Hwi Yeol Park

Microstructure and piezoelectric properties of 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 ceramics

For 0.95(Na0.5K0.5)NbO3–0.05BaTiO3 (0.95NKN-0.05BT) ceramics sintered at 1040–1075°C, abnormal grain growth occurred but the grain size decreased when the sintering temperature exceeded 1075°C. The dielectric constant (ϵ3T∕ϵ3), electromechanical coupling factor (kp), and piezoelectric constant (d33) were considerably increased with increasing relative density and grain size. Evaporation of Na2O deteriorated the piezoelectric properties by decreasing the resistivity. To minimize Na2O evaporation, specimens were muffled with 0.95NKN-0.05BT powders during the sintering. Improved piezoelectric properties of d33=225pC∕N, kp=36%, and ϵ3T∕ϵ3=1058 were obtained for specimen sintered at 1060°C for 2h with muffling.

Microstructure and piezoelectric properties of lead-free (1−x)(Na0.5K0.5)NbO3-xCaTiO3 ceramics

The crystal structure of (1−x)(Na0.5K0.5)NbO3-xCaTiO3 ceramics began to change from orthorhombic to tetragonal at x≥0.03, then became a morphotropic phase boundary in which both the orthogonal and tetragonal phases coexisted at 0.03<x<0.07, and was converted to tetragonal structure at x≥0.07. The Curie temperature decreased with increasing x and the specimen became a relaxor at x≥0.03. A dense microstructure with large grains was developed at 0.04<x<0.07 due to liquid phase sintering. The relative density, grain size, and liquid phase considerably influenced the piezoelectric properties of the specimens. High piezoelectric properties of d33=241, kp=0.41, and e3T/eo=1316 were obtained from the 0.95NKN-0.05CT ceramics sintered at 1050 °C for 2 h, indicating that the 0.95NKN-0.05 ceramic is a good candidate material for lead-free piezoelectric ceramics.

Microstructure and piezoelectric properties of the CuO-added (Na0.5K0.5)(Nb0.97Sb0.03)O3 lead-free piezoelectric ceramics

The presence of the Sb5+ ion increased the piezoelectric properties of the (Na0.5K0.5)NbO3 (NKN) ceramics by enhancing the poling state as identified by the increased phase angle (θ). The increased θ value was explained by the increased ferroelectricity due to the large electronegativity of the Sb5+ ion and the increased off centering of the cation due to the small ionic size of the Sb5+ ion. The (Na0.5K0.5)(Nb0.97Sb0.03)O3[NK(N0.97S0.03)] ceramics containing CuO were well sintered even at 960 °C. A small amount of the Cu2+ ions was incorporated into the matrix of the NK(N0.97S0.03) ceramic and behaved as a hardener. The kp, Qm, d33, and e3T/e0 values of the CuO-added NK(N0.97S0.03) ceramics were much higher than those of the CuO-added NKN ceramics due to the Sb5+ ion’s enhancement of the piezoelectric properties by increasing the poling state of the specimens. The high piezoelectric properties of kp=0.41, Qm=1333, d33=111 pC/N, and e3T/e0=324 were obtained from the 2.0 mol % CuO-added NK(N0.97S0.03) cera...

Microstructure and piezoelectric properties of (Na 0.5 K 0.5 )NbO 3 lead-free piezoelectric ceramics with V 2 O 5 addition

Various amounts of Nb₂O₅ in the (Na_0.5K_0.5) NbO₃ (NKN) ceramic were replaced by V₂O₅ to decrease its sintering temperature to below 950°C. A small V₂O₅ content resulted in a dense microstructure with an increased grain size for the specimen sintered at 900°C due to the presence of a liquid phase. When V₂O₅ was added to the NKN ceramics, their orthorhombic-to-tetragonal transition temperature increased from 178°C to around 200°C. However, their Curie temperature decreased from 402°C to around 330°C. The k_p, _ε3 ^T/_ε0, and Q_m values increased with V₂O₅ addition, probably due to the increased density and poling state, which was identified by the phase angle. The specimen with x = 0.05, sintered at 900°C for 8 h, exhibited the following piezoelectric properties: k_p = 0.32, _ε3 ^T/_ε0 = 245, d₃₃ = 120 (pC/N), and Q_m = 232.

High Power (Na0.5K0.5)NbO3-Based Lead-Free Piezoelectric Transformer

Though there is a lot of research related with lead-free piezoelectric materials, high power characteristics of the candidate lead-free materials have not been looked into in the literature under equilibrium conditions. This paper reports high power characteristics of a sodium potassium niobate (NKN) based ceramic under equilibrium conditions, and its application as a candidate material for piezoelectric transformers compared with hard lead–zirconate–titanate (PZT). (Na0.5K0.5)(Nb0.97Sb0.03)O3 was prepared with 1.5 mol % CuO addition. Disk-shaped samples were sintered with conventional sintering methods. High power characteristics were investigated with our high power piezoelectric characterization system (HiPoCS). Distinctly different from PZTs, the NKN ceramics did not exhibit a decrease in mechanical quality factor (Qm) with increasing vibration velocity (up to 0.4 m/s). Ring-dot piezoelectric transformers made from a disk shaped NKN ceramic revealed power density as high as 25 W/cm3, almost comparable to that obtained for the conventional PZT transformers. In conclusion, NKN ceramics possess good high power characteristics, which are satisfactorily applicable to piezoelectric transformers.

Low Temperature Sintering of the Alkali-Niobate Ceramics

(Na1-xKx)NbO3 (NKN)-based lead-free ceramics have been extensively investigated for the replacement of Pb(Zr,Ti)O3 (PZT)-based ceramics because of their high piezoelectric properties. However, the high sintering temperatures of NKN-based ceramics caused Na2O evaporation during sintering. Moreover, the Na2O evaporation reduced the polling efficiency and changed the microstructure and eventually degraded the piezoelectric properties of the NKN-based ceramics. Therefore, because the Na2O evaporation occurred at temperatures above 1,000°C, it is necessary to find a method to reduce the sintering temperature of the NKN-based ceramics to below 1,000°C in order to obtain reliable piezoelectric properties. In general, two methods have been used to reduce the sintering temperature of NKN-based ceramics (1) addition of sintering aids and (2) spark plasma sintering (SPS). In this chapter, detailed discussions about these two methods will be presented.