Nickel ferrite (NiFe₂O₄) ceramics were prepared by combustion in air and sintered using different two-step sintering processes, classified as condition I and condition II. Both conditions employed a first-step temperature (T₁) of 1200 °C for 1 minute. In the second step, the temperature (T₂) was reduced to 1150 °C, with no holding time for condition I and a 240-minute hold for condition II. XRD analysis confirmed the formation of a single-phase NiFe₂O₄ with no phase transformations during sintering. NiFe₂O₄ ceramic from condition II exhibited higher density (3.71 ± 0.04 g/cm³) and flexural strength (17.32 ± 0.52 MPa) than those from condition I (3.43 ± 0.06 g/cm³ and 10.19 ± 0.83 MPa). Despite an 8% increase in density and an 11.68% reduction in porosity, the strength improved by ~52%, highlighting the role of extended holding time. However, the higher porosity and specific surface area (0.76 m²/g) of NiFe₂O₄ ceramic from condition I enhanced gas-sensing properties. The NiFe₂O₄ ceramics responded consistently to variations in LPG flow rate, indicating p-type semiconductor behavior. Ceramic from condition I showed stronger and faster responses to LPG at room temperature (S = 0.76, t₉₀ = 20.47 s, t_rec = 1.29 min) than ceramic sintered from condition II (S = 0.46). The sensitivity of the ceramic from condition I at various LPG concentrations was 7.31 × 10^-6 ppm^-1.

Keywords: Nickel ferrite, Combustion synthesis, Two-step sintering, P-type semiconductor, Gas sensor.