Thermodynamic Measurement of Calcium Phosphates by Double Knudsen Cell Mass Spectrometry
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The Minerals, Metals & Materials Society and ASM International 2009
I.
INTRODUCTION
THERMODYNAMIC properties of oxide systems have previously been evaluated by the measurement of heat capacity, chemical equilibrium method, and other techniques. Each method has advantages and drawbacks, and systems and experimental conditions to which these methods can be applied are limited. Double Knudsen cell mass spectrometry was developed as a means of directly measuring thermodynamic data of metals, alloys, and intermetallic compounds.[1–6] However, this method often has not been employed to determine the thermodynamic properties of oxide systems, since it is difficult to interpret fragment ions yielded during a measurement and to control oxygen potential in an experimental system even though this potential is one of the most important factors in such a measurement.[1,7,8] It is anticipated that use of double Knudsen cell mass spectrometry will provide more accurate data on thermodynamics and experimental conditions of oxide systems than can be learned by currently established methods. In this study, thermodynamic properties of tetracalcium phosphate, (CaO)4P2O5, and tri-calcium phosphate, (CaO)3P2O5, were investigated by applying this technique. Many researchers have reported thermodynamic data of these compounds, as determined by chemical equilibrium methods[9–13] or EMF,[14,15] since TAKASHI NAGAI and MASAO MIYAKE, Research Associates, and MASAFUMI MAEDA, Professor, are with the Institute of Industrial Science, the University of Tokyo, Tokyo 153-8505, Japan. Contact e-mail: [email protected] Manuscript submitted October 25, 2008. Article published online May 30, 2009. 544—VOLUME 40B, AUGUST 2009
the compounds are products of de-phosphorization reaction in steelmaking, and their thermodynamic properties are critical in understanding the behavior of phosphorus in steelmaking thermodynamically. The results obtained have confirmed the validity of measurement of these oxide system properties by comparing the thermodynamic properties of (CaO)4P2O5 and (CaO)3P2O5 with this method and those by other methods.
II.
EXPERIMENTAL
A. Double Knudsen Cell Mass Spectrometry Knudsen cell mass spectrometry was developed to measure vapor pressures of metals, alloys, and compounds. In this method, vapor pressure is measured by a mass spectrometer as ion current, which is proportional to the pressure. The pressure, pi, and ion current of i-species, Ii, measured by mass spectrometer are related through the following fundamental equation:[16] pi ¼
T Ii Si
½1
where T is the absolute temperature of a specimen and Si is the device-dependent constant, which includes such factors as ionization cross section and efficiency of the ion detector. The usual accuracy of the data determined by single Knudsen cell mass spectrometry is not always adequate, since the constancy of the device-dependent constant Si is difficult to maintain over a long period and reproducibility of each experimental run may not be adequate. To overcome this problem, a tech
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