【作者】 刘宇锋；

【导师】 凌志远；

【作者基本信息】 华南理工大学 ， 微电子学与固体电子学， 2018， 博士

【摘要】 弛豫铁电体从发现至今已有半个多世纪,由于具备优异的介电、压电和热释电等特性使得其在换能器、精密定位、储能、制冷等应用上具有广阔的应用前景,并获得了广泛的研究。然而,在纳米畴提高压电性的机理、弛豫铁电体的微观结构特征等问题方面依然存在许多疑问和争议。本论文以PLZT为研究对象,采用固相反应法合成了Sb₂O₅掺杂的(Pb_1-xLa_x)(Zr_yTi_1-y)_1-x/4O₃（其中x=5 mol%⁸ mol%,y=56 mol%⁶0 mol%）压电陶瓷。室温介电和压电特性表征发现,在La>6 mol%、Zr/Ti>58/42的区域内,大部分组成都具有优异的介电和压电性能(d₃₃>700 pC/N,ε_r>5000)。最优的综合性能在6.5/59/41处取得,此时ε_r=6076,tanδ=0.025,d₃₃=821 pC/N,k_p=0.70和Q_m=40。为了探究其强压电性的机理,本论文在上述实验基础上,固定Zr/Ti摩尔比为59/41,详细研究了La含量从0 mol%增大至6.5 mol%时,PLZT的相结构、介电和压电性能,以及弛豫特性的变化。研究表明,随着La含量的增加,PLZT的相结构依次经历三方相到四方相、四方相到伪立方相的转变,PLZT逐渐由正常铁电体转变为弛豫铁电体。当La≥6 mol%时,样品极化后依然呈现出明显的弛豫特性。与此对应的是,此时ε_r、d₃₃和k_p都快速增大,Q_m则快速减小。由双极性电场下的电滞回线表征发现,当La≥6 mol%时,在2 kV/mm下的剩余极化强度P_r与最大极化强度P_max的比值P_r/P_max急剧减小;所有极化样品在单极性电场下的极化响应都存在非线性区（低电场）和线性区（高电场）,但当La≥6 mol%时,样品在更低的电场下表现出更显著的非线性响应,且这些非线性响应是一个可逆的过程。这些都表明La≥6 mol%样品的电畴处于不稳定的状态,施加很小的电场即可使极化强度矢量发生旋转或相结构发生变化,因而使得样品具有很强的介电和压电响应。而在极化样品中依然存在的纳米畴是这一不稳定性的来源。弛豫铁电体和正常铁电体的去极化过程一般被认为是不同的,弛豫铁电体的去极化是长程有序性破缺形成短程有序的过程,而正常铁电体中则涉及铁电相到顺电相的转变。本论文中采用谐振特性温谱和介电温谱详细研究了PLZT从正铁电体转变为弛豫铁电体过程中去极化温度的演变历程。研究表明,所有样品的谐振频率f_r和反谐振频率f_a随着温度升高都存在一个最小值,在f_a最小值对应的温度T_min-a处,（35）f/f_r（（35）f=f_r-f_a）和（35）|Z|均趋于0,表明样品在T_min-a处发生了去极化。结合介电温谱发现,介电损耗峰温度T_loss与T_min-a吻合的较好。在La=6.5 mol%样品中,T_loss与介电异常峰温度T_d*不重合,说明T_d*不是去极化温度。T_d*的出现可能与畴的解耦有关,而T_loss则对应于宏观极化强度的消失,表明样品的去极化过程包含两个阶段:畴的解耦和畴取向的无序化。这种去极化过程存在于所有样品中。随着La含量增大,T_loss逐渐偏离介电峰值温度T_m,且即使在La=0 mol%样品中,T_loss≠T_m,说明所有极化样品去极化后,至少在T_loss^T_m温区内,其极化强度矢量（微区）处于无序的状态。极化前后的介电损耗温谱在T_loss处存在相似的异常变化,且在T_loss以上,极化前后的损耗温谱重合,这进一步证实了在T_loss处去极化的发生,也说明极化前的样品在T_loss处也会发生畴构型的转变。结合以上实验结果可推知,正常铁电体和弛豫铁电体在去极化和冻结过程方面并未有本质上的差别,正常铁电体转变为弛豫铁电体是一个渐进的过程,而T_m和T_loss间的温差（35）T可作为样品弛豫程度的表征参数。为了提高样品的压电性,在前述所有实验样品中均添加了1 wt%的Sb₂O₅。然而目前Sb₂O₅对x/59/41弛豫特性和压电性的作用机理依然不清楚。本论文中详细研究了La₂O₃和Sb₂O₅共掺杂对PZT的相结构、弛豫特性和电性能的影响。研究发现,随着Sb₂O₅含量的增加,x/59/41的相结构均往伪立方相转变。Sb₂O₅在x/59/41中的固溶度随着La含量的增加而减小。Sb₂O₅的掺杂使得PZT介电峰展宽,但样品无明显弛豫特征;随着La含量的增加,Sb₂O₅的掺杂可使PLZT呈现出显著的弛豫特性。当La（28）2 mol%时,极化后样品的频率色散现象消失;当La=5 mol%,Sb₂O₅≥1.2 wt%时,极化样品的介电温谱中出现介电异常峰,但在介电异常峰温度以下无频率色散现象;而当La=6.5 mol%时,极化样品在介电异常峰以下仍然呈现出明显的频率色散现象。在电性能方面,总体而言,La含量的影响远大于Sb₂O₅掺杂量的影响,随着La含量的增加,ε_r、d₃₃和kp都快速增大,Q_m和P_r/P_max则快速减小。当La≤2 mol%时,ε_r、d₃₃、k_p、Q_m和P_r/P_max均随着Sb₂O₅掺杂量的增加而变化不大;而在La>2 mol%样品中,当极化后样品弛豫程度明显增强时,ε_r和d₃₃随着Sb₂O₅掺杂量的增加而较快的增大。当出现烧绿石相时,样品的弛豫程度、介电和压电特性均明显减弱。更多还原

【Abstract】 Relaxor has been found for over half a century.Because of excellent dielectric,piezoelectric and pyroelectric properties,relaxor possesses potential applications in transducer,precision positioning,energy storage,refrigeration,etc.and has been studied widely.However,there still exist some queries and controversies in relaxor,such as the contribution of nanodomains to piezoelectricity,the underlying structure of relaxor,and so on.In present thesis,Sb₂O₅ modified(Pb_1-x-x La_x)(Zr_y Ti_1-y)_1-x/4O₃（where x=5 mol%⁸ mol%,y=56 mol%⁶0 mol%）ceramics were synthesized by solid state reaction method.It is found that most of compositions with La>6 mol%and Zr/Ti>58/42 possess excellent dielectric and piezoelectric properties(d₃₃>700 pC/N,ε_r>5000).The optimal properties withε_r=6076,tanδ=0.025,d₃₃=821 pC/N,k_p=0.70 and Q_m=40 is obtained in 6.5/59/41.In order to make clear the mechanism of the strong piezoelectricity of these compositions,the crystal structures,electrical properties and relaxation behaviors of x/59/41 were studied as a function of La content from 0 mol%to 6.5 mol%.It is found that with increasing La content,the crystal structures of x/59/41 transform from rhombohedral to tetragonal and then to cubic phases.Meanwhile,x/59/41 undergoes a transition from normal to relaxor ferroelectrics.The obvious relaxation behavior is presented in poled samples with La≥6 mol%.Correspondingly,ε_r,d₃₃and k_p increase rapidly while Q_m decreases sharply when La≥6 mol%.The hysteresis loops show that when La≥6 mol%,the ratio of remanent polarization P_r to maximum polarization P_max under an electric field of 2 kV/mm decreases rapidly.And for all poled samples,there exists nonlinear（low electric field）and linear（high electric field）regions in the responses of polarization to unipolar electric field.The most important thing is that the nonlinear responses in the samples with La≥6 mol%are much more obvious and occur at a much lower electric field than that in low La content samples.Furthermore,these nonlinear responses are reversible.These results indicate that the domains of La≥6 mol%samples are in an unstable state.A much small electric field can trigger rotation of polarization vectors or change of crystal structure,resulting in a strong dielectric and piezoelectric responses.The nanodomains existing in poled samples are exactly the source of the unstability.Generally,it is though that depolarization processes in normal and relaxor ferroelectrics are different.The depolarization process in relaxor is relative to the breaking up of long-range polar ordering and formation of short-range polar ordering.But,in normal ferroelectric,this process is connected with phase transition between ferroelectric and paraelectric phases.In present thesis,temperature dependent resonant and dielectric properties were used to study the evolution of depolarization temperature of PLZT from normal to ralxor ferroelectrics.It is found that with increasing temperature,resonant frequencies f_r and anti-resonant frequencies f_a of all samples exist a minimum value.（35）f/f_r（（35）f/f_r=f_r-f_a）and（35）|Z|are close to 0 at the temperature T_min-a corresponding to the minimum value of f_a,indicating that depolarization occurs at T_min-a.Importantly,dielectric loss peak temperature T_loss is much in accord with T_min-a.In La=6.5 mol%sample,T_loss is not equal to dielectric anomalous peak temperature T_d*,suggesting that T_d*is not a depolarization temperature.The appearance of T_d*probably is related to the decoupling of domains while T_loss corresponds to the loss of macro-polarization.It implies that the depolarization process in La=6.5 ml%sample contains two steps,i.e.decoupling and disorientation of domains.These two steps may exist in all samples.With increasing La content,T_loss deviates from T_m gradually.It is noted that T_loss is not equal toT_m even in La=0 mol%sample,indicating that after depolarization,polarization vectors are disordered in all samples at least at T_loss^T_m.Similar anomalous changes in dielectric loss of unpoled and poled samples are found at T_loss,suggesting that domain configuration changes at T_loss in unpoled samples either.Above T_loss,the dielectric loss of unpoled sample is identical to the value of poled sample,implying that depolarization occurs at T_loss.Based on these results,it can be deduced that there is no essential difference between normal and relaxor ferroelectrics in depolarization and freezing processes.The transition from normal to relaxor ferroelectrics is a gradual process and the temperature difference（35）T between T_m and T_loss can be a parameter to describe the relaxation degree.In order to improve piezoelectricity of samples,1 wt%Sb₂O₅ were added into samples in above experiment.However,it is still unclear about the effect of Sb₂O₅ on the relaxation behavior and piezoelectricity of x/59/41.In present thesis,these effects were studied in detail.It is found that with increasing Sb₂O₅ content,the crystal structures of x/59/41 transform into pseudo-cubic phase.The solid solubility of Sb₂O₅ in x/59/41 decreases with increasing La content.The dielectric peak of PZT become diffuse with the effect of Sb₂O₅,but no frequency dispersion can be observed.With increasing La content,the addition of Sb₂O₅ can induce obvious relaxation behavior in PLZT.After poled,the frequency dispersion disappears in La=2 mol%samples.When La=5 mol%and Sb₂O₅≥1.2 wt%,anomalous dielectric peak can be observed in the dielectric spectra of poled samples,but no frequency dispersion is presented below anomalous peak temperature.However,when La=6.5 mol%,strong frequency dispersion maintains after the samples are poled.La content shows greater influence on electrical properties than Sb₂O₅ content.With increasing La content,ε_r,d₃₃ and kp increase rapidly while Q_m and P_r/P_max decrease fastly.When La≤2 mol%,ε_r,d₃₃,k_p,Q_m and P_r/P_max change a little with varying Sb₂O₅ content.In La>2 mol%samples,ε_r and d₃₃ increase fastly with increasing Sb₂O₅ content,if the relaxation behavior of samples become more obvious.The relaxation behavior,dielectric and piezoelectric properties degenerate obviously when pyrochlore phase appears in samples.更多还原