Study Of Temperature-stable Piezoelectric Properties Of (K,Na,Li)(Nb,Ta)O₃ Ceramics

Piezoelectric ceramics as a type of the most important functional materials are used in many practical applications, such as transducers, actuators, resonators and filters. Pb(Zr,Ti)O3-based piezoelectric ceramics have been popularly utilized for several decades in many electronic devices, because of their superior piezoelectric performance. However, they are listed as restricted materials by regulations, due to the toxicity of the lead oxide used in the production process, so there are increasing demands for environmentally benign lead-free alternatives.It is widely believed that (K,Na)NbO3 (KNN)-based ceramics may be the promising candidates as the lesd-free piezoelectric materials. However, the enhanced piezoelectric properties in KNN-based ceramics have been recently found to closely relate with the occurrence of an orthorhombic-tetragonal phase transition around room temperature. As a result, strong piezoelectric temperature dependence is usually observed, which is often very undesirable for practical applications. In this work, the author mainly studied piezoelectric temperature dependence and domain structure of KNN-based ceramicsIn the first part of this paper, (K,Na,Li)(Nb,Ta)O3 (KNLNT) ceramics were prepared by a conventional solid-state reaction technique. The influence of K/Na, microstructures, crystal structure, dielectric properties, piezoelectric properties and piezoelectric temperature dependence were investigated. The result indicates that the ratio of K/Na has an influence on the coexistence temperature range of orthorhombic-tetragonal phases and piezoelectric temperature dependence. The ceramic with a composition of (K0.55Na0.45)0.965Li0.035Nb0.80Ta0.20O3 was in particular found to have an outstanding piezoelectric performance with dd3=262 pC/N,kp=0.53, k33=0.63 andε'=1290, tanδ=1.9% at room temperature. In spite of that the orthorhombic-tetragonal phase transition is around 30℃, temperature stability of its electromechanical coupling coefficients is very good over the common usage temperature interval between-40 and 85℃. Furthermore, the piezoelectric properties remained almost unchanged in the severe thermal ageing test down to-150℃and up to 300℃Moreover, it has been also found that KNLNT ceramics show the characteristic of extermely temperature-stable piezoelectric properties in orthorhombic phase. For the ceramics with compositions of x=0.50 or x=0.55, their piezoelectric properties are almost temperature-independent over the temperature range from-150℃to 50℃.In consideration of that the orthorhombic phase possesses much better piezoelectric temperature stabilities than the tetragonal phase, we further attempted to decrease the amount of Li in KNLNT ceramic to shift the orthorhombic-tetragonal phase transition to high temperatures. As a result, we obtained the ceramic with composition of (K0.5Na0.5)0.975Li0.025Nb0.80Ta0.20O3, whose electromechanical coupling coefficients shows excellent temperature-stable properties over a broad temperature rage from-150℃to 120℃.In the second part of this paper, a method to observe domain structure of KNN-based ceramics was successfully developed. We used polishing and acid-etching technique and made observation under optical polarizing microscope.