Grain Growth of Microtubes During Sintering in Semiconducting Ba3Ce3−xBixTi5Nb5O30 (x=0.5, 1.0, 2.0, and 3.0) Ceramics

Well grown microtubes of Ba₃Ce_{3? x} Bi _x Ti₅Nb₅O₃₀ ( x =0.5, 1.0, 2.0, and 3.0) have been observed in the sintered ceramics. The microtubes have octagonal shapes at the cylindrical ends, hollow cores with diameters of 2–3 μm, lengths over a few tens of micrometers and wall thickness of 0.5–1.0 μm. In morphology, the tubes are very straight and are apparently welded to each other on the sides. The tubular growth as well as increase of wall thickness is observed to be more with greater Bi content, but when Ce is fully substituted by Bi, the tubes have grown into solid rods without hollowness. The growth spirals at the ends of tubes as seen by scanning electron microscope indicate that growth takes place in the longitudinal direction of the tubes. Observation of growth spirals on grains of a sintered ceramic is unusual. In fact, such features are usually seen in liquid phase growth and, therefore, we propose here grain growth in partially fluid conditions. The phase identification by X-ray diffraction shows that the compound has a tetragonal tungsten bronze type structure. The resistivity measurements above room temperature indicate that these compounds are semiconductors and both resistivity and activation energy increase with Bi content.