SPS processing of bismuth-layer structured ferroelectric ceramics yielding highly textured microstructures

The spark plasma sintering (SPS) behaviour of nano-sized Bi4Ti3O12 (BIT) and micron-sized CaBi2Nb2O9 (CBNO) powders is described. The densification process of both powders is very rapid, i.e. the densification occurs within a very narrow time interval (2–3 min using a heating rate of 100 °C min−1 and a pressure of 50 MPa). The BIT powder exhibits a lower densification onset temperature (∼650 °C) and higher maximum shrinkage rate (8.9 × 10−3 s−1 at 780 °C) than that of the CBNO powder (∼825 °C and 4.5 × 10−3 s−1 at 950 °C). Isothermal compaction studies revealed that fully dense nano-sized BIT compacts could be obtained within the temperature region 750 °C < Tiso < 850 °C while for Tiso > 850 °C compacts containing elongated platelet grains are formed. A new preparation route to produce highly textured compacts is described in detail. Appropriate pre-forms are prepared by spark plasma sintering (SPS) and these fully dense compacts are subject to superplastic deformation in the SPS unit to achieve a total compressive strain of ∼60%. This strain was achieved within a period of 1.5 min and with a maximum strain rates of 1.1 × 10−2 s−1 achieved at ∼840 °C and 1.3 × 10−2 s−1 at 1020 °C for the BIT and CBNO compacts, respectively. The X-ray studies showed that the Lotgering orientation factors of grains in the deformed BIT and CBNO compacts are 99% and 70%. The formation of highly textured compacts is suggested to be governed by a superplastic deformation-induced directional dynamic ripening mechanism.