Grain-growth-controlled densification during reactive sintering of refractory Cr-substituted hollandite waste forms

Cr-substituted hollandite ceramics are promising crystalline waste forms for immobilizing radioactive cesium, yet achieving high densification is challenging within temperature windows that mitigate Cs loss. Here, we develop a practical, sintering-aid-assisted reactive sintering route to densify Ba 0.667 Cs 0.667 Cr 2 Ti 6 O 16 at relatively low temperature and to identify the key factors controlling densification. Nine commonly used sintering aids were screened individually, and CuO was found to be uniquely effective. Guided by this screening, we correlate thermal behavior, crystallographic evolution, and microstructural development to clarify the role of CuO. Thermal analysis shows that CuO shifts BaCO 3 -related mass-loss/endothermic events to a lower temperature, indicating earlier BaCO 3 decarbonation and CO 2 release. Rietveld refinement of the X-ray diffraction data further reveals systematic crystallographic evolution, consistent with limited Cu substitution and associated defect-compensation effects that may enhance mass transport. These effects coincide with a marked change in grain-growth behavior, where anisotropic growth is suppressed, and grains become more equiaxed, improving packing efficiency and enabling grain-growth-controlled densification. The resulting increase in relative density is accompanied by significantly reduced Cs release, indicating improved chemical durability. Overall, this work establishes CuO-enabled reactive sintering as a practical low-temperature processing strategy for refractory Cr-hollandite waste forms and highlights grain-growth control as a key microstructural mechanism linking processing, densification, and durability.