Research On Fabrication And Properties Of KNN Lead-free Based Piezoceramics By Two-step Sintering

Piezoceramics are widely used as the smart materials in many components including sensors, actuators and transducers. The traditional piezoceramics are mostly Pb-based perovskite materials. However, due to the toxicity of the lead oxide during the production process, use and disposal, It is important to replace them using the lead-free for the society and economy. Especially, K_0.5Na₍0.5NbO₃ (abbreviated KNN) piezoceramics with the higher Curie temperatures, better piezoelectric properties, are attracting more and more attention as the most promising candidate. But the KNN piezoceramics are difficult in the conventional sintering owing to the volatilization of K and Na, which will decrease the densities and piezoelectric properties. So the article is studied to improve the properties of the KNN-based piezoceramics by Li,Ta,Sb co-doping and the new sintering procedure of two-step sintering.The subject research and results as following:(1). The results of the pure KNN specimens by the two-step sintering showed that the higher density ceramics with regular grain size were showed by the two-step sintering. The optimized sintering procedure as following: T1=1100oC, v1=2oC /min, T2=980oC, t2=10h. In the optimal condition, the excellent properties of d₃₃=121pC/N, kp=35.2%,εr=815, Pr =9.24μC/cm2 are achieved, which are 30% than the specimens by the conventional sintering.(2). The effects of Sb2O3 and Sb2O5 addition on (K_0.44Na_0.52Li_0.04)(Nb_0.9-xTa_0.1Sb_x)O₃) lead-free piezoceramics by the conventional sintering were studied. The piezoelectric properties are the best near the MPB composition range. In the Sb2O3 addition, the specimens achieved the higher density and remnant polarization, and the grain size of the specimens impact on the remnant polarization. In the optimal condition, the excellent properties are: dopant of Sb2O3 with the value at x=0.04, d₃₃=248 pC/N, Pr=19.2μC/cm2,εr=1254, kp=41%; dopant of Sb2O5 with the value at x=0.05, d₃₃=253 pC/N, Pr=15.5μC/cm2,εr=1290, kp=37.4%.(3). The properties of (K_0.44Na_0.52Li_0.04)(Nb0.86Ta0.1Sb0.04)O3 with the addition of Sb2O3 fabricated by the different two-step sintering procedures were studied. The optimized sintering procedure as following: 1140oC /1000oC/10h. In the optimal condition, the excellent properties of d₃₃=310pC/N, kp=52.14%,εr=1346, Pr=22.93μC/cm2 are achieved.(4). To reduce the use of the Ta, (K_0.48-xNa_0.52Li_x)(Nb_0.93-xTa_xSb_0.07)O₃ lead-free piezoceramics by the conventional sintering were studied. The optimal raw material of Ta at x=0.0375 was revealed. At the optimal sintering procedure: 1120oC, 3h, the excellent properties of the ceramics with the grain size of 1.1μm: d₃₃=320pC/N, kp=47%,εr=1909, Pr=22.2μC/cm2 are achieved.(5). The properties of the (K_0.4425Na_0.52Li_0.0375)(Nb_0.8925Ta_0.0375Sb_0.07)O₃ piezoceramics fabricated by the different two-step sintering procedures were studied. The optimized sintering procedure as following: 1120oC/10min/1000oC /5h. In the optimal condition, the excellent properties of d₃₃=261pC/N, kp=47.6%,εr=1659, Pr=17.67μC/cm2. But the grain size of the ceramics with 0.9μm was smaller than the conventional sintering.So the KNN-based piezoceramics were successfully manufactured via the two-step sintering process which is a new method with the advantage of Easy-to-use and just only simple equipment. From the results, the two-step sintering can be used in the low temperature sintering, and the higher piezoelectric properties of the KNN-based lead-free piezoceramics with the uniform and smaller grain size could be fabricated by the two-step sintering.