Study On The Piezoelectric Properties And The Cut Directions Of KNN Based Single Crystal

Lead-free KNN based single crystal is expected to replace PZT piezoelectric ceramics and PMN-PT crystal in life and production due to its excellent piezoelectric properties and high Curie temperature.When KNN based crystals are used in piezoelectric devices,it is necessary to know the piezoelectric properties and specific vibration modes of the crystal.In this paper,the following two studies are carried out on KNN based single crystal:Firstly,the distribution characteristics of the piezoelectric modulus and the electromechanical coupling coefficients in 3D space of KNN based crystal are analyzed by coordinate rotation method;Secondly,by solving Christoffel equation under coordinate rotation,the mode of acoustic wave is calculated and the cutting direction of single mode is determined.The results are as follows:For the KNNT(3m)single crystal with rhombohedral phase,when ? is 0° and ? is-51°,the maximum values of the longitudinal piezoelectric properties d33 and k33 are 70.8 pC/N and 0.459,respectively.When(?/?)is(-180°/-29°)and(0°/37.5°),the maximum values of d15 and k15 are 160.3 pC/N and 0.625,which are 1.15 and 1.1 times of the original values,respectively.The piezoelectric properties of tetragonal KNNTL-Mn?KNN-TL and KNNT single crystals have a similar law.When the rotation angle(?/?)is(0°/53.8°)and(-135°/64.5°),the maximum values of d24 and k24 of the KNNTL-Mn single crystal are 607.4 pC/N and 0.831,which are close to the values of d24 and k24 of the PZT-5H piezoelectric ceramics.The maximum values of d24 and k24 of the KNN-TL single crystal are 370.9 pC/N and 0.573,which are 2.17 and 1.3 times of the original values,respectively.The maximum values of d24 and k24 of the KNNT single crystal are 176.1 pC/N and 0.647,which are 3.9 and 2.76 times larger than the original values.The shear piezoelectric properties d24 and k24 of these three crystals are excellent.For the single mode cutting type of the KNNT(3 m)single crystal,the quasi-shear mode k"z of Y-?(??[147°,152°])cut can reach 0.574,the k value of the quasi-longitudinal mode is close to 0,and the polarization direction is within 5°,which are available cutting directions.Z-cut can obtain pure longitudinal mode,the value of k"z is 0.283,v is 6774.4 m/s,which is also an available cutting direction.Only X-cut,Y-cut and Z-cut are available cut directions for single-mode cutting type of tetragonal KNNTL-Mn?KNN-TL and KNNT single crystals.The pure shear mode can be obtained by X-cut and Y-cut of each crystal,and the values are equal;the pure longitudinal mode can be obtained by Z-cut.In X-cutting and Y-cutting,the comprehensive properties of the KNN-TL single crystal are better,the value of k"z is 0.441,the wave velocity is 3906.8 m/s,and the error is available within 3°.In Z-cutting,the properties of the KNNT single crystal are better,k"z can reach 0.636,v is 6742.3 m/s,and it is available to cut in the error range of 2°.The orientation dependence of the piezoelectric properties and the available cutting type of rhombohedral KNNT(3m)and tetragonal KNNTL-Mn?KNN-TL?KNNT single crystals were obtained in this studying result.The result will promote the application of KNN-based crystals in piezoelectric devices,and it's beneficial to the selection of piezoelectric materials.