定向凝固单晶冰的取向确定与选晶

基于六方冰晶偏振光学特性,定义了用于确定冰晶晶体取向的三个参数:光轴倾角α,消光角β和与冰晶基面(0001)面内晶体学择优方向1120与温度梯度的夹角γ,提出了定量判定冰晶晶体取向的理论基础,并在定向凝固平台上采用偏光显微镜成功实现了冰晶晶体取向的精确主动控制,获得了任意取向的单晶冰.本文成功解决了冰晶的定向凝固晶体取向确定和选择的难题,为冰晶生长过程中相关理论问题的研究提供了有效的途径.

Orientation determination and manipulation of single ice crystal via unidirectional solidification

The growth of ice crystal has been widely investigated by researchers from various fields, but efficient method that can meet the experimental requirements for identifying and reproducing the ice crystal with specific orientation is still lacking. In this paper, an ice crystal can be characterized with unique orientation information, where tilt angle of optical axis α, extinction angle β and the angle γ relative to preferred orientation 〈1120〉 in the basal plane (0001) and the direction of temperature gradient G are determined based on the properties of optic polarization of hexagonal ice in the directional solidification. An integrated criterion for determining the orientation of hexagonal ice is proposed by combining the crystal optics and solidification interface morphology. Precise manipulation of the orientation of single ice crystal is achieved by using a step-by-step method via a unidirectional platform combined with a polarized optical microscope. Three coordinate systems are established to achieve the manipulation of ice. They are the microscope coordinate system termed as “A-P-L”, where A, P and L refer to the directions of analyzer, polarizer and incident beam of the optical microscope, respectively, the specimen box coordinate system named “xyz”, and the crystallographic coordinate system described by the optical axis and 〈1120〉 in the basal plane (0001). Ice crystals are all confined in a series of glass specimen boxes filled with KCl solution (0.2 mol/L) and the growth sequence of the single ice crystal from one specimen box to another is specially designed to ensure the specific orientation relations among specimen boxes, and the orientation relations among the specimen boxes are adjusted according to the integrated criterion. Single ice crystals with three typical orientations (α₃=90°, β_{3 a}=0°; α₃=90°, β_3b=90°; α₄=90°, β₄ dose not exist, γ ≈ 33°) relative to the microscope coordinate A-P-L are obtained, and their morphological characteristics of S/L interface are observed in situ under different pulling velocities (10.3 μm/s, 13.4 μm/s and 100 μm/s, respectively). In this paper we successfully solve the problem of orientation determination and manipulation of ice orientation in the study of directional solidification of ice crystal, which may provide an effective experimental approach for investigating the theoretical problems concerning ice crystal growth.