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The high density of aluminum nanocrystals (>10 21 m −3 ) that develop during the primary crystallization in Al-based metallic glasses indicates a high nucleation rate (∼10 18 m −3 s −1 ). Several studies have been advanced to account for the primary crystallization behavior, but none have been developed to completely describe the reaction kinetics. Recently, structural analysis by fluctuation electron microscopy has demonstrated the presence of the Al-like medium range order (MRO) regions as a spatial heterogeneity in as-spun Al 88 Y 7 Fe 5 metallic glass that is representative for the class of Al-based amorphous alloys that develop Al nanocrystals during primary crystallization. From the structural characterization, an MRO seeded nucleation configuration is established, whereby the Al nanocrystals are catalyzed by the MRO core to decrease the nucleation barrier. The MRO seeded nucleation model and the kinetic data from the delay time ( τ) measurement provide a full accounting of the evolution of the Al nanocrystal density (N v ) during the primary crystallization under isothermal annealing treatments. Moreover, the calculated values of the steady state nucleation rates ( J ss ) predicted by the nucleation model agree with the experimental results. Moreover, the model satisfies constraints on the structural, thermodynamic, and kinetic parameters, such as the critical nucleus size, the interface energy, and the volume-free energy driving force that are essential for a fully self-consistent nucleation kinetics analysis. The nucleation kinetics model can be applied more broadly to materials that are characterized by the presence of spatial heterogeneities.
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Formation and ripening of SrTiO3 nanocrystals on SiO2
The crystallization of complex oxide thin films on amorphous substrates presents a significant challenge because of the lack of long-range order in these substrates and the subsequent difficulty in controlling crystal growth. Nanocrystals with similar crystal structure have the potential to serve as nucleation sites for crystallization and can facilitate this integration. Isolated nanocrystals of strontium titanate (SrTiO3) can be produced on amorphous SiO2 surfaces through crystallization and ripening of initially amorphous layers of SrTiO3. The resulting SrTiO3 nanocrystals exhibit characteristic lateral radii ranging from tens to hundreds of nm and a consistent average height of 1–2 nm across this range. The area density and mean radii of the nanocrystals can be selected by adjusting the deposition and heating parameters, including the amount of deposited SrTiO3 and the heating duration. The heating-time dependence of the area density and mean radii of the nanocrystals is consistent with predictions based on Ostwald ripening kinetics. The selection of these parameters facilitates the use of SrTiO3 nanocrystals as nucleation sites to crystallize the subsequently deposited layer.
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- PAR ID:
- 10611743
- Publisher / Repository:
- American Institute of Physics
- Date Published:
- Journal Name:
- APL Materials
- Volume:
- 13
- Issue:
- 7
- ISSN:
- 2166-532X
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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