Effects of spark-plasma sintering on the piezoelectric properties of high-density (1 - X)(Na_0.5K_0.5) NbO₃- xLiTaO₃ ceramics

High-density (1-x)(Na_0.5K_0.5)NbO₃- xLiTaO₃ [(1-x)NKN-xLT] ceramics with x = 0 - 0.2 were successfully fabricated by spark-plasma sintering (SPS). The effect of SPS on the microstructure and the piezoelectric properties of the fabricated NKN-LT ceramics are discussed and compared with those of the same NKN-LT ceramics prepared by conventional mixed-oxide ceramic processing. The morphotropic phase boundary (MPB) between the orthorhombic and tetragonal phases for the NKN-LT series was found to be in the vicinity of x = 0.06 - 0.08. The MPB was shifted to the slightly LT-rich side of that previously reported by excess addition of 2.5 mol% K₂CO₃. The dielectric, ferroelectric and piezoelectric properties of the NKN-LT ceramics fabricated by using the SPS process were excellent compared to those for the conventionally prepared sample. The values of ε₃₃^T/ε₀, k_p and P_r were 1.9, 1.4 and 2.3 times larger than the values for the conventionally prepared sample, respectively. These improvements in the ferroelectric and piezoelectric properties of the present ceramics are shown to originate from both the slight excess addition of K₂CO₃ and the particle orientation imparted by SPS.