Optical, thermal, and crystallization characteristics of metallic glasses



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Materials with large photo-thermal energy conversion efficiency are essential for a wide variety of applications spanning from medicine to renewable energy. Photoexcitation is an effective approach to generate controlled and localized heat at relatively low optical powers. However, lateral heat diffusion to the surrounding illuminated areas accompanied by low photo-thermal energy conversion efficiency remains a challenge for metallic surfaces. Surface nanoengineering has proven to be a successful approach to further absorption and heat generation. In this dissertation, we investigate the optical, thermal and crystallization properties of arrays of metallic glass nanowires. Integrated diffuse reflectance measurements and rigorous coupled wave analysis simulations at different angles of incidence and different wavelengths were used to characterize the absorption properties of metallic glass nanowires with different length-to-diameter aspect ratios. Our measurements and simulations revealed the importance of the nanowire profile (vertically aligned or bundled) in the reflectance of nanopatterned metallic glasses. Furthermore, the scattering properties of metallic glass nanowires were investigated using speckle imaging. Statistical analysis of the speckle images obtained from samples with different aspect ratios, amorphicity and crystallinity revealed distinct features of these scatterers. These features are then used in various machine learning algorithms for classification of new speckle images. High accuracy of the classifier algorithms enabled a speckle analysis-based method for characterization of the topology and the structural state of metallic glasses. We further show that pronounced spatial heat localization and high temperatures can be achieved with arrays of amorphous metallic glass nanowires whose geometry can be readily tailored through thermoplastic molding. Thermography measurements revealed marked temperature contrast between illuminated and non-illuminated areas even under low optical power excitation conditions. This attribute allowed for generating legible photo-induced thermal patterns on textured metallic glass surfaces. We have also demonstrated that the optical absorption and coupled heat conversion in the near infrared can be enhanced by tailoring the metallic glass nanowire topology. Infrared thermography measurements and heat transport simulations reveal that the photo-induced temperature rise can be amplified by increasing the length of nanowires and decreasing the thickness of the supporting substrate. Temperatures above 500°C can be rapidly achieved to induce a controlled phase transformation from amorphous to crystalline state in metallic glass nanowires while maintaining their geometrical integrity. We utilized the photo-induced temperature rise of metallic glass nanowires in optical ignition application as demonstrated by the ignition example of thermite powder. We further explore the effects of structural state (amorphous and crystalline) on the optical characteristics of the metallic glass nanowires. The optical constants of unpatterned amorphous and crystalline samples were measured using spectroscopic ellipsometry. Thus, an optical characterization method based on ellipsometry was introduced to verify the crystallinity of metallic glasses.



Nanowires, Amorphous materials, Amorphous metals, Thermal efficiency, Infrared imaging, Optical absorption, Speckle imaging