Anne Marie Huntz

Self-diffusion in cr2o3 I. Chromium diffusion in single crystals

Abstract Chromium self-diffusion coefficients in chromia (Cr2O3) single crystals were determined by both ion implantation and thick-film methods, using the 54Cr and 50Cr isotopes. The concentration profiles were established by secondary-ion mass spectrometry, and the diffusion coefficients were computed using a solution of Ficks law taking into account evaporation and exchange at the surface. Chromium diffusion was studied as a function of temperature T and oxygen pressure Po2. Both methods lead to diffusion coefficients of the same order of magnitude. The diffusion coefficients are lower than those given in the literature and do not depend on the oxygen pressure; they are well described by the relation

Diffusion of18O in massive Cr2O3 and in Cr2O3 scales at 900°C and its relation to the oxidation kinetics of chromia forming alloys

The lattice and grain-boundary diffusion coefficients of18O atPO2=0.1 atm and at 900°C were determined in massive Cr2O3 and in Cr2O3 scales which were grown on a Ni−30Cr alloy. The diffusion profiles were established by SIMS and analyzed considering two domains in the case of polycrystalline Cr2O3 (massive or scales), the first one relative to apparent diffusion and the second to grain-boundary diffusion. A ridge model is proposed for Cr2O3 scales to modify thef value, fraction of sites associated with the grain boundary. With such a model,f is equal to 0.0006 and 0.0005 for the scales formed during 15 hr and 165 hr, respectively. The oxygen-lattice diffusion coefficients determined in Cr2O3 scales are in very good agreement with those in massive Cr2O3. With some assumptions, our diffusion data lead to a calculated parabolic oxidation constant equal to the experimental one. Scale growth occurs by countercurrent diffusion of oxygen and chromium, mainly by grain-boundary diffusion.

Self-diffusion in cr2o3 II. Oxygen diffusion in single crystals

Abstract Oxygen self-diffusion in chromia (Cr2O3) single crystals was studied as a function of oxygen pressure at 1100°C, using the gas-solid isotope exchange method. The diffusion experiments were performed in H2-H2 18O atmospheres. After the diffusion anneal, the 18O diffusion profiles were determined by secondary ion mass spectrometry and the diffusion coefficients were computed using a general solution for the Ficks second law, taking into account evaporation and exchange at the surface. Our results show that oxygen diffusion coefficients at 1100°C do not depend on the oxygen pressure and are smaller than values given in the literature. Comparison with results concerning chromium self-diffusion in the same Cr2O3 single crystals clearly indicates that oxygen diffusion is faster than chromium diffusion.

Self-diffusion in cr2o3 III. Chromium and oxygen grain-boundary diffusion in polycrystals

Abstract Chromium and oxygen grain-boundary self-diffusion coefficients in Cr2O3 polycrystals were determined by a thick-film method, using the 54Cr isotope and isotopic exchange with 18O. Depth profiling was made by secondary-ion mass spectrometry, and the diffusion coefficients were computed using the Whipple-Le Claire equation for type B intergranular diffusion. Grain-boundary self-diffusion coefficients were determined as a function of temperature and oxygen pressure. The grain-boundary self-diffusion coefficients are lower than the few results given in the literature and do not seem to depend on the oxygen pressure. Moreover, it is found that oxygen grain-boundary diffusion is faster than chromium grain-boundary diffusion.

Relation between impurities and oxide-scale growth mechanisms on Ni-34Cr and Ni-20Cr alloys. I. Influence of C, Mn, and Si

The oxidation behavior of Ni-Cr alloys (34 and 20 wt.% Cr) was investigated between 850 and 1200°C in oxygen for a maximum duration of about 70 hr. The oxide-growth mechanism is a diffusion process controlled by either outward diffusion of chromium in Cr2O3 (Ni-34Cr alloy) or by an increase in grain size (Ni-20Cr alloy). In the case of the Ni-34Cr alloy, low values of chromium diffusion were found for the growth of Cr2O3 by taking into account the general equation of Wagner. The influence of impurities (Si, C, Mn, Ni) diffusing from the underlying alloy is analyzed because of their doping effect in the outer oxide scales.

Comparative study of high temperature oxidation behaviour in AISI 304 and AISI 439 stainless steels

This work deals with a comparison of high temperature oxidation behaviour in AISI 304 austenitic and AISI 439 ferritic stainless steels. The oxidation experiments were performed between 850 and 950 °C, in oxygen and Ar (100 vpm H 2 ). In most cases, it was formed a Cr 2 O 3 protective scale, whose growth kinetics follows a parabolic law. The exception was for the the AISI 304 steel, at 950 °C, in oxygen atmosphere, which forms an iron oxide external layer. The oxidation resistance of the AISI 439 does not depend on the atmosphere. The AISI 304 has the same oxidation resistance in both atmospheres, at 850 °C, but at higher temperatures, its oxidation rate strongly increases in oxygen atmosphere. Concerning the performance of these steels under oxidation, our results show that the AISI 439 steel has higher oxidation resistance in oxidizing atmosphere, above 850 °C, while, in low pO 2 atmosphere, the AISI 304 steel has higher oxidation resistance than the AISI 439, in all the temperature range investigated.

Measurement of iron self-diffusion in hematite single crystals by secondary ion-mass spectrometry (SIMS) and comparison with cation self-diffusion in corundum-structure oxides

Iron bulk self-diffusion coefficients were measured in Fe2O3 single crystals using original methodology based on the utilization of 57Fe stable isotope as iron tracer and depth profiling by secondary ion-mass spectrometry (SIMS). The iron self-diffusion coefficients were measured along and perpendicular to the c-axis, between 900 and 1100°C, in an oxygen atmosphere. Along the c-axis, the coefficients can be described by D ∥ c (cm2/s) = 5.2 × 106 exp[−510 (kJ/mol)/RT], and are close to reliable data, available in the literature, obtained by means of radioactive techniques. Perpendicular to the c-axis, D ⊥c (cm2/s) =  83 exp[−430 (kJ/mol)/RT], and the coefficients are smaller than coefficients along the c-axis. The data are compared with previously results of cation bulk self-diffusion in Cr2O3 and Al2O3 single crystals.

High temperature oxidation behavior of the AISI 430A and AISI 430E stainless steels in Ar/H2/H2O atmosphere

atm. The microstructure and the composition of the oxide scales were analysed by scanning electronic microscopy (SEM) and energy dispersive spectroscopy (EDS). Different oxidation behaviors in AISI 430A and AISI 430E stainless steels were observed. At 850 °C, the oxidation of the 430A steel is greater than that of the 430E steel, but above 900 °C the oxidation of the 430A steels is lower than that of the 430E steel. The oxidation rate of the 430A steel shows low dependence on temperature, while the oxidation of the 430E follows an Arrhenius law, with an activation energy corresponding to the chromia scale growth.

Oxygen diffusion in Bi2O3-doped ZnO

In order to clarify the influence of Bi-doping on oxygen diffusion in ZnO, the bulk and grain boundary oxygen diffusion coefficients were measured in Bi2O3-doped ZnO polycrystals by means of the gas-solid exchange method using the isotope 18O as the oxygen tracer. The experiments were performed on ZnO sintered samples containing 0.1, 0.3 and 0.5 mol% Bi2O3. The diffusion annealings were performed at 942, 1000 and 1092 °C, in an Ar+18O2 atmosphere under an oxygen partial pressure of 0.2 atm. After the diffusion annealings, the 18O diffusion profiles were established by secondary ion mass spectrometry (SIMS). The results show an increase in the oxygen diffusion in the Bi2O3-doped ZnO, when compared to the oxygen diffusion in the undoped ZnO polycrystal under the same experimental conditions, both in bulk and in grain-boundaries. Moreover, it was observed that the higher the Bi2O3 concentration, the higher the oxygen diffusion. These results suggest that the incorporation of Bi2O3 increases the interstitial oxygen concentration which agrees with an interstitial diffusion mechanism both in bulk and in grain-boundaries.

Diffusion Study of Cerium and Gadolinium in Single- and Polycrystalline Yttria-Stabilized Zirconia

Yttria-stabilized zirconia (YSZ) ceramic is considered as an attractive matrix for nuclear applications, such as inert matrix for the destruction of excess plutonium or good host material for nuclear waste storage. Some actinide elements in high-level radioactive wastes can be simulated by cerium as tetravalent actinide, and gadolinium as trivalent actinide or neutron absorber. The present work is focused on the diffusion study of Ce and Gd in YSZ single crystal and high density polycrystals. A thin film of Ce or Gd was deposited either by spin-coating method or by physical vapour deposition on the surface of polished samples. The diffusion experiments were performed from 1173 to 1673 K under air. The Ce or Gd diffusion profiles were determined by secondary ion mass spectrometry. The experiments led to the determination of effective diffusion coefficient, Deff, bulk and grain boundary diffusion coefficients, DB and DGB. The dependence of these diffusion coefficients on temperature is described by means of Arrhenius equations and the diffusivity is compared with literature.