Water-assisted alignment plus secondary cold lsostatic pressing: achieving high-performance pure h-BN ceramics by pressureless sintering

Hexagonal boron nitride (h-BN) ceramics possess high thermal conductivity, excellent electrical insulation, and good thermal and chemical stability, showing great potential for high-end electronics and thermal management. Current industrial production relies on hot pressing, which limits product size and yield with high costs. Pressureless sintering is simple, low-cost, and suitable for large or complex shapes. However, due to the extremely low self-diffusion coefficient of h-BN, densification via pressureless sintering is difficult, with relative densities typically below 90%. Therefore, achieving densification of h-BN by pressureless sintering has remained a key challenge for over half a century since its first synthesis.

Recently, a team of material scientists led by Gang He from Tianjin University of Technology, China reported the fabrication, microstructure, density, and thermal conductivity of high-density h-BN ceramics via a secondary cold isostatic pressing assisted pressureless sintering process.

This study not only elucidates the formation mechanism of the unique layered oriented structure and excellent thermal conductivity of h-BN ceramics, but also provides a cost-effective and easily implementable strategy for preparing high density h-BN ceramics.

The team published their work in Journal of Advanced Ceramics on April 27, 2026.

“In this report, we adopted a new process for pressureless sintering of high density h-BN ceramics, namely the ‘water lubrication and secondary cold isostatic pressing-assisted pressureless sintering process’, to prepare h-BN ceramics with a layered oriented structure. This method was inspired by recent successes in achieving room-temperature densification of h-BN nanosheets through the lubrication effect of water. Given that the intrinsic tendency of h-BN to form a ‘card-house structure’ at high temperatures cannot be eliminated, we asked whether the technical path of ‘densification at room temperature and structural strengthening at high temperature’ could be pursued. To this end, in consideration of industrial production needs, we collaborated with a company to use commercial micron-sized h-BN powder as the raw material. Water was added to promote densification during forming, followed by a secondary cold isostatic pressing process to solve the density reduction problem caused by water evaporation. Finally, pure h-BN ceramics with properties comparable to those of hot-pressed counterparts were prepared by pressureless sintering.” said Gang He, professor at the Institute of Functional Crystals, Tianjin University of Technology (China), an expert whose research interests focus on the field of functional ceramic materials and composites.

“The h-BN ceramic is composed of flaky h-BN grains stacked and aligned with their c-axis parallel to the uniaxial pressure direction, exhibiting a highly preferred orientation structure with an orientation preference index of -436 and a relative density of 91.90%.” said Gang He.

“Due to its well-packed layered structure and the absence of introduced impurities, the prepared h-BN ceramic exhibits outstanding in-plane thermal conductivity (57.16 W·m^-1·K^-1) and dielectric constant (4.73). This makes it a promising candidate for applications such as electronic packaging materials.” said Gang He.

However, more delicate research works are still needed to further explore the secondary cold isostatic pressing process for the preparation of h-BN ceramics. Although the current process is suitable for low-cost industrial production of large-sized h-BN ceramics with thermal and dielectric properties comparable to conventional hot-pressed products, challenges remain in mechanical properties. Future work should systematically investigate the mechanical properties, aiming for breakthroughs in high-strength application scenarios. In this regard, Gang He also provided an objective assessment and put forward practical and feasible improvement plans.

Other contributors include Shuxuan Wang, Yuqi Yang, and Lei Yang from the Institute of Functional Crystals, Tianjin University of Technology in Tianjin, China; Xiaodong Li and Jiakai Cao from Novoray Co. Ltd in Lianyungang, China.

About Author

Gang He is a professor at the Institute of Functional Crystals, Tianjin University of Technology, China. He received his PhD degree in Engineering from the University of Chinese Academy of Sciences in 2015. In July 2015, he joined the Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, and was transferred to the Institute of Functional Crystals, Tianjin University of Technology in December 2021. His research focuses on advanced ceramic powders, processing science and engineering applications of optical functional ceramics.

Funding

The work was supported by the National Natural Science Foundation of China (Nos. 12374030 and 12304455).

About Journal of Advanced Ceramics

Journal of Advanced Ceramics (JAC) is an international academic journal that presents the state-of-the-art results of theoretical and experimental studies on the processing, structure, and properties of advanced ceramics and ceramic-based composites. JAC is Fully Open Access, monthly published by Tsinghua University Press, and exclusively available via SciOpen. JAC’s 2024 IF is 16.6, ranking in Top 1 (1/34, Q1) among all journals in “Materials Science, Ceramics” category, and its 2024 CiteScore is 25.9 (5/130) in Scopus database. ResearchGate homepage: https://www.researchgate.net/journal/Journal-of-Advanced-Ceramics-2227-8508