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ABSTRACT: Tantalum oxide films produced from anodic oxidation and thermal oxidation at 400 and 800°C have been investigated with respect
to long- and short-range structure by X-ray diffraction and X-ray absorption spectroscopy, respectively. Films formed at 800°C
show orthorhombic structure
with greater disorder compared to the
powder reference, where six O atoms were observed at an average radial distances of
powder) and
Å (800°C Ta oxide film). Lower temperature films consist primarily of metastable
and appear to be relatively stable for extended times of treatment with a first shell not unlike that of
based on a linear combination analysis. Furthermore, using a linear combination of
and α-Ta metal, the amorphous anodic oxide films studied as a function of thickness were observed to exhibit a relatively
ordered local structure consistent with the high-temperature (i.e., 800°C) phase. This work demonstrates that the nanoscale oxide films formed on tantalum exhibit an ordered local structure,
reflecting the very compact nature that enhances its corrosion resistance. © 2005 The Electrochemical Society. All rights
reserved.
Journal of The Electrochemical Society. 01/2005; 152(2):B60-B64.
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ABSTRACT: In order to predict the stable and metastable phases of the bcc metals in the block of the Periodic Table defined by groups 5B to 6B and periods 4 to 6, as well as the structure dependence of their transport properties, we have performed full potential computations of the total energies per unit cell as a function of the c/a ratio at constant experimental volume. In all cases, a metastable body centered tetragonal (bct) phase was predicted from the calculations. The total energy differences between the calculated stable and metastable phases ranged from 0.09 eV/cell (vanadium) to 0.39 eV/cell (tungsten). The trends in resistivity as a function of structure and atomic number are discussed in terms of a model of electron transport in metals. Theoretical calculations of the electrical resistivity and other transport properties show that bct phases derived from group 5B elements are more conductive than the corresponding bcc phases, while bct phases formed from group 6B elements are less conductive than the corresponding bcc phases. Special attention is paid to the phases of tantalum where we show that the frequently observed beta phase is not a simple tetragonal distortion of bcc tantalum.
07/2002;
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ABSTRACT: Intraparticle surface diffusion is an important and rate-limiting process in the sorption of metal ions to microporous sorbents such as those of hydrous amorphous Al (HAO), Fe (HFO), and Mn (HMO) oxides; these minerals are abundant in the environment, exhibiting a high affinity for metal contaminants. In aquatic systems representative of natural environments, internal micropore surfaces of HAO, HFO, and HMO can account for 40 to 90% of the sorption sites. Surface diffusivities have been observed to range between 10(-16) and 10(-10) cm2 s(-1) for metals including Sr, Cd, Zn, and Ni. The combination of significant microporosity and small diffusivities results in the amorphous oxides acting as natural attenuating sinks.
Journal of Colloid and Interface Science 04/2002; 247(2):259-65. · 3.07 Impact Factor
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ABSTRACT: Commercial and industrial activities are being found to have
significant effects on natural ecological systems, increasing the
importance of evaluating these activities in relation to the earth's
"carrying capacity". Several types of environmental impact commonly
associated with business activities have been selected for evaluation.
These include global warming, stratospheric ozone depletion,
acidification, eutrophication, photochemical ozone formation, resource
depletion, human toxicity, and eco-toxicity. For each type of impact,
numerous sources of information have been used to estimate the carrying
capacity at the scale appropriate for the impact. Various scientific
models also have been employed. The carrying capacity estimates are a
primary input to the sustainability target method (STM), an
environmental performance metric that relates carrying capacity to
individual products and services. The STM is a basis for using carrying
capacity along with product impact and value to provide a practical
sustainability target for businesses
Electronics and the Environment, 2002 IEEE International Symposium on; 02/2002
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ABSTRACT: Adsorption mechanisms of Zn2+ and Ni2+ on hydrous manganese oxide (HMO) were investigated using XAS. Analyses reveal that both metal ions retain their primary hydration shell when sorbed to HMO, which is consistent with physical sorption. These local structures are invariant of pH (3.0-7.0) and adsorbate loading (10(-4) - 10(-2) mol (g of HMO)-1), suggesting one average type of adsorption mechanism. In addition to the first shell, a second shell was observed with 6-8 O atoms at 3.34 A for Ni and 8-10 O atoms at 3.49 A for Zn. The lack of Ni or Zn contributions in the second shell eliminates the possibility of surface precipitation or polymerization. Likewise, the absence of Mn atoms in the second shell suggests outer-sphere adsorption. Interestingly, the local structure of Ni and Zn sorbed to HMO did not change with reaction time when as much as 90% of the sorbed contribution was due to the slow sorption process, thus supporting intraparticle diffusion as the rate-limiting mechanism. This result demonstrates that adsorption sites located along the micropore walls of HMO are similar to ones located on the external surface. Overall, metals from the same group in the Periodic Table appear to form similar adsorption complexes with HMO.
Environmental Science and Technology 12/2001; 35(22):4515-21. · 5.23 Impact Factor
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ABSTRACT: Intraparticle diffusion in microporous amorphous oxides of aluminum, iron, and manganese affects contaminant mobility and bioavailability in soils and sediments. This sorption is a lengthy process, as such, predictive methods to assess thermodynamic and transport parameters would be useful. Based on enthalpies observed in recent work, adsorption of Zn, Cd, and Sr to amorphous oxides is a physical type of reaction where the metal ions retain their waters of hydration. Consequently the adsorbate-surface interactions are a function of electrostatic forces of attraction. Accordingly, knowing the hydrated radius and the hydration number of a metal cation, a correlation is used to predict enthalpy and hence affinity. Using the resulting enthalpy and the Polanyi relation, the activation energy was evaluated for Ni and Ca. This Polanyi relationship reveals that for a given metal the activation energies with respect to these oxides are comparable. Additionally, metals of the same periodic group appear to form similar sorption complexes with a particular oxide and therefore have an equivalent Polanyi constant, alpha. Assuming a sinusoidal function describes the surface potential along the oxide surface, the surface diffusivity was predicted from the site activation theory. In this work, the predicted sorption parameters proved to be equivalent to experimental ones given the associated errors.
Environmental Science and Technology 06/2001; 35(9):1779-84. · 5.23 Impact Factor
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ABSTRACT: The mobility and bioavailability of Cd and Zn in soils and sediments are affected by contaminant distribution mechanisms. One important process is sorption to hydrous aluminum, iron, and manganese oxides, which are ubiquitous in soils and sediments as both discrete particles and coatings and exhibit a high affinity for these metals. Mechanistic models are required for accurately assessing risks to populations and in the long-term management of contaminated soils and sediments. This research demonstrates intraparticle diffusion is the rate-limiting step in the sorption of Cd and Zn to microporous oxides. Furthermore, as much as 90% of the total sorption sites on the oxides reside on the micropore walls. Because long-term experiments require a lengthy period of time, predictive methods would be useful for determining surface diffusivities. Theoretically, surface diffusivities can be predicted from site activation theory, which is based on the random walk model where atoms or molecules vibrate at localized sites along the surface. Once the vibrating ion has sufficient energy, it will jump to a neighboring site. For a given metal, the associated activation energy was observed to be equivalent for all three oxides; in an effort to predict this energy, a correlation is presented between the adsorption enthalpy and the adsorbate hydrated radius. For each oxide, the Polanyi constant (α) that relates adsorption enthalpy and activation energy was equivalent for the transition metals studied.
04/2000;
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ABSTRACT: The corrosion behaviour of magnetron sputtered α- and β-Ta coated AISI 4340 steels was studied with potentiodynamic polarization and electrochemical impedance spectroscopy. The coating porosity was observed to decrease with increasing coating thickness. For coatings less than 10 μm thick (α- or β-Ta), porosity was significant and open pores resulted in severe localized corrosion of the steel substrate, coating delamination, and overall coating failure. Additionally, the β-Ta coatings were more susceptible than the α-phase to delamination. As for the 50 and 100 μm thick α-Ta coatings, the electrochemical impedance behaviour was comparable to that of Ta foil, demonstrating the coating viability and corrosion resistance.
Corrosion Science.
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ABSTRACT: The corrosion behaviour of α- and β-Ta coatings was compared to that of Cr coatings produced by cylindrical magnetron sputtering as well as electrodeposition. For Ta coatings, the corrosion process was dominated by dissolution of the steel substrate through open pores, however, at the end of 5 days, coating degradation was not observed. In contrast, while open pores were not observed with the Cr coatings, the corrosion resistance decreased as a function of time under acidic conditions, as a result of the protective oxide dissolution. Initially, however, the sputtered Cr coating exhibited improved corrosion resistance over the electrodeposited one, potentially due to the surface structure of the oxide film. Both Cr coatings showed similar transpassive behaviour to that of pure Cr; the current density of the sputtered Cr coating was observed to be greater than that of the electrodeposited one in the transpassive region due to the extensive droplet-like defects.
Surface and Coatings Technology.
