铝酸盐长余辉材料的制备方法进展

对碱土金属铝酸盐体系的长余辉荧光材料的光谱特性以及近几年研究较为关注的几种制备方法进行了详细的介绍说明,比较了各材料体系的荧光性能以及各种制备方法的优劣,并对新的合成方法进行了展望。     

Processing of Amorphous Calcium Aluminate Powders at < 900°C

Aluminum chelate and calcium nitrate precursors were used to synthesize amorphous calcium aluminate powders by solgel processing at < 900°C. The method of preparation and results of characterization of the gels by thermogravimetric analysis, X-ray diffraction, scanning Auger microscopy, and single-point BET analysis are presented. An optimum heat-treatment schedule consisting of heating the gel to 900°C at 5°C/min and holding for 16 h was developed to produce highly reactive, X-ray amorphous calcium aluminate powders.     

Development of magnetic recyclable spinel photocatalysts with enhanced sunlight-driven degradation of industrial dyes

Nanocrystalline nickel and copper-substituted zinc aluminate spinel powders (Ni_xZn_1−xAl₂O₄ and Cu_xZn_1−xAl₂O₄) with different additional ion concentrations (0 ≤ x ≤ 1) were successfully synthesized by the sol-gel auto combustion method using diethanolamine (DEA) as a fuel. The structures, chemical bonds, morphologies, composition, surface area, and optical properties including the magnetic behavior of the obtained samples were investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), the Brunauer-Emmett-Teller (BET) method, UV-visible diffuse reflectance spectroscopy (UV-DRS), and vibrating sample magnetometer (VSM). All characterization results confirmed that a single-phase spinel structure was obtained for all calcined aluminate powders with various crystallite sizes and lattice constants. The band gap energy (E_g) of all modified aluminates is in the range of 2.99-3.15 eV, which was found to be much lower than that of the pure sample (5.60 eV). These results indicate that the Ni²⁺ and Cu²⁺-substituted ZnAl₂O₄ samples could be effectively photoexcited by both the ultraviolet and visible light. Evaluation of the samples to determine the photocatalytic activity was carried out through investigation of the way the four main pollutant types decompose when irradiated by sunlight. These pollutants were rhodamine B (RhB), methylene blue (MB), methyl orange (MO), and methyl red (MR). For all optimum samples of organic dyes, the efficiency of photocatalytic degradation achieved 96% by the end of 150 min. Furthermore, each of the modified photocatalysts could be reused and showed a high degree of stability. According to magnetic measurements, the S-shaped curve of ferrimagnetism can arise in those samples with the optimum concentration, although pure ZnAl₂O₄ exhibits diamagnetic properties. In comparison to pure ZnAl₂O₄, the modified samples exhibit high enhancement in the remanent magnetization (M_r), which indicates that it is easy to recover those magnetic photocatalyst through the use of an external magnetic field application. These findings therefore serve as a strong indication that ZnAl₂O₄ powders substituted by both Ni²⁺ and Cu²⁺ may offer the capability to serve in environmentally beneficial harvesting of solar energy.     

Crystal growth and phase formation of high-entropy rare-earth monoclinic aluminates

For the first time, high-entropy rare-earth monoclinic aluminate crystals were grown via directional solidification using the micro-pulling-down method. Five high-entropy compositions were formulated with a general formula RE₄Al₂O₉, where RE is an equiatomic mixture of five rare-earth elements. The rare-earth elements included were Lu, Yb, Er, Y, Ho, Dy, Tb, Gd, Eu, Sm, Nd, and La. High-temperature powder X-ray diffraction and Rietveld structure refinement indicated that all crystals were a single monoclinic phase and that rare-earth average ionic radius did not affect phase purity. At room temperature, the refined lattice parameters increased consistently with increasing average ionic radii of the five compositions. One of the crystals had a typical high-temperature phase transition of single-RE RE₄Al₂O₉ in the range of 1100–1150°C, which consisted of a lattice contraction upon heating. Differential scanning calorimetry indicated a thermal event corresponding to that phase transition. Electron probe microanalysis revealed Al-rich inclusions on the surface of the crystals. Crystals containing Tb had dark surface features that became lighter after annealing in a reducing atmosphere, which indicated that Tb⁴⁺ may be responsible for the dark features.     

Thermoanalytical and infrared spectroscopy investigations of some mineral pastes containing organic polymers

The interaction in the presence of water between organic polymers [two polyvinyl alcohol acetate (PVA1 and PVA2)-, one polyvinyl acetate (PVAc)-, and one acryloethyl-metacrylomethyl (R)-type copolymers] and inorganic materials (3CaO·Al₂O₃+CaSO₄·2H₂O, 12CaO·7Al₂O₃, and CaO·Al₂O₃) was investigated using complex thermal analyses and infrared (IR) spectroscopy. The DTA curves of the 1-to 28-day-hydrated organo-mineral composite samples showed some significant differences in comparison with those of the hydrated inorganic materials. The most important differences consist in the presence of some exothermic peaks that are not attributable to the pure organic polymers. More than that, none of the specific effects of the pure organic polymers are evidenced on the DTA curves of the hydrated organo-mineral composite materials. This, in connection with their IR spectra, which clearly evidence the disappearance of some investigated polymers specific IR bands, can be considered as an indirect evidence of the cross-linking of polymer chains via metal ions.     

Cathodoluminescence imaging—the study of yttrium aluminates distribution in aluminum nitride

Cathodoluminescence imaging is a technique which may combine a high-resolution spectroscopic method with a high-spatial resolution electron imaging technique to characterize chemical identity as well as physical size, shape, and spatial distribution of the materials which have cathodoluminescence. Cathodoluminescence imaging has been used to characterize yttrium aluminates in aluminum nitride ceramics. By digitally combining cathodoluminescence and backscat-tering electron images, it is possible not only to distinguish between different yttrium aluminates, but also to determine the size, shape, and spatial distribution of these phases. This information provides a route to understanding the local residual oxygen concentration before sintering, the amount of sintering aid to be added, and possible sintering schedule. Furthermore, this information may be related to other properties of aluminum nitride such as residual oxygen concentration and thermal conductivity.     

Synthesis of NiAl2O4 with high surface area as precursor of Ni nanoparticles for hydrogen production

NiAl₂O₄ with high surface area was synthesized by the combustion method, evaluating the effect of urea/nitrate (U/N) ratio. The use of a stoichiometric U/N ratio resulted in a material with high surface area and homogeneous nanocrystallites, while the excess of fuel resulted in a non-porous material with low surface area. The formation of superficial Ni nanoparticles resulted in excellent stability on CO₂ methane reforming. This can be attributed to the rearrangement of nickel in the aluminate matrix and the migration of nickel particles through carbon filaments at the surface during the activation process.     

Influence of fabrication conditions on the structural characteristics and the magnetic properties of FeAl2O4

We report a systematic evaluation of the magnetic properties of FeAl₂O₄ focusing on the relationship between the fabrication conditions and its structural characteristics, in order to improve ceramics processing in applications of this material. For this purpose, the most important factor to control is the inversion parameter, expressed as y in the (Fe_1−yAl_y)(Al_1−y/2Fe_y/2)₂O₄ composition, which is relatively high for the spinel aluminate of a transition metal. The magnetic properties of these samples all show the spin glass phenomenon at low temperatures, and the cusp temperature depends systematically on this y value. This means that the evaluation of these magnetic properties will be an effective way to predict some characteristics of product FeAl₂O₄. Additionally, this study found an anomaly in the structural and magnetic characteristics of FeAl₂O₄ fabricated at a low temperature. This is thought to originate in a tiny and a small amount of impurity. It will be key for discussing the quality of chemically synthesized FeAl₂O₄, which is typically produced at low temperatures.     

Deterioration in the Final-Stage Sintering of Magnesium Aluminate Spinel

Sintering of magnesium aluminate spinel of the MgO-excess, stoichiometric, and Al₂O₃-excess compositions has been investigated under vacuum and in air for the effect of low oxygen partial pressures. Densification enhancement of the surface layer is due to MgO evaporation which generates oxygen vacancies in the host crystal lattice. Regions of different grain sizes are observed from samples sintered under both conditions. Microstructural features of pairwise breakup of particle chains representing differential sintering are characteristic of the less-densified sample interior. The densification improved initially and yet was retarded in the intermediate sintering stage when the density exceeded 75% with vacuum-sintering owing to MgO evaporation.     

Effect of Rapid Solidification on Microstructural Evolution in MgO–MgAl2O4

The response of the MgO–20 and 30 mol% Al₂O₃ compositions to rapid solidification has been studied. The oxides were twin-roller quenched and the resulting flakes were characterized by X-ray diffraction and transmission electron microscopy. The results indicate that metastable extensions of spinel and periclase occurred and the microstructural pathway was determined from the final microstructure. The flakes having MgO–20 mol% Al₂O₃ show a dendritic structure consisting of periclase and spinel. In the MgO–30 mol% Al₂O₃ composition, the liquid transforms to spinel partitionlessly. The spinel is believed to undergo decomposition by a modulation in composition, and the resulting microstructure consists of spinel and periclase. Kinetic and thermodynamic aspects of phase selection have been rationalized based on the metastable extensions of the different phase fields in the phase diagram. It has been proposed that composition fluctuations in spinel are stabilized because of the formation of disordered phases with a continuous range of order parameter on the tetrahedral sublattice.