Research On Fabrication Of Lead-free Piezoelectric Ceramic And Ultrasound Transducer Based On 3D Printing Technology

Piezoelectric ceramic ultrasonic transducers are widely used in medical,industrial,transportation and military fields due to their good piezoelectric properties and low manufacturing costs.In the design of this kind of transducers,the shape and distribution of piezoelectric ceramics play a crucial role in the performance of the transducer.However,due to the high hardness and brittleness of ceramics,it is difficult to manufacture piezoelectric ceramic transducers with spatial heterogeneity to meet the needs of special structures by traditional mechanical processing.3D printing technology has the advantages of fast moulding speed,high moulding accuracy and no moulds required during the moulding process,making it easy to realise individual and complex fabrication requirements,but as the technology is based on a layer-by-layer stacking manufacturing principle,the internal structure and performance parameters of the printed ceramics vary in different directions.In addition to the preparation process,the components of piezoelectric ceramics also have a large impact on the application of transducers,of which,lead-free piezoelectric ceramic components do not contain heavy pollution of lead,will not cause harm to humans and the environment,in line with the development direction of piezoelectric ceramics in the future,and the transparent lead-free piezoelectric ceramics ultrasonic transducers have great prospects for application in transparent touch,photoacoustic imaging,self-powered touch screens,transparent robots,etc.In summary,this thesis mainly focuses on 3D printed lead-free piezoelectric ceramics and ultrasonic transducers,comparing the axial-radial differences of printed piezoelectric ceramics and investigating the fabrication and application of transparent lead-free piezoelectric ceramic ultrasonic transducers,the main research content are as follows:（1）Several square-shaped BTO piezoelectric ceramics were fabricated.The differences between the axial and radial directions of the ceramics,including mechanical properties,structure,microstructure and electrical properties,were investigated,and the effects of the cold isostatic pressing process on the axial and radial directions were compared.The results show that the axial and radial directions are pure perovskite structures,with no secondary phases,but the longitudinal and transverse sound velocities(V₇)～4787m/s,V～2413m/s),relative dielectric permittivity(ε_rt～2546),piezoelectric constant(d₃₃～208p C/N)and electromechanical coupling coefficients（k_p～0.276）are higher in the axial direction than in the radial direction,while the Curie temperature（T_c～117.5℃）,bulk modulus（83.83GPa）and dielectric loss（tanδ～0.032）are lower than in the radial direction,and the axial section has less pores and larger grain size.When the green body is processed by cold isostatic pressing,the above parameters were significantly improved and the differences were significantly reduced.Based on the ceramic,a 12-array element-shaped array piezoelectric transducer with a centre frequency of 2.09 MHz and a-6d B relative bandwidth of 14.05%was designed and fabricated.The acoustic simulation results show that the transducer has good self-focusing effect and the focal region is～7mm in depth direction,which can meet some applications requiring higher ultrasonic energy（2）The[Li_0.04(K_0.5Na_0.5)_0.96]Nb O₃-1.5%La₂O₃-0.5%Sm₂O₃ piezoelectric ceramics were fabricated to investigate the effects of rare earth La/Sm,sintering parameters on the structural,optical and electrical properties of the ceramics.The results show that the ceramics have a pseudo-cubic phase structure.The ceramics were fabricated at 1120℃for 6 h have less pores,homogeneous grain size,compact arrangement,the liquid phase was generated at the grain interstices,and the piezoelectric properties were good(d₃₃～120p C/N,k_t～0.36).The ceramic exhibits strong red-yellow light emission when excited by 407nm laser.In addition,with proper regulation of the sintering temperature,the transmittance of the ceramics reached 36%in the wavelength range of 500-850 nm.（3）The[Li_0.04(K_0.5Na_0.5)_0.96]Nb O₃-0.5%Er₂O₃piezoelectric ceramics were fabricated to investigate the effects of rare earth Er,sintering parameters on the structural,optical and electrical properties of the ceramics.The results show that the ceramics have a perovskite structure with the coexistence of orthotropic and tetragonal phases.Similarly,the ceramics fabricated at 1120°C for 6h have good properties(d₃₃～97p C/N,k_t～0.36),low dielectric loss（tanδ～0.06）and high Curie temperature（T_c～281.8℃）.When excited by 980nm laser,the component ceramics show significant emission peaks in both the visible and near-infrared regions,the intensity of which increases with increasing holding time.In the wavelength range of 500-850nm,the transmittance of the printed ceramics reached 34.3%,and the holding time and preparation process have a significant influence on the transmittance.Finally,a transparent piezoelectric transducer was fabricated based on the transparent piezoelectric ceramics,which has a centre frequency of 10.56MHz and a-6d B relative bandwidth of 18.26%.The transducer effectively couples a variety of functions into one,which can greatly improve the detection accuracy.