Fabrication, phase formation and microstructure of Ni4Nb2O9 ceramics by   using two-stage sintering technique
  The potential utilization of two-stage sintering for the production of highly dense and pure nickel diniobate (Ni4Nb2O9) ceramics with low firing temperature was demonstrated. Effects of designed sintering conditions on phase formation, densification and microstructure of the ceramics were characterized by X-ray diffraction (XRD), Archimedes method and scanning electron microscopy (SEM), respectively. It has been found that minor phase of columbite NiNb2O6 tended to form together with the desired Ni4Nb2O9 phase, depending on sintering conditions. The optimization of sintering conditions could lead to a single-phase Ni4Nb2O9 ceramics with orthorhombic structure. The ceramics doubly sintered at 950/1250 {\deg}C for 4 h exhibited maximum density value of 92%. Microstructures with denser angular grain-packing were generally found in both sets of the sintered Ni4Nb2O9 ceramics. However, the grains were irregular in shape when the samples are sintered at 1050/1250 {\deg}C. Two-stage sintering was also found to enhance ferroelectric behavior of Ni4Nb2O9 ceramic. 
Orawan Khamman, Jiraporn Jainumpone, Anucha Watcharapasorn, Supon   Ananta
cond-mat.mtrl-sci