加载中…奥斯特瓦尔德熟化Ostwald Ripening
(2011-05-27 09:19:19)
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杂谈 |
“一个材料如果内部有许多晶粒，则为多晶材料；若仅为一个晶粒组成，则为单晶。”
形成单晶结构的方式有几种，Ostwald
ripening是最经典的一种，就是一楼所说的“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 which
describes the evolution of an inhomogenous structure over time.
When a phase precipitates out of a solid, energetic factors will
drive some precipitates to grow, drawing from the others, which
shrink. If this process continues, eventually fewer and larger
crystals form inside the solid that have smaller and smaller
surface-to-volume ratios compared to the smaller particles, thus
reducing the energy of the entire system.
The phenomenon was first described by Wilhelm Ostwald in 1896.[1]
In geology, it is the textural coarsening, ageing or growth of phenocrysts and crystals in solid rock which is below the solidus temperature. It is often ascribed as a process in the formation of orthoclase megacrysts, as an alternative to the physical processes governing crystal growth from nucleation and growth rate thermochemical limitations.
In oil-in-water emulsion-type polymerizations, Ostwald ripening is the diffusion of monomer from smaller to larger droplets due to the greater solubility of the single monomer molecules in the larger monomer droplets. The rate of this diffusion process is linked to the solubility of the monomer in the continuous (water) phase of the emulsion. Ostwald ripening is a key mechanism in the destabilization of emulsions (for example, by creaming and sedimentation).
An everyday example of Ostwald ripening is the recrystallization of water within ice cream which gives old ice cream a gritty, crunchy texture. The larger ice crystals grow competitively with the smaller within the ice cream, thereby destabilizing the homogeneity of the emulsion.
In chemistry, the term refers to the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones. In the process, many small crystals formed initially slowly disappear, except for a few that grow larger, at the expense of the small crystals. The smaller crystals act as fuel for the growth of bigger crystals. The process of Ostwald Ripening is fundamental in modern technology for the solution synthesis of quantum dots.
This spontaneous process occurs because larger particles are more energetically favored than smaller particles. While the formation of many small particles is kinetically favored, (i.e. they nucleate more easily) large particles are thermodynamically favored. This is because small particles have a larger surface area to volume ratio than large particles and are consequently easier to produce. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. Large particles, with their greater volume to surface area ratio, therefore represent a lower energy state. Hence, many small particles will attain a lower energy state if transformed into large particles and this is what we see in Ostwald ripening.
The phenomenon was first described by Wilhelm Ostwald in 1896.[1]
In geology, it is the textural coarsening, ageing or growth of phenocrysts and crystals in solid rock which is below the solidus temperature. It is often ascribed as a process in the formation of orthoclase megacrysts, as an alternative to the physical processes governing crystal growth from nucleation and growth rate thermochemical limitations.
In oil-in-water emulsion-type polymerizations, Ostwald ripening is the diffusion of monomer from smaller to larger droplets due to the greater solubility of the single monomer molecules in the larger monomer droplets. The rate of this diffusion process is linked to the solubility of the monomer in the continuous (water) phase of the emulsion. Ostwald ripening is a key mechanism in the destabilization of emulsions (for example, by creaming and sedimentation).
An everyday example of Ostwald ripening is the recrystallization of water within ice cream which gives old ice cream a gritty, crunchy texture. The larger ice crystals grow competitively with the smaller within the ice cream, thereby destabilizing the homogeneity of the emulsion.
In chemistry, the term refers to the growth of larger crystals from those of smaller size which have a higher solubility than the larger ones. In the process, many small crystals formed initially slowly disappear, except for a few that grow larger, at the expense of the small crystals. The smaller crystals act as fuel for the growth of bigger crystals. The process of Ostwald Ripening is fundamental in modern technology for the solution synthesis of quantum dots.
This spontaneous process occurs because larger particles are more energetically favored than smaller particles. While the formation of many small particles is kinetically favored, (i.e. they nucleate more easily) large particles are thermodynamically favored. This is because small particles have a larger surface area to volume ratio than large particles and are consequently easier to produce. Molecules on the surface are energetically less stable than the ones already well ordered and packed in the interior. Large particles, with their greater volume to surface area ratio, therefore represent a lower energy state. Hence, many small particles will attain a lower energy state if transformed into large particles and this is what we see in Ostwald ripening.
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