The influence of transformation twins on the seismic-frequency elastic and anelastic properties of perovskite:: dynamical mechanical analysis of single crystal LaAlO3
The influence of transformation twins on the seismic-frequency elastic and anelastic properties of perovskite:: dynamical mechanical analysis of single crystal LaAlO3
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DOI：
10.1016/s0031-9201(02)00190-5
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发表时间：
2002-12-22
影响因子：
2.3
通讯作者：
Redfern, SA
中科院分区：
文献类型：
--
作者：
Harrison, RJ;Redfern, SA
用动态力学分析技术研究了LaAlO_3单晶的低频力学性能随温度、频率和外加力的变化规律。LaAlO_3在550℃以下发生立方-菱方相变。低温三方相的力学响应主要是相变孪晶的粘性运动，导致储能模比高温立方相降低10倍(超弹性软化)，衰减显著增加。观察到200℃以下的超弹性软化，低于此温度，磁畴壁的迁移率显著降低，导致储能模数迅速增加，并出现显著的衰减峰值(磁畴壁冻结)。在冻结温度附近，储能随频率的变化可用修正的Burgers型模型精确描述，活化能服从正态分布，平均值分别为84.1(1)kJ/mol和S.D.10.3(L)kJ/mol。这一激活能表明，磁化壁钉扎主要是由氧真空引起的。对动态力-挠度曲线的详细分析揭示了三种不同的机械响应区域。在弹性状态下，域壁被钉扎，不能移动。弹性响应与斜率呈线性关系，斜率由晶格的固有刚度、钉扎势的初始磁化率和钉扎点之间孪生壁的弯曲决定。在超弹性状态下，磁化壁解锁和位移的大小由施加的力和恢复力之间的平衡决定。表观超弹性模量值与自发应变无关，因此与温度无关。在较高的外力值下，相邻的域壁相互接触，防止进一步的超弹性变形(饱和)。饱和区域的应变与自发应变成比例关系，由此产生的弹性模量与温度密切相关。讨论了磁化壁运动对矿物地震性质的可能影响。如果将这些结果直接转移到形成地幔的(Mg，Fe)(Si，Al)O-3钙钛矿上，典型地震波的应变幅度将足以引起超弹性软化。然而，氧空位对磁畴壁的钉扎导致了在地幔温度下非常短的驰豫时间。如果换算成(Mg，Fe)(Si，Al)O-3，它们将太短，不能达到显著的地震衰减。然而，由于缺陷、杂质和真实地幔钙钛矿中的晶界对铁弹性磁畴壁的钉扎增加，或者氧空位扩散的显著正激活体积，将足以将弛豫时间增加到导致地震波衰减的值。(C)2002 Elsevier Science B.V.保留所有权利。
The low-frequency mechanical properties of single crystal LaAlO3 have been investigated as a function of temperature, frequency and applied force using the technique of dynamical mechanical analysis (DMA) in three-point bend geometry. LaAlO3 undergoes a cubic to rhombohedral phase transition below 550degreesC. The mechanical response in the low-temperature rhombohedral phase is shown to be dominated by the viscous motion of transformation twin domain walls, resulting in a factor of 10 decrease in the storage modulus relative to the high-temperature cubic phase (super-elastic softening) and a significant increase in attenuation. Super-elastic softening is observed down to 200degreesC, below which the mobility of the domain walls decreases markedly, causing a rapid increase in storage modulus and a pronounced peak in attenuation (domain wall freezing). The frequency dependence of the storage modulus close to the freezing temperature is accurately described by a modified Burgers model with a Gaussian distribution of activation energies with mean value 84.1 (1) kJ/mol and S.D. 10.3(l) kJ/mol. This activation energy suggests that domain walls are pinned predominantly by oxygen vacancies.Detailed analysis of the dynamic force-deflection curves reveals three distinct regimes of mechanical response. In the elastic regime, the domain walls are pinned and unable to move. The elastic response is linear with a slope determined by the intrinsic stiffness of the lattice, the initial susceptibility of the pinning potential and the bending of twin walls between the pinning sites. In the super-elastic regime, the domain walls unpin and displace by an amount determined by the balance between the applied and restoring forces. The value of the apparent super-elastic modulus is shown to be independent of the spontaneous strain and hence independent of temperature. At high values of the applied force, adjacent domain walls come into contact with each other and prevent further super-elastic deformation (saturation). The strain in the saturation regime scales with the spontaneous strain and the resulting modulus is strongly temperature dependent.The possible effects of domain wall motion on the seismic properties of minerals are discussed. It is concluded that, if these results are directly transferred to mantle-forming (Mg, Fe)(Si, AI)O-3 perovskite, the strain amplitude of a typical seismic wave would be sufficient to cause super-elastic softening. However, pinning of domain walls by oxygen vacancies leads to very short relaxation times at mantle temperatures. If translated to (Mg, Fe)(Si, Al)O-3, these would be too short to amount to significant seismic attenuation. Increased pinning of ferroelastic domain walls by defects, impurities and grain boundaries in real mantle perovskite, or a significant positive activation volume for oxygen vacancy diffusion, would be sufficient to increase the relaxation time to values resulting in seismic wave attenuation, however. (C) 2002 Elsevier Science B.V. All rights reserved.