Effect of Calcination Conditions and Excess Alkali Carbonate on the Phase Formation and Particle Morphology of Na_0.5K_0.5NbO₃ Powders

Abstract

Sodium‐potassium niobate [Na0.5K0.5NbO3] powders were prepared following the conventional mixed oxide method. An orthorhombic XRD pattern, consistent with single‐phase Na0.5K0.5NbO3, was obtained after calcination at 900°C for 6 h. Introducing 5 mol% excess Na2CO3 and K2CO3 into the starting mixture allowed milder calcination conditions to be used, for example 800°C for 2 h. Primary particles in 5 mol% excess samples were cuboid, with maximum sizes of ∼2.5 μm. Equiaxed 0.3–0.4‐μm particles were formed for non‐excess powders, and also for powders prepared with 1 and 3 mol% excess alkali carbonates. The results suggest liquid formation during calcination of the excess 5‐mol% starting powders.

Sintering of sub-micron K 0.5 Na 0.5 NbO 3 powders fabricated by spray pyrolysis

“…12) as expected from shift of the equilibria given by Eqs. (3) to (5). Higher alkali loss is therefore expected when sintered under reducing or inert conditions.…”

“…For 2 and 1 % alkali excess, the amount of liquid is less than the critical amount of liquid for particle rearrangement. The formation of a low temperature liquid phase has also been discussed for alkali-rich KNN prepared by a solid state route 5,15 and KNbO3 42 powders. In these studies the liquid phase were formed at somewhat higher temperatures, but this may reflect variation in the K/Na ratio in the liquid phase.…”

“…5,15 Synthesizing KNN using conventional solid state reaction techniques often requires excessive calcination temperature and duration, making alkali evaporation possible during calcination as well as sintering. Reducing the particle size of the precursors allows calcination at lower temperatures, but requires high-energy milling (e.g.…”

“…2 The preparation of dense and stoichiometric KNN ceramics 3,4 has however been a persistent challenge due to the hygroscopic nature of alkali carbonate precursors, 5 volatility of alkali oxides [6][7][8] and narrow sintering interval close to the solidus temperature. [9][10][11] As denser ceramics typically yield improved piezoelectric coefficients, many have endeavored to improve performance through optimizing synthesis and sintering procedures.…”

Strain Mechanisms in Lead-Free Ferroelectrics for Actuators

“…279 A controlled atmosphere and humidity of the raw materials prior to calcination, during calcination, and also during sintering should be considered for obtaining the desired stoichiometry. [286][287][288][289] Additional problems arise also during sintering due to an extremely narrow sintering window in terms of temperature and dwelling time [290][291][292] , early activation of surface diffusion that reduces the driving force for densification 273 , and high volatilization of K + above 1000 °C. 154,288 The processing of KNN-based materials remains an open research area in which new techniques such as pressure assisted sintering, spark plasma, and microwave sintering, and others are being investigated for developing reproducible and dense materials with high functional properties.…”