Preparation and characterization of (Ba,Cs)(M,Ti)8O16 (M = Al3+, Fe3+, Ga3+, Cr3+, Sc3+, Mg2+) hollandite ceramics developed for radioactive cesium immobilization

Among the different matrices proposed for selective and durable immobilization of radioactive cesium, (Ba x,Cs y)(M,Ti)8O16 hollandite ceramics, with x + y < 2 and M = divalent or trivalent cation appeared as the best candidates. In this study, hollandite ceramics were prepared using oxide route from oxide, carbonate and nitrate powders with and without Cs for different cations M (Al3+, Cr3+, Ga3+, Fe3+, Mg2+, Sc3+) of increasing size, in order to evaluate the effect of composition on ceramics microstructure and structure and on cesium incorporation. To reduce the risks of Cs vaporization during synthesis, calcined powders were sintered in air at moderate temperature (1200 oC). This oxide route appeared as an alternative to the alkoxide route generally proposed to prepare hollandite waste form. For y = 0, single phase Ba x(M,Ti)8O16 was obtained only for M3+ = Al3+, Cr3+ and Fe3+. For y ≠ 0 and Fe3+, all cesium was incorporated in hollandite and ceramic was well densified. For Cr3+ and Ga3+, only 46% and 63%, respectively, of Cs were retained in hollandite phase. For these samples, a high fraction of Cs was either evaporated and/or concentrated in a Cs-rich parasitic phase. Mixed hollandite samples with M3+ = Ga3+ + Al3+ and M3+ = Fe3+ + Al3+ were also synthesized and the best results regarding Cs immobilization and ceramic density were obtained with iron + aluminum but the sample porosity was higher than that of the sample containing only iron. All results were discussed by considering cations size and refractory character of oxides and hollandite ceramics