Microstructural and phase evolution of BaTiO3–BaO–B2O3–Bi2O3 glass composites during pressureless and spark plasma sintering

BaTiO 3 –BaO–B 2 O 3 –Bi 2 O 3 (BBB) glass composites were investigated to clarify how BBB content and sintering route control low-temperature densification, pore evolution, glass-derived phase redistribution, and crystalline phase development. Densification was assessed using 15 and 30 wt% BBB glass with pressureless sintering (PS) at 600–800 °C and spark plasma sintering (SPS) at 500–800 °C. Monolithic BaTiO 3 exhibited limited densification at 800 °C, suggesting that solid-state diffusion was inadequate for consolidation. The addition of BBB facilitated the formation of a transient liquid phase, enhancing glass softening and pore filling. Densification improved with temperature and BBB content under PS, although residual porosity remained, particularly for 15 wt% BBB. SPS significantly modified microstructural evolution, achieving a relative density of 98.28% for 30 wt% BBB at 800 °C due to SPS-assisted liquid redistribution. SEM analysis revealed a shift from interconnected pores to isolated pinholes, with Bi-rich BBB regions redistributing in the BaTiO 3 framework. Tetragonal BaTiO 3 was the primary crystalline phase, while 30 wt% BBB, after SPS, formed secondary borate phases. These results establish a processing–microstructure–phase relationship for BaTiO 3 –BBB glass composites and show that BBB-assisted SPS enables low-temperature densification.