Observations during Al:Zr composite particle combustion in varied gas environments

This work presents observations of Al:Zr bi-metallic composite particle combustion in multiple gas environments using particles that were synthesized by both arrested reactive ball milling and physical vapor deposition (PVD). We report on combustion and microexplosion behavior using high speed videography and emission spectroscopy in four different environments: air, Ar + O2, Ar + N2, and Ar. We report multiple final morphologies including thin, hollow-shelled combustion products that imply gaseous expansion of the particles during combustion at temperatures ranging from 2700 to 3500 K. Scanning electron microscopy and energy dispersive spectroscopy are performed on these post-reaction products to analyze their morphologies and elemental compositions. We also report evidence of Al burning in the vapor-phase in air and in Ar + O2. Ball milled and PVD particles microexplode extensively and repeatedly in air. In the Ar + O2 environment, ball milled particles show no secondary or tertiary explosions, while the PVD particles rarely microexplode. In Ar + N2 and Ar environments, the particles react to form hot intermetallic particles but show very little combustion and no microexplosions. We propose a two-phase reaction mechanism for Al:Zr in which Al vaporizes and combusts in the vapor phase, then exhibits a critical shift to a condensed phase reaction and burns similarly to pure Zr.