Study of the mechanism for controlled crystallization of BaF 2 under two kinds of monolayers
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M. Rappaz Department of Materials, Ecole Polytechnique Federal Lausanne, Lausanne, Switzerland (Received 29 January 2001; accepted 5 June 2001)
Pulsed heating experiments that measure high-temperature thermophysical properties using pyrometric measurement of the temperature–time history of metal specimens rapidly heated by passage of electric current have a 30-year history at the National Institute of Standards and Technology. In recent years, efforts have been made to move beyond the limitations of the standard technique of using costly, black-body geometry specimens. Specifically, simultaneous polarimetry measurement of the spectral emissivity has permitted study of sheet and wire specimens. This paper presents the results of two efforts to expand beyond the macroscopically monolithic, single-phase materials of all previous studies. In the first study the melting temperatures of coatings, including Ti and Ti(Al) alloys, deposited on higher melting Mo substrates are measured. In the second study the melting temperatures of substrates, Ti and Cr, covered by higher melting W and Mo coatings are measured.
I. INTRODUCTION
Pulsed heating experiments based on pyrometric and electrical measurement of the temperature–time history of metal specimens rapidly heated by passage of electric current have been used to obtain melting temperatures, specific heats, and latent heats at the National Institute of Standards and Technology (NIST) for the past 30 years.1,2 Electrical and magnetic issues related to the pulsed current in these experiments are detailed in Ref. 2 in particular; for the currents and pulse times in these experiments, spatial variation of current density due to such effects is negligible. Early experiments utilized tubular specimens with small sighting holes to approximate black-body conditions. The pyrometer is focused on the hole, and the experimentally measured pyrometer voltages can be directly converted to true temperatures without knowledge of the spectral emissivity of the specimen at the pyrometer wavelength. Such experiments provide thermodynamic data for elemental materials at elevated temperatures, with uncertainties in determination of, e.g., melting temperature as small as those associated with the pyrometric determination of temperature, typically approximately 8 K (2) at temperatures near 2000 K. A significant drawback of such specially prepared specimens is their cost. Some recent efforts attempt to move beyond the limitations of this standard technique. Specifically, polarimetric measurement of the spectral emissivity of the J. Mater. Res., Vol. 16, No. 8, Aug 2001
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specimen surface, concurrent with pyrometric measurement of the radiance temperature on the same surface, has permitted study of sheet and wire specimens3 rather than costly black-body geometry specimens. Such a technique permitted a study of the impact of heating rate and grain size on the melting behavior exhibited by a Nb–47 mass% Ti alloy.4 The savings do come with othe
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