Reviewed by Danielle Ellis, B.Sc.Sep 23 2024
A group of material scientists from Sun Yat-sen University in China, under the direction of Dr. Zhilin Tian and Bin Li, created a porous Si3N4 ceramic with homogeneous, fine structures by employing a dual-solvent templating and freeze-casting technique, as reported in a study that was published in the Journal of Advanced Ceramics on August 26th, 2024.
Porous Si3N4 ceramics with uniform and fine structures were achieved by the dual-solvent templating combined freeze-casting method. Image Credit: Journal of Advanced Ceramics, Tsinghua University Press
Radomes and wave-transmitting antenna windows are critical structural components in aircraft that protect radar antennas from external interference while maintaining reliable communication. At present, the most widely used wave-transmitting materials are oxide and nitride-based ceramics. Si3N4 ceramics, with their high melting point and superior mechanical properties, are seen as promising candidates for hypersonic vehicle applications.
However, the dielectric and thermal insulation properties of dense Si3N4 ceramics must be improved to meet the precise guidance and thermal protection requirements of high-speed flight. Adjusting the microstructure can improve the dielectric, mechanical, and thermal properties of wave-permeable materials.
As a result, porous Si3N4 ceramics are critical in the fabrication of radomes and antenna windows for hypersonic aircraft, which must withstand extreme temperatures and stresses while maintaining low dielectric constants for proper radar functionality. Traditionally, balancing mechanical strength with wave transmission efficiency has been difficult, necessitating novel solutions to improve both aspects.
This method allows for precise control of pore size and structure, both of which have a significant impact on the performance of high-temperature wave-transparent materials. Conventional methods such as direct foaming, gel casting, and organic template impregnation frequently fail to achieve the required uniformity and regularity in pore structure.
However, the new method easily converts anisotropic prismatic pores into isotropic spherical pores. This innovation results in a synergistic improvement of mechanical, thermal, and dielectric properties in porous Si3N4 ceramics.
In this study, we developed porous Si3N4 ceramics with uniform pore structures using a dual-solvent templating and freeze-casting method. The resulting ceramics have a porosity of 56% and exhibit impressive mechanical properties, with bending and compressive strengths of 95±14.8 MPa and 132±4.5 MPa, respectively.
Zhilin Tian, Professor, Sun Yat-sen University
Tian added, “The dual-solvent system enriches the diversity of pore structures, which is closely tied to the solvent ratio. As the tert-butyl alcohol content decreases, the pore structure transitions from prismatic to spherical. Moreover, with increased camphene content, the pore diameter initially decreases before increasing again. When the solvent ratio is 1:2, the average pore diameter reaches its minimum, due to competition between the solvent crystals for growth space. More tert-butyl alcohol results in larger crystal and pore sizes, while reduced tert-butyl alcohol allows smaller camphene crystals to dominate, leading to smaller pores.”
He further added, “Our dual-solvent templating method offers unprecedented control over pore size and structure, which are critical for creating high-performance ceramics for aerospace applications. The combination of tert-butyl alcohol and camphene as templates allows us to achieve isotropic spherical pores, greatly enhancing both mechanical strength and thermal properties. The competition between solvent crystals helps achieve optimal pore size, leading to improved crack deflection and energy absorption under high-speed flight conditions.”
This study paves the way for future advances in wave-transparent materials. According to Professor Tian, the next steps are to scale up the manufacturing process and fine-tune the material's properties to meet the specific requirements of various aerospace applications.
Tian noted. “Our ultimate goal is to develop a class of ceramics that can be utilized in a wide range of extreme environments.”
National Natural Science Foundation of China (Grant No. 52202078), the Leading Talent Project of the National Special Support Program (2022WRLJ003), Guangdong Basic and Applied Basic Research Foundation for Distinguished Young Scholars (Grant No. 2021B1515020083), and Guangdong Basic and Applied Basic Research Foundation (Grant Nos. 2021A1515110293 and 2022A1515012201) supported the study.
Journal Reference:
Liu, Y., et al. (2024) Synergistic promotion of dielectric and thermomechanical properties of porous Si3N4 ceramics by a dual-solvent template method. Journal of Advanced Ceramics. doi.org/10.26599/JAC.2024.9220962