Research On Polymer Transducers

Ultrasonic technology has been widely used in the fields of industry,agriculture,medical science and environmental protection.As a device that realizes the mutual conversion of electric energy and sound energy,the transducer plays a decisive role in the performance of the ultrasonic vibration system.Among the many forms of transducers,composite rod transducer is utilized more in high-power applications such as ultrasonic cleaning,ultrasonic machining,ultrasonic welding,and enhancing chemical reactions.The traditional composite rod transducer is composed of metal vibrating bodies and piezoelectric ceramics,which has the characteristics of high electroacoustic efficiency,high electromechanical coupling coefficient and large power capacity.With the continuous development of ultrasonic applications,many ideas have been proposed for the development direction and application scenarios of the transducer.In 2017,the US Defense Advanced Research Projects Agency proposed the"mechanical antennas",which directly excite low-frequency electromagnetic waves through the mechanical movement of specific materials such as electrets or permanent magnets to solve the problems of existing low-frequency communication systems.It is a new type of low-frequency electromagnetic transmission technology.In the very low frequency(VLF,<30kHz)communication,the composite rod transducer is expected to become a fine choice for mechanical motion driving source.But the front head of the transducer used to connect the electret is easy to be polarized and generate an inverted electric dipole moment,which leads to the reduction of efficiency of the mechanical antennas.Therefore,thoughts of using new materials to develop transducers to meet the requirements of mechanical antennas are proposed.In view of the above-mentioned problems,by comparing different materials,the polymer material polyphenylene sulfide(PPS)with relatively large storage modulus and small loss factor is selected as the front head of the transducer.Details are as follows:(1)On the basis of expounding and testing the mechanical properties of polymers,an analysis of dynamic mechanical parameters based on the design of transducer is proposed.The mechanical characteristics of viscoelastic materials are between the ideal elastic materials and the ideal viscous materials,and there is a phase difference between the stress and the strain.At this time,the elastic modulus is in the complex form,which is called the complex modulus.The mechanical properties of viscoelastic materials have a great impact on temperature and frequency and follow the principle of time-temperature superposition.Before employment of polymer,the mechanical property spectrum that changes with temperature or frequency should be clarified.The dynamic mechanical analysis method,modified piezoelectric ultrasonic composite oscillator technique,resonance measurement method and wave-speed method are applied to run the tests of mechanical parameters of the PPS sample respectively.A high-frequency mechanical parameter analysis method based on the design of the transducer is proposed to analyze the accuracy of the aforementioned tests.The results show that the dynamic mechanical analysis method obtains the most accurate mechanical parameters,while the modified piezoelectric ultrasonic composite oscillator technique has the characteristics of short test time,simple test process,low cost,and relatively high accuracy.(2)The design method of the transducer is given through the lumped parameter model,distributed parameter model and finite element model.The current application scenarios and classification of the transducer are summarized.Based on different theories,the lumped parameter model,distributed parameter model and finite element model of the transducer are introduced.The lumped parameter model equates the transducer to an ideal mass and elastic element.It is a simplest type of one-dimensional theoretical model,which is convenient for understanding,designing and evaluating the transducer,but the accuracy of this model is poor.The distributed parameter model emphasizes the continuous distribution of each component in the overall structure.The electromechanical equivalent circuit of the transducer is derived from the one-dimensional longitudinal vibration theory and the electromechanical and piezoelectric equations of piezoelectric ceramics.The finite element model divides the entire transducer into many small elements,connects them through a mesh,calculates the derived variables corresponding to each degree of freedom by solving the degrees of freedom of the elements,and then combines everything into an entire model.(3)The polymer is used as the materials of vibration bodies of the transducer,and the longitudinal wave propagation velocity in the polymer rod is studied.Based on this,the polymer-based transducer is designed,and the electromechanical equivalent circuit of the polymer-based transducer with central bolt is studied.The resonance frequency equation of the whole polymer-based transducer is deduced,and the resonance frequency of the polymer-based transducer is calculated from this.In previous studies,the design was simplified by dividing the transducer into two parts from the position of the node,so the resonance frequency of the transducer could be easily obtained.When the position of the node is unknown,it is more complicated to solve the entire equivalent circuit.Through the delta-star transformation,the simplified frequency equation of the overall transducer is obtained.The mechanical parameters of PPS are substituted into the design of the transducer,and the resonance frequencies and the dissipated powers of PPS-based transducers are calculated.(4)The 1#transducer whose front head made of PPS and the rear end made of steel,and the 2#transducer whose front head and rear end both made of PPS have been developed and tested.Experiments show that the measured resonance frequency is consistent with the theoretically calculated resonance frequency.The vibration amplitude of the front surface of the two transducers is proportional to the applied voltage,and the local temperature rise is obvious at the place where the strain is larger.The 1#transducer has better working performance due to its larger vibration-to-voltage ratio and smaller average dissipated power.Compare with the traditional metal material transducer of the same resonance frequency,when the material and size of the rear end are the same,using PPS as the material of the front head can significantly improve the front-to-rear vibration amplitude ratio.