Furnace-Dependent Phase Evolution of Ball-Milled BaTiO₃ Powders under Nominally Identical Calcination Conditions

In ceramic synthesis, the thermal history actually experienced by a powder may differ substantially from the programmed furnace schedule, with direct consequences for phase formation and reproducibility. This study shows that identical nominal calcination schedules can produce different BaTiO₃ reaction outcomes when applied in different furnaces. A single high-energy ball-milled precursor batch was divided into three portions and calcined at 1200 °C for 8 h using the same programmed heating and cooling rates. XRD and Rietveld refinement identified tetragonal BaTiO₃ (P4mm) as the dominant phase in all samples. However, the powder treated in Furnace 1 showed weak Ba-rich secondary phases, smaller crystallites, higher microstrain, and poorer refinement statistics, whereas samples from Furnaces 2 and 3 were phase-pure within the XRD detection limit and exhibited larger, less strained crystallites. Direct thermocouple measurements revealed the origin of these differences. Furnaces 2 and 3 closely maintained the programmed 1200 °C set point, while the sample position in Furnace 1 reached only approximately 1028 °C. SEM revealed a finer, more compact microstructure for Furnace 1 and coarser, more heterogeneous grains for Furnaces 2 and 3. EDS mapping confirmed homogeneous Ba, Ti, and O distributions in all powders, although quantitative EDS indicated a higher Ba/Ti ratio in the Furnace 1 sample. XPS showed comparable BaTiO₃-type surface chemical environments and modest surface carbonation for all samples. These results establish furnace identity and actual temperature distribution as critical processing variables that should be monitored and explicitly reported in reproducibility-sensitive BaTiO₃ and ceramic-synthesis studies.