Oxidation Kinetics Of Metals Research Articles

Impact of Granularity on the Oxidation Kinetics of Copper

Polycrystalline copper films with mean grain sizes varying by three orders of magnitude are prepared to correlate morphology and oxidation kinetics of metals. The films are oxidized to cuprous oxide at well‐defined pressure and temperature conditions, monitoring the progress of the reaction by in situ optical transmission spectroscopy. The oxidation kinetics is retrieved by fitting the optical data to a three‐layer Cu2O/Cu2O–Cu/Cu model with the central layer accounting for the inhomogeneity of the oxidation front due to grain boundaries. The analysis reveals the highest oxidation rates for Cu films with finest granularity, demonstrating the decisive role of grain boundaries for Cu mass transport during oxidation. Moreover, reaction rates are found to differ by one order of magnitude for oxidation along the grain boundaries and into the crystalline Cu grains. In fact, slow oxidation into the Cu crystallites is responsible for an incomplete metal‐oxide conversion in the case of coarse‐granular films. The experiments demonstrate the direct interplay between morphology and oxidation kinetics for metals.

Localized surface plasmon sensing based investigation of nanoscale metal oxidation kinetics

The localized surface plasmon resonance (LSPR) of nanoparticles can be a powerful and sensitive probe of chemical changes in nanoscale volumes. Here we have used the LSPR of silver (Ag) to study the oxidation kinetics of nanoscopic volumes of cobalt (Co) metal. Bimetal nanoparticles of the immiscible Co–Ag system prepared by pulsed laser dewetting were aged in ambient air and the resulting changes to the LSPR signal and bandwidth were used to probe the oxidation kinetics. Co was found to preferentially oxidize first. This resulted in a significant enhancement by a factor of 8 or more in the lifetime of stable Ag plasmons over that of pure Ag. Theoretical modeling based on optical mean field approximation was able to predict the oxidation lifetimes and could help design stable Ag-based plasmonic nanoparticles for sensing applications.

Potentiostatic Oxide Growth Kinetics on Ni-Cr and Co-Cr Alloys: Potential and pH Dependences

Oxide growth kinetics on the Ni-Cr-Fe alloy Inconel 600 and the Co-Cr alloy Stellite 6 under potentiostatic polarization have been investigated by current measurements augmented by ex-situ surface analyses. The results reveal a mechanism for metal oxidation and oxide formation that is common to both alloys. The reaction thermodynamics for the oxidation of a metal determine whether a certain metal oxidation can or cannot occur. However, the metal oxidation proceeds via two competing pathways, oxide formation and metal ion dissolution. At pH 10.6 where the solubilities of FeII, NiII or CoII species are near their minima, oxide formation is favoured over metal ion dissolution. As the oxide grows, the rate of metal oxidation decreases with time due to an increase in the electrochemical potential barrier. The oxide formation occurs sequentially; the conversion of the preformed Cr2O3 film to chromite (FeCr2O4 or CoCr2O4) proceeds before the next layers of Fe3O4/NiFe2O4 and NiO/Ni(OH)2 grow on Inconel 600, or CoO/Co(OH)2 grows on Stellite 6. The effect of a different EAPP is to limit the oxidation sequence. The pH does not directly affect the driving force for metal oxidation but it strongly influences the relative rates of oxide formation and metal dissolution, thereby affecting metal oxidation kinetics.

Metal oxidation kinetics and the transition from thin to thick films

We report an investigation of growth kinetics and transition from thin to thick films during metal oxidation. In the thin film limit (<20 nm), Cabrera and Mott's theory is usually adopted by explicitly considering ionic drift through the oxide in response to electric fields, where the growth kinetics follow an inverse logarithmic law log(dl/dt) is proportional to 1/l. It is generally accepted that Wagner's theory, involving self-diffusion, is valid only in the limit of thick film regime (>1 μm) and leads to parabolic growth kinetics dl/dt is proportional to 1/l, where l is the oxide film thickness. Theory presented here unifies the two models and provides a complete description of oxidation including the transition from thin to thick film. The range of validity of Cabrera and Mott's theory and Wagner's theory can be well defined in terms of the Debye-Hückel screening length. The transition from drift-dominated ionic transport for thin film to diffusion-dominated transport for thick film is found to strictly follow the direct logarithmic law log(dl/dt) is proportional to -l that is frequently observed in many experiments.

In situ X-ray diffraction study of self-forming barriers from a Cu–Mn alloy in 100 nm Cu/low- k damascene interconnects using synchrotron radiation

An in situ study of self-forming barriers from a Cu–Mn alloy was performed to investigate the barrier growth using X-ray diffraction on damascene lines. The associated evolution in interconnect texture and Cu stress was also observed. The shift in Cu diffraction peak position was used to determine the change in Mn concentration and hence, estimate the thickness of the MnSi x O y barrier. The observed peak shift followed a log( t) behaviour and is described well by metal oxidation kinetics, following the field enhanced diffusion model. We used multiple anneal temperatures to study the activation of the formation process, demonstrating a faster barrier formation with higher ion excitation. A strong [1 1 1] Cu texture was shown to develop during the anneal in contrast to traditional PVD barrier systems. Finally, the stress in the 100 nm Cu lines was calculated, observing a large in-plane relaxation when using a self-forming barrier due to reduced confinement.

A procedure for rapid studies of the metal oxidation kinetics at high temperatures

A procedure for rapid studies of the metal oxidation kinetics in one experiment with one sample in a wide high-temperature range is described. In this experiment, the temperature of the sample is sequentially raised step by step and its mass increment is continuously recorded in the isotherm. The proposed procedure differs from the thermogravimetric isothermic method for studying the kinetics of the high-temperature oxidation of metals (when it is necessary to conduct multiple isothermic experiments with a corresponding number of samples) in substantially decreased time and facilities required for executing the studies. To check this procedure, an iodide zirconium sample was oxidized in an air + argon mixture in a 1380-to 1720-K temperature range. The obtained results agree with the available reference data.

Metal/Oxide Interfacial Reactions: Oxidation of Metals on SrTiO3 (100) and TiO2 (110).

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Metal/Oxide Interfacial Reactions: Oxidation of Metals on SrTiO3 (100) and TiO2 (110)

We studied chemical reactions between ultrathin metal films (Al, Cr, Fe, Mo) and single-crystal oxides (SrTiO3 (100), TiO2 (110)) with X-ray photoelectron spectroscopy (XPS). The work function of the metal and the electron density in the oxide strongly influence the reaction onset temperature (T(RO)), where metal oxidation is first observed, and the rate of metal oxidation at the metal/oxide interfaces. The Fermi levels of the two contacting phases affect both the space charges formed at the interfaces and the diffusion of ionic defects across the interfaces. These processes, which determine metal oxidation kinetics at relatively low temperatures, can be understood in the framework of the Cabrera-Mott theory. The results suggest that the interfacial reactivity is tunable by modifying the Fermi level (E(F)) of both contacting phases. This effect is of great technological importance for a variety of devices with heterophase boundaries.

Yttrium silicate formation on silicon: Effect of silicon preoxidation and nitridation on interface reaction kinetics

The effects of oxygen and nitrogen pretreatments on interface reaction kinetics during yttrium silicate formation on silicon are described. X-ray photoelectron spectroscopy (XPS) and medium energy ion scattering (MEIS) are used to determine chemical bonding and composition of films formed by oxidation of yttrium deposited on silicon. Capacitance–voltage testing is used to determine the quality of the dielectric and the electrical thickness. The effect of ultrathin silicon oxide, nitrided oxide, and nitrided silicon interfaces on metal oxidation kinetics is also described. When yttrium is deposited on clean silicon and oxidized, XPS and MEIS indicate significant mixing of the metal and the silicon, resulting in a film with Y–O–Si bonding and composition close to yttrium orthosilicate (Y2O3⋅SiO2). A thin (∼10 Å) in situ preoxidation step is not sufficient to impede the metal/silicon reaction, whereas a nitrided silicon interface significantly reduces the silicon consumption rate, and the resulting film is close to Y2O3. The mechanisms described for yttrium are expected to occur in a variety of oxide and silicate deposition processes of interest for high-k dielectrics. Therefore, in addition to thermodynamic stability, understanding the relative rates of elementary reaction steps in film formation is critical to control composition and structure at the dielectric/Si interface.

A System for Measurement of the Oxidation Kinetics of Metals Based on Solid-State Electrochemistry

A novel system for measuring oxidation kinetics, based on solid-stateelectrochemistry, has been designed and developed. In this system, thepressure of the oxidizing gas was measured by a gas-pressure sensor andkept constant in the course of oxidation using aPt|ZrO2(Y2O3)|Pt oxygenpump. The mass of oxygen consumed was calculated by integrating the electriccurrent flowing through the solid-state electrochemical oxygen pump overtime. The oxidation kinetics curve could be plotted and displayed on thecomputer automatically and continuously. The reliability of, and operatingconditions for, the system have been determined. The results showed that thesystem can be used for accurate measurement of the kinetics of oxidation ofmetals in oxygen at pressures of 0.05 to 1 atm and over a wide range oftemperatures. The accuracy of measurement was close to 0.01 mg and could beimproved with further development.

Kinetic Analysis of High‐Temperature Oxidation of Metals Accompanied by Scale Volatilization

An equation suitable for analyzing the high‐temperature oxidation kinetics of metals in the case where scale volatilization is inevitable is presented. The equation derived , where is the parabolic rate constant, the mass fraction of the metal Me in the scale oxide , and the volatilization rate constant of the oxide, has such a simple form as to be used for easily separating the mass gain into the scale growth due to diffusion and its volatilization by curve‐fitting the equation for mass gain data. The limitations of the application of this equation are discussed, and its validity is verified by its application to experimental data reported for alloys. © 1999 The Electrochemical Society. All rights reserved.

Effect of the segregation-induced potential barrier on gas/solid kinetics. Impact on corrosion kinetics of Zircaloy

The surface vs. bulk defect chemistry of zirconia is considered in terms of electrical properties, such as electrical conductivity (bulk) and work function (surface). The difference between the two is considered in terms of segregation-induced concentration gradients. It is postulated that the transport of charged defects through the interface layer of zirconia may be influenced strongly by the local electric field, which is formed within this layer across the segregation-induced concentration gradients. The effect of this field on the oxidation kinetics of metals and alloys, such as Zircaloy, is considered.

Electron stimulated oxidation of the Ni(111) surface: Dependence on substrate temperature and incident electron energy

The effect of surface temperature on the rate of oxidation of the Ni(111) surface with and without electron irradiation has been determined for temperatures between 120 and 340 K. The oxidation rate in the presence of an electron beam demonstrates an inverse dependence on the substrate temperature, while without an electron beam we observe a decrease in oxidation rate with decreasing substrate temperature, decreasing almost to zero at 120 K. Similar rates are observed near room temperature for the two cases. We have found that oxidation of this surface can be well described by either of two rate expressions: one that relates the oxide growth rate to the rate of lateral growth of two dimensional oxide islands, and another that is first order in oxide and oxygen coverages at the surface. The physical implications of each model are discussed in terms of the nucleation sites created by the electron beam, and the rate constants for oxidation at these nucleation sites. We present evidence that the nucleation sites created by the electron beam are metastable, with an unusually long half-life of about 600±150 s. We have also investigated the dependence of the cross section for nucleation center creation as a function of incident electron energy at constant electron flux and constant oxygen exposure. The energy dependencies of the cross section for nucleation center creation and the yield of secondary electrons produced by irradiation from the incident electron beam are compared, leading to consideration of the role that such secondary electrons may have in the creation of nucleation centers. The results presented herein delineate the correct low temperature oxidation kinetics for Ni(111) in the absence of perturbing electrons. They also provide a cautionary note for experiments which use electron-based probes, or optical probes which generate intense swarms of electrons, for studying the oxidation kinetics of metals, and perhaps other classes of interfacial reactions.

Reactive evaporation of low-defect density hafnia

Motivation for this work includes observations at Lawrence Livermore National Laboratory of a correlation between laser damage thresholds and both the absorption and the nodular-defect density of coatings. Activated oxygen is used to increase the metal-oxidation kinetics at the coated surface during electron-beam deposition. A series of hafnia layers are made with various conditions: two µ-wave configuations, two sources (hafnium and hafnia), and two reactive oxygen pressures. Laser damage thresholds (1064-nm, 10-ns pulses), absorption (at 511 nm), and nodular-defect densities from these coatings are reported. The damage thresholds are observed to increase as the absorption of the coatings decreases. However, no significant increase in damage thresholds are observed with the coatings made from a low nodular-defect density source material (hafnium). Hafnia coatings can be made from hafnium sources that have lower nodular-defect densities, lower absorption, and damage thresholds thatare comparable with coatings made from a conventional hafnia source.

Metal oxidation kinetics from the viewpoint of a physicist: the microscopic motion of charged defects through oxides

Metal oxidation kinetics from the viewpoint of a physicist: the microscopic motion of charged defects through oxides

Influence of interference effects in oxide films on the kinetics of laser heating of metals

A theoretical model is developed for the oxidation of metals by cw laser radiation allowing for interference effects in the oxide film. The problem is resolved by jointly solving a parabolic equation for the metal oxidation kinetics, a heat conduction equation allowing for convective and radiative losses, and also an equation which relates the absorbtivity of the oxide-metal system to the thickness and optical constants of the oxide film and the cold absorptivity of the metal. An analysis is made of the heating kinetics from room temperature to the melting point of a thermally thin plate and a semi-infinite target. The results of the calculations show good agreement with the experimental data on the oxidation of copper by cw CO2 laser radiation.

Influence on the oxidation kinetics of metals by control of the structure of oxide scales

An explanation of the deviation from the parabolic law is the treatment which considers both shortcircuit and lattice diffusion in the oxide scale. In this study we examine how the oxidation kinetics are influenced by changing the structure of the scale of copper oxide in order to confirm the role of short-circuit diffusion in determining the oxidation rate. In addition we explain the oxidation kinetics of copper and nickel by using a model of the scale structure which includes recrystallization and grain growth. Results are as follows: (1) The nucleation and growth behavior of oxide have a direct effect on the structure and in turn the oxidation kinetics due to short-circuit diffusion. (2) A modified treatment is valid in the region where volume diffusion and short-circuit diffusion play an important role in which it is necessary to consider the scale structure such as the grain size distribution and the boundary width. (3) When recrystallization takes place it is necessary to consider the model of a two-layered scale structure which is different in properties and morphology. (4) In this region the rate curves are S-shaped when oxide recrystallization takes place and exhibit a transition from a parabolic to an nth-power relationship (n>2) when grain growth takes place.

Calculations of parabolic rate constants for metal oxidation

The conditions necessary to determine the oxidation kinetics of metals and alloys are discussed quantitatively, and a new method of calculating the rational rate constants was suggested. It was shown that the size and the shape of a metal sample has an effect on the kinetics of oxidation. Further, it was established that the values of the parabolic rate constants of corrosion, calculated from the empirical Pilling and Bedworth equation, contain a serious systematic error, if changes in the metallic core surface area during the oxidation process are neglected.

A bakeable vacuum microbalance for determining metal oxidation kinetics

Details are given of a vacuum microbalance used to study the oxidation of metals at high temperatures over a pressure range of 10−6–760 Torr. The balance is a modified Cahn RG Electrobalance housed inside a stainless steel chamber, which forms part of a bakeable all-metal UHV system. All possible electrical components were taken outside the vacuum system and all internal electrical connections were replaced by spot-welded platinum wire. After bakeout at 100 °C pressures of &amp;lt;5×10−8 Torr were obtained, the outgassing rate from the balance, walls, etc., being ∼8×10−8 Torr·liter·sec−1. Low oxygen pressures were maintained by a Granville-Phillips automatic pressure controller and either a conventional wire-wound resistance furnace or a radiant heating furnace using quartz-iodine lamps was employed to heat the samples. The performance of the balance is illustrated by some results on the oxidation kinetics of iron and nickel at 500 and 700 °C. The total noise level during an experiment was typically about 3 μg.

The effect of metal vacancies on the oxidation kinetics of metals

When transport through a strongly adherent oxide film on a metal is via cations, vacancies are generated in the metal at the metal-oxide interface. If the concentration of vacancies at the metaloxide interface rises above the equilibrium concentration, the reverse reaction at that interface (i.e. metal ions going from the oxide into the metal) will be enhanced. This will have the effect of reducing the oxidation rate thereby altering the oxidation kinetics. Rate equations are calculated for a variety of limiting cases. For the cases where no internal vacancy sinks are present in the metal the thickness of the metal being oxidized enters the rate equations as a parameter.