Mass Spectrometric Study of Phase Changes in Aluminum, Praseodymium, and Neodymium

Abstract

A sensitive method has been developed for measuring rapidly the latent heat of vaporization or sublimation of metals of low volatility. Atoms from the saturated vapor emerge from a small Knudsen‐type effusion cell and impinge upon a hot filament. A fraction of the incident atoms re‐evaporate from the filament as ions and are detected by a mass spectrometer. The Clausius‐Clapeyron equation is used to obtain the latent heat from the dependence of ion current upon cell temperature. Application has been made to aluminum, praseodymium, and neodymium. The work on aluminum yielded an enthalpy of sublimation at 298°K of 78.0±0.4 kcal/M. Some anomalous behavior was observed in the rare earths. Preliminary molar enthalpy values are: Pr, 79.3±2.0 kcal (vaporization at 1380°K); Nd, 70.6±2.0 kcal (vaporization at 1348°K) and 74.1±2.0 kcal (sublimation at 1236°K). An estimate of the heat of fusion of Nd yielded 3.5±1.0 kcal/M. The method also provided a convenient measurement of first‐order phase transition temperatures. The melting points of Pr and Nd were found to be 1192±2°K and 1292±2°K respectively. In addition, a solid‐to‐solid phase transition was found in Nd at 1142±2°K.