Navegando por Autor "Millot, F."

Agora exibindo 1 - 5 de 5

Effect of grain-boundaries on uranium and oxygen diffusion in polycrystalline UO2.
(2000) Sabioni, Antônio Claret Soares; Ferraz, Wilmar Barbosa; Millot, F.
The influence of grain-boundaries on uranium and oxygen difusion in polycrystalline UO2 has been investigated. Our results show that between 1498°C and 1697°C, in H2 atmosphere, uranium difusion in UO2 grain-boundaries is about ®ve orders of magnitude greater than uranium volume di€usion, in the same experimental conditions. Between 605°C and 750°C, in H2/N2/H2O atmosphere, the oxygen difusion coe cients measured in polycrystalline and single crystalline UO2 are similar, and correspond to the volume difusion.
First study of uranium self-diffusion in UO2 by SIMS.
(1998) Sabioni, Antônio Claret Soares; Ferraz, Wilmar Barbosa; Millot, F.
Uranium self-di€usion coe cients were measured in UO2 single crystals through an original methodology based on the utilization of 235U as tracer and depth pro®ling by secondary ion mass spectrometry (SIMS). The values of the selfdi €usion coe cients measured in this work, between 1498 and 1697°C, in H2 atmosphere, can be described by the following Arrhenius relation: D (cm2/s).8.54 ´ 10ÿ7exp[)4.4(eV)/kT], and are lower than those previously determined by other authors in `nearly stochiometric' UO2 in the same temperature range. Ó 1998 Elsevier Science B.V. All rights reserved.
Oxygen self-diffusion in a cordierite glass.
(1998) Sabioni, Antônio Claret Soares; Zanotto, Edgar Dutra; Millot, F.; Tuller, H. L.
Oxygen self-di€usion measurements determined below and above the glass transition temperature in a cordierite glass are reported. The di€usion experiments were performed by the solid±gas isotope exchange method using 18O as a tracer in H2±H2 18O and Ar/18O2 atmospheres. The 18O di€usion pro®les were established by secondary ion mass spectrometry (SIMS) after appropriate di€usion annealing. The oxygen di€usion coe cients changed at the glass transition temperature as expected. The atmosphere a€ected the oxygen di€usivity in agreement with other studies. The di€usion data were useful for comparisons with di€usion controlled process in the same glass. There is a di€erence between the activation enthalpies for crystal growth, viscous ¯ow, and oxygen self-di€usion. Hence, bond-breaking and molecular reorientation required for crystallization and the atomic transport mechanism involved in viscous ¯ow are not correlated in any obvious way to oxygen di€usion in cordierite glass. Ó 1998 Elsevier Science B.V. All rights reserved.
Self-diffusion in a-Fe203 natural single crystals.
(1999) Amami, B. A.; Addou, Mohammed; Millot, F.; Sabioni, Antônio Claret Soares; Monty, C.
Measurements of 180 self-diffusion in hematite (Fe203) natural single crystals have been carried out as a function of temperature at constant partial pressure ao2 = 6.5"10 -2 in the temperature range 890 to 1227 ~ The ao2 dependence of the oxygen self-diffusion coefficient at fixed temperature T = 1150 ~ has also been deduced in the ao2 range 4.5" 10 .4 - 6.5" 10 1. The concentration profiles were established by secondary-ion mass spectrometry; several profiles exhibit curvatures or long tails; volume diffusion coefficients were computed from the first part of the profiles using a solution taking into account the evaporation and the exchange at the surface. The results are well described by - 0 26 ( 542 (kJ/mol) ] D o (cm2/s) = 2.7.108 ao2' exp,- ~- J From fitting a grain boundary diffusion solution to the profile tails, the oxygen self-diffusion coefficient in sub-boundaries has been deduced. They are well described by - 0 4 ( 911 (kJ/mol) / D" o (cm2/s) = 3.2.1025 ao2' exp,- RT J Experiments performed introducing simultaneously 180 and 57Fe provided comparative values of the self-diffusion coefficients in volume: iron is slower than oxygen in this system showing that the concentrations of atomic point defects in the iron sublattice are lower than the concentrations of atomic point defects in the oxygen sublattice. The iron self-diffusion values obtained at T > 940 ~ can be described by DFe (cm2/s) 9.2 101~ -056 (578(kJ/mol) ) = 9 ao~ exp,- The exponent-1/4 observed for the oxygen activity dependence of the oxygen self-diffusion in the bulk has been interpreted considering that singly charged oxygen vacancies V~ are involved in the oxygen diffusion mechanism. Oxygen activity dependence of iron self-diffusion is not known accurately but the best agreement with the point defect population model is obtained considering that iron self-diffusion occurs both via neutral interstitals Fe x and charged ones.
Study of oxygen self-diffusion in oxides by ion beam techniques : comparison between nuclear reaction analysis and SIMS.
(1993) Sabioni, Antônio Claret Soares; Freire Júnior, Fernando Lázaro; Leite Filho, Carlos Vieira Barros; Amami, B. A.; Dolin, C.; Monty, C.; Millot, F.
Oxygen self-diffusion in Fe,_,0 single crystals has been studied by means of the isotope exchange method in Hz/Hz”0 atmospheres. The ‘*O concentration profiles were determined by using two different techniques based on ion beams: secondary ion mass spectrometry (SIMS) and nuclear reaction analysis (NRA); the latter by means of the narrow nuclear resonance reaction ‘“O(p, o)“N at 629 keV, r = 2.1 keV. The diffusion coefficients obtained from the analysis of the measured profiles are in good agreement with those found in the literature. To compare the depth resolution and sensitivity of both techniques, the “0 profiles of implanted Cr,O, single crystals (Erso = 50 keV, fluence = 1.6 X lOI ions/cm’) were also measured.