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[求助]
晶核長大成大顆粒 大顆粒后又變成小顆粒 生長機理
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| 生長材料過程中，晶核慢慢長大，由小顆粒變成大顆粒，隨著時間的延長，大顆粒又變成小尺寸的顆粒，而且都是晶態(tài)的，哪位蟲友遇到過類似情況知道其中的生長機理，望賜教！不勝感激！ |
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liu2004m
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4樓2012-05-17 16:53:15
liu2004m
榮譽版主 (文壇精英)
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2樓2012-05-17 11:47:54
3樓2012-05-17 15:56:41
ltsstu
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【答案】應(yīng)助回帖
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441741128: 金幣+5, 博學(xué)EPI+1, ★★★很有幫助, 謝謝 大晶粒變小應(yīng)該是dissolution-recrystallization rule了吧 2012-05-18 16:37:20
441741128: 金幣+5, 博學(xué)EPI+1, ★★★很有幫助, 謝謝 大晶粒變小應(yīng)該是dissolution-recrystallization rule了吧 2012-05-18 16:37:20
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Ostwald ripening是一種材料生長的機理，簡單點說就是材料從分子階段開始，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長，這個階段不會再形成新的晶核了，只是晶核生長的越來越大， 形成單晶結(jié)構(gòu)的方式有幾種，Ostwald ripening是最經(jīng)典的一種，“從液態(tài)轉(zhuǎn)變?yōu)楣虘B(tài)的過程首先要成核，然后生長，這個過程叫晶粒的成核長大。晶粒內(nèi)分子、原子都是有規(guī)則地排列的，所以一個晶粒就是單晶”。同時最近幾年，Banfiled又提出了一種新的晶體生長機制也能形成單晶結(jié)構(gòu)，oriented attachment, 多個取向不一致的單晶納米顆，通過粒子的旋轉(zhuǎn)，使得晶格取向一致，向后通過定向附著生長（oreinted attachment）使這些小單晶生長成為一個大單晶，當(dāng)然定向附著的過程出難免會出現(xiàn)一些位錯和缺陷，這種生長機理形成的單晶的特點同Ostwald ripening不同，OR形成的單晶大多是規(guī)則的，給材料本身晶體結(jié)構(gòu)相關(guān)，而OA形成的單晶結(jié)構(gòu)在形貌上則沒有限制，任何形狀和結(jié)構(gòu)的單晶材料都能通過此機理形成。還有，Alivisatos最近報道的Kirkendall Effect 也能形成單晶結(jié)構(gòu)，在其論文中報道了通過這種機理形成的直徑只有幾十個納米的單晶空心球，這種結(jié)構(gòu)以傳統(tǒng)的Ostwald ripening來看貌似是不可實現(xiàn)的，但通過別的生長機理就能成為現(xiàn)實。 Ostwald發(fā)生的過程包括小于一個臨界尺寸的粒子的溶解,然后質(zhì)量轉(zhuǎn)移到大于這個臨界尺寸的粒子上. Ostwald過程不同于dissolution-recrystallization過程,因為它強調(diào)的是小粒子的溶解,大粒子依靠攝取小粒子的質(zhì)量進行生長.Ostwald 過程發(fā)生的驅(qū)動力是粒子相總表面積的降低產(chǎn)生的總界面自由能的降低. 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樓2012-05-17 21:44:48
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