Microsecond Laser Polarimetry for Emissivity Measurements on Liquid Metals at High Temperatures and Pressures

Casting process simulations have become very advanced in recent years as computer programs incorporate fluid flow, heat transfer, stress analysis and microstructural modeling. For these simulations, a large variety of thermophysical properties must be known as input parameters in order to achieve accurate results. Unfortunately, the required thermophysical data of cast metals and mold materials are partially not available with sufficient precision. A photopolarimeter for the determination of the normal spectral emissivity at a wavelength of 685 nm was integrated into the existing fast ohmic pulse heating system for the measurement of thermophysical properties of liquid metals. This polarimeter is characterized by the reduction of the polarization state measurement to the measurement of four intensities of a laser beam reflected at the sample surface. Since in this setup no mobile parts are necessary, measurements at high-speed experiments can be carried out in the sub- second time range. Is the polarization state of the incident laser beam known and the polarization state of the reflected beam measured by the polarimeter, the optical parameters refractive index and extinction coefficient can be calculated by means of ellipsometric standard equations. From this quantities the normal spectral emissivity is obtained in a simple way. Within this project it was possible for the first time to determine the normal spectral emissivity of some selected metals in a wide temperature range in the liquid state. These data help to reduce the measurement uncertainties for pyrometric temperature determination when measuring thermophysical properties. The data determined within these experiments such as specific heat capacity, specific electrical resistivity, thermal conductivity and thermal diffusivity are needed in the automotive industry and in the foundry industry to model liquid metal processing operations such as casting and welding, to understand, simulate and design new processing equipment such as facilities for the growth of single crystals from the melt, for obtaining phase diagrams, for obtaining temperature reference points, for accurate application of potential accidents in the design of safe nuclear reactors, and for aerospace techniques. As results of this project the normal spectral emissivity of the metals Niobium, Nickel, Tantalum, Tungsten and Copper as well as of the superalloy Inconel 718 from melting up to several hundred degrees into liquid state were obtained. Due to these measurements the accuracy of the obtained thermophysical properties in the liquid phase such as specific heat capacity, specific electrical resistivity, heat of fusion, thermal conductivity and thermal diffusivity could be improved essential.