Solvothermal Synthesis and Formation Mechanism of Potassium Sodium Niobate Mesocrystals Under Low Alkaline Conditions

Abstract

Pure-phase (K, Na)NbO3 (KNN) powders with orthorhombic symmetry were successfully synthesized by solvothermal method using isopropanol as solvent, without the addition of water. The as-prepared powders were characterized by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometry to show the variation of phase, morphology, size distribution and chemical composition under different synthetic conditions, such as fill factors (FF) of the solvothermal system and alkalinity of the starting solution. Compared with the traditional hydrothermal method and the so-called solvothermal method (water aided in fact), small grains with well crystallinity were obtained using 100% isopropanol as reaction medium. The results indicate that both fill factor and alkalinity have significant effects on the phase structure and size distribution of the as-obtained KNN powders. Pure orthorhombic perovskite-structured KNN powders with a grain size of 100 nm were synthesized at the following condition: reaction time, 16 h; reaction temperature, 240 °C; fill factor, 70%; and alkalinity, 1 M. Small grains (~100 nm) tend to form mesocrystals (~10 µm) with tetrakaidecahedron structures, and the possible formation mechanism was proposed. The solvothermal method without the addition of water is a promising alternative to synthesize pure and refined powders under mild reaction conditions.

Selective Phase Control of Dopant-Free Potassium Sodium Niobate Perovskites in Solution

“…Nevertheless, pure KNN ceramics without any dopants prepared by the high-temperature solid-state reaction have yielded an orthorhombic phase, because the phase transition is rapid and reversible. Until today, almost all KNN (including KN and NN) particles prepared by the solution-based synthesis have also shown the most thermodynamically stable orthorhombic phase even after controlling reaction conditions such as alkalinity, molar ratio between constituents, changing organic solvents, and applying microwave. − A few studies separately reported the possible formation of metastable monoclinic KNbO 3 and cubic NaNbO 3 as compositional end members in KNN by solution synthesis ( x = 0 or 1); , the simultaneous control of both phase and stoichiometry has not been achieved particularly in the case of KNN solid solutions owing to the different solubility and reactivity of Na and K niobate salts in solution …”

Improved sintering activity and piezoelectric properties of PZT ceramics from hydrothermally synthesized powders with Pb excess

“…In the alkali hydrothermal environment, there may exist in numerous A-site deficiencies in ABO 3 -type compounds induced by OH -or H 3 O ? [20,21]. In our previous work, it's evidenced that higher alkali concentration would cause more severe Pb-deficiency in A-sites [18].…”

Hydrothermal synthesis of potassium–sodium niobate powders

“…16 Uneven volatilization of such oxides usually causes the formation of secondary Na-rich phases, 17 that is, the deviations from the target stoichiometry, and the formation of non-perovskite structures. 18 Taking into consideration of the mentioned challenges, alternative routes including molten salt synthesis, [19][20][21] solgel processing, [22][23][24] co-precipitation, 25 spray pyrolysis, 26 and hydro/solvothermal reaction [27][28][29][30] have been proposed. Among those, hydrothermal synthesis is of particular interest since it provides high crystallinity particles at moderate temperatures.…”

“…Taking into consideration of the mentioned challenges, alternative routes including molten salt synthesis, 19–21 sol–gel processing, 22–24 co‐precipitation, 25 spray pyrolysis, 26 and hydro/solvothermal reaction 27–30 have been proposed. Among those, hydrothermal synthesis is of particular interest since it provides high crystallinity particles at moderate temperatures 29 .…”