Preparation And Physical Properties Of Two Niobate Lead-free Piezoelectric Ceramics

The application of traditional lead-based piezoelectric ceramics which contain high lead are strongly challenged because the lead can cause serious damage to human and environment. As a result, the development of lead-free and environmentally-friendly piezoelectric ceramics becomes an urgent and of great practical significance task. In this paper, two type of lead-free piezoelectric ceramics of (K_xNa_1-x)_1-yLi_yNbO₃ and Sr_2-xCa_x NaNb₅O₁₅(abbreviated to KNLN and SCNN, respectively) were fabricated by solid-state reaction technique. Microstructure, crystallographic stucture, dielectric and piezoelectric properties of those two type of ceramics were investigated systematically.The (K,Na)NbO₃ (KNN) based ceramics have been paid much attention because of their enhanced piezoelectric properties. At present, the investigation on KNN-based ceramics is concentrated on two aspects, (1) exploring the KNN-based compositions with more enhanced piezoelectric properties, and (2) researching on factors which improve the piezoelectric properties of KNN-based ceramics, such as morphotropic phase boundary (MPB) and polymorphic phase transition.In first part of this paper, the variety of polymorphic structural phase transition(PPT) and the influence of PPT to the piezoelectric properties were mainly discussed. In (K_0.50Na_0.50)_1-yLi_yNbO₃ (y=0-0.08) ceramics, with the increase of Li content y the experimental result of PPT and the piezoelectric properties are generally in agreement with that in previous literature. In (K_xNa_1-x)_0.94Li_0.06NbO₃(x=0.30-0. 70) ceramics, with the increase of K content x it exists an orthorhombic-tetragonal- orthorhombic phase transition and the phase boundaries are located at x=0.40 and x=0.60, respectively. Two ceramic samples of (K_0.50Na_0.50)_0.935Li_0.065NbO₃ and (K_0.45Na_0.55)_0.94Li_0.06NbO₃ show the best piezoelectric properties of d₃₃>200pC/N and k_p>45%. The orthorhombic-tetragonal phase transition temperatures of these two samples are 27℃and 35℃, respectively. It can be speculated that the nice piezoelectric properties of those two samples are mainly related to the existence of orthorhombic-tetragonal phase transition at room temperature.In second part of this paper, the Sr_2-xCa_xNaNb₅O₁₅ piezoelectric ceramics with tungsten bronze structure show the piezoelectric properties of d₃₃-70pC/N, dielectric constantε₁>1400, dielectric loss tgδ<1% and relatice densityρ>92%. For SCNN ceramics, the relative density, the average grain size and the piezoelectric properties begin to decrease when the Ca²⁺ content exceeds 0.15. The ceramic sample of Sr_1.9Ca_0.1NaNb₅O₁₅ reveals the better piezoelectric properties of d₃₃=74pC/N, k_p=8.2% and Q_m=923.