Nanocrystalline, porous periclase aggregates as product of brucite dehydration
Nanocrystalline, porous periclase aggregates as product of brucite dehydration
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作为水镁石脱水产物的纳米晶多孔方镁石聚集体
DOI：
10.1127/0935-1221/01/0013-0329
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发表时间：
2001
影响因子：
2.1
通讯作者：
F. Langenhorst
中科院分区：
文献类型：
--
作者：
P. Aken;F. Langenhorst
用透射电子显微镜原位研究了电子束诱导水镁石脱水的过程。在电子束作用下，水镁石的六方晶片立即分解，在a和c方向的形貌收缩分别为5%和10-20%。体积收缩首先发生在边缘，然后影响到颗粒的中心。电子能量低损失谱显示，局域质量厚度同时变化50-55%。综合这些数据，得出脱水材料中的孔隙率为37.5-50%。分解产物由大量细小的氧化镁微晶和空洞组成。电子衍射表明水镁石与[0001]Bru//[111]MgO和[1120]Bru//[110]MgO之间存在拓扑定向关系。由于脱水材料的孔隙率略小于最大理论孔隙率(54%)，因此只有一小部分空隙被输送出集合体。
氧原子局域环境的信息是从扩展能量损失精细结构(EXELFS)和能量损失近边结构(ELNE)得到的。在脱水过程中，氧的配位数从水镁石的3增加到氧化镁的6。在瞬变状态下，德拜-沃勒因子达到最大值，表明处于高度无序的中间状态。这些数据使我们能够利用Avrami方程来模拟失水和检验反应动力学。水镁石的分解被解释为一个复杂的三阶段过程：(I)它首先通过界面控制的过程进行，从水镁石的边缘开始；水通过基准面逃逸。(Ii)脱水的晶格在边缘坍塌，而核心区仍然水合。为了进一步使颗粒脱水，空隙必须相互连接并以网络的形式重新排列，以减缓分解。在这个阶段，这个过程是扩散控制的。(Iii)最后，毛孔相互连通并到达表面。脱水加速，并再次是界面控制的过程。
在沃尔夫冈·F·穆勒教授60岁生日之际献给他
Transmission electron microscopy (TEM) techniques were employed to in situ study the electron-beam induced dehydration of brucite Mg(OH)2. Under the electron beam, the hexagonal platelets of brucite immediately decompose and show a morphological shrinkage of 5% and 10–20% in the a and c directions, respectively. The volume contraction occurs first in the rim and then affects the center of grains. Electron energy low-loss spectra reveal a simultaneous change in the local mass thickness of 50–55%. Combining these data, it follows that the porosity in the dehydrated material is 37.5–50%. The decomposition product is composed of numerous, tiny MgO crystallites and voids. Electron diffraction reveals a topotactic relationship between brucite and MgO with [0001]Bru // [111]MgO and [1120]Bru // [110]MgO. Since the porosity of the dehydrated material is slightly smaller than the maximum theoretical porosity (54%), only a small fraction of the voids is transported out of aggregates.
Information on the local environment of the oxygen atoms was derived from extended energy-loss fine (EXELFS) and energy-loss near-edge structures (ELNES). In the time course of dehydration the coordination number of oxygen shows the expected increase from 3 for brucite to 6 for MgO. In a transient state the Debye-Waller factor reaches a maximum, indicating a highly disordered intermediate state. These data allow us to model the water loss and to examine reaction kinetics applying the Avrami equation. The decomposition of brucite is interpreted as a complex three-stage process: ( i ) It proceeds first via an interface-controlled process, starting at the rim of brucite; water escapes through the basal plane. ( ii ) The dehydrated lattice collapses then at the rim, whereas the core region is still hydrated. To further dehydrate the grain, the voids have to interconnect and rearrange in the form of a network slowing down the decomposition. At this stage, the process is diffusion-controlled. ( iii ) Finally, the pores are interconnected and reach the surface. The dehydration accelerates and is again an interface-controlled process.
Dedicated to Prof. Dr.Wolfgang F.Muller on the occasion of his 60th birthday