In Situ TEM Study of Electron Beam Stimulated Organization Of Three-Dimensional Void Superlattice in CAF 2
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IN SITU TEM STUDY OF ELECTRON BEAM STIMULATED ORGANIZATION OF THREE-DIMENSIONAL VOID SUPERLATTICE IN CAF2 T.H. Ding1, S. Zhu1, L.M. Wang1,2,3 1 Department of Nuclear Engineering & Radiological Sciences, 2 Department of Geological Science, 3Department of Materials Science and Engineering, University of Michigan, Ann Arbor, MI 48109-2104, [email protected] ABSTRACT CaF2 is widely adopted as deep-UV window material and thin film optical coating. It has been known that ordered defect superlattices may form in electron irradiated CaF2.[1] However, the nature of the defects on the superlattice point (i.e., Ca colloids or voids) has not been certain and the self-organization mechanism has not been fully understood. In this study, single crystal CaF2 were irradiated under 200 keV electron beam at room temperature with in situ TEM observation of the dynamic process of defect ordering. The superlattice reached steady state after an electron dose of 1×1021e-/cm2 with void radius about 5 nm. Videos recorded during the in-situ observation reveal the dynamic self-organization process of the void superlattice. Coalescence was prevalent at the initial stages. Migration and preferential growth were dominant at the final stages of the superlattice formation. At a dose higher than 3×1021 e-/cm2 the superlattice structure was destroyed. These critical doses of void superlattice formation and deformation seem to be independent of dose rate. The anisotropic diffusivity of the migrating “molecules” may explain the superlattice formation. INTRODUCTION Solids under bombardment with energetic particles such as ions, neutrons or electrons often turn to states far from equilibrium. Ordering of irradiation-induced defects by “self” organization is an interesting phenomenon observed among such solids.[2] These ordered structures include periodic defect walls, bubble lattices, void lattices and periodic composition variations in solids. First observation of voids ordering in ion and neutron irradiated metal was in Mo in 1971[2]. Later, void superlattices have been observed in Ni, Al, and Nb pure metals, after high energy ion or neutron irradiation.[3] Void superlattices were also found in some alloys, for example, in stainless steel. Furthermore, stainless steel is the only metal in which ordered voids can be induced by electron irradiation. [4] Besides void lattices, cluster arrays formed by neutron irradiation in Ni were also observed [5]. After that, experimental observations have become available on periodic defect wall formation under light ion irradiation in Cu and Ni [6] as well as bubble lattice formation in many metals under inert gas implantation [3, 7]. Observations of gas bubbles ordering have been made on Cu, stainless steel, Ni, Au, V, Mo, Ti and Co, by low energy inert gas irradiation [3, 7]. Similar phenomena were also observed in ceramics. When single crystals of calcium fluorite CaF2 are irradiated with electron in transmission electron microscope (TEM), ordering of the radiation induced defects occurs which leads to for
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