【作者】 张立学；

【导师】 任晓兵； 金志浩；

【作者基本信息】 西安交通大学 ， 材料科学与工程， 2008， 博士

【摘要】 铁电晶体作为一类重要的电介质材料,具有较高的介电常数,显著的热释电和压电效应,广泛应用于从尖端技术到日常生活的多个领域。点缺陷（空位,掺杂等）在铁电晶体中的存在强烈的影响了其几乎所有的物理性质,并且与铁电体中许多未解决的问题,特别是铁电时效现象（指性能随时间发生变化）密切相关。一方面,时效现象的存在严重降低了铁电器件的可靠性与稳定性。另一方面,出现时效问题的机理还不能用已有的铁电理论解释。为了找出时效现象的根本原因,迄今为止人们提出了很多模型,但是仍然未有一个统一的观点。因此时效问题的研究对于铁电材料的理论研究和应用前景都有着重要的意义。最近,报道指出晶体中点缺陷具有短程有序对称性遵循的普适性原理（简称点缺陷对称性原理）。将该点缺陷对称性原理应用于铁电体中,能够合理的解释在时效的掺杂铁电单晶中通过可逆畴翻转实现巨大的可回复电致应变的试验结果。更为重要的是,这一原理为理解点缺陷在铁电时效中的作用提供了全新的思路。基于此,本文利用点缺陷对称性原理,系统地研究掺杂缺陷的钛酸钡（BaTiO₃）铁电体中的时效现象,以期给出铁电时效的统一微观解释。首先,基于点缺陷对称性原理,在时效的铁电单晶中已经获得巨大的可回复电致应变,这表明时效现象对铁电晶体的应用可以带来有益的影响。但是从工业角度出发,人们更关心铁电多晶材料,因为其容易批量生产。因此我们将此原理首次应用到工业中更为关心的多晶陶瓷材料中。实验表明,室温时效的1.0mol%Mn掺杂钛酸锶钡(Ba_0.95Sr_0.05TiO₃)多晶陶瓷材料,在3.0kV/mm电场下可以获得0.12-0.15%的大应变,与传统的压电陶瓷Pb（Zr, Ti）O₃ （PZT）所产生的压电效应应变相当。此结果表明基于点缺陷对称性原理可以在时效的环境友好（无铅）铁电陶瓷材料中产生可回复电致应变效应,这为此新的应变效应进入实际应用迈出了关键的一步。其次,时效铁电体中的巨大可回复电致应变是基于点缺陷对称性原理所带来的可逆畴翻转机制产生的。实验上宏观的可回复应变已经在单晶及多晶样品中都观察到,但是,导致巨大电致应变效应的可逆畴翻转过程本身还未得到直接证明。因此本文采用原位畴观察实验对时效的Mn掺杂BaTiO₃单晶样品在电场下的畴演变过程进行观察,并且把介观的畴翻转行为和宏观的铁电体的极化转向及巨大电致应变之间相互关系结合起来。结果表明:时效过的多畴样品,加电场翻转为单畴,但是电场去除后又能回到原来多畴状态,此现象对应于宏观的可回复应变;这一实验结果为可逆畴翻转机制提供了最直接的介观证据。再次,时效现象及其相关的可回复电致应变和可逆畴翻转等多尺度现象都由基于点缺陷对称性原理的模型给出了很好的解释。但是历史上,有关时效现象的解释存在很多模型,并未有一个统一的观点。主要争论点在于时效现象究竟是畴壁效应还是畴体积效应。为了解决这一问题,我们制备了单畴的Mn掺杂BaTiO₃单晶,并研究其时效现象。实验表明掺杂BaTiO₃单晶的单畴样品也有时效效应,单畴状态在电场循环中被稳定。这一结果充分地支持了体效应是时效产生的最基本原因。并且体效应实际来源于普适的点缺陷对称性原理。采用点缺陷对称性原理的微观模型,可以解释所有已知的铁电时效现象,包括大信号电滞回线及小信号介电压电的时效（由于点缺陷效应使得畴壁在小电场下的移动性降低）,从而表明铁电时效存在统一微观解释。最后,基于点缺陷对称性原理的时效模型可以解释时效效应因掺杂位置和掺杂种类不同而不同的现象。一方面,我们实验表明A位K离子受主掺杂BaTiO₃单晶的时效弱于B位Mn离子掺杂的样品。这一现象,在已有的文献中并未报道过。另一方面,时效现象只在受主掺杂晶体中存在,但在La或Nb离子等施主掺杂的BaTiO₃晶体中几乎没有时效现象。此已知现象也没有很好的微观解释。基于点缺陷对称性原理的半定量化时效模型可以对此作出解释:A位受主掺杂带来的较小的点缺陷对称性遵循驱动力决定了它的弱时效效应,而施主掺杂带来的点缺陷对称性遵循力太弱,不能克服阳离子空位缺陷短程扩散所受到的阻力,因而没有时效现象。这些解释表明可以从理论上判断不同体系时效的强弱有无,对于跟时效有关的实际应用具有重要的指导意义。以上研究表明,基于点缺陷对称性原理,在无铅铁电材料,特别是陶瓷材料中产生大的可回复应变,从而表明实际的工业应用潜能;并且铁电时效问题也得到了统一的清晰微观解释,这加深了对缺陷物理的理解从而为铁电晶体中其他点缺陷有关的问题提供了解决思路。更多还原

【Abstract】 Ferroelectric materials have been widely applied in industrial fields as capacitors, sensors, and transducers, etc. due to their functional properties. Despite their usefulness, there have been a number of long-standing unsolved problems to be solved. Most of these problems are related to the point defects, which are inevitably introduced either intentionally as dopants or unintentionally as impurities. Among these problems, the ferroelectric aging effect （the gradual change of properties with time） is a well observed but a puzzling phenomenon in ferroelectrics. Aging strongly affects the stability and reliability of ferroelectric devices; however, it can not be understood by conventional theory. To explore the origin of aging, historically, a number of models had been proposed. Unfortunately, a unified microscopic explanation remains unclear. Thus the study of ferroelectric aging is an important issue both for applications and for a fundamental understanding.Recently, a general symmetry-conforming principle for point defects （defect symmetry principle for short） has been proposed in ferroelectrics. Research work based on this principle has generated exiting discoveries （large recoverable electrostrain） in aged ferroelectrics; and more importantly, it shows potential to microscopically understand the ferroelectric aging. Therefore, in the present work, we utilize such a principle to systematically investigate the aging in doped BaTiO₃ ferroelectrics, aiming to give a unified microscopic explanation on ferroelectric aging.Firstly, based on defect symmetry principle, large recoverable electrostrain had been achieved in aged ferroelectric single crystals, which indicating aging is not always a detrimental effect; it may be even“useful”for applications. However, in practical viewpoint, poly-crystals/ceramics are suitable for mass production. So we investigated aging-induced strain effect in ceramic samples. It is found that, after aging 1.0mol% Mn-doped (Ba_0.95Sr_0.05)TiO₃ ceramics can generate a large recoverable electrostrain of about 0.12-0.15% , which is comparable to good piezoelectric Pb（Zr,Ti）O₃ （PZT） ceramics. This result demonstrates the potential of our approach in achieving large recoverable electrostrain in environmental-friendly （Pb-free） ceramics and thus is a crucial step towards the real application of this novel strain effect.Secondly, the aging-induced recoverable strain effect comes from reversible domain switching mechanism based on defect symmetry principle. Experimentally the macroscopic strain effect has been obtained in aged ferroelectric single and poly crystals. However, the reversible domain-switching process itself is yet to be directly verified. So we performed in-situ domain observation for an aged Mn-doped BaTiO₃ single crystal and simultaneously measured its polarization （P）-field （E） hysteresis loop. Besides, electrostrain behavior of the sample was also characterized at the same time. It was found that the aged sample shows a remarkable reversible domain switching during electric field cycling; it corresponds well to the giant recoverable electrostrain effect. This provides direct mesoscopic evidence for defect-mediated reversible domain-switching mechanism.Thirdly, aging and its related multi-scale phenomena had been well explained by our aging model based on defect symmetry principle . However, historically there exist several other aging models without consensus. The main controversy comes from whether aging is due to domain wall pinning effect or due to a volume effect. To solve this problem, we made a single-domain （domain-wall-free） Mn-doped BaTiO₃ single crystal and found that aging effect exists even in the single-domain sample: the single domain state was stabilized during electric field cycling. This direct evidence strongly suggests that volume effect is the governing mechanism for the ferroelectric aging. Furthermore, the microscopic origin of volume effect is provided by defect symmetry principle. Such an aging model explains both the aging in hysteresis loop （large signal properties） and the aging in dielectric and piezoelectric constants （small signal properties）, thus proving a unified microscopic explanation for all ferroelectric aging.Finally, the unified microscopic aging model based on defect symmetry principle can clearly explain the drastic change in aging effect dependent on doping site and doping type. Our experimental results showed that A-site K acceptor doped BaTiO₃ crystal has a weaker aging compared with B-site Mn acceptor doped one. Such a phenomenon is yet to be reported so far. Besides, it is well known that aging exists only in acceptor doped BaTiO₃ crystal, while La or Nb donor doped BaTiO₃ crystal has no aging effect. A microscopic explanation on the non-existence of aging in donor-doped case has been missing so far, but our work provides a simple explanation. According to our semi-quantitative aging model, the weaker aging in A-site acceptor doped sample is due to the small symmetry-conforming force of A-site related defects, while donor doped sample has no aging effect is because symmetry-conforming force of donor related defects is too small to overcome the large migration barrier of defects. This indicates the necessary conditions of aging, which can be a guideline for how to reduce or enhance the aging effect. This guideline may be important in the application of ferroelectric materials.In conclusion, based on the defect symmetry principle, large recoverable electrostrain has been obtained in environment-friendly ferroelectrics, especially in economic ceramics, thus being important to applications; furthermore, ferroelectric aging is well explained by a unified microscopic model, which have been verified with a number of critical experiments. The microscopic understanding of the aging may provide insights into other defect-related problems in ferroelectrics.更多还原