Flash sintering and dielectric properties of K0.5Na0.5NbO3

Abstract

K0.5Na0.5NbO3 (KNN) of 4-mu m average particle size was flash sintered in parallel plate capacitor configuration (PPCC) under 100 V/mm (0.8 mA/mm(2) cut-off) with 10 degrees C/min heating rate in air. Precipitous densification occurred from T-f = 662-670 degrees C (T-f-furnace temperature) in 60 s with 55 mW/mm(3) peak power absorption at T-f = 667 degrees C, resulting in 98% dense ceramic of < 5-mu m grains size. No grain growth was observed. A cubic grain morphology was inherited from the starting KNN powder. No sign of liquid phase formation on grain boundaries (GBs) was detected. A 147 degrees C rise above T-f = 667 degrees C due to Joule heating is predicted. At 25 degrees C and 1 kHz, relative permittivity (epsilon(r)) and dielectric loss (tan(delta)) are 4096 and 2.9%, respectively. From 0.1 to 100 kHz, epsilon(r) and tan(delta) undergo relaxation, reaching epsilon(r)( )= 577 and tan(delta) = 0.2% at 1000 kHz. Variation of epsilon(r) with temperature revealed Amm2 -> P4mm and P4mm -> Pm3m transitions at similar to 204 degrees C and 409 degrees C, respectively. As per Curie-Weiss analysis, Curie temperature and Curie-Weiss constant are theta = 375 degrees C and C = 3.1 x 10(-5)degrees C for the P4mm -> Pm3m transition, respectively. The transition is first order as T-tr > theta with a diffusiveness exponent gamma = 1.04, indicating normal ferroelectric behavior. The low-frequency dielectric relaxation is attributed to the space charge in the vicinity of GBs, which is a remnant of flash sintering. Joule heating cannot alone account for the densification in 60 s, which we substantiate within the framework of phase equilibrium and diffusion kinetics. We conjecture that oxygen vacancies start ionizing at T-f = 662 degrees C, couple with the applied electric field via the Lorentz force, and increase their electrochemical potential, causing ultrafast densification. The effects of sample shape, size, and PPCC contributing to high sintered density are also discussed.