Co-continuous composites from cold sintering

Cold sintering enables the fabrication of ceramic matrix-polymer composites through low temperature densification by employing a transient solvent under moderate pressure to drive diffusional processes. This innovative processing allows the integration of seemingly incompatible components in a single step to provide new possibilities for tailored multifunctional composites. However, the microstructure of these cold-sintered composites is controlled by a complex interplay between solubility, evaporation, plastic flow and compaction of the inorganic particles. Pressure solution creep process densifies the inorganic particulates through the dissolution, transport and precipitation at the interfaces of the particulates under applied stresses and slightly elevated temperatures. Through selection of ceramic (gypsum and MgO) and polymer (polypropylene and polymethyl methacrylate) materials that differ in densification mechanisms, new insights are gleaned about how material selection impacts the morphology and mechanical behavior of cold-sintered composites. Cold sintering of gypsum leads to a well-densified ceramic, while rapid hydration of MgO leads to minimal densification of the inorganic phase. This difference in ceramic densification affected characteristics of the composites, including the polymer distribution, phase connectivity, and mechanical performance. The high compaction of gypsum during cold sintering facilitated polymer infiltration between particles to form co-continuous phases on cold sintering. In contrast, the limited densification of MgO did not promote flow of polymer and produced isolated polymer domains that led to poor mechanical performance in the cold sintered composites. Although the cold sintering temperature impacts the rheology of the polymer phase to alter the infiltration of the plastic between the inorganic phase during processing, the primary factor dictating the formation of co-continuous phases and corresponding good mechanical performance is signficant densification of the ceramic during cold sintering. The processing temperature and material interactions between the polymer and inorganic phases also impact the morphology of cold sintered ceramic-polymer composites. The combination of materials selection and cold sintering processing parameters provide routes to control morphology for engineering composites with cold sintering with a key heuristic identified here that the inclusion of the polymer cannot overcome poor sintering of the ceramic and densification (compaction) during cold sintering appears to drive the flow and developed connectivity of the polymer phase.