Oxidation of nickel and transport properties of nickel oxide

Oxidation kinetics of high purity nickel, as well as the nonstoichiometry and chemical diffusion in nickel oxide have been studied as a function of temperature (1373-1673 K) and oxygen pressure (10-10⁵ Pa) using microthermogravimetric techniques. In order to eliminate the possible participation of grain boundary diffusion in scale growth at lower temperatures, the oxidation rate measurements have always been started at the highest temperature (1673 K), when coarse-grained scale was formed, and the temperature and pressure dependence of the oxidation rate was determined by step-wise lowering the temperature of such pre-oxidized sample. Nonstoichiometry and the chemical diffusion coefficient in Ni_1−yO have also been determined on such coarse-grained oxide samples, obtained by complete oxidation of nickel at highest temperature (1673 K). It has been found, that under such conditions oxidation of nickel follows strictly the parabolic rate law, and the parabolic rate constant of this reaction is the following function of temperature and oxygen pressure: The results of nonstoichiometry measurements, in turn, may be described by the following relationship Finally, chemical diffusion coefficient in Ni_1−yO has been found to be independent on oxygen activity, indicating that the mobility of point defects in this oxide does not depend on their concentration, being the following function of temperature: It has been shown, that the parabolic rate constants of nickel oxidation, calculated from nonstoichiometry and chemical diffusion data are in excellent agreement with experimentally determined k_p values. All these results clearly indicate that the predominant defects in nonstoichiometric nickel oxide (Ni_1−yO) are double ionized cation vacancies and electron holes and the oxide scale on nickel growths by the outward volume diffusion of cations.     

Pressure effects on aluminium oxidation kinetics using X-ray photoelectron spectroscopy and parallel factor analysis

The effects of water vapour pressure on oxidation kinetics of aluminium have been studied using X-ray photoelectron spectroscopy (XPS) and three-way parallel factor analysis (PARAFAC). While the first technique is a powerful experimental tool for surface oxidation studies, the PARAFAC technique is a sophisticated analytical tool for analysing XPS data. The XPS Al(2p) and O(1s) core level have been used to follow the oxide film growth on clean surfaces at room temperature as a function of oxidation time (ranging from 1 to 60 min) and pressure of water vapour (ranging from 2.0×10⁻⁶ to 6.5×10⁻⁴ Pa). The growth of thin oxide films on aluminium surfaces has been found to follow the Cabrera–Mott inverse logarithmic law in all pressure ranges studied. The pressure effects have shown that the defect formation reaction at the oxide film/gas interface is the rate determining process in the aluminium oxidation. The pressure dependence of oxidation kinetics can be explained on the basis of metal vacancies in the defect structure of thin aluminium oxide films.     

Effect of various lanthanum sol-gel coatings on the 330Cb (Fe-35Ni-18Cr-1Nb-2Si) oxidation at 900 °C

The influence of a lanthanum sol-gel coating on the oxide scale adherence has been studied during the 330Cb (Fe-35Ni-18Cr-1Nb-2Si) oxidation at 900 °C, in air. The alloy oxidation is performed in order to generate a protective chromia scale acting as a good barrier against carburization. Argon annealing of lanthanum sol-gel coatings have been performed at various temperatures in order to find the best conditions to insure the scale adherence. Kinetic results show that lanthanum sol-gel coatings lead to a lower oxidation rate compared to blank specimens. Thermal cycling tests on lanthanum the sol-gel coated specimen show that the oxide scale formed at 900 °C, in air, is adherent.     

Oxidation of titanium between 25°C and 400°C

The oxidation of Ti between 25°C and 400°C has been studied in an ultra high vacuum system with ellipsometry, Auger spectroscopy and surface potential difference. The surface potential difference is primarily sensitive to the adsorbed layer of oxygen, the Auger spectrometer is sensitive to the oxygen and Ti concentration at the oxide-gas interface and the ellipsometer is sensitive to the film thickness. Consequently, it was possible to follow the various processes separately during oxidation and oxide dissolution. Oxygen adsorbs in a percusor state as a molecule which dissociates and adsorbs in the atomic state. Reaction of atomic oxygen with Ti at the oxide-gas interface causes the stoichiometry to vary with the temperature, oxygen pressure and film thickness. At low temperatures and oxygen pressure the initial film growth follows a logarithmic time law. The predominant transport species is cationic. After initial oxidation, at higher temperatures and oxygen pressure, the predominant transport species is anionic. At constant temperature and oxygen pressure the oxide grows until the rate of growth is equal to the rate of dissolution into the metal. At this point the film thickness remains constant with time. Doping the Ti surface with Au results in a decrease in the oxide dissolution process due to blocking of interstitial diffusion paths by Au atoms.     

The influence of aliovalent impurities on the oxidation kinetics of nickel at high temperatures

The influence of chromium and sodium on the nickel oxidation kinetics has been studied as a function of temperature (1373-1673 K) and oxygen activity (10−10⁵ Pa O₂), using microthermogravimetric techniques. It has been shown that the oxidation of Ni-Cr and Ni-Na alloys, like that of pure nickel, follows strictly the parabolic rate law being thus diffusion controlled. In agreement with the defect model of Ni_1−yO, it has been found that the oxidation rate of the Ni-Cr alloy is higher than that of pure nickel, the reaction rate is pressure independent and the activation energy of this process is lower. This implies that the concentration of double ionized cation vacancies in a Ni_1−yO-Cr₂O₃ solid solution is fixed on a constant level by trivalent chromium ions, substitutionally incorporated into the cation sublattice of this oxide. In the case of the Ni-Na alloy, on the other hand, the oxidation rate is lower than that of pure nickel, the activation energy is higher and the oxidation rate increases more rapidly with oxygen pressure. These results can again be explained in terms of the doping effect, by assuming that univalent sodium ions dissolve substitutionally in the cation sublattice of nickel oxide.     

Investigation by X-ray photoelectron spectroscopy of the transient oxidation of NiAl

X-ray photoelectron spectroscopy (XPS) has been used to study the oxidation of NiAl in oxygen at atmospheric pressure. Prior to oxidation, the native oxide scale on the specimen was removed by ion sputtering and the specimens were (pre-)heated in vacuum before exposure to oxygen. At low oxidation temperatures (<750 K) scales consisted of Al₂O₃ and NiAl₂O₄, with a thin surface layer of NiO, but at higher temperatures were of Al₂O₃, apart from about 0.5 at % Ni. The Ni content in the latter case was constant throughout the scale and did not increase dramatically near the alloy/oxide interface. In the experimental conditions used in this study, initial formation of NiO and NiAl₂O₄ seems to be avoided at the higher oxidation temperatures ( > 750 K).     

Oxidation and faceting of polycrystalline tungsten ribbons

We have measured the oxidation rate of tungsten and the evaporation rate of tungsten oxide in the temperature range from 900 to 1200 K at an oxygen pressure from 5 × 10^?4 to 5 × 10^?3 Torr. The oxidation rate increases steadily with coverage in the whole range studied. The evaporation rate decreases at high pressure and is strongly dependent on the initial conditions of the experiments. These kinetic measurements support a qualitative model of oxidation. The surface is composed of oxide islands surrounded by oxide-free regions covered only by chemisorbed oxygen atoms. On the bare regions beside the chemisorbed oxygen atoms we suppose the existence of a dilute chemisorbed oxide layer which can either enter the condensed oxide phase or evaporate. The number of the growing islands is set up at the beginning of the reaction and does not increase further. This model, consistent with kinetic results during oxidation, has been proposed first to explain results obtained by Auger electron spectroscopy and thermal desorption spectroscopy under vacuum. Faceting is particularly important in the early stages of the experiment because it can hinder the nucleation of the oxide which is a necessary step for growth. In a narrow range of temperature and oxygen pressure this inhibited nucleation leads to an enhanced evaporation rate so that the growth rate is lower. Recording this growth rate allows us to follow faceting. The parameters studied are the oxygen coverage and the temperature, experimental results are in agreement with LEED and RHEED results. Reconstruction and faceting are discussed and are believed to be caused by a smoothing of the surface during the chemisorption step.     

The influence of implanted yttrium on the cyclic oxidation behaviour of 304 stainless steel

High-temperature alloys are frequently used in power plants, gasification systems, petrochemical industry, combustion processes and in aerospace applications. Depending on the application, materials are subjected to corrosive atmospheres and thermal cycling. In the present work, thermal cycling was carried out in order to study the influence of implanted yttrium on the oxide scale adherence on 304 steel specimens oxidised in air at 1273 K. In situ X-ray diffraction indicates that the oxides formed at 1273 K are different on blank specimens compared to implanted specimens. Glancing angle XRD allows to analyse the oxide scale composition after cooling to room temperature.Experimental results show that yttrium implantation at a nominal dose of 10¹⁷ ions cm⁻² does not improve significantly the cyclic oxidation behaviour of the austenitic AISI 304 steel. However, it appears that yttrium implantation remarkably enhance the oxidation resistance during isothermal oxidation. It reduces the transient oxidation stage and the parabolic oxidation rate constant by one order of magnitude.     

The oxidation and co reduction kinetics of a platinum surface

A thermogravimetric technique was used to study the oxidation and CO reduction kinetics of Pt as a function of temperature, T. The measurements were performed on Pt powder (particle sizes 0.8–2.5 μm) at 1 atm pressure. After exposure to 1 atm of oxygen at 600 K for 1 h, the total uptake of oxygen by the powder amounted to less than one oxygen atom per Pt surface atom and followed a logarithmic growth law. For 400 < T < 600 K, the logarithmic rate constant, K₀, could be described by an Arrhenius law with an apparent activation energy of 2.6 kcal/mole. Above 600 K, K₀ slowly decreased, an effect believed to be associated with the dissociation of the oxide between 650 and 850 K. Oxidation isotherms were calculated using the low pressure oxygen sticking coefficient data of Hopster et al. The calculated and measured oxidation isotherms were found to be in remarkable agreement. The CO reduction data were more difficult to analyze but showed that the reduction rate had a stronger temperature dependence (~11 kcal/mole) than the oxidation rate. For427 < T<487 K, the general time scale of the reduction process was 10–50 min. Typical durations of the oxidation and CO reduction processes are consistent with the periods observed in studies of the oscillatory rate of CO oxidation over a Pt catalyst.     

激光熔覆原位合成Nb(C,N)陶瓷颗粒增强铁基金属涂层

采用预涂粉末激光熔覆技术,在42CrMo基体上制备出原位合成Nb(C, N)颗粒增强的铁基复合涂层。X射线及扫描电镜分析结果表明:激光熔覆获得的涂层基体为耐氧化、耐蚀性良好的Fe-Cr细晶组织及少量的-Fe相,原位合成的Nb（C, N）呈块状弥散分布在基体上。进一步的磨损试验表明:这些颗粒增强相极大增强了抗磨损性能,与未熔覆的母材相比,其磨损失重仅为母材的1/9左右; 涂层在750 ℃恒温氧化条件下具有较好的抗氧化性能,氧化层主要由NbO1.1,Cr2O3相组成; 母材的氧化产物为Fe2O3,容易脱落,保护性能较差; 激光熔覆涂层的氧化膜厚度仅为未涂层的1/5。     

Understanding the mechanism of aluminium nanoparticle oxidation

Aluminium nanoparticles have gained importance in the last decade because of their increased reactivity as compared with traditional micron-sized particle. The physics of burning of aluminium nanoparticle is expected to be different than that of micron-sized particles, and the current article is motivated by these differences. We have previously measured the size resolved reactivity of nanoaluminium by single-particle mass spectrometry, to which we now add transmission electron microscope (TEM) and an on-line density measurement. The latter two studies revealed the presence of hollow particles following oxidation of nanoaluminium and indicating the significance of diffusion of aluminium in the overall process. Based on experimental evidence, we believe that aluminium nanoparticle oxidation occurs in two regimes. Prior to melting of aluminium slow oxidation occurs through the diffusion of oxygen through the aluminium oxide shell. Above the melting point, we transition to a fast oxidation regime whereby both aluminium and oxygen diffuse through the oxide shell to enhance the oxidation rate.

We also develop a phenomenological model for nanoaluminium oxidation that accounts for the experimentally observed rates, the fact that both fuel and oxidizer are diffusing, and a new effect related to internal pressure gradients. The latter phenomen is based on molecular dynamic simulations suggesting that there are large pressure gradients present inside these particles, with the aluminium core under a positive pressure and the aluminium oxide shell under a negative pressure. We have considered the effect of these pressure gradients on the oxidation process. A power law relation was obtained (t ∝ r ^{1.6± 0.1}) between the time required for oxidation and particle radius.     

InP清洁表面上电子感应吸附氧的研究

用俄歇电子能谱观察清洁InP表面在电子束照射下与真空室中H₂O和O₂的相互作用,发现水蒸汽所引起的电子束感应吸附氧的作用比氧气要明显得多。在吸附氧的同时,In和P的俄歇信号也发生变化,分析其过程为一种氧化过程,氧一开始先同表面的In结合成为氧化铟,随后向表面以内透入,并不断同In和P结合,氧化层的厚度随时间几乎是线性地增加的。与InP的自体氧化层的俄歇深度分布相比较,二者极为相似,所不同的是,电子束感应吸附的氧不足以使表面层中的磷全部氧化,而ESO的氧化层中磷的 关键词：     

Initial oxidation of Ni(1 1 1) observed by electron emission microscopy: PEEM and MEEM

The initial oxidation of Ni(1 1 1) in the temperature range of 550-700 K has been monitored by photoelectron emission microscopy (PEEM) and metastable-atom electron emission microscopy (MEEM). The PEEM and MEEM images show uniform patterns for the chemisorbed overlayer, reflecting the electronic homogeneity as seen at the μm scale. During the nucleation and lateral growth of oxide, however, the μm-scale pattern due to the formation of oxide domains appears and its evolution depends strongly on the substrate temperature and dose pressure of gaseous O₂. Our data indicate that the high-temperature oxidation is regarded as a successive multi-nucleation process in a reaction-diffusion field.     

High-temperature oxidation studies of cold-sprayed Ni-20Cr and Ni-50Cr coatings on SAE 213-T22 boiler steel

The high-temperature oxidation behavior of cold-sprayed Ni-20Cr and Ni-50Cr coatings on SAE 213-T22 boiler steel has been investigated at 900 °C in air under cyclic heating and cooling conditions for 50 cycles. The kinetics of oxidation of coated and bare boiler steel has been established with the help of weight change measurements. It was observed that all the coated and bare steels obeyed parabolic rate law of oxidation. X-ray diffraction, FE-SEM/EDAX and X-ray mapping techniques were used to analyse the oxidation products of the coated and uncoated boiler steel. The uncoated steel suffered corrosion in the form of intense spalling and peeling of its oxide scale, which was perhaps due to the formation of unprotective Fe₂O₃ oxide scale. Both the coatings showed better resistance to the air oxidation as compared to the uncoated steel. The Ni-50Cr coating was found to be more protective than the Ni-20Cr-coated steel. The formation of oxides and spinels of nickel and chromium may be contributing to the development of air oxidation resistance in the coatings.     

磷通过热生长二氧化硅层在硅中的扩散

用四探针测量薄层电导方法及阳极氧化去层技术,测定了磷在硅中扩散的具体分布,在恒表面浓度下,它们偏离余误差函数分布。如认为这是由于扩散系数是杂质浓度的函数,实验得到了当杂质浓度大于10¹⁹原子/厘米³时,扩散系数随杂质浓度增加而增大的强烈依赖关系。用同样方法测定了磷通过二氧化硅层后在硅中扩散的具体分布,研究了这些杂质分布的特性,实验表明,不同厚度的氧化层在1300℃高温下仍具有掩蔽效应,在完全掩蔽失效时间附近,杂质分布的共同特点是表面浓度较低(～10¹⁷原子/厘米³)、结较浅(～1微米)。对不同厚度的氧化层,经过足够的时间后,硅中表面浓度不受氧化层厚度的影响,而只由扩散源的蒸气压决定。磷通过氧化层后扩散的具体分布情况还与扩散源的性质、条件等密切相关。扩散过程中观察到的氧化层厚度增长有可能影响表面附近杂质的具体分布情况。     

Surface oxidation of liquid Sn

We report the results of an X-ray scattering study that reveals oxidation kinetics and formation of a previously unreported crystalline phase of SnO at the liquid-vapour interface of Sn. Our experiments reveal that the pure liquid Sn surface does not react with molecular oxygen below an activation pressure of ∼5.0 × 10⁻⁶ Torr. Above that pressure a rough solid Sn oxide grows over the liquid metal surface. Once the activation pressure has been exceeded the oxidation proceeds at pressures below the oxidation pressure threshold. The observed diffraction pattern associated with the surface oxidation does not match any of the known Sn oxide phases. The data have an explicit signature of the face-centred cubic structure, however it requires lattice parameters that are about 9% smaller than those reported for cubic structures of high-pressure phases of Sn oxides.     

The oxidation of gallium nitride epilayers in dry oxygen

The oxidation of GaN epilayers in dry oxygen has been investigated. The GaN epilayers, about 1 7m thick, were grown on (0001) sapphire substrates by rapid thermal process/low pressure metalorganic chemical vapor deposition. Bulk /-2/ X-ray diffraction (XRD) data showed that the slight oxidation of GaN began to occur at 800 °C for 6 h. The oxide was identified as the monoclinic #-Ga₂O₃. The GaN epilayers were completely oxidized at 1050 °C for 4 h or at 1100 °C for 1 h. For all samples, the strongest oxide's peak is (11-3), and (30-6) followed. There is a rapid oxidation process in the initial stage of oxidation, and a relatively slow process followed when the temperature was over 1000 °C. The oxidation of two stages was limited by the rate of an interfacial reaction mechanism and by the diffusion mechanism, respectively. When the temperature reaches 1100 °C, the oxidation rate is very fast, which is considered as the results of the GaN decomposition at high temperature under atmosphere. The oxide layers were also observed by a scanning electron microscope, which shows a rough oxide surface and an expansion of the volume of 40%. The photoluminescence (PL) seriously influenced by the oxidation is also discussed.     

Stress analysis of local blisters coupling Raman spectroscopy and X-ray diffraction. Correlation between experimental results and continuous damage modelling for buckling in an iron oxide/phosphated iron system

In this present work, local stress development in the iron oxide layers growing on phosphated α-Fe at 400 °C in ambient air is investigated by Raman spectroscopy. Coupled with X-ray diffraction it enables to obtain directly local stresses’ maps in the oxide layers. Use of Raman spectroscopy allows obtaining better accuracy on mechanical behaviour at local scale. This characterisation technique is very useful to study systems developing mechanical heterogeneities on surface, especially in case of buckling phenomenon. Investigations on particular local blisters have been done to measure some characteristic lengths at local scale. From local measurements, we are able to evaluate general effect of buckling from simplified scale transition. So, a macroscopic approach has been performed to calculate global stress evolution of the oxide layer, based on continuous damage mechanics. Consequently, it leads to good comparison between modelling and experimental values (global stresses versus oxidation time) in α-Fe₂O₃ oxide.     

Theory of oxidation/reduction-induced valence transformations of metal ion dopants in oxide crystals mediated by oxide-vacancy diffusion: II. Kinetic analysis

We consider theoretically valence transformations of doping metal ions in oxide crystals induced by oxidation and reduction obtained by changes in the ambient oxygen partial pressure. Three types of oxygen vacancies are assumed to mediate transformations: neutral, singly ionized, and doubly ionized. In the companion part I paper we provide thermodynamic analyses yielding concentration relations among the oxygen vacancy, metal ions, holes and electrons, as functions of the ambient oxygen pressure. In the present companion part II paper we provide time dependent concentration profiles of the various species and reaction rate profiles. The diffusion exhibits a complex behavior; under some conditions, it may be described by a constant diffusivity, and is symmetric with respect to oxidation and reduction. However, under a wide range of conditions, the ionic state changes are highly asymmetric with respect to oxidation and reduction. For example, in the case of a neutral vacancy, a very narrow reaction front may establish during reduction. In the inverse (oxidation) process, however, the different species' profiles are quite smooth.     

Oxidation of the (100) surface of a NiFe alloy

The initial stages of oxidation of the (100) surface of a single crystal alloy specimen of approximate atomic composition Ni 59, Fe 41 (at%) have been studied by Auger spectroscopy and electron diffraction techniques. The clean alloy surface shows only a slight iron enrichment over the temperature range of the oxidation studies (373–873 K). Oxidation studies were performed over the O₂ pressure range 5 × 10^?9 to 1 × 10^?6 Torr. Within these experimental conditions the rate of oxygen uptake was found to be linear in pressure and essentially independent of temperature. LEED studies showed that a chemisorbed c(2 × 2) structure preceded the formation of surface oxides. The interaction of oxygen with the surface induced a marked segregation of iron and this was particularly pronounced at elevated temperatures. Chemical shifts were observed in the low energy Ni and Fe Auger spectra during oxidation; these were similar to those previously observed in separate studies of the oxidation of pure Ni and of pure Fe. At the higher temperatures the initial oxide layer grew epitaxially apparently as a (111) cubic oxide on the (100) substrate. The Ni to Fe concentration ratio in oxides several layers thick was found to depend on the temperature of the reaction; at higher temperatures the oxide were more Fe-rich. The Fe to Ni ratio in oxides produced at lower temperatures could be increased by annealing. At large O₂ exposures (about 5000 L) a transition was observed in the structure of the oxide layer.