Phase transitional behavior, microstructure, and electrical properties in Ta-modified [(K0.458Na0.542)0.96Li0.04] NbO3 lead-free piezoelectric ceramics

Abstract

Lead-free [(K0.458Na0.542)0.96Li0.04] (Nb1−xTax)O3 ceramics were prepared by ordinary sintering technique. The effects of Ta content on the phase transitional behavior, Raman spectrum, microstructure, and dielectric, piezoelectric, and ferroelectric properties of the ceramics were investigated. X-ray diffraction results indicate that the phase structure undergoes a transition from orthorhombic to tetragonal phase with the increase in x. The small and large peaks observed in the Raman spectra can all be attributed to the internal modes of the NbO6 octahedron, and both ν5 and ν1 modes slightly shift to lower frequency numbers by increasing x. Ta substitution for Nb leads to the disappearance of the abnormal grain growth behavior, inhibits the grain growth, and improves the density of the ceramics. Increasing x leads to the different variations of dielectric constants before and after poling and makes the ceramics more relaxorlike. The proper substitution of Ta shifts the polymorphic phase transition (at To-T) to near room temperature, and a coexistence of orthorhombic and tetragonal phases is formed, which leads to significant enhancements of the electrical properties. For the ceramics with x=0.15, the electrical properties become optimum, which are as follows: piezoelectric coefficient d33=298 pC/N, electromechanical coupling coefficient kp=0.52, dielectric constant εr=1195, dielectric loss tan δ=0.016, To-T=35 °C, Curie temperature TC=366 °C, remanent polarization Pr=28.68 μC/cm2, and coercive field Ec=7.14 kV/cm. These properties are much higher than those of previously reported (K,Na,Li)(Nb,Ta)O3 systems with K:Na=1:1.

Crystal structure and orthorhombic–tetragonal phase transition of nanoscale (Li0.06Na0.47K0.47)NbO3

“…For the reason that PPT is temperature dependent, many recent studies concern about designing different composites to turn the O-T phase transition from about 200 • C to room temperature. 4,5 Of all the NKN based ceramics, (Li 0.06 Na 0.47 K 0.47 )NbO 3 (LNKN) is one of the most intensively studied system due to the PPT structure near room temperature. The LNKN ceramics show the large piezoelectric constant d 33 over 200 pC/N, and are considered as a new generation materials for lead-free piezoelectric devices.…”

Phase coexistence and high electrical properties in (KxNa0.96−xLi0.04)(Nb0.85Ta0.15)O3 piezoelectric ceramics

“…After that, many investigations have been carried out on ͑K,Na,Li͒ ͑Nb, Ta͒O 3 ceramics. [8][9][10][11][12][13][14] However, most of them were based on an equal or nearly equal molar ratio of K/Na. This situation might be explained by that the recent researches on ͑K,Na,Li͒ ͑Nb, Ta͒O 3 ceramics have been influenced to a great extent by the results of very early investigations.…”

Origin of discrepancy between electrical and mechanical anomalies in lead-free (K,Na)NbO3 -based ceramics

“…We will discuss the three observed regions at ∼250, ∼600, and 860 cm −1 , which correspond to different vibrational modes of the NbO 6 octahedron and directly reflect the phase transitions [30][31][32]. The Raman bands consist of five vibrational modes; the intermediate wave-number modes, A 1g (ν1) (∼615 cm −1 ) and E g (ν2) (∼555 cm −1 ), are associated with O-Nb-O bond stretching modes, while low-and highwave-number modes, F 2u (ν6) (∼200 cm −1 ), F 2g (ν5) (∼260 cm −1 ), and the combination of A 1g (ν1) plus F 2g (ν5) (∼860 cm −1 ), are related to O-Nb-O bending modes [31,[33][34][35][36]. The evolution of the position of peaks F 2u (ν6), F 2g (ν5), E g (ν2), A 1g (ν1), and A 1g (ν1) + F 2g (ν5) with temperature is shown in Fig.…”