Lightweight, hierarchically porous ceramics are widely used from catalyst supports to thermal insulation to photonics and electronics. But fabricating these structures requires expensive nanofabrication methods or two-photon lithography to create polymer templates for coating in ceramic materials. Etching, foaming agents, and freeze-drying can also create porous ceramics, but more recently additive manufacturing and self-assembly of block copolymers promise new fabrication possibilities. Now researchers from Air Force Research Laboratory, UES, University of Massachusetts, State University of New York, and Harvard University have combined block copolymer self-assembly with 3D printing to fabricate ceramics with porosity ranging from the nano- to the microscale [Bowen et al., Materials Today (2022), https://doi.org/10.1016/j.mattod.2022.07.002].
“We were interested in exploring the creation of hierarchically structured ceramics, over large areas, using a combined self-assembly and 3D printing approach,” explains Matthew B. Dickerson of the Air Force Research Laboratory, who led the work. Block copolymers contain two or more chemically distinct polymer blocks that self-assemble into nanostructures. If combined with other polymers, block copolymers can act as a template. Dickerson and his colleagues exploited this phenomenon with pre-ceramic polymers, which form solid ceramic materials when heat-treated. The team created an ink containing block copolymers (namely poly(n-butyl acrylate) or PnBA and poly(methylmethacrylate) or PMMA) and a pre-ceramic polymer (polycarbosilane or PCS) suitable for direct ink write 3D printing. When the block copolymer is burned out of the 3D-printed structure during pyrolysis, a silicon oxycarbide and silicon carbide ceramic with hierarchical porosity and unique mechanical properties is left behind. “The ceramic has a shape reminiscent of coral and, as such, we termed the ceramic to have a ‘nanocoral’ morphology,” says Dickerson. “Significantly, these hierarchically structured nanocoral ceramics demonstrate a combination of robust mechanical energy absorption and strength,” he adds. The low-density material has the mechanical energy absorption properties of a metal alloy foam and significantly lower thermal conductivity than bulk, non-porous ceramics. Moreover, the printed polymer architectures can be bent, twisted or folded into complex shapes such as bowties, waves, cylinders, or even novel geometries reminiscent of ‘paper’ airplanes that are preserved after pyrolysis, enabling the fabrication of an almost infinite range of unusual ceramic shapes. “These architectures may be useful for mechanical energy absorption applications, particularly in higher temperature environments,” points out Dickerson, “[or] be leveraged for functional applications such as catalysis and energy storage.”