High Q_m In (K,Na)NbO₃-based Piezoelectric Ceramics

Piezoelectric materials, which are important functional materials for mutual transformation between mechanical energy and electrical energy, are extensively used in functional devices, such as resonators, wave filters, piezoelectric transformers, sensors and ultrasonic transducers. Because of their excellent piezoelectric properties, simple preparation technology and low manufacturing costs, the lead zirconate titanate piezoelectric ceramics (PZT ceramics for short) have been playing a dominate role among piezoelectric materials for several decades. However, with a lead tolerance of about 60%, when sintered, employed and disused, PZT ceramics will produce heavy metal pollution, which is harm to human health. Several years ago, European Union published the WEEE/RoHS/ELV Bill, which brought a revolution for lead-free electronic products around the world. So it is of great practical significance to find lead-free and environmentally-friendly piezoelectric ceramics substituting for PZT ceramics.Because of their powerful ferroelectric properties, strong piezoelectric properties, high Curie temperature and other advantages, KNN ceramics are the most promising lead-free piezoelectric ceramics to substitute PZT ceramics. The KNN lead-free ceramics reported are mostly "soft" piezoelectric ceramics, while the piezoelectric ceramics applied to high frequency devices, wave filters and actuators should be "hard" piezoelectric ceramics, which not only have a particular piezoelectric constant d33 and higher electromechanical coupling factor kP, but also have higher mechanical quality factor and lower dielectric loss. The currently reported KNN lead-free piezoelectric ceramics rarely can meet the requirements. So revolving around doping modification of KNN piezoelectric ceramics with high Qm and low loss, this paper systematically studies on their microscopic appearance, crystal structure, dielectric and piezoelectric properties, while simply explains their physical mechanism.This article fabricates KNN-KCT lead-free piezoelectric ceramics by traditional solid state reaction technique. Firstly, The K/Na ratio influence on the physical properties in (KxNa1-x)NbO3-0.5mol% KCT ceramics is investigated in the compositional range of x=0.30-0.70, and the optimal K/Na ratio of the piezoelectric ceramics is obtained. When K/Na ratio is x=0.5, the specimen shows excellent performances with relative density=98.5%, Qm=1980, dielectric loss tanδ=0.3%, d33=95pC/N,=40%. Secondly, on the basis of the K/Na proportion(x=0.5), change the addition of KCT to study on the microscopic structure, dielectric property and piezoelectric property of the KNN-KCT ceramics. Then the optimal intermingled quantity is found to be 0.75mol%and the properties of the specimen are as follows: Qm is 2350, loss is 4%, d33 is 95pC/N and kp is 0.40. This is because the substitution of the Cu2+forthe Nb5+in B location makes the inside of the material produce Oxygen vacancy to electrically balance the crystal structure, and the Cu2+and Oxygen vacancy forms a defective dipole that prohibits the polarized deflection of the KNN and makes the material become harder.This article further fabricates KNN-0.75mol% KCT-xmol% Bi2O3 ceramics and systematically investigates the impact of Bi2O3 on KNN-0.75mol%KCT ceramics. When the Bi2O3 content<0.5, the d33 and kp are significantly elevated along with the increasing of the intermingled quantity. There should be two reasons for this phenomenon:one is the doping of Bi2O3 makes the ceramics intensify and improves the Sintering properties of the ceramics; the other is the substitution of Bi3+for K+,Na+produces A-site ionic vacancies, which can release stresses brought about by the reorientation of non-1800 domains, so that the non-1800 domains deflect easier and adequately in the polarization process, and the piezoelectric properties of the ceramics are improved significantly. The Qm of specimen is not increasing with the addition of Bi2O3 On the contrary, after the doping of Bi2O3, the loss of the sample increases from 0.5% to 2%, while Qm is decreased from 2300 to below 231.Through the systematical study of KNN based piezoelectric ceramics, I have find the basic law affecting the mechanical quality factors, losses and so on, and get the optimal doping and technological conditions, which provides reliable foundation for experiments and theories.