PVD Coating of Mg–AZ31 by Thin Layer of Al and Al–Si

Although magnesium alloys have the advantage of high specific strength, they have poor atmospheric corrosion resistance. An important method of improving the corrosion resistance is by applying a coating layer. In this work, the physical vapor deposition (PVD) technique is used for coating a magnesium (Mg) AZ31 sheet substrate with a thin layer of high purity aluminum (Al) and Al–12.6% Si. Aluminum is expected to be suitable as a coating layer on Mg sheets, due to its corrosion resistance and its formability. Before coating, the substrate was subjected to several consecutive surface preparations, including sand-blasting, mechanical grinding, mirror-like polishing, ultrasonic etching, and finally ion etching by magnetron sputtering (MS). PVD coating was conducted using a PVD machine with max electron beam power and voltage of 100 kW and 40 kV, respectively. This was either with or without plasma activation, and with variable substrate speeds ranging between 10 and 70 mm/s. During MS ion etching and coating, the substrate temperature increased. The substrate temperature increased with the application of plasma activation and with lower substrate speeds. The coating-layer thickness varied inversely with substrate speed. A thinner coat with finer morphology was obtained in the case of plasma activation. Other results included coating layer characteristics, diffusion between the AZ 31 substrate and the Al coating layer, adhesion of the coating layer to the substrate, and corrosion resistance by a humidity test.     

Effect of cryolite on formation of Al2O3/Al composites produced by oxidation of molten Al–Mg alloy

Abstract

The oxidation of molten Al–5 wt-%Mg and Al–5 wt-%Mg–0·5 wt-%cryolite (Na₃AlF₆) has been investigated by carrying out crucible corrosion tests under thermal gradient conditions. Two morphologies of Al₂O₃/Al composite were observed: ‘large balls’ in the absence of cryolite; and small mushrooms in the presence of this salt. Microstructural analysis and thermogravimetric measurements were also carried out to study the effect of cryolite on the oxidation mechanism. The results showed that both composite morphologies are the result of the same directed metal oxidation (DIMOX) process (DIMOX is a trademark of Lanxide Corporation, Newark, DE). Thermogravimetric analysis also confirmed that two morphologies formed under different oxidation kinetic conditions.     

Study on the adsorption–desorption behaviours of La3+, Sm3+, and Y3+ by HEHEHP extraction resin in chloride medium

The adsorption–desorption behaviours of La³⁺, Sm³⁺, and Y³⁺ on 2-ethylhexylphosphonic acid mono-2-ethyl hexyl ester (HEHEHP) extraction resin were investigated in a chloride medium. The effects of reaction time, pH, hydrochloric acid concentration, flow rate, and adsorption mechanism were studied in detail using static and dynamic experimental methods. The results show that La³⁺ is the first to reach adsorption equilibrium, followed by Sm³⁺ and Y³⁺; the highest adsorption capacities of La³⁺, Sm³⁺, and Y³⁺ are obtained at solution pH of 4 and in the order of Y³⁺ > Sm³⁺ > La³⁺. La³⁺, Sm³⁺, and Y³⁺ can be eluted by 0.3, 0.5, and 3 mol/L hydrochloric acid, respectively, indicating that La³⁺ elutes most easily, and Y³⁺ is difficult. Moreover, the flow rate has little effect on the peak of rare earth desorption. The adsorption process is chemisorption; the slope method indicates that for each mol Sm³⁺ or Y³⁺ ion adsorbed, 3 mol H⁺ ions are simultaneously released, but two for La³⁺ under the given pH conditions. It is expected to provide theoretical and technical support for preparation of ultra-high purity rare earth by HEHEHP extraction resin.     

Effect of Al2O3 particles on mechanical and tribological properties of Al–Mg dual-matrix nanocomposites

This article aims to manufacture homogenous dual-matrix Al–Mg/Al₂O₃ nanocomposite from their raw materials and give insight into the correlation between powder morphology, crystallite structure and their mechanical and tribological properties. Al–Mg dual-matrix reinforced with micro/nano Al₂O₃ particles was manufactured by a novel double high-energy ball milling process followed by a cold consolidation and sintering. Microstructure and phase composition of the prepared samples were characterized using FE-SEM, EDS and XRD inspections. Mechanical and wear properties were characterized using compression and sliding wear tests. The results showed that a milling of Mg with Al₂O₃ particles in an initial step before mixing with Al has the beneficial of well dispersion of Al₂O₃ nanoparticles in Al–Mg dual matrix. The Al–Mg dual matrix reinforced with nano-size Al₂O₃ showed 3.29-times smaller crystallite size than pure Al. Moreover, the hardness and compressive strength are enhanced by adding nano-size Al₂O₃ with Al–Mg dual matrix composite while the ductility is maintained relatively high. Additionally, the wear rate of this composite was reduced by a factor of 2.7 compared to pure Al. The reduced crystallite size, the dispersion of Al₂O₃ nanoparticles and the formation of (Al–Mg)_ss were the main improvement factors for mechanical and wear properties.     

A comparative study of the magnetic and microwave properties of Al3+ and In3+ substituted Mg–Mn ferrites

The effect of substitution of diamagnetic Al³⁺ and In³⁺ ions for partial Fe³⁺ ions in a spinel lattice on the magnetic and microwave properties of magnesium–manganese (Mg–Mn) ferrites has been studied. Three kinds of Mg–Mn based ferrites with compositions of Mg_0.9Mn_0.1Fe₂O₄, Mg_0.9Mn_0.1Al_0.1Fe_1.9O₄, and Mg_0.9Mn_0.1In_0.1Fe_1.9O₄ were prepared by the solid-state reaction route. Each mixture of high-purity starting materials (oxide powders) in stoichiometric amounts was calcined at 1100 °C for 4 h, and the debinded green compacts were sintered at 1350 °C for 4 h. XRD examination confirmed that the sintered ferrite samples had a single-phase cubic spinel structure. The incorporation of Al³⁺ or In³⁺ ions in place of Fe³⁺ ions in Mg–Mn ferrites increased the average particle size, decreased the Curie temperature, and resulted in a broader resonance linewidth as compared to un-substituted Mg–Mn ferrites in the X-band. In this study, the In³⁺ substituted Mg–Mn ferrites exhibited the highest saturation magnetization of 35.7 emu/g, the lowest coercivity of 4.1 Oe, and the highest Q×f value of 1050 GHz at a frequency of 6.5 GHz.     

Physico-chemical and catalytic properties of Mg–Al hydrotalcite and Mg–Al mixed oxide supported copper catalysts

The Mg–Al hydrotalcite (HT) and Mg–Al mixed oxide supported copper catalysts containing 3–3.5 wt.% copper in finely dispersed form were synthesized and characterized. The effect of support nature on physico-chemical and catalytic properties of supported copper species were studied. The loading of copper on the supports was observed to be influencing the surface acidic, basic and reducibility properties, and catalytic behavior in dehydrogenation of benzyl alcohol. The high basicity and intercalated copper ions in Mg–Al hydrotalcite supported copper sample showed multifunctional activity in catalytic transformations of alcohols (primary, secondary and aromatic alcohols).     

Effect of metal oxides on electrochemical performances of La–Mg–Ni-based alloys

Metal oxides (TiO₂, Er₂O₃ and ZnO) were added to La–Mg–Ni-based hydrogen storage alloy electrodes and their effects on the structural and electrochemical properties were studied. The charge efficiency, especially at high charge current density was greatly ameliorated, and the high rate charge capability at 1440 mA g⁻¹ increased from 85.1% (blank) to 94.1% (TiO₂), 93.3% (Er₂O₃) and 90.5% (ZnO). The high temperature dischargeability was also improved in case of TiO₂, Er₂O₃ and ZnO additives. These additives suppressed formation of Mg(OH)₂ and La(OH)₃ during charge/discharge process and therefore the cycling stability was improved. The discharge capacity retention at the 200th cycle increased from 72.9% (blank) to 79.6% (TiO₂), 87.5% (Er₂O₃) and 77.9% (ZnO).     

Highly selective synthesis of 2‐butoxy ethanol over Mg/Al/V mixed oxides catalysts derived from hydrotalcites

The catalytic activity of a series of mixed oxides obtained by the thermal decomposition of hydrotalcite‐like precursors was assessed for the alkoxylation of n-butanol with ethylene oxide. The calcination products of a decavanadate intercalated magnesium–aluminium layered double hydroxide were shown to possess extremely high activity for the alkoxylation reaction achieving up to 100% conversion in batch reaction. In all cases, the catalysts exhibit a much higher selectivity towards the monoglycol adduct than that obtained with the industrial catalyst. This revised version was published online in July 2006 with corrections to the Cover Date.     

The effect of electrolyte flow rate and temperature on corrosion and protection of Al–2.5 Mg alloy by (+)-catechin

The effects of flow rate and temperature on the corrosion behaviour of the Al–2.5 Mg alloy in a 3% NaCl solution and the inhibiting efficiency of (+)-catechin on the corrosion of the same alloy have been examined. Measurements were carried out in a flow-through cell, at different flow rates (v ₁ = 0.0029 m s⁻¹, v ₂ = 0.0059 m s⁻¹ and v ₃ = 0.0118 m s⁻¹) and temperatures (20, 30, 40 °C). Electrochemical parameters for the Al–2.5 Mg alloy were determined by polarisation techniques and electrochemical impedance spectroscopy (EIS). Increased flow rate and temperature cause a stronger corrosion attack on the alloy. The addition of (+)-catechin inhibited corrosion at all temperatures and flow rates. The inhibitor efficiency decreased with increase in flow rate and temperature.     

Coil-coated Zn–Mg and Zn–Al–Mg: Effect of climatic parameters on the corrosion at cut edges

We studied the effect of temperature, wet/dry cycling, pH, and the type and concentration of the corrosion activator on cut edge corrosion of painted Zn–15Mg and Zn–1.5Al–1.5Mg coated steel. In most accelerated tests, paint delamination and red rust formation were reduced compared to hot dip galvanised steel (HDG), and Zn–15Mg outperformed Zn–1.5Al–1.5Mg; however, Zn–1.5Al–1.5Mg showed better results when exposed outdoors. The alloyed materials were particularly resistant when HDG was prone to elevated corrosion, i.e. under permanent wetness, at higher temperatures, with high chloride loadings and in the presence of sulphate. Oxygen reduction on steel cut edges was inhibited by the alloying elements.     

Synthesis of cyclohexanone-formaldehyde resin catalyzed by rehydrated Mg–Al hydrotalcite

Cyclohexanone-formaldehyde resin (CFR) was synthesized over rehydrated Mg–Al hydrotalcite (HT). Therein, Mg–Al HT was rehydrated from Mg–Al HT of good crystallinity using a liquid method. Rehydrated Mg–Al HT (HT-r) shows certain catalytic activity for aldol condensation and catalytic activity is dramatically improved in the presence of hexadecyl trimethyl ammonium bromide (CTAB) because CTAB as a phase transfer catalyst is helpful to enhance the mass transfer among three phases. Both higher temperature and longer reaction time are preferable to increase the molecular weight of CFR. Though the catalytic activity of HT-r decreases as the reutilization cycle increases, the deactivated HT-r can be easily regenerated upon calcining and rehydrating.     

Effect of solid loading on properties of Y2O3–Al2O3–Nd2O3 powder mixtures obtained by planetary ball milling and ceramics based on them

The effect of the solid loading (41–50 wt%) of the slurry on granulometric composition and physico-chemical characteristics of Y₂O₃–Al₂O₃–Nd₂O₃ powder mixtures obtained by planetary ball milling has been studied for the first time. It was shown that the particle size distribution of powder, its Zeta potential, and specific surface area depend on the solid loading of the milled slurry and, consequently, on the interparticle distance during milling. The interparticle distance decreases from 200 nm to 142 nm with an increase of solid loading in the range of 41–50 wt%. It was shown that for the solid loading of 47 wt%, the convergence of particles to a distance comparable to their median diameter promotes subsequent clustering of particles. This facilitates the sintering of highly-homogenous ceramics. It was found that solid loadings in the 46–50 wt% range is useful for obtaining high-quality Nd:YAG transparent ceramics. The lowest optical losses optical losses of 1 × 10^?3 cm^?1 and the highest in-line transmittance of 84.1%@1064 nm were obtained for 1 at.% Nd:YAG transparent ceramics (22 × 3 × 4 mm³) prepared from slurries with 47 wt% solid loading (taking all other ball milling parameters fixed). If the interparticle distance in the powder is higher (solid loading of 41 wt%) than the median particle diameter, the ceramics are characterized by significant residual porosity due to the survival of large particles (insufficient milling).     

Influence of iron addition on the microstructure and the electrochemical corrosion of Al–Zn–Mg alloys

The effect of Fe addition on the microstructural properties and the corrosion resistance of Al–Zn–Mg alloys submitted to different heat treatments (cast, annealed and aged), has been studied in chloride solutions using optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray (EDX), cyclic polarization (CP) and open circuit potential (o.c.p.) measurements. The presence of 0.3% Fe in the alloy limited the growth of the MgZn₂ precipitates, both in the annealed and in the quenched specimens. No effect of Cr on the grain size in the presence of Fe was found because of the accumulation of Cr in the Fe-rich particles. Fe in the Al–Zn–Mg alloys also made them more susceptible to pitting. Pitting occurred mainly near the Fe-rich particles both, under o.c.p. conditions in O₂-saturated solutions and during the CP.     

Role of Pt Oxidation State on the Activity and Selectivity for Crotonaldehyde Hydrogenation Over Pt–Sn/Al2O3–La and Pt–Pb/Al2O3–La Catalysts

Pt–Sn/ALa10 and Pt–Pb/ALa10 catalysts (10 wt% La₂O₃) were studied in the selective hydrogenation of crotonaldehyde. Oxidized Pt²⁺ and reduced Pt⁰ species were identified by XPS on the bimetallic catalysts. High selectivity to crotylalcohol was obtained on the Pt–Pb/ALa10 catalyst where an electron transfer effect from Pb to Pt was proposed. For the Pt–Sn/ALa10 catalyst the formation of Pt–SnO _x –La₂O₃ complexes showing low activity and low selectivity was inferred.     

Preparation and sealing effects of Mg–Al–Si–Ba–O-based glass-ceramic coatings on NTC thermistors

Herein, Mg–Al–Si–Ba–O-based glass ceramics were studied as potential candidates to protect Mn–Co–Ni–O-based negative temperature coefficient (NTC) thermistors at high temperatures such as 900 °C. The ceramics were prepared in three glass formulations (1#: 15MgO–15Al₂O₃-44.7SiO₂–25BaO, 2#: 17MgO–17Al₂O₃–41SiO₂–25BaO and 3#: 17MgO–17Al₂O₃–41SiO₂–20BaO–5Y₂O₃ (in mol%)) and their glass-transition temperatures (T_g) were determined using the differential scanning calorimetry (DSC) method. Scanning electronic microscopy (SEM) and X-ray diffraction (XRD) were used to characterize the parent glasses and glass-ceramic coatings. The sealing effects of the glass ceramics were examined by conducting an insulation test. The glass-ceramic sealing structures were subjected to 1000 thermal shock cycles at temperatures varying from room temperature to 900 °C. Notably, the sealing structure of glass-ceramic coating 1# was compact at a T_g of 760.9 °C. The glass-ceramic coatings effectively maintained the NTC properties of the sensitive ceramics in all three formulations. Interestingly, the glass-ceramic coating 3# containing Y₂O₃ demonstrated an increase in electrical resistance. Both the NTC thermistors coated with 1# and 2# glass formulations successfully passed 1000 thermal shock cycles without visible failures, and their resistance change ratios were well below the requisite 20%.     

Effect of adding Y2O3 on structural and mechanical properties of Al2O3–ZrO2 ceramics

The effects of adding 1–8 wt% Y₂O₃ on phase formation and fracture toughness of Al₂O₃–xZrO₂–Y₂O₃(AZY) ceramics were studied. Phase formations of the samples were characterized by the X-ray diffraction (XRD) technique. It was found that the major phase was rhombohedral-Al₂O₃, while the minor phase consisted of the monoclinic-ZrO₂, tetragonal-ZrO₂ and monoclinic-Y₂O₃. It was found that Y₂O₃ contents did not clearly influence grain shape of AZY ceramics. The results obtained from the microhardness test could be used to evaluate the fracture toughness. It was found that the smaller grains had high fracture toughness. The maximum fracture toughness of 4.827 MPa m^1/2 was obtained from 4 wt% Y₂O₃. Refinement of lattice parameters using Rietveld analysis revealed the quantitative phases of AZY ceramics. This shows that under adding Y₂O₃ conditions the proportion of tetragonal-ZrO₂ phase plays an important role for the mechanical properties of AZY ceramics.     

Effect of Er3+ substitution on the quality of Mg–Fe spinel pigments

Inorganic pigments containing erbium cations and based on the spinel structure of MgFe₂O₄ were prepared. This type of spinel compound provides pigments of red to brown colour; an increase in the content of trivalent erbium cations results in pigments of a light brown hue. The pigments displayed good resistance to sunlight but this was reduced as the content of erbium increased. Pigments prepared using mechanoactivation possessed good resistance to sunlight over a range of erbium cation content.     

Ultrafast formation of Al2O3–Y3Al5O12 eutectic ceramic by flash sintering

In this study, a dense Al₂O₃–Y₃Al₅O₁₂ (YAG) ceramic was synthesized by flash sintering a powder mixture of Al₂O₃ and Y₂O₃ in less than 150 seconds at a furnace temperature of 1350°C. The resultant ceramic has a well-defined eutectic structure consisting of alternating Al₂O₃ and YAG layers. The hardness and fracture toughness of the ceramic were measured to be 18.5 GPa and 4.3 MPa.m^1/2, respectively. These values are comparable to those of similar eutectic ceramics made by directional solidification techniques. The results suggest a new method for making high-performance eutectic ceramics, which could be applied in other systems.     

Effect of CaO doping on the properties and crystallization behavior of Y2O3–Al2O3–SiO2 glass

Nowadays, Y₂O₃–Al₂O₃–SiO₂ (YAS) glass joining is considered to be a promising scheme for nuclear-grade continuous silicon carbide (SiC) fiber reinforced SiC matrix composites (SiC/SiC). CaO has great potential for nuclear applications since it has low reactivity and low decay rate under nuclear irradiation. In this paper, the effect of CaO doping on the structure, thermophysical properties, and crystallization behavior of YAS glass was systematically studied. As the CaO doping content increased, the number of bridge oxygens and the viscosity at high temperatures reduced gradually. After heat treatment at 1400 °C, the main phases in YAS glass were β-Y₂Si₂O₇, mullite, and SiO₂ (coexistence of crystalline and glass phases), while that with 3.0% CaO doping turned into a single glassy phase under the same treatment conditions. Moreover, a structural model and the modification mechanism were proposed, which provided a theoretical basis for the subsequent component design and optimization.     

Raman and X-ray photoelectron spectroscopy study on the influence of La2O3 on the melt structure of SiO2–CaO–Al2O3–MgO

In order to gain more insights into the influence of rare earth elements on the melt structure of SiO₂–CaO–Al₂O₃–MgO glass ceramics, Raman and X-ray photoelectron spectroscopy techniques were used to study the influence of La₂O₃ on the Si–O/Al–O tetrahedron structure within SiO₂–CaO–Al₂O₃–MgO–quenched glass samples in this study. Results showed that some Raman peak shapes at low frequencies (200–840 cm^?1) changed significantly after the addition of La₂O₃, compared to the high frequency (840–1200 cm^?1) region that corresponds to the [SiO₄] structure, suggesting that the depolymerization of the low-frequency T–O–T (T=Si or Al) structure was more prevalent with La³⁺ addition. Besides, the depolymerization extent of the Si–O/Al–O tetrahedral network varied when the melt composition altered. Most notably, depolymerization is the most significant at a low CaO/SiO₂ ratio (0.25) and a high Al₂O₃ content (8%). Meanwhile, La³⁺ can promote the transformation of Si–O–Si and Al–O–Al bonds to the Si–O–Al ones, thereby forming a complex ionic cluster network interwoven with Si–O and Al–O tetrahedrons.