Processing Bi2Te3 via Nanosecond Pulsed Laser - Numerical and Experimental Studies Open Access
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Initial experimental and numerical investigations into processing Bi2Te3, a common thermoelectric material, via nanosecond pulsed laser were carried out. Single scan tracks in Bi2Te3 powder compacts were produced via nanosecond pulsed laser at a scan speed of 40 mm/s and average powers of 1, 2, 3, 4 and 5 W. A depressed central region along with humps of partially ejected material on the edge of the processed region are observed for average powers between 3-5 W. Partial melt ejection driven by recoil pressure and constrained by surface tension is proposed as a mechanism for the observed molten pool shape. Overall, between this work and previous work, a small window of average powers, from 3-5 W, showed significant subsurface melting. Lower average laser powers produced no subsurface consolidation and higher intensities, similar average powers with smaller beam diameters, produced excessive melt ejection, cutting and splatter. Other morphological features in the single scan tracks surfaces and cross-sections, such as micro-cracking and surface porosity, were observed via SEM imaging. Finally, the microstructure of the processed region was observed, showing a very fine microstructure with thin-tall columnar grains and some evidence of preferred orientation. In addition to the experimental studies, a simplified numerical model including heat transfer and phase change was developed. The model showed peak temperatures which supported the possible impact of evaporation/recoil pressure on the size and shape of the processed region. Numerically observed molten pool durations were short on the scale of 0.1 ms to 10 ms and highly sensitive to the choice of absorptivity and thermal conductivity used to model the powder compact. Molten pool size and shape, observed in the model, failed to match, accurately, the experimentally observed processed region dimensions. Neglected physics, such as evaporation, recoil pressure, and Marangoni convection, along with possible inaccuracies in thermophysical and optical properties, are all likely factors impacting the mismatch between numerically predicted molten pool size and the experimentally observed processed region size. A notable result of the numerical model is the sensitivity of the model to the thermal conductivity of the powder compact. The temperature dependent thermal conductivity of thermoelectric materials, such as Bi2Te3, could be used to alter solidification rates through pre-heating the build chamber and/or substrate.