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A method for the extension of the measurement of the vapor pressure of metals to include the range of pressure frora 10 to 10/sup -8/ ram was devised. The method is applicable to metals which forra a stable carbide in the range of temperature to be determined by the Langmuir method of measurement, and the use of the higher pressure region is limited only by the normal imudsen restriction of a characterizable vapor species. The study of the lower pressure region was accoraplished by the use of a large evaporation area surrounded by a condenser. The use of the condenser allowed the possibility of using a large evaporation area while the total weight supported by the balance was small, and thus allowed the sensitivity of the balance to be greater. The use of the normal Knudsen method in the higher pressure region was done as a check on the other techniques and this method was extended to still higher pressures by the use of a tube orifice, requiring a large Clausing factor correction. By the use of the above methods the vapor pressure of holmium metal was studied from 650 to 1750 deg C, so that the vaporization as wellmore » as the sublimation process could be observed. The data yielded the heat of sublimation (70.6 plus or minus 0.5 kcal/mole), the heat of vaporization (64.7 plus or minus 0.5 kcal/mole) and the heat of fusion (4.6 plus or minus 0.6 kcal/raole). No evidence of a transformation was observed, but the scatter of the data in the temperature region immediately below the melting point of the metal is such that the existence of phenomena of this type is not excluded. The use of the carbide formation as a means to retain holmium vapor on the condenser necessitated the consideration of the thermal stability of the carbide of holmium, since the re-evaporation of any of the metal would lead to erroneous readings. To this end, it was determined that the carbide formed on the condenser is the dicarbide, and that at elevated temperatures, this can be decomposed to yield the metal and graphite. The equilibrium decomposition pressure of the dicarbide was measured by the Knudsen method. The sesquicarbide was prepared and the decomposition pressure determined by the Knudsen method. The sesquicarbide decomposes to the dicarbide and holmium vapor as evidenced by the x-ray diffraction patterns of the residue of the decomposition pressure determinations. An attempt to measure the decomposition pressure of a third holmium --carbon compound, as indicated by Gschniedner, was unsuccessful due to the inability to identify the phases present in the decomposition products.« less

Vapor pressure of holmium metal and decomposition pressures of holmium carbides