Image Credit: Roman Dombrowski/Shutterstock.com
Exposure to microwave radiation generates atomic rearrangements in ceramic oxide, opening up new possibilities to use electromagnetic radiation to create bespoke materials. A recent study has revealed the relationship between an applied electromagnetic field and the resultant atomic structure, paving the way to engineering ceramics at the atomic scale.
Microwave ovens and ceramicware are ubiquitous around us. Microwave radiation is the part of the electromagnetic field in the frequency range 0.3 to 300 GHz. Electromagnetic fields have been known to have an influence on the synthesis of materials, for example, an applied electromagnetic field can reduce the temperature required for atoms to assemble into crystalline structures.
How exactly matter interacts with the electromagnetic fields that it is exposed to remains unknown. Studies have indicated that electromagnetic fields can disrupt equilibrium atomic arrangements, leading to changes in material properties. This has been hypothesized to occur in two ways:
- Thermal effects such as more efficient or rapid heating of the material occur, and/or
- Non-thermal effects such as defect generation or atom transport occur.
Distinguishing between these two effects has so far proven challenging due to experimental limitations.
Scientists have now experimentally verified that microwave radiation of 2.45 GHz (the same frequency as a typical microwave oven in the home) can directly alter the local atomic structure of ceramic oxides to generate non-equilibrium structures that are not formed when the materials are not exposed to radiation.
This study demonstrates that the applied radiation alters local electric field intensities and generates oxygen defects (missing atoms, clustering, or misalignments in the atomic structure) that change the local structure, which cannot be accounted for by thermal effects.
Professor Reeja-Jayan and her colleagues at Carnegie Mellon University, along with researchers at the Brookhaven National Laboratory in the US, have designed a microwave reactor that can irradiate a ceramic thin film simultaneously with microwave radiation as well as X-rays, enabling the study of the structure of a site known to be exposed to radiation.
The local atomic structure of the ceramic can be deduced from the X-rays scattered by the material. This novel in-situ technique can hence provide information regarding the dynamic changes taking place in the material when exposed to microwave radiation.
The researchers have overcome a major experimental bottleneck caused by the difficulty of irradiating a growing ceramic with both X-rays and microwave radiation.
This work adds an important technique to the experimental repertoire of material scientists, enabling them to monitor dynamic changes in materials exposed to microwaves.
Microwave Radiation Changes Local Atomic Structure
Although microwaves have been known to cause atoms to assemble into crystalline microstructures at lower temperatures, this study provides the first experimental evidence that microwaves achieve this by generating non-equilibrium atomic arrangements.
Using the pair distribution function (PDF) analysis, the researchers identified both well-ordered crystalline atomic structures and disordered amorphous structures, unlike the conventionally used X-ray diffraction (XRD) technique that only detects crystalline structures.
This work shows that during the formation of tin oxide (SnO2) nanoparticles when exposed to microwaves, rapid formation of the crystalline phase is preceded by large displacements of oxygen atoms.
The magnitude of the displacements is directly proportional to the strength of the local electric fields caused by the microwaves. This evidence proves that the applied field primarily affects the oxygen sublattice in the ceramic and the oxygen defects. They, therefore, generate further atomic rearrangement.
From this investigation of local structure and local electric field intensities, the effect of the microwave radiation on the atomic structure could be decoupled from the thermal effects of the radiation, which is significant progress achieved in this field.
Further Insights to Create Bespoke Ceramics
This novel technique and the insights gained thereof, can lead to the utilization of electromagnetic waves to manipulate ceramic nanoparticles at the atomic level, tailoring the structure (and consequently the properties) of ceramics.
The mechanistic understanding gained in this study can extend the use of ceramics for applications such as lithium-ion batteries, solar cells, catalysis, or for molecularly blending ceramics with delicate polymers for flexible electronics.
Not only can external electromagnetic fields be used to create materials with non-equilibrium microstructures, but this can also be achieved at lower temperatures than conventional routes. In addition, such energy-efficient material processing methods assisted by electromagnetic fields will be critical to achieving a green manufacturing economy.
References and Further Reading
N. Nakamura, Laisuo Su, Jianming Bai, Sanjit Ghose, B. Reeja-Jayan, In situ synchrotron pair distribution function analysis to monitor synthetic pathways under electromagnetic excitation, Journal of Materials Chemistry A, 2020, 8, 15909-15918, Available at: https://doi.org/10.1039/D0TA03721D
N. Nakamura and B. Reeja-Jayan, “Synchrotron X-ray Characterization of Materials Synthesized Under Microwave Irradiation,” Journal of Materials Research, 2019, 34, 194- 205, Available at: https://doi.org/10.1557/jmr.2018.465
S. K. Jha, X.L. Phuah, J. Luo, C.P. Grigoropoulos, H. Wang, E. García, B. Reeja‐Jayan, “The Effects of External Fields in Ceramic Sintering,” Journal of the American Ceramic Society, 2019, 102, 5-31, Available at: https://doi.org/10.1111/jace.16061