A Subnanoscale Study of Segregation at CdO/Ag(Au) Heterophase Interfaces
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A Subnanoscale Study of Segregation at CdO/Ag(Au) Heterophase Interfaces Jason T. Sebastian, Olof C. Hellman and David N. Seidman Department of Materials Science and Engineering Northwestern University 2225 N. Campus Drive Evanston, IL 60208-3108, U.S.A. ABSTRACT Three-dimensional atom-probe (3DAP) microscopy permits the atom-by-atom reconstruction of a small volume (typically 10 nm x 10 nm x 100 nm) of a material with respect to both the positions and chemical identities of individual atoms. It is, therefore, ideally suited for the study of solute segregation at internal heterophase interfaces. We present recent results of a 3DAP microscopy study of solute segregation at ceramic/metal (C/M) heterophase interfaces prepared by internal oxidation. In particular, results on the CdO/Ag(Au) (where Au is the segregating species) system are presented. In the 3DAP atomic reconstructions, the interfaces of nanometer-size CdO ceramic particles are delineated as Cd isoconcentration surfaces. The distribution of the segregating species as a function of distance to the isoconcentration surfaces is determined with the proximity histogram (or proxigram) method. Two interfaces are investigated in detail. The first shows no appreciable Au segregation, while the second exhibits segregation with a Gibbsian interfacial excess of 1.65 nm-2 at 650°C. INTRODUCTION Three-dimensional atom-probe (3DAP) microscopy [1-3] is unique in its capability of measuring quantitatively and directly the segregation of specific elements at internal interfaces without data deconvolution and without recourse to standards. The technique relies upon the enhanced electric field at the apex of a sharply polished, needle-shaped specimen to which a high voltage is applied. The strong electric field (on the order of tens of volts per nanometer) results in the field evaporation of ions from the tip of a specimen in a highly controlled manner. The chemical identities of these ions are determined by time-of-flight mass spectrometry and a twodimensional position-sensitive detector records the positions of the impact of individual ions. The chemical and position data is then used to create a three-dimensional atomic reconstruction of the analyzed volume. A typical reconstructed volume has dimensions of 10 nm x 10 nm x 100 nm and includes approximately 500,000 atoms. The overall detection and positioning efficiency of the 3DAP is approximately 60-65%. That is, 60-65% of the atoms in the specimen are represented in the atomic reconstruction. 3DAP microscopy has only been applied recently to the study of segregation at ceramic/metal interfaces [4, 5]. These studies rely on recent advances in 3DAP data analysis. Specifically, the so-called proximity histogram or “proxigram” method has been developed [6] to investigate the segregation of segregating atoms at topologically complicated interfaces for data obtained from 3DAP reconstructions. This powerful method also allows for the direct determination of the Gibbsian interfacial excess ( Γ ) of a segregating species at such i
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