A metastable Fe(A) termination at the Fe3O4(001) surface

A metastable Fe(A) terminated Fe₃O₄(001) surface was prepared by tailoring the surface preparation conditions. STM, LEIS and LEED are utilized to demonstrate that annealing the Ar⁺ sputtered surface to 350 °C produces an Fe(A) terminated surface with a (√2 × √2)R45° superstructure. Within the superstructure both single Fe atoms and Fe dimer species are observed. The surface is reoxidized upon annealing to higher temperatures, eventually leading to the recovery of the energetically favorable Jahn–Teller distorted surface at 700 °C. The ability to reproducibly prepare the Fe(A) termination in this simple manner will allow investigations into the structure–function relationship for this important technological material.     

Fe alloying effect on the performance of the Ni anode in hydrogen fuel

Composite materials of YSZ (yttria-stabilized zirconia) with various Ni–Fe alloys were synthesized and evaluated as the solid oxide fuel cell (SOFC) anode using a 200-µm thick YSZ electrolyte as support and YSZ +La_0.8Sr_0.2MnO₃ (LSM) as cathode. The single cell with the YSZ + Ni_0.75Fe_0.25 anode exhibited the highest performance among all the investigated cells, e.g. a peak power density of 403, 337, 218 and 112 mW/cm² was achieved with H₂ fuel at 900, 850, 800 and 750 °C, respectively. The composite anode with the Ni_0.75Fe_0.25 alloy also had the lowest polarization resistance of 0.55 Ω·cm² at 800 °C among all the alloy compositions, indicating that this specific alloy offered a better anode composition than pure Ni. The possible mechanism for the improved performance of Ni with the Fe alloying addition towards H₂ oxidation was discussed.     

Application of laser in seam welding of dissimilar steel to aluminium joints for thick structural components

Laser welding-brazing technique, using a continuous wave (CW) fibre laser with 8000 W of maximum power, was applied in conduction mode to join 2 mm thick steel (XF350) to 6 mm thick aluminium (AA5083-H22), in a lap joint configuration with steel on the top. The steel surface was irradiated by the laser and the heat was conducted through the steel plate to the steel-aluminium interface, where the aluminium melts and wets the steel surface. The welded samples were defect free and the weld micrographs revealed presence of a brittle intermetallic compounds (IMC) layer resulting from reaction of Fe and Al atoms. Energy Dispersive Spectroscopy (EDS) analysis indicated the stoichiometry of the IMC as Fe₂Al₅ and FeAl₃, the former with maximum microhardness measured of 1145 HV 0.025/10. The IMC layer thickness varied between 4 to 21 μm depending upon the laser processing parameters. The IMC layer showed an exponential growth pattern with the applied specific point energy (E_sp) at a constant power density (PD). Higher PD values accelerate the IMC layer growth. The mechanical shear strength showed a narrow band of variation in all the samples (with the maximum value registered at 31.3 kN), with a marginal increase in the applied E_sp. This could be explained by the fact that increasing the E_sp results into an increase in the wetting and thereby the bonded area in the steel-aluminium interface.     

Polyphosphazene based composite polymer electrolytes

Composite salt-in-polymer electrolyte membranes were prepared from poly[(bis(2-methoxyethyl)amino)_1−x(n-propylamino)_x-phosphazene] (BMEAP) with dissolved LiCF₃SO₃ and dispersed Al₂O₃ nanoparticles (40 nm). Membranes with good mechanical stability were obtained. Low ionic conductivities were found in particle free membranes with maximum conductivities at 10 wt.% LiCF₃SO₃ ranging from 3.1 × 10^− 7 S/cm at 30 °C to 1.8 × 10^− 5 S/cm at 90 °C. For the composite membranes, addition of 2 wt.% Al₂O₃ nanoparticles leads to a steep increase of the conductivity by almost two orders of magnitude as compared to the homogeneous membranes. The highest room temperature conductivity for the investigated BMEAP–LiCF₃SO₃–Al₂O₃ composite systems was 10^− 5 S/cm.     

Identification of iron oxide phases in thin films grown on Al2O3(0 0 0 1) by Raman spectroscopy and X-ray diffraction

We report on the identification of Fe₃O₄ (magnetite) and α-Fe₂O₃ (hematite) in iron oxide thin films grown on α-Al₂O₃(0 0 0 1) by evaporation of Fe in an O₂-atmosphere with a thickness of a few unit cells. The phases were observed by Raman spectroscopy and confirmed by X-ray diffraction (XRD). Magnetite appeared independently from the substrate temperature and could not be completely removed by post-annealing in an oxygen atmosphere as observed by X-ray diffraction. In the temperature range between 400 °C and 500 °C the X-ray diffraction shows that predominantly hematite is formed, the Raman spectrum shows a mixture of magnetite and hematite. At both lower and higher substrate temperatures (300 °C and 600 °C) only magnetite was observed. After post-annealing in an O₂-atmosphere of 5 × 10^?5 mbar only hematite was detectable in the Raman spectrum.     

Thermoluminescence,infrared reflectivity and electron paramagnetic resonance properties of hemimorphite

Silicate mineral hemimorphite has been investigated concerning its TL, IR and EPR properties. A broad TL peak around 180 °C and a weaker and narrower peak around 360 °C were found in a sample annealed at 600 °C for 1 h and then irradiated. The deconvolution using the CGCD method revealed peaks around 132, 169, 222 and 367 °C. The reflectivity measurements showed several bands in the NIR region due to H₂O, OH and Al–OH complexes. No band was observed in the visible region. The thermal treatments were carried out from ∼110 to 940 °C and dehydration was observed, first causing a diminishing optical absorption in general and the disappearance of water and hydroxyl absorption bands. The EPR spectrum of natural hemimorphite, presented Cu²⁺ signals at g = 2.4 and g = 2.1 plus E₁′ signal superposed to Fe³⁺ signal around g = 2.0.     

Thermostability and magnetic properties of Sm2(Fe,M)17(C,N)y (M=Ga,Al and Si)

The effect of heat treatments on the interstitial nitrides and carbides prepared from Sm₂(Fe,M)₁₇ and Sm₂Fe₁₅Ga₂C₂ by gas–solid reaction (GSR) was investigated in order to understand more about the influence of the substitutions M=Ga, Al and Si on the thermostability of these compounds. X-ray diffraction measurements of the heat-treated samples have shown that all of these substitutions improve the thermostability of the Th₂Zn₁₇-type interstitial structure, with Al having the smallest influence and Si and Ga having a stronger influence on the thermostability. Contrary to the nitrides, the carbides prepared by GSR can be significantly stabilized with an increasing amount of the stabilizing elements. Already for a small amount of the substitution element, e.g. Sm₂Fe_16.5Ga_0.5, the 2 : 17-type interstitial phase was stabilized up to 750°C. Carburization at temperatures above 500°C was possible for the stabilized 2 : 17 compounds making possible much shorter carburization times for high C contents. Investigations on Sm₂Fe₁₅Ga₂C_y have revealed a dependence of the thermostability on the amount of absorbed C and on the processing route for the interstitial modification by GSR. A coercivity μ_0JH_c up to about 2.8 T was obtained for Sm₂Fe₁₅Ga₂C₂N showing the magnetic potential of these modified compounds.     

Proton conductivity of Al(H2PO4)3–H3PO4 composites at intermediate temperature

Composites of Al(H₂PO₄)₃ and H₃PO₄ were synthesised by soft chemical methods with different Al/P ratios. The Al(H₂PO₄)₃ obtained was found to have a hexagonal symmetry with parameter a = 13.687(3)Å, c = 9.1328(1)Å. The conductivity of this material was measured by a.c. impedance spectroscopy between 100 °C and 200 °C in different atmospheres. The conductivity of pure Al(H₂PO₄)₃ in air is in the order of 10^? 6–10^? 7 S/cm between 100 and 200 °C. For samples containing small excess of H₃PO₄, much higher conductivity was observed. The impedance responses of the composites were found to be similar with AlH₂P₃O₁₀·nH₂O under different relative humidity. The conductivity of Al(H₂PO₄)₃–H₃PO₄ composite with Al/P = 1/3.5 reached 6.6 mS/cm at 200 °C in wet 5% H₂. The extra acid is found to play a key role in enhancing the conductivity of Al(H₂PO₄)₃–H₃PO₄ composite at the surface region of the Al(H₂PO₄)₃ in a core shell type behaviour. 0.7% excess of H₃PO₄ can increase the conductivity by three orders of magnitude. These composites might be alternative electrolytes for intermediate temperature fuel cells and other electrochemical devices. Conductivity (9.5 mS/cm) changed little, when the sample was held at 175 °C for over 100 h as the conductivity stabilised.     

Structural and magnetic properties of thin epitaxial Fe films on (1 1 0) GaAs prepared by metalorganic chemical vapor deposition

Iron films have been grown on (1 1 0) GaAs substrates by atmospheric pressure metalorganic chemical vapor deposition at substrate temperatures (T_s) between 135°C and 400°C. X-ray diffraction (XRD) analysis showed that the Fe films grown at T_s between 200°C and 330°C were single crystals. Amorphous films were observed at T_s below 200°C and it was not possible to deposit films at T_s above 330°C. The full-width at half-maximum of the rocking curves showed that crystalline qualities were improved at T_s above 270°C. Single crystalline Fe films grown at different substrate temperature showed different structural behaviors in XRD measurements. Iron films grown at T_s between 200°C and 300°C showed bulk α-Fe like behavior regardless of film thickness (100–6400 Å). Meanwhile, Fe films grown at 330°C (144 and 300 Å) showed a biaxially compressed strain between substrate and epilayer, resulting in an expanded inter-planar spacing along the growth direction. Magnetization measurements showed that Fe films (>200 Å) grown at 280°C and 330°C were ferromagnetic with the in-plane easy axis along the [1 1 0] direction. For the thinner Fe films (⩽200 Å) regardless of growth temperature, square loops along the [1 0 0] easy axis were very weak and broad.     

Solid state interactions in nano-sized oxides

Comparative study of reactivity of nano- and micro-sized alumina and nickel oxide, obtained by the electrical explosion of metal wires in oxidizing atmosphere, was carried out for the reactions NiO + MoO₃, NiO + Al₂O₃, and Al₂O₃ + Bi₂O₃ by coupled anneals of ceramics, measurements of the conductivity of individual oxides and raw oxide mixtures, X-ray diffraction and differential thermal analysis. The total conductivity of nano-structured oxides was found lower than that of micro-structured ceramics. Mixing bismuth oxide with nano-structured alumina leads to stabilization of the low temperature polymorph α-Bi₂O₃ up to 780 °C. The diffusion permeability of NiMoO₄ layer grown at the surface of NiO ceramics, having submicron grains, was found 2 times lower if compared to NiMoO₄ grown at micro-sized NiO ceramics. NiO and Al₂O₃ nano-powders preserve the high reactivity even when heated up to 1000 °C. The results are discussed in terms of size effects on the solid state reactivity of oxides.     

Surface reactions of TiCl4 and Al(CH3)3 on GaAs(100) during the first half-cycle of atomic layer deposition

GaAs(100) was exposed to pulses of trimethylaluminum (TMA, Al(CH₃)₃) and titanium tetrachloride (TiCl₄) to mimic the first half-cycle of atomic layer deposition (ALD). Both precursors removed the 9.0 ± 1.6 Å-thick mixed oxide consisting primarily of As₂O₃ with a small Ga₂O component that was left on the surface after aqueous HF treatment and vacuum annealing. In its place, TMA deposited an Al₂O₃ layer, but TiCl₄ exposure left Cl atoms adsorbed to an elemental As layer. This suggests that oxygen was removed by the formation of a volatile oxychloride species. A small TiO₂ coverage of approximately 0.04 monolayer remained on the surface for deposition temperatures of 89 °C to 135 °C, but no TiO₂ was present from 170 °C to 230 °C. The adsorbed Cl layer chemically passivated the surface at these temperatures and blocked TiO₂ deposition even after 50 full ALD cycles of TiCl₄ and water vapor. The Cl and As layers desorbed simultaneously at higher temperature producing peaks in the temperature programmed desorption spectrum in the range 237–297 °C. This allowed TiO₂ deposition at 300 °C in single TiCl₄ pulse experiments. On the native oxide-covered surface where there was a higher proportional Ga oxide composition, TiCl₄ exposure deposited TiO₂.     

Formation of nickel and nickel,zinc-bearing ferrite in aqueous suspension by air oxidation

Stoichiometric Ni-bearing ferrite was formed by air oxidation of an iron(II) hydroxide suspension at an initial Ni : Fe_tot mol ratio (r_Ni) of 0.20 : 2.80 at pH 10.0 and 65°C. Most of products formed at r_Ni=0.40 : 2.60 and 0.60 : 2.40 were Ni-bearing ferrites, of which vacancies of Fe³⁺ ion on the lattice points may be considered. Only Ni, Zn-bearing ferrites were formed in the suspensions at initial (Ni + Zn)  : Fe_tot mol ratios (r_{Ni + Zn}) of 0.20 : 2.80–0.60 : 2.40 at pH 10.0 and 65°C. At higher r_Ni or r_{Ni + Zn} by-products containing Ni, Fe and O₄²⁻ were formed. The formation of the by-products was depressed in the suspensions containing chloride ions in the place of sulfate ions.     

Oxygen adsorption properties of YBaCo4O7-type compounds

The oxygen adsorption and desorption of newly found compounds RBaCo₄O₇ (R = Y, Dy–Lu, In) were investigated by thermogravimetry (TG) in the temperature range from room temperature to 1100 °C. The influence of Co replaced by Zn and Fe on the oxygen diffusion properties of YBaCo₄O₇ was also studied. TG results showed clearly that all RBaCo₄O₇ compounds basically experience two oxygen adsorption and desorption processes in the temperature range 20∼1100 °C in oxygen flow. One happens at about 200∼450 °C and the another happens at about 660∼1050 °C. The differences between the resulting states by adsorbing oxygen at lower and high temperature were discussed based on the X-ray diffraction (XRD) patterns and TG data. We showed evidence that the oxygen adsorption at the lower temperature has a small activation energy, while the oxygen adsorption at the higher temperature has a large activation energy. The oxygen adsorbed at high temperature will destroy the RBaCo₄O₇ structure. Zn substituting in the YBaCo_4 − xZn_xO₇ influences the oxygen diffusion behavior prominently, the amount of oxygen adsorbed becomes increasingly weak with the increase of Zn content and disappears completely for the samples with x ≥ 2.0. However, replacement of Co by Fe has little effect on the oxygen absorption process.     

Magnetic properties of nanocrystalline Fe86.7Zr3.3B4Ag6 thin film

The changes of magnetic properties with annealing temperature were studied in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film. The thin films were deposited by a DC magnetron sputtering method, annealed at 300–700°C for 1 h in vacuum under a field of 1.5 kOe parallel to the film plane, and then furnace-cooled. As a result, it has been found that the Ag addition to Fe–Zr–B amorphous thin films resulted in the decrease of crystallization temperature to 400°C due to promoted crystallization ability. Also, it gave rise to formation of fine BCC α-Fe crystalline precipitates with a grain size smaller than 10 nm in the amorphous matrix near 400°C, and led to prominent enhancement in the magnetic properties of the Fe_86.7Zr_3.3B₄Ag₆ thin films. Significantly, excellent magnetic properties such as a saturation magnetization of 1.7 T, a coercive force of 1 Oe and a permeability of 7800 at 50 MHz were obtained in the amorphous Fe_86.7Zr_3.3B₄Ag₆ thin film containing 7.2 nm-size BCC α-Fe, which was annealed at 400°C. Also, core loss of 1.4 W cm⁻³ (B_m=0.1 T) at 1 MHz in the thin film was obtained, and it is a much lower value than had been obtained in any existing soft magnetic materials. Such excellent properties are inferred to originate from the uniform dispersion of nano-size BCC α-Fe in the amorphous matrix.     

Sonocatalytic rapid degradation of Congo red dye from aqueous solution using magnetic Fe0/polyaniline nanofibers

Nano-sized magnetic Fe⁰/polyaniline (Fe⁰/PANI) nanofibers were used as an effective material for sonocatalytic degradation of organic anionic Congo red (CR) dye. Fe⁰/PANI_, was synthesized via reductive deposition of nano-Fe⁰ onto the PANI nanofibers at room temperature. Prepared catalyst was characterized using HR-TEM, FE-SEM, XRD, FTIR instruments. The efficacy of catalyst in removing CR was assessed colorimetrically using UV–visible spectroscopy under different experimental conditions such as % of Fe⁰ loading into the composite material, solution pH, initial concentration of dye, catalyst dosage, temperature and ultrasonic power. The optimum conditions for sonocatalytic degradation of CR were obtained at catalyst concentrations = 500 mg.L⁻¹, concentration of CR = 200 ppm, solution pH = neutral (7.0), temperature = 30 °C, % of Fe⁰ loading = 30% and 500 W ultrasonic power. The experimental results showed that ultrasonic process could remove 98% of Congo red within 30 min with higher Q_max value (Q_max = 446.4 at 25 °C). The rate of degradation of CR dye was much faster in this ultrasonic technique rather than conventional adsorption process. The degradation efficiency declined with the addition of common inorganic salts (NaCl, Na₂CO₃, Na₂SO₄ and Na₃PO₄). The rate of degradation suppressed more with increasing salt concentration. Kinetic and isotherm studies indicated that the degradation of CR provides pseudo-second order rate kinetic and Langmuir isotherm model compared to all other models tested. The excellent high degradation capacity of Fe⁰/PANI under ultrasonic irradiation can be explained on the basis of the formation of active hydroxyl radicals (OH) and subsequently a series of free radical reactions.     

Electrical conductivity and thermal expansion of the oxy-cuspidine Gd4Al2O9 substituted with Ca and Sr

The series of Gd_4 ? xM_xAl₂O_9 ? x/2 (M = Ca, Sr) with x = 0, 0.01, 0.05, 0.10 and 0.25 was prepared by the citrate complexation method. Both Gd_4 ? xCa_xAl₂O_9 ? x/2 and Gd_4 ? xSr_xAl₂O_9 ? x/2 show the monoclinic cuspidine structure with space group of P2₁/c up to 0.05–0.1 and 0.01–0.05 mol for Ca and Sr, respectively. Beyond the substitution limit of Gd₄Al₂O₉, GdAlO₃ and SrGd₂Al₂O₇ appear as additional phases. The highest electrical conductivity obtained at 900 °C yielded σ = 1.49 × 10^? 4 S/cm for Gd_3.95Ca_0.05Al₂O_8.98. In comparison, the conductivity of pure Gd₄Al₂O₉ was σ = 1.73 × 10^? 5 S/cm. The conductivities determined are in a similar range as those of other cuspidine materials investigated previously. The thermal expansion coefficient of Gd₄Al₂O₉ at 1000 °C was 7.4 × 10^? 6 K^? 1. The phase transition between 1100 and 1200 °C reported earlier changes with increasing substitution of Ca and Sr.     

Interfacial microstructure and growth mechanism of Al4C3 in Grf/Al composites fabricated by liquid pressure method

In this study, Gr_f/Al composite was fabricated by liquid pressure method. The diffusion layer and the nucleation and growth of Al₄C₃ were observed at the interface of Gr_f/Al composites by TEM and HRTEM. The growth mechanism of Al₄C₃ was analyzed in detail by crystallography theory. It was found that Al₄C₃ had no phase relations with the carbon fiber. (0 0 0 1) layer of Al₄C₃ was parallel with main growth direction. Both the diffusion layer at the interface and crystal structure of Al₄C₃ affected the shape of Al₄C₃. At a certain position, Al₄C₃ could connect two fibers when the fibers were close to each other.     

Thermal expansion behavior and chemical compatibility of BaxSr1−xCo1−yFeyO3−δ with 8YSZ and 20GDC

Perovskite oxides of the composition Ba_xSr_1−xCo_1−yFe_yO_3−δ(BSCF) were synthesized via a modified Pechini method and characterized by X-ray diffraction, dilatometry and thermogravimetry. Investigations revealed that single-phase perovskites with cubic structure can be obtained for x ≤ 0.6 and 0.2 ≤ y ≤ 1.0. The as-synthesized BSCF powders can be sintered in several hours to nearly full density at temperatures of over 1180 °C. Thermal expansion curves of dense BSCF samples show nonlinear behavior with sudden increase in thermal expansion rate between about 500 °C and 650 °C, due mainly to the loss of lattice oxygen caused by the reduction of Co⁴⁺ and Fe⁴⁺ to lower valence states. Thermal expansion coefficients (TECs) of BSCF were measured to be 19.2–22.9 × 10^− 6 K^− 1 between 25 °C and 850 °C. Investigations showed further that Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ is chemically compatible with 8YSZ and 20GDC for temperatures up to 800 °C, above which severe reactions were detected. After being heat-treated with 8YSZ or 20GDC for 5 h above 1000 °C, Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3−δ was completely converted to phases like SrCoO_3−δ, BaCeO₃, BaZrO₃, etc.     

The initial growth structure of Ni1-xFex (x=0.6–0.8) films dc-biased plasma-sputter-deposited on Ni/MgO(0 0 1), and on Fe/MgO(0 0 1)

Cross sectional and plane-view transmission electron microscopy (X- and PV-TEM) were used to investigate the initial growth phase of 5, 10, 20 and 40 nm thick Ni_1-xFe_x (x=0.6–0.8) films, prepared on MgO(0 0 1) covered with a buffer layer of Fe or Ni as well as on naked MgO(0 0 1). The 100 nm thick buffer layers of Fe and Ni were pre-grown on MgO(0 0 1). All of Ni_0.20Fe_0.80, Ni_0.40Fe_0.60, Fe and Ni films could be epitaxially grown at 250°C by dc-biased plasma sputtering at 2.9 kV in pure Ar gas.The films of Ni_0.20Fe_0.80 and Ni_0.40Fe_0.60 were grown in their own stable phase, bcc and fcc on MgO(0 0 1), respectively. However, Ni_0.20Fe_0.80 film could be grown in fcc phase pseudomorphic with Ni(0 0 1) up to 20 nm thick on Ni/MgO(0 0 1), while Ni_0.40Fe_0.60 film in bcc phase pseudomorphic with Fe(0 0 1) up to 10 nm thick on Fe/MgO(0 0 1). With increasing thickness, their growth phases transformed into their own stable phases. Whether or not the pseudomorphic phase may be induced and what its critical thickness may be should depend primarily on the lattice misfit between the crystal planes in contact. The growth mode of Ni_0.40Fe_0.60 film was investigated more in details to be compared with the simulations of the average strain energy versus thickness and with those of the critical thickness of the pseudomorphic films versus the lattice misfit between the contacted crystal planes.     

Sono-synthesis and characterization of bimetallic Ni–Co/Al2O3–MgO nanocatalyst: Effects of metal content on catalytic properties and activity for hydrogen production via CO2 reforming of CH4

Sono-dispersion of Ni, Co and Ni–Co over Al₂O₃–MgO with Al/Mg ratio of 1.5 was prepared and tested for dry reforming of methane. The samples were characterized by XRD, FESEM, PSD, EDX, TEM, BET and FTIR analyses. In order to assess the effect of ultrasound irradiation, Ni–Co/Al₂O₃–MgO with Co content of 8% prepared via sonochemistry and impregnation methods. The sono-synthesized sample showed better textural properties and higher activity than that of impregnated one. Comparison of XRD patterns indicated that the NiO peaks became broader by increasing Co content over the support. The FESEM images displayed the particles are small and well-dispersed as a result of sonochemistry method. Also, EDX analysis demonstrated better dispersion of Ni and Co as a result of sonochemistry method in confirmation of XRD analysis. The sono-synthesized Ni–Co/Al₂O₃–MgO as a superior nanocatalyst with Co content of 3% illustrates much higher conversions (97.5% and 99% for CH₄ and CO₂ at 850 °C), yields (94% and 96% for H₂ and CO at 850 °C) and 0.97 of H₂/CO molar ratio in all samples using an equimolar feed ratio at 850 °C. During the 1200 min stability test, H₂/CO molar ratio remained constant for the superior nanocatalyst.