Comparative study of cold assisted and conventional sintering of (1-2x) K0.5Na0.5NbO3-xBaTiO3-xBiFeO3 multiferroic ceramics

(1-2x) K0.5Na0.5NbO3-xBaTiO3-xBiFeO3 (KNN-BTO-BFO) lead-free ceramics fabricated by Conventional Sintering (ConS) and Cold Assisted Sintering (CAS) were studied using X-ray diffraction, scanning electron microscopy and their dielectric, piezoelectric and magnetic properties characterised. Orthorhombic KNN transformed to tetragonal for compositions with x ≥ 0.03 for ConS and CAS. The relative density attained for CAS (1080–1100°C) samples was consistently higher than ConS (1140–1180°C) for equivalent compositions, despite the lower sintering temperature. The higher relative density in CAS with respect to ConS samples is attributed to the higher (‘super’) green density achieved through cold sintering in comparison with dry pressing. The permittivity maxima for undoped KNN occurred at 420°C and 400°C, respectively, for CAS and ConS samples with ConS samples exhibiting a broader permittivity maximum at the orthorhombic to tetragonal transition and higher dielectric loss, particularly above 190°C. The permittivity maxima decreased in temperature and broadened as a function of × in each case. For x = 0.01, the piezoelectric coefficient was maximum (d33 = 125 pC/N) for CAS with coupling coefficient, kp = 0.3. A magnetization of 0.09 emu/gm was obtained for CAS at x = 0.05, with only 0.06 emu/gm for ConS, likely due to the lower density for the latter sintering method. These preliminary results demonstrate that KNN-based ceramics may be densified using CAS at temperatures lower than conventional whilst still retaining promising values of permittivity, piezoelectric coefficient, and magnetization.