A comparative study on citrate sol-gel and combustion synthesis methods of CoAl2O4 spinel

CoAl₂O₄ spinel was successfully synthesized by combustion synthesis method using glycine and urea by 1:1 molar ratio as fuels and sol-gel process using citric acid as a chelating agent. The as-synthesized powders were calcined at desired temperatures to obtain CoAl₂O₄ spinel as a single phase. X-ray diffraction, thermogravimetric, and differential thermal analysis results revealed that the formation of CoAl₂O₄ spinel in combustion method needs 300°C higher temperatures than those of sol-gel. Scanning electron microscopy and transmission electron microscopy analysis results revealed that “sol-gel spinel” had nanometric particle size which was smaller than those of “combustion spinel.” Temperature programed reduction with hydrogen and Fourier transform infrared spectroscopy results declared that there was a little residual cobalt oxide in combustion spinel while there is no oxide resided in “sol-gel spinel.” Consequently, the sol-gel method has more benefit in synthesizing spinel with sulfate precursors than combustion.     

Structural,microstructural and electrochemical studies on LiMn2-x(GdAl)xO4 with spinel structure as cathode material for Li-ion batteries

Gd and Al co-doped LiMn_2-x(GdAl)_xO₄ (x?=?0, 0.01, 0.02, 0.03, 0.04 and 0.05) materials with spinel structure were synthesized by sol–gel method. Powder X-ray diffraction results confirm the formation of cubic spinel structure and average particle sizes are found to be between 80 and 110?nm from FE-SEM and TEM analysis. Decrease in peak potential difference as a function of doping in Cyclic Voltammetry results establishes enhancement in Li⁺ intercalation and de-intercalation. Electrochemical Impedance Spectroscopy (EIS) results showed that accumulation of charges on electrode has improved with doping over pristine samples. At a doping of x?=?0.02 charge transfer resistance values were found to be least. First cycle charge–discharge profiles for LiMn_1.96(GdAl)_0.02O₄ shows 139.2?mAh/g discharge capacity over other doped derivatives and pure LiMn₂O₄ (119.6?mAh/g) in aqueous Li₂SO₄ electrolyte. Doping of x?=?0.02 exhibit good cycling performance with only a total 4% capacity loss after 30 cycles.     

The effect of synthesis conditions on the physicochemical properties of magnesium aluminate materials

Magnesium aluminate-based materials were prepared by applying different methods: (i) mechanochemical milling of the initial mixture of magnesium and aluminium nitrate powders (in appropriate stoichiometric amounts) followed by heat treatment at temperatures of 650 °C and 850 °C and (ii) melting of the mixture of nitrate precursors at 240 °C followed by thermal treatment at 650 °C, 750 °C and 850 °C. The effect of synthesis method on the structure and morphology of the obtained solids was studied by using various techniques such as: nitrogen adsorption-desorption isotherms, powder XRD, IR spectroscopy and SEM. It was shown that the mechanochemical milling performed before calcination procedure leads to obtaining of nanocrystalline magnesium aluminate spinel phase at lower temperature of 650 °C in comparison with the method using thermal treatment only (at 750 °C). The obtained nanomaterials exhibit mesoporous structure.     

Photocatalytic desulfurization of dibenzothiophene by NiCo2O4 nanospinel obtained by an oxidative precipitation process modeling and optimization

This paper reports for the first time, synthesis of NiCo₂O₄ nano spinel by green oxidative precipitation and its performance in photocatalytic desulfurization of dibenzothiophene (DBT) from gas oil model at different process conditions under visible light. The as-produced nanostructure was characterized by X-ray diffraction, Fourier transform infrared, UV–VIS diffuse reflective spectroscopy, energy-dispersive X-ray spectrometry and scanning electron microscopy. The experiments for the study of different photocatalytic conditions were designed by response surface methodology and a second-order regression model was developed with a determination coefficient (R²) of 0.9769. Pareto analysis predicted that the relative importance of process factors for DBT removal is as follows: irradiation time?>?reaction temperature?>?photocatalyst dosage?>?DBT concentration. The promising results for DBT removal were concluded by photo desulfurization over the nano spinel. The study confirmed that nano spinels could be an alternative and cheap photocatalyst for desulfurization from the oil products.     

NiMn2O4尖晶石氧化物阴极的制备及电化学性能研究

高性能阴极材料的开发对推动中温固体氧化物燃料电池（intermediate temperature solid oxide fuel cells, IT-SOFCs）的发展具有重要意义。本文采用溶胶-凝胶法制备了尖晶石型NiMn₂O₄（NMO）电子-离子混合导体材料，并作为IT-SOFCs阴极进行了系统的研究，通过X射线衍射表征确定NMO材料呈稳定的立方相结构，并采用电导弛豫方法对其氧离子传导能力进行了研究。发现NMO具有优秀的氧离子传导能力，为其电化学性能提供了保障。对称电池的电化学阻抗谱测试结果表明，800℃时NMO阴极材料的界面电阻值为0.27 Ω·cm^-2，同时作为阳极支撑型SOFC的阴极材料进行放电时的最大功率密度可以达到864.9 mW·cm^-2。上述结果表明，NiMn₂O₄是一种极具潜力的IT-SOFCs阴极材料。     

Preparation and electrocatalytic properties of spinel CoxFe3-xO4 nanoparticles

The catalysts are often used in fuel cells and metal-air batteries to speed up electrochemical reactions. In this study, we prepared CoFe₂O₄ nanoparticles with mainly inverse spinel structure and FeCo₂O₄ nanoparticles with mainly spinel structure as bifunctional catalysts by hydrothermal method. After annealing at 350 °C, pure CoFe₂O₄ and FeCo₂O₄ nanoparticles with uniform size distribution have been obtained. The CoFe₂O₄ nanoparticles showed high current density of 5.5 mA/cm² at −0.8 V in the ORR test. It's low Tafel slope of 83.0 mV/dec further confirmed the excellent ORR catalytic properties of CoFe₂O₄ nanoparticles. Furthermore, the CoFe₂O₄ nanoparticles also showed good OER properties with satisfied current density of 35.7 mV/cm² at l.0 V and low OER Tafel slope of 71.0 mV/dec. Both the ORR and OER properties of CoFe₂O₄ nanoparticles showed good time stability which were compared with FeCo₂O₄ nanoparticles. These results indicated that CoFe₂O₄ nanoparticles with mainly inverse spinel structure had better electrocatalytic performance than FeCo₂O₄ nanoparticles with mainly spinel structure. The CoFe₂O₄ nanoparticles with mainly inverse spinel structure show a significant potential application in rechargeable battery.     

CaO-SiO2-FeO-Cr2O3-MgO-MnO渣系物相组成的热力学计算

摘要：在含铬铁水转炉冶炼过程中，Cr很容易被氧化成Cr2O3进入渣中，并与渣中其他成分反应生成高熔点含铬尖晶石。采用FactSage热力学软件计算了CaO-SiO2-FeO-Cr2O3-MgO-MnO转炉渣系在冶炼温度1300～1700℃下的物相组成，研究了Cr2O3、FeO和碱度对炉渣中尖晶石相含量的影响规律。研究结果表明，温度和渣系成分都会影响炉渣的物相组成。渣系中含有Cr2O3时，物相中均含有MgCr2O4、FeCr2O4和MgFe2O4尖晶石相，尖晶石相的总含量随着Cr2O3和碱度的增加而增加，随着炉温的升高而减少。温度为1300～1500℃时，炉渣中尖晶石含量随着FeO的增加而增加；温度为1500～1700℃时，尖晶石含量随着FeO的增加而略有减少。在温度小于1500℃的转炉冶炼前中期，炉渣物相组成中尖晶石相所占比例较大，易造成化渣不良或者炉渣粘稠，影响转炉冶炼工艺的顺行。     

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.     

Influence of composition and microstructure on transparency and diffusivity in ion-exchangeable spinel glass-ceramics

Ion-exchangeable, transparent spinel glass-ceramics are presented and discussed here for the first time. To retain transparency with increasing crystallinity, spinel glass-ceramics must have uniform crystallization of small (~9 nm) crystallites, not large spherulitic structures comprised of small crystallites. To obtain such a uniform microstructure, the amount of total nucleating agents (ZrO₂ + TiO₂) in the precursor glass composition must be greater than 5 mol%. With small changes in composition and significant differences in microstructure, the demarcation between transparent and opaque glass-ceramics is distinct as is the decrease in K diffusivity during ion-exchange from the transparent (14.7 microns²/h) to the opaque (11.2 microns²/h) compositions. Understanding how to retain transparency during ceramming and increase diffusivity during chemical strengthening is critical in designing materials for many real-world applications. Ion-exchangeable, transparent spinel glass-ceramics are presented and discussed here for the first time. To retain transparency with increasing crystallinity, spinel glass-ceramics must have uniform crystallization of small (~9 nm) crystallites, not large spherulitic structures comprised of small crystallites. To obtain such a uniform microstructure, the amount of total nucleating agents (ZrO₂ + TiO₂) in the precursor glass composition must be greater than 5 mol%. With small changes in composition and significant differences in microstructure, the demarcation between transparent and opaque glass-ceramics is distinct as is the decrease in K diffusivity during ion-exchange from the transparent (14.7 microns²/h) to the opaque (11.2 microns²/h) compositions. Understanding how to retain transparency during ceramming and increase diffusivity during chemical strengthening is critical in designing materials for many real-world applications.     

Thermodynamic calculation and microstructure characterization of spinel formation in MgO–Al2O3–C refractories

In this paper, by simulating the gas phase conditions inside the MgO–Al₂O₃–C refractories during continuous casting process and combining with thermodynamic analysis, as well as SEM analysis, the gas-gas and gas-solid formation of MA spinel were clarified in carbon containing refractories. Thermodynamic calculations showed that gas partial pressure of CO, O₂ and Mg could meet the formation and stable existence conditions of MA spinel in MgO–Al₂O₃–C refractories under service environment, and nitrogen could not affect the formation of MA spinel at 1550 °C in the thermodynamic condition. The formation processes of MA spinel were analyzed experimentally under embedding carbon atmosphere. The carbon-coated alumina powders in MgO–Al₂O₃–C refractories prevented the direct contact between magnesia and alumina. Mg gas was formed by carbon thermal reaction, then reacted with alumina (gas-solid) and gas containing aluminum (gas-gas) to generate MA spinel. Through gas-gas or gas-solid reaction, the formation of MA spinel was effectively controlled. By means of SEM analysis, a two-layer structure with dense outer spinel layer and loose inner layer was formed in MgO–Al₂O₃–C refractories.