with pyrolysis temperature. At 600 °C, the hardness values are low due to incomplete carbonization, as the cellular framework remains partially intact. As the temperature rises to 800 °C and 1,000 °C, the hardness significantly increases, reflecting the enhanced carbonization and formation of a more crystalline carbon network. Notably, the axial hardness consistently exceeds transverse hardness across all species and temperatures, with the difference becoming more pronounced at higher pyrolysis temperatures. This increased anisotropy is due to the preferential alignment of carbon structures along the axial direction. For example, at 1,000 °C, the axial hardness of maple biochar nearly doubles compared to 600 °C, emphasizing the impact of pyrolysis temperature on mechanical properties. Furthermore, the study finds that the hardness values vary significantly across wood species at 1,000 °C, with African ironwood exhibiting the highest hardness values in both directions, and hemlock showing the lowest hardness overall. The study also identifies a strong correlation between bulk density and hardness, particularly in the axial direction (R² = 0.84), highlighting the role of density in controlling hardness. Additionally, the carbon fraction, determined through SEM analysis, correlates with increased axial hardness, reinforcing the importance of carbon content in enhancing the material's mechanical strength.
The study provides valuable insights into controlling and enhancing the mechanical performance of monolithic biochar. By optimizing pyrolysis temperatures and selecting appropriate feedstocks, biochar can be tailored for various applications, such as ultra-hard biochar for robust electrodes or structural components that require high load-bearing capacity, and highly anisotropic biochar for directional-flow filters or composites that demand strength along specific axes. This research paves the way for integrating biochar into next-generation technologies, ranging from energy storage devices to environmental filtration systems, by leveraging its unique mechanical properties.
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References
DOI
10.48130/bchax-0025-0007
Original Source URL
https://doi.org/10.48130/bchax-0025-0007
Funding information
This work was supported by the Natural Science and Engineering Research Council of Canada (NSERC), and the Low Carbon Renewable Materials Centre at the University of Toronto.
About Biochar X
Biochar X is an open access, online-only journal aims to transcend traditional disciplinary boundaries by providing a multidisciplinary platform for the exchange of cutting-edge research in both fundamental and applied aspects of biochar. The journal is dedicated to supporting the global biochar research community by offering an innovative, efficient, and professional outlet for sharing new findings and perspectives. Its core focus lies in the discovery of novel insights and the development of emerging applications in the rapidly growing field of biochar science.