Formation of ultradispersed systems based on sodium polyaluminates from thermal plasma flows

Abstract

The process of synthesis of sodium b-alumina in a low-temperature nitric plasma is investigated. It is established that the main product of the plasmachemical reaction is sodium polyaluminate Na 2 O × nAl 2 O 3 of an unknown structure, whose homogeneity range is within the interval of n = 3.5 -8.0. The specifics of phase transformations in heating products of plasmachemical synthesis in the temperature interval of 600 -1400°C are considered. The formation of the phase with the b-alumina structure (b-and b²-Al 2 O 3 ) proceeds in several stages with the formation of intermediate metastable sodium polyaluminates.High-density ceramics based on sodium b-alumina is used as a solid electrolyte in thermoelectrochemical generators and batteries with a sodium anode and in other electrochemical devices.Lately substantial advance has been made in the plasmachemical synthesis of sodium polyaluminates (SPA) (USSR Inventor's Certif. No. 1510302) [1, 2], which provides vast opportunities for producing a solid electrolyte with high ionic conductivity within the range of compositions depleted in alkali elements. Meeting these requirements is essential for ensuring high electrochemical and corrosion resistance in ceramic electrolytes. A new trend in research is related to the development of thin-film solid electrolytes for thermoelectrochemical generators using plasma spray-deposition methods.The present study describes the results of studying physicochemical transformations occurring at various stages of plasmachemical synthesis of SPA and identifying the significance of particular reaction parameters for target properties of solid electrolytes.The development of b-ceramics with high service parameters is closely related to the search for new technologies of synthesis of single-phase materials based on structurally similar modifications of sodium b-alumina of the hexagonal (b-Al 2 O 3 ) and rhombohedral (b²-Al 2 O 3 ) symmetry with the idealized formulas b-NaAl 11 O 17 and b²-NaAl 5 O 8 . Their reaction activity is determined by their chemical and thermal prehistory and mainly by the state of their main component Al 2 O 3 participating in the reaction. Therefore, our purpose at the initial state of research was to identify the conditions for the formation of ultradispersed alumina powder from a thermal plasma flow.The experiments were performed on an experimental plasma test plant for producing oxide powders based on a VChI-63/5.28 IG-L01 high-frequency generator of 105 kW. Nitrogen (or air) was the plasma-forming gas. To increase the degree of dispersion of synthesized products and ensure their complete oxidation, cold air was additionally supplied into the reactor.The experiments were performed on several SPA compositions with content of sodium oxide varying from 3 to 27% (here and elsewhere wt.%).The initial materials were aluminum powders, aluminum oxides (or hydroxides), and sodium carbonate (or bicarbonate). Lithium and (or) magnesium oxide additives stabilizing the structure of b²-polyaluminate were introduced in the amoun...