DETERMINATION OF THE MELTING POINT AND THE EMISSIVITY OF REFRACTORY METALS WITH A SIX-WAVELENGTH PYROMETER
A six-colour pyrometer has been used to measure the melting point and the emissivity of nine refractory metals: hafnium, niobium, molybdenum, rhenium, rhodium, tantalum, tungsten, vanadium and zirconium. In several metals and conductive ceramics the isotherms of the emissivity are found to cross at a particular wavelength, or within a very narrow interval. This effect holds up to and beyond the melting point; in fact, it is found that the slope of a plot of emissivity against wavelength changes sign when the metal goes from the solid to the liquid phase. This inversion occurs at or near the melting point, where the investigated metals behave like greybodies in the interval 500 - 1000 nm. Although the physical meaning of this property is still unclear, its technological interest for calibration of multiwavelength pyrometers at high temperatures is evident, especially under difficult experimental conditions, typically when the sample is heated in optically absorbing atmospheres.
Bibliographic Reference: ARTICLE: HIGH TEMPERATURES-HIGH PRESSURES, VOL 21 (1989), PP. 139-148
Record Number: 1989128010000 / Last updated on: 1990-04-01
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