Experimental crystallization of gallium: ultrasonic measurements of elastic anisotropy and implications for the inner core
Experimental crystallization of gallium: ultrasonic measurements of elastic anisotropy and implications for the inner core
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DOI：
10.1016/s0031-9201(01)00298-9
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发表时间：
2002-02-25
影响因子：
2.3
通讯作者：
Olson, P
中科院分区：
文献类型：
--
作者：
Brito, D;Elbert, D;Olson, P
我们提出了超声波测量的弹性各向异性镓进行定向凝固中存在的强加的热梯度，旋转，对流，湍流，和磁场。同时在现场测量的温度和压缩波的速度被用来跟踪在凝固过程中的结晶前沿。我们发现，个别凝固镓样品总是多晶和弹性各向异性，与晶粒在凝固方向拉长。在单个固体样品中测得的压缩波各向异性范围为单晶值的20%至80%，这取决于实验条件。我们还发现弹性各向异性的量随单个样品中的位置而变化。基于在相似环境条件下进行的多个实验的系综平均值，我们发现固体中的弹性各向异性的方向对热梯度方向敏感，而各向异性的量对熔体中初始成核的存在或不存在最敏感。显示平均各向异性的实验具有与重力和热梯度对准的超声快轴。强各向异性固体的结果时，成核晶粒存在于初始熔体中，而较小的或零平均各向异性的结果时，成核晶粒最初不存在。我们已经研究过的其他外部因素，如湍流和磁场，要么没有可测量的影响，要么倾向于减少固体的各向异性。我们的结果表明，在地球内核结晶过程中，新形成的固体中平均各向异性的取向主要由径向凝固控制，而各向异性的大小可能受到预先存在的内核纹理的影响。(C)2002 Elsevier Science B.V保留所有权利。
We present ultrasonic measurements of elastic anisotropy in gallium undergoing directional solidification in the presence of imposed thermal gradients, rotation, convection, turbulence, and magnetic fields. Simultaneous in situ measurements of temperature and compressional wave speed are used to track the crystallization front during solidification. We find that individual solidified gallium samples are always polycrystalline and elastically anisotropic, with grains elongated in the solidification direction. The measured compressional wave anisotropy in individual solid samples ranges from 20 to 80% of the single crystal values, depending on experimental conditions. We also find the amount of elastic anisotropy varies with position in an individual sample. Based on ensemble averages from multiple experiments made under similar environmental conditions, we find the direction of elastic anisotropy in the solid is sensitive to the thermal: gradient direction, while the amount of anisotropy is most sensitive to the presence or absence of initial nucleation in the melt. Experiments that show average anisotropy have the ultrasonically fast axis aligned with gravity and the thermal gradient. Strongly anisotropic solids result when nucleation grains are present in the initial melt, whereas smaller or zero average anisotropy results when nucleation grains are initially absent. Other externally imposed factors we have examined, such as turbulence and magnetic fields, have either no measurable influence or tend to reduce the amount of anisotropy of the solid. Our results suggest that during crystallization of Earth's inner core, the orientation of average anisotropy in the newly formed solid is controlled primarily by radial solidification, while the amount of anisotropy may be influenced by pre-existing inner core texture. (C) 2002 Elsevier Science B.V All rights reserved.