POSITION SENSITIVE ATOM PROBE STUDIES OF THE COMPOSITION OF Ω AND θ' PRECIPITATES IN Al-Cu-Mg-Ag ALLOYS

Low concentration additions of silver to Al-Cu-Mg alloys can lead to the formation of platelike precipitates, designated Ω, is formed on {111} planes. Although the morphology of these precipitates has been extensively studied, little information exists about the chemistry of the Ω precipitates, and in particular, about local elemental segregation. In this paper we report the results obtained in a series of experiments carried out in the Oxford atom probe laboratory on the microchemistry of both Ω and θ' precipitates in an Al-Cu-Mg-Ag alloy. The Ω precipitates are found to be richer in magnesium and silver than the θ' precipitates, but are found to have a lower copper content. Studies of coherent interfaces of the Ω precipitates suggest that there is no preferential segregation of magnesium or silver to these interfaces

STRUCTURAL AND ELECTRICAL EFFECTS OF DOPANT SEGREGATION TO SILICON GRAIN BOUNDARIES

L'analyse STEM à haute résolution de ségrégation aux joints de grains d'arsenic dans le silicium polycristallin a démontré qu'il y a forte ségrégation après chauffage entre 700 et 1000°C. La variation de la ségrégation avec la température permet le calcul de l'énergie moyenne de ségrégation d'atomes As aux joints de grains du silicium (0,65eV/atome) et la concentration de saturation moyenne (12at%). Nous considérons deux espèces possibles de sites de ségrégation en insistant sur les effets électriques de la ségrégation à ces sites.High resolution STEM analysis of grain boundary segregation of arsenic in CVD grown polysilicon has shown that there is substantial equilibrium segregation after annealing at temperatures between 700 and 1000°C. The variation of extent of segregation with temperature allows calculation of the average binding energy of arsenic atoms to silicon grain boundaries (0.65eV/atom) and the average saturation concentration in the boundaries (12at%). 2 possible segregation sites are discussed with emphasis on the electrical effects of segregation to them

The effect of composition in controlling the O content in (Bi,Pb)2 Sr2 Ca2Cu3Oy thin films

The effect of composition in controlling the O content of textured polycrystalline Bi2-xPbxSr2Ca2Cu3Oy thin films grown on MgO(001) substrates by RF magnetron sputtering has been studied. Films of composition Bi1.5Pb1.5Sr1.25Ca1.85Cu3Ox sintered at high temperatures (862 °C) for 30 min in air were cooled down to different low temperatures (700-150 °C) with a cooling rate of 1 °C min-1 and subsequently air quenched to room temperature. A small decrease of Tc from 109.5 K to 105.5 K was observed with decreasing quenching temperature, i.e. with increasing O content. The same effect has also been observed in films sintered and cooled in different partial O2 atmospheres. However, it was found that this is not the case for slightly Bi-rich (Bi:1.82) as-deposited film compositions and films doped with excess Pb (Pb: more than 0.26 in sintered films). It is suggested that the O content can be changed at ambient pressure in air for optimization of the charge carrier concentration. The contradictory results in the literature concerning the variation of Tc with O content for the Bi-2223 phase can now be understood as being due to differences in the film composition or initial O content of the films

Materials for semiconductor devices

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Analysis of nanometer–sized precipitates using atom probe techniques

Positive-sensitive detection has been combined with time-of-flight mass spectrometry in the atom probe field-ion microscope to yield a system in which both chemical identity and spatial information are obtained for individual ions field evaporated from the specimen surface. This allows the variations in composition originally present in the sample to be reconstructed in three dimensions with subnanometer resolution. From these data, selected region analyses are easily performed, permitting the accurate determination of precipitate compositions without the difficulties of alignment that can plague conventional atom probe studies of very fine precipitates. The position-sensitive atom probe technique is described, together with some examples of its application in the study of precipitation in maraging steels, aluminium alloys, and the iron-copper system. © 1990