Metal carboxylate precursors for ceramic materials and waste forms for radionuclides
Description
Novel metal carboxylate precursors for ceramic materials have been developed which provide extremely low deposition temperatures and/or precise control of metal stoichiometry Low temperature metal oxide precursors based on metal complexes of $\alpha$-oximino carboxylate ligand pyruvic acid oxime (PAO) have been developed. Complexes of this ligand with a variety of metal cations, including main group elements, transition metals and rare-earths have been prepared. Results from X-ray analysis of a number of these complexes demonstrate that the ligand forms five membered metallocyclic structures with the complexed metal cation through one oxygen of the carboxylate group and the nitrogen from the oximino moiety. These complexes decompose cleanly within the range of 76 to 251$\sp\circ$C with simultaneous generation of the volatile decomposition products, water, carbon dioxide and acetonitrile. Based on these experimental data, a mechanism for the decomposition of the metal complexes of the $\alpha$-oximino carboxylate ligands has been proposed, which includes the formation of a six-membered cyclic transition state. It has been established that improvement of the solubility characteristics of these PAO complexes as well as lowering of their decomposition temperatures can be achieved by variation of the substituents R and R$\sp\prime$ in the molecule of the $\alpha$-oximino carboxylate ligand RC(=NR$\sp\prime$)COOH. One of these ligands t-BuC(=NOH)COOH was identified as exceptionally promising in terms of both solubility and low decomposition temperatures Precise stoichiometry precursors for ferrite and perovskite materials have been designed, which fix the metal to metal ratios by crystallization processes. High purity mixed metal oxalates of the type MTiO(Ox)2 $\cdot$ xH$\sb2$O (M = Sr, Ba) have been prepared in a simple procedure which includes ion exchange of the starting material K2TiO(Ox)2 $\cdot$ 2H$\sb2$O to the corresponding proteo-derivative H2TiO(Ox)2 $\cdot$ xH$\sb2$O and its subsequent reaction with Ba$\sp{2+}$ and Sr$\sp{2+}$ containing salts. These relatively insoluble mixed metal oxalates have been converted by reflux in an excess of methoxyacetic acid to methoxyacetates, having significantly higher solubility in water and organic solvents Precise stoichiometry mixed ferrite precursors have been prepared in a similar procedure, in which the readily available starting material NaFeEDTA has been ion exchanged to the corresponding proteo-derivative and subsequently reacted with water soluble salts of divalent metals such as Ni, Mg, Co and Cu. This procedure yields the mixed metal ferrite precursors as crystalline compounds in excellent purity without any sodium contamination The MOD approach has also been utilized for encapsulation of radionuclides into Synroc mineral phases. Three different MOD procedures have been designed which use different carboxylate ligands and encapsulation procedures. The first two approaches utilize NaPAO/disodium fumarate and sodium polyacrylate to precipitate the radionuclides and the Synroc forming elements from aqueous solution. Firing of this precipitate to ceramics results in the encapsulation of all but the alkali metal radionuclides into Synroc mineral phases. The third procedure involves conversion of the nitrate salts (in which form the radionuclides in radwaste are usually present) to acetates by reduction of the nitrate ions with excess acetaldehyde in acetic acid. Removal of the acetic acid solvent furnishes an amorphous acetate precursor, the pyrolysis of which at 800$\sp\circ$C leads to encapsulation of all of the radionuclides into the Synroc matrix. (Abstract shortened by UMI.)