Antônio Claret Soares Sabioni

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

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.

Oxygen diffusion in pure and doped ZnO

O diffusion profiles were determined by secondary ion mass spectrometry. The results of the experiments show that oxygen diffusion in Li-doped ZnO is similar to the oxygen diffusion in pure ZnO, while in Al-doped ZnO the oxygen diffusion is enhanced in relation to that observed in pure ZnO, in the same experimental conditions. Based on these results is proposed an interstitial mechanism for oxygen diffusion in ZnO. Moreover, it was found that oxygen grain-boundary diffusion is ca. 3 to 4 orders of magnitude greater than oxygen volume diffusion in pure and doped ZnO, which means that the grain-boundary is a fast path for oxygen diffusion in ZnO.

Effect of grain-boundaries on uranium and oxygen diffusion in polycrystalline UO2

Abstract The influence of grain-boundaries on uranium and oxygen diffusion in polycrystalline UO2 has been investigated. Our results show that between 1498°C and 1697°C, in H2 atmosphere, uranium diffusion in UO2 grain-boundaries is about five orders of magnitude greater than uranium volume diffusion, in the same experimental conditions. Between 605°C and 750°C, in H2/N2/H2O atmosphere, the oxygen diffusion coefficients measured in polycrystalline and single crystalline UO2 are similar, and correspond to the volume diffusion.

First study of manganese diffusion in Cr2O3 polycrystals and thin films by SIMS

Chromia layers are formed on many industrial alloys and act as a protective barrier against the corrosion of the materials by limiting the diffusion of oxygen and cations. Most of these alloys contain manganese as an impurity, and manganese oxides are often found at the outer surface of the oxide films. In order to clarify the oxidation mechanism and to check if chromia acts as a barrier, manganese diffusion in chromia was studied in both polycrystals and oxide films formed by oxidation of Ni–30Cr alloy in the temperature range 700–1100°C at an oxygen pressure of 10−4 atm. After deposition of Mn on the chromia surface and a diffusing treatment, the manganese penetration profiles were established by secondary ion mass spectrometry. In all cases, the diffusion profiles showed two domains. For the first domain, using the solution of Ficks law for diffusion from a thick film into a semi-infinite medium, bulk diffusion coefficients were determined. For the second domain, the Le Claire model allowed the grain boundary diffusion parameter (αD gbδ) to be obtained. Manganese diffusion does not vary significantly according to the nature and microstructure of chromia. The activation energy of grain boundary diffusion is not far from that obtained for bulk diffusion, probably on account of segregation phenomena. Manganese diffusion was compared to cationic self-diffusion and iron diffusion, and related to the protective character of chromia.

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.

Diffusion of nickel in single- and polycrystalline Cr2O3

Chromia protective layers are formed on many industrial alloys to prevent corrosion by oxidation. Their role is to limit the inward diffusion of oxygen and the outward diffusion of cations. A number of chromia-forming alloys contain nickel as a component, such as steels, FeNiCr and NiCr alloys. To ascertain if chromia is a barrier to outward diffusion, nickel diffusion in chromia was studied in both single crystals and polycrystals in the temperature range 900–1100°C at an oxygen pressure of 10−4 atm (argon + 100 ppm O2). A nickel film of ∼35 nm thick was deposited on the chromia surface and, after diffusing treatment, nickel penetration profiles were established by secondary ion mass spectrometry (SIMS). Two diffusion domains appear in polycrystals, the first domain is assigned to bulk diffusion and the second is due to diffusion along grain boundaries. For the bulk diffusion domain and diffusion in single crystals, using a solution of Ficks second law for diffusion from a thick film, bulk diffusion coefficients were determined at 900 and 1000°C. At the higher temperature, a solution of Ficks second law for diffusion from a thin film could be used. For the second domain in polycrystals, Le Claires model allowed the grain boundary diffusion parameter (αD gb δ) to be established. Nickel bulk diffusion does not vary significantly according to the microstructure of chromia. The activation energy of grain boundary diffusion is slightly greater than the activation energy of bulk diffusion, probably on account of segregation phenomena. Nickel diffusion was compared with cationic self-diffusion and with literature data on Fe and Mn heterodiffusion in the bulk and along grain boundaries. All results were analyzed in relation to the oxidation process of stainless steel.

Spectroscopic studies of spessartine from Brazilian pegmatites

Abstract Mössbauer spectra (MS) at room temperature (RT) and Fourier-transform infrared (FTIR) spectra in the OH stretching region were acquired for natural spessartine-almandine garnet samples from different Brazilian pegmatites, including the complex, zoned Alto Mirador pegmatite, the simple, zoned Escondido pegmatite, and the simple Poaiá pegmatite. From MS, it is obvious that in the samples from the Alto Mirador pegmatite, ferrous iron is present at the dodecahedral site and ferric iron at the octahedral site of the garnet structure. The Fe2+/Fe3+ ratio is comparable for all samples from this pegmatite, i.e., ~10%, implying the same geological history, namely similar oxygen fugacities at the moment of garnet formation. In the simple pegmatites, however, almost no ferric iron was detected. On the basis of the multi-band FTIR spectra the nature of the point defects involved in the process of hydrogen uptake in spessartine-almandine garnet has been elucidated, if not completely clarified. The behavior of the absorption bands provides no evidence for multiple incorporation mechanisms, hence the hydrogarnet substitution is proposed to be the only mechanism for the incorporation of hydrogen in the samples studied. As geological setting is difficult to separate from chemical composition, FTIR spectra have the potential to provide useful information on some aspects of the geological history of the samples.