Dynamical Effects of the Oxide Layer in Aluminum Nanoenergetic Materials

Dynamical effects of the aluminum nanopowder oxide layer are investigated in nanoenergetic materials consisting of nitrocellulose (NC) oxidizer containing embedded ~ 60 nm diameter Al having thinner (2.5 nm) or thicker (6 nm) oxide layers. Following laser flash‐heating, a hot spot is formed near each Al particle. The mean distance of reaction propagation d_rxn from the hot spot through the nitrocellulose is determined with ~100 nm resolution. With 100 ps pulses a shock propagation mechanism is dominant, and with 10–25 ns pulses reaction a thermal explosion mechanism is dominant. When higher energy picosecond pulses are used, d_rxn is observed to be significantly increased with thicker oxide layers, but using nanosecond pulses d_rxn is slightly decreased with thicker oxide layers. This oxide layer enhancement of d_rxn with picosecond pulses is attributed to the thicker oxide layer confining the hot Al for several tens of picoseconds, resulting in a larger shock wave. This work supports the view of the oxide layer as deadweight for slower heating rate processes such as combustion, but it suggests a thicker oxide layer may be of some benefit for extremely high heating rate processes involved in detonation or high speed deflagration of nanoenergetic materials.     

Zinc impregnation of the anodic oxidation layer of 1050 and 2024 aluminium alloys

The porous oxide layer obtained by phosphoric anodic oxidation (PAO) of 1050 and 2024T3 aluminium alloys is modified by impregnation with zinc under alternating voltage. The resulting current against applied voltage relationship shows that a threshold voltage is required to deposit the zinc. Beyond a low critical voltage, V _{c L}, zinc electrocrystallization starts near the barrier layer and grows with time through the porous oxide layer whatever the alloy used as substrate. For the 2024T3 alloy, beyond a high critical voltage V _{c H}, S.I.M.S. analysis shows that zinc is also present on top of the oxide layer. The distribution of zinc particles depends on the porous layer morphology: formation of zinc needles in a columnar form (1050 alloy) and dispersion of zinc particles in a disorganized structure (2024T3 alloy). The polarization curves obtained in a 3 wt % NaCl solution show a decrease in anodic and cathodic currents indicating a protective effect of zinc impregnation, confirmed by electrochemical impedance analysis.     

Effect of silicon alloying addition on the corrosion behaviour of aluminium in some aqueous media

The corrosion behaviour of three Al–Si alloys was studied after galvanostatic passivation in 0.1 M sodium tartrate, sulfate and borate solutions using EIS techniques. The degree of passivation depends on the anion type, the degree of polarization and the alloy composition. It was also found that increase in pH led to a decrease in polarization resistance R _p. The effect of formation voltage, V _f, on the growth and dissolution kinetics of the oxide grown on the alloys was studied. The polarization resistance value increases as V _f increases up to a certain value; above this the R _p value decreases. This critical V _f depends on the alloy composition and the test solution. The kinetics of oxide layer dissolution in the absence and presence of Cl^– ions was also studied. Increase in immersion time leads to a more severe attack by Cl^– ions as shown by the decrease in the value of R _p. At low Cl^– ion concentration the value of R _p is higher than that in chloride ion free sulfate solutions, because the rate of passive film repair is much higher than that of barrier layer dissolution. However, at high Cl^– ion concentration penetration of Cl^– through defects in the barrier layer leads to formation of an oxyhalide layer.     

Mechanism and Kinetics of Oxidation of ZrN Ceramics

Oxidation of ZrN ceramics from 973–1373 K under static conditions reveals parabolic rate behavior, indicative of a diffusion‐controlled process. In‐situ high temperature powder XRD found the oxidation mechanism begins with destabilization of ZrN through formation of a ZrN_1?x phase with oxide peaks initially detected at around 773 K. The zirconium oxide layer was found to be monoclinic by in‐situ XRD with no evidence of tetragonal or cubic polymorphs present to 1023 K. Bulk ceramic samples oxidized at 1173 and 1273 K underwent slower oxidation than those oxidized at 973 and 1073 K. This change in oxidation rate and hence mechanism was due to formation of a denser c‐ZrO₂ polymorph stabilized by nitrogen defects. This N‐doped dense ZrO₂ layer acts as a diffusion barrier to oxygen diffusion. However, at an oxidation temperature of 1373 K this layer is no longer protective due to increased diffusion through it resulting in grain boundary oxidation.     

Effect of oxidation on room temperature strength of ZrB2- and HfB2- based ultra-high temperature ceramics

The effect of oxidation on room temperature (RT) flexure strength degradation in SiC-reinforced ultra-high temperature ceramics (UHTCs) and La₂O₃-doped UHTCs has been characterised in the temperature range 1400–1600°C for oxidation times of up to 32?h. Flaw healing was identified for oxide scale thicknesses 50 μm. Two oxide scale configurations have been proposed to minimise RT strength degradation. The most promising is the scale with a porous layer containing non-interconnected porosity (85–90% dense) of either MeLa₂O₇ or MeO_xC_y (Me?=?Zr or Hf).     

Effect of temperature on the formation and dissolution of anodic oxide films on titanium in acid solutions

The effect of electrolyte and current density on the growth rate of the oxide film on titanium were studied by following the voltage-time characteristics. The barrier oxide grows to greater thickness at lower pH and higher current density. The growth of the oxide in acid medium is lowered by an increase in temperature. In 0.5n H₂SO₄ the oxide grows to greater thicknesses than that grown in 0.5n HCLO₄. This is due to a relatively higher rate of dissolution in HClO₄ during the oxide-growth/oxide-dissolution process during the anodization. The effect of temperature on the dissolution of the oxide previously grown to 12.5 V is followed in 0.5n H₂SO₄ by impedance and potential measurements. The oxide, which is of duplex nature, dissolves with a rate that increases with increasing temperature. The results indicate that the rate of dissolution of the outer layer is affected by temperature more than that of the inner layer, probably owing to higher porosity of the former. The heat of activation, -H, was estimated to be 110.5 kJ mol^–1.     

Effect of Initial Oxide Layer on Ignition and Combustion of Boron Powder

Only few data exist for experimental studies on ignition and combustion of boron particles with initial oxide thickness. The oxidation, ignition and combustion characteristics including the onset temperatures, weight gain, apparent activation energy, emission spectra during combustion, and ignition delay time of crystalline boron powders with different initial oxide thickness (x₀) were studied by a laser ignition and thermogravimetric (TG) analyses. Simulations of the kinetics of oxide layer during boron ignition were conducted using a common model. The results indicated that the onset temperature was approximately 775 °C, independent of x₀. The total weight gain decreased with increasing x_0, whereas the weight gain at 775 °C did not change. The apparent activation energy was found to be insensitive to x₀ and had a constant value of about 210 kJ mol⁻¹. The intensity of the emission spectra gradually decreased while the ignition delay time increased with increasing x₀. Numerical simulation showed that the removal rate of oxide layer enhanced with increasing x₀. The experimental results revealed that the oxidation of boron powder was no diffusion‐controlled process at low temperatures. But the diffusion of oxygen could become important to the oxidation reaction at high temperatures     

The anodic oxide film on iron in neutral solution

The composition of the anodic passive oxide film on iron in neutral solution has been investigated by cathodic reduction, chemical analysis and ellipsometry. The cathodic reduction using a borate solution of pH 6·35 containing arsenic trioxide as inhibitor estimates iron in the film to be all iron (III), indicating that no magnetite layer is present. Oxygen in the film is estimated from the ellipsometric thickness to be in excess of the stoichiometric ferric oxide, suggesting the presence of bound water. The average composition is represented as Fe₂O₃.0·4H₂O, in which hydrogen may be replaced partly with iron-ion vacancy. The anodic oxide film is composed of an inner anhydrous ferric oxide layer, which thickens with the potential and an outer layer of hydrous ferric oxide whose thickness depends on the condition of passivation and environment.     

Oxidation of the copper alloy with pure copper plating

The oxidation failure of a copper alloy lead frame with/without a copper plating layer was investigated. The oxidation rate and adhesion strength of oxide films on copper alloy substrates were studied by measuring the thickness and by carrying out peel tests. The adhesion strength of the oxide film was mainly influenced by the composition but not the thickness of the oxide film. The highest adhesion strength was obtained when the oxide film was composed mainly of Cu₂O. When the thickness of the copper preplated layer was over 0.165?μm, the Cu atoms of the preplated copper were available for oxidation. Thus the oxidation process was within the copper preplated layer, and the main product of the oxidation was Cu₂O. It was found that the large column grain of the oxide film on the copper alloy with a copper plated layer, favored the diffusion of copper or oxygen atoms that led to the formation of Cu₂O, and lead to higher adhesion strength. This indicated that the oxidation resistance of a copper alloy lead frame can be effectively improved by electroplating copper.     

Polished carbon electrodes for improving the fluorine production process

Improvements in the fluorine production process may be obtained by forming a ternary conducting layer CF _x M _y at the surface of the carbon anodes (M is a metal: Li, Mg, Al,...). Electron transfer at the interface is thus enhanced and the wettability of the electrode by the KF-2HF melt is improved. In this paper, we show that polishing the carbon electrodes with alumina or magnesia actually dopes the carbon by mechanical insertion of the latter compound. Scanning electron microscopy experiments show that particles of aluminium oxide or magnesium oxide are inserted during the polishing process; they give the corresponding fluorides during electrolysis in the KF-2HF melt. A surfacic ternary conducting compound is formed, as in the case of electrodes doped by additions of LiF or AlF₃ into the melt. The electrochemical behavior of the polished electrodes has been studied by cyclic voltammetry experiments and impedance measurements.     

Electrochemical behaviour of titanium in fluoride-containing saliva

The effect of fluoride on the electrochemical behaviour of titanium was studied. Open circuit potentials, breakdown potentials (E _b) and potentiostatic transient currents were measured in synthetic salivas of different compositions. Optical and scanning electron microscopic observations were also made. Results show that the growth rate of Ti oxide layer is affected by fluoride anions and tensile stresses are developed. The OCP/time relationship of Ti immersed in salivas A and B obeys a logarithmic law which depends on the saliva composition. The E _b value is influenced by the thickness of the oxide layer, by the composition of the saliva (including fluoride concentration), and by the technique utilised for its evaluation. Thus, results reported in the literature, which seem to be contradictory, could be explained taking into account the experimental conditions assayed. A careful control of the titanium-containing dental materials should be made after long treatments with fluoride-containing prophylactic products or when fluoride-releasing restorative materials are present in the vicinity.     

A nonlinear kinetic model introduced for the corrosion inhibitive properties of some organic inhibitors

Corrosion kinetics of low-carbon steel in hydrochloric acid was studied at various concentrations of mimosa tannin inhibitor. This system was subjected to impedance spectroscopy and quasi steady-state polarization. The inhibition efficiency, was derived from the corrosion current, i _corr and charge transfer resistance, R _ct data. The fractional surface coverage as a function of the inhibitor concentration was calculated from the rate of hydrogen evolution reaction (h.e.r.) at constant cathodic potential. Based on the theoretical model and the observed experimental relationship between the ratio of the corrosion current densities in the uninhibited and the inhibited systems and the surface coverage, the relative influences of the geometric blocking action and the energy effect of the inhibitor on the corrosion process were estimated. Fitting of the nonlinear model to the experimental data was carried out by the Levnberg–Marquardt nonlinear fit method implemented into the programming system Mathematica ^®. Restructuring of the adsorbed layer and change in the orientation of adsorbed inhibitor molecules upon the increase of surface coverage was assumed on the basis of the experimentally observed functional relationship of the double layer capacitance and the surface coverage. The results were explained with respect to the molecular properties of the inhibitor – geometry and size of the molecule, electronic orbital structure and dipole moment.     

On the composition and structure of thin layers of titanium oxide on platinum surfaces

This work reviews the available data about the structure and composition of thin layers of titanium oxide on the surface of platinum. These systems can be prepared by vapor deposition and subsequent oxidation of Ti on a massive Pt substrate or by oxidation at low pressure of the Pt₃Ti alloy. For both types of substrate, the XPS data show that two distinct Ti states in the oxide layer can be detected. The structure and the composition of the oxide layer varies depending on the substrate composition and on the conditions of preparation. On pure Pt, the preferential formation of a defective TiO_2–x oxide was observed. On Pt₃Ti, the oxide film a layer was formed of TiO of thickness of the order of a single atomic layer and of crystallites of nearly stoichiometry TiO₂.     

The impedance properties of the oxide film on the Ni–Cr–Mo Alloy-22 in neutral concentrated sodium chloride solution

The oxide film properties on Alloy-22 in the applied potential (E) range −600 mV to 600 mV (vs. saturated KCl, Ag/AgCl reference electrode) were characterized by Electrochemical Impedance Spectroscopy (EIS) in near neutral pH, 5 M NaCl solutions, at 30 °C. The impedance properties of the film were compared to the chromium content of the film determined by X-ray photoelectron spectroscopy (XPS). The oxide film properties on Alloy-22 may be divided into three applied potential (E) ranges: −600 mV ≤ E < −300 mV, −300 mV ≤ E ≤ 300 mV, and E > 300 mV. For the range −600 mV ≤ E < −300 mV the film resistance (R_film) increases with potential accompanied by an increase in Cr₂O₃ content; in the range −300 mV ≤ E ≤ 300 mV, R_film values and the Cr₂O₃ content of the oxide film achieve their maximum values; for E > 300 mV, a decrease in both R_film and Cr₂O₃ is observed accompanied by a significant increase in Cr(OH)₃. Comparison of the impedance properties for Alloy-22 to those of Ni–Cr alloys indicate that the barrier layer oxide on Alloy-22 contains a lower number of less mobile defects, most likely Cr interstitials. Destruction of the barrier layer for E > 300 mV leads to the formation of a thicker, less protective bilayer, which is high in Mo content.     

Electrochemical investigation of the passive behaviour of biomaterials based on Ag–Sn and Cu–Zn–Al in carbonate buffer in the absence and presence of chloride

The electrochemical behaviour of biomaterials based on Cu–Zn–Al (cubic Cu₃Zn phase) and Ag–Sn (orthorhombic Ag₃Sn and hexagonal Ag₄Sn phases) alloys was investigated in carbonate buffer solutions (pH 9.66) in the absence and presence of chloride, using different electrochemical techniques. Analyses of the open circuit potential and the potentiodynamic polarisation curves showed that the passivation domain and the corrosion parameters depend on alloy composition and chloride concentration. Chronoamperometric studies showed that passivation kinetics and corrosion of the passive film are both well described by a linear ln(i) versus ln(t) relation. The passive film formed on the Ag–Sn alloy is less susceptible to corrosion when compared to the Cu–Zn–Al system. The impedance data obtained in the passive region for the Cu–Zn–Al alloy showed that the passive layer is compact. In contrast, the impedance data obtained for the Ag–Sn alloy showed that the passive layer is formed by a compact oxide layer covered by a porous oxide gel layer. Mott–Schottky analysis showed that the passive film formed on the Cu–Zn–Al alloy behaves as a p-type semiconductor.     

Modeling gold/iron oxide interface system

The effect of gold particle size and Au/FeO _x interface on the electronic properties and catalytic activity using samples of Au/SiO₂/Si(100), Au/FeO _x /SiO₂/Si(100), FeO _x /Au/SiO₂/Si(100) has been modelled. Nanosize gold particles of varying size were fabricated by deposition of a 10 nm thick gold film onto SiO₂/Si(100) substrate by electron beam evaporation followed by modification using low energy Ar⁺ ion bombardment or Ar⁺ ion implantation. These modifications formed Au islands of decreasing size accompanied by the strong redistribution of the Au 5d valence band structure determined by ultraviolet and X-ray photoelectron spectroscopy (UPS, XPS) and increased activity in catalytic CO oxidation. The gold/iron oxide interface was prepared by deposition of iron oxide using pulsed laser deposition (PLD). The structural properties of gold and iron oxide were characterized by XPS, atomic force microscopy (AFM), transmission electron microscopy (TEM) and secondary ion mass spectroscopy (SIMS). Generally, the formation of gold/iron oxide interface increases the catalytic activity in CO oxidation regardless of the sequence of deposition, namely either Au/FeO _x /SiO₂/Si(100) or FeO _x /Au/SiO₂/Si(100) is formed. Furthermore, the interface formed is operative in determining the catalytic activity even if gold is not exposed to the surface, but it is located underneath the iron oxide layer. This is a promoting effect of the Au nanoparticles, which is more efficient than that of the bulk like Au films.     

Zirconium carbide oxidation: Kinetics and oxygen diffusion through the intermediate layer

Oxidation of hot‐pressed ZrC was investigated in air in the 1073‐1373 K range. The kinetics were linear at 1073 K, whereas at higher temperature samples initially followed linear kinetics before undergoing rapid oxidation leading to a Maltese cross shape of the oxide. The linear kinetics at 1073 K was governed by inward oxygen diffusion through an intermediate layer of constant thickness between ZrC and ZrO₂ which was comprised of amorphous carbon and ZrO₂ nanocrystals. Diffusion of oxygen through the intermediate layer was measured to be 9 × 10^?10 cm² s^?1 using ¹⁸O as a tracer in a double oxidation experiment in ¹⁶O/¹⁸O. Oxidation at 1073 and 1173 K produced samples made of m‐ZrO₂ and either t‐ or c‐ZrO₂ with an adherent intermediate layer made of amorphous carbon and ZrO₂, whereas oxidation at 1273 and 1373 K produced samples with a voluminous oxide made of m‐ZrO₂ showing a gap between ZrC and the oxide. A substoichiometric zirconia layer was found at the gap at 1273 K and no carbon uptake was detected in this layer when compared with the top oxide layer. The loss of the intermediate layer and the slowdown of the linear rate constant (g m^?2 s^?1) at 1273 K compared to 1173 K was correlated with the preferential oxidation of carbon at the intermediate layer which would leave as CO and/or CO₂ leaving a gap between ZrC and substoichiometric zirconia.     

A novel IrO<Subscript>2</Subscript> electrode with iridium–titanium oxide interlayers from a mixture of TiN nanoparticle and H<Subscript>2</Subscript>IrCl<Subscript>6</Subscript> solution

A novel IrO₂ anode on titanium substrate with iridium–titanium oxide interlayer (Ti/IrO _x –TiO₂/IrO₂) was prepared and investigated for oxygen evolution. IrO _x –TiO₂ interlayer was coated on titanium substrate by impregnation-thermal decomposition method from a mixture of TiN nanoparticles and H₂IrCl₆ solution at 500 °C. The results showed that the service life of Ti/IrO _x –TiO₂/IrO₂ was a factor of six times longer than that of Ti/IrO₂, which was attributed to the IrO _x –TiO₂ interlayer, it could form a metastable solid solution between IrO _x and thin titanium oxide layer on titanium substrate during calcination. The interlayer contributed to the decrease in migration rate of oxygen atom or molecule toward substrate and the increase in bonding force among IrO₂ layer, interlayer, and substrate. Therefore, besides keeping high electrocatalytic activity, the service life of Ti/IrO _x –TiO₂/IrO₂ electrode was greatly improved, and its overall electrocatalytic performance for oxygen evolution was increased as well.     

Study of bio-synthesised transition nano metal oxides dispersed polyester composites as enhanced protectors for mild steel in 0.5 M H2SO4 medium-a green approach

A facile, efficient and eco-friendly synthesis of transition nano metal oxides like zinc oxide (ZnO), copper oxide (CuO), tin oxide (SnO) from Persea americana. Mill was executed followed by ex-situ polymerization favoring poly (glycerol succinate) (PGS)/nano metal oxide composite for the first time. The incorporation of nanofillers was confirmed by Fourier transform infrared spectroscopy (FT-IR), X-ray diffraction spectrometer (XRD), Scanning electron microscopy (SEM) and Energy dispersive spectroscopy (EDS). As a comparative approach, the extent of metal dissolution was evaluated in presence of precursor as well as nano dispersed forms for various concentrations (10, 50, 100, 500, 1000 ppm) under the cluster of electrochemical and non-electrochemical techniques. Enhanced inhibition efficiency of 97% was observed in the case of CuO dispersion which was additionally supported by XRD particle size determination. The increase in charge transfer resistance (R_ct) and a decrease in corrosion current (I_corr) elicited from electrochemical measurements strictly proved the role of metal – oxide nanoparticles. Further, the mode of inhibitor was found to be predominantly cathodic.     

Coating metal oxide particles via the combustion of deposited polymer precursors

Water-swellable chelated polymers were synthesized, starting either from (1) In(III) or In(III) and Sn(II) as central ions and N-trimethoxysilylpropylethylenediamine or ethylenediamine each together with ethylenediaminetetraacetic acid (EDTA) as coordination ligands, or from (2) Ag(I) as the central ion and ethylene glycol (EG) and EDTA as coordination ligands, as follows: The nitrate(s) of the above metal ions together with the corresponding ligands were dissolved in water, and the solution was concentrated by heating to carry out the chelating polymerization. After cooling, the polymer was dried and ground to a fine powder, which was then mixed with a metal oxide powder by grinding in the presence of a small amount of water. A paste was thus obtained, which, after drying, was calcined at 200°C and subsequently at 750°C. The polymer became a sticky gel at 200°C, which adhered to the surface of the metal oxide particles; it was converted to an inorganic coating, In₂O₃—SnO₂—SiO₂, In₂O₃—SnO₂, SnO₂, or Ag, during the subsequent calcination at 750°C. Two metal oxide powders, namely, the electrically conductive In₂O₃ and the nonconductive SnO₂, were used as substrates. Scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS) indicated that the substrate particles were coated after calcination by a multicomponent oxide or silver layer. Temperature-resistant electrically antistatic film could be prepared by using the metal oxide coated In₂O₃ particles as pigments and polypropylsiloxane as the binder. The Ag-coated SnO₂ powder had a conductivity σ = 1.0 × 10⁻³ S cm⁻¹ at 8.6 vol % Ag, while the mechanically mixed powders of Ag and SnO₂ exhibited a conductivity of 2.0 × 10⁻⁷ S cm⁻¹ at 16 vol %. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:1891–1903, 1998