【作者】 彭珏；

【导师】 陈敬中； 罗豪甦；

【作者基本信息】 中国地质大学 ， 矿物学、岩石学、矿床学， 2005， 博士

【摘要】 医用超声换能器中应用较多的压电材料长期以来一直是压电陶瓷。但是压电陶瓷是多晶体，使用频率受到限制，由于陶瓷抗拉强度低，本身性脆，具有一定时间的老化性，其参数随时间的增加和环境温度改变时变化显著，甚至影响正常使用。近年来新发现的弛豫铁电单晶压电材料（1-x）Pb(Mg_1／3Nb_2／3)O₃-xPbTiO₃（简称PMN-PT）和（1-x）Pb(Zn_1／3Nb_2／3)O₃-xPbTiO₃（简称PZN-PT），其中x为PbTiO₃的摩尔分数，具有比锆钛酸铅(Pb(Zr_1-xTi_x)O₃，简称PZT)系压电陶瓷更高的机电耦合系数(k₃₃＞92％)和压电系数d₃₃（～2000pC／N），介电损耗因子tan δ仅为PZT-5H陶瓷的1／3。压电单晶的介电损耗比较小，机电耦合系数大，频带宽等优异性能，使其在医用超声成像，尤其是新的谐波成像或多频成像技术中有着非常诱人的应用前景。在目前大尺寸高质量PMN-PT单晶已经成功制备，晶片尺寸及性能的一致性已经满足多种医用超声换能器的应用需要的基础上，需要充分挖掘并发挥PMN-PT单晶优异的压电性能并实现其在原型器件中的应用。 本论文主要对（1-x）PMN-xPT单晶的两个组分，即x=0.30、0.31进行了系统的性能研究。系统表征了沿<001>_cub方向极化的0.70PMN-0.30PT单晶的弹性、介电、压电及声学性能，并分析单晶性能的温度稳定性。由于三方相0.70PMN-0.30PT单晶沿<001>_cub方向极化后，极性畴由原来的8个等效的<111>_cub方向变为靠近外场的4个等效<111>_cub方向，从而形成工程化畴结构，宏观上呈现4mm对称。因此这里表征的Z切0.70PMN-0.30PT单晶均按照4mm点群处理，选择五种切型的单晶，表征了11个独立的材料参数，包括3个压电常量，6个弹性常量及2个介电常数，这将有助于其应用方案设计、元件挑选、性能研究以及器件制备时的模拟分析。<001>_cub方向极化后的0.70PMN-0.30PT单晶，机电耦合系数k₃₃值～92％，k₃₁～89.5％，k₃₁<110>～76％及k_t～62％，均高于PZT系压电陶瓷，具有很好的应用潜力。比较了<001>_cub极化的PMN-PT单晶材料性能的各向异性，发现当组分偏离准同型相界（MPB）时，弛豫铁电单晶的弹性刚度系数的变化并不明显，然而弹性顺度系数变化明显，而且优异的压电性能是与大的s₃₃值相关的，组分的不同仅对S₁₃^E／S₁₂^E的影响比较明显。 对PMN-PT单晶压电性能温度稳定性的研究表明，实际使用的最高温度受升温过程中的第一个结构转变温度限制，而PT含量越高（即T_m越高）铁电相变T_rt越低。所以器件研究中要求既保证单晶较高的压电性能，又需要尽可能高的温度使用范围，PT组分控制在0.28～0.31之间，都可以获得稳定而优异的压电性能，适合医用换能器制作应用。 采用谐振-反谐振法系统研究了PMN-PT单晶的厚度剪切模式压电性能的晶体取向依更多还原

【Abstract】 The piezoelectric materials most commonly used in transducers for medical imaging are soft piezoelectric ceramics such PZT-5A and PZT-5H. However they are polycrystalline ceramics, brittleness and temporal aging, the operating frequency is finite and some parameters are variable with the going of time and changing of environmental temperature which will impede their regular operation. Recently, relaxor ferroelectric single crystals of （1-x）Pb(Mg_1/3Nb_2/3)O₃-xPbTiO₃ （hereafter as PMN-PT） and（1-x）Pb(Zn_1/3Nb_2/3)O₃-xPbTiO₃（hereafter as PZN-PT） are provided with superior piezoelectric properties than Pb(Zr_1-xTi_x)O₃ （hereafter as PZT） piezoelectric ceramics. These single crystals have ultrahigh electromechanical coupling factor in the longitudinal mode (k₃₃>92%) and piezoelectric constant (d₃₃-2000pC/N), but lower dielectric loss tan8 is 1/3 of PZT-5H.Such excellent performances make single crystals promising candidates for medical imaging, especially in the developing harmonic imaging and multi-frequency imaging technique. It is critical to discover the outstanding properties of PMN-PT single crystals sufficiently and realize the application in the medical ultrasonic transducer based on grown successfully in batched with high quality, large size and consistent performances.In this dissertation, as grown （1-x） PMN-xPT single crystals were prepared with composition x is 0.30, 0.31 respectively, for a detail study. The investigation of elastic, dielectric piezoelectric, and acoustic performances in <001>-poled 0.70Pb (Mg_1/3Nb_2/3)O₃-0.30PbTiO₃ （0.70PMN-0.30PT） single crystals using a resonance technique were present, and the temperature stabilities also were analyzed. With poling the single crystals with rhombohedral symmetry along [001] of parent cubic axis, eight equivalent spontaneous polarization <111>_cub directions turn to four <11 l>_cub directions, which make them into desired multidomain structures with the effective tetragonal 4mm symmetry. The complete set of constants was determined for this <001>-poled domain engineered system by the 4mm symmetry. There are total of 11 independent physical constants describing the elasto-piezo-dielectric matrices for this symmetry.So there are six elastic constants( s_ij^E ,s_ij^D ,c_ij^D ,c_ij^D , ij=11, 12, 13, 33, 44, 66), three piezoelectric constants( e_mi , d_mi , g_mi , h_mi , mi = 15, 31, 33), and two dielectric更多还原