High‐Temperature Instability of Li‐ and Ta‐Modified (K,Na)NbO₃ Piezoceramics

Abstract

This paper addresses the high‐temperature instability of Li‐ and Ta‐modified (K,Na)NbO3 piezoceramics. The grains with abnormal size evolve out of the fine matrix grains during high‐temperature annealing. They are found to be precipitates with a tetragonal tungsten bronze structure, which result from the volatilization and segregation of the alkali metal elements. With the growth of the abnormal grains the composition of the perovskite matrix phase also changes remarkably, as has been suggested by EDX analysis (for Na) and electric measurements (for Li). These variations lead to a large increase in the tetragonal/orthorhombic phase transition temperature and appreciable variations in the dielectric, ferroelectric, and piezoelectric properties of the ceramic samples. Control of the volatilization of the alkali metal elements can efficiently depress the abnormal grain growth and the compositional segregation.

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

“…(3). Both the shape and the segregation of the secondary phase we observe in sintered KNN (Fig.11), similar as reported by Wang et al, 55 can be rationalized by the formation of a liquid phase. This liquid phase is formed when the solidus line is crossed during sintering at high temperatures.…”

“…The K/Na ratio in the liquid phase is also different from the nominal stoichiometry due the higher melting point of NaNbO3, and a inhomogeneous distribution of K and Na in the sintered materials is expected as previously observed by several groups. 11,55,57 If large amounts of liquid phase are formed, more rearrangement can occur, and the secondary phase segregates into clusters during normal cooling rates (Fig. 11(c) and (d)).…”

Influence of sintering temperature on electrical properties of (K0.4425Na0.52Li0.0375)(Nb0.8825Sb0.07Ta0.0475)O3 ceramics without phase transition induced by sintering temperature

“…The secondary phase could be assigned to the tetragonal tungsten-bronze (TTB) type structure phase, which does not disappear as the composition changes. The occurrence of the TTB secondary phase is attributed to the volatilization and segregation of the alkali elements during the sintering process for Li/Ta-modified KNN material [23], which induces B-site ion excess that is accommodated through TTB phase formation. With the increase in temperature, the (002) and (200) peaks shift toward lower angles, which is attributed to the easy volatilization of sodium and potassium during high-temperature sintering.…”

Dielectric, ferroelectrics properties and impedance spectroscopy analysis of the [(Na0.535K0.480)0.966Li0.058](Nb0.90Ta0.10)O3-based lead-free ceramics

“…[14][15][16][17][18] For all of them, a general trend can be evidenced: (i) the evaporation of (at least) one of the alkali constituents of the MPB involves an evolution of the chemical composition and the formation of secondary phases resulting in poor properties and (ii) this mechanism is associated with abnormal gain growth, these two phenomena being probably correlated. 19,20 However, the control of the alkali content in such samples is also a key point in order to obtain suitable properties.…”