Boquan Zhu

The relationship between the pore size distribution and the thermo-mechanical properties of high alumina refractory castables

Abstract A series of high alumina refractory castables were prepared via casting with tabular alumina aggregates (0.088 – 5 mm), fine powders, and calcium aluminate cement as starting materials. The effect of the median diameter of alumina (D50 = 7.26 μm, 5.33 μm, 2.37 μm) on the thermo-mechanical properties of the refractory castables was investigated. The results indicate that the decrease in the alumina particle size from 7.26 μm to 2.37 μm has little influence on both the apparent porosity and bulk density of castables. However, the median pore size of castables fired after 1 600 °C decreased drastically from 4.7 μm to 2.4 μm correspondingly, which led to significant growth in the strength and thermal shock resistance of castables. The cold modulus of rupture and crushing strength were increased by 201 % and 120 %, respectively. At the same time, the hot modulus of rupture and elastic modulus were also increased by 143 % and 127 %, respectively. The residual elastic modulus was enhanced twice after three thermal cycles.

In-situ formation of MgO whiskers and CNTs and their influence on the mechanical properties of low-carbon MgO–C refractory composites

Abstract The influence of the addition of Ni(NO3)2 · 6H2O on the microstructure and mechanical properties of low-carbon MgO–C refractory composites was investigated. The results indicated that MgO whiskers and carbon nanotubes could be generated in low-carbon MgO–C refractory composites with Ni(NO3)2 · 6H2O in a reducing atmosphere at 1 200 °C. The magnitudes of the cold crushing strength, the cold modulus of rupture, the fracture displacement, and the residual cold crushing strength are increased by 47 %, 66 %, 8 % and 26 %, respectively. The MgO–C refractory composites with the addition of 0.6 wt.% Ni(NO3)2 · 6H2O result in better overall properties. The MgO whiskers were generated via a dissolution–diffusion–precipitation mechanism, and their growth is in accordance with the vapour–liquid–solid model.

In-situ catalytic growth of MgAl2O4 spinel whiskers in MgO–C refractories

Abstract Upon application of nano-sized metallic Ni particles as catalyst, the in-situ synthesis mechanism of spinel whiskers in MgO–C refractories was studied. Their phase composition and morphology were determined by means of X-ray diffraction and scanning electron microscopy supported by energy dispersive spectroscopy. The results show that when the catalyst of nano-sized Ni was added in MgO–C refractories, the granular MgAl2O4 (MA) spinels transformed into the shape of whiskers at 1 200 °C. The presence of Ni catalyst can accelerate the generation of Mg vapor, which can react with Al vapor to form MA spinel whiskers. Through dissolution and precipitation, MA spinel crystals nucleate directly and grow into whiskers from the catalytic droplets of nano-sized metallic Ni particles. The growth of spinel whiskers follows a typical vapor–liquid–solid (V–L–S) growth mechanism.

Effect of Colloidal Silica on the Hydration Behavior of Calcium Aluminate Cement

The effect of colloidal silica (CS) on the hydrate phases and microstructure evolution of calcium aluminate cement (CAC) was investigated. Samples hydrated with CS were obtained and characterized by X-ray diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM), Fourier Transform Infrared spectroscopy (FT-IR), hydration heat measurement and Nuclear Magnetic Resonance (NMR). The results revealed that SiO2 nanoparticles may affect the hydrates crystallization process. There was a compact structure in the CAC paste with CS, while petal-shaped hydrates with a porous structure were formed in the pure CAC paste. The maximum value of electrical conductivity for CAC paste with CS suggested that the early stage of hydration for CAC was accelerated. However, the hydration heat curves revealed that the late stage of the CAC hydration process was inhibited, and the hydration degree was reduced, this result was in accordance with Thermogravimetry-Differential scanning calorimetry(TG-DSC) curves. The fitting results of hydration heat curves further showed that the hydration degree at NG (nucleation and crystal growth) process stage was promoted, while it was limited at the phase boundaries stage, and the diffusion stage in the hydration reaction was brought forward due to the addition of CS. According to these results and analyses, the differences in the hydration process for CAC with and without CS can be attributed to the distribution and nucleation effect of SiO2 nanoparticles.

Research Status and Prospect on Vanadium-Based Catalysts for NH3-SCR Denitration

Selective catalytic reduction of NOx with NH3 is one of the most widely used technologies in denitration. Vanadium-based catalysts have been extensively studied for the deNOx process. V2O5/WO3(MoO3)TiO2 as a commercial catalyst has excellent catalytic activity in the medium temperature range. However, it has usually faced several problems in practical industrial applications, including narrow windows of operation temperatures, and the deactivation of catalysts. The modification of vanadium-based catalysts will be the focus in future research. In this paper, the chemical composition of vanadium-based catalysts, catalytic mechanism, the broadening of the temperature range, and the improvement of erosion resistance are reviewed. Furthermore, the effects of four major systems of copper, iron, cerium and manganese on the modification of vanadium-based catalysts are introduced and analyzed. It is worth noting that the addition of modified elements as promoters has greatly improved the catalytic performance. They can enhance the surface acidity, which leads to the increasing adsorption capacity of NH3. Surface defects and oxygen vacancies have also been increased, resulting in more active sites. Finally, the future development of vanadium-based catalysts for denitration is prospected. It is indicated that the main purpose for the research of vanadium-based modification will help to obtain safe, environmentally friendly, efficient, and economical catalysts.

Oxidation resistance and wettability of graphite/SiC composite

Abstract A graphite/SiC composite was synthesized at different calcination temperatures using microsilica and carboxymethylated cellulose. The oxidation resistance and wettability (with water) of graphite/SiC were investigated. The results showed that carboxymethylated cellulose could react with microsilica to form a coating of SiC on the surface of graphite at elevated temperatures. Consequently, SiO2 phase was converted into SiC phase above 1 600 °C. The microstructure of the SiC coating on graphite became denser with the increase in temperature. Thermogravimetric curves revealed that the weight loss of graphite was approximately 97.3 wt.% whereas the value decreased to 29.78 wt.% when SiC was formed. Differential scanning calorimetry analysis showed that the SiC coating decreased the enthalpy of the carbon oxidation reaction from 12.02 kJ g−1 to 1.14 kJ g−1, confirming excellent oxidation resistance. Furthermore, the water contact angle of graphite was approximately 78.5° whereas that of the graphite/SiC composite was reduced to 43°. The study of the formation of graphite/SiC composite showed that SiO2 could be reduced using carboxymethylated cellulose to SiO (g), which was deposited on the graphite to form SiC coating.

Effects of La–Zn substituent and calcination temperature on the microstructure and magnetic properties of Sr-ferrites

Abstract In this study, La–Zn-substituted SrFe12O19 ferrites were synthesized using the traditional ceramic process. The by-products of iron oxide scales from a steel plant were used as the main raw materials. The influence of the La–Zn substituent and the calcination temperature on the microstructure and magnetic properties of Sr1−xLaxFe12−xZnxO19 ferrites was investigated. The results showed that with the increase in the x value, the crystalline lattice constant of the a- and c-axes and the cell volume decreased. There was no α-Fe2O3 phase in the ferrites when the value of x was 0.3. The corresponding saturation magnetization (Ms) and remnant magnetization (Mr) values were, respectively, about 65 emu g−1 and 39.5 emu g−1. Both values of Ms and Mr rise to the maximum value. When the calcination temperature was reduced from 1200°C to 1150°C, the average particle size decreased from 0.9 μm to 0.7 μm and Ms remained at 65 emu g−1. However, the coercivity increased from 2690 Oe to 3100 Oe.

Effect of Nano Metallic Fe on Al 5 O 6 N Synthesis in Al 2 O 3 -C Refractories

Al2O3-C refractory samples were prepared using corundum, flake graphite, Al and Si powders and phenolic resin as raw materials. The in-situ synthesis mechanism of Al5O6N in Al2O3-C refractories was studied and the effect of adding nano metallic Fe on the formation and microstructure of Al5O6N phase was also investigated. Thermodynamic calculations show that Al5O6N can potentially be synthesized in a reducing atmosphere at 1600°C over a narrow range of N2 and CO pressures. Experimental results proved that Al5O6N phase was synthesized after 1600°C heat treatment when nano Fe was added to Al2O3-C refractories. As the content of nano Fe increases, Al5O6N particles show disappearance of oriented growth, with octahedron shapes turning into spherical agglomerations.