The preparations of (K0.5Na0.5)NbO3 (KNN)‐based ceramics were studied as lead‐free piezoelectric materials. The authors found that the addition of CuO greatly enhanced the sinterability of the KNN‐based ceramics. The sinterability and piezoelectric properties of these ceramics were dependent upon the A/B ratio, CuO doping, and the formation of a solid solution with KTaO3. Perovskite (K0.5Na0.5)xNbO3 (x=A/B ratio) was synthesized with A/B ratios of 1.00 and 1.05 by CuO doping, while (K0.5Na0.5)NbO3 contained K4CuNb8O23 as a second phase with A/B ratios below 0.98. Although the A site‐rich (K0.5Na0.5)NbO3 (x=1.00 and 1.05) ceramics exhibited deliquescence, the A site‐poor (K0.5Na0.5)NbO3 (x≤0.98) ceramics with K4CuNb8O23 had higher densities without deliquescence. K4CuNb8O23 also improved Qm, which reached 1400 for the (K0.5Na0.5)1.0NbO3 doped with 0.5 mol% K4CuNb8O23. The formation of a solid solution with KTaO3 raised the melting point of the system and also improved its sinterability. The kp and Qm of (K0.5Na0.5)0.97(Nb0.95Ta0.05)O3 with CuO were 0.41 and 1400, respectively.

( K 0.5 Na 0.5 ) ( Nb 1 − x Ta x ) O 3  (KNNT)-based ceramics have been synthesized via a solid-state reaction. In this study, KNNT-based ceramics were sintered at atmospheric pressure by adding small amounts of sintering aid [0.38mol% K5.4Cu1.3Ta10O29]. The (K0.5Na0.5)NbO3 (KNN) ceramics which synthesized by using the K5.4Cu1.3Ta10O29 showed “hard” piezoelectric characteristics such as a high mechanical quality factor (Qm=1300) and low tanδ (0.4%). The quantitative effects of Ta on the electrical properties of (K0.5Na0.5)(Nb1−xTax)O3–K5.4Cu1.3Ta10O29 ceramics were also examined. Ta substitution provides “soft” piezoelectric characteristics and a large electrostrictive effect for KNN, which resulted in an improvement in kp, εr, strain, and d33 of KNNT ceramics depending upon the Ta content. Especially, (K0.5Na0.5)(Nb0.7Ta0.3)O3 showed the maximum strain and d33 values of 0.11% (at 40kV∕cm) and 270pm∕V (at 30–40kV∕cm), respectively. This strain level is comparable to that of hard Pb(Zr,Ti)O3 ceramics.

Dense (K,Na)NbO 3 (KNN)-based ceramics were developed by optimizing the sintering conditions with newly developed K 4 CuNb 8 O 23 (KCN) as a sintering aid. The sinterability and electrical properties of KNN ceramics were investigated as a function of KCN concentration. The density of KNN ceramics increased monotonically with increasing the KCN contents, and reached the highest value of 4.40 g/cm 3 with the addition of 0.5 mol% KCN. This is due to the formation of liquid phase, which significantly promotes the densification of KNN ceramics. The Curie temperature (T c ) of pure KNN ceramics was 420 C and decreased with increasing the KCN content. With the addition of 0.5 mol% KCN, planar mode electromechanical coupling factor (k p ) and mechanical quality factor (Q m ) of KNN ceramics also reached the highest values of 0.39 and 1200, respectively. The KNN ceramics prepared with 0.5 mol% KCN exhibited a large field-induced strain of 0.09% at 40 kV/cm and the piezoelectric constant d 33 of 180 pm/V.

(K,Na,Li)NbO3 (KNN) ceramics with 0.38 mol % K5.4Cu1.3Ta10O29 (KCT) have been prepared by a solid state reaction and their electrical properties were examined. The Curie temperature of (K,Na,Li)NbO3 increased with increasing Li substitution in both the Na and K sites. The 6 mol % Li-substituted sample showed a tetragonal phase at room temperature. Furthermore, a 1–2 mol % Li substitution for Na or K in (K0.5Na0.5)NbO3 improved several of its electrical properties. Most importantly, (K0.49Li0.01Na0.5)NbO3 attained high k p and Q m values of 0.43 and 2000, respectively. The value of the field-induced strain was also increased by substituting Li into (K0.5Na0.5)NbO3. The piezoelectric constant d 33 of (K0.5Li0.02Na0.48)NbO3 was approximately 200 pm/V, which was calculated from the slope of the field-induced strain curve at 30–40 kV/cm under a unipolar driving field.

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