In situ STM study of the duplex passive films formed on Cu(1 1 1) and Cu(0 0 1) in 0.1 M NaOH

In situ electrochemical scanning tunneling microscopy (ECSTM) investigations of the anodic Cu(I)/Cu(II) duplex passive layers grown on Cu(1 1 1) and Cu(0 0 1) in 0.1 M NaOH are reported. The outer Cu(II) part of the duplex film formed on both substrates is crystalline with a terrace and step topography. The observed lattices are consistent with a bulk-like terminated CuO(0 0 1) surface on both substrates. This common crystallographic orientation is explained by the hydroxylation of the otherwise polar and unstable oxide surface at the passive film/electrolyte interface. The epitaxy of the oxide layers is governed by the parallel alignment of the close packed directions of the CuO outer layers and Cu₂O inner layers on both substrates. A granular and amorphous layer covering the crystalline CuO(0 0 1) oxide has been observed on Cu(0 0 1) but not on Cu(1 1 1). It is assigned to a film of copper hydroxide corrosion products formed by a dissolution-precipitation mechanism. Its absence on the passivated Cu(1 1 1) surface is explained by the higher stability of the Cu₂O(1 1 1) precursor oxide formed on this substrate in the initial stages of growth of the duplex passive film, resulting in a lower amount of dissolved copper.     

Strain relief: Mainspring of Ge semiconducting nanostructures growth on LaAlO3(0 0 1)

We report on the initial growth mechanisms of Ge on LaAlO₃(0 0 1), a crystalline oxide with a high dielectric constant (high-κ material). Chemical and structural properties were investigated in situ, through X-ray photoelectron spectroscopy and reflection high-energy electron diffraction, and ex situ by using high-resolution transmission electron microscopy. Ge was deposited by molecular beam epitaxy at 600 °C on a c(2 × 2) reconstructed LaAlO₃(0 0 1) surface. At this temperature, a Volmer-Weber growth mode is observed due to a lower LaAlO₃(0 0 1) surface free energy. It is characterized by the immediate formation of crystalline nano-islands. The Ge islands are relaxed and present an abrupt interface with the substrate. Some of them exhibit a preferential relationship in their heteroepitaxy, where the Ge(0 0 1) planes are parallel to the LaAlO₃(0 0 1) ones, but rotated by 45° in the [0 0 1] direction. An additional rotation of 6° with respect to the growth axis is also observed, which compensates partially for the strain produced by the high lattice parameter mismatch (∼5%) between the semiconductor and the oxide.     

Dependence of magnetic properties on the film thickness and Pt atomic fraction of post-annealed Fe1−xPtx-C granular films on MgO(1 0 0) and SiO2/Si(1 0 0) substrates

The Fe_1−xPt_x-C granular films with different Pt atomic fractions (0.09 ≤ x ≤ 0.52) and film thicknesses (5 nm ≤ t ≤ 100 nm) were deposited on MgO(1 0 0) and SiO₂/Si(1 0 0) substrates by facing-target sputtering and post-annealing. With the increasing x, the ordered L1₀ FePt grains form. All of the films are ferromagnetic, and the easy axis is in the film plane. With the decrease of t, the films turn from hard ferromagnetic to soft ferromagnetic. The maximum coercivity of the 100-nm thick Fe_1−xPt_x-C granular films measured at a 10-kOe field is 3.7 kOe at x = 0.48. The coercivity of the Fe_0.56Pt_0.44-C granular films increases, and the magnetization measured at a 10-kOe field decreases with the increasing t. The reversal mechanism of the 100-nm thick Fe_1−xPt_x-C granular films turns from the domain wall motion to the Stoner-Wohlfarth rotation mode as x increases. However, the reversal mechanism of the Fe₅₆Pt₄₄-C granular films with different t approaches the Stoner-Wohlfarth rotation mode, and is film-thickness independent.     

Hot corrosion and oxidation behavior of a novel Pt + Hf-modified γ′-Ni3Al + γ-Ni-based coating

A new type of Pt + Hf-modified γ′-Ni₃Al + γ-Ni-based coating has been developed in which deposition involves Pt electroplating followed by combined aluminizing and hafnizing using a pack cementation process. Cyclic oxidation testing of both Pt + Hf-modified γ′ + γ and Pt-modified β-NiAl coatings at 1150 °C (2102 °F), in air, resulted in the formation of a continuous and adherent α-Al₂O₃ scale; however, the latter developed unwanted surface undulations after thermal cycling. Type I (i.e. 900 °C/1652 °F) and Type II (i.e. 705 °C/1300 °F) hot corrosion behavior of the Pt + Hf-modified γ′ + γ coating were studied and compared to Pt-modified β and γ + β-CoCrAlY coatings. Both types of hot corrosion conditions were simulated by depositing Na₂SO₄ salt on the coated samples and then exposing the samples to a laboratory-based furnace rig. It was found that the Pt + Hf-modified γ′ + γ and Pt-modified β coatings exhibited superior Type II hot corrosion resistance compared to the γ + β-CoCrAlY coating; while the Pt + Hf-modified γ′ + γ and γ + β-CoCrAlY coatings showed improved Type I hot corrosion performance than the Pt-modified β.     

Adsorption of quinonediimines on aluminum(1 0 0): A computational and FT-IR study

Density functional theory (DFT) calculations were performed on a series of symmetrically substituted quinonediimines, both free and adsorbed on a 42 atom aluminum(1 0 0) lattice. Enthalpies of adsorption for quinonediimines in the emeraldine base oxidation state were calculated to vary from −4.07 to −8.47 kcal mol⁻¹, with the emeraldine salt value at −78.00 kcal mol⁻¹. The enthalpy of absorption for the pernigraniline is calculated to be 30 kcal mol⁻¹. Adsorption onto the aluminum surface caused a change in hybridization of the central imine nitrogen from sp² to sp³, with concomitant formation of a coordinate covalent bond to two electron deficient aluminum atoms of the surface. Experimental IR studies of three quinonediimines show shifts in absorption bands consistent with strong interaction with the aluminum surface, and consistent with frequencies calculations using DFT.     

Anisotropy compensation and high low-field magnetostriction of epoxy/Tb1−xHox(Fe0.8Co0.2)2 composites (0.60 ≤ x ≤ 1.0)

The anisotropy compensation and magnetostrictive properties of Tb_1−xHo_x(Fe_0.8Co_0.2)₂ (0.60 ≤ x ≤ 1.0) alloys have been investigated. The easy magnetization direction (EMD) at room temperature rotates from the 〈1 1 1〉 axis (x ≤ 0.75) to the 〈1 0 0〉 axis (x ≥ 0.90) through an intermediate state 〈1 1 0〉, subjected to the anisotropy compensation between Tb³⁺ and Ho³⁺ ions. Composition anisotropy compensation is realized near x = 0.75. The Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ alloy has a minimum anisotropy and a large spontaneous magnetostriction coefficient λ₁₁₁ (≈740 ppm) at room temperature. The strong 〈1 1 1〉-oriented 1-3 epoxy-bonded composite has been fabricated by curing under a moderate magnetic field. A high low-field magnetostriction of about 400 ppm at 3 kOe is obtained for the 1-3 epoxy/Tb_0.25Ho_0.75(Fe_0.8Co_0.2)₂ composite with 40-vol% alloy particles, which can be attributed to the low magnetic anisotropy, EMD lying along 〈1 1 1〉 direction, the strong 〈1 1 1〉-textured orientation and the chain structure.     

Ab initio study of the interaction of chlorides with defect-free hydroxylated NiO surfaces

DFT periodic calculations have been performed to model the adsorption and sub-surface insertion of Cl atoms at increasing coverage on undefective hydroxylated NiO(1 1 1) terraces characteristic of the surface of the passive film on nickel. The results evidence that structural relaxation, splitting the mixed topmost OH/Cl anionic plane, releases the repulsive interactions induced by Cl adsorption, but the effect decreases with increasing Cl coverage with a loss of ∼1.1 eV in stability at surface saturation. Cl sub-surface insertion in the first inner anionic plane also releases the repulsive interactions in the topmost plane but the effect becomes favourable only at high Cl coverage because of inter-layer mixing after surface reconstruction. At low Cl coverage, an unrelaxed diatomic Ni-OH layer weakly bonded to the oxide and likely to dissolve is formed after sub-surface insertion, suggesting a hybrid mechanism of local oxide thinning. Adsorption of Cl by place exchange is energetically favourable but does not seem to promote dissolution. Insertion into the lattice may promote dissolution but it is energetically favourable only at high Cl surface coverage, which is not a likely situation due to Cl repulsive interaction on the surface. For undefective terraces of the passive film, the reported calculations do not confirm the existing hypotheses of adsorption-induced oxide thinning or chloride sub-surface penetration.     

Laser surface alloying of aluminium with WC + Co + NiCr for improved wear resistance

In the present study, laser surface alloying of aluminium with WC + Co + NiCr (in the ratio of 70:15:15) has been conducted using a 5 kW continuous wave (CW) Nd:YAG laser (at a beam diameter of 0.003 m), with the output power ranging from 3 to 3.5 kW and scan speed from 0.012 m/s to 0.04 m/s by simultaneous feeding of precursor powder (at a flow rate of 1 × 10^− 5 kg/s) and using He shroud at a gas flow rate of 3 × 10^− 6 m³/s. The effect of laser power and scan speed on the characteristics (microstructures, phases and composition) and properties (wear and corrosion resistance) of the surface alloyed layer have been investigated in details. Laser surface alloying leads to development of fine grained aluminium with the dispersion of WC, W₂C, Al₄C₃, Al₉Co₂, Al₃Ni, Cr₂₃C₆, and Co₆W₆C. The microhardness of the alloyed zone is significantly improved to a maximum value of 650 VHN as compared to 22 VHN of the as-received aluminium substrate. The mechanism of microhardness enhancement has been established. The fretting wear behavior of the alloyed zone was evaluated against WC by Ball-on-disc wear testing unit and the mechanism of wear was established.     

The nature of the third-element effect in the oxidation of Fe-xCr-3 at.% Al alloys in 1 atm O2 at 1000 °C

The oxidation of an Fe-Al alloy containing 3 at.% Al and of four ternary Fe-Cr-Al alloys with the same Al content plus 2, 3, 5 or 10 at.% Cr has been studied in 1 atm O₂ at 1000 °C. Both Fe-3Al and Fe-2Cr-3Al formed external iron-rich scales associated with an internal oxidation of Al or of Cr+Al. The addition of 3 at.% Cr to Fe-3Al was able to stop the internal oxidation of Al only on a fraction of the alloy surface covered by scales containing mixtures of the oxides of the three alloy components, but not beneath the iron-rich oxide nodules which covered the remaining alloy surface. Fe-5Cr-3Al formed very irregular external scales where areas covered by a thin protective oxide layer alternated with others covered by thick scales containing mixtures of the oxides of the three alloy components, undergrown by a thin layer rich in Cr and Al, while internal oxidation was completely absent. Conversely, Fe-10Cr-3Al formed very thin, slowly-growing external Al₂O₃scales, providing an example of third-element effect (TEE). However, the TEE due to the Cr addition to Fe-3Al was not directly associated with a prevention of the internal oxidation of Al, but rather with the inhibition of the growth of external scales containing iron oxides. This behavior has been interpreted on the basis of a qualitative oxidation map for ternary Fe-Cr-Al alloys taking into account the existence of a complete solid solubility between Cr₂O₃ and Al₂O₃.     

Formation, fast oxidation and thermodynamic data of Fe(II) hydroxychlorides

Iron(II) hydroxide and hydroxychloride precipitates were obtained by mixing FeCl₂ · 4H₂O and NaOH aqueous solutions with various concentration ratios R′ = [Cl⁻]/[OH⁻] = 2 [FeCl₂]/[NaOH] at [NaOH] = 0.4 mol L⁻¹. They were analysed by Infrared spectroscopy after 24 h of ageing at room temperature. Fe(OH)₂ was obtained alone only for the smallest values of R′, typically R′ ? 1.16. β-Fe₂(OH)₃Cl formed as soon as R′ ? 1.40 and was obtained alone for R′ ? 2.25. The initial precipitates were oxidised by addition of a small amount of hydrogen peroxide (5 mL of an aqueous solution containing approximately 30 vol% H₂O₂) instead of O₂. The action of H₂O₂ on Fe(OH)₂ gave rise to δ-FeOOH as already reported. Its action on Fe(II) hydroxychlorides gave rise to akaganéite β-FeO_1−2x(OH)_1+xCl_x. A transformation of the two-phase system found at R′ = 1.5 after long ageing times (6 months) was observed and β-Fe₂(OH)₃Cl remained alone. This slow transformation of Fe(OH)₂ into β-Fe₂(OH)₃Cl may explain why β-Fe₂(OH)₃Cl was only reported as a corrosion product on iron archaeological artefacts. Finally, the respective domains of stability of Fe(OH)₂ and β-Fe₂(OH)₃Cl were demarcated and an estimation of the standard Gibbs free energy of formation of β-Fe₂(OH)₃Cl could be given: .     

Effect of the sintering temperature on the properties of nanocrystalline Ca1−xSmxMnO3 (0 ≤ x ≤ 0.4) powders

Nanocrystalline Ca_1−xSm_xMnO₃ (0 ≤ x ≤ 0.4) manganites were prepared by a soft chemical method (Pechini method) followed by auto-combustion and sintering in air at 1073 or 1473 K. Single-phase powders with general composition Ca_1−xSm_xMnO₃ were obtained after 18 h annealing. The particle and grain sizes of the substituted Sm-manganites did not exhibit variation with samarium content, but increase with increasing the sintering temperature. All manganites show two active IR vibrational modes near 400 and 600 cm⁻¹ characteristic of the BO₆ octahedron vibrations.For the samples sintered at T_s = 1473 K, the partial substitution of calcium by samarium in the CaMnO₃ phase induces a marked decrease in the electrical resistivity, in the temperature range of 300-900 K, and at the same time a metal-to-insulator transition occurs; for T_s = 1073 K all the samples present semiconductor behaviour. With the increase of the annealing temperature the grain size increases and a metal-semiconductor transition appears. The results can be ascribed to the Mn⁴⁺/Mn³⁺ ratio and particle grain size. The effects of particle size on the electrical properties can be attributed to the domain status, changes in the Mn-O-Mn bond angle and Mn-O bond length.     

Oxidation of a quaternary three-phase Cu-20Ni-20Cr-5Fe alloy at 700-900 °C in 1 atm of pure O2

The oxidation of a quaternary Cu-Ni-Cr-Fe alloy containing approximately 20 at.% Ni, 20 at.% Cr and 5 at.% Fe, balance Cu (Cu-20Ni-20Cr-5Fe), was studied at 700-900 °C in 1 atm of pure oxygen. The alloy is composed of a mixture of three phases, where the lightest α phase with the largest Cu content forms the matrix, while the other two, much richer in Cr, form a dispersion of isolated particles. At variance with the ternary three-phase Cu-20Ni-20Cr alloy examined previously, which was unable to form protective chromia scales over the alloy surface even after an extended period of oxidation, the present alloy formed complex external scales containing mixtures of the oxides of the various components plus a deep internal region containing a mixture of alloy and oxide phases. With time, a very irregular and thin but essentially continuous chromia layer formed at the bottom of the mixed internal oxidation region, producing a gradual decrease of the oxidation rate. Thus, the addition of 5 at.% Fe to Cu-20Ni-20Cr alloy is able to decrease the critical Cr content required to form the most stable oxide and promotes the formation of a continuous chromia scale under a lower Cr content in spite of the simultaneous presence of three different phases.     

Thermoelectric properties of Cu1−xPtxFeO2 (0.0 ≤ x ≤ 0.05) delafossite-type transition oxide

The samples of Cu_1−xPt_xFeO₂ (0 ≤ x ≤ 0.05) delafossite were synthesized by solid state reaction method for studying thermoelectric properties. The properties of Seebeck coefficient, electrical conductivity and thermal conductivity were measured in the high temperature ranging from 300 to 960 K. The results of Seebeck coefficient, electrical conductivity and power factor were increased with increasing Pt substitution and temperature. The thermal conductivity was decreased from 5.8 to 3.5 W/mK with increasing the temperature from 300 to 960 K. An important results, the highest value of power factor and ZT is 2.0 × 10⁻⁴ W/mK² and 0.05, respectively, for x = 0.05 at 960 K.     

Perpendicular magnetic anisotropy in 70 nm CoFe2O4 thin films fabricated on SiO2/Si(1 0 0) by the sol-gel method

Cobalt ferrite CoFe₂O₄ films were fabricated on SiO₂/Si(1 0 0) by the sol-gel method. Films crystallized at/above 600 °C are stoichiometric as expected. With increase of the annealing temperature from 600 °C to 750 °C, the columnar grain size of CoFe₂O₄ film increases from 13 nm to 50 nm, resulting in surface roughness increasing from 0.46 nm to 2.55 nm. Magnetic hysteresis loops in both in-plane and out-of-plane directions, at different annealing temperatures, indicate that the films annealed at 750 °C exhibit obvious perpendicular magnetic anisotropy. Simultaneously, with the annealing temperature increasing from 600 °C to 750 °C, the out of plane coercivity increases from 1 kOe to 2.4 kOe and the corresponding saturation magnetization increases from 200 emu/cm³ to 283 emu/cm³. In addition, all crystallized films exhibit cluster-like structured magnetic domains.     

Magnetomechanical properties of epoxy-bonded Sm1−xNdxFe1.55 (0 ≤ x ≤ 0.56) pseudo-1-3 magnetostrictive particulate composites

Pseudo-1-3 magnetostrictive particulate composites consisting of light rare earth (Sm and Nd)-based magnetostrictive Sm_1−xNd_xFe_1.55 particles with the Nd content x of 0-0.56 and randomly distributed sizes of 10-180 μm embedded and aligned in a passive epoxy matrix are fabricated using the particulate volume fraction of 0.5. The quasistatic magnetomechanical properties of the composites are investigated and the results are compared with their monolithic alloys for various x. The composites exhibit similar qualitative trends in properties with the alloys for all x. The Sm_0.92Nd_0.08Fe_1.55 composite shows a large unsaturated magnetostriction λ of −530 ppm at 500 kA/m and a high piezomagnetic coefficient d₃₃ of −2.0 nm/A at 100 kA/m as a result of the magnetocrystalline anisotropy compensation between Sm³⁺ and Nd³⁺ ions in the Sm_0.92Nd_0.08Fe_1.55 alloy.     

Corrosion behaviour of magnesium/aluminium alloys in 3.5 wt.% NaCl

Corrosion behaviour of commercial magnesium/aluminium alloys (AZ31, AZ80 and AZ91D) was investigated by electrochemical and gravimetric tests in 3.5 wt.% NaCl at 25 °C. Corrosion products were analysed by scanning electron microscopy, energy dispersive X-ray analysis and low-angle X-ray diffraction. Corrosion damage was mainly caused by formation of a Mg(OH)₂ corrosion layer. AZ80 and AZ91D alloys revealed the highest corrosion resistance. The relatively fine β-phase (Mg₁₇Al₁₂) network and the aluminium enrichment produced on the corroded surface were the key factors limiting progression of the corrosion attack. Preferential attack was located at the matrix/β-phase and matrix/MnAl intermetallic compounds interfaces.     

The effect of Si additions on the high temperature oxidation of a ternary Ni-10Cr-4Al alloy in 1 atm O2 at 1100 °C

The oxidation of four Ni-10Cr-ySi-4Al alloys was studied at 1100 °C to examine the effects of Si additions (from 2 to 6 at.%) on the behavior of the alloy Ni-10Cr-4Al. Addition of 2 at.% Si prevented completely nickel oxidation, but could not form alumina scales. Larger Si additions produced alumina only over part of the alloy surface (about 20% with 4 at.% Si and 30% with 6 at.% Si), but could not prevent completely the internal oxidation of Al. The results are interpreted by extending to quaternary alloys the mechanism of the third-element effect already proposed for ternary alloys.     

Ferromagnetism in ZnTe:Cr film grown on Si(1 0 0)

Zn_1−xCr_xTe (x = 0.0 and 0.05) films were grown on Si(1 0 0) substrate by using thermal evaporation method. The structure of the films was investigated by X-ray diffraction and it showed the formation of ZnCrTe phase with an amorphous background, which indicated poor crystallinity. Composition analysis by XPS disclosed the presence of antiferromagnetic Cr₂O₃ and Cr precipitates. Magnetic domains were observed by using magnetic force microscopy at ambient temperature and the result showed anisotropic domains with an average size of 3.5 nm. Magnetic field dependence of magnetic moment measurements showed obvious hysteresis loop with a coercive field of 121 Oe at 300 K. Temperature dependence of magnetic moment showed short-range ferromagnetic order. The Curie temperature was estimated to be 354.5 K.     

Numerical simulation of corrosion process with two-step charge transfer mechanism in the Cu/CuSO4 + H2SO4 system

The corrosion process in the Cu/CuSO₄ + H₂SO₄ system is considered as the sum of two coupled single-electron electrochemical reactions that occur simultaneously and independently on the surface of the copper electrode. Our numerical model incorporates diffusion and migration of solution species including cuprous ions, as well as the chemical equilibria for copper sulphate and sulphuric acid dissociation. Numerical simulations are compared with the trends discovered during experimental investigation of copper corrosion in similar systems.