Thermodynamic measurement of Nd–Fe system by double Knudsen cell mass spectrometry

The activities of Nd in the Nd–Fe alloys were determined by double Knudsen cell mass spectrometry. The activities of Nd in Nd–Fe alloys with Nd concentration from 8.5 to 93 at% were determined at 1373–1523 K, measuring the ion current of vapor of Nd in equilibrium with melts, Nd2Fe17 + γFe, Nd2Fe17 + liquid, and γFe + liquid. The standard Gibbs energy of formation of Nd2Fe17 was calculated from the activities of Nd obtained from the measurement, as follows: 2Nd(l)+17Fe(γ)=Nd2Fe17(s),   ΔGf Nd2Fe17°=−238,000+130T±(21,000) J/mol   (1373–1473 K).The activities of Fe in Nd–Fe alloys were calculated based on the Gibbs–Duhem equation.