2) 同样为了满足Bi3+的成键要求, 氧八面体绕a轴旋转, 这一旋转会因Bi(2)–O(2)键的作用而进一
图5 Bi3TiNbO9沿<100>方向的投影图[22]
Fig. 5 Projection of Bi3TiNbO9 along <100> direction [22]
步增强, 同时这也导致了O(2)的过价(over-bonding), 为了抵消这一作用氧八面体会在c方向被拉长, 最终导致c轴的变长。
BLSF是一种典型的位移型铁电体, 目前研究显示其自发极化主要来自三部分贡献: I) B位离子的移动; II) 氧八面体沿c轴倾侧; III) 氧八面体在a-b面内的旋转。
而各离子对极化的作用, 可以通过结构中每个离子相对顺电相的偏移量来计算, 公式如下[39-40]:
s
=()/
i i i i
P m x Q e V
×Δ×
∑(4)
其中m i是各位重复度,
i
x
Δ为原子相对于顺电相的偏移量, Q i e为第i种离子的电荷数, V是单胞体积。通过计算发现, 不同体系极化产生根源的不同, 如CaBi2Ta2O9结构氧八面体中的O(5)2-离子的偏移对自发极化的贡献很大; 而在SrBi2Ta2O9结构中, (Bi2O2)2+中的Bi3+也对极化有很大的贡献[40]。
2.2性能及其优化
BLSF虽然有许多优势, 但也存在着剩余极化小、压电活性低、矫顽场大以及高温介电损耗高等缺点, 使其应用受到一定的限制。为了提高其性能, 人们进行了大量探索并积累了丰富的经验。目前常用的方法包括掺杂改性、构筑共生结构以及织构化等。
2.2.1 掺杂改性
掺杂取代是BLSF最常用的改性方法之一。早期的研究表明, BLSF的居里温度(T c)与类钙钛矿层中A位离子有关: A位离子半径越小或电负性越大, 居里温度越高[41]。但压电活性受取代离子半径、电负性等多重因素影响, 关系较为复杂[42-43]。
A位掺杂在BLSF改性的报道近年来层出不穷, 镧系元素是最常用的掺杂取代离子[39, 44-60]。镧系元素可以不同程度地提高Bi4Ti3O12的铁电性, 并且对其疲劳特性的改善亦有明显效果[44-45]。一般研究认为是类钙钛矿层中A位的Bi3+被取代, 减少了高温
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下由于铋的挥发而产生氧空位的几率。另外, Chon 等[45]通过解析Nd3+掺杂的BiT的晶体结构, 发现Nd3+掺杂使Ti4+的偏移方向发生了变化, 即自发极化主要沿c轴分布。Yau等[61]通过Raman光谱在掺La3+的BiT中也得到了类似结论。
B位掺杂在m=2、3层的铋层状化合物中也得到广泛研究[62-63]。对未掺杂BiT研究表明其容易产生氧空位[64], 且为p型半导体[65], 其漏电流相对较大, 对极化不利。采用B位施主元素掺杂可以有效提高BiT电阻率[66-70], 从而提高其铁电压电性能。
对A、B位复合掺杂也有较多研究[71-75]。Takayuki等[74]通过La3+, Nd3+与V5+复合掺杂, 发现复合掺杂后的薄膜相对于未掺杂或者单一元素掺杂的薄膜具有更高的剩余极化。因此, 他们提出了位置工程(site engineering)的概念, 认为A位离子掺杂降低材料的居里温度至沉积温度以下, B位掺杂能解钉扎, 因此, A、B位复合掺杂可以有效提高薄膜的铁电性能。另外有人通过对铋层状陶瓷的A、B位同时掺杂取代, 得到了高压电活性和较高T c 的材料[71]。
2.2.2 构筑共生结构
如前所述, 不同层数的BLSF可以沿c轴交替排列形成一种超晶格结构, 利用这两种不同结构单元之间的相互作用, 能够提高BLSF的铁电性能[30-31,76-77]。如Noguchi等[76]首先报道了Bi4Ti3O12-SrBi4Ti4O15共生铁电陶瓷的剩余极化为15 μC/cm2, 高于在同样工艺条件下制备的Bi4Ti3O12和SrBi4Ti4O15。Kobayashi等[30-31]对比研究了BiT、BaBi4Ti4O15 (BaBT)和BiT-BaBT, 发现无论是陶瓷还是单晶, BiT-BaBT的剩余极化都要比BiT和BaBT大, 且单晶BiT-BaBT沿a(b)方向的剩余极化达到了52 μC/cm2。从结构出发, Yi等[77]对比分析了Bi3TiNbO9-Bi4Ti3O12、Bi3TiNbO9和Bi4Ti3O12结构中氧八面体的旋转方式, 发现沿a-c面相对c轴的倾角αx的增加对共生结构的铁电性有重要贡献, 虽然BTN-BiT的自发极化低于BiT, 但相对BTN大的自发极化特别是比BiT更容易进行的畴反转行为使得BTN-BiT陶瓷表现出比BTN和BiT要大的剩余极化。
2.2.3 织构化
受对称性的影响, BLSF的晶粒通常呈片状, 其电学性能亦具有极为明显的各向异性, 自发极化主要位于a-b面内。晶粒方向随机分布的陶瓷难以最大限度地利用其性能, 因此织构化技术被引入到BLSF的改性中。常采用的织构化方法可简单分为两类: 一类是基于热、力、电、磁场的热锻/压、SPS、磁定向等方法[78-83]; 一类是基于原料形状的模板法[84-89]。
Takenaka等[78-79]采用热压处理得到明显织构的BiT陶瓷, 其垂直于织构化方向的P r达37 μC/cm2, 远大于普通BiT陶瓷的9.3 μC/cm2。在热压基础上, Fuierer等[80]将电场引入到BiT陶瓷的织构化过程中, 发现在电场作用下陶瓷实现了三个方向的织构, 表现出类似单晶体的性能特征。Shen等[81]则巧妙地利用纳米陶瓷颗粒在脉冲电场/电流下超塑性变形这一特性, 采用SPS实现了BiT陶瓷的织构化, 其P r 达27 μC/cm2。除此之外, Chen等[82-83]对强磁场处理过的BiT陶瓷织构化行为进行了研究, 发现在灌浆成型时毛细管力的作用下, 样品上表面到下表面逐渐表现出不同的织构特性, 且这种影响随初始晶粒形状的变化而变化。
模板法制备织构化BLSF陶瓷也是一种常用方法。Duran等[86]以片状KSr2Nb5O15为模板, 采用流延–叠层的方法制备出织构化的Sr0.9Nd0.1Bi2Nb2O9陶瓷, 其P r达20.3 μC/cm2, d33最高达84 pC/N。Takeuchi等[84-85]则采用反应模板法, 以片状BiT为模板, 利用BiT与Bi2O3、CaCO3、TiO2和Na2CO3的反应获得了织构化的CaBi4Ti4O15和Na0.475Ca0.05Bi0.4475Ti4O15陶瓷, 对比普通陶瓷, 前者的d33增加了200%, 达到45 pC/N, 后者则增加了238%, 达到44 pC/N。
值得一提的是, BLSF在其它一些钙钛矿及类钙钛矿陶瓷的织构化中也有重要应用。2004年Saito 等[90]以片状NaNbO3为模板, 制备出d33达416 pC/N 的(K0.44Na0.52Li0.04)(Nb0.84Ta0.10Sb0.06)O3织构化陶瓷, 这一结果作为铌酸盐体系压电性能研究的一个重大突破, 一度吸引了大量学者关注[91]。在这一研究中, Bi2.5Na3.5Nb5O18(m=5)作为前驱体在熔盐法制备片状NaNbO3模板的过程中起到了不可取代的作用[92]。相关研究显示[93], 这一方法也适合于制备片状的Na0.5Bi0.5TiO3模板。此外, 直接以片状的铋层状粉体为反应模板也是常用的方法, Wu等[94]以BiT为模板制备出了织构化的(Na0.5Bi0.5)0.94Ba0.06TiO3陶瓷。
3 BLSF铁电薄膜
BLSF薄膜的研究最早始于上世纪70年代初, Takei等[95]采用二极反应溅射法分别在Pt和MgO衬底上制备出BiT薄膜。Wu和Sugibuchi等[96-97]先后将BiT薄膜应用于铁电场效应晶体管(Field Effect
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Transistor, FET), 在硅基片上制备出可循环读写105次而不表现出疲劳的FET, 并发现其具有低的开路电压以及优异的集成电路兼容性, 因而认为BiT在只读存储器领域有着重要的应用前景。然而由于种种原因, 这一结果在之后的20年间并没有得到足够的重视, 直到上世纪1995年, De Araujo等[98]在Nature期刊上发表了一篇关于非易失性铁电存储器的文章, 报道SrBi2Ta2O9(SBT)和BaBi2Ta2O9 (BBT)薄膜经1012次读写仍不表现疲劳, 同时其漏电流低于相同条件下PZT薄膜的百分之一, 这一结果从此在铋层状铁电薄膜乃至陶瓷领域中掀起了一股以非易失性铁电存储器为背景的研究热潮, 并取得了大量成果。
在材料体系上, 1999年Park等[44]首先报道了剩余极化达12 μC/cm2且经3×1010循环读写之后仍不出现疲劳的Bi3.25La0.75Ti3O12(BLT)薄膜。之后, 2002年同样是在BiT体系上, Chon等[45]通过Nd掺杂制备出P r达51.5 μC/cm2, 且经6.5×1010循环读写之后仍不出现疲劳的Bi3.25Nd0.75Ti3O12(BNT)薄膜, 从而为BLSF在非易失性铁电存储器领域中的应用奠定了基础。
在机制上, 如何解释BLSF这种优异的抗疲劳性从一开始就受到了广泛关注。Ding等[99-101]分别在抗疲劳性能优异的SBT和BLT晶粒中观察到了大量反相畴界(APB), 并通过原位观察发现这些反相畴有助于极化反转时在薄膜内部诱发新畴的形核, 能够有效地减轻电极附近区域的形核负担, 从而降低这一区域的过早疲劳, 因此认为APB的存在是铋层状铁电薄膜优异的抗疲劳性的根源。此外, Su等[102]发现90°畴密度较高的材料对应的疲劳性好, 他们认为90°畴在极化过程中会吸收电荷缺陷, 90°畴密度低的材料其畴壁很快被钉扎, 从而影响到抗疲劳性能。
薄膜制备上, BLSF晶粒通常为c轴取向或随机取向, 这既对薄膜的极化性能不利, 也对纳米尺度下记忆单元间极化特性的一致不利, 因此制备一致的非c轴取向, 特别是具有a(b)轴取向的薄膜显得尤为重要。虽然大量文献[102-107]报道了类似(118)、(104)取向的BLT薄膜和(116)、(103)取向的SBT薄膜, 但具有a(b)轴取向的薄膜则相对少见。Moon等[108]报道具有a(b)轴取向的SBT薄膜, 但这一薄膜并非生长在电极材料之上, 因此不具备实用性。此后, Lee等[109]以SrRuO3为底电极, 在Y 稳定的ZrO衬底上成功制备出了具有a(b)轴取向的BLT薄膜(图6), 从而攻克了这一难题。
4 BLSF的拓展性研究
在传统铁电压电性能研究的同时, 近年来在BLSF的研究中出现了一些与其铁电压电性相关的新的研究方向, 包括以多铁为背景的磁–电性能研究以及以太阳能电池为背景的铁电光伏特性研究。
多铁性研究主要集中在以Bi5Ti3FeO15 (BTF, m=4)结构为基的体系上, 由于Fe3+–O–Fe3+负的超交换作用, 因而BTF理论上表现出反铁磁性。Mao等[110]报道BTF陶瓷的P r=5.9 μC/cm2, 并发现其表现出弱的铁磁性, 认为结构中少量的Fe2+离子是导致这一现象的根本原因。利用Co3+–O–Fe3+间正的交互作用, Mao、Hu和Wang等分别[111-113]对Co掺杂BTF (BFCT) 陶瓷的磁–电性能进行了研究, 均发现Co 掺杂可以将剩余磁化强度提高3个数量级, 并能在一定范围内增加材料剩余极化。而Sun等[114]对Bi5Fe0.5Co0.5Ti3O15薄膜的研究显示, 其P r=15.8 μC/cm2, M r=2.6 emu/cm3, 且观察到了磁介电效应, 0.6 T/ 100 kHz下对应MDC=0.39%。在Co掺杂的基础上, Yang和Mao等分别[115-116]对稀土Nd、La掺杂BFCT(BNFCT, BLFCT)薄膜的磁–电性能进行了报道, BNFCT薄膜的M r=165 memu/g, Yang等[115]认为如此大的剩余磁化强度可能和小半径的Nd3+和Co3+
图6 c-轴取向Bi3.25La0.75Ti3O12(BLT)多晶薄膜AFM形貌照片(a)和BLT晶粒<010>方向高分辨像(b)[109] Fig. 6 AFM topography image of BLT film (a) and HRTEM image of the BLT grain along <010> direction (b) [109]
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离子的取代有关, 但仍需进一步研究。BLFCT薄膜的M r=25.6 memu/g, P r=7.7 μC/cm2, 且在室温下表现出明显的磁介电效应, 1 T/1 kHz下MDC=8.1%, 远远大于BFCT薄膜。从这些结果可以看出, BLSF 在多铁领域有着重要的应用前景, 但目前还处于起步阶段, 大量工作有待开展。
铁电光伏效应是光生电子–空穴在铁电体内电场作用下发生分离形成稳定光生电流或开路电压的现象, 这一现象在上世纪70年代被发现[118], 由于铁电体内电场比p-n节内建电场大一个数量级, 且作为一种特殊体效应其光伏电压比p-n结光伏电池高2~4个数量级, 达103~105 V/cm, 近年来受到广泛关注[119-123]。但受其较大禁带宽度的影响, 铁电体内部电子–空穴处于紧束缚状态很容易发生复合, 体电导很低, 光伏效应一直比较差, 因此设计具有窄禁带宽度的铁电体成为一个研究重点。近年来, BLSF在这方面的应用开始受到关注, Choi等[124]对LaTMO3(TM=Al、Ti、V、Cr、Mn、Co、Ni)系列掺杂Bi4Ti3O12薄膜的禁带宽度进行了研究, 发现LaCoO3掺杂产生显著作用, 能够将禁带宽度降低近1 eV。如图7显示[117], LaCoO3掺杂时La部分取代了 (Bi2O2)2+中一侧的Bi, 采用第一性原理计算发现禁带宽度的减小是Co填充O空位附近的B位后引起周围Bi离子6p轨道能态降低的结果, 而对薄膜光电导性能的测试也表明铋层状结构是一类值得进一步开发的光电子材料。
总体而言, 无论是多铁研究还是铁电光伏特性的研究本质上都需要克服相同的矛盾, 即磁性/体电导的提高以及铁电性的保留, 这就要求在材料设计中必须充分考虑到这一矛盾, BLSF作为这两个领域开发较晚的材料体系, 应该在充分利用其结构优势的同时引入其它体系中较为成熟的理论, 这样才能实现更合理的设计。
5总结与展望
本文简要介绍了BLSF的研究背景和研究内容, 从结构、性能及研究动态三个方面构建了BLSF的研究框架, 并详细分析了各方向的进展以及遗留问题。我们认为BLSF至少在以下几方面还有待进一步的研究:
1) 结构的认识和新结构的设计。结构是一切研究的基础, 随着研究的积累, 对铋层状结构的认识不断深入, 但其结构稳定性问题仍是一个核心问题。随着m值的增大, 铋层状结构特别是其共生结构的超晶格有望逐渐表现出纳米异质结的某些
图7 Bi4Ti3O12(BiT)、Bi4Ti3O12-2CoLaO3(1B2L)原子分辨率Z 衬度像以及1B2L局部EELS面扫(a)和第一性原理计算BiT 及Co掺杂BiT结构电子态密度(b)[117]
Fig. 7 Atomic resolution Z-contrast STEM images of BiT and 1B2L. The elemental maps of 1B2L for Ti and La visualized by EELS are also shown(a) and the caculated electronic density of states for BiT and Co-doped BiT structure(b)[117]
特点[12], 这在一定程度上促使了一些新结构的诞生, 但受结构稳定性的影响, 相关的设计并不十分顺利。同时, 有序–无序一直都是铋层状结构乃至其它共生体系中一个普遍的现象[34-35], 但却未得到很好的解释, 对这个问题的研究既有利于加深对结构或物相的认识, 也有利于新结构的设计。
2) 铁电薄膜关键问题的突破。虽然BLSF铁电薄膜在诸多方面表现出了相对优异的性能, 但仍存在一些问题和技术障碍, 包括较高的制备加工温度、仍需解决的疲劳和老化问题以及有待深入研究的相关物理问题等等[125]。
3) 性能裁剪。就结构而言, 铋层状结构特别是其共生结构有着天然的可裁剪性, 这对材料的多功能化有着重要的意义, 但这一特质到目前为止尚未得到很好利用。BLSF在多铁及铁电光伏特性中的应用可以说是不同应用领域交叉的产物, 从这两个
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研究方向面临的问题来说, 如何在借鉴成熟的理论方法的同时充分利用铋层状结构的优势(如可裁剪性)是一个值得考虑的问题; 而就方法而言, 将BLSF的铁电性进一步拓展到其它的领域中有可能获得意想不到的效果。
4) 第一性原理计算的应用。第一性原理计算是揭示组成–构–性能之间内在联系的一种有效方法, 可以应用于前述三个方面的研究, 帮助人们理解一些现象的本质[126], 甚至实现真正意义上的材料设计。由于BLSF十分复杂, 单胞原子数大多几十甚至远远超过一百, 计算量很大, 因此第一性原理在BLSF中的应用并不普遍。但随着信息技术的迅猛发展, 这种方法也越来越多的被用于BLSF的研究, 相信在不久的将来其必定能在BLSF的研究中发挥更加突出的作用。
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