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Grain size has a significant influence on the performances of materials. Cooling rate is a key process parameter for controlling the size of crystal grain. Real-time observations of crystallization process on an atomic scale under different cooling rates are helpful for an in-depth understanding of this scientific issue. However, it is very difficult to observe directly the crystallization process on an atomic scale because it is small in size and fast in motion. Over last decades, colloidal suspension has attracted many researches attention as a model system of condensed matter to investigate phase transition kinetics at a particle scale level because colloidal particles are micrometer-sized and their thermal motions can be directly visualized and measured with an optical microscope. Thermo-sensitive poly-N-isopropylacrylamide (PNIPAM) colloidal suspension is one of the model systems and its phase transition can be easily controlled by temperature. In this paper, the PNIPAM colloidal system is used to make the real-time observation of the influence of the cooling rate on crystal grain size. Firstly, the crystal nucleation and growth process of PNIPAM colloidal suspension at a cooling rate of 30.0 ℃/h is observed with a high-resolution transmission microscope. It is found that liquid-solid phase transition of the PNIPAM colloidal suspension begins from a sudden transient nucleation, followed by a rapid grain growth as temperature decreases. The variation of crystal phase fraction with temperature undergoes three stages: slow, rapid and slow. In the initial stage, nuclei are limited and the growth driving force is low, therefore the crystal phase fraction changes slowly. In the middle stage, as temperature decreases, the growth driving force further increases and the crystal phase fraction increases rapidly. In the final stage, the crystal grains begin to adjoin with each other and the left liquid volume becomes less and less, so the crystal phase fraction increases in a slow mode again. Secondly, the PNIPAM colloidal crystal under different cooling rates from 0.5 ℃/h to 30.0 ℃/h is observed with Bragg diffraction technique. The grain size of PNIPAM crystal is also measured. It is found that the size of PNIPAM colloidal crystal grain decreases with the increase of cooling rate and the relationship between the grain size and the cooling rate obeys a power-law formula, which is also used to well describe the effect of cooling rate on grain size in metallic system. This suggests that the crystallization behavior of PNIPAM colloidal system under continuous cooling is similar to those of metallic systems. However, the fitted power-law pre-factor of PNIPAM colloidal system is very different from those of the metallic systems because the sizes and motions of PNIPAM particles are much larger and slower than those of atoms, respectively.
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Keywords:
- crystallization /
- nucleation /
- colloid /
- solidification microstructure
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[16] Han Y L 2013 Physics 42 160 (in Chinese) [韩一龙 2013 物理 42 160]
[17] Liu L, Xu S H, Liu J, Duan L, Sun Z W, Liu R X, Dong P 2006 Acta Phys. Sin. 55 6168 (in Chinese) [刘蕾, 徐升华, 刘捷, 段俐, 孙祉伟, 刘忍肖, 董鹏 2006 物理学报 55 6168]
[18] Xu S H, Zhou H W, Sun Z W, Xie J C 2010 Phys. Rev. E 82 010401
[19] Wu J Z, Zhou B, Hu Z B 2003 Phys. Rev. Lett. 90 048304
[20] Tang S J, Hu Z B, Cheng Z D, Wu J Z 2004 Langmuir 20 8858
[21] Gong T Y, Shen J Y, Hu Z B, Marquez M, Cheng Z D 2007 Langmuir 23 2919
[22] Okubo T, Suzuki D, Shibata K, Tsuchida A 2012 Colloid Polym. Sci. 290 1403
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[1] Hahn E N, Meyers M A 2015 Mater. Sci. Eng. A 646 101
[2] Lu J, Zeng X Q, Ding W J 2008 Light Met. 8 59 (in Chinese) [路君, 曾小勤, 丁文江 2008 轻金属 8 59]
[3] Zuo Y B, Cui J Z, Zhao Z H, Zhu Q F, Qu F, Wang X J 2008 Chin. J. Rare Met. 32 589 (in Chinese) [左玉波, 崔建忠, 赵志浩, 朱庆丰, 福 屈, 王向杰 2008 稀有金属 32 589]
[4] Liu D, Liu Y, Huang Y, Song R, Chen M 2014 Prog. Nat. Sci. 24 452
[5] Hosseini V A, Shabestari S G, Gholizadeh R 2013 Mater. Design 50 7
[6] Easton M A, StJohn D H 2008 Mater. Sci. Eng. A 486 8
[7] Quested T E, Greer A L 2004 Acta Mater. 52 3859
[8] Zhou L L, Liu R S, Hou Z Y, Tian Z A, Lin Y, Liu Q H 2008 Acta Phys. Sin. 57 3653 (in Chinese) [周丽丽, 刘让苏, 侯兆阳, 田泽安, 林艳, 刘全慧 2008 物理学报 57 3653]
[9] Li G J, Wang Q, Cao Y Z, L X, Li D G, He J C 2011 Acta Phys. Sin. 60 093601 (in Chinese) [李国建, 王强, 曹永泽, 吕逍, 李东刚, 赫冀成 2011 物理学报 60 093601]
[10] Jian Z Y, Li N, Chang F E, Fang W, Zhao Z W, Dong G Z, Jie W Q 2012 Acta Metall. Sin. 48 703 (in Chinese) [坚增运, 李娜, 常芳娥, 方雯, 赵志伟, 董广志, 介万奇 2012 金属学报 48 703]
[11] Yang T, Zhang J, Long J, Long Q H, Chen Z 2014 Chin. Phys. B 23 088109
[12] Granasy L, Tegze G, Toth G I, Pusztai T 2011 Philos. Mag. 91 123
[13] Gasser U, Weeks E R, Schofield A, Pusey P N, Weitz D A 2001 Science 292 258
[14] Franke M, Lederer A, Schope H J 2011 Soft Matter 7 11267
[15] Tan P, Xu N, Xu L 2014 Nat. Phys. 10 73
[16] Han Y L 2013 Physics 42 160 (in Chinese) [韩一龙 2013 物理 42 160]
[17] Liu L, Xu S H, Liu J, Duan L, Sun Z W, Liu R X, Dong P 2006 Acta Phys. Sin. 55 6168 (in Chinese) [刘蕾, 徐升华, 刘捷, 段俐, 孙祉伟, 刘忍肖, 董鹏 2006 物理学报 55 6168]
[18] Xu S H, Zhou H W, Sun Z W, Xie J C 2010 Phys. Rev. E 82 010401
[19] Wu J Z, Zhou B, Hu Z B 2003 Phys. Rev. Lett. 90 048304
[20] Tang S J, Hu Z B, Cheng Z D, Wu J Z 2004 Langmuir 20 8858
[21] Gong T Y, Shen J Y, Hu Z B, Marquez M, Cheng Z D 2007 Langmuir 23 2919
[22] Okubo T, Suzuki D, Shibata K, Tsuchida A 2012 Colloid Polym. Sci. 290 1403
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