Laser-Induced Melting of Co-C Eutectic Cells as a New Research Tool

Abstract

A new laser-based technique to examine heat transfer and energetics of phase transitions in metal–carbon fixed points and potentially to improve the quality of phase transitions in furnaces with poor uniformity is reported. Being reproducible below 0.1 K, metal–carbon fixed points are increasingly used as reference standards for the calibration of thermocouples and radiation thermometers. At NMIA, the Co–C eutectic point is used for the calibration of thermocouples, with the fixed point traceable to the International Temperature Scale (ITS-90) using radiation thermometry. For thermocouple use, these cells are deep inside a high-uniformity furnace, easily obtaining excellent melting plateaus. However, when used with radiation thermometers, the essential large viewing cone to the crucible restricts the furnace depth and introduces large heat losses from the front furnace zone, affecting the quality of the phase transition. Short $\hbox {CO}_{2}$ laser bursts have been used to illuminate the cavity of a conventional Co–C fixed-point cell during various points in its melting phase transition. The laser is employed to partially melt the metal at the rear of the crucible providing a liquid–solid interface close to the region being observed by the reference pyrometer. As the laser power is known, a quantitative estimate of $(166 \pm 30) \hbox { J}{\cdot }\hbox {g}^{-1}$ can be made for the Co–C latent heat of fusion. Using a single laser pulse during a furnace-induced melt, a plateau up to 8 min is observed before the crucible resumes a characteristic conventional melt curve. Although this plateau is satisfyingly flat, well within 100 mK, it is observed that the plateau is laser energy dependent and elevates from the conventional melt “inflection-point” value.