Magnetic composites based on hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides

Materials containing hybrid spheres of aluminum oxide and superparamagnetic nanoparticles of iron oxides were obtained from a chemical precursor prepared by admixing chitosan and iron and aluminum hydroxides. The oxides were first characterized with scanning electron microscopy, X-ray diffraction, and Mössbauer spectroscopy. Scanning electron microscopy micrographs showed the size distribution of the resulting spheres to be highly homogeneous. The occurrence of nano-composites containing aluminum oxides and iron oxides was confirmed from powder X-ray diffraction patterns; except for the sample with no aluminum, the superparamagnetic relaxation due to iron oxide particles were observed from Mössbauer spectra obtained at 298 and 110 K; the onset six line-spectrum collected at 20 K indicates a magnetic ordering related to the blocking relaxation effect for significant portion of small spheres in the sample with a molar ratio Al:Fe of 2:1.     

Difference in high-temperature oxidation resistance of amorphous Zr-Si-N and W-Si-N films with a high Si content

The high-temperature oxidation resistance of amorphous Zr-Si-N and W-Si-N films with a high Si content (≥20 at.%) deposited by reactive dc magnetron sputtering at different partial pressures of nitrogen was systematically investigated by means of a symmetrical high-resolution thermogravimetry in a flowing air up to an annealing temperature of 1300 °C (a temperature limit for Si(1 0 0) substrate). Additional analyses including X-ray diffraction (XRD), light optical microscopy (LOM), scanning electron microscopy (SEM), atomic force microscopy (AFM), and microhardness measurement were carried out as well. The obtained results showed (i) an excellent high-temperature oxidation resistance of the Zr-Si-N films up to 1300 °C, (ii) a considerably lower oxidation resistance of the W-Si-N films. The W-Si-N films are completely oxidized at 800 °C with a subsequent volatilization of unstable WO_x oxides. On the other hand, the Zr-Si-N films are oxidized only very slightly on the surface, where a stable oxide barrier layer preventing further inward oxygen diffusion is formed. The thickness of the oxide layer is only about of 3% of the total film thickness. The phase composition, thermal stability of individual phases and amorphous structure were found to be key factors to achieve a high oxidation resistance.     

Investigation of the structure and surface characteristics of Cu-Ni-M(III) mixed oxides (M = Al, Cr and In) prepared from layered double hydroxide precursors

The Cu-Ni-M(III) mixed oxides (M = Al, Cr and In) were prepared by calcination of layered double hydroxide precursors with Cu²⁺/Ni²⁺/M³⁺ ratio of 1/2/1. The materials were characterized by means of powder X-ray diffraction (XRD), Fourier transform infrared (FT-IR), thermogravimetric-differential thermal analysis (TG-DTA), X-ray photoelectron spectroscopy (XPS) and low temperature N₂ adsorption-desorption experiments. The results indicated that calcination of precursors at 500 °C gave rise to mixed metal oxides including simple oxides and composite oxides, and the composition distributions of obtained oxides depended on the nature of trivalent cation in precursors. Under mild experimental conditions (atmospheric pressure and 25 °C), oxidation of aqueous phenol solutions by hydrogen peroxide exhibited that the Cr-containing mixed oxide achieved the highest conversion of phenol owing to the presence of more amount of composite oxide phase containing active copper centers, while the aluminum-containing one could significantly enhance deep oxidation of phenol into smaller molecules owing to the presence of more surface oxygen species.     

Microstructural behavior on particle surfaces and interfaces in Sm2Fe17N3 powder compacts during low-temperature sintering

Sm₂Fe₁₇N₃ sintered compacts were prepared below 450 °C by a high-pressure current sintering technique. The coercivity of the sintered compacts decreased linearly as the sintering temperature increased. Transmission electron microscopic analyses indicated that thin Fe-rich layers containing α-Fe phases were formed just inside the initial oxide layer on the particle surfaces and interfaces in the sintered samples. The generation of α-Fe phases was supposed to cause the coercivity decrease. In addition, X-ray photoelectron spectroscopy analysis revealed that Fe₂O₃ and FeO contained in the oxide layer of the raw powder disappeared subsequent to heat treatment. These results suggested that the α-Fe phases were generated by the oxidation–reduction reaction between the initial iron oxides and the primary Sm₂Fe₁₇N₃ phase but not by thermal decomposition or exogenous oxidation during sintering. This mechanism was supported by the fact that extending the sintering time did not result in any further decrease in the coercivity.     

Controlled preparation of well-ordered transition metal oxide layers on a metallic surface

A method to create various well-ordered two dimensional transition metal oxide films on a metallic substrate has been exploited. The formation of an intermediate amorphous layer with controllable metal-oxygen stoichiometry serves as an important precursor condition for the final transformation into a mono-phase, crystalline oxide layer via mild annealing. As a key ingredient serves a Cu₃Au(1 0 0) substrate covered by oxygen. The flat Cu-O topmost layer stops completely intermixing of the substrate material with the subsequently evaporated transition metal film. Likewise the wetting of the surface is considerably enhanced and a homogeneous oxidation of the film is strongly promoted. The proposed technique appears to be widely efficient for preparation of various two dimensional oxide films covering the entire Cu₃Au(1 0 0) substrate. Its usefulness is demonstrated successfully for vanadium, niobium and molybdenum to produce a set of single-phase transition metal oxides of different stoichiometry and geometrical structure. All created oxides are found to be thermally stable at least up to a substrate temperature of 800 K.     

Evolution of steel surface composition with heating in vacuum and in air

X-ray photoelectron spectroscopy (XPS) has been used to investigate the changes in surface composition of three steels as they have undergone heating. The steels were mild steel, and two austenitic stainless steels, commonly designated 304 and 316 stainless steels. XPS measurements were made on the untreated samples, and then following heating for 30 min in vacuo and in a 1 × 10⁻⁶ Torr partial pressure of air, at temperatures between 100 °C and 600 °C.Mild steel behaves differently to the two stainless steels under the heating conditions. In mild steel the iron content of the surface increased, with oxygen and carbon decreasing, as a function of increasing temperature. The chemical state of the iron also changed from oxide at low temperatures, to metallic at temperatures above 450 °C.In both stainless steels the amount of iron present in the surface decreased with increasing temperature. The decrease in iron at the surface was accompanied by an increase in the amount of chromium at the steel surface. At temperatures above 450 °C the iron in both 304 and 316 stainless steels showed significant contributions from metallic iron, whilst the chromium present was in an oxide state. In 316 stainless steel heated to 600 °C there was some metallic chromium present in the surface layer.The surfaces heated in air showed the least variation in composition, with the major change being the loss of carbon from the surfaces following heating above 300 °C. There was also a minor increase in the concentration of chromium present on both the stainless steels heated under these conditions. There was also little change in the oxidation state of the iron and chromium present on the surface of these steels. There was some evidence of the thickening of the surface oxides as seen by the loss of the lower binding energy signal in the iron or chromium core level scans.The surfaces heated in vacuum showed a similar trend in the concentration of carbon on the surfaces, however the overall concentration of oxygen decreased throughout the heating of these steels. There were also significant changes in the oxidation state of the iron and chromium on these surfaces with significant amounts or iron and chromium present in the metallic form following heating up to 600 °C.It appears that the carbon contamination on the surfaces plays an important role in the fate of the surface oxide layer for all of the steels heated in a vacuum environment.     

Comparative study of morphology and surface composition of Al–Cr–Fe alloy powders produced by water and gas atomisation technologies

The morphology and surface composition of Al–Cr–Fe alloy powders of 0–63 and 63–100 μm size fractions, produced by gas and water atomisation, have been studied by scanning electron microscopy and Auger electron spectroscopy. While gas atomised particles are of spherical shape, water atomised powders are usually irregular in shape with a complex branched relief. The morphology and composition of surface oxides have been estimated. The surface oxide film is composed of aluminium oxides/hydroxides and contains no Fe and Cr atoms. Two to five water molecules are associated with one Al₂O₃ molecule on the surface of powders. The surface oxide film has a non-uniform thickness, with thick oxide islands separated by thinner oxide film. The parameters of the surface film morphology, such as the island coverage, the oxygen content and the thin film thickness, depend on the atomisation technology used and powder size fraction. Heavily and weakly oxidised powder groups present in all powder fractions are distinguished by Auger spectra analysis. Relationships between heavily and weakly oxidised powder groups are discussed as a function of atomisation technology and size fraction.     

Superparamagnetic nanocomposites based on poly(styrene-b-ethylene/butylene-b-styrene)/cobalt ferrite compositions

Magnetic nanoparticles were created in or around the sulfonated (s) polystyrene domains in a phase separated poly[styrene-b-(ethylene-co-butylene)-b-styrene)] block copolymer (BCP) using an in situ inorganic precipitation procedure. The sBCP was neutralized with a mixed iron/cobalt chloride electrolyte and the doped samples were converted to their oxides by reaction with sodium hydroxide and further washing with water. Transmission electron microscopy indicated the presence of nanoparticles in the 5–25 nm size range. The metal oxide particle structures were studied using select area electron diffraction, which revealed that they are of the cobalt iron oxide composition (CoFe₂O₄). These nanocomposites were shown, using a superconducting quantum interference device magnetometer, to be superparamagnetic at 300 K and ferrimagnetic at 5 K. Nanocomposites consisting of smaller particles have a blocking temperature of 70 K, whereas it was 140 K for larger particles.     

Characterisation of advanced metal particle recording media pigments

Advanced metal particle (MP) pigments used in high density digital storage applications on flexible substrates have been characterised using a wide range of techniques. We have examined the chemical composition of the particles and find that the core consists of an iron–cobalt alloy in a structure BCC, analogous to that of α-Fe. Surface layers on the particle used to provide oxidation resistance, have been found to contain some form of oxides of Co, Al and Y as well as iron oxide which we find to be in a spinel form. From Mössbauer spectroscopy we have determined the proportion of the iron atoms in the oxide layer and find that by controlling the percentage of Y in the surface, a much thinner oxide layer results with a corresponding increase in the overall saturation magnetisation. From high resolution TEM images we have determined the particle size of each sample and compared those values with those obtained from X-ray line broadening. We find that in each case the X-ray ‘size' is about a quarter of the physical size excluding the oxide layer. This indicates the polycrystalline character of the particles and the crystallite size is in broad agreement with the activation volume of magnetisation reversal determined from an analysis of magnetic viscosity in the materials. These results are indicative of incoherent magnetisation reversal in the particles which may account for the lower than expected coercivity values in such pigments.     

The surface nanostructure of pure iron after combined electrochemical passivation and thermal annealing treatments

The properties of the surface oxide film on pure iron after electrochemical passivation and thermal annealing treatments were investigated using a variety of techniques. Passivation was carried out with an applied potential of 800 mV (vs Ag/AgCl) for 15 min in a pH 8.4 borate buffer solution at 30 °C, whilst annealing was carried out in air in an electric furnace at temperatures up to 300 °C. Analysis of the surface properties was then carried out using X-ray diffraction to determine oxide composition, a spectroscopic ellipsometer to measure the optical properties and oxide thickness, and a scanning probe microscope to measure the surface roughness using tapping mode AFM and to observe the nanoscale structure using constant height mode STM.     

Spectroscopic characterization of Ni films on sub-10-nm silica layers: Thermal metamorphosis and chemical bonding

The thermal evolution in the chemical and physical characteristics of the Ni film of thickness 1-50 nm deposited on silica of thickness less than 10 nm was studied. The chemical composition of silica affected the thermal behavior of the Ni overlayer substantially. Nickel deposited on native oxide may diffuse downward into native oxide during annealing and was oxidized. It mainly produced Ni₃O₂ and silicides below 150 °C. Increasing the temperature to 300 °C caused further oxidation of Ni to yield NiO. The sub-10-nm silicon dioxide layer, on the other hand, can inhibit the diffusion of Ni atoms downward when the Ni-deposited sample was annealed. Instead, these atoms aggregated into small particles on the surface at elevated temperatures, causing the substrate to be exposed. The size of the particles produced can be controlled, as it increased almost linearly with the thickness of the Ni film deposited in the low thickness regime. The thinner Ni films yielded smaller, round nanoparticles with better dispersity. The particles formed were strongly adhered to the silicon dioxide surface. The bulk of the particles formed was mainly metallic. Exposing to the air of the Ni particles formed on silicon dioxide mainly produces Ni₂O₃ on the particles.     

The growth of Au/Pd/alumina/Cu-Al system studied by SRPES

An ultra-thin alumina layer grown on Cu-9at.%Al (1 1 1) surface was studied using synchrotron radiation photoelectron spectroscopy (SRPES), X-ray photoelectron spectroscopy (XPS) and low energy electron diffraction (LEED). By deconvolving SRPES spectra of the Al 2p doublet, four components belonging to metallic as well as oxide phases were recognized. Pd-Au alloy formation was confirmed by SRPES measurement during Pd and Au deposition. The study of the system's thermal stability reveals diffusion of Pd and Au atoms through the alumina layer. While Au atoms start to diffuse under the alumina layer at 670 K, Pd atoms are forming Pd-Al surface alloy at this temperature. The diffusion of Pd atoms through alumina occurs when sample was heated over 770 K. Alumina layer was stable even after heating the sample at 870 K, but its structure was corrupted probably due to the diffusion of metal atoms.     

The electrical properties of metal-oxide-semiconductor devices fabricated on the chemically etched n-InP substrate

The electrical properties of the Cu/n-InP and Al/n-InP Schottky barrier diodes (SBDs) with and without the interfacial oxide layer have been investigated by using current-voltage (I-V) measurements. The oxide layer on chemically cleaned indium phosphide (InP) surface has been obtained by exposure to water vapor at 1 ml/min at 200 °C before metal evaporation. The chemical composition of surface oxides grown on the InP is investigated using X-ray photoelectron spectroscopy (XPS). Phosphorus is present as In(PO₃)₃, InPO₄, P₂O₅ and P₄O₁₀. The values of 0.437 ± 0.007 and 0.438 ± 0.003 eV for the barrier height of the reference Cu/n-InP and Al/n-InP SBDs were obtained, respectively. Furthermore, the values of 0.700 ± 0.030 and 0.517 ± 0.023 eV for the barrier height of the oxidized Cu/n-InP and Al/n-InP SBD were obtained, respectively. The transport properties of the metal-semiconductor contacts have been observed to be significantly affected by the presence of the interfacial oxide layer. Devices built on the oxidized surfaces show improved characteristics compared with those built on chemically cleaned surfaces. The chemical reactivity of the metal with oxide and n-InP is important to the formation of the Schottky barriers. The reactive metal Al gave a low barrier height due to the reduction of oxide and reaction with InP. The transmission coefficients for the oxidized Cu/n-InP and Al/n-InP are equal to 2.23 × 10⁻⁵ and 4.60 × 10⁻², respectively.     

Sc2O3-W matrix impregnated cathode with spherical grains

Sc₂O₃-W matrix cathodes have been prepared by using a liquid-liquid doping method combined with high-temperature sintering. The microstructure and physical behavior of active substances of scandia-doped tungsten matrix and impregnated cathode has been studied by SEM and AES methods. The results show that the matrix has a homogeneous structure composed of W grains with spherical shape and superfine Sc₂O₃ particles dispersed uniformly over and among W grains. After impregnation, this Sc-type impregnated cathode has high emission capability. Space-charge-limited current density could reach 52 A/cm² at 850 °C_b. The high emission results from a Ba-Sc-O active layer with a thickness of about 80 nm, which is formed at the cathode surface during the activation period. Both the decrease of the thickness of active surface layer and the decrease of the content of Sc at the surface could lead to the degradation of current density during operation.     

Effect of alkaline cleaning and activation on aluminum alloy 7075-T6

The effect of alkaline cleaning and activation on the composition and thickness of the oxide layer on aluminum alloy 7075-T6 was studied. E-pH diagrams were developed to predict the effect of alkaline cleaning and activation solutions on the stability of the oxide surface layers. The thickness of the native oxide layer was determined to be ∼30 nm by Auger electron spectroscopy depth profiling analysis. The outer ∼20 nm was rich in magnesium while the remaining ∼10 nm was rich in aluminum. Cleaning in a 9.1 pH alkaline solution was found to remove the magnesium-rich layer and leave behind an aluminum-rich oxide layer ∼10 nm thick. Activation in alkaline solutions of NaOH (pH > 12.9) or Na₂CO₃ (pH > 11.5) produced an oxide that was ∼20 to 60 nm thick and rich in magnesium. Alkaline cleaning and activation altered the oxide composition and thickness making it possible for deposition of thicker cerium-based conversion coatings (∼100 to 250 nm) compared to only alkaline cleaning (∼30 nm), with application of one spray cycle of deposition solution.     

Formation of interface and surface oxides on supported Pd nanoparticles

We have quantitatively studied the interaction between oxygen and an Fe₃O₄-supported Pd model catalyst by molecular beam (MB) methods, time resolved IR reflection absorption spectroscopy (TR-IRAS) and photoelectron spectroscopy (PES) using synchrotron radiation. The well-shaped Pd particles were prepared in situ by metal evaporation and growth under ultrahigh vacuum (UHV) conditions on a well-ordered Fe₃O₄ film on Pt(1 1 1).It is found that for oxidation temperatures up to 450 K oxygen predominantly chemisorbs on metallic Pd whereas at 500 K and above (∼10⁻⁶ mbar effective oxygen pressure) large amounts of Pd oxide are formed. These Pd oxide species preferentially form a thin layer at the particle/support interface, stabilized by the iron-oxide support. Their formation and reduction is fully reversible. Upon decomposition, oxygen is released which migrates back onto the metallic part of the Pd surface. In consequence, the Pd interface oxide layer acts as an oxygen reservoir, the capacity of which by far exceeds the amount of chemisorbed oxygen on the metallic surface.Additionally, Pd surface oxides can also be formed at temperatures above 500 K. The extent of surface oxide formation critically depends on the oxidation temperature. This effect is addressed to different onset temperatures for oxidation of the particle facets and sites. It is shown that the presence of Pd surface oxides sensitively modifies the adsorption and reaction properties of the model catalyst, i.e. by lowering the CO adsorption energy and CO oxidation probability. Still, a complete reduction of the Pd surface oxides can be obtained by extended CO exposure, fully reestablishing the metallic Pd surface.     

Iron-based soft magnetic composites with Mn–Zn ferrite nanoparticles coating obtained by sol–gel method

This paper focuses on iron-based soft magnetic composites which were synthesized by utilizing Mn–Zn ferrite nanoparticles to coat iron powder. The nanocrystalline iron powders, with an average particle diameter of 20 nm, were obtained via the sol–gel method. Scanning electron microscopy, energy dispersive X-ray spectroscopy and distribution maps show that the iron particle surface is covered with a thin layer of Mn–Zn ferrites. Mn–Zn ferrite uniformly coated the surface of the powder particles, resulting in a reduced imaginary permeability, increased electrical resistivity and a higher operating frequency of the synthesized magnets. Mn–Zn ferrite coated samples have higher permeability and lower magnetic loss when compared with the non-magnetic epoxy resin coated compacts. The real part of permeability increases by 33.5% when compared with the epoxy resin coated samples at 10 kHz. The effects of heat treatment temperature on crystalline phase formation and on the magnetic properties of the Mn–Zn ferrite were investigated via X-ray diffraction and a vibrating sample magnetometer. Ferrites decomposed to FeO and MnO after annealing above 400 °C in nitrogen; thus it is the optimum annealing temperature to attain the desired permeability.     

Physico-chemical and electrical properties of rapid thermal oxides on Ge-rich SiGe heterolayers

Rapid thermal oxidation of high-Ge content (Ge-rich) Si_1−xGe_x (x = 0.85) layers in dry O₂ ambient has been investigated. High-resolution X-ray diffraction (HRXRD) and strain-sensitive two-dimensional reciprocal space mapping X-ray diffractometry (2D-RSM) are employed to investigate strain relaxation and composition of as-grown SiGe alloy layers. Characterizations of ultra thin oxides (∼6-8 nm) have been performed using Fourier transform infrared spectroscopy (FTIR) and high-resolution X-ray photoelectron spectroscopy (HRXPS). Formation of mixed oxide i.e., (SiO₂ + GeO₂) and pile-up of Ge at the oxide/Si_1−xGe_x interface have been observed. Enhancement in Ge segregation and reduction of oxide thickness with increasing oxidation temperature are reported. Interface properties and leakage current behavior of the rapid thermal oxides have been studied by capacitance-voltage (C-V) and current-voltage (J-V) techniques using metal-oxide-semiconductor capacitor (MOSCAP) structures and the results are reported.     

Self-assembled monolayers of flufenaminate anions on mild steel surface formed in aqueous solution

Adsorption of derivative of phenylanthranilic acid - flufenamic acid (FFA) on the “oxide-free” and oxidized surface of mild steel in neutral borate buffer solution was studied by ellipsometry and XPS. Anodic polarization curves reveal that complete suppression of the anodic dissolution of iron is achieved at FFA concentration C_in = 3.8 mM. Besides, adding FFA substantially shifts the pitting potential from 0.06 V to 0.67 V. Ellipsometric studies have shown that at the applied potential −0.65 V, when the surface is free from the oxide layer, FFA forms monomolecular layer. To characterize the surface layers formed after exposing the sample in 5 mM FFA solution the XPS was used to assess the composition and the thickness of the layers. Using the intensities of the Fe 2p, Fe 3p, N 1s, F 1s, O 1s and C 1s and analyzing the angle resolved XPS data the FFA molecules have been shown to form monomolecular layer in which FFA is (vertically or slightly inclined) anchored by iron cations through oxygen atoms of carboxyl group to the surface and the fluorine atoms of CF₃ groups form the utmost layer. Similar orientation is also assumed for FFA molecules adsorbed on the oxidized iron surface. It seems that the layer formed by FFA or similar molecules may serve a robust interface for grafting other substances on such a functionalized surface.     

A study of the 42CrMo4 steel surface by quantitative XPS electron spectroscopy

Quantitative X-ray photoelectron spectroscopy was used to characterize the native oxide film formed on 42CrMo4 steel surface by air exposure in normal conditions. In order to determine the thickness and composition of the oxide layer we have used a stacking layer model together with experimental XPS sputtering depth profiling. At a nanoscale study, to obtain quantitative results one must take into account fundamental parameters like the attenuation depth of photoelectrons. We have found that both lepidocrocit (γ-FeOOH) and magnetite (Fe₃O₄) were present and the total thickness of the oxide layer was 16 monolayers.