Professor Xiaoyan Li from Tsinghua University and Academician Yongjun Tian from Yanshan University: Significant Progress Achieved in the Field of Ultra-Hard, Room-Temperature Conductive Materials — Published in *Nature Materials*

Materials that combine high strength, high hardness, and room-temperature conductivity are high-performance materials highly sought after in various scientific and industrial fields. While traditional metals boast excellent electrical conductivity, they typically exhibit relatively low yield strength and tend to soften at elevated temperatures. In contrast, ceramics generally offer superior strength, hardness, wear resistance, and thermal stability—qualities that make them ideal for demanding applications. However, most ceramics are poor conductors of electricity. To enhance their conductivity, ceramics can be doped with specific elements or reinforced with a second, electrically conductive phase. Yet, the limited diffusion rates of dopants within ceramics restrict the achievable doping concentrations, thereby capping the potential improvement in electrical performance. Moreover, compared to monolithic ceramics, conductive ceramic composites often suffer from a trade-off between mechanical properties and conductivity, as the weaker interfacial interactions between the matrix and the second phase hinder simultaneous optimization of both attributes. As a result, developing materials that seamlessly integrate exceptional mechanical performance with outstanding electrical conductivity remains one of the most pressing and critical scientific challenges in the field of materials science.

Carbon exists in three hybridization states—sp, sp², and sp³—which allow it to form allotropic variants with vastly different properties. These include soft, conductive graphite and ultra-hard, insulating diamond. By combining two or more carbon-based materials, it’s possible to harness the unique advantages of each component, resulting in composites that exhibit exceptional mechanical performance along with outstanding electrical conductivity. This approach aims to create materials that seamlessly integrate both superior strength and remarkable conductivity. Traditionally, carbon/carbon composites have been primarily composed of sp²-hybridized carbon materials, such as carbon fibers embedded in pyrolytic carbon matrices. While these materials typically display high electrical conductivity and impressive strength, their mechanical properties remain challenging to further enhance—largely due to the weak van der Waals forces that exist either within or between the constituent components. One potential solution is to incorporate diamond into these composites. However, diamond’s covalent bonds are exceptionally strong and fully saturated, making it nearly impossible to break them chemically. As a result, directly integrating diamond with other carbon materials remains a significant technical hurdle.

Figure 1. Microstructure and interfacial architecture of the nanodiamond/disordered multilayer graphene composite material

In response to the issues and challenges mentioned above, Tsinghua University School of Aerospace and Aeronautics Professor Li Xiaoyan The research team and Yanshan University Academician Tian Yongjun Teamwork ， By selecting commercial pyrolytic carbon as the precursor, high-pressure/high-temperature experiments were conducted on the precursor under high pressure (25 GPa) and a narrow temperature range (1050–1150°C), successfully yielding a novel carbon/carbon composite material: a self-grown nanodiamond/disordered multilayer graphene composite. This composite material consists of ultrafine nanodiamonds with an average particle size of approximately 4.8 nm, uniformly and independently embedded within a disordered multilayer graphene matrix. By carefully controlling the temperature, it is possible to tailor the composite to achieve diamond volume fractions ranging from 20% to 70%. Atomic-resolution high-angle annular dark-field imaging and molecular dynamics simulations (as shown in Figure 1) reveal the formation of non-coherent interfaces between the nanodiamonds and the multilayer graphene, shedding light on a unique mechanism driving the transformation of amorphous carbon into diamond crystals. This process primarily involves nucleation of nanocrystalline diamond followed by diffusion-controlled growth. The unique microstructure and interfacial characteristics of this novel carbon/carbon composite endow it with exceptional overall performance: its Knoop hardness reaches up to 53 GPa, surpassing that of conventional superhard material cubic boron nitride; its micropillar uniaxial compressive strength exceeds twice that of SiC when tested using the same method; and at room temperature, its electrical conductivity ranges from 670 to 1240 S/m—comparable to the highest-performing conductive ceramic materials (as illustrated in Figure 2).

This ultra-hard, ultra-strong, and highly conductive all-carbon-based composite material outperforms all currently known conductive ceramics and carbon/carbon composites, making it a promising candidate for critical national applications such as aerospace and advanced micro- and nanoelectronic devices. The research also unveils the mechanism behind the amorphous-to-diamond phase transition in carbon, offering valuable insights into how phase transformations in carbon materials could be harnessed to develop next-generation, high-performance materials. Furthermore, large-scale molecular dynamics simulations were employed to model the compressive deformation behavior of nano-diamond/multilayer graphene composites, shedding light—at the atomic level—on the intrinsic reasons behind the material’s exceptional hardness and strength: specifically, the presence of nano-diamonds effectively inhibits the propagation and expansion of shear bands within the graphene matrix (Figure 3). Additionally, percolation theory and rule-of-mixtures models were used to predict the electrical conductivity and hardness of composites with varying diamond contents, and the theoretical predictions aligned perfectly with experimental measurements.

Figure 2. Comparison of Mechanical Properties and Conductivity Between Nanodiamond/Disordered Multilayer Graphene Composites and Other Materials

Figure 3. A large-scale molecular dynamics simulation demonstrates that, during the compressive deformation of the composite material, nanodiamonds effectively inhibit the propagation and expansion of shear bands within the graphene matrix.

The recent breakthrough has been published as a paper in *Nature Materials*, titled "Ultrastrong Conductive In-Situ Composite Composed of Nanodiamonds Incoherently Embedded in Disordered Multi-Layer Graphene." Meanwhile, the research team was invited to contribute a Research Briefing to *Nature Materials* titled "Combining the Properties of Nanodiamonds and Disordered Multi-Layer Graphene," which highlights both the study’s key findings and the fascinating story behind the research. The briefing also features feedback from reviewers, who remarked: "This is an intriguing paper on a unique nanodiamond–graphene composite, demonstrating that optimizing the synthesis of glassy carbon can yield a material with exceptional strength and electrical conductivity. The study is compelling, and the characterization of the composite reaches cutting-edge standards."

Tsinghua University School of Aerospace Professor Li Xiaoyan Yanshan University Academician Tian Yongjun Yanshan University Professor Zhao Zhisheng And from Aalborg University in Denmark Professor Yue Yuanzheng Co-corresponding author of this article. Postdoctoral Fellow, School of Aerospace Engineering, Tsinghua University Li Zihé , Doctoral graduates of the Class of 2022 Wang Yujia • A 2019 doctoral graduate from Yanshan University Ma Mengdong • PhD student, Class of 2019, School of Aeronautics and Astronautics, Tsinghua University Ma Hua Chun And Yanshan University Professor Hu Wentao Co-first author of the paper. This research was supported by projects including the National Natural Science Foundation of China’s Basic Science Center Program, Major Research Plan, and Innovation Group Project, among others.

Paper link: https://www.nature.com/articles/s41563-022-01425-9

Source: Tsinghua University News Network

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