Reaction mechanism and microstructure evolution of reaction sintered <Emphasis Type="Italic">h</Emphasis>-BN

Hexagonal boron nitride ceramic (h-BN) based on the nitridation of B powders was obtained by reaction sintering method. The effects of sintering temperature on the mechanical properties and microstructure of the resultant products were investigated and the reaction mechanism was discussed. Results showed that the reaction between B and N₂ occurred vigorously at temperatures ranging from 1 000 °C to 1 300 °C, which resulted in the generation of t-BN. When the temperature exceeded 1 450 °C, transformation from t-BN to h-BN began to occur. As the sintering temperature increased, the spherical particles of t-BN gradually transformed into fine sheet particles of h-BN. These particles subsequently displayed a compact arrangement to achieve a more uniform microstructure, thereby increasing the strength.     

Numerical simulation of the multicomponent mass transfer during Bridgman growth of CdZnTe crystal using Maxwell-Stefan diffusion model

To reveal the complicated mechanism of the multicomponent mass transfer during the growth of ternary compound semiconductors, a numerical model based on Maxwell-Stefan equations was developed to simulate the Bridgman growth of CdZnTe crystal. The Maxwell-Stefan diffusion coefficients in the melt were estimated. Distributions of Zn, Cd, and Te were calculated with variable ampoule traveling rate and diffusion coefficients. The experimental results show that Zn in melt near the growth interface decreases and diffuses from the bulk melt to the growth interface. For Cd, the situation is just the opposite. The coupling effects of Zn and Cd diffusions result in an uphill diffusion of Te at the beginning of the growth. Throughout the growth, the concentration of Te in the melt keeps low near the growth interface but high far from the growth interface. Increasing the ampoule traveling rate will aggravate the segregation of Zn and Cd, and hence deteriorate the uniformity of Te. We also find that not only the diffusion coefficients but also the ratios between them have significant influence on the species diffusions.     

Diffusion characteristic of sulfate anion in surface region of cement mortar at early stage

The SO_4~(2-)concentration distributions in surface region of cement mortar immersed in sulfate solution at early stage were measured by layered sampling method combined with chemical analysis, and the diffusion coefficients of SO_4~(2-)anions in surface region of mortar into internal area were calculated by means of instantaneous plane diffusion theory. The experimental results showed that the SO_4~(2-)concentration gradually reduced when the diffusion depth increased in the surface region of mortar. Diffusion coefficient(D) was relevant with the concentration and kind of environmental sulfate solution, which reduced with immersion time at the beginning, and then rose slowly after a period of time. The calculation of initial diffusion coefficient(D_0) and starting time of deterioration(t_∞) caused by sulfate attack was further attempted based on the data of diffusion coefficient, and it was found that D_0 and t∞ were all relevant with concentrations of sulfate and different kind of sulfate as well.     

Effects of CaO and Na<Subscript>2</Subscript>CO<Subscript>3</Subscript> on the reduction of high silicon iron ores

The effects of CaO and Na₂CO₃ on the reduction of high silicon iron ores at 1 250 °C were studied. The experimental results showed that the metallization rate was significantly hindered by the addition of CaO and Na₂CO₃, particularly at the early stage of roasting, compared to the rate without additives. In the absence of additives, iron oxides were quickly reduced to metallic iron, and fayalite was difficult to form. When CaO and Na₂CO₃ were added, the low reducible iron-containing silicate compounds formed and melted, subsequently retarding the metallization process. The inhibition of Na₂CO₃ was more noticeable than that of CaO, and higher Na₂CO₃ doses resulted in stronger inhibition of the increased metallization rate. However, when Na₂CO₃ was added prior to CaO, the liquid phase formed, which facilitated the growth of the metallic phase. To reinforce the separation of the metallic phase and slag, an appropriate amount of liquid phase generated during the reduction is necessary. It was shown that when 10% CaO and 10% Na₂CO₃ were added, a high metallization rate and larger metallic iron particles were obtained, thus further decreasing the required Na₂CO₃ dosage.     

Mineralogy and thermal analysis of natural pozzolana opal shale with nano-pores

Thermodynamic stability, microvoid distribution and phases transformation of natural pozzolana opal shale(POS) were studied systematically in this work. XRD analysis showed that opal-CT, including microcrystal cristobalite and tridymite, is a major component of POS. DTA and FT-IR indicated that there were many hydroxyl groups and acid sites on the surface of amorphous SiO_2 materials. FE-SEM analysis exhibited amorphous SiO_2 particles(opal-A) covering over stacking sequences microcrystal cristobalite and tridymite. Meanwhile, MIP analysis demonstrated that porosity and pore size distribution of POS remained uniform below 600 ℃. Because stable porous microstructure is a key factor in improving photocatalyst activity, POS is suited to preparing highly active supported.     

Wear behavior of <Emphasis Type="Italic">in-situ</Emphasis> TiC particles reinforced aluminum matrix composite

The microstructure, tensile property and wear resistance of 7075 aluminum matrix composite reinforced with TiC particles prepared by in-situ reaction casting were investigated. The effect of TiC reinforcement on wear behavior was analyzed. The wear mechanism was also discussed. A micro-mechanism model of reaction kinetics for synthesis of TiC was acquired. Results show that TiC could increase the tensile and yield strength, but decrease the elongation. Besides, TiC particles improve the property of wear resistance of 7075 aluminum alloy. The wear mechanisms include abrasive wear and adhesive wear in wear test process.     

Solvothermal synthesis and electrochemical properties of octahedral cobalt oxide decorated with Ag<Subscript>2</Subscript>O

Octahedral CoO with nanostructures decorated with Ag nanoparticles was prepared via a facile solvothermal approach. After being annealed at 500 °C for 1 h, an electrochemical capacitor material of Co₃O₄ decorated with Ag₂O was obtained. The cyclic voltammetry and galvanostatic charge-discharge were used to evaluate the electrochemical properties of the as-prepared products. The results indicated that the as-prepared samples exhibited fine pseudo-capacitive performance, and the surface modifications of Ag₂O can significantly increase the capacitance of the Co₃O₄ material. The specific capacitance of Ag₂O/Co₃O₄ composite electrode was up to 217.6 F·g^?1, which was 3.35 times as high as that of pure Co₃O₄. Moreover, Ag₂O/Co₃O₄ composite showed an excellent cycle performance, and 65.3% of specific capacitance was maintained after 200 cycles.     

Photocatalytic activity of porous magnesium oxychloride cement combined with AC/TiO<Subscript>2</Subscript> composites

As a decorative material, magnesium oxychloride cement was used as a photocatalyst supporter to purify the pollutants indoors. Due to excellent adsorption properties of activated carbon (AC), the photocatalytic composties, TiO₂/AC, were prepared and introduced into the porous magnesium oxychloride cement (PMOC) substrate to composite a sort of photocatalytic cementitious material (PCM). The optimal composite processes were assessed by gas chromatograph, using toluene as the target. By comparing the perspective of toluene purification and thorough decomposition, it can be found that the optimal mass ratio for TiO₂/AC composites is 4/25, and the heat treatment to TiO₂/AC sample at 350 °C can play the optimal synergetic role of adsorbents in photocatalytic process. The synergistic effect of TiO₂, AC and magnesium oxychloride cement (MOC) was also evaluated by gas chromatograph. One-take molding process was adopted to introduce the TiO₂/AC into PMOC substrate, and its optimal mass fraction was 4 wt%, while the appropriate density of substrate was 0.35 g/cm3. Toluene degradation showed that the prepared PCM can degrade pollutants efficiently. The appropriate treatment process of TiO₂/AC, mass of TiO₂/AC, substrate density, and stable pore structure should be coordinated to maximize the adsorption-photodegradation performance. The combination of photocatalytic materials, adsorbents, and building materials provided a new idea for the application of photocatalysis.     

Effect of friction reclaimed materials of waste brake-shoe on basic performance of mortar

Discarded train brake shoes mainly consist of steel-backed friction material. To be better reutilized, its essential features and its interaction in cement-based material need to be studied. Consequently, particle size analysis, SEM, IR and TGA were used to investigate two types of waste brake shoes, i e, mechanical grinding friction reclaimed material of waste brake-shoe (G-FRMWBS) and pyrolysis-friction reclaimed materials of waste brake-shoe (P-FRMWBS). The latter exhibited less organic content, larger range of particle size distribution and smaller medium particle diameter. Both types contained inorganic particles of spherical and irregular shapes, striped with steel fiber. Upon isometric substituting fine aggregates, G-FRMWBS lifted the strength of mortar effectively that was increased by 16.6% and 17.5% when the replacing rate was 5%; the value went up to 19.2% and 19.2% when the replacing rate was 10%. Moreover, inclusion of FRMWBS enhanced the chloride penetration resistance, and optimized the pore characteristic and ITZ (interfacial transition zone) as well.     

Microstructure and mechanical properties of Nb-B bearing low carbon steel plate: Ultrafast cooling <Emphasis Type="Italic">versus</Emphasis> accelerated cooling

The microstructure and mechanical properties of low carbon bainite high strength steel plate were studied via different cooling paths at the pilot scale. There was a significant increase in mechanical properties, and notably, the yield strength, tensile strength, and toughness at -40 °C for the tested steel processed by ultra-fast cooling were 126 MPa, 98 MPa and 69 J, respectively, in relation to steel processed by accelerated cooling. The ultra-fast cooling rate not only refined the microstructure, precipitates, and martensiteaustenite (M/A) islands, but also contributed to the refinement of microstructure in thick plates. The large size M/A constituents formed at lower cooling rate experienced stress concentration and were potential sites for crack initiation, which led to deterioration of low-temperature impact toughness. In contrast, the acicular ferrite and lath bainite with high fraction of high-angle grain boundaries were formed in steel processed by ultra-fast cooling, which retarded cleavage crack propagation.