Synthesis and characterization of MgAl2O4 spinel precursor sol prepared by inorganic salts

Magnesium aluminate (MgAl₂O₄) spinel precursor sol was prepared using aluminum nitrate and magnesium nitrate as the precursors for alumina and magnesia, respectively. The obtained sol owned a translucent, homogenous, and stable appearance with a density of 1.19 g/cm³, and at the temperatures above 60 °C, converted to a soft and clear gel. The measured pH of sol was in the range of 3–4, and at the calcination temperature of 1000 °C, the solid content of 6% was reached. TGA/DSC analysis was utilized to study the thermal behavior of the sol. Fourier transforms infrared spectroscopy (FTIR) was applied to recognize the existing bounds in the dried and calcined sol. X-ray diffraction patterns revealed the formation of single-phase MgAl₂O₄ up to 600 °C. According to the FESEM images, the grain size for the sol calcined at 1000 °C was estimated at around 50 nm.     

Hierarchical MgAl2O4 supported Pt-Sn as a highly thermostable catalyst for propane dehydrogenation

Highly thermal stability is an essential property for propane dehydrogenation (PDH) catalysts that need to be regenerated frequently in oxidative atmosphere. A kind of hierarchical MgAl₂O₄ with flower-like morphology is prepared by alcohothermal method and used to support Pt and Pt-Sn. The materials are characterized by ICP-AES, SEM, TEM, XRD, Py-IR, and N₂ physi-sorption. Hierarchical MgAl₂O₄ supported Pt shows great thermal stability under oxidizing atmosphere at 800 °C. Supported Pt-Sn catalyst for PDH shows high stability of performance during 10 cycles of dehydrogenation-regeneration runs. The high stability could originate from the existence of relatively abundant MgAl₂O₄(111) facets on MgAl₂O₄.     

MgAl2O4 spinel synthesis by combustion and detonation reactions: A thermochemical evaluation

The production of MgAl₂O₄ spinel is evaluated using a thermochemical program, THOR, which calculates the equilibrium products resulting from the adiabatic combustion (isobaric and isochoric) or detonation of energetic materials. A classic metalized emulsion was used (ammonium nitrate, fuel oil, aluminium, water and glass microballoons), to which MgO was added. The Al/MgO proportion and the reaction regime were varied, the maximum spinel yield being achieved for the detonation of a fuel-rich emulsion with 5Al:3MgO (mol). In parallel, MgAl₂O₄ was experimentally synthesized by solution combustion of Al and Mg nitrates with various urea contents. The same reactions were simulated with THOR and the results obtained for products type and amounts were found to be in good agreement. Additionally, THOR simulations provided clear explanations for the experimental observations, which can be of invaluable help in the selection of fuel type and content in solution combustion synthesis of any given mixed oxide.     

Effect of calcination temperature on isobutane dehydrogenation over Mo/MgAl2O4 catalysts

The catalytic activity of a novel catalyst Mo/MgAl₂O₄ was studied in isobutane dehydrogenation, and the effect of calcination temperature has been comprehensively investigated. For catalysts with MoO₃ loading below monolayer coverage (5Mo/MgAl₂O₄), isobutane conversion increases with calcination temperature, while the selectivity to isobutene decreases. An opposite trend is observed for catalysts with MoO₃ loading above monolayer coverage (30Mo/MgAl₂O₄). In combination with characterization results, it can be concluded that calcination temperature affects catalyst structure and properties in many aspects including surface area, acidity and reducibility, as well as the interaction between Mo species and support, and thus influences the catalytic behaviors.     

Sintering behavior of magnesium aluminate spinel MgAl2O4 synthesized by different methods

This paper is focusing basically on the ceramic technology, of which several methods for the synthesis of MgAl₂O₄ have been investigated. The synthesis conditions regarding the powders cleanliness, microstructure, and sintering parameters of MgAl₂O₄ were studied. MgAl₂O₄ powder was synthesized via conventional solid-state route using different milling process: vertical attrition milling, WAB as a high-energy horizontal attrition milling, and Pulverisette as a planetary ball miller, and via solution combustion route using Urea, Glycine, and a mixture of Urea/Glycine. Urea and Glycine was used as fuel. The white powders were obtained for all solid-state routes and for Urea-combustion technique. The black and gray powders were obtained in the case of combustion technique, respectively, using a fuel of Glycine and Glycine/Urea mixture. The obtained powders and pellets were characterized by XRD, SEM, and Dilatometry. The results show that, among all the solid-state route processes, wet attrition milling gives the better and clean spinel phase. The WAB milling and Pulverisette miller introduce a contamination by some yttria-stabilized zirconia balls in the corresponding powder. Furthermore, the flash combustion technique permit to have nanoparticles with a dense spinel phase of MgAl₂O₄ and with lower sintering temperature in less time and with no calcination step.     

Synthesis and characterization of nano MgAl2O4 spinel by the co-precipitated method

Magnesium aluminate (MgAl₂O₄) spinel powders were prepared by co-precipitation of stoichiometric amounts of magnesium and aluminum chlorides at 80 °C. Some sintering aids such as ZnO and MnO₂ were added in the form of chlorides during the precipitation to study their effect on densification. The co-precipitated materials were a mixture of Mg–Al double hydroxide with the presence of few amounts of gibbsite and brucite. After heat-treatment of the precipitated powders up to 1000 °C, a crystalline spinel powder was obtained. The presence of 0.5, 1, 2 and 3 wt.% of ZnO or MnO₂ as sintering aids increased sinterability after firing up to 1550 °C. The highest density was obtained for the samples containing 2 wt.% ZnO or 3 wt.% MnO₂ which reached about >94 and 96% theoretical density (TD), respectively. The mechanical properties increased by adding ZnO or MnO₂, an exception being the sample containing 0.5 wt.% of ZnO for which relatively smaller value were obtained.     

Preparation and properties of MgAl2O4 spinel ceramics by double-doped CeO2 and La2O3

Magnesium aluminate spinel (MgAl₂O₄) ceramics are high-performance and carbon-free materials widely used in both military and civilian fields. However, it is usually challenging to densify during the solid-state sintering process. The excellent properties of some rare earth oxides have been proved to promote the densification of MgAl₂O₄ spinel ceramics. But the mechanism of promoting sintering is not clear. In the present work, MgAl₂O₄ spinel ceramics have been successfully fabricated by co-doping CeO₂ and La₂O₃ via a single-stage solid-state reaction sintering. The effects of addition amounts of CeO₂ and La₂O₃ on phase compositions, microstructures, sintering characteristics, cold compressive strength, and thermal shock resistance of as-prepared MgAl₂O₄ spinel ceramics were systematically investigated. The results show that by co-doping CeO₂ and La₂O₃ can increase the defect concentration due to the lattice distortion. This could promote the movement of Al³⁺ and Mg²⁺ at high temperature, which is beneficial to the formation of more secondary MgAl₂O₄ spinel. t-ZrO₂ with more Ce⁴⁺ filling between spinel grains could prevent the growth of grains and promote the densification, besides the new-formed LaAlO₃ that was mainly distributed along the grain boundary of the MgAl₂O₄ phase, both of which were favorable for the formation of dense microstructure of MgAl₂O₄ spinel materials. At the same time, the formation of more secondary MgAl₂O₄ spinel and sintering densification also improve the mechanical properties of spinel ceramics. La³⁺ will segregate to the spinel grain boundary, preventing grain boundary movement and absorbing the main crack's fracture energy. With 3 wt% CeO₂ and 3 wt% La₂O₃ co-doping, the bulk density of the sample increased from 3.02 g∙cm⁻³ to 3.55 g∙cm⁻³; the apparent porosity decreased from 12.21% to 9.97%; the cold compressive strength increased from 172.88 MPa to 189.54 MPa; and the residual strength retention ratio after thermal shock increased from 84.92% to 89.15%.     

SO2 abatement over nanocrystalline MgAl2O4 spinel-supported catalysts

Thermally stable magnesium-rich MgAl₂O₄ spinel with mesoporous nanostructures and high surface area has been prepared by co-precipitation and post hydrothermal treatment, using glucose as organic template. Physical and chemical properties were characterized by XRD, N₂ sorption, TG, FTIR, SEM, and TEM. The synthesized MgAl₂O₄ showed a surface area of 324 m² g^?1 and centralized mesopore distribution (ca. 3.3 nm pore width) after calcination at 700 °C for 3 h. The prepared MgAl₂O₄ were impregnated with metal oxides as sulfur transfer catalysts for high-temperature SO₂ adsorption reaction. The results showed that ferric doped MgAl₂O₄ had the highest SO₂ pick-up capacity up to 58 % and best regeneration up to 81 %. These results showed that thermally stable nanostructured MgAl₂O₄ are a promising candidate as catalyst for desulfurization in fluid catalytic cracking process.     

Production and characterization of transparent MgAl2O4 prepared by hot pressing

Hot pressing has been investigated for the production of transparent MgAl₂O₄ aimed at the scaling up of the process. Other assessed techniques (hot isostatic pressing, spark plasma sintering) can hardly be used for the production of flat components with large dimensions and good planarity.Hot pressing of stoichiometric Al₂O₃–MgO powder mixtures has been preferred to the direct pressing of spinel powder for the readily availability of pure powders and to exploit the thermodynamic driving force of the spinel formation. LiF has been used as sintering additive.A thermodynamic investigation of the reactions involving LiF, MgO and Al₂O₃ has helped in the comprehension of the densification mechanisms affecting the transparency of spinel. Transparencies up to 70% in the visible range (highest value 78% at 1100 nm) have been obtained. Suitable soakings have been added for promoting the initial liquid phase sintering and the release of LiF through formation of vapour phases.     

Sintering behavior and microstructure of (1-x)MgAl2O4-xMg2TiO4 spinel solid solutions prepared by isostructural heterogeneous nucleation

Magnesium aluminate (MgAl₂O₄) spinel has grasped considerable attention in high-temperature application by right of its excellent properties. However, the poor sintering behavior of MgAl₂O₄ is detrimental to its further development. In the present work, the application of isostructural heterogeneous nucleation method provides a novel idea for optimizing the sintering behavior of refractory materials. A series of (1-x)MgAl₂O₄-xMg₂TiO₄ (x = 0, 0.02, 0.04, 0.06, 0.08, and 0.1) spinel solid solutions with a present ration of components were fabricated from light calcined magnesia, reactive alumina and pre-preparation Mg₂TiO₄. The effect of Mg₂TiO₄ heterogeneous nucleating agent on the crystalline phase, densification, and microstructure evolution of MgAl₂O₄–Mg₂TiO₄ spinel solid solutions was studied. The XRD, XPS, and EDS results showed that Mg₂TiO₄ entered the lattice of MgAl₂O₄ to form a spinel solid solution, and the heterovalent substitution process was identified, where Ti⁴⁺ and Mg²⁺ ions of larger radius in the Mg₂TiO₄ replaced the Al³⁺ of smaller radius in the MgAl₂O₄. For the sample at x = 0.08, the spinel solid solutions exhibited the optimized densification with a relative density of 93.3%, an apparent porosity of 1.2%, and a compressive strength of 84.5 MPa. A significant increase in densification was related to the lattice distortion induced by ion size mismatch during the heterovalent substitution, thus accelerating the diffusion rate of Mg²⁺ and Al³⁺ ions in the spinelisation state. Moreover, the solid solubility content of Ti⁴⁺ in the MgAl₂O₄–Mg₂TiO₄ spinel solid solutions had a significant effect on the grain morphologies. The Mg₂TiO₄ heterogeneous nucleating agent significantly increased the spinelisation rate of MgAl₂O₄ spinel with negligible effect on densification.     

MgAl2O4 spinel powders from oxide one pot synthesis (OOPS) process for ceramic humidity sensors

MgAl₂O₄ spinel powders can be prepared via the oxide one pot synthesis (OOPS) process. Porous ceramic oxide bodies can be produced with these powders and used as humidity-sensing materials. Pellets were characterized by SEM and their electrical properties were measured using impedance spectroscopy in the frequency range from 10⁻² to 10⁵ Hz at different relative humidity (RH) levels in the range 4–90%. The spinel pellets made of powder from the OOPS process exhibited good humidity sensitivity with a linear response of the logarithm of resistance with RH in the whole RH range tested.     

Luminescence of MgAl2O4 and ZnAl2O4 spinel ceramics containing some 3d ions

The luminescence properties of ceramic phosphors based on two spinel hosts MgAl₂O₄ and ZnAl₂O₄ doped with manganese ions have been studied. It has been found that the spectral properties of these phosphors can be strongly varied by changing synthesis conditions. Both types of doped ceramic spinel can serve as efficient Mn²⁺ green-emitting phosphors having peak emissions at 525 and 510 nm, respectively. Mn-doped MgAl₂O₄ spinel can also be prepared as an efficient Mn⁴⁺ red-emitting phosphor having peak emission at ~651 nm by using specific temperatures of heat treatment in air. It has also been shown that the conversion of Mn²⁺ to Mn⁴⁺ and viсe versa, as well as the coexistence of Mn²⁺ green and Mn⁴⁺ red emissions, can be accomplished by properly chosen annealing conditions of the same initially synthesized MgAl₂O₄:Mn sample. Manganese doped MgAl₂O₄ spinel with an optimal intensity ratio of green and red emissions can be a promising single-phase bicolor phosphor suitable for the development of warm white phosphor-converted LED lamps. On the other hand, it has been determined that perfectly normal ZnAl₂O₄ spinel cannot be doped with Mn⁴⁺ ions in contrast to partially inverse MgAl₂O₄ spinel. However, ZnAl₂O₄ samples unintentionally doped with impurity Cr³⁺ ions show emission spectra in the far-red region with well pronounced R, N and vibronic lines of Cr³⁺ luminescence due to the perfect normal spinel structure of synthesized ZnAl₂O₄ ceramics. Also, by partially substituting Al³⁺ cations for Mg²⁺ in ZnAl₂O₄ there is an opportunity to obtain Mn⁴⁺ doped or Mn⁴⁺/Cr³⁺ codoped far-red emitting phosphors which can be suitable for indoor plant growth lighting sources.     

Optimization of support technology for the production of industrial microspheric alumina-chromia catalysts for paraffin dehydrogenation

A systematic physical and chemical analysis of different industrial microspherical supports based on thermochemically activated aluminum trihydrate (TCA-ATH) has been performed in order to improve and optimize supports for the production of isobutane dehydrogenation alumina-chromia catalysts. It has been demonstrated that the phase composition of TCA-ATH products and their structure plays an important role in their synthesis of catalysts on their basis. According to results of physical and chemical research of supports supplied by different producers, an improvement of the industrial technology has been proposed. Preliminary thermal treatment of industrial microspheric products of thermochemical activation of aluminum trihydrate at 550°C until full dehydration of gibbsite and destruction of micropores has been recommended. It has been shown that thermal treatment does not influence the strength of the microgranules of the supports and the catalysts on the basis of them. Alumina-chromia catalysts synthesized employing thermally treated supports are more catalytically active than those based on initial supports. The yield of isobutene in the reaction of isobutane dehydrogenation increases from 40–45 to 44–48 wt %.     

高性能镁铝尖晶石粉体的制备技术进展

综述了目前常用的制备高性能镁铝尖晶石粉体的各种方法的工艺过程、特点及其产物的性能特征. 分析指出,纯度和粒度是粉体最重要的两个性能指标;降低合成温度、简化工艺过程是今后制备技术发展的趋 势;金属醇盐可能成为获得高纯度产物最有应用前景的前驱物;水热处理、溶剂蒸发、超临界干燥等物理手段是 解决粒度最有效的途径.     

Solid-state combustion synthesis of spinel LiMn2O4 using glucose as a fuel

Spinel LiMn₂O₄ cathode material was rapidly synthesized in 1 h by solid-state combustion synthesis using metal carbonates as metal ion sources and glucose as a fuel. The effect of different amounts of glucose on the structure and electrochemical performance of as-prepared LiMn₂O₄ was investigated by X-ray diffraction (XRD), scanning electron micrographs (SEM), galvanostatic charge–discharge test, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). LiMn₂O₄ spinel was identified as the main crystalline phase with the presence of minor Mn₃O₄. The amount of glucose greatly affected the formation of Mn₃O₄. The optimal content of glucose was found to be 10 wt%. Under this condition, the Mn₃O₄ peaks almost disappeared, and high-purity spinel LiMn₂O₄ was obtained. Its initial discharge specific capacity of was 125.9 mAh/g, and discharge specific capacity retained at 105.2 mAh/g after 40 cycles. The detail influence of glucose on the electrochemical activity, reversibility and cycling performance of LiMn₂O₄ was discussed.     

Optimum ratio of K2O to CeO2 in a wet-chemical method prepared catalysts for ethylbenzene dehydrogenation

Fe₂O₃–K₂O–CeO₂ catalysts with various ratios of K₂O to CeO₂ were prepared by the wet-chemical method. Their phase compositions, reducibility, valence states of elements and catalytic activities for ethylbenzene dehydrogenation were studied. The results demonstrated that when the weight ratio of K₂O: CeO₂ was 1.40, the catalyst had highest ethylbenzene conversion and styrene selectivity, which were attributed to the optimization of active phase content and electron transfer ability, etc. Further, higher CeO₂ content not only enhanced styrene selectivity, but also prolonged the life cycle of catalysts.     

在尖晶石结合浇注料中尖晶石的结构分析

普通的水合镁盐和廉价的一水软铝石溶胶的混合物被认为是有经济效益的活性纳米尖晶石（1∶1）粉末的前身。它经机械化学处理后,在控制pH值、温度和时间的条件下凝胶,并在低温下煅烧。利用X-射线衍射（XRD）、红外光谱、差热分析、质子核磁共振（NMR）和透射电子显微镜来研究水合尖晶石中低纳米晶体域的初期形成。骨料表面上（OH）基的薄吸附层保护了尖晶石纳米粒子。对传统Al2O3尖晶石和Al2O3-MgO浇注料的选择属性和将尖晶石细粉用化学途径以相同的配方制出的浇注料进行比较。在有商用尖晶石的耐火浇注料中添加过量铝粉的情况下,其冷热强度最好,而通过活性氧化镁和沉淀的尖晶石细粉结合的工业尖晶石不是非常令人满意。溶胶-凝胶尖晶石的性能与含有78%氧化铝的预制尖晶石具有可比性。     

MgAl2O4 nanopowder synthesis by microwave assisted high energy ball-milling

This paper reports the development of a new process for the synthesis of spinel nano powder via microwave assisted high energy ball milling of a powder mixture containing Al(OH)₃ and Mg(OH)₂. X-ray diffraction (XRD), Simultaneous thermal analysis (STA), FTIR spectrometer, BET and scanning electron microscopy (SEM) techniques were utilized to characterize the as-milled and annealed samples. X-ray diffraction results provide evidence for the formation of a completely amorphous phase after milling for 8 h. It is found that highly ordered MgAl₂O₄ spinel can be obtained by calcination the as-milled powder over 800 °C. Also, SEM observations of synthesized powders showed that the particle size of powders lies in the nano meter range compared with the BET results (about 28–149 nm). The DTA–TG analyses were carried out to investigate the effect of microwave heating on the synthesis temperature compared to the conventional heat treatment by conventional furnace. Synthesis of powders with different heating methods showed that microwave heating reduces the synthesis temperature by about 200 °C.     

Nano-structured MgAl2O4 spinel consolidated by high pressure spark plasma sintering (HPSPS)

Nano-structured transparent polycrystalline magnesium aluminate spinel (PMAS) was fabricated using a high pressure (up to 1000 MPa) spark plasma sintering (HPSPS) apparatus and various properties of the spinel, such as transparency, micro-structure and mechanical properties (specifically, hardness and fracture toughness), were tested. Using a creep densification model, it was concluded that densification in the final stage of HPSPS is controlled by grain boundary sliding (GBS), rather than by oxygen diffusion. The average grain size of PMAS fabricated under 400 MPa pressure at 1200 °C was about 170 nm, while for samples fabricated under 1000 MPa at 1000 °C the average grain size was remarkably smaller (about 50 nm). HRTEM analysis clearly demonstrated clean grain boundaries and triple points with no evidence for the existence of amorphous regions. Fully dense specimens displayed in-line transmittance higher than 80%. It was moreover established that hardness and fracture toughness values did not depend on the indentation load applied. Finally, hardness values for grains sized between tens of microns and tens of nm strictly followed the Hall-Petch relationship.     

Aqueous slip casting and hydrolysis assisted solidification of MgAl2O4 spinel ceramics

Abstract

Abstract

This paper reports on synthesis of MgAl₂O₄ spinel (MAS) powders with six different chemical compositions and the consolidation of the synthesised MAS powders following an aqueous slip casting and hydrolysis assisted solidification (HAS) routes. The synthesised MAS powders were surface passivated against hydrolysis before being dispersed in distilled water to obtain suspensions with 41–45?vol.‐% solid loading using suitable dispersing agents. In the case of the HAS process, the consolidation of suspensions was achieved in non‐porous moulds under ambient conditions by the incorporation of AlN equivalent to 1–5?wt‐%Al₂O₃ into the suspension. The stoichiometric MAS powder consolidated by slip casting and dry pressing routes was sintered along with those consolidated by HAS route at 1550–1650°C for 1?h. Various characterisation techniques were utilised to evaluate the effect of composition and consolidation technique on slurry characteristics and sintered properties of MAS ceramics.