Effects of Second Phase and Defect on Electrical Properties in Bi_0.5Na_0.5-xK_xTiO₃ Lead-Free Piezoelectric Ceramics

Abstract

Second phase and defect formation mechanism of (Bi0.5(Na1-xKx)0.5)TiO3 (BNKT100x) ceramics were investigated using electron microscopy, x-ray photoelectron spectroscope (XPS) and electrical properties measurements. Experimental results indicated that second phase formation induces Bi-rich regions and compositional inhomogeneity within matrix due to thermodynamic stability of potassium titanate. Ti valence transition for BNKT ceramics sintered in air might be ascribed to formation of the secondary phase, rather than simply attributed to volatilization of bismuth. Li substitution at A-site in BNKT ceramics suppresses formation of the second phase and Ti valence transition. Appropriate Li doped BNKT ceramics suppress oxygen vacancies and titanium valence transition, and therefore decrease the leakage current.

Low-temperature synthesis of bismuth titanate by modified citrate amorphous method

“…The bismuth oxide melting temperature is 824 °C, and since we do not observe this transition with the DSC analysis, either during heating or cooling, we can conclude that there is no significant formation of Bi 2 O 3 . Bismuth volatilisation is a critical issue J o u r n a l P r e -p r o o f when it is in the oxide form [34], due to its low melting temperature, and consequently, high vapour pressure. However, when the BiT is formed, the bismuth is more stable [20,34] due to its higher melting temperature (1195 °C) [23].…”

“…Bismuth volatilisation is a critical issue J o u r n a l P r e -p r o o f when it is in the oxide form [34], due to its low melting temperature, and consequently, high vapour pressure. However, when the BiT is formed, the bismuth is more stable [20,34] due to its higher melting temperature (1195 °C) [23]. We can thus presume that Bi loss does not occur as the BiT forms directly, rather than through intermediate oxide phases.…”

“…Both samples display equiaxial grains with no formations that could resemble secondary phases, in agreement with the observations from the XRD results. A calcination at 700 °C is significantly lower than the typical temperature used to obtain single-phase BiT ceramics [11,12,[38][39][40][41][42]13,14,16,[18][19][20]22,34]. Lowering the temperature of thermal J o u r n a l P r e -p r o o f treatments for BiT is important to mitigate bismuth volatilisation, and to also enable the abatement of the energetic impact in the material´s synthesis.…”

“…Yet, the distinct conduction mechanisms for the plate boundary and crystalline grain enforces the single-phase characteristic. This because the segregation of second phases are known to induce higher electronic conductivity at grain boundaries [20,34]. The dielectric constant and loss factor can be directly calculated from impedance measurements using the equations ( 1) and (2):…”