Ceramic oxides and carbides find widespread use in technologies ranging from solar cells and electronics to high-durability, impact-resistant surfaces for military and aerospace applications. However, current ceramic manufacturing processes emit up to 80 mg/m3 of particulate matter and 470 mg/m3 of NO2 gas (compared with a national average of 0.1 mg/m3 particulate matter and 75 mg/m3 NO2), in addition to CO2 and sulfur oxides.
Ceramic-processing steps – like sintering of particles into dense parts – also require high temperatures (500-2500°C), making these processes highly energy-intensive. Because of these drawbacks, new ceramic manufacturing processes that avoid high-temperature steps must be developed. Indeed, the first pottery was shaped and fired in a high-temperature oven around 13,000 BCE. Even in 2020, the basic process remains the same and incorporates high-temperature (sintering) steps that limit the integration of ceramics with new soft materials like polymers.
Externally applied electric and electromagnetic fields can offer a low-temperature alternative to conventional thermal curing/sintering methods for energy-efficient ceramic processing. However, a lot remains to be understood about the fundamental mechanisms underlying interactions of these fields with materials such as ceramics. In particular, we need to quantify how atomic ordering and resultant crystal structures change under the conditions where fields are applied.
Our experiments use time-varying electromagnetic fields in microwave radiation to crystallize and sinter ceramic oxides like TiO2 and ZrO2 at 150-250°C in 30 minutes (versus 500-1500°C in multiple hours conventionally). Our experiments can grow both particles in a solution and thin solid films on a substrate such as glass.
The experiments use interfaces (such as conducting layers on a substrate) to localize and intensify the field impinging on the sample. In this way, we are able to address a long-standing challenge for experimentally studying field-induced forces by accurately mapping localized fields, temperatures, charge transport and reaction kinetics within a localized region instead of in a bulk media.
Using high-resolution synchrotron X-ray studies, we were able to demonstrate the first experimental evidence that 2.45-GHz microwave fields stabilize different atomic structural arrangements or phase(s) in ceramics like TiO2 compared to conventional, high-temperature furnace-based synthesis. These results provide definitive proof that these fields can induce structural changes, leading to crystallization of ceramic oxides like TiO2 at temperatures as low as 150°C.
Our work thus far lays the theoretical foundations for deploying external fields as a new processing tool to access high-temperature ceramic phases with minimal thermal input, allowing us to explore regions of phase space, microstructures and properties not accessible via conventional synthesis routes.
Understanding these fundamental mechanisms behind field-matter coupling when applying electromagnetic fields during the synthesis of materials such as ceramics can be applied to devise novel additive manufacturing (3D printing) of ceramic materials (e.g., Al2O3, SiC), which is not as mature a technology as printing of polymers (plastics) and metals. 3D-printed ceramics find use in areas as diverse as energy, environment, transportation, aerospace, telecommunications and healthcare.