Defect-Mediated Anisotropic Lattice Expansion in Ceramics as Evidence for Nonthermal Coupling between Electromagnetic Fields and Matter

SK Jha and N Nakamura and SY Zhang and LS Su and PM Smith and XL Phuah and H Wang and HY Wang and JS Okasinski and AJH McGaughey and B Reeja- Jayan, ADVANCED ENGINEERING MATERIALS, 21, 1900762 (2019).

DOI: 10.1002/adem.201900762

Electromagnetic (EM) fields can trigger a range of surprising responses in materials. Microwave radiation (MWR), a type of EM field in the frequency range of 0.3-300 GHz, can lower the synthesis temperature required for ceramics such as TiO2 and induces mixed amorphous- crystalline phase compositions. To better understand the effects of MWR on matter, structural changes during microwave heating and MWR-assisted synthesis using in situ synchrotron X-ray diffraction are studied. Anisotropic expansion-contraction of lattice parameters under microwave- radiation is observed, which contradicts the results from conventional thermal heating. When as-received TiO2 powders are heated with MWR, an instantaneous decrease in the intensities of diffraction peaks indicates decrystallization/amorphization. High-resolution electron microscopy supports these observations. Raman spectroscopy and X-ray photoemission spectroscopy indicate increased defect-generation under microwave exposure. Molecular dynamics simulations on the anatase phase of TiO2 suggests that introducing an oxygen vacancy can lead to the formation of an interstitial-vacancy pair resulting in anisotropic expansion- contraction of the lattice. These unique responses of ceramics under externally applied fields provide direct evidence for nonthermal coupling between EM fields and matter. Understanding such nonthermal, field-driven processes has implications in engineering low-temperature processes for integrating ceramics with polymers for flexible electronics, energy harnessing, and storage applications.

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