Development of Lead-Free (Na0.5K0.5)NbO3–Based Piezoelectric Ceramics with High Mechanical Quality Factor and Their Applications on Piezoelectric Transformers

• Main Authors: Song-LingYang, 楊松齡
• Other Authors: Sheng-Yuan Chu
• Format: Others
• Language: en_US
• Published: 2013
• Online Access: http://ndltd.ncl.edu.tw/handle/92031592020309912398

博士 === 國立成功大學 === 電機工程學系碩博士班 === 101 === In this thesis, development of lead-free (Na0.5K0.5)NbO3-based (NKN-based) piezoelectric ceramics with high mechanical quality factor (Qm) and their application on piezoelectric transformers, was investigated. Sintering aid CuTa2O6 (CT) compound was developed and then doped into NKN ceramics. A high bulk density (4.595 g/cm3) and electromechanical coupling factors (kp, kt) were obtained. Moreover, the mechanical quality factor (Qm) also increased from 67 to 1,550 as the concentrations of CT doping from 0 to 0.5 mole %. NKN ceramics with sintering aid CT doping showed good piezoelectric properties: kp: 42.5 %; kt: 49.1%; Qm: 1,550; and d33: 96 pC/N. On the other hand, the compensation for NKN ceramics was also an important role to affect the microstructure and piezoelectric properties. In CT-doped NKN ceramics, the concentration of oxygen vacancy dominates the magnitude of Qm value. However, non-stoichiometry NKN with CT doping showed the Qm value did not correspond to a higher concentration of oxygen vacancies. This reason guesses that the oxygen vacancies were induced from the defect of A-site and the replacement of Nb ions by Cu ions. The magnitude of Qm value was directly attributed to the formation of oxygen vacancy which was induced as the high valence ion (Nb5+) be substituted by low valence ion (Cu2+). Copper oxide was also a good dopant for enhancing the piezoelectric properties of NKN ceramics. The microstructure of CuO-doped NKN (NKNCx) ceramics prepared using the conventional mixed oxide method (MO method) exhibited obviously inhomogeneous microstructure. In contrast, the two-step calcination process (TC method) improved the compositional homogeneity as well as the electrical properties. The Qm value and dielectric constant (ε33T/ε0) of NKNCx ceramics prepared using the TC method were therefore by 21 % and 25 % better, respectively. The internal bias field and activation energy were measured and calculated to confirm the presence of oxygen vacancies. The ceramics prepared using the TC method exhibited the formation of more oxygen vacancies, resulting in an increase in the internal bias field. In addition, low activation energy corresponded to high Qm value. The maximum Qm value of NKNCx ceramics prepared using the TC method was more than 2,000 and other properties, including the bulk density, dielectric loss, kp, d33 and ε33T/ε0, were 4.488 g/cm3, 0.15 %, 41.5 %, 95 pC/N and 280, respectively. Unlike CuO, ZnO-doped NKN (NKNZx) ceramics could not significantly enhance the piezoelectric properties of the samples. The Qm values of the NKNZx ceramics were not as high as those of NKNCx ceramics due to a lack of oxygen vacancies in the former. The internal bias field was used to demonstrate the presence of oxygen vacancies. In ZnO-doped NKN ceramics, the piezoelectric properties only improved slightly because ZnO used as a sintering aid which enhanced the densification of samples. A high Qm value was obtained for CuO-doped NKN ceramics mainly due to the formation of oxygen vacancies. The performances of piezoelectric transformers (PTs) made with two substrates were compared (the ceramics prepared using the MO and TC methods). Experimental results showed that the output power and temperature stability of PTs were enhanced due to lower resonant impedance of the ceramics prepared using the TC method. In addition, the output power of PTs was more affected by the resonant impedance than by the mechanical quality factor (Qm) of the ceramics. For application on ballasts, PTs with different electrode areas were fabricated using the CuO-doped NKN ceramics prepared using the TC method. Considering the efficiency, voltage gain, and raising temperature of PTs at a load resistance of 1 kΩ, PTs with an electrode with an inner diameter of 15 mm were combined with the circuit design for driving a 13-W T5 fluorescent lamp. A raising temperature of 6 oC and a total efficiency of 82.4 % (PT and circuit) were obtained using the present PTs. This output power in the lead-free disk-type PTs was the best reported so far.