Influence of alumina shell on nano aluminum melting temperature depression
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As particle size reduces from the micron- to the nano-scale, physical properties of the material can be affected and impact thermal and reaction dynamics of the particles. In the case of nano-aluminum particles encased in an alumina passivation shell, as particle size decreases the shell strength approaches theoretical. Also, nano particles can exhibit a melting temperature depression following the Gibbs-Thomson relationship based on surface tension effects. Understanding how shell strength and surface tension influence each other is an objective of this study. Specifically, the effect of the alumina shell on the nano aluminum melting temperature depression is examined using thermal analysis techniques. Nano aluminum particles of various particle sizes ranging from 17 to 108 nm having virtually undamaged alumina shells were selected for this study. Melting temperatures for each of these powders were measured using differential scanning calorimetry. These measured values were compared with theoretical melting temperatures calculated using the Gibbs-Thomson equation. It was observed that the melting temperatures of alumina encapsulated nano-aluminum particles matched qualitatively with the theoretical trend but not quantitatively. For example, melting temperatures of nano-aluminum particles with undamaged shells exhibited melting temperatures on average 5 K greater than theoretical predictions. The alumina shells of these particles were then damaged mechanically by grinding them between two cylindrical dies in Hydraulic-press. Melting temperatures of the mechanically damaged particles were measured using differential scanning calorimetry and found to have reduced melting temperatures when compared to undamaged particles. The melting temperatures of nano-aluminum particles with damaged shells were in better agreement with theoretical values. Using the difference in melting temperatures of damaged and undamaged powders pressure build-up within the aluminum core was calculated and compared with the pressures calculated using elasticity theory. The comparison showed that the pressure build-up in most of the particles was due to the interfacial surface energies between alumina-aluminum, alumin-air and solid-liquid aluminum.