Influence of Dislocations on Deuteride Formation in Single Crystal Palladium
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Influence of Dislocations on Deuteride Formation in Single Crystal Palladium W.C. Chen and Brent J. Heuser Dept. Nuclear, Plasma, and Radiological Engineering, University of Illinois 214 Nuclear Engineering Lab, 103 S. Goodwin Ave., Urbana, IL 61801 ABSTRACT The effect of dislocations on deuteride formation in the Pd-D system has been investigated with small-angle neutron scattering (SANS). Deuteride precipitation morphologies in well-annealed single crystal Pd and deformed single crystal Pd have been determined with SANS. High densities of dislocations were introduced into the deformed sample by cycling across the deuteride miscibility gap. The characteristic response from small, 20-30 Angstrom thick, plate-like particles has been observed in the SANS measurement of both sample types. We believe these small plates act as precursors to the formation of much larger, tens-of-microns thick incoherent plate-like particles. These much larger deuteride plates have a preferred habit plane that depends on sample type; (100) for the well-annealed matrix and (110) for the deformed matrix. INTRODUCTION The pressure hysteresis in metal-hydrogen systems is attributed to irreversible dislocation generation which takes place due to volumetric misfit and the associated growth of incoherent particles, during both hydride formation and decomposition, inducing a separation of the absorption and desorption isotherm branches [1-3]. Precipitate morphology characteristics have been investigated using TEM in Pd-H system [4,5]. However no in-situ morphology measurements with hydrogen concentration control along the pressure-composition (PC) isotherm have been reported. Small-angle scattering techniques are suited to characterize the microstructure of dilute two-phase systems where the determination of a characteristic size for the precipitates is possible [6, 7]. The purpose of this paper is to investigate the particle morphology characteristics during the deuteride precipitation of the first cycle and the second cycle in the Pd-D system at 353 K using small-angle neutron scattering. The effects of dislocations formed during the phase transformation of the first cycle on the absorption of the following cycle are discussed. EXPERIMENTAL Undeformed, single crystal Pd samples were prepared from a 99.99% pure single crystal ingot grown by Metal Crystals and Oxides of Cambridge, UK. The ingot was grown using the Czochralski method with a [110] cylinder axis and a 1.0 cm diameter. The single crystal Pd wafer allows us to determine the precipitate orientation relative to the host lattice during the deuteride precipitation and decomposition. The crystallographic orientations of the samples were determined by x-ray diffraction. The reference sample consisted of two identically-oriented wafers, each 1.0 cm diameter and 0.16 cm thick. This increased the surface-to-volume ratio, thereby facilitating deuterium Y8.4.1
loading from the gas phase. The deformed sample consisted of a single wafer 1.0 cm diameter and 0.16 cm thick which was deformed by cycl
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