TEMPERATURE ANOMALIES OF THE THERMIONIC EMISSION OF ALKALI METAL CATIONS FROM THE SURFACE OF PILLARED MONTMORRILLONITE

Thermal emission of alkali metal ions M+ (M=Li, Na, K, Rb, Cs) from the surface of pristine montmorillonite (MM), isolated from bentonite of Dash-Salakhli deposit, and synthesized samples of Al13-, Al30-, and Al/Ce-PMM (PMM stands for pillared montmorillonite) was studied using a special technique of high-temperature mass spectrometry. The measurements were performed on a single focusing sector type magnet mass spectrometer MI-1201 (angle 90°, radius of curvature 200 mm) with a system of ion currents registration based on a secondary electron multiplier R595 (Hamamatsu) and a picoammeter 6485 (Keythley). For thermionic emission measurements, highly dispersed powders of the MM and PMM were molded into tablets in the form of disks with a diameter of 12 mm and a thickness of 1 mm. Ions emitted in vacuum (10-5 Pa) from the heated tablet surface were drawn by a weak electric field with a strength of ~105 V/m generated between the tablet holder and the collimator electrode. The dynamic range of emission current density (j) measurements was 2 10-17-2 10-11 A cm-2 in the temperature range of 770-940 K. The temperature dependences of j have distinctive features in each case: monotonic character of j change (MM and Al13-PMM) and reversible at cooling and heating anomaly of j change (Al30-PMM and Al/Ce-PMM) - a jump-like fall of current at temperature increase, and vice versa, a jump-like increase at decrease. This effect manifests itself in an opposite way depending on the ionic radius of the emitted ions: for Al30-PMM it increases with increasing M+ radius (observation interval 805-830 K), and for Al/Ce-PMM it decreases (860-885); in the case of Al/Ce-PMM for Li+ there is an inversion of the anomaly: with increasing temperature there is a jump-like growth of j(Li+). The results are interpreted in terms of the dependence of the thermionic emission efficiency on phase transformations of the ensemble of pillars, whose surface structure in the interlayer space of montmorillonite determines the migration rate of alkali ions to the emitting surface, as well as the mutual influence of the self-diffusion coefficients of M+ ions in the interlayer space of montmorillonite. For citation: Motalov V.B., Ovchinnikov N.L., Butman M.F. Temperature anomalies of the thermionic emission of alkali metal cations from the surface of pillared montmorrillonite. ChemChemTech [Izv. Vyssh. Uchebn. Zaved. Khim. Khim. Tekhnol.]. 2025. V. 68. N 7. P. 85-95. DOI: 10.6060/ivkkt.20256807.7146.