Mass spectrometric investigation of two-component systems of complex vapour composition by the isothermal evaporation method

Abstract

The Isothermal Evaporation Method (IEM) and its application to the investigation of two-component systems is considered. The isothermal evaporation process should be carried out rather slowly (1–3% mole per hour). Under this condition the process can be considered as a sequence of equilibrium states. Ion current intensities are measured as a function of time. Taken in such a way the ion current time dependences are transposed into dependences of the partial pressures on the melt composition. The mathematical part of IEM which makes possible this transformation is based on two well-know equations, viz. Knudsen's equation and the Gibbs—Duhem equation. Using EIM the following two-component systems were studied: NaFMF n (where M = Be, Mg, Ba, Al, Sc, Y, La, Zr), KFAlF 3, LiFAlF 3 and LiFScF 3. In the equilibrium vapour of these systems four types of molecules were found: AB  NaBeF 3, NaAlF 4, NaScF 4, NaYF 4, NaZrF 4, LiAlF 4, LiScF 4 KAlF 4; A 2B 2(NaBeF 3) 2, (NaAlF 4) 2, (LiAlF 4) 2, (KAlF 4) 2; A 2B  Na 2BeF 4, Li 2AlF 4, possibly Na 2AlF 5; AB 2  NaV 2F 7. The mass spectra of the main positive ions, the relative ionization cross-sections, heats and entropies of dissociation of these molecules are given. Investigation of two-composition systems of complex vapour composition leads to the conclusion that some of the thermodynamic relations can usefully be written in a form which takes into account the complex vapour composition. Forms of Gibbs—Duhem's and Schreder's equations are considered. The relations which are true at the extreme points of the isothermal function ▪ are given. Examples illustrating the mass-spectrometric application of the given thermodynamic relations are given.