Approche des mecanismes reactionnels mis en jeu au cours d'une insertion par stades dans le graphite
Approche des mecanismes reactionnels mis en jeu au cours d'une insertion par stades dans le graphite
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DOI：
10.1016/0008-6223(88)90100-5
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发表时间：
1988
期刊：
影响因子：
10.9
通讯作者：
P. Lagrange
中科院分区：
文献类型：
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作者：
P. Lagrange
分析了从试剂扩散开碳平面并进入膨胀间隙的时刻起，分阶段嵌入石墨的现象。有必要非常清楚地区分两个完全不同但同时又高度不可分离的过程：它们在整个嵌入化学反应中同时出现。为了描述这两个基本过程，我们使用了著名的Daumas-Hérold模型，其主要目的是将所有的石墨间隔铸造成一个完全对称的零件。有人提出了一个扩散模型，但它与实验现象不一致，必须予以拒绝。相反，必须采用对应于夹层生长的模型，夹层的浓度是均匀的，并通过生长前沿在石墨层间移动。图1显示了一维空间中的问题（x为单空间变量）。图2显示了在明确定义的时刻，一方面对于扩散模型（2A），另一方面对于可移动生长前沿模型（2B），石墨间隔中的插层物质的浓度。图3A和图3B分别表示均匀浓度线（分别在扩散模型和可移动生长前沿模型中）。最后，图4显示了生长前沿随时间的移动速率。第二个过程是已经插入的层在石墨层间的滑动。这种滑动必然出现在插入过程中，以便局部恢复阶段的特征序列。图5展示了在阶段3化合物变为阶段2化合物的情况下的这种现象。因此，可以估计插层物的全部分数部分，其必须滑动以从任何更高级的化合物形成饱和的插层物（表1）。为了明确已经插入层的滑动的概念，有趣的是定义"滑动量"（notedg），其在一定量插入物的滑动期间测量在碳平面上覆盖该量的面积与滑动期间覆盖的距离的乘积。为了说明计算，提出了一个提供圆柱对称性的模型：图6显示了Daumas-Hérold模型中的第4阶段化合物。这种对称性的好处是便于计算滑动量（图7），计算结果汇总在表2中，它给出了从第10级→第9级，第9级→第8级，...，最后从第2级→第1级的g值。图8提供了gn的演变（根据起始阶段，对应于变更阶段→阶段1的幻灯片总量）。当n变为无穷大时（改变原始石墨→第1阶段饱和石墨），n-euslyn变为无穷大。因此，有必要允许滑动过程不从插入的开始出现，而是仅在几个插入层之间出现相互作用时出现。
The phenomenon of intercalation by stages into graphite is analyzed from the moment when the reagent spreads apart the carbon planes and penetrates into the expanded intervals. It is necessary to distinguish very clearly two processes completely distinct and meanwhile highly indissociable: they appear simultaneously throughout the chemical reaction of intercalation. In order to describe these two basic processes, the well-known Daumas-Hérold model is used, whose main interest is to cast all the graphitic intervals for a perfectly symmetrical part.The first process is the growth of the intercalated layer. A diffusive model has been proposed, but it is inconsistent with the experimental phenomenon and must be rejected. It is necessary to adopt on the contrary a model corresponding to the growth of an intercalated layer, whose concentration is uniform and moving about in the graphitic interval by means of a growth front.Figure 1 shows the problem in a monodimensional space (xis the single-space variable). Figure 2 exhibits, at a well-defined moment, the concentration in intercalated species in the graphitic interval for the diffusive model on one hand (2A), and for the model of a movable growth front on the other hand (2B). Figures 3A and 3B represent in a diagramxvstthe lines of uniform concentration (respectively, in the diffusive model and in the model of a movable growth front). Finally, Fig. 4 exhibits the rate of shifting of the growth front in terms of the time.The second process is the sliding of the already intercalated layers in the graphitic intervals. This sliding appears necessarily during the intercalation in order to restore locally the characteristic sequences of the stages. Figure 5 exhibits this phenomenon in the case of the change of a stage 3 compound into a stage 2 compound. Thus it is possible to estimate the whole fractional part of intercalate, which has to slide in order to form the saturated phasis from any compound of higher stage (Table 1). In order to make clear this notion of sliding of the already intercalated layers, it is interesting to define a “quantity of slide” (notedg), which measures, during the sliding of a quantity of intercalate, the product of the area, that covers this quantity on a carbon plane, by the distance covered during the sliding. A model offering a cylindrical symmetry is proposed in order to illustrate the calculation: Fig. 6 shows thus a stage 4 compound in the Daumas-Hérold model. The interest of such a symmetry is to allow an easy calculation of the quantity of slide (Fig. 7).The results of this calculation are gathered in Table 2; it gives the values ofgfor the changes stage 10 → stage 9, stage 9 → stage 8, …, and finally stage 2 → stage 1. Figure 8 supplies the evolution ofgn(total quantity of slide corresponding to the change stagen→ stage 1 in terms of the starting stagen. Whennbecomes infinite (change pristine graphite → stage 1 saturated phasis), simultaneouslygnbecomes infinite. Consequently, it is necessary to allow that the process of sliding do not appear from the outset of the intercalation but only when appears a mutual interaction between the several intercalated layers.