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晶核长大成大颗粒 大颗粒后又变成小颗粒 生长机理
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| 生长材料过程中，晶核慢慢长大，由小颗粒变成大颗粒，随着时间的延长，大颗粒又变成小尺寸的颗粒，而且都是晶态的，哪位虫友遇到过类似情况知道其中的生长机理，望赐教！不胜感激！ |
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ltsstu
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441741128: 金币+5, 博学EPI+1, ★★★很有帮助, 谢谢 大晶粒变小应该是dissolution-recrystallization rule了吧 2012-05-18 16:37:20
441741128: 金币+5, 博学EPI+1, ★★★很有帮助, 谢谢 大晶粒变小应该是dissolution-recrystallization rule了吧 2012-05-18 16:37:20
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Ostwald ripening是一种材料生长的机理，简单点说就是材料从分子阶段开始，首先形成一定尺寸的晶核，然后所有的分子都依附于晶核生长，这个阶段不会再形成新的晶核了，只是晶核生长的越来越大， 形成单晶结构的方式有几种，Ostwald ripening是最经典的一种，“从液态转变为固态的过程首先要成核，然后生长，这个过程叫晶粒的成核长大。晶粒内分子、原子都是有规则地排列的，所以一个晶粒就是单晶”。同时最近几年，Banfiled又提出了一种新的晶体生长机制也能形成单晶结构，oriented attachment, 多个取向不一致的单晶纳米颗，通过粒子的旋转，使得晶格取向一致，向后通过定向附着生长（oreinted attachment）使这些小单晶生长成为一个大单晶，当然定向附着的过程出难免会出现一些位错和缺陷，这种生长机理形成的单晶的特点同Ostwald ripening不同，OR形成的单晶大多是规则的，给材料本身晶体结构相关，而OA形成的单晶结构在形貌上则没有限制，任何形状和结构的单晶材料都能通过此机理形成。还有，Alivisatos最近报道的Kirkendall Effect 也能形成单晶结构，在其论文中报道了通过这种机理形成的直径只有几十个纳米的单晶空心球，这种结构以传统的Ostwald ripening来看貌似是不可实现的，但通过别的生长机理就能成为现实。 Ostwald发生的过程包括小于一个临界尺寸的粒子的溶解,然后质量转移到大于这个临界尺寸的粒子上. Ostwald过程不同于dissolution-recrystallization过程,因为它强调的是小粒子的溶解,大粒子依靠摄取小粒子的质量进行生长.Ostwald 过程发生的驱动力是粒子相总表面积的降低产生的总界面自由能的降低. Ostwald ripening is an observed phenomenon in solid solutions or liquid sols which describes the change of an inhomogeneous structure over time. In other words, over time, small crystals or sol particles dissolve, and redeposit onto larger crystals or sol particles. Dissolution of small crystals or sol particles and the redeposition of the dissolved species on the surfaces of larger crystals or sol particles was first described by Wilhelm Ostwald in 1896. Ostwald ripening is generally found in water-in-oil emulsions, while flocculation is found in oil-in-water emulsions . This thermodynamically-driven spontaneous process occurs because larger particles are more energetically favored than smaller particles. This stems from the fact that molecules on the surface of a particle are energetically less stable than the ones in the interior. Consider a cubic crystal of atoms: all the atoms inside are bonded to 6 neighbors and are quite stable, but atoms on the surface are only bonded to 5 neighbors or less, which makes these surface atoms less stable. Large particles are more energetically favorable since, continuing with our example, more atoms are bonded to 6 neighbors and fewer atoms are at the unfavorable surface. As the system tries to lower its overall energy, molecules on the surface of a small particle (energetically unfavorable, with only 3 or 4 or 5 bonded neighbors) will tend to detach from the particle, as per the Kelvin equation, and diffuse into the solution. When all small particles do this, it increases the concentration of free atoms in solution. When the free atoms in solution are supersaturated, the free atoms have a tendency to condense on the surface of larger particles. Therefore, all smaller particles shrink, while larger particles grow, and overall the average size will increase. After an infinite amount of time, the entire population of particles will have become one, huge, spherical particle to minimize the total surface area |
5楼2012-05-17 21:44:48
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2楼2012-05-17 11:47:54
3楼2012-05-17 15:56:41
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4楼2012-05-17 16:53:15
