| Summary: | 博士 === 南台科技大學 === 電子工程系 === 99 === Optimizing the dynamic resonant characteristics of ultrasonic therapeutic transducers depends most importantly on fine-grain piezoelectric ceramics with good resonant properties and good temperature stability properties. Therefore, the modified PMnN-PZT based piezoelectric ceramics of composition Pb0.99Sr0.01Ca0.01 [(Mn1/3Nb2/3)x-(1-x)(ZryTi1-y)]O3 (PCSMNZT-x) (0.02≦x≦0.07, 0.47≦y≦0.54) were synthesized by B-site oxide precursor method (BO method) and conventional mixed-oxide method (MO method). The influence of PMnN content on the microstructure, electrical, temperature stability and mechanical properties of the samples synthesized using these two methods were systematically investigated and compared. Results showed that the samples synthesized using the BO method not only lowered the sintering temperature (from 1260 oC to 1150 oC) without adding sintering aids, but also exhibited fine-grain structure, enhanced piezoelectric properties, and temperature stability of resonance frequency (TCF) and clamped capacitance (TCC), which were superior to those of MO-type piezoelectric ceramics. The obvious improvement is due to the fine-grain microstructure and chemical homogeneity.
A trade-off composition with x = 0.035 were chosen for MO and BO types specimens based on the experimental results. Two types of 1-MHz narrow-band therapeutic transducers were fabricated according to electro-acoustic energy transfer mechanism. Their dynamic resonant characteristics were examined as the transducers are driven by a power driver with open-loop control. Results showed that the transducers with the BO-type piezoceramics had better dynamic characteristics, i.e. time stability (e.g. aging rate of resonant frequency at thickness mode is 0.26 %/decade cycle), and aging rate of clamped capacitance (0.55%/decade cycle) and lower degradation in input electrical power (decreased 30 % electrical power). The partial derivative ratio of the transfer function model (PDR model) was proposed and simulated the powder degradation in the ultrasonic system. The fitted result of power degradation was well consistent with the results of that of MO- and BO-types transducers.
To develop the low temperature-sintered ceramics below 1000 oC to meet the energy saving and multilayer devices, the Pb0.98Ca0.01Sr0.01[(Mn1/3Nb2/3)0.054Cu0.006- (Zr0.52Ti0.48)0.94]O3 (PCSMNCZT) + x wt. % ZnO (PMNCZT-10x) ceramics were prepared using the modified BO method. The results showed that ZnO additive increased the tetragonality ratio, the tetragonal phase fraction, and the long-range order degree of crystal structure. In addition, the electromechanical coupling factors, and the temperature stability of resonance frequency and resonant resistance were improved at low sintering temperature 950-980 oC. Besides, the normalized value decrease of space charge variation can reflect the decrease in Qm value.
For the environmental protection, developing the lead-free piezoelectric ceramics for power transducer applications had become into urgent issues. Therefore, the lead-free ceramics with non-stoichiometric composition of (Na0.5K0.5)0.998Ca0.001Nb1+x/5O3 + 1 mole% CuO (NKCN-x-C1) were synthesized by the mixed-oxide method Experimental results showed that the grains in the microstructures slightly decreased with increasing the excess Nb contents, and the corresponding bulk densities were enhanced for NKCN-x-C1 ceramics. Furthermore, the low dielectric loss (tan δ) of 0.003, high electro-mechanical coupling coefficient (kt) of 0.49 (in thickness mode), high mechanical quality factor (Qm) of 2800, and internal dipolar field (Ei) of 5.4 KV/cm were obtained. The high Ei value induced the high Qm value due to the defect dipole increase.
Based on transmission coefficient of intensity, piezoelectric and temperature stability properties, the 3 MHz LS and LF therapeutic transducers were fabricated using the low temperature sintering ceramics with x = 0.5, and lead-free ceramics (x = 0.03), respectively. Their corresponding properties showed that the effective coupling factor keff2 of 5.6 % and 5.5 %, and electro-acoustic efficiency of 34.1 % and 15 %. Furthermore, the electro-acoustic power was compared to normal temperature sintering (NS), and low temperature sintering (LS) lead-based transducers. Results were found that the electro-mechanical power of LF transducer is inferior to that of NS and LS transducers.
Keywords: dynamic resonant characteristics; piezoelectric ceramics; chemical homogeneity; electro-acoustic efficiency; non-stoichiometric composition
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