Determining the Stoichiometry of (K,Na)NbO3 Using Optimized Energy-Dispersive X-Ray Spectroscopy and Electron Energy-Loss Spectroscopy Analyses in a Transmission Electron Microscope

Abstract

This paper describes an optimized analytical procedure for determining the composition of alkaline niobate-based lead-free piezoceramics by energy-dispersive X-ray spectroscopy (EDXS) in a transmission electron microscope (TEM). To discriminate the material-specific composition from the artifacts introduced during the EDXS/TEM analyses, the effects of radiation damage and the absorption of the characteristic X-ray lines were studied in detail. The optimized, quantitative EDXS analysis was tested on sodium potassium niobate with the nominal composition K 0.5 Na 0.5 NbO 3 (KNN) by applying KNbO 3 and NaNbO 3 as standards. The results obtained indicate that KNbO 3 is more radiation sensitive than NaNbO 3 . A similar degradation was confirmed in KNN, where the loss of potassium was higher than that of sodium. In addition, the degradation of the KNN was associated with a severe oxygen loss and the reduction of niobium. To achieve highly reliable, quantitative analyses and to preserve good counting statistics, the optimum conditions for the EDXS/TEM analyses were found at an electron dose rate of 0.4 pA/nm 2 and an acquisition time of around 200 s in the specimen thickness range between 30 and 200 nm. II. Experimental ProceduresKNbO 3 was prepared by milling potassium tartrate (K 2 C 4 H 4 O 6 Á 0.5H 2 O, 499.5%, Fluka,

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

“…A problem during EDS/WDS analysis is, however, misinterpretation of secondary phase grains (with lower solubility of Na than K) as matrix grains and electron beam related decomposition. 50 Decreasing the partial pressure of oxygen during the heat treatment of KNN clearly increases the vapor pressure of alkali elements (Fig. 12) as expected from shift of the equilibria given by Eqs.…”

Giant d 33 in nonstoichiometric (K,Na)NbO 3 -based lead-free ceramics

“…Unfortunately, the d 33 of KNN is still inferior to that of PZT [1][2][3][4][5][6][7][8][9][10][11][12]. Some attempts have been conducted to further promote the electrical properties of KNN-based ceramics in addition to the construction of phase boundaries [1][2][3][4][5][6][7][8][9][10][11][12], by, for example, varying the stoichiometry [13,14], preparation method [1,15] and sintering temperature [13,16].…”

“…It is well known that dense KNN ceramics must endure a high processing temperature of $1100°C [17], and consequently a fluctuation of chemical composition cannot be avoided owing to the evaporation of the alkali elements [13,14]. However, compositional fluctuation seriously affects the electrical properties of KNN ceramics, gradually becoming a major issue in the spite of the significant breakthroughs in their synthesis and characterization [13,14].…”

“…However, compositional fluctuation seriously affects the electrical properties of KNN ceramics, gradually becoming a major issue in the spite of the significant breakthroughs in their synthesis and characterization [13,14]. The deviation from stoichiometry during synthesis may result from the strongly hygroscopic nature of the alkaline carbonates [13,14].…”

“…The deviation from stoichiometry during synthesis may result from the strongly hygroscopic nature of the alkaline carbonates [13,14]. As a result, preventing the loss of the alkali metals in KNN during calcination and sintering is a critical issue [13,14]. Various methods have been used to suppress the fluctuations in the composition of KNN-based ceramics [12,18,19].…”