고체 반응성에 미치는 회합체 거동과 구조적 결함의 영향

최청송

서강대학교 화학공학과

Effects of Clustering Behavior and Structural Defects

on Reactivity of Solids

Cheong-Song Choi

Department of Chemical Engineering, Sogang University

Introduction

Nanostructure loosely define particles, grains, functional structures, and devices with dimensions in the 1 to 100 nanometer range. The fundamental physical properties of materials are remarkably altered as the size of their constituent grains decreases to a nanometer scale. These materials with at least one characteristic length scale confined to the nanometer range offer unique and entirely different properties compared with conventional micro-size materials because of their distinctive size, shape, surface chemistry, and topology. But the characteristic length scale is not the only important feature of a nanostructured material. From a microsturctural standpoint, the correspondingly high defect densities and the reduced dimensionality may be just as relevant to understanding the material's properties. The relationship between the reaction mechanism and the character of the influence of defects on the reactivity of solids can be utilized in solving the control problem of the reaction rate[1-5]. Then changing the methods of the preparation of the crystal or using various methods of its preliminary treatment will make it possible to change the concentration of those defects that are most important for a given reaction to control the process. The understanding of the reactivity of crystalline solids has gradually grown from the increasing knowledge of the solid state, including the regular lattice structure as well as the different lattice defects. The discussion in this investigation is confined to urea and urea inclusion compound (UICs) system that are well defined from the solid state point of view.

Solute Clustering and Nanostructural Patterns

In order to confirm the phenomenon of solute clustering, RDF(radial distribution function) was applied[6-7]. When the solution saturated at 20□ is suddenly brought into a highly supersaturated condition by rapid cooling, H2O-H2O interaction (2.95□) and hydration of urea (3.75□ and 4.75□) are strong, respectively. In the case of the solution saturated at 40□, the peak at 4.5□ ([eqn: { (C-N-H-O)}_{U }]), which increases with concentration, results from urea molecular clusters. In the RDF of the solution at 60□(rapid cooling), a peak at 2.98□ is quite resolved and is ascribed to [eqn: { (N-H-O)}_{U }] band, which gives responsible proof of the existence of urea clusters. And from the metastable  solution prepared from a saturated solution at 60□, peak at 2.98□ and peak at 4.5□ are not resolved, which mean that it is difficult to find the solute clustering behavior.

To illustrate all that was said above, it is necessary to demonstrate the behavior of urea molecular clusters during forming UIC in a aqueous urea solution saturated using Patterson function. From this, it is noted that after adding the infinitesimal amount of n-dodecane to the saturated solution at 20□, the size of 40□ was gradually decreased and the one of 85□ was increased with time. Generally in SAXS, the increasing intensity of the scattering means that a higher ordered phase was formed from a homogeneous  phase. This result indicates that 85□ clathrate-like clusters apparently undergo increase in degree of clustering, which occurs at the expense of 40□ urea clusters.

According to the results described above, the existence of clathrate-like cluster was confirmed. The characteristics of urea only appeared at initial state, and when the reaction time reached 60 minutes the UICs character composed of the peak at 2.93□([eqn: { (N-H-O)}_{A }]) and at 3.5□(NA-NA) could be found from the RDF.

As the solution becomes supersaturated, some of the clusters coalesce and/or grow to form larger clusters that may also have a higher internal solute concentration. At the end of the growth of fluctuation of the spinodal decomposition process, the system has reached a local equilibrium, a surface tension between the domains can be identified, and other types of growth, related to the interaction between domains, can be proceed ; that is, the percolation threshold is likely to have a predominant role, deciding whether the clusters can be interconnected or not. If it is so, new types of growth, through capillary instability, can form and accelerate the growth very much. However, if the difference between phases is small, as the case for structurally similar phases with similar orientations and lattice parameters, the differences may be accommodated by an elastic component or by an array of misfit dislocations. If the parameter, which is a combination of elastic constants, depends on the crystallographic direction, the associated strain energy also becomes an isotropic. Therefore, the locus of the spinodal may also vary with the crystallographic direction. SEM photograph shows the (110) plane of urea crystal from spinodal decomposition, consisting of a lamella structure of nanophase, which form preferentially along [001].

It could be deduced that the elastic strain energy attains a minimum for composition wave along [001]. As predicted, the growth rate is highest along the [001].

Structural Modification in Solids

The residual stresses in the activated urea crystals appear that the (111) plane which has high lattice density is rarely activated by means of chemical or mechanical treatment. The deformed solid urea via the thermal activation method reveals a narrow range in the residual stresses. And the relative order of activities can be also evaluated using the lowering of melting point due to the surface and volume effects of solid clusters in the activated solid urea. From the viewpoint of the activity, the solid urea from the spinodal-type decomposition is very active because it is composed of high dislocation density with uniform elastic strain. To ascertain the increase in reactivity by the defects and some degrees of the internal homogeneity of solid state, the thermal decomposition reaction system was applied. From this approach, the solid urea by the thermal activation method can be explained by two dimensional phase boundary model[8], but if the conversion is over 70%, the mechanism is changed, while the solid urea by the spinodal decomposition method can be exactly determined by two dimensional phase boundary model from the start of the reaction to the end.

Reactivity of Nanostructured Solid Urea

Nanostructured solid clusters resulting from imperfection of urea crystals are said to differ from corresponding bulk solid urea in that their surface area are relatively large. When the cluster becomes small enough to be classed as a superfine(1 to 100nm) substance then its total surface area also becomes extremely large and its surface energy becomes high. On the other hand, when the nanostructure solid clusters are formed, it is possible that a few of these are much smaller than a reactive critical size. At such centers the molecular process of formation of the UIC can be readily occurred. The lower melting point of the clusters or their unusual crystalline nanostructure may be attributable to this surface effect. So the melt phase can be formed via the heat of adsorption of n-dodecane. But the solid urea crystals from the spinodal type decomposition method induce fast and complete formation of the inclusion compound for the reason that the average size of their cluster nearly comes to the reactive critical value.

It is clear from these results that in the course of the reaction of UIC formation the existence of an interface between the starting material (tetragonal urea) and the solid product(hexagonal urea or UIC) leads to a change in the reactivity of the unreacted tetragonal urea crystal in the boundary region, because of a change in lattice parameters. And the strains at the boundary between the two solid phases cause an increase in the dislocation density near the phase border. Thus, the difference in the lattice parameters is responsible for the autolocalization of the process.

Conclusion

In order to regulate the spatial development and generate the effective type of defects acting as potential centers of reaction,(UIC formation), the application of the spinodal-type decomposition process is much better than that of physical treatment.

Owing to the great variety of possible phases and defects, to their spatial arrangement, and not least to their mutual interactions, understanding  the relationship between microstructure and properties remains one of the greatest challenges in material science.

References

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