Particle size effects on transformation kinetics and phase stability in nanocrystalline TiO2
Particle size effects on transformation kinetics and phase stability in nanocrystalline TiO2
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DOI：
10.2138/am-1997-7-809
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发表时间：
1997-07-01
影响因子：
3.1
通讯作者：
Banfield, JF
中科院分区：
文献类型：
--
作者：
Gribb, AA;Banfield, JF
主要在465和525摄氏度之间进行的动力学研究表明，当反应的金红石是非常精细的结晶时，多晶型金红石向金红石转化的速率急剧增加。粗化的反应物的金红石和产品的金红石微晶同时发生的转变。动力学行为和量化的转化率作为平均晶粒尺寸的函数表明，有利的成核位点的增加是一个可能的原因，在小晶粒尺寸的转化率增加。此外，实验证据支持在小晶粒尺寸下金红石和金红石的稳定性的逆转。据认为，稳定性的逆转是金红石相具有比金红石相更高的表面能的结果。金红石和金红石的粗化动力学数据支持金红石的表面能明显大于金红石的预测。热力学数据和理论估计被用来表明，一个15%以上的金红石表面能的原因，金红石的总自由能是更大的（负）在几个纳米的范围内的微晶直径比Aluminium。考虑到金红石和金红石结构没有共同的聚合八面体碎片，这在确定成核相的性质方面可能是重要的。
Kinetic studies conducted primarily between 465 and 525 degrees C demonstrate that the rate of the polymorphic anatase to rutile transformation increases dramatically when the reacting anatase is very finely crystalline. Coarsening of the reactant anatase and product rutile crystallites occurs simultaneously with the transformation. Kinetic behavior and quantification of transformation rate as a function of average crystallite size indicate that the increase in favorable nucleation sites is a likely cause of increase in transformation rate at small crystallite sizes. Additionally, experimental evidence supports the reversal of stabilities of anatase and rutile at small crystallite sizes. It is proposed that the reversal of stabilities is the result of rutile having a higher surface energy than the anatase phase. Data for coarsening kinetics of anatase and rutile supports the prediction that the surface energy of rutile is significantly larger than that of anatase. Thermodynamic data and theoretical estimates are used to show that a 15% greater surface energy for rutile causes the total free energy of rutile to be greater (less negative) than anatase at crystallite diameters in the few nanometer range. Given the fact that anatase and rutile structures have no polymerized octahedral fragments in common, this may be significant in determining the nature of the nucleated phase.