The effect of magnesium compounds (MgO and MgAl2O4) on the synthesis of Lanthanum magnesium hexaaluminate (LaMgAl11O19) by solid-state reaction method

In the present study, the lanthanum magnesium hexaaluminate (LaMgAl₁₁O₁₉)(LaMA) powder was synthesized by the solid–state reaction method using two types of magnesium compounds, including magnesium oxide (MgO) and magnesium aluminate (MgAl₂O₄) spinel (MAS). The effect of substitution of magnesium oxide with MAS on the synthesis temperature, intermediate compounds and morphology of synthesized powders were investigated. The microstructural results showed that the intermediate compounds of lanthanum aluminate (LaAlO₃), aluminum oxide and MAS were formed in the presence of magnesium oxide, whereas in the latter case, the LaAlO₃ intermediate phase was not observed and La₄Al₂MgO₁₀ was formed at about 810 °C. Also in both cases, a single LaMA phase with the platelet-like morphology was formed. The thickness of the LaMA platelets decreased from 300 nm to 125 nm and the synthesis temperature increased from 1330 °C to 1355 °C, by replacing MgO with MAS.     

Preparation of Al2O3 and MgAl2O4-based samples with tailored macroporous structures

In this work we successfully obtained freeze-cast alumina (Al₂O₃) and magnesium aluminate spinel (MgAl₂O₄) samples. Camphene was used as the freezing vehicle in this study. The specimens prepared herein were examined by Archimedes tests, scanning electron microscopy, and X-ray powder diffraction. Cold crushing tests were also carried out at room temperature. It was observed that the pore structure of Al₂O₃ samples can be tailored by changing the solid loading and freezing rate; the higher the solid loading and freezing rate, the finer the pore structure of the freeze-cast sample. MgAl₂O₄-based specimens were fabricated by keeping the solid loading in the starting slurry at 30 vol% and using liquid nitrogen as the cooling agent. The material obtained from a 60 Al₂O₃?40 MgO slurry showed a spinel amount of about 90%, an expressive total porosity (63 ± 3%), and a significant cold crushing strength (58 ± 6 MPa). In addition, this material exhibited the finest pore structure among the composition studied herein, showing a mean pore size of about 4 µm.     

Highest UV–vis transparency of MgAl2O4 spinel ceramics prepared by hot pressing with LiF

Reaction sintering of MgO and Al₂O₃ with addition of LiF as sintering additive was used to prepare MgAl₂O₄ spinel ceramic by hot pressing. The process parameter (temperature, pressure, dwell time), the stoichiometric ratio of MgO to Al₂O₃ and the selection of the alumina raw powder are equally important for highest transparency of the spinel ceramic. With this optimization highest transparency of 86% in the visible range at λ = 640 nm together with UV transmission of 62% at 200 nm for spinel ceramic with 4 mm thickness was reached.     

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.     

Dilatometric sintering study of fine-grained ceramics from ultradispersed admixture composed of Ce0.09Zr0.91O2 and MgO–Al2O3

The experiment was carried out to produce fine-grained ceramics with a grain size of less than 5 μm. Ultradispersed oxide mixture MgO–Al₂O₃ (weight ratio MgO/Al₂O₃ value was 3/97) and solid solution Ce_0.09Zr_0.91O_2?δ were used as initial nanopowders with a crystallite size less than 10 nm. Dilatometric investigation was carried out at the temperature interval 1100–1550 °C using three temperature modes, included various heating and cooling rates and isothermal plots. Initial metal oxide powders were obtained by modified sol–gel technique using N-containing organic compounds for sol stabilization. It was shown that the role of MgO in nanopowdery composition for sintering is to accelerate the sintering due to the formation of the liquid phase with spinel MgAl₆O₁₀. It was determined, that the presence of interim isotherms on the temperature rise curves does not impact the rate and quality of sintering.     

Investigation on the anti-reduction mechanism of Ti4+ in high dielectric constant system Ca0.9La0.067TiO3 by doping with Al2O3

Ca_0.9La_0.067TiO₃ (abbreviated as CLT) ceramics doped with different amount of Al₂O₃ were prepared via the solid state reaction method. The anti-reduction mechanism of Ti⁴⁺ in CLT ceramics was carefully investigated. X-ray diffraction (XRD) was used to analyze the phase composition and lattice structure. Meanwhile, the Rietveld method was taken to calculate the lattice parameters. X-ray photoelectron spectroscopy (XPS) was employed to study the valence variation of Ti ions in CLT ceramics without and with Al₂O₃. The results showed that Al³⁺ substituted for Ti⁴⁺ to form solid solution and the solid solubility limit of Al³⁺ is near 1.11 mol%. Furthermore, the reduction of Ti⁴⁺ in CLT ceramics was restrained by acceptor doping process and the Q × f values of CLT ceramics were improved significantly. The CLT ceramic doped with 1.11 mol% Al₂O₃ exhibited good microwave dielectric properties: ε_r = 141, Q × f = 6848 GHz, τ_f = 576 ppm/°C.     

Solvent free synthesis of chalcone and flavanone over zinc oxide supported metal oxide catalysts

Liquid phase Claisen–Schmidt condensation between 2′-hydroxyacetophenone and benzaldehyde to form 2′-hydroxychalcone, followed by intramolecular cyclisation to form flavanone was carried out over zinc oxide supported metal oxide catalysts under solvent free condition. The reaction was carried out over ZnO supported MgO, BaO, K₂O and Na₂O catalysts with 0.2 g of each catalyst at 140 °C for 3 h. Magnesium oxide impregnated zinc oxide was observed to offer higher conversion of 2′-hydroxyacetophenone than other catalysts. Further MgO impregnated with various other supports such as HZSM-5, Al₂O₃ and SiO₂ were also used for the reaction to assess the suitability of the support. The order of activity of the support is ZnO > SiO₂ > Al₂O₃ > HZSM-5. Various weight percentage of MgO was loaded on ZnO to optimize maximum efficiency of the catalyst system. The impregnation of MgO (wt%) in ZnO was optimized for better conversion of 2′-hydroxyacetophenone. The effect of temperature and catalyst loading was studied for the reaction.     

In situ synchrotron powder diffraction study of the setting reaction kinetics of magnesium-potassium phosphate cements

This work reports a kinetic study of the formation of magnesium-potassium phosphate cements accomplished using in-situ synchrotron powder diffraction. The reaction: MgO + KH₂PO₄ + 5H₂O → MgKPO₄ · 6H₂O was followed in situ in the attempt of contributing to explain the overall mechanism and assess the influence of periclase (MgO) grain size and calcination temperature (1400-1600 °C) on the reaction kinetics. Numerical kinetic parameters for the setting reaction have been provided for the first time. The best fit to the kinetic data was obtained using a weighted nonlinear model fitting method with two kinetic equations, representing two consecutive, partially overlapping processes. MgO decomposition could be described by a first order (F1) model followed by a Jander diffusion (D3) controlled model. Crystallization of the product of reaction was modelled using an Avrami model (A_n) followed by a first order (F1) chemical reaction. A reaction mechanism accounting for such results has been proposed.     

Microstructure and mechanical properties of silicon carbide pressureless sintered with oxide additives

It is known that SiC powders can be densified at relatively low temperatures (1850–2000 °C) with some oxide additions. In this work the densification behavior, microstructure and mechanical properties (bending strength, fracture toughness, hardness) of SiC ceramics pressureless sintered with different additions chosen from oxide groups: Al₂O₃ + Y₂O₃, Al₂O₃ + Y₂O₃ + MgO, were investigated. It was found that oxide additives facilitate densification of sinters and significantly improve mechanical properties of SiC ceramics. The best activating oxide additions have been identified.     

Crystallisation of amorphous spray-dried precursors in the Al2O3–SiO2 system

The crystallisation of amorphous precursors has been studied in the whole range of composition in the Al₂O₃–SiO₂ system. The amorphous precursors have been obtained by hydrolysing TEOS directly in a diluted aqueous solution of aluminium nitrate, spray drying the clear solution and heating the resulting powder. Up to 70 mol % Al₂O₃, only mullite crystallises around 980–1000 °C; between 70 and 80 mol % Al₂O₃ mullite and spinel crystallise together; and for more than 80 mol % Al₂O₃ only spinel is formed. In the 70–80 mol % Al₂O₃ range of composition, when both mullite and spinel crystallise, low heating favours the crystallisation of mullite and it is nearly possible to crystallise only mullite from a 75 mol % Al₂O₃ sample. By rapid heating it is also possible to crystallise only spinel from the same 75 mol % Al₂O₃ precursor. The enthalpy and the activation energy for crystallisation are maximum for 60–80 mol % Al₂O₃. Heating the samples up to 1700 °C for 1 h, the phase equilibrium is not reached, particularly when both mullite and spinel crystallise together, and θ-Al₂O₃ is still present.     

Synthesis and characterization of a MgO-MgAl2O4-ZrO2 composite with a continuous network microstructure

Using analytically pure MgO, analytically pure Al₂O₃ and analytically pure ZrO₂ as raw materials, Mg_4.68Al_2.64Zr_1.68O₁₂ was prepared at 1993 K for 10 h, and then, a MgO-MgAl₂O₄-ZrO₂ composite with a continuous network was successfully obtained by controlling the cooling rate based on the in-situ decomposition reaction of Mg_4.68Al_2.64Zr_1.68O₁₂ at temperatures below 1887 K. The three phases of MgO, MgAl₂O₄ and ZrO₂ are highly dispersed in this continuous network microstructure, with ZrO₂ intertwined by MgO and MgAl₂O₄ and micropores with a size of less than 2 µm. Furthermore, the synthesis mechanism of Mg_4.68Al_2.64Zr_1.68O₁₂ is given as follows: first, MgAl₂O₄ is synthesized using the reaction: MgO(s)+Al₂O₃(s)=MgAl₂O₄(s) at temperatures below 1894 K; and then, Mg_4.68Al_2.64Zr_1.68O₁₂ is further prepared through MgO and ZrO₂ diffusion and dissolution into MgAl₂O₄ at temperatures above 1894 K, for example, at 1923 K or 1993 K in this work.     

Effect of Al2O3-SiO2 substrate on gas-sensing properties of TiO2 based lambda sensor at high temperature

Alumina is widely used as substrate material for oxygen lambda sensors. It has been reported that response properties of sensors are highly dependent on surface state of substrates. In this work, Al₂O₃-x%SiO₂ (x = 2–30) substrates were specially prepared for TiO₂ based lambda sensors for high temperature operation. Structure and surface state of prepared substrates were characterized by XRD, SEM and XPS. TiO₂ sensing film was prepared by screen printing method. Results indicated that sensors fabricated on Al₂O₃-10%SiO₂ substrate exhibited the best sensing properties. Moreover, final steady-state voltages of all sensors were limited to less than 100 mV at 600–800 ℃.     

Interaction of Al2O3-rich slag with MgO–C refractories during VOD refining—MgO and spinel layer formation at the slag/refractory interface

The interaction mechanisms between a pitch-bonded MgO–C refractory and an Al₂O₃ rich (～15 wt%) stainless steelmaking slag were investigated by rotating finger tests in a vacuum induction furnace. A porous MgO layer (instead of a dense MgO layer) was observed at the hot face of the MgO–C bricks. This implies that under the present low oxygen pressure conditions, the oxygen supply from the slag is insufficient to meet the demand of reoxidising the entire amount of Mg vapor generated from the MgO–C reaction to form a fully dense MgO layer. A Mg(Al,Cr)₂O₄ spinel layer with zoning was found at the slag/brick interface in the top slag zone specimen of Test 3 (CHS3). Based on the thermodynamic analyses with and experimental data, a mechanism of Mg(Al,Cr)₂O₄ spinel formation is proposed. Initially, hot face periclase grains take up Cr₂O₃, and to a much lesser extent, Al₂O₃ from the slag. The further diffusion of Cr₂O₃ and Al₂O₃ from the slag establishes a spinel layer of three distinct compositions of the type MgAl_2(1?x)Cr_2xO₄, with x decreasing when moving from the interior to the exterior spinel layer. Due to the low oxygen pressures, the thermodynamically less stable, dissolved Cr₂O₃ in the hot face periclase decomposes and forms chromium-rich metal droplets.     

The effect of solvent water content on the dielectric properties of Al2O3 films grown by atmospheric pressure mist-CVD

Aluminum oxide (Al₂O₃) dielectric layers were grown by a mist-chemical vapor deposition (mist-CVD) process at 300 °C, using solvent mixtures containing acetone and water. As the acetone to water ratio was varied from 9:1 to 7:3, the leakage current of Al₂O₃ at an electric field of 7 MV/cm² decreased from 9.0 × 10^?7 to 4.4 × 10^?10 A/cm², and the dielectric constant increased from 6.03 to 6.85 with improved hysteresis during capacitance-voltage measurements. Consequently, the most robust Al₂O₃ films were obtained at an acetone to water ratio of 7:3, with a dielectric constant (κ) close to the ideal value 7.0, and a breakdown field of approximately 9 MV/cm. Thin film transistors (TFTs) incorporating In-Sn-Zn-O (ITZO) as the semiconductor were fabricated with the Al₂O₃ (7:3) dielectric onto p⁺⁺-Si substrates. The devices exhibit high electrical performance, with a high field effect mobility of 42.7 cm²V^?1s^?1, and a small subthreshold swing (S.S.) value of 0.44 V/decade.     

Degradation mechanisms of magnesia-carbon refractories by high-alumina stainless steel slags under vacuum

The corrosion behaviour of a pitch-bonded magnesia-carbon refractory by an Al₂O₃ rich (∼15 wt.%) stainless steelmaking slag was investigated by rotating finger tests in a vacuum induction furnace at high temperature (>1650 °C) and low oxygen partial pressure (1.5–4.3 × 10⁻¹⁰ atm). This study confirms the poor slagline behaviour of MgO-C bricks industrially observed in VOD ladles. Higher temperatures and longer exposure times lead to more severe slag infiltration and direct MgO dissolution. The intrinsic MgO-C reaction is the major decarburisation mechanism, while extrinsic decarburisation by oxygen from the atmosphere and/or reducible slag components (CrO_x, FeO_x) was limited. Three kinds of metallic particles with different size, shape, location, composition and origin were observed in the refractory specimens. Concurrently, the thermodynamic conditions for the formation of a protective Mg(Al,Cr)₂O₄ spinel layer at the slag/refractory interface are investigated. The industrial relevance of this spinel layer formation is discussed with respect to the chosen Al₂O₃ level. Guidelines are proposed to minimise MgO refractory dissolution in VOD slaglines.     

Enhanced production of benzyl alcohol in the gas phase continuous hydrogenation of benzaldehyde over Au/Al2O3

Exclusive hydrogenation of benzaldehyde to benzyl alcohol in gas phase continuous operation (393–413 K, 1 atm) was achieved over Au/Al₂O₃, Au/TiO₂ and Au/ZrO₂. Synthesis of Au/Al₂O₃ by deposition–precipitation generated a narrower distribution (2–8 nm) of smaller (mean = 4.3 nm) Au particles relative to impregnation (1–21 nm, mean = 7.9 nm) with increased H₂ uptake under reaction conditions and higher benzaldehyde turnover. Switching reactant carrier from ethanol to water resulted in a significant enhancement of selective hydrogenation rate over Au/Al₂O₃ with 100% benzyl alcohol yield, attributed to increased available reactive hydrogen. This response extends to reaction over Au/TiO₂ and Au/ZrO₂.     

Facile modified synthesis and intermediate temperature electrical properties of Sn0.95Al0.05P2O7/KSn2(PO4)3 composite electrolyte

In this study, Sn_0.95Al_0.05P₂O₇ and a novel dense Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite electrolytes were synthesized. The structural characterization of X–ray diffraction (XRD) and microstructual properties of scanning electron microscopy (SEM) were carried out. The XRD results indicated that an in-situ reaction between Sn_0.95Al_0.05P₂O₇ and inorganic melt salt take place to form the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ composite. The intermediate temperature electrical properties were determined by using impedance spectroscopy, oxygen concentration cell and hydrogen concentration discharge cell. Finally, the H₂/O₂ fuel cell using the Sn_0.95Al_0.05P₂O₇/KSn₂(PO₄)₃ as electrolyte membrane was constructed and the obtained maximum power output densities were 67.7 mW cm^?2 and 142.1 mW cm^?2 at 650 °C and 700 °C, respectively.     

Evolution of elastic properties and microstructural changes versus temperature in bonding phases of alumina and alumina–magnesia refractory castables

The microstructural evolutions of high alumina refractory concretes, based on the systems CaO–Al₂O₃ and CaO–Al₂O₃–MgO, have been studied by the way of ultrasonic high temperature measurements. Since such a refractory concrete can be considered as a composite material with two constituents, a continuous matrix (so called bonding phase) and aggregates, investigations of matrices made of mixtures containing cement, reactive alumina and/or magnesia, constitute a preliminary study which is presented in this paper. The elastic behaviour of these matrices has been followed from room temperature to 1550 °C via a specific ultrasonic method. During the first thermal treatment, different changes of slope are observed in the curve E = f(T). Between 200 °C and 400 °C, dehydration mechanisms involve a microstructural reorganisation correlated with a strong decrease of the elastic properties. At high temperature, the Young's modulus evolutions are associated with the expansive formations of CA₂^b and/or in-situ spinel at 1100 °C and then CA₆ (see endnote b) at 1450 °C, which directly depend on the CaO/Al₂O₃ and MgO/Al₂O₃ ratios in the mix. The forming of bond linkage between CA₆ and in-situ spinel grains in the matrix is believed to enhance the elastic properties at high temperature.     

Influence of the structure of MgO·nAl2O3 spinel lattices on transparent ceramics processing and properties

It is demonstrated that a complete elimination of pores on sintering is governed not only by the size of the ceramic powder particles and by the homogeneity of their mutual coordination but similarly strongly by the state of the crystal lattice: with different cation disorder at fixed stoichiometry (n = 1) the sintering temperatures may differ by as much as 200 °C at constant powder particle size and equal homogeneity of the green bodies. Additionally, the impact of stoichiometry was investigated over the range between n = 1 and n = 3 with retarded reactive sintering at moderately increased Al₂O₃ concentrations but promoted densification of alumina-rich compositions. Taking advantage of the observed effects, sintered spinel ceramics were derived by reactive sintering of undoped MgO/Al₂O₃ mixtures resulting in an in-line transmittance which equals spinel single crystals of similar composition from 200 nm wave length up to the IR range.     

Photooxidation of iodide ion on some semiconductor and non-semiconductor surfaces

In 80% aqueous ethanol, TiO₂ (anatase), ZrO₂, ZnO, V₂O₅, Fe₂O₃ and Al₂O₃ photocatalyze the oxidation of iodide ion but CdO and CdS do not; the wavelength of illumination is 365 nm. However, Fe₂O₃ fails to bring in a sustainable photocatalysis in 60% aqueous ethanol. The photooxidation of iodide ion on TiO₂, ZrO₂, ZnO, V₂O₅ and Al₂O₃ in 60% aqueous ethanol was studied as a function of [I⁻], amount of catalyst suspended, airflow rate, light intensity and solvent composition. The metal oxides examined show sustainable photocatalytic activity. Iodine formation is larger with illumination at 254 nm than at 365 nm. The mechanisms of photocatalysis on semiconductor and non-semiconductor surfaces have been discussed. Photocatalytic generation of iodine has been analyzed using a kinetic model. The photocatalytic efficiencies are of the order V₂O₅ > TiO₂ > ZrO₂ > ZnO > Al₂O₃ and V₂O₅ > TiO₂ > ZrO₂ > ZnO=Fe₂O₃ > Al₂O₃ in 60% and 80% aqueous ethanol.