Efficient production of hydrogen and multi-walled carbon nanotubes from ethanol over Fe/Al2O3 catalysts

Fe/Al₂O₃ catalysts with different Fe loadings (10-90 mol%) were prepared by hydrothermal method. Ethanol decomposition was studied over these Fe/Al₂O₃ catalysts at temperatures between 500 and 800 °C to produce hydrogen and multi-walled carbon nanotubes (MWCNTs) at the same time. The results showed that the catalytic activity and stability of Fe/Al₂O₃ depended strongly on the Fe loading and reaction temperature. The Fe(30 mol%)/Al₂O₃ and Fe(40 mol%)/Al₂O₃ were both the effective catalyst for ethanol decomposition into hydrogen and MWCNTs at 600 °C. Several reaction pathways were proposed to explain ethanol decomposition to produce hydrogen and carbon (including nanotube) at the same time.     

Mixtures of carbon and Ni/Al2O3 as catalysts for the microwave-assisted CO2 reforming of CH4

In this work, the microwave-assisted CO₂ reforming of CH₄ over mixtures of carbonaceous materials and an in-lab prepared Ni/Al₂O₃ was studied. Ni/Al₂O₃ is not heated by microwave radiation, and for this reason, microwave receptors, such as carbonaceous materials, must be mixed with this catalyst. In order to evaluate the role of the carbonaceous component of the blend, two different carbonaceous materials were used: an activated carbon, FY5, and a metallurgical coke, CQ. The carbonaceous component acted not only as microwave receptor but also as catalyst and, consequently, it influenced the catalytic activity of the mixture. FY5 + Ni/Al₂O₃ was found to be a better catalyst than CQ + Ni/Al₂O₃, since FY5 on its own showed a better catalytic activity than CQ. Ni/FY5, which consists of Ni impregnated directly onto the microwave receptor, was also evaluated as a catalyst. It was found that the catalytic activity of the mixture FY5 + Ni/Al₂O₃ was better than that of Ni/FY5. Finally, the influence of the heating device on the catalytic activity of FY5 + Ni/Al₂O₃ was studied. Conversions over FY5 + Ni/Al₂O₃ and microwave heating were found to be similar to conversions over Ni/Al₂O₃ and conventional heating.     

Preparation of in situ grown silicon carbide whiskers onto graphite for application in Al2O3-C refractories

C-SiC composite powders were prepared by salt-assisted synthesis from Si powders, graphite, and a molten salt medium (NaCl and NaF) with the molar ratio of Si/C =?1/2 at 1300?°C for 3?h. After the C-SiC composite powders part and complete replacement of the graphite, the mechanical properties, oxidation resistance and slag-corrosion resistance of the Al₂O₃-C materials were studied by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS), as well as with dedicated equipment. The results indicated that SiC whiskers, with lengths of 10–50?nm, formed on the surface of the flake graphite, and the activation energy of oxidation of the C-SiC composite powder increased by 45.72?kJ?mol^?1 as compared to that of flake graphite. Furthermore, the decarburization area and slag erosion area of the Al₂O₃-C material decreased after 3?wt% of C-SiC composite powder was substituted for the flake graphite. Meanwhile, the cold modulus of rupture was maintained when 3?wt% of C-SiC composite powder was added. This improved both the oxidation and slag resistance of the Al₂O₃-C materials.     

The effect of precursor composition and sintering additives on the formation of β-sialon from Al, Si and Al2O3 powders

A study was performed to investigate the effect of increasing the Al or Al₂O₃ precursor content, above the stoichiometric amount, on the formation of β-sialon by pressureless sintering of Al, Si and Al₂O₃ powders in flowing nitrogen gas. The effect of adding Y₂O₃ or Fe to the precursor mixture, on the β-sialon formation, was also studied. The phase morphology and yield produced by the various compositions were examined using X-ray diffraction (XRD). Additional Al₂O₃ decreases the β-sialon phase yield and results in a greater amount of Al₂O₃ in the final sintered material. Additional Al improved the conversion to β-sialon up to a maximum of 4 wt% Al beyond which the β-sialon:15R sialon ratio in the sintered material decreases. 1 wt% Y₂O₃ was determined to be the optimum sintering additive content, as yttrium aluminium garnet (YAG) was found to be present in materials formed from higher Y₂O₃ containing precursors. The presence of Fe in the precursor powder retards the formation of β-sialon by preferentially forming Fe silicides at low temperatures, thus depleting the reaction system of elemental Si, favouring the formation of 15R sialon.     

High temperature wettability and corrosion of ZrO2, Al2O3, Al2O3-C,MgO and MgAlON ceramic substrates by an AZ91 magnesium alloy melt

Wettability and interfacial reactivity of various ceramics (ZrO₂, Al₂O₃, Al₂O₃-C, MgO, MgAlON) in contact with molten AZ91 were investigated in this paper. MgAlON and Al₂O₃-C are new materials to be used in magnesium melt filtration, a comparative study of its wettability and corrosion resistance is performed to evaluate their applicability. Modified sessile-drop tests were conducted, measuring contact angles between the ceramic substrates and molten AZ91 droplets at 680 °C; contact surfaces were analyzed by means of scanning electron microscopy. MgAlON showed the largest contact angles, indicating lowest wettability. Al₂O₃, Al₂O₃-C, and MgO proved non-wettable as well. Al₂O₃- and ZrO₂-containing substrates underwent noticeable interface reactions with molten AZ91.     

Evolution mechanism of MgAlON in the Al-Al2O3–MgO composite at 1800 °C in flowing nitrogen

The Al-Al₂O₃–MgO composite was calcined at 1800 ℃ in flowing nitrogen, using fused corundum, tabular alumina, activated α-Al₂O₃, high purity magnesia and metal aluminum powder as raw materials and magnesium aluminate sol as binding agent. The calcined sample was characterized and analyzed by XRD, SEM and EDS, and evolution mechanism of MgAlON was studied. Formation mechanism of MgAlON can be described as follows. At elevated temperatures in flowing nitrogen, Al(g)/Al₂O(g) diffuses and transfers along pores or gaps in the Al-Al₂O₃–MgO composite, being nitrided as AlN, and then AlN reacts with Al₂O₃-rich spinel to form granular MgAlON. In gas-gas reaction system, Al(g)/Al₂O(g), N₂(g), Mg(g) and O₂(g) react to form flake MgAlON.     

Sol–gel synthesis of YAG/Al2O3 long fibres from water solvent systems

Y₃Al₅O₁₂(YAG)/Al₂O₃ long fibres were prepared by a sol–gel method using water as the solvent. They were synthesized from aluminium nitrate and chloride solutions, aluminium salt, aluminium metal and Y₂O₃. The starting materials were dissolved by refluxing at 100°C for 2–18 h and were then condensed. The fibre spinnability was examined by a hand drawing method using a glass rod. In the nitrate solution system, the composition range available for fibre preparation was very limited because nitrate ions decomposed during the refluxing, raising the solution pH and precipitating the Y component. On the other hand, the composition range of the fibres prepared from the chloride system was 0/10⩽YAG/Al₂O₃⩽6/4 (volume ratio) and was wider than that from the nitrate system. The YAG/Al₂O₃ fibres prepared by firing at 1300°C became denser with faster heating rates. The grain size in the fired fibres was small, below the firing temperature at 1400°C, but increased greatly above that temperature.     

Preparation and electrical properties of graphene nanosheet/Al2O3 composites

Fully dense graphene nanosheet(GNS)/Al₂O₃ composites with homogeneously distributed GNSs of thicknesses ranging from 2.5 to 20 nm have been fabricated from ball milled expanded graphite and Al₂O₃ by spark plasma sintering. The percolation threshold of electrical conductivity of the as-prepared GNS/Al₂O₃ composites is around 3 vol.%, and this new composite outperforms most of carbon nanotube/Al₂O₃ composites in electrical conductivity. The temperature dependence of electrical conductivity indicated that the as-prepared composites behaved as a semimetal in a temperature range from 2 to 300 K.     

Corrosion mechanisms of Al2O3/MgAl2O4 by V2O5, NiO, Fe2O3 and vanadium slag

The effects of V₂O₅, NiO, Fe₂O₃ and vanadium slag on the corrosion of Al₂O₃ and MgAl₂O₄ have been investigated. The specimens of Al₂O₃ and MgAl₂O₄ with the respective oxides above mentioned were heated at 10 °C/min from room temperature up to three different temperatures: 1400, 1450 and 1500 °C. The corrosion mechanisms of each system were followed by XRD and SEM analyses. The results obtained showed that Al₂O₃ was less affected by the studied oxides than MgAl₂O₄. Alumina was only attacked by NiO forming NiAl₂O₄ spinel, while the MgAl₂O₄ spinel was attacked by V₂O₅ forming MgV₂O₆. It was also observed that Fe₂O₃ and Mg, Ni, V and Fe present in the vanadium slag diffused into Al₂O₃. On the other hand, the Fe₂O₃ and Ca, S, Si, Na, Mg, V and Fe diffused into the MgAl₂O₄ structure. Finally, the results obtained were compared with those predicted by the FactSage software.     

Preparation of TiC-Ti3AlC composite coated graphite flakes and their improved oxidation resistance

The weak oxidation resistance has severely hindered graphite from various high temperature applications, therefore in this paper a molten salt technique was proposed to prepare titanium aluminium carbide based coatings on graphite flakes to overcome this problem. The resultant TiC-Ti₃AlC coated graphite showed higher peak oxidation temperature (~?900?°C) than the uncoated one (700?°C), suggesting that the coatings will afford graphite with superior oxidation resistance. Such improvements can be largely ascribed to not only the homogenous and crack-free TiC-Ti₃AlC coatings and their resultant relicts of TiO₂ and Al₂O₃, but also the molten salt technique for preparing the coatings. Especially the molten salts offer fast dissolution/melting/dispersion of Ti/Al powders, rapid reaction with graphite in the salt melts, and homogenous growth of the carbide coatings on the surface of graphite flakes at as low temperatures as 950–1150?°C.     

The effects of processing parameters on formation of nano-spinel (MgAl2O4) from LiCl moltensalt

Nano-MgAl₂O₄ particles were successfully synthesized at 850 °C using the molten-salt method, and the effects of processing parameters, such as temperature, holding time and amount of salt on the crystallization of MgAl₂O₄ were investigated. Nano-alumina, magnesia and lithium chloride were used as starting materials. LiCl molten salt provided a liquid medium for reaction of Al₂O₃ and MgO to form MgAl₂O₄. The results demonstrated that MgAl₂O₄ started to form at about 650 °C and that, after the temperature was increased to 1000 °C, the amounts of MgAl₂O₄ in the resultant powders increased with a concomitant decrease in Al₂O₃ and MgO contents. After washing with hot-distilled water, the samples heated for 3 h at 850 °C were single-phase MgAl₂O₄ with 30–50 nm particle size. Furthermore, the synthesized MgAl₂O₄ particles retained the size and morphology of the Al₂O₃ powders, which indicated that a template formation mechanism dominated the formation of MgAl₂O₄ by molten-salt method.     

Evaluation of hot corrosion behavior of CSZ,CSZ/micro Al2O3 and CSZ/nano Al2O3 plasma sprayed thermal barrier coatings

Hot corrosion is one of the main destructive factors in thermal barrier coatings (TBCs) which come as a result of molten salt effect on the coating–gas interface. Hot corrosion behavior of three types of plasma sprayed TBCs was evaluated: usual CSZ, layer composite of CSZ/Micro Al₂O₃ and layer composite of CSZ/Nano Al₂O₃ in which Al₂O₃ was as a topcoat on CSZ layer. Hot corrosion studies of plasma sprayed thermal barrier coatings (TBCs) were conducted in 45 wt% Na₂SO₄+55 wt% V₂O₅ molten salt at 1050 °C for 40 h. The graded microstructure of the coatings was examined using scanning electron microscope (SEM) and X-ray diffractometer (XRD) before and after hot corrosion test. The results showed that no damage and hot corrosion products was found on the surface of CSZ/Nano Al₂O₃ coating and monoclinic ZrO₂ fraction was lower in CSZ/Micro Al₂O₃ coating in comparison with usual CSZ. reaction of molten salts with stabilizers of zirconia (Y₂O₃ and CeO₂) that accompanied by formation of monoclinic zirconia, irregular shape crystals of YVO₄, CeVO₄ and semi-cubic crystals of CeO₂ as hot corrosion products, caused the degradation of CSZ coating in usual CSZ and CSZ/Micro Al₂O₃ coating.     

Rapid fabrication of bulk graded Al2O3/YAG/YSZ eutectics by combustion synthesis under ultra-high-gravity field

A rapid and simple way of producing bulk graded Al₂O₃/YAG/YSZ ternary eutectics was investigated. The combustion reaction between Al/Fe₂O₃/Y₂O₃/ZrO₂ led to the formation of molten mixtures consisting of Al₂O₃/YAG/YSZ, and the subsequent separation of the ceramic melt from the iron melt was realized under ultra-high-gravity field, followed by the solidification of the ceramic melt. The as-solidified ceramic ingot sank into the iron melt, where an instantaneous isostatic pressure about 2 MPa was exerted on the around of the ceramic ingot resulting in an enhanced degree of densification. Microstructure analysis demonstrated that densified ceramic was composed of Al₂O₃/YAG/YSZ ternary eutectics. The phase composition, morphologies, hardness and fracture toughness of the eutectic product changed gradually along the direction of high gravity field. The maximum density of the eutectic ceramic was 97.32%, correspondingly, the maximum value of the Vickers hardness and fracture toughness reached 17.82 GPa and 5.51 MPa m^1/2, respectively.     

Assessment of thermodynamic stability of sapphire in eutectic molten chloride environment

The continued development of molten salt reactors and concentrated solar power plants requires highly efficient and stable instruments that can efficiently monitor the chemical conditions of the molten salt during long-term operation in both the fuel and coolant/heat transfer fluid loops. Sapphire (Al₂O₃) fibers have shown tremendous potential due to inherent radiation resistance and a broader operational range of temperature. In this work, computational thermodynamic modeling (CALPHAD) using the ThermoCalc software in conjunction with the SGTE (Scientific Group Thermodata Europe) Molten Salts (SALT1) and Pure Substances (Pure5) databases is applied to understand the compatibility of Al₂O₃ fibers with NaCl-MgCl₂ eutectic molten salt in the temperature range of 1500–2500 K. The thermodynamic calculations show that sapphire fibers are not expected to be stable over the long-term when exposed to molten chloride salts at these temperatures. To improve the stability of these diagnostic fibers in molten salt environments, various pure metallic elements were evaluated as potential cladding materials for Al₂O₃ fibers. Based on the thermodynamic analysis, molybdenum (Mo) and nickel (Ni) could be effective cladding materials to enhance the stability of Al₂O₃ in NaCl-MgCl₂ chloride salt molten bath in the desired temperature range. Thus, the presence of Mo and Ni cladding can provide a protective coating against the corrosive molten salts, thus improving the stability of Al₂O₃. Additionally, it is also shown that Al₂O₃ remains stable up to 2400 K in the presence of preexisting Al₂MgO₄ and Al₂NiO₄ in the eutectic molten chloride bath environment.     

Fabrication of Al2O3@BaFe12O19 core-shell powder by a modified heterogeneous precipitation method

Core-shell structural composites are promising to quench the desire for low-cost, lightweight and multifunctional magnetic materials, but the fabrication of highly pure shell nanoparticles with a desired morphology on a heterogeneous nuclear is still a big challenge. This study gives a successful illustration by in-situ synthesizing BaFe₁₂O₁₉ nanoparticles as a shell on Al₂O₃ core powders using BaCl₂ and FeCl₃ as precursors, instead of commonly used Ba(NO₃)₂ and Fe(NO₃)₃, by a modified heterogeneous precipitation process followed by calcination for crystallization. After calcination of the precipitants from the raw precursor mixture of BaCl₂:FeCl₃ for 1:10 at 800?°C, and for 1:8.5 at 900?°C, respectively, pure BaFe₁₂O₁₉ formed on pristine Al₂O₃, mostly with a hexagonal plane, about 100–200?nm in basal diameter and 20–100?nm in thickness. The highest saturation magnetization and coercivity of the coated Al₂O₃ @?BaFe₁₂O₁₉ powders was 23.3?emu/g and 5149?Oe, quite higher than that of the direct-mixed Al₂O₃-BaFe₁₂O₁₉ composite powder, and BaFe₁₂O₁₉-containing composites reported in literature, which could be attributed to the hexagonal shape of BaFe₁₂O₁₉ and uniform shell dispersion on Al₂O₃ core. The formation mechanism for the Al₂O₃@BaFe₁₂O₁₉ powders was discussed in detail.     

Comparative bioelectrochemical study of two types of myoglobin layer-by-layer films with alumina: Vapor-surface sol-gel deposited Al2O3 films versus Al2O3 nanoparticle films

Two types of layer-by-layer films of myoglobin (Mb) and Al₂O₃ were assembled on different surfaces and compared in Mb electrochemistry and bioelectrocatalysis. One type, designated as {SG-Al₂O₃/Mb}_n, was assembled by alternate deposition of Al₂O₃ by vapor-surface sol-gel method from liquid aluminum butoxide and Mb by adsorption from its solution. Another type, designated as {NP-Al₂O₃/Mb}_n, was constructed by alternate adsorption of Al₂O₃ nanoparticles and Mb from their dispersion or solution in the conventional layer-by-layer way. Quartz crystal microbalance (QCM), UV-vis spectroscopy, and cyclic voltammetry (CV) were used to monitor the growth of the two types of {Al₂O₃/Mb}_n films. UV-vis and IR spectroscopy demonstrated that Mb in both types of {Al₂O₃/Mb}_n films retained its near native structure. While both Mb films assembled on pyrolytic graphite (PG) electrodes exhibited a pair of well-defined, nearly reversible CV reduction-oxidation peaks for Mb heme Fe(III)/Fe(II) redox couple and good electrocatalytic reactivity toward reduction of oxygen and hydrogen peroxide, the {SG-Al₂O₃/Mb}_n films demonstrated distinct advantages over the {NP-Al₂O₃/Mb}_n films in larger maximum surface concentration of electroactive Mb and better biocatalytic performances. This may be mainly attributed to the higher porosity of {SG-Al₂O₃/Mb}_n films than that of {NP-Al₂O₃/Mb}_n films, which may be beneficial to counterion transport in the charge-hopping mechanism and the diffusion of catalytic substrates through the films.     

In situ synthesis of Al2O3–SiC powders via molten-salt-assisted aluminum/carbothermal reduction method

Al₂O₃–SiC composite powder (ASCP) was successfully synthesized using a novel molten-salt-assisted aluminum/carbothermal reduction (MS-ACTR) method with silica fume, aluminum powder, and carbon black as raw materials; NaCl–KCl was used as the molten salt medium. The effects of the synthesis temperature and salt-reactant ratio on the phase composition and microstructure were investigated. The results showed that the Al₂O₃–SiC content increased with an increase in molten salt temperature, and the salt–reactant ratio in the range of 1.5:1–2.5:1 had an impact on the fabrication of ASCP. The optimum condition for synthesizing ASCP from NaCl–KCl molten salt consisted of maintaining the temperature at 1573 K for 4 h. The chemical reaction thermodynamics and growth mechanism indicate that the molten salt plays an important role in the formation of SiC whiskers by following the vapor-solid growth mode in the MS-ACTR treatment. This study demonstrates that the addition of molten salt as a reaction medium is a promising approach for synthesizing high-melting-point composite powders at low temperatures.     

EPI-type zeolite synthesis from Greek sulphocalcic fly ashes promoted by H2O2 solutions

EPI-type zeolite was synthesised from Greek sulphocalcic lignite fly ashes. They consist mainly of SiO₂, CaO and Al₂O₃, while the SiO₂/Al₂O₃ ratio was found to be 2.74. The activation was performed by 30% H₂O₂ in an open system. Zeolite formation was observed only when activated products aged at 95 °C. The resulting materials were characterised by means of PXRD, FT-IR and SEM-EDS. PXRD and FT-IR results are in good agreement, confirming the zeolite formation. The role of H₂O₂ as a dominant factor in the zeolite synthesis is attributed to the oxidation of Fe(II) to Fe(III) and to the oxidative action on the unburned organic mater of the fly ash to prevent the reduction of Fe(III) to Fe(II). Fe(III) is proposed to participate in the reaction with Si-OH and HO-Al groups in the preliminary steps, resulting to the formation of an intermediate group [Fe-(H⁺)O(O-Si)-Al] which then gives Si-O-Al groups and Si-O-Fe groups to a lesser extent, both of which lead to a zeolite structure. Formation of the latter group explains the presence of Fe(III) in the zeolite crystal structure.     

LiF对Na3AlF6-Al2O3熔盐电解阴极过程的影响

电解铝工业中铝在阴极上析出,铝析出反应的机理对电解铝生产具有理论指导意义。在运用循环伏安法研究的基础上,通过理论计算,对Na₃AlF₆-Al₂O₃和Na₃AlF₆-Al₂O₃-LiF体系中金属铝在钨电极上的电化学沉积行为以及铝钨金属间化合物的形成机理进行了研究。结果表明：两体系中铝钨金属间化合物在50 mV·s^-1≤ν≤150 mV·s^-1扫描速率下的形成过程是受扩散控制的准可逆过程。在化合物形成的过程中,两体系中Al³⁺的扩散系数从4.54×10^-9 cm²·s^-1增长到5.71×10^-9 cm²·s^-1,Al³⁺反应的活化能分别为11.14 kJ·mol^-1和10.47 kJ·mol^-1。在Na₃AlF₆-Al₂O₃-LiF体系的还原过程中,Li并没有还原析出,而在氧化过程中Al在金属间化合物中的氧化电流增大;在恒电流电解时,Al-W金属间化合物并不溶于熔盐中,会附着在工作电极表面,LiF的加入会使电极表面的WAl₄量变小,取而代之的是Al₂O₃的增加,说明LiF的加入使电解更加稳定,抑制了电极表面WAl₄的生长。     

Cation distributions and magnetism of Al-substituted CoFe2O4 - NiFe2O4 solid solutions synthesized by sol-gel auto-combustion method

Aluminium substituted cobalt-nickel ferrite nanoparticles were synthesized by citrate gel auto-combustion method followed by annealing at 1000?°C for 1?h in air. Scanning electron micrographs of all the samples show crystalline particles of irregular morphology with a small variation in particle sizes (~ 110–160?nm). From the analysis of the X-ray diffraction results we observed that the unit cell parameter decreases linearly with increase in aluminium concentration due to the smaller ionic radius of the Al³⁺ ions substituting the other cations such as Co²⁺, Ni²⁺ and Fe³⁺ ions in the compounds. The room temperature Mössbauer spectra of the samples show Zeeman split sextet patterns corresponding to the tetrahedral (Th) and octahedral (Oh) interstitial iron (Fe³⁺) cations. The observed magnetic hyperfine field (B_hf) decreases with increase in Al-concentration due to the distribution of diamagnetic Al³⁺ in the environment of ⁵⁷Fe probe atoms. The saturation magnetization measured by Vibrating Sample Magnetometer (VSM) shows a similar trend like that of B_hf. The distributions of the cations obtained from the Rietveld refinement and Mössbauer spectroscopy results indicate an increase in Fe³⁺(Th)/Fe³⁺(Oh) occupancy-ratio on increasing Al³⁺ concentration, and Ni²⁺ cations prefer the octahedral site, whereas Co²⁺ and Al³⁺ ions redistribute themselves in tetrahedral and octahedral sites, in the ratio 2:3.