How “Impermeable” Carbon Becomes Permeable under Pressure

Synthetic nanostructured diamond capsules (NDC) provide a general solution to preserve otherwise “untamable” high pressure volatiles to ambient conditions, enabling detailed characterization and potential applications ranging from materials science to energy research. A critical process in NDC synthesis is the pressure-driven diffusion of volatiles into the enclosed nanopores of disordered carbon precursors, most commonly glassy carbon. Yet glassy carbon is highly impermeable to gases under ambient conditions, leaving a fundamental question unresolved: how do gases enter such an apparently sealed material under pressure?

A new study by a joined team from the Center for High Pressure Science and Technology Advanced Research (HPSTAR) and Shanghai Advanced Research in Physical Sciences (SHARPS) led by Dr. Qiaoshi Zeng, published in the Journal of the American Chemical Society, provides a clear answer. By combining in situ high-pressure synchrotron X-ray diffraction, small-angle X-ray scattering, and electron microscopy, the research team was able to track helium or argon gas uptake in real time while the glassy carbon and amorphous carbon nanospheres were compressed in a diamond anvil cell.

Contrary to long-standing assumptions, the results show that gases do not diffuse through the relatively wide interlayer space between graphene-like layers inside glassy carbon. Instead, gas transport occurs through intrinsic disordered structural defects within the carbon network. These structural defects can act as fast pathways for gas transport under high pressure, analogous to “short-circuit” diffusion along grain boundaries in crystalline solids at ambient conditions.

Caption: Pressure drives gas diffusion into the nanopores of glassy carbon and carbon nanospheres.

The study further shows that that this phenomenon is not unique to glassy carbon. Even modest pressures (<0.3 GPa) are sufficient to drive gas diffusion into a wide range of amorphous carbon nanostructures, including hollow carbon nanospheres with very different atomic arrangements. This finding demonstrates that pressure-induced gas permeability is a general property of disordered sp²-bonded carbon, rather than a special feature of a single material.

By uncovering the structural mechanism behind pressure-induced permeability, this work fills a long-standing gap in understanding gas transport in non-crystalline carbon. More importantly, it provides practical design guidelines for selecting and engineering carbon precursors for NDCs. With tunable pore architectures and defect networks, disordered carbon materials can be tailored to trap specific high-pressure volatiles, opening new opportunities for fundamental research and advanced technologies that rely on high-pressure volatile materials.

近期发展的金刚石纳米压力舱（Nanostructured Diamond Capsules，NDC）技术为在常压条件下保存挥发性物质(气体、液体)的高压态提供了一种通用解决方案，为在常压环境对其开展深入研究和获得应用提供了途径。在NDC的制备过程中，一个关键步骤是：在高压作用下，将气体、液体等压入非晶碳前驱体内部的密闭纳米孔洞中。然而，作为NDC最常用的前驱体材料之一，玻璃碳在常压下具有极低的气体渗透性，这一看似矛盾的现象缺乏清晰的微观结构机制解释,阻碍了NDC技术的进一步发展。近日，来自北京高压科学研究中心和上海前瞻物质科学研究院的曾桥石研究员带领的研究团队利用原位高压同步辐射X射线衍射、小角X射线散射及电镜等手段，研究了氦气和氩气在玻璃碳及非晶碳纳米球中的扩散行为，揭示了无序碳材料在高压条件下实现气体有效渗透的原子结构通道。相关成果于近日在线发表在《美国化学会志》。