Study On The Physical Properties Of Composties Based On 1-3 Relaxor Ferroelctric Single Crystal

Piezoelctic element is the most essential part of an ultrasound transducer,its properties directly deterimines the transducer performance.In recent years,outstanding piezoelectric property was obtained in relaxor-based ferroelectric single crystals with ABO₃ perovskite structure.Pb(Mg_1/3Nb_2/3)O₃-x Pb Ti O₃（PMN-x PT）single crystal is one of the best candidates,outstanding piezoelectric and electromechanical coupling were found when they are poled along the pseudo-cubic[001]direction to form a multidomain state(d₃₃2000pC/N,d₃₁-2000pC/N,k₃₃0.92,k₃₁1 0.94,k₁₅5 0.97),therefore,they are used as the active phase in 1-3relaxor-based ferroelectric single crystal composites.Compared with bulk PMN-x PT single crystal,which is fragile,having high acoustic impedance and nonpure thickness mode,1-3 composites are superior in thickness electromechanical coupling factor k_t,lower spurious modes,lower acoustic impedance,and much broader bandwidth.Although there are vast literature focused on the understanding of the PMN-x PT single crystal,there are few studies about the influence of the kerf filler on the composite performance degration,domain configurations,and the relationship between them.In this thesis,we have investigated the 1-3 relaxor-based ferroelectric single crystal composites in several aspects as detailed below.In the first part,a unified formula has been derived to quantitatively describe the electromechanical coupling factor for the thickness mode of 1-3 piezoelectric composites with rectangular rod.The effect of different kerf filler on the electromechanical coupling factor is analyzed by varying the aspect ratio of piezoelectric rod and the elastic stiffness of kerf filler.The results show that the coupling factor decreases with the increase of the elastic stiffness of the kerf filler.In addition,we have also compared the current rectangular rod model with previous cylinder model for 1-3 piezoelectric composites,it was found that the circular cross-section piezoelectric rod tends to provide higher electromechanical coupling factor than the square rod when the aspect ratio is between 0.3 and 4 in the 1-3composites with the same volume fraction,this could be very important for high frequency transducers for which the aspect ratio cannot be made very large.Then,“hard”epoxy（Epotek301）and“soft”epoxy（Stycast）were chosen as kerf fillers,and the 1-3 relaxor-based ferroelectric single crystal composites were made using PMN-0.26 PT and PMN-0.33 PT single crystal pillars embedded in a polymer matrix by modified dice and fill method.The two types of composites were poled at both room temperature and 70°C to exame the influence of poling procedure to the composite performance.Using the translucent nature of PMN-x PT single crystals,we have studied the stress distribution and domain configuration change during poling inside the crystal rods by using polarizing light microscopy（PLM）and piezoresponse force microscopy（PFM）.Besides,analytic stress distribution equations based on cylinder rod were modified to explain the physical principle and to predict the stress distribution for square rods case,which was verified by finite element simulations.Particular emphasis is placed on the domain configuration change in crystal-rods and the variation of interface stress between crystal and polymer with temperature.When the PMN-xPT single crystal composites are working under a high electric driving field,the induced heat and large displacement may cause mechanical and piezoelectric property degradation.To understand this point,the piezoelectric and dielectric performance degradation with temperature were investigated.Besides,combine the PLM and electric polarization fatigue measurements,the change of remanant polarization and coercive field for both single crystals and composites with fatigue cycles were studied and the results showed that the percentage change has the following trend:PMN-0.26 PT bulk single crystal>1-3 PMN-0.26PT/rubber composites>1-3 PMN-0.26PT/epoxy composites,i.e.,1-3 relaxor ferroelectric single cryatal composites are more fatigue-resistant compared to bulk single crystal.Finally,the fatigue mechanism of bulk single crystal and composite s are discussed.