A Research On The Spectral Emissivity Of Molybdenum

Molybdenum is widely used in steel manufacture,electronics industry,agriculture,chemical engineering,and pharmaceuticals industry,thanks to its good properties such as high biological activity,high melting point,high boiling point,low coefficient of linear expansion,and excellent elastic modulus.Metal working industry needs accurate thermophysical data as input parameters for numerical simulation to optimize casting processes.As one of the most important thermophysical properties,the temperature of a metal is usually measured by radiation thermometry in high-temperature environment.The accuracy of thermometry mainly relies on precise emissivity of material,which varies sensitively with measurement condition and is difficult to be measured.In this paper,the surface emissivity of molybdenum at high temperature is taken as the research object.The self-built spectral emissivity measuring device and the polarization direction emissivity measuring device are used to study the emissivity of molybdenum at different temperatures with the oxidation process,and the reasons of changes in emissivity are explored.A device for measuring the normal emissivity of materials at a temperature of 673-973 K was built according to the direct measurement method.The infrared normal spectral emissivity of the oxidized molybdenum was measured during thermal oxidation process,and the integral emissivity was calculated from the data of spectral emissivity.It is found that the surface oxidation has a remarkable effect on the spectral emissivity of molybdenum,and the spectral emissivity curves become more fluctuant with the increase in oxidation time.The integral emissivity grows exponentially with the oxidation time at 773 K,remains almost constant at 823 K,and fluctuates at 873 and 923 K.The X-ray fluorescence spectrometer,the X-ray diffraction,and the scanning electron microscopy were employed to analyze the changes in surface composition and surface morphology.The results show that the most probable reason for the variation of integral emissivity is the change in surface roughness caused by the variation in the size and shape of oxide particle on specimen surface.A device for measuring the polarized emissivity of materials was built by the energy ratio function.The P-polarization and S-polarized spectral emissivity of molybdenum changed with angle at different temperatures were measured,and the relationship between polarized emissivity and unpolarized spectral emissivity was explored.The normal emissivity obtained by the polarization direction emissivity measuring device is substantially the same as the normal spectral emissivity value obtained by the spectral emissivity measuring device composed of the grating monochromator at the same wavelength,and the two devices mutual corroboration.The trends of the polarization direction emissivity of molybdenum at different temperatures are basically the same.At 773 K,the polarized emissivity at different wavelengths have greater difference.The larger the wavelength,the larger the emissivity value is.That because there is a small signalto-noise ratio and a large measurement error at shorter wavelengths at lower temperatures,especially at short wavelengths,and after the temperature rises,the difference between the emissivity caused by the wavelength is increasing.The polarization emissivity at 1550 nm with angle is larger than the values at the other two wavelengths at 823?873 and 923 K,but the overall difference is small.