Spark Plasma Sintering of Tungsten Bronze Sr2−xCaxNaNb5O15 (x= 0.1) Piezoelectric Ceramics: I, Processing and Microstructure

Tungsten bronze Sr2−xCaxNaNb5O15 (SCNN, x= 0.1) piezoelectric ceramics were prepared by a novel super‐fast sintering technique, i.e., spark plasma sintering (SPS). Sintering was conducted at temperatures ranging from 1000° to 1200°C under vacuum under 35 MPa. The heating rate varied from 30° to 300°C/min and the soaking time was in the range of 5 to 60 min. The microstructure and the phase identification were analyzed via scanning electron microscopy (SEM) and X‐ray diffraction (XRD). The density of the sintered ceramics increased with an increase in sintering temperature as well as in heating rate. Materials with a near‐theoretical density could be attained by sintering at 1200°C for 5 min with a heating rate of 300°C/min, exhibiting a homogeneous and fine‐grained microstructure (grain size about 2.7 μm) and a uniform translucency. A tetragonal tungsten bronze phase was identified in ceramics sintered at temperatures above 1200°C, whereas the tungsten bronze phase transformed to a SrNb2O6‐type phase in the materials sintered at temperatures below 1200°C. XRD patterns demonstrated a preferred crystallographic orientation of the SCNN grains with their c‐axes perpendicular to the pressing direction.