Energy-Tunable X-Ray Diffraction in Polycrystalline Materials: a Look at Microstructure in Seashells
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Energy-Tunable X-Ray Diffraction in Polycrystalline Materials: a Look at Microstructure in Seashells Emil Zolotoyabko1 and John P. Quintana DND-CAT Research Center, Northwestern University, APS/ANL Sector 5, Building 432 A, 9700 S. Cass Ave., Argonne, IL 60439-4857, U.S.A. 1 Department of Materials Engineering, Technion-IIT, Haifa 32000, Israel ABSTRACT We developed a depth-sensitive x-ray diffraction technique in which diffraction profiles are measured at x-ray energies that are varied by small steps. The method is intended for synchrotron beam lines and provides non-destructive mapping of structural characteristics in inhomogeneous polycrystalline materials. Depth resolution is achieved due to an energy dependence of the x-ray penetration length. Application of this technique to seashells allowed us to extract spatial distributions of preferred orientation and strain components, which revealed pronounced variations of the shell microstructure in three dimensions. The results shed light on “engineering solutions” by mollusk. The developed technique can be used to characterize various laminated structures and composite materials. INTRODUCTION Most x-ray structural studies are performed at fixed x-ray energy. This allows us to measure the lattice parameters in homogeneous crystalline materials with an unsurpassed precision of 10-5, routinely [1], and up to 10-8 in special cases [2]. Recently, high-resolution x-ray diffraction techniques have been established with an aim to provide depth-resolved profiles of lattice parameters and interplanar spacings (d-spacings) in nearly perfect multilayered single crystalline structures which are widely used in modern microelectronics and optoelectronics. This powerful method is based on the accurate measurement and simulation of the fine interference features of diffraction profile in the vicinity of the Bragg angle [1, 3]. However, it can not be applied to the entire “world” of inhomogeneous polycrystalline materials because the coherency length of x-ray interactions is restricted by the individual grain size, and the interference features are lost. At the same time, the measurement of structural characteristics with depth resolution in polycrystalline materials is one of the important problems in materials science. However, extracting depth-resolved structural information meets difficulties due to the relatively large x-ray penetration depths. The complications increase for hard x-rays which are used to measure thicker samples [4]. As an example, one can mention energy-dispersive x-ray diffraction [5] which utilizes high energy white radiation and registration of the scattered beam at fixed angles by means of an energysensitive detector. In this method, the diffracting lozenge in the direction of the incident beam can reach hundreds of microns. The problem is partially solved by using kinds of triangulation technique [6,7]. Due to these methods, remarkable progress in xray microscopy has recently been achieved [8-11]. While it is possible to separate diffraction signals from in
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