Studies On New Lead-Free Piezoelectric Ceramics And Their Applications In Filters

Lead-free piezoelectric ceramics [Bi_0.5(Na_1-x-yK_xLi_y)_0.5]TiO₃(abbreviated as BNKLT) were prepared by conventional ceramic method. XRD analysis shows that the ceramic samples prepared by the rolling forming technique and sintered between 1075-1150℃for 2h have good crystallization and display single-phase perovskite structure, which is the same as those prepared by the dry-pressing forming technique. The microstructure of the ceramic material was characterized. The properties of the ceramic material, including dielectric and piezoelectric properties under different fabrication conditions, were measured. With the ceramic material systems, the studies about the single disc middle frequency filter, the multi-section ladder middle frequency passband filters, and the energy trapped mode high frequency filters were developed. Through the work above, the main conclusions with innovative significance were got as follows: (1) The ceramic samples utilizing the rolling forming technique have high densification without any pore and the crystalline grains are replete. The aging properties of the ceramic samples utilizing the rolling forming technique were measured. Under the aging between 150-170 ℃for 2h, the dielectric and piezoelectric properties of BNKLT-2 lead-free piezoelectric ceramics who has a lower K and Li concentration haven't decreased much, which indicates that severe depolarization hasn't appeared in the BNKLT-2 lead-free piezoelectric ceramics under aging processes, those can still be used. (2)Piezoelectric properties of both BNKLT-1 and BNKLT-2 ceramic discs under the dry-pressing forming fabrication and the rolling forming fabrication were measured. The binder additives needed by the rolling forming technique are as high as 20%, which is far more than those needed by the dry-pressing forming technique, which results in the decrease of the ceramic densification and the piezoelectric constant. Piezoelectric constant d33 of dry-pressing disc of BNKLT-1 ceramics is above 170pC/N, and piezoelectric constant d33 of rolling disc is about 150pC/N. Piezoelectric constant d33 of BNKLT-2 ceramics is smaller than that of BNKLT-1 ceramics, which is about 118 pC/N. The planner electromechanical coupling factor kp of BNKLT-1 ceramic disc is higher than that of BNKLT-2 ceramic disc. However, the mechanical quality factor Qm of BNKLT-1 ceramic disc is lower than that of BNKLT-2 ceramic disc. (3) Using the skeletal vibration mode, the single disc middle frequency filters with the center frequency about 530KHz is fabricated. The loss insertion is about 3dB, and the bandwidth is about 8.8KHz. The left and right choose character is good, which can safely satisfy the same series of SFU standard of the Murata Company and can be put into the applications. (4) Using the full electrode radial vibration mode of the resonance samples, the multi-section ladder passband filters were fabricated. Through the suitable combination, the passband filters have good choose character with the loss insertion no more than 6dB and the out-band attenuation more than 30dB. The center frequency is about 460KHz. (5) Based on the theory of the energy trapped mode, the energy trapped mode high frequency filters with the center frequency about 6.2MHz were designed and studied. The filters were packaged under 150℃, which didn't bring the electricproperties of the ceramic material severe decrease. This shows that the lead-free piezoelectric ceramic system has high using temperature. The loss insertion is 27dB, which is much higher than that of the SFE series of the lead-based filter. Which indicates that the energy trapped mode high frequency filters made from lead-free piezoelectric ceramics could be further studied.