Emissivity Measurements on liquid Metals and Alloys

The results on the normal spectral emissivity of several liquid elements investigated within the frame of project P12775-PHY fit into three general groups - those where the emissivity values drop at the onset of melting to a minimum and then stay constant for the rest of the liquid phase, those where emissivity values drop at the onset of melting to a minimum and then increase again with rising temperatures, and a third group, where the emissivity values drop at the onset of melting and then further decrease in the liquid phase with increasing temperatures. The HAGEN-RUBENS relation associates for conducting materials in the infrared region emissivity to electrical resistivity. If similar relations wore found to be valid for different metals and alloys in a wave-length range of about 700 nm, emissivity values could be computed from electrical resistivity data. This would be significant, since resistivity for liquid metals can be measured much easier than the emissivity. Using temperature values which are improved by emissivity data, all temperature dependent thermophysical properties of liquid metals and alloys can be obtained with much higher accuracy than available up to now. For the first part of this project the existence of some kind of numerical HAGEN-RUBENS-relation should be investigated for liquid metals by simultaneous measurements of electrical resistivity and normal spectral emissivity at 684,5 nm for several industrial relevant elements such as Fe, Ni, Co, Cu, Ti, V, Nb, Zr, Ir, Pt, Mo, Ta, Re, and W. As especially foundry industry needs very accurate thermophysical properties of liquid metals and even much more of liquid alloys as input data for their calculations on rapid prototyping and to optimize different casting processes, the second part of this project will deal with the systematic investigation of the dependence of emissivity from concentration, when two pure elements are alloyed, such as Fe - Ni or W - Re. Furthermore some selected ternary alloys should be investigated such as 6Al 4V 90Ti and stainless steel. The thermophysical properties measured will include heat capacity, (measured by DSC from 300 K up to 1500 K, and by pulse heating above 1500 K) enthalpy, electrical resistivity, density, thermal diffusivity and thermal conductivity from 300 K up to the end of the stable liquid phase as functions of temperature.

Optical and thermophysical properties of pure metals up to the melting point and in the liquid phase are of general interest for technological applications, especially if these metals are commonly applied. Many metals are used either as pure metals or as alloying components in a vast amount of industry sectors. Due to their excessive usage in industry an ongoing need for new and more accurate data exists. Within this project, the determination of thermophysical properties has been successfully performed on pure metals and alloys. Based on an ohmic pulse-heating apparatus, properties of conducting materials can be obtained from temperatures of about 1200 K, which is in the solid state for most metals and alloys up to temperatures of about 5000 K in the liquid state. The range of accessible thermophysical properties includes specific enthalpy, electrical resistivity, isothermal heat capacity, thermal conductivity, and thermal diffusivity. To enable an accurate temperature measurement over such a vast range pyrometric temperature detection based on Planck`s radiation law is used. Furthermore, an ellipsometric device (s-DOAP) is utilized to detect normal spectral emissivity close to the wavelength (650 nm) of the pyrometer. These results are very important to avoid uncertainties arising from the unknown behaviour of emissivity at melting and in the liquid phase. Measurements of normal spectral emissivity at 684.5 nm for pure metals have been performed on Ag, Au, Cu, Fe, Ni, Co, Ti, V, Nb, Zr, Ir, Pt, Pd, Mo, Hf, Ta, Re and W by means of the mentioned s-Division-Of-Amplitude- Photopolarimeter. Furthermore, multi-wavelength pyrometry is used to extend the emissivity measurements for W, Re, Ta, Mo and Nb to wavelengths of 902 nm and 1570 nm respectively, yielding additional information on the liquid-phase behaviour of emissivity at these two wavelengths. The most recent measurement results show, that the Hagen-Rubens-relation, which relates the emissivity to the electrical resistivity on the basis of the free electron model (for wavelengths > 10 m), is not applicable to wavelengths below 10 m, in particular for wavelengths in the visible spectra. Experiments are also conducted on several different alloy systems such as: Ag-Cu, Au-Ni, Cu-Ni, Ni-Ti, Fe-Ni, Mo-Re, W-Re, Ti-6Al-4V. Using the laser polarimeter at 684.5nm and radiation pyrometers at 650 and 1570 nm, the dependence of the normal spectral emissivity of these systems on their elemental composition is studied as a function of temperature in the liquid phase. It is found that the emissivity value of the alloys cannot be estimated a priori just on the basis of the emissivity values of their constituent metals and the mixing ratio. Values that lie above, below, or in between the emissivity values of the individual alloy constituents are observed, in a non- predictable fashion. For the first time, a detailed uncertainty analysis for all measurement quantities of the pulse-heating system is carried out based on the ISO Guide to the Expression of Uncertainty in Measurement (GUM).