Sustainable Seashell Waste-Derived Calcium Niobate: Structural, Morphological, and Thermophysical Properties

Calcium niobate ceramics were synthesized by the solid-state reaction of seashell-derived CaCO3 and Nb2O5 at a molar ratio of 4:1 over the temperature range of 800–1200 °C for 8 h. This work provides a correlated structural, morphological, and thermophysical characterization of the resulting calcium niobate phases. X-ray diffraction coupled with Rietveld refinement revealed a temperature-dependent phase transformation from a CaNb2O6-rich composition at lower synthesis temperatures to a Ca4Nb2O9-rich material at higher temperatures. At 1200 °C, Ca4Nb2O9 was obtained as the predominant phase (96.16 ± 1.81%), with only a minor Ca2Nb2O7 contribution. This phase evolution was accompanied by an increase in the Ca4Nb2O9 crystallite size from 39.49 ± 0.19 nm at 1000 °C to 50.38 ± 0.24 nm at 1200 °C. SEM analysis showed a concurrent morphological transformation from agglomerated and irregular particles to well-defined elongated and platelet-like particles, indicating enhanced crystallization and grain growth. Thermophysical properties were determined using open photoacoustic cell and laser-flash techniques. Thermal diffusivity ranged from 1.86 to 3.12 × 10−7 m2 s−1, heat capacity from 0.79 to 1.30 × 104 JK−1 m−3, thermal conductivity from 0.15 to 0.41 Wm−1 K−1, and thermal effusivity from 0.36 to 0.73 kWs1/2 m−2 K−1. The combined results establish a clear synthesis–structure–property relationship and identify the Ca4Nb2O9-rich ceramic obtained at 1200 °C as a promising low-thermal-conductivity material. The use of seashell waste additionally provides a renewable calcium precursor for the synthesis.