Properties of Ni1−xFex (0.1 < x < 0.9) and Invar (x = 0.64) alloys obtained by electrodeposition

Electrodeposition of Ni_1−xFe_x (x = 0.1-0.9) films was carried out from a chloride plating solution containing saccharin as an organic additive at a constant current density (5 mA/cm²) and a controlled pH of 2.5. X-ray diffraction studies revealed the existence of an fcc, or γ phase, in the range of 10-58 wt.% Fe, a mixed fcc/bcc phase in the range of 59-60 wt.% Fe, and a bcc, or α phase in the range of 64-90 wt.% Fe. The saturation magnetization, B_s, of electrodeposited Ni_1−xFe_x alloys at the room temperature was found to increase with the increase of Fe-content and follows the Slater-Pauling curve, but deviates from as-cast bulk NiFe alloys. The coefficient of thermal expansion, CTE, of electrodeposited alloys at room temperature also deviates from as-cast bulk NiFe alloys. Annealing of α-Ni₃₆Fe₆₄ alloy results in a martensitic α → γ phase transformation, which takes place between 300 and 400 °C. It was demonstrated that thermal treatment above 400 °C was necessary to obtain magnetic and mechanical properties similar to those to conventional Invar alloy. Annealing of α-Ni₃₆Fe₆₄ alloy at 700 °C brings about a decrease of B_s from 1.75 to 0.45 T. By controlling the annealing conditions of α → γ martensitic transformation, it is possible to adjust the CTE of Ni₃₆Fe₆₄ alloy over the broad limits from 2.7 to 8.7 × 10⁻⁶/°C.     

Electrocatalytic oxidation of urea by nanostructured nickel/cobalt hydroxide electrodes

The present paper describes the catalytic oxidation of urea performed by nickel hydroxide and nickel/cobalt hydroxide modified electrodes by using both electrodeposited films and nanoparticles. The incorporation of Co foreign atoms leads to a slight increase in sensitivity besides the shift in redox process, avoiding the oxygen reaction. Nanostructured Ni₈₀Co₂₀(OH)₂ was synthesized by sonochemical route producing 5 nm diameter particles characterized by high-resolution transmission electron microscopy (HRTEM) being immobilized onto electrode by using the electrostatic Layer-by-layer technique, yielding attractive modified electrodes for sensor development.     

Interfacial Fe(III)-hydroxide formation during Fe-Pt alloy deposition

Fe-Pt films with an Fe/Pt ratio close to one can be electrodeposited from an FeSO₄-H₂PtCl₆-Na₂SO₄electrolyte. At the deposition potential, the hydrogen evolution and the reduction of the Pt complex are diffusion limited, and Fe overpotential deposition has not yet set in. The sources of the Fe incorporation are iron hydroxide formation together with Fe underpotential deposition due to Fe-Pt alloy formation. Mössbauer measurements show that the iron in the iron hydroxide is predominantly Fe(III). For stoichiometry reasons, a Pt-rich Fe-Pt phase must be present in addition to the Fe(III)-hydroxide. The Fe³⁺ that takes part in the hydroxide formation is produced in the electrolyte by the oxidation of Fe²⁺ by the complexed Pt ion. This exchange reaction results in a significantly higher Fe³⁺ content in the FeSO₄-H₂PtCl₆-Na₂SO₄ electrolyte in comparison to the same electrolyte without H₂PtCl₆. Fe(III)-hydroxide formation can be depressed by adding citric acid, that acts as buffering and complexing agent. This leads to a lower iron content of the deposits. The Fe/Pt ratio close to one that is needed for hard magnetic properties can, however, only be achieved with a significant incorporation of iron hydroxide.     

Effects of the electrodeposition potential and temperature on the electrochemical capacitance behavior of ordered mesoporous cobalt hydroxide films

Novel ordered mesoporous cobalt hydroxide film (designated H_I-e Co(OH)₂) has been successfully electrodeposited from cobalt nitrate dissolved in the aqueous domains of the hexagonal lyotropic liquid crystalline phase of Brij 56. Experimental electrodeposition parameters such as deposition potentials and deposition temperatures are varied to analyze their influences on the electrochemical capacitor behavior. The films are physically characterized by scanning electron microscopy (SEM) and transmission electron microscopy (TEM) to determine the effects of deposition potentials and temperatures on the surface morphology and nanostructure. Electrochemical techniques such as cyclic voltammetry (CV) and chronopotentiometry are applied to systematically investigate the effects of deposition potentials and temperatures on the capacitance of the films. The results demonstrated that the capacitive performance of the H_I-e Co(OH)₂ film achieved the highest value when it is electrodeposited at −0.75 V under the deposition temperature of 50 °C.     

Stress evolution in CoxFe1−x (x = 0.33-0.87) electrodeposited films

Electrodeposition of 0.5 μm thick Co_xFe_1−x (x = 0.33-0.87) films was carried out from a sulfate/chloride plating solution containing saccharin as an organic additive at constant current density and a controlled pH 2.3. The increase of Fe²⁺ concentrations in plating solution resulted in an increase of Fe-content and tensile stress in Co_xFe_1−x films, which is accompanied by a decrease of plating rate. Several possible origins for generation of tensile stress include the following: interfacial stress between CoFe films and Cu-substrate, crystal texture and grain size, coalescence and stress evolution during film growth, and hydrogen adsorption/desorption. The adsorption/desorption mechanism of hydrogen seems to be the most likely dominant stress mechanism. The relationship between increase of the tensile stress and decrease of plating rate was discussed.     

CoFeRh alloys: Part 2. Electrodeposition of CoFeRh alloys with high saturation magnetic flux density and high corrosion resistance

A Co₃₈Fe₆₁Rh₁ alloy with high saturation magnetic flux density, Bs, of 2.4 T was produced by electrodeposition from a solution containing RhCl₃, NH₄Cl, H₃BO₃, CoSO₄, FeSO₄, saccharin, NaLS (Na-lauryl sulfate) at pH 2.0. This alloy showed excellent corrosion resistance, which was comparable to sputtered 2.4 T Co₄₀Fe₆₀ alloy. It has been demonstrated that CoFeRh films with Bs = 2.3-2.4 T can be obtained at lower current density and lower concentration of RhCl₃ in the plating solution. The increase of current density and RhCl₃ concentration brings about increase of Rh-content in CoFeRh films with higher corrosion resistance, but lower Bs value. The co-electrodeposition of Co and Fe at the controlled potential occurs at more positive potentials (−0.6 to −0.8 V vs. SCE) in the presence of RhCl₃ in the plating solution. This phenomenon is possibly driven by the negative enthalpy of CoFeRh formation and/or through induced co-deposition of CoFeRh from a mixed-metal complex as an electroactive intermediate. It is also demonstrated that the decrease of the Bs value of CoFeRh alloys from 2.4 to 1.2 T is linked to the increase of nonmagnetic elements, i.e. Rh, O, H, S, C, N, and Cl, in the CoFeRh deposit.     

Electrooxidation/electroreduction processes at composite iron hydroxide layers in carbonate-bicarbonate buffers

The electrooxidation/electroreduction processes at precipitated iron hydroxide layers on platinum electrodes have been studied in carbonate-bicarbonate buffers at 25°C by using electrochemical methods. The initial characteristics and properties of the hydrous iron hydroxide were changed by varying the precipitation conditions of the chemically formed active materials. Different potentialtime perturbation programs were employed to analyse the contribution of redox couples within the composite iron hydroxide layers on platinum electrodes and the corresponding electrochemical responses were compared with results obtained for massive iron electrodes in the same solutions. The complex electroreduction and electrooxidation processes are discussed on the basis of a reaction model which takes into account the incorporation of FeCO₃ in the hydrous iron hydroxide layer and its oxidation to FeOOH species, which in turn can participate in electroreduction reactions yielding Fe₃O₄, Fe(OH)₂, and soluble Fe(II) species.     

Electrochemical behaviour of FexCo3?xO4 with (x?=?0, 1, 2 and 3) oxides thin film electrodes in alkaline medium

Spinel-type Fe _x Co_3−x O₄ thin films with x = 0, 1, 2 and 3 were prepared, on stainless steel supports, using the thermal decomposition method at 400 °C. The X-ray diffraction patterns show the presence of a spinel-type structure with a low crystallinity. The electrochemical behaviour was investigated in 1 M KOH, using open-circuit-potential measurements and cyclic voltammetry. The studies allowed to observe the redox reactions occurring at the Fe _x Co_3−x O₄ (x = 1 and 2) oxide surface, namely Fe₃O₄/Fe(OH)₂ or Fe₃O₄/Fe₂O₃, Co₃O₄/CoOOH, Co(OH)₂/CoOOH and CoO₂/CoOOH by comparing the experimental data with those obtained for the Co₃O₄ and Fe₃O₄ films as well as with those referred to in the literature. The results show that iron ions play the major role in the solid-state surface redox transitions in the negative potential range, whereas the cobalt ions are the key species in the positive potential range. However, the contribution of each component, although small, has to be considered in both potential regions.     

Nanostructured α- and β-cobalt hydroxide thin films

A new perspective in the use of electrochemical methods to deposit cobalt hydroxide thin films is presented. Ordered arrays of α-Co(OH)₂ (hydrotalcite-like (Co-HT)) and β-Co(OH)₂ nanoparticles were synthesized on transparent conductive oxide (TCO) substrates by localized cathodic electrogeneration of hydroxyl via the reduction of NO₂⁻ (or NO₃⁻) ion precursors in solution containing Co²⁺ in very low concentration. The thin films, analyzed by X-ray diffraction and scanning electron microscopy were found to be composed of vertically oriented platelets with the crystallographic c-axis parallel to the substrate surface. Turbostratic disorder was not observed in the films. UV/Vis spectra and thermal gravimetric analyses (TGA) indicated distinct variation between the Co-HT structures. Films deposited at 60 °C using a nitrite precursor generated uniform, vibrant-green mixed-valence Co-HT (Co²⁺/Co³⁺). Nitrate precursors yielded a “hydroxyl-deficient” Co-HT (Co²⁺ only). Films deposited at 95 °C in nitrate solution yielded β-Co(OH)₂. The films obtained in presence of nitrite were thicker than those obtained in nitrate. They were formed of β-Co(OH)₂ and contained traces of Co-HT.     

The electrochemical and electrocatalytic behaviour of glassy metals

The suitability of a selection of amorphous alloys as electrocatalysts or as inhibitors for hydrogen evolution (HE) was investigated in 1 m KOH at 25 °C. Mild basic conditions were chosen so as to make direct comparison with other data, where available. The alloys studied were the known glassy alloys Fe₆₇Co₁₈B₁₄Si₁, Co₆₆Fe₄Si₁₆B₁₂Mo₂, Fe₄₀Ni₄₀B₂₀ and Fe₄₀Ni₄₀P₁₄B₆ and an entirely new glassy alloy Zr_73.22Ti_19.71Cu_1.24Fe_5.83. The electrochemical techniques of slow sweep anodic and cathodic polarisation were used, in conjunction with the surface analysis techniques of scanning electron microscopy (SEM) and X-ray analysis, to characterise the alloys and new data has been obtained for all alloys. The glassy alloys were tested in their as-polished state, as well as after surface activation, by ex situ chemical (acid etching) and in situ electrochemical (anodic oxidation in base) pre-treatment. The least corrosion resistant composition, Fe₆₇Co₁₈B₁₄Si₁, displayed the highest activity for HE in the as-polished state and only a minor improvement resulted from surface pre-treatment. Corrosion resistance was partly characterised by the degree to which the passive region increased and the passive region current decreased as a function of pre-treatment. The most corrosion resistant alloy, Zr_73.22Ti_19.71Cu_1.24Fe_5.83, displayed the poorest activity for HE in the as-polished state, but a significant improvement resulted from surface activation by in situ anodic oxidation in basic media. Surface activation by acid pre-treatment reduced the corrosion resistance of the Zr_73.22Ti_19.71 Cu_1.24Fe_5.83 alloy and was, therefore, a non-viable and destructive procedure. However, acid pre-treatment was effective in substantially activating the glassy Co₆₆Fe₄Si₁₆B₁₂Mo₂ and Fe₄₀Ni₄₀P₁₄B₆ alloys towards HE and did not alter the corrosion properties of these compositions. A novel technique for mounting thin alloy specimens has been developed, using an insulating photo-resist coating, resulting in sharply defined electrode edges.     

Preparation of highly effective ferric hydroxide supported noble metal catalysts for CO oxidations: From gold to palladium

Ferric hydroxide supported Pd catalyst prepared by a simple co-precipitation method without calcinations at elevated temperatures and only reduced at 50 °C possessed unexpectedly higher activity for CO oxidations even compared with that of supported Au catalysts. XRD and TEM results showed that the support was mixture of Fe(OH)_x and Fe₃O₄ and Pd was highly dispersed on it. XPS results showed that Pd existed as mixture of oxidation and metal state. The large amount of OH group on the supports may play an important role in O adsorption and activation.     

Synthesis and characterization of Cu–Co–Fe hydrotalcites and their calcined products

A series of new Cu–Co–Fe compounds with the general formula Cu _x Co_2−x Fe₁HT (x = 0, 0.5, 1, 1.5 and 2) has been prepared by hydrotalcite coprecipitation method. The presence of hydrotalcite phase is revealed by XRD analysis for x values of 0, 0.5 and 1. When the copper quantity is higher than 1, the malachite phase is preferentially formed. These results are confirmed by TG-DTA, FT-IR and XPS analysis. After calcination at 500 °C in air of all samples, XRD analysis reveals the presence of spinel phases such as Co₃O₄, CoFe₂O₄, CuFe₂O₄, Cu _x Co _y O₄ in the solids, monoclinic CuO phase when the copper content is greater or equal to 1 and haematite phase for the sample where x is equal to 2. The presence of these phases is also confirmed by XPS results. For comparison, a Co₂Fe₁OH sample has been synthesized by classical coprecipitation method and although Co₂Fe₁HT sample and Co₂Fe₁OH form a similar phase after calcination at 500 °C, Co₂Fe₁HT500 presents a higher BET value than Co₂Fe₁OH500 sample.     

Electroformed iron and FeCo alloy

Iron and FeCo alloys were electroformed from additive-free acidic chloride baths. Film stress and magnetic properties were strongly influenced by deposition current density and operating temperature. In general, low film stress and low coercivity (H_C) was achieved with low current density and high operating temperature baths. SEM micrographs indicated that these conditions promote large grain growth. Coercivity of electroformed iron films linearly increased with increasing film stress, indicating that magnetoelastic energy is a dominant anisotropy. The addition of 0.25 M CaCl₂ improves current efficiency while maintaining low film stress. The lowest iron film stress of 5 MPa was achieved from 1.5 M FeCl₂ in the absence of CaCl₂ at 20 mA cm⁻² with a current efficiency of 91%. A “normal” codeposition of FeCo was observed in acidic chloride baths, where the deposition rate of Co²⁺ was faster than Fe²⁺. Film compositions also played an important role in magnetic properties of FeCo films in addition to film stress. Magnetic saturation (M_S) of FeCo films increased linearly with an increase in deposited Fe content. High magnetic saturation with low-stress (M_S of 2.3 T and σ=70 MPa) were achieved from 71Fe29Co films.     

Magnetic properties of randomly oriented BaM, SrM, Co2Y, Co2Z and Co2W hexagonal ferrite fibres

The microstructure and magnetic properties of randomly oriented BaFe₁₂O₁₉, SrFe₁₂O₁₉, Ba₂Co₂Fe₁₂O₂₂, Ba₃Co₂Fe₂₄O₄₁, Ba₃Ca_0.3Co₂Fe₂₄O₄₁ and BaCo₂Fe₁₆O₂₇ hexaferrite fibres were characterised. 2D and 3D AFM and MFM images were taken of a single BaM fibre. Magnetic properties of random ferrite fibres compared well to expected values for polycrystalline ceramics. The little-characterised Co₂W ferrite was found to have M_s and H_c similar to that of Co₂Z. Relatively small applied fields of <0.05 T were required to reverse the magnetisation of all the soft hexaplana ferrite fibres, and all had H_c < 40 kA/m, becoming demagnetised in fields <0.025 T. Random Co₂W fibres had a high M_r/M_s ratio of 0.56, (greater than M ferrites), despite being very magnetically soft (low coercivity), due to the unusual “lobed” shape of their hysteresis loop, which was attributed to their fibrous nature, and elongated growth of the grains along the fibre axis. Co₂Z had the lowest H_c of all the ferroxplana fibres.     

Multi-doped bismuth ferrite thin films with enhanced multiferroic properties

Thin films of multi-doped bismuth ferrite, Bi_0.97−xLa_xSr_0.03Fe_0·94Mn_0·04Co_0·02O₃ (BL_xSFMC, x = 0.00–0.18), are synthesized on a fluorine-doped tin oxide (FTO)/glass substrate. The structure and multiferroic properties of the film samples are characterized and tested. The results indicate that on doping, the structure of the BL_xSFMC film changes has been changed. The concentrations of both oxygen vacancies and Fe²⁺ are decreased. The BL_0.18SFMC thin film exhibits Ohmic conduction, which reduces the influence of the built-in electric field E_bi of the space-charge region at the interface between an Au electrode and the BL_xSFMC during polarization. The BL_0.18SFMC thin film also exhibits enhanced ferroelectric properties than the undoped film, with a higher residual polarization of 188 μC/cm² and a higher squareness ratio of 1.21. Meanwhile, the reduced number of oxygen vacancies also reduces the Fe²⁺/Fe³⁺ ratio, thereby enhancing the Dzyaloshinskii–Moriya interaction of Fe–O–Fe bonds, and so the BL_0.18SFMC thin film exhibits enhanced ferromagnetism, with a saturation magnetization of M_s ≈ 3.94 emu/cm³. Thus, multi-ion doping can improve both the ferroelectric and ferromagnetic properties of BL_xSFMC thin films.     

FexCo0.5−xNi0.5-SDC anodes for low-temperature solid oxide fuel cells

Trimetal alloys, Fe_xCo_0.5−xNi_0.5 (x = 0.1, 0.2, 0.25, 0.3, 0.4), were studied as anodes for low-temperature solid oxide fuel cells (LT-SOFCs) based on GDC (Ce_0.9Gd_0.1O_1.95) electrolytes. The alloys were formed by in situ reduction of Fe_xCo_0.5−xNi_0.5O_y composites, which were synthesized using a glycine-nitrate technique. Symmetrical cells consisted of Fe_xCo_0.5−xNi_0.5-SDC electrodes and GDC electrolytes, and single cells consisted of Fe_xCo_0.5−xNi_0.5-SDC (Ce_0.8Sm_0.2O_1.9) anodes, GDC electrolytes, and SSC (Sm_0.5Sr_0.5CoO₃)-SDC cathodes were prepared using a co-pressing and co-firing process. Interfacial polarization resistances and I-V curves of these cells were measured at temperature from 450 to 600 °C. With Fe_0.25Co_0.25Ni_0.5-SDC as anodes, the cells showed the lowest interfacial resistance and highest power density. For example, at 600 °C, the resistance was about 0.11 Ω cm² and power density was about 750 mW cm⁻² when humidified (3% H₂O) hydrogen was used as fuel and stationary air as oxidant. Further, the cell performance was improved when the molar ratio of Fe:Co:Ni approached 1:1:2, i.e. Fe_0.25Co_0.25Ni_0.5. In addition, higher power density and lower interfacial resistance were obtained for cells with the Fe_0.25Co_0.25Ni_0.5-SDC anodes comparing to that with Ni-SDC anodes, which have been usually used for LT-SOFCs. The promising performance of Fe_xCo_0.5−xNi_0.5 as anodes suggests that trimetallic anodes are worth considering for SOFCs that operate at low-temperature.     

Electro-precipitation of magnetite nanoparticles: An electrochemical study

Nanoparticles of magnetites (Fe₃O₄) are synthesized with a new process based on electro-precipitation in ethanol medium. A mechanism pathway is proposed consisting of a Fe(OH)₃ precipitation followed by the reduction of iron hydroxide to magnetite in the presence of hydroxyl ions which are generated at the cathode.     

Electrocatalytic properties of Fe-based bulk metallic glasses for hydrogen evolution reaction

The electrocatalytic activity of newly developed Fe-based bulk metallic glasses was evaluated by polarization and impedance measurements in deaerated 1M KOH solution at room temperature. The electrocatalytic activity as well as the glass-forming ability of the Fe₇₃C₃Si_7.3B_8.5P_5.7Mo_2.5 alloy was improved by the partial substitution of Fe with 13.5 at% Co. In hydrogen evolution reaction, the metallic glasses exhibited higher activity than the activity of G14 (Fe₆₀B₁₀Si₁₀Co₂₀) metallic glass, which has been reported to be comparable to the activity of Pt electrode. The activity was further improved after a chemical pretreatment in 1 M HF solution for 1 min due to the formation of cobalt-rich layer with a large effective surface area.     

Magnetic and phonon transitions in B-site Co doped BiFeO3 ceramics

Magnetic susceptibility and phonons have been characterized in multiferroic Bi(Fe_1−xCo_x)O_3−δ ceramics for x=0.0, 0.05, and 0.10 (BFO100xCo) as functions of temperature. A preferred (100) crystallographic orientation and increasing average oxygen vacancies were observed in BFO5Co and BFO10Co. The Fe and Co K-edge synchrotron X-ray absorptions revealed mixed valences of Fe³⁺, Fe⁴⁺, Co²⁺, and Co³⁺ ions in BFO5Co and BFO10Co, which exhibit a ferromagnetic (or ferrimagnetic) phase below room temperature due to appearance of ferromagnetic B–O–B (B=Fe and Co) superexchange interactions. Field–cooled (FC) and zero–field–cooled (ZFC) magnetic susceptibilities exhibit a significant spin-glass splitting below room temperature in BFO5Co and BFO10Co. Two Raman-active phonon anomalies at ~170 K (or 200 K) and ~260 K were attributed to the Fe³⁺–O–Co³⁺ and Co³⁺–O–Co³⁺ magnetic orderings, respectively. This work suggests that the low-spin Co²⁺–O–Co²⁺, Fe³⁺–O–Fe³⁺ (or Fe⁴⁺), and high-spin Co²⁺–O–Co²⁺ superexchange interactions are responsible for phonon anomalies at ~290 (or ~300 K), ~400, and ~470 K (or ~520 K) in BFO5Co and BFO10Co.     

Carbonate and sulphate green rusts—Mechanisms of oxidation and reduction

The oxidation and reduction of carbonate, GR(CO₃), and sulphate, GR(SO₄), green rusts (GR) have been studied through electrochemical techniques, electrochemical quartz crystal microbalance (EQCM), FTIR, XRD and SEM. The used samples were made of thin films electrodeposited on gold substrate. The results from the present work, from our previous studies and from literature were compiled in order to establish a general scheme for the formation and transformation pathways involving carbonate or sulphate green rusts. Depending on experimental conditions, two routes of redox transformations occur. The first one corresponds to reaction via solution and leads to the formation of ferric products such as goethite or lepidocrocite (oxidation) or to the release of Fe^II ions into the solution (reduction) with soluble Fe^II-Fe^III complexes acting as intermediate species. The second way is solid-state reaction that involve conversion of lattice Fe²⁺ into Fe³⁺ and deprotonation of OH groups in octahedra sheets (solid-state oxidation) or conversion of lattice Fe³⁺ into Fe²⁺ and protonation of OH groups (solid-state reduction). The solid-state oxidation implies the complete transformation of GR(CO₃) or GR(SO₄) to ferric oxyhydroxycarbonate exGRc-Fe(III) or ferric oxyhydroxysulphate exGRs-Fe(III), for which the following formulas can be proposed, Fe^III₆(OH)_(12−2y)(O)_(2+y)(H₂O)_(y)(CO₃) or Fe^III₆(OH)_(12−2z)(O)_(2+z)(H₂O)_(6+z)(SO₄) with 0 ≤ y or z ≤ 2. The solid-state reduction gives ferrous hydroxycarbonate exGRc-Fe(II) or ferrous hydroxysulphate exGRs-Fe(II), which may have the following chemical formulas, [Fe^II₆(OH)₁₀(H₂O)₂]·[CO₃, 2H₂O] or [Fe^II₆(OH)₁₀(H₂O)₂]·[SO₄, 8H₂O].