Jialin Sun

Morphology of α-Si3N4 in Fe–Si3N4 prepared via flash combustion

The state and formation mechanism of α-Si3N4 in Fe–Si3N4 prepared by flash combustion were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy. The results indicate that α-Si3N4 crystals exist only in the Fe–Si3N4 dense areas. When FeSi75 particles react with N2, which generates substantial heat, a large number of Si solid particles evaporate. The product between Si gas and N2 is a mixture of α-Si3N4 and β-Si3N4. At the later stage of the flash combustion process, α-Si3N4 crystals dissolve and reprecipitate as β-Si3N4 and the β-Si3N4 crystals grow outward from the dense areas in the product pool. As the temperature decreases, the α-Si3N4 crystals cool before transforming into β-Si3N4 crystals in the dense areas of Fe–Si3N4. The phase composition of flash-combustion-synthesized Fe–Si3N4 is controllable through manipulation of the gas-phase reaction in the early stage and the α→β transformation in the later stage.

Reaction behavior of trace oxygen during combustion of falling FeSi75 powder in a nitrogen flow

To explore the reaction behavior of trace oxygen during the flash combustion process of falling FeSi75 powder in a nitrogen flow, a flash-combustion-synthesized Fe-Si3N4 sample was heat-treated to remove SiO2. The samples before and after the treatment were investigated by X-ray diffraction, scanning electron microscopy, and transmission electron microscopy, and the formation mechanism of SiO2 was investigated. The results show that SiO2 in the Fe-Si3N4 is mainly located on the surface or around the Si3N4 particles in dense areas, existing in both crystalline and amorphous states; when the FeSi75 particles, which are less than 0.074 mm in size, fell in up-flowing hot N2 stream, trace oxygen in the N2 stream did not significantly hinder the nitridation of FeSi75 particles as it was consumed by the surface oxidation of the generated Si3N4 particles to form SiO2. At the reaction zone, the oxidation of Si3N4 particles decreased the oxygen partial pressure in the N2 stream and greatly reduced the opportunity for FeSi75 particles to be oxidized into SiO2; by virtue of the SiO2 film developed on the surface, the Si3N4 particles adhered to each other and formed dense areas in the material.

Influence of Microstructure on Formation of Deterioration Layer in Periclase-Hercynite Bricks

The microstructure of the original layer and the cement melt-penetrated layer of a used periclase-hercynite brick from a cement rotary kiln with a daily output of 5000 tons for 12 months was studied by XRD, SEM, EDS, and a mercury porosimeter. The results show that the cation diffusion between hercynite and periclase particles in the brick at high temperatures decreases the pore size of the brick. The pore size in the original layer is located mainly in the range of 4 – 20 μm; the decreased pore size increases the penetration resistance of the cement melt to the inside of the brick and makes the cement melt react with the pore walls better. The components of the matrix pore walls such as MgO and Al2O3 dissolve in the cement melt, enhancing the hot properties of the penetrated melt, decreasing the penetration depth, and slowing the formation of the deterioration layer. The pore structure and the element distribution endow the brick with good thermal shock resistance.

Understanding the relationship between dopant and ionic transport in yttria-doped ceria-zirconia

Probing the underlying conduction mechanism with a reliable and detailed description is important for optimizing solid oxide electrolyte materials. Herein, an equation has been proposed for the first time to quantitatively predict the vacancy preference for a given cation in a fluorite structure. The results derived from the equation are consistent with NMR observations. By investigating the local structure and conductivity in CeO2-ZrO2-Y2O3 ternary system from both molecular dynamics simulations and experiments, we have developed a theoretical framework to elucidate the mechanism of the dopant properties on ion transport in yttria-doped ceria-zirconia system. Lower vacancy preference mismatch between the host and a dopant is found to give rise to a better homogeneity of vacancy distribution, indicated by less vacancy cluster formation thereby resulting in a higher conductivity. This comprehension concerning the dopant properties-conductivity correlation should help in the development of more effective and selective oxide electrolyte for the fuel cells and other applications.

One step sintering of homogenized bauxite raw material and kinetic study

A one-step sintering process of bauxite raw material from direct mining was completed, and the kinetics of this process was analyzed thoroughly. The results show that the sintering kinetics of bauxite raw material exhibits the liquid-phase sintering behavior. A small portion of impurities existed in the raw material act as a liquid phase. After X-ray diffraction analyses, scanning electron microscopy observations, and kinetics calculations, sintering temperature and heating duration were determined as the two major factors contributing to the sintering process and densification of bauxite ore. An elevated heating temperature and longer duration favor the densification process. The major obstacle for the densification of bauxite material is attributed to the formation of the enclosed blowhole during liquid-phase sintering.

3D nano-arrays of silver nanoparticles and graphene quantum dots with excellent surface-enhanced Raman scattering

ABSTRACT Active surface-enhanced Raman scattering (SERS) substrates, 3D nano-arrays of Ag nanoparticles (NPs) and graphene quantum dots (GQDs), were prepared using a photochemical approach and an electrophoresis deposition technique with the formation mechanism addressed. The GQDs (ca. 6 nm average) fit into the inter-particle gaps between Ag NPs, as verified by their scanning electron microscopy and high-resolution transmission electron microscopy. This deliberately designed 3D assembly of Ag NPs and GQDs could promote the synergistic effects of both components to further enhance the SERS performances according to both electromagnetic mechanism and chemical mechanism. Preliminary experiments show that the 3D substrates exhibited strong SERS signals comparing with bare Si substrates. This work provides a promising way for 3D SERS substrates.

Shear-thickening behavior of Fe-ZSM5 zeolite slurry and its removal with alumina/boehmites

A cryogenic scanning electron microscopy (cryo-SEM) technique was used to explore the shear-thickening behavior of Fe-ZSM5 zeolite pastes and to discover its underlying mechanism. Bare Fe-ZSM5 zeolite samples were found to contain agglomerations, which may break the flow of the pastes and cause shear-thickening behaviors. However, the shear-thickening behaviors can be eliminated by the addition of halloysite and various boehmites because of improved particle packing. Furthermore, compared with pure Fe-ZSM5 zeolite samples and its composite samples with halloysite, the samples with boehmite (Pural SB or Disperal) additions exhibited network structures in their cryo-SEM images; these structures could facilitate the storage and release of flow water, smooth paste flow, and avoid shear-thickening. By contrast, another boehmite (Versal 250) formed agglomerations rather than network structures after being added to the Fe-ZSM5 zeolite paste and resulted in shear-thickening behavior. Consequently, the results suggest that these network structures play key roles in eliminating the shear-thickening behavior.

A new synthetic route to MgO–MgAl2O4–ZrO2 highly dispersed composite material through formation of Mg5Al2.4Zr1.7O12 metastable phase: synthesis and physical properties

Mg5Al2.4Zr1.7O12 metastable phase was successfully synthesized from analytical-grade MgO, α-Al2O3, MgAl2O4, and ZrO2 under an N2 atmosphere. The sintering temperature was varied from 1650 to 1780°C, and the highest amount of Mg5Al2.4Zr1.7O12 appeared in the composite material when the sintering temperature was 1760°C. According to our research of the formation mechanism of Mg5Al2.4Zr1.7O12, the formation and growth of MgAl2O4 dominated when the temperature was not higher than 1650°C. When the temperature was higher than 1650°C, MgO and ZrO2 tended to diffuse into MgAl2O4 and the Mg5Al2.4Zr1.7O12 solid solution was formed. When the temperature reached 1760°C, the formation of Mg5Al2.4Zr1.7O12 was completed. The effect of MgAl2O4 spinel crystals was also studied, and their introduction into the composite material promoted the formation and growth of Mg5Al2.4Zr1.7O12. A highly dispersed MgO–MgAl2O4–ZrO2 composite material was prepared through the decomposition of the Mg5Al2.4Zr1.7O12 metastable phase. The as-prepared composite material showed improved overall physical properties because of the good dispersion of MgO, MgAl2O4, and ZrO2 phases.

A Three-Dimensional Porous Conducting Polymer Composite with Ultralow Density and Highly Sensitive Pressure Sensing Properties

An ultralight conducting polyaniline/SiC/polyacrylonitrile PANI/SiC/PAN composite was fabricated by in situ polymerization of aniline monomer on the surface of fibers in SiC/PAN aerogel. The SiC/PAN aerogel was obtained by electrospinning, freeze-drying, and heat treatment. The ingredient, morphology, structure, and electrical properties of the aerogel before and after in situ polymerization were investigated by X-ray powder diffraction XRD, Fourier transform infrared spectroscopy FT-IR, scanning electron microscope SEM, and voltage-current characteristic measurement. The thermostability of PANI/SiC/PAN composite was investigated by thermogravimetric analysis TGA and electrical resistance measured at different temperatures. The density of the PANI/SiC/PAN composite was approximately 0.211 g cm−3, the porosity was 76.44%, and the conductivity was 0.013 S m−1. The pressure sensing properties were evaluated at room temperature. The electrical resistance of as-prepared sample decreased gradually with the increase of pressure. Furthermore, the pressure sensing process was reversible and the response time was short about 1 s. This composite may have application in pressure sensor field.