INFLUENCES OF LITHIUM/CAESIUM ADDITIONAL/SUBSTITUTIONAL DOPING ON PHASE FORMATION, MICROSTRUCTURE AND ELECTRICAL PROPERTIES OF (K0.5Na0.5)(Nb0.7Ta0.3)O3 CERAMICS

Abstract

The effect of the firing conditions on the phase formation, microstructure, and electrical properties of (K0.5Na0.5)(Nb0.7Ta0.3)O3 (KNNT) ceramics synthesized by the solid-state combustion technique using glycine as fuel has been investigated. All samples were calcined at 600 to 800°C for 2 h and sintered at 1150 to 1190°C for 2 to 5 h. Pure KNNT powders were produced after calcination at 600°C for 2 h. The average particle size increased when the calcination temperature was increased. The KNNT powder calcined at 600°C for 2 h showed rather square morphology with average particle size of ~160 nm. The x-ray diffraction (XRD) analysis results for the ceramics revealed the presence of orthorhombic (O) and tetragonal (T) phases in all samples. When sintering at 1150°C for 4 h, the O:T ratio was 50:50, as verified by the Rietveld refinement technique. The average grain size, density values, and dielectric properties tended to increase when the dwell time was increased from 2 to 4 h, but then degraded. The KNNT ceramic produced at the optimum firing condition (1150°C for 4 h) showed good crystalline morphology, the highest density (ρ = 5.28 g/cm3), the highest dielectric constant (εC = 5002), and good ferroelectric behavior (Pr = 18.50 µC/cm2 and Ec = 9.04 kV/cm).

Then, the effects of Cs+ substitution and direct doping in (K0.5Na0.5)(Nb0.7Ta0.3)O3 (KNNT) ceramics, on the crystal structure, microstructure and electrical properties were investigated. Both the KNNT with Cs+ substitution (K0.5-xCsxNNT) and addition (KNNT-xCs) (x=0, 0.01, 0.02, 0.03 and 0.04 mol.%) were synthesized by the solid-state combustion technique using glycine as fuel. All samples were sintered at 1130-1150°C for 4 h. The x-ray diffraction (XRD) analysis for the ceramics revealed the presence of O and T phases in all samples. Increasing both the Cs+ substitution and doping amounts, enhanced orthorhombicity, as verified by the Rietveld refinement technique. It was found that Cs+ doping, either substitutional or additional, strongly decreased their density, dielectric and ferroelectric properties. The undoped KNNT ceramic exhibited well-saturated P-E hysteresis loop. With Cs+ doping, the samples became unsaturated and a leakage current was produced. The KNNT-xCs ceramics demonstrated higher density and dielectric properties than the K0.5-xCsxNNT ceramics.

After that, lead-free (K0.5Na0.5)(Nb0.7Ta0.3)O3 (KNNT) ceramics with Li+ substitution (KN0.5-xLixNT) and direct (KNNT-xLi) doping at x=0, 0.01, 0.02, 0.03 and 0.04 mol.% were synthesized by the solid-state combustion route. The phase, microstructure, dielectric and ferroelectric properties of the ceramics were examined. The XRD pattern of the ceramics revealed orthorhombic and tetragonal phases were in all specimens. The Rietveld refinement procedure showed that increasing either the Li+ substitution or doping levels enhanced the amount of the tetragonal phase. It was found that Li+ doping, either substitutional or additional, enhanced the Curie temperature (TC) by increasing the tetragonal distortion, while the dielectric constant (εC) decreased. The good remanent P-E loops of the KN0.5-xLixNT ceramics was found with x=0.01 (Pr~10.89 µC/cm2 and EC~13.09 kV/cm), while for KNNT-xLi ceramics, it was obtained with x=0.02 (Pr~15.65 µC/cm2 and EC~11.46 kV/cm), which were confirmed by remanent P-E hysteresis measurements.

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