High temperature fiber optic laser-induced breakdown spectroscopy sensor for analysis of molten alloy constituents

Abstract

A fiber optic (FO) laser-induced breakdown spectroscopy (LIBS) sensor that measures the on-line, in situ elemental composition of a molten alloy inside the melt in a furnace is described. This sensor has applications as a process monitor and control tool for glass, aluminum, and steel melters. The sensor is based on the transmission of laser energy through a multimode optical fiber. The laser radiation from the fiber is collimated and finally focused inside the aluminum melt in the furnace by a specially designed stainless steel holder that holds the collimating and focusing lens. Atomic emission from sparks from the laser plasma is collected by the same stainless steel lens holder and transmitted back through the optical fiber and finally fed into the entrance slit of the spectrograph. The present design of the stainless steel holder is useful for obtaining a collimated LIBS signal over a long distance (the distance between the focusing and collimated lenses is more than 200 cm). Parametric studies such as sample-to-lens distance and the effect of the angle of incidence of the laser beam on the sample surface were performed. Calibration curves for minor elements were obtained for solid Al alloys as well as for a molten Al alloy in the laboratory furnace by inserting the FO LIBS probe inside the molten material. The performance of the probe was also tested by inserting the stainless steel holder into the melt at a 45° angle, which is necessary for collecting LIBS data in an industrial furnace. LIBS spectra in different spectral regions were recorded in the pilot furnace during different tests where known amounts of minor elements were added to the melt. The results obtained from this sensor for different Al alloys demonstrate the usefulness of this sensor to monitor the concentration of different constituents of the molten Al alloy in an industrial furnace.