Measurements of Vapor Pressures of Sc, La, and Ce by Multi-Knudsen Cell Mass Spectrometry
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I.
INTRODUCTION
RARE earth (RE) elements have been increasingly used in new technological applications such as electronics, optoelectronics, supermagnets, and superconductors, and increase in the use of these elements in new applications is expected to continue. Scandium (Sc) has been used in metal halide lamps and Sc-Al alloys,[1,2] lanthanum (La) is used in hydrogen storage alloys,[3,4] and cerium (Ce) is used as abrasives and magnets.[5,6] However, reliable thermodynamic properties of these RE metals available in the literature are limited, although they are of particular importance in development of a high-temperature process. The lack of data seems to be from experimental difficulties caused by the high reactivity of the metals. Recently, we demonstrated that the vapor pressure of yttrium (Y) could be accurately measured by multi-Knudsen cell mass spectrometry.[7] In this method, measurements are carried out in a high vacuum in a short time, and thus, significant oxidation or contamination of the RE metal during the measurements can be avoided.[7] In the present study, vapor pressures of Sc at temperatures from 1373 K to 1573 K and those of La and Ce from 1473 K to 1573 K were determined in a similar way. The Gibbs energies of formation of gaseous elements were derived from the vapor pressures. Ackermann and Rauth[8] and Habermann and Danne[9] reported on the vapor pressure measurements of these metals at high temperature (more than 1520 K for Sc and more than 1874 K for La and Ce). The thermodynamic properties of these metals in a wide range of temperature are listed in a book edited by Barin and Knaeke.[10] However, they are just an estimation based on research conducted in the late 1950s, and thus, revalidation of the data is necessary. WOONG-HEE HAN, Graduate Student, TAKASHI NAGAI, Research Associate, MASAO MIYAKE, Research Associate, and MASAFUMI MAEDA, Professor, are with the Institute of Industrial Science, The University of Tokyo, Meguro-Ku, Tokyo 153-8505, Japan. Contact e-mail: [email protected]. Manuscript submitted May 20, 2009. Article published online August 14, 2009. 656—VOLUME 40B, OCTOBER 2009
II.
EXPERIMENTAL
A. Multi-Knudsen Cell Mass Spectrometry In Knudsen cell mass spectrometry, the vapor pressure of a substance i in a Knudsen cell pi and ion current of i-species Ii measured by a mass spectrometer are related through the following fundamental equation[11]: Ii Pi ¼ b T ri
½1
where T is the absolute temperature of a specimen, b is the device-dependent constant, and ri is the ionization cross section of the i-species. Hence, if the values of b and ri are known, vapor pressure can be obtained by the measuring ion current. However, the usual accuracy of the data determined by this method is not always sufficient, since the constancy of the devicedependent constant b is difficult to maintain over a long period and the reproducibility of each experimental run may not be good. To overcome this problem, a multi-Knudsen cell method was adopted that uses several Knudsen cells and allows mea
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