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Shashkov, Dmitriy Alexandrovich |
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Atomic-Scale Studies of the Structure and Chemistry of Ceramic/Metal Heterophase Interfaces |
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Book Whole |
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1997 |
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275 |
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Materials containing metallic and ceramic phases play an important role in modern technology. The structure and chemistry, however, of ceramic/metal interfaces are not well understood. This study, therefore, was conducted to characterize fully the atomic-scale structure and chemistry of metal oxide/metal
interfaces produced by internal oxidation of dilute binary and ternary metallic alloys. Ternary alloys were oxidized under conditions whereby only one of the two solute species is oxidized. The remaining solute species segregates to the
ceramic precipitate/matrix interfaces.
Scanning transmission electron microscopy was used to determine the atomic structure and chemistry of the ceramic/metal interfaces. Electron energy loss and energy-dispersive X-ray spectroscopies were used to determine the chemistry and nature of bonding at the interfaces. Atom-probe microscopy was used to measure quantitatively the chemical composition of the interfaces.
Four alloys were internally oxidized: Pd-2.3 at.% Mg, Cu-2.5 at.% Mg, Cu-2.5 at.% Mg-0.8 at.% Ag, and Ag-1.5 at.% Cd-1 at.% Au. MgO precipitates are formed in Pd, Cu and Cu(Ag) matrices and CdO precipitates are formed in a Ag(Au) matrix. In all cases, the precipitates are octahedral-shaped, bound by {222} planes, and maintain a cube-on-cube orientation relationship with the
matrix.
Atom-probe microscopy revealed a significant level of segregation in both ternary systems. The Gibbsian excess of solute at the interface (Γsolute) was directly calculated. The values of Γsolute are (4.0±1.9)×1014 atoms cm-2 for the {222}MgO/Cu(Ag) and (3.0±1.0)×1014 atoms cm-2 for the {222}CdO/Ag(Au) interfaces.
Scanning transmission electron microscopy of {222}MgO/Cu(Ag)
interfaces showed that the interfaces are semicoherent and contain misfit dislocations spaced at 1.45±0.19 nm. Double and quadruple-height steps were observed. Stand-off misfit dislocations were found at one {111} interplanar distance (0.208 nm) in the metal matrix. It was proven by measurements of the
interface separation that the interface is terminated by oxygen ions. Silver enrichment at this interface was detected by Z-contrast imaging and by electron energy loss spectroscopy (EELS).
A study of {222}MgO/Cu interfaces by EELS revealed that this interface is terminated by oxygen ions and showed that copper remains metallic at the interface, introducing electronic states inside the band gap of MgO. A study of {222}MgO/Pd interfaces by energy-dispersive X-ray spectroscopy (EDX), however, could not reveal the interfacial chemistry due to experimental limitations. The conditions necessary for the successful interface analysis by EDX are examined. |
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Northwestern University |
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Ph.D. thesis |
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no |
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NU @ karnesky @ |
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149 |
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