Nondestructive Evaluation Of Homogeneity Of Piezoelectric Materials

Ultra high piezoelectric properties of relaxor-based single crystals make them"materials of choice" for next generation electroacoustic transducers.Up to now,the problem of homogeneity of relaxor-based single crystals has not been completely solved.The non-homogeneity of compositions leads to the instability of the local performance of single crystals,which hinder their application in the acoustic engineering.To ensure the performance of the acoustic devices using relaxor-based single crystals,the homogeneity of these single crystrals should be nondestructively evaluated first.Existing methods for evaluating the homogeneity of piezoelectric materials include the electron probe microanalyzer technology(EPMA),multi-probe detection technology and thermal wave technology,etc.The advantage of EPMA is that it can directly analyze the chemical composition distribution of the sample.However,it can only test the thin sample.The multi-probe detection technology can simultaneously measure the Curie temperature Tc,electromechanical coupling coefficient kt and dielectric coefficient of several small samples cut from different areas of a larger piezoelectric sample,so as to evaluate the homogeneity of the sample.During the evaluation,the sample should be cut into several small samples.Therefore,it is a destructive evaluation technique.The thermal wave technology can quantitatively evaluate the distribution of piezoelectric coefficient d33 of piezoelectric samples along the thickness direction.At present,there are still very few methods for evaluating the homogeneity of piezoelectric materials.To promote the application of novel piezoelectric materials such as relaxor-based single crystals in acoustic transducers,developing other efficient and low-cost nondestructive evaluation(NDE)methods for evaluating the homogeneity of piezoelectric materials are urgently needed.In this thesis,the resonant ultrasound spectroscopy(RUS)and electric impedance spectroscopy(EIS)techniques are applied to the NDE of the homogeneity of piezoelectric materials for the first time.The principle of RUS for evaluating the homogeneity of piezoelectric samples is as follows:The resonance frequencies of piezoelectric samples are not only depend on their geometrical and material coefficients,but also on their boundary conditions.If a surface of the piezoelectric sample is attached to an electrode,it is equipotential;otherwise,it is free.For the rectangular parallelepiped piezoelectric sample with a 4mm,6mm or mm2 point group symmetry,one of its vertexes is taken as the coordinate origin,and three mutually perpendicular edges connected to the vertex are taken as x,y and z axes.Two surfaces of the sample perpendicular to the x direction are assumed as S1 and S2.If the sample is perfect and homogeneous along the x direction,the resonance frequencies of the sample,surface S1 of which is attached with an electrode,should be identical to those of the sample,surface S2 of which is attached with an electrode,otherwise,the sample is nonhomogeneous along the x direction.Similarly,the homogeneity of the sample along the y or z direction can be evaluated.Therefore,the NDE of the homogeneity of the piezoelectric sample can be realized by measuring the resonant ultrasound spectra of the piezoelectric sample under different boundary conditions,and comparing resonance frequencies identified from them.The principle of EIS for evaluating the homogeneity of the piezoelectric sample is as follows:The electric resonance frequencies and anti-resonance frequencise of piezoelectric samples are not only depend on their geometrical and material constants,but also on their boundary conditions.For the rectangular parallelepiped piezoelectric sample with a 4mm,6mm or mm2 point group symmetry,the geometric center of the sample is assumed as the coordinate origin.Moreover,x,y and z axes are parallel to the length,width and height directions,respectively.There are two surfaces of the sample perpendicular to the z direction.The parts of these two surfaces satisfying x<0 and x>0 are set as G1 and G2,respectively.If the sample is perfect and homogeneous along the x direction,the electric resonance frequencies and anti-resonance frequencies of the sample,the surface G1 of which is attached with an electrode,are identical to those of the sample,the surface G2 of which is attached with an electrode,otherwise,the sample is nonhomogeneous along the x direction.Note that the surface G1 is composed of two parts parallel to each other,as well as the surface G2.Similarly,the homogeneity of the sample along the y or z direction can be evaluated.Therefore,the NDE of the homogeneity of the piezoelectric sample can be realized by measuring the electric impedance spectra of the piezoelectric sample under different boundary conditions,and comparing the resonance frequencies and anti-resonance frequencies identified from them.RUS-NDE and EIS-NDE do not need to cut the sample into thin pieces.Moreover,they are easy to be carried out with low costs.In this thesis,RUS-NDE and EIS-NDE are employed to evaluate the homogeneity of PZT-8,[001]c poled Mn-doped 23Pb(In1/2Nb1/2)O348Pb(Mg1/3Nb2/3)O3-29PbTiO3(23PIN-48PMN-29PT:Mn)single crystal and[011]c poled Pb(Zn1/3Nb2/3)O3-4.75PbTiO3(PZN-4.75PT)single crystal samples.The results obtained by RUS-NDE and EIS-NDE are consistent.In addition,the PZT-8 and 23PIN-48PMN-29PT:Mn samples were cut into four subsamples.Then the elastic constants C44D,C55D and C66E,dielectric constants ?11T,?22T,?33T,?11S,?22S and ?33S,and piezoelectric constant d33 of each subsample were measured.The measurement results of subsamples are consistent with those obtained by RUS-NDE and EIS-NDE,which proves the effectiveness and reliability of RUS-NDE and EIS-NDE.The innovation of this thesis is as follows:(1)RUS is introduced to the NDE of the homogeneity of piezoelectric materials.This method can realize the NDE of the homogeneity of the rectangular parallelepiped piezoelectric sample with a 4mm,6mm or mm2 point group symmetry by measuring the resonant ultrasound spectra of the sample under different boundary conditions,and comparing the resonance frequencies identified from them.(2)EIS is introduced to the NDE of the homogeneity of piezoelectric materials.This method can realize the NDE of the homogeneity of the rectangular parallelepiped piezoelectric sample with a 4mm,6mm or mm2 point group symmetry by measuring the electric impedance spectra of the sample under different boundary conditions,and comparing the resonance frequencies and anti-resonance frequencies identified from them.