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G Da Costa

Publications and source records attributed to G Da Costa.

5 recordsLinked to original sources
PubMed2004

Application of Fourier transform and autocorrelation to cluster identification in the three-dimensional atom probe.

Because of the increasing number of collected atoms (up to millions) in the three-dimensional atom probe, derivation of chemical or structural information from the direct observation of three-dimensional images is becoming more and more difficult. New data analysis tools are thus required. Application of a discrete Fourier transform algorithm to three-dimensional atom probe datasets provides information that is not easily accessible in real space. Derivation of mean particle size from Fourier intensities or from three-dimensional autocorrelation is an example. These powerful methods can be used to detect and image nano-segregations. Using three-dimensional 'bright-field' imaging, single nano-segregations were isolated from the surrounding matrix of an iron-copper alloy. Measurement of the inner concentration within clusters is, therefore, straightforward. Theoretical aspects related to filtering in reciprocal space are developed.

Journal Article↗
PubMed2001

Structural analyses in three-dimensional atom probe: a Fourier transform approach.

The three-dimensional atom probe (3DAP) technique gives the elemental identities and the position of atoms within the small volume analysed (on the order of 10 x 10 x 100 nm(3)). The large number of atoms collected (up to two million) and the excellent spatial resolution of this instrument allows the observation of some crystallographic features of phases chemically identified. This paper shows that the application of a discrete Fourier transform algorithm to a 3DAP dataset provides information that is not easily accessible in real space. The derivation of the mean size of particles from Fourier intensities is an example. Using 3D 'dark-field' imaging, single ordered grains were isolated from the disordered matrix of a ternary alloy. Moreover, the intrinsic spatial resolution of the instrument was evaluated by this method for pure metal; the resolution reaches 0.2 nm laterally and 0.06 nm in depth. This excellent resolution is shown to be sufficient to give access to the crystalline lattice. The use of image filtering in the reciprocal space enables for atomic columns to be imaged the first time from 3DAP data.

Journal Article↗
PubMedDate not supplied

Effects of incidence angles of ions on the mass resolution of an energy compensated 3D atom probe.

We have used a first-order reflectron lens in an optical tomographic atom probe in order to improve the mass resolution. Calculations have been performed to determine the effect of second-order errors in ion energy and incidence angle on the performance of the lens. By applying a correction procedure based on the results of these calculations, we have been able to improve experimental mass resolution by 30%.

Journal Article↗
PubMedDate not supplied

Hydrostatic pressure effects on eel mitochondrial functioning and membrane fluidity.

Aerobic metabolism which is required for long swimming activities during the eel's spawning migration at depth, is a potential target for pressure effects due to its components located in the inner mitochondrial membrane (respiratory chain and oxidative phosphorylation). Previous studies have evidenced that eels are able to acclimatize to pressure through membrane fluidity adjustment. However these studies were performed on the premigratory stage (yellow stage), which never encounters high pressure. Metamorphosis (silvering) seems to preadapt eels (at the silver stage) to most of the environmental changes they will encounter during migration. Is it also true for pressure resistance? This study shows that yellow eels exhibit a higher pressure sensitivity than silver eels (compression effects). The acclimatization period (21 days at 10.1 MPa) cancels the differences in pressure sensitivity and in aerobic metabolism observed at 0.1 MPa between the two stages. The mechanisms, which take place in yellow eels during acclimatization to high pressure, appear to be already present in silver eels before pressure exposure. Indeed at 0.1 MPa, silver eels exhibit higher membrane fluidity and proportions of membrane polyunsaturated fatty acids. Metamorphosis, by improving membrane fluidity, seems to allow silver eels to cope with hydrostatic pressure without spending energy in acclimatization processes.

Acclimatization↗