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).     

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%.     

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.     

Effects of oxide addition on the microstructure and mechanical properties of lamellar SiC scaffolds and Al–Si–Mg/SiC composites prepared by freeze casting and pressureless infiltration

Silicate-bonded porous SiC scaffolds with lamellar structures were prepared by freeze casting and liquid-phase sintering. It was found that the viscosity and solidification velocity of SiC water-based slurries with 30 vol% solid loading decreased with increasing Al₂O₃–MgO (AM) addition. As the AM content increased from 10 to 30 wt%, the lamellae of the sintered scaffolds became denser and the porosity decreased from 69±0.5% to 62±0.5%, while the compressive strength improved from 25±2 to 51±2 MPa. The dynamics of pressureless infiltration for an Al–12 Si–10 Mg alloy on the SiC porous scaffold was measured and the composites with lamellar-interpenetrated structures were successfully produced. Both the compressive strength and the elastic modulus of the composites increased with increasing AM content. The maximum strength reached 952±24 MPa and the highest elastic modulus about 156 GPa, respectively, in a longitudinal direction, increasing about 32% and 11% as compared with those of the composites without AM.     

Microstructural and mechanical behavior of multi-walled carbon nanotubes reinforced Al–Mg–Si alloy composites in aging treatment

Microstructural and mechanical behavior of heat treatable Al–Mg–Si (6XXX series) alloy composites reinforced with multi-wall carbon nanotubes (MWCNTs) fabricated by powder metallurgy process were investigated by SEM-EDS, XRD, tensile test and Vicker’s hardness test. As-extruded P/M 6063 alloy composites with CNT reinforcements indicated a small increment of mechanical strength compared to the monolithic 6063 alloy with no CNT before T6 heat treatment. When T6 heat treatment was applied to the specimens, the 6063 composite with CNTs showed a noticeable decrease of yield stress (YS) improvement, compared to the monolithic Al alloy. It means that Mg₂Si precipitates hardening effect by the artificial aging treatment was insufficient for the composite containing CNTs. This was mainly because Mg alloying elements were diffused around CNTs and consumed to form Al₂MgC₂ compounds, and resulted in the incomplete matrix strengthening behavior by Mg₂Si precipitation after the aging treatment.     

Experimental Investigations on Microstructure,Properties and Workability Behavior of Zinc Oxide Reinforced Al–Si–Mg Matrix Composites

Silicon - Al-Si based alloy matrix composites are now broadly utilized by the industrial sectors like automobile, structural, aerospace and more practical industrial applications due to its...     

Microstructural evolution and enhanced mechanical properties of 95Al2O3–Yb/Mg/Si ceramics by optimizing sintering temperature

Designed component of 0.95Al₂O₃–0.015Yb₂O₃–0.01MgO–0.025SiO₂ (95Al₂O₃–Yb/Mg/Si) ceramics were prepared by the traditional mixed-oxide method in the sintering temperature range of 1450–1700 °C. The influence of sintering temperature on the crystal structure, densification, microstructure, mechanical, friction and wear properties of 95Al₂O₃-YbMgSi ceramics were systematically investigated. XRD and SEM analysis results revealed that the increase in sintering temperature was very beneficial to eliminate the pores, increase the density and grain size, which obeys the common grain growth law. Both the flexural strength and hardness of obtained samples were increased almost linearly when the sintering temperature increased from 1450 °C to 1700 °C. The ceramics sintered at 1650 °C showed the optimum properties: H_v = 1812.3, σ = 151.3 MPa, μ = 0.41, ρ = 3.72 g/cm³ and K_c = 8.06e^?5 mm³/N·m, respectively. Furthermore, the results of friction and wear experiments suggested that the 95Al₂O₃–Yb/Mg/Si ceramic prepared at the optimizing sintering process exhibited more stable friction state and lower wear degree under non-lubricated conditions. The enhanced mechanical properties could be attributed to their structure densification, pore elimination and gain growth with the increase of sintering temperature.     

Structure and surface properties of Al2O3–TiO2 ceramic coated AZ31 magnesium alloy

Magnesium and its alloys are the engineering materials which have the potential ability to be able to used widely particularly in the automotive, aerospace and in the biomedical sectors, especially thanks to their features such as lightness, specific strength that they have and biocompatibility. However, due to their poor wear resistance and corrosion resistance, the areas of usage are being restricted. This situation prevents Mg alloys to be used without any surface protection despite their good mechanical properties such as high strength/weight ratio. In this study, plasma spraying method is used to improve the poor corrosion resistance of AZ31 Mg alloy. Al₂O₃–13 wt% TiO₂ (AT13) and Al₂O₃–40 wt% TiO₂ (AT40) composite ceramic coatings were coated successfully on the surfaces of AZ31 Mg samples. The wear properties of the AT13 and AT40 coated samples were investigated for tribological applications. Surface morphology and microstructure of the duplex treated samples were analyzed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). The microhardness value of the uncoated AZ31 Mg alloy is 40±3 HV_0.1, while the microhardness values of the AT40 and AT13 coatings are enhanced to 800±39 HV_0.1 and 1500±35 HV_0.1, respectively.     

Influence of additives on microstructure and property of microarc oxidized Mg–Si–O coatings

Ceramic coatings were fabricated on ZK60 magnesium alloy substrate by microarc oxidation (MAO) in Na₂SiO₃–KOH base electrolyte with four kinds of additives (i.e. KF, NH₄HF₂, C₃H₈O₃ and H₂O₂). The effects of these additives on microstructure and property of coatings were investigated. The surface morphology, phase composition and corrosion resistance of the ceramic coatings were analyzed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and simulation body fluid (SBF) immersion test respectively. It is found that different additives can change the spark discharge phenomenon during microarc oxidation. It is proved that both potassium fluoride (KF) and ammonium bifluoride (NH₄HF₂) promote discharge and accelerate reaction while the introduce of glycerol (C₃H₈O₃) leads to the refining of sparks and reduction of thermal effects. Results also demonstrate that the introduce of hydrogen peroxide (H₂O₂) contributes to the increase of coating surface roughness and enlargement of surface micropore size. XRD results indicate that the ceramic coatings are mainly composed of Mg₂SiO₄, MgSiO₃ and SiO₂. The introduce of H₂O₂ hinders the reaction between SiO₂ and MgO and creates favorable conditions for the formation of the MgO phase. The ceramic coatings formed in base electrolyte containing 7 g/L NH₄HF₂ and 5 mL/L C₃H₈O₃ exhibit the highest corrosion resistance.     

Determination of Characteristics of Thin‐Layer Thermoprotective Coatings by Solving Inverse Heat‐ and Mass‐Transfer Problems

Measurement data on unsteady temperature fields in decomposing thermoprotective coatings and on mass losses in a hightemperature inertgas (nitrogen) flow are reported. With the use of analytical and numerical methods for solving boundary and coefficient inverse heat and masstransfer problems, thermophysical and kinetic characteristics of thermoprotective materials made by two modifications of chlorosulfonated polyethylene and powdered polypropylene filler are found.     

In situ hot elastic modulus evolution of MgO–C refractories containing Al,Si or Al–Mg antioxidants

Metals and alloys (such as Al, Si, Al–Mg and Al–Si) are commonly added to MgO–C refractory bricks as antioxidants due to their effectiveness to prevent carbon oxidation (in the 600–1400 °C range) and their low cost. These additives act at different temperatures and react with refractory components and gases in the environment, inducing significant changes in the resultant microstructure and affecting the overall thermo-mechanical performance of these products. This work addresses the evaluation of physical properties, cold and hot mechanical resistance, as well as in situ hot elastic modulus (E) measurements in the temperature range of 30–1400 °C for MgO–C bricks containing antioxidants (Al, Si or Al–Mg alloy) in a reducing atmosphere. Cured and fired samples of the designed formulations were evaluated throughout 1 or 2 heating-cooling cycles. Despite the improved mechanical behavior (higher cold crushing strength and hot modulus of rupture) of the antioxidant-containing formulations, compared to the additive-free MgO–C sample, the interaction of the selected additives with the refractory components and CO_(g) led to a generation of phases (i.e., Al₄C₃, Al₂O₃, SiC, SiO₂, MgAl₂O₄) that could not be well accommodated in the microstructure. Consequently, the in situ E drop was observed during cooling (mainly below 600 °C) of the antioxidant-containing sample due to crack and flaw formations. Si and Al–Mg were the most promising antioxidants, whereas the Al-containing composition showed the highest E damage level after two heating/cooling cycles up to 1400 °C for cured samples. Based on the elastic modulus profiles with the temperature, the results also indicated the best working conditions for these ceramic materials.     

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.     

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.     

The preparation and properties of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics

Two series (N-9 and N-18 series) of zirconia-doped Y–Si–Al–O–N oxynitride glasses and glass-ceramics were designed. Nominal compositions of the glass samples in equivalent percent (eq%) are xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 91O: 9 N and xZr: (24–0.25x)Y: (15–0.25x)Al: (61–0.5x)Si: 82O: 18 N (x=0, 2, 4, 6), respectively. The obtained samples were characterized by differential thermal analysis (DTA), X-ray diffraction (XRD), fourier transform infrared spectroscopy (FTIR) and scanning electron microscopy (SEM). Densities, Vickers hardness, fracture toughness, glass transition temperature, and thermal expansion coefficient data were established for each sample. Effect of Zr and N content on glass network structure, thermal and mechanical properties was investigated. It was found that the addition of zirconia is effective in preparing Y–Si–Al–O–N oxynitride glasses with lower glass transition temperature and higher hardness.     

Epoxidation of Cyclohexene With H2O2 and Acetonitrile Catalyzed by Mg–Al Hydrotalcite and Cobalt Modified Hydrotalcites

The paper presents a comparison between the results obtained for cyclohexene epoxidation in the presence of base catalysts: Mg_0/75Al_0.25-hydrotalcite (HT), the corresponding mixed oxide (CHT), the reconstructed hydrotalcite (RHT), a hydrotalcite containing cobalt in the brucite-type layer ((Co-Mg_0.75)Al_0.25)) and one containing cobalt complex-species supported on HT carrier (Co-complex/HT). The selective conversion of cyclohexene to the corresponding epoxide is closely related to the base strength of the catalysts.     

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.     

The influence of the mixed alkaline earth effect on the structure and properties of (Ca,Mg)–Si–Al–O–N glasses

Alkaline earth oxynitride glasses of (Ca, Mg)–Si–Al–O–N with different CaO/(CaO + MgO) molar ratios (0, 0.25, 0.5, 0.75, and 1) were successfully prepared using the sol-gel method, and their structural compositions were characterised by Raman and FT-IR techniques. The glass dynamic properties of thermal expansion coefficient, glass transition temperature (T_g), and static properties of density, molar volume, Vickers hardness and compressive strength were systematically measured and analysed. The results showed that the static properties exhibited an overall regular change as the CaO/(CaO + MgO) ratio gradually increased, while the dynamic properties had an obvious mixed alkaline earth effect, which represented the appearance of an extreme value point in CaO/(CaO + MgO) mole ratios of 0.25 and 0.75, respectively. The typical thermal expansion coefficient and T_g of mixed alkaline earth oxynitride glasses deviated far from the linear connection between single alkaline earth oxynitride glasses. Raman spectra and infrared spectra revealed that the ratio value of the Q³/(Q²+Q⁴) decreased (Qⁿ: n = no. of bridging anions joining SiO₄ tetrahedra) in the mixed alkaline earth oxynitride glasses with increasing the amount of Ca, confirming that Ca decreased the crosslinking between individual tetrahedra via the transformation of Q³ species into Q² and Q⁴ species.     

Preparation of Highly Adhesive and Superhydrophobic Epoxy-Based Thin Film by Sol–Gel Process

In this study the preparation of a superhydrophobic epoxy-based thin film with excellent high adhesion properties was carried out by mixing epoxy polymer solution, MTMOS solution, and silica powder. It was found that the adhesion between the film and a substrate could be improved by adding PET fibers to the above solution. The characteristic properties of the as-prepared films were analyzed by contact angle measurements, scanning electron microscopy (SEM) and by atomic force microscopy (AFM). The adhesion between the film and the substrate was estimated by the cross-cut method. The experimental parameters were: the mole fraction of MTMOS, weight fraction of silica powder, weight fraction of PET fibers, and the curing temperature. The result indicated that the contact angle and the sliding angle of the hybrid film were 153° and 4°, respectively, and the adhesion test result was very good (5B degree), while the mole fraction of MTMOS was 0.4, the silica powder concentration was 30 wt%, and the PET fibers concentration was 0.15 wt%, with the curing temperature in the range of 130–180°C.