Effects of heat treatment on the microstructure of amorphous boron carbide coating deposited on graphite substrates by chemical vapor deposition

A two-layer boron carbide coating is deposited on a graphite substrate by chemical vapor deposition from a CH4/BCl3/H-2 precursor mixture at a low temperature of 950 degrees C and a reduced pressure of 10 KPa. Coated substrates are annealed at 1600 degrees C, 1700 degrees C, 1800 degrees C, 1900 degrees C and 2000 degrees C in high purity argon for 2 h, respectively. Structural evolution of the coatings is explored by electron microscopy and spectroscopy. Results demonstrate that the as-deposited coating is composed of pyrolytic carbon and amorphous boron carbide. A composition gradient of B and C is induced in each deposition. After annealing, B4C crystallites precipitate out of the amorphous boron carbide and grow to several hundreds nanometers by receiving B and C from boron-doped pyrolytic carbon. Energy-dispersive spectroscopy proves that the crystallization is controlled by element diffusion activated by high temperature annealing, after that a larger concentration gradient of B and C is induced in the coating. Quantified Raman spectrum identifies a graphitization enhancement of pyrolytic carbon. Transmission electron microscopy exhibits an epitaxial growth of B4C at layer/layer interface of the annealed coatings. Mechanism concerning the structural evolution on the basis of the experimental results is proposed. (C) 2010 Elsevier B.V. All rights reserved.National Natural Science Foundation of China [50532010, 90405015

An Experimental Study of Gas Flow Regime and Pressure Drop in a Random Packed Bed with Sinter Particles

The gas flow regime and pressure drop in a gas–solid packed bed with irregularly shaped sinter particles were experimentally investigated. Using a self-made experimental facility for data measurement, the gas flow pressure drop in a sinter bed layer was determined for various experimental conditions. According to the changes in the modified coefficients, α and β, for different flow regimes, the flow transitions from one regime to another in packed beds with different particle diameters were described. Furthermore, the pressure drop correlations for different flow regimes were determined, and the reliabilities of the obtained correlations were contrastively analyzed. The results show that, when the particle diameter is constant, the modified pressure drop per unit height, ΔP/Hu, increases linearly with the increasing gas superficial velocity. When the gas superficial velocity is larger than 1.15 m/s under atmospheric conditions, the gas flow regime in the sinter bed layer is the turbulent flow. Compared with the experimental correlation of the whole flow, the pressure drop correlations obtained by the piecewise fitting method provided a better prediction of the experimental values, and the average deviations of the obtained correlations for the Forchheimer flow and the turbulent flow were 5.31% and 4.07%, respectively

An Experimental Study of Gas Flow Regime and Pressure Drop in a Random Packed Bed with Sinter Particles

The gas flow regime and pressure drop in a gas–solid packed bed with irregularly shaped sinter particles were experimentally investigated. Using a self-made experimental facility for data measurement, the gas flow pressure drop in a sinter bed layer was determined for various experimental conditions. According to the changes in the modified coefficients, α and β, for different flow regimes, the flow transitions from one regime to another in packed beds with different particle diameters were described. Furthermore, the pressure drop correlations for different flow regimes were determined, and the reliabilities of the obtained correlations were contrastively analyzed. The results show that, when the particle diameter is constant, the modified pressure drop per unit height, ΔP/Hu, increases linearly with the increasing gas superficial velocity. When the gas superficial velocity is larger than 1.15 m/s under atmospheric conditions, the gas flow regime in the sinter bed layer is the turbulent flow. Compared with the experimental correlation of the whole flow, the pressure drop correlations obtained by the piecewise fitting method provided a better prediction of the experimental values, and the average deviations of the obtained correlations for the Forchheimer flow and the turbulent flow were 5.31% and 4.07%, respectively

Energy and Exergy Efficiency Analysis of Fluid Flow and Heat Transfer in Sinter Vertical Cooler

In order to fully understand the energy and exergy transfer processes in sinter vertical coolers, a simulation model of the fluid flow and heat transfer in a vertical cooler was established, and energy and exergy efficiency analyses of the gas–solid heat transfer in a vertical cooler were conducted in detail. Based on the calculation method of the whole working condition, the suitable operational parameters of the vertical cooler were obtained by setting the net exergy efficiency in the vertical cooler as the indicator function. The results show that both the quantity of sinter waste heat recovery (SWHR) and energy efficiency increased as the air flow rate (AFR) increased, and they decreased as the air inlet temperature (AIT) increased. The increase in the sinter inlet temperature (SIT) resulted in an increase in the quantity of SWHR and a decrease in energy efficiency. The air net exergy had the maximum value as the AFR increased, and it only increased monotonically as the SIT and AIT increased. The net exergy efficiency reached the maximum value as the AFR and AIT increased, and the increase in the SIT only resulted in a decrease in the net exergy efficiency. When the sinter annual production of a 360 m2 sintering machine was taken as the processing capacity of the vertical cooler, the suitable operational parameters of the vertical cooler were 190 kg/s for the AFR, and 353 K for the AIT

Characteristics and anticorrosion performance of Fe-doped TiO2 films by liquid phase deposition method

Fe-doped TiO2 thin films were fabricated by liquid phase deposition (LPD) method, using Fe(III) nitrate as both Fe element source and fluoride scavenger instead of commonly-used boric acid (H3BO 3). Scanning electron microscopy (SEM), X-ray diffraction (XRD), and UV-vis spectrum were employed to examine the effects of Fe element on morphology, structure and optical characteristics of TiO2 films. The as-prepared films were served as photoanode applied to photogenerated cathodic protection of SUS304 stainless steel (304SS). It was observed that the photoelectrochemical properties of the as-prepared films were enhanced with the addition of Fe element compared to the undoped TiO2 film. The highest photoactivity was achieved for Ti13Fe (Fe/Ti = 3 molar ratio) film prepared in precursor bath containing 0.02 M TiF4 + 0.06 M Fe(NO 3)3 under white-light illumination. The effective anticorrosion behaviors can be attributed to the Fe element incorporation which decreases the probability of photogenerated charge-carrier recombination and extends the light response range of Fe-doped TiO2 films appeared to visible-light region. ? 2014 Elsevier B.V

Photogenerated cathodic protection of stainless steel by liquid-phase-deposited sodium polyacrylate/TiO2 hybrid films

Sodium polyacrylate/TiO2 hybrid films that served as photoanodes for cathodic protection application were prepared by liquid phase deposition. Under white-light illumination, the open-circuit potential of the hybrid films coupled with SUS304 stainless steel could shift to a more negative value and offer an effective photogenerated cathodic protection for stainless steel. Moreover, the hybrid films also exhibited stronger photocurrents in both the ultraviolet-light and visible-light regions compared to that of control TiO2 films. In summary, the addition of sodium polyacrylate could greatly improve the photogenerated cathodic protection properties of the liquid-phase-deposited TiO2 films. (C) 2012 Elsevier Ltd. All rights reserved

Preparation and characterization of freestanding SiC(Ti, B) films derived from polycarbosilane with TiN and B as additives

Freestanding SiC(Ti, B) films with high temperature resistance were fabricated from polymer precursor of polycarbosilane (PCS) blended with 0.26 wt% TiN and 0.74 wt% B powders. Results reveal that SiC(Ti, B) films with good mechanical properties are uniform and dense. After high temperature annealing at 1500 degrees C in argon, SiC(Ti, B) films exhibit better high temperature resistance as compared to SiC films without additives, which implies their potential applications in ultra-high temperatures (exceeding 1500 degrees C) microelectromechanical systems (MEMS). Sintering additives are effective in suppressing the growth of SiC crystals and decreasing the content of oxygen and free carbon, which is normally beneficial to enhance high temperature resistance of films. (C) 2012 Elsevier Ltd. All rights reserved.Science and Technology Program of Fujian Province [2009H0038]; Fundamental Research Funds for the Central Universities [2010121053]; Aeronautical Science Foundation of China [2011ZF68004]; Specialized Research Fund for the Doctoral Program of Higher Education [20110121120033

Oxidation behavior of Hi-Nicalon SiC monofilament fibres in air and O-2-H2O-Ar atmospheres

An equipment including aircraft engine simulated environmental system and sample loading stage was manufactured for annealing Hi-Nicalon monofilament fibres in corrosive environments. The fibres were exposed in air and 14% H2O/8% O-2/78% Ar atmospheres at 1200 degrees C for 1-100 h, respectively. Experimental results indicate that tensile strength decreases, whereas oxide layer thickness and grain size of SiO2 increase with rising annealing time. The specimens show brittle/flat fracture, most of cracks initiate at the interface between SiC and SiO2 region of fibres. The control effect of water vapor on formation and structural evolution of the passive film was discussed. (C) 2011 Elsevier Ltd. All rights reserved.Science and Technology Program of Fujian Province [2009H0038]; Fundamental Research Funds for the Central Universities [201012 1053]; Aeronautical Science Foundation of China [2011ZF68004]; Specialized Research Fund for the Doctoral Program of Higher Education [20110121120033

Determination of Pressure Drop Correlation for Air Flow through Packed Bed of Sinter Particles in Terms of Euler Number

In order to clearly understand the air flow resistance characteristics in vertical tanks for sinter waste heat recovery in the steel industry, experimental research on the air flow pressure drop (FPD) performance in a sinter bed layer (BL) was conducted. Based on a self-made experimental device, the measurement values of air FPD for different experimental conditions were determined firstly, and then the concept of Euler number (Eu) in heat exchangers was introduced into the study of air FPD in BL; the change rules of Eu under different particle diameters were analyzed. Finally, the air FPD correlation in sinter BL was obtained and described in the form of Eu, and the error analysis of obtained air FPD correlation was performed. The results show that, the air FPD increases as a second power relationship with the increase in air superficial velocity when the particle diameter is constant. The decrease amplitude of Eu gradually dwindles when increasing the Reynolds number (Re), and the decrease in the Eu shows a reciprocal relationship with the Re. As the bed geometry factor increases, the FPD coefficient, A, decreases as an exponential relationship, while the FPD coefficient, B, increases as a first power relationship. The obtained air FPD correlation in the form of Eu in the experiment is well compatible with the measurement values, and the mean deviation of obtained correlation is 4.67%, showing good originality