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Cluster Science Group

Research Period:@Fiscal 1992-1997(6 years)
AIST Spetial Research ProgramiInternational Basic Researchj

ĦObject

@The Cluster Science Group is dedicated to studies on clarifying the cluster properties which play crucial roles in changes of materials status (condensation,crystallization,phase separation and so on) and in chemical reactions (combustion and synthesis in gas phase) and on exploiting the effective utilization of newly found properties. In order for materials to exist stably in isolation, it is essential that more than certain number of atoms or molecules are assembled. In case of clusters where the number of constituent atoms/molecules is limited, the proportion of atoms and molecules located on the surface becomes greater than otherwise, and enhanced reactivity may be expected. In fact, clusters grow or disintegrate on collision with other clusters, or undergo reaction or transfomation. That is, the most significant feature of clusters is their failure of existing in stable isolation, and this aspect is characterrized by the disignation of "dynamic cluster".

There are two approaches for studying the characteristics of dynamic cluster:(1)to identify new structures and properties resulting from assembling of atoms and molecules, and analyze the causative mechanism by tracing changes in cluster properties while increasing the number of constituent atoms/molecules from monomer to dimer,trimer and so on. And (2) to investigate how materials properties change as ultra-fine particles (smallest possible bulk entities) is divided into smaller parts.

The forces binding constituent units of cluster range from very weak one to strong one. In decreasing order of strength,there are covalent bonds between carbon or silicon atoms,bouds between metal atoms,hydrogen bond playing an active role in water,and very weak Van der Waals force.

Clusters can exist in collisionless condition in the vacuum,in frequently changing condition in the liquid or solution,or on freezed condition on a surface or in a solid. Since clusters can not persist stably in isolation,its is necessary to stabilize them before achieving their effective utilization.The stabilization requires some substrate,and if the interaction between the cluster and the substrate is adequately weak,the cluster characteristics dominates to provide clues for studying it.On the other hand,when the substrate interaction prevails,the cluster behaves differently with the interaction incorporated.Characterizing clusters under strong substrate interaction constitutes one of important subgoals of cluster science.

The objective of this project is to elaborate the cluster characteristics on the basis of these studies,and to contribute to the progress of the materials science.Some experiments can not be implemented at present,and for explaining some of experimental results,the theoretical analysis including the computer simulation will play a significant role.