2, Columbia University, New York, New York, United States
3, Purdue University, West Lafayette, Indiana, United States
4, Brookhaven National Laboratory, Islip, New York, United States
Electromagnetic (EM) radiation can significantly affect ceramic synthesis and processing, inducing rapid, low-temperature crystallization, non-equilibrium phase formation, and altered material properties. In particular, microwave radiation (MWR)-assisted synthesis has demonstrated the ability to crystallize high-temperature ceramic phases at significantly lower temperatures than conventionally required and impact local atomic ordering relative to furnace-based techniques. However, the mechanisms underlying these effects are not well understood. To understand these mechanisms, it is necessary to characterize EM field effects not only on crystalline phase formation, but also on non-crystalline local atomic order due to defects or amorphous components. It has been theorized that defect generation and transport may play an important role in promoting the effects seen under EM field exposure. Therefore, the relationship between synthesis and processing parameters (e.g., temperature, pressure) and the type, concentration, and response of defects is critical to the efficient design of these materials. An increased understanding of the potential mechanisms underlying EM field-assisted techniques will create the opportunity to utilize these methods to stabilize non-equilibrium phases or phase mixtures with desirable material properties.
To better analyze how EM fields influence phase formation and transitions, we utilize X-ray pair distribution function (PDF) analysis complimented by transmission electron microscopy (TEM) and Raman spectroscopy to study the crystallization and local atomic order in ZrO2, TiO2, and ZnO thin films. We demonstrate that the low-temperature crystallization and phase transitions observed during MWR-assisted synthesis are not a result of purely thermal effects. In all materials studied, we find that MWR helps to crystallize phases at much lower temperatures than conventionally required, and that the local order differs in MWR and furnace-grown films. This indicates a clear effect of MWR exposure on phase formation. By analyzing the local atomic order across various synthesis conditions, we find that thermal effects (e.g., reaction temperature, heating rate) do not fully explain the phase transitions observed. From our results, we are able to gain insight into the importance of defects and potential mechanisms underlying field-assisted phase formation.