Effect of segmented cracks on TGO growth and life of thick thermal barrier coating under isothermal oxidation conditions

This paper is devoted to a comparative study on the isothermal oxidation of thick thermal barrier coating (TTBC) with and without segmented cracks produced by atmospheric plasma spray (APS) process. Accordingly, the growth of thermally grown oxide (TGO) and its effect on the degradation of the coating were investigated. Thick top coat in both segmented crack and conventional thick TBC reduced the double layered TGO growth rate slightly. The segmented crack thick TBC demonstrated longer isothermal oxidation life in comparison with that of the conventional thick TBC at 1100 °C. The dominant failure mechanism was spallation due to lateral cracking within the TGO and/or within TBC near the TGO layer, called mixed failure. Stress, and consequently strain, induced on the TTBC due to progressive TGO growth, seems to be primarily responsible for the crack initiation and propagation leading to the coating failure. Increment of elastic energy stored within the top coat due to the increasing of TGO thickness, finally causes thick thermal barrier coating failure in high temperature isothermal oxidation.     

医用钛合金微弧氧化膜的制备及其生物相容性研究

利用微弧氧化技术在Ti合金表面制备了医用羟基磷灰石(HA)膜,研究了HA膜在模拟体液中的生物相容性,通过SEM观察了HA膜在模拟体液中浸泡不同时间的表面形貌,并利用EDs测试了HA膜浸泡前后的Ca、P原子分数.结果表明,HA膜在模拟体液中浸泡后,体液的pH变化不大,而经过溶解-重结晶,新生成的HA晶粒发育更完整,更利于...     

Ti75钛合金微弧氧化膜的制备及性能

在含硅酸钠10g/L、六偏磷酸钠2g/L、柠檬酸钠2g/L和添加剂4g/L的电解液中,以微弧氧化技术在Ti75钛合金的表面成功制备了微弧氧化膜。采用涡流测厚仪、扫描电镜、X射线衍射仪、显微硬度计等手段研究了微弧氧化电压和时间对钛合金微弧氧化膜厚度的影响,分析了氧化膜层的表面形貌、组成、硬度、耐蚀性能及高温性能。结果表明,微弧氧化膜层主要由金红石型和锐钛矿型二氧化钛组成。氧化电压升高,膜层厚度增加;氧化时间延长,初期膜层厚度增加明显,20min后膜厚增加减缓。经微弧氧化处理后,钛合金的硬度、耐蚀性能和高温抗氧化性能均得到了明显改善。     

The effect of gas diffusion layer compression on gas bypass and water slug motion in parallel gas flow channels

Water slugs form in the gas flow channels of polymer electrolyte membrane fuel cells (PEMFCs) which hinder reactant transport to the catalyst layer. We report a study correlating video images of slug formation and motion with pressure/flow measurements in parallel gas flow channels. Slugs move when the differential gas pressure exceeds the force to advance the contact lines of the slug with the channel walls. Water slugs can divert the gas flow through the gas diffusion layer (GDL) beneath the ribs to adjacent channels. The flow diversion can cause slugs to stop moving. Slug size and motion has been correlated with in situ GDL permeabilities as functions of GDL compression. Compression reduces the GDL permeability under the ribs much more than the GDL permeability under the channel. A model is presented to describe the spatio‐temporal location of slugs in a PEMFC flow field. © 2014 American Institute of Chemical Engineers AIChE J, 61: 355–367, 2015     

基于两相流流型的平行流冷凝器整体仿真模型与实验验证

引言平行流冷凝器作为一种新颖的换热器加之其不同于传统翅片管换热器的结构,因此其传热流动性能尚处研究之中。尤其是平行流冷凝器制冷剂侧微通道两相流机制及其转变不同于常规的管子,以往关于平行流冷凝器整体仿真模型的研究中,微通道两相流数值模拟大都采用传统大管径模型或者修正     

High-temperature oxidation of composites based on titanium carbonitride and double titanium-chromium carbide

The authors consider resistance to high-temperature oxidation in composite materials based on titanium carbonitride and double titanium-chromium carbide with metal binders, as well as titanium carbonitride with an aluminum nitride additive, with and without a binder. The oxidation resistance of double titanium-chromium carbide powders with a metal binder is studied in their initial and clad forms. Micro-x-ray-spectral and petrographic analysis is used to identify the composition of sample surfaces after oxidation. __________ Translated from Novye Ogneupory, No. 8, pp. 42–46, August, 2006.     

Microstructure and oxidation behavior of NiCr-chromium carbides coating prepared by powder-fed laser cladding on titanium aluminide substrate

In the present study, a NiCr–Cr₃C₂ powder mixture was prepared by mechanical alloying and then coated on titanium aluminide substrates by the powder-fed laser cladding process using a set of optimum parameters. The high temperature oxidation behavior of the substrate and coating was studied by isothermal annealing at 900 °C for 5 h. It was found that the microstructure of the coating is composed of γ solid solution with different chromium carbide phases (Cr₃C₂, Cr₇C₃ and Cr₂₃C₆). The presence of different chromium carbides in the microstructure of coating can be attributed to the partial melting of primary Cr₃C₂ and the formation of non-equilibrium carbide phases during rapid cooling of laser cladding. The NiCr-chromium carbide laser cladded coating samples showed superior oxidation resistance compared to the substrate. The oxidation mechanism of both coating and substrate follow the parabolic law, where the parabolic rate constant of the coating was 20% of that of the substrate at 900 °C. Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) and Grazing Angle X-Ray Diffraction (GAXRD) analysis revealed that the surface of the oxide layer formed on the NiCr-chromium carbides coating and the substrate is mostly composed of Cr₂O₃ and TiO₂, respectively.     

多孔双相钙磷钛合金微弧氧化膜层的制备及生物相容性

为克服钛合金生物膜层单一相的缺陷,以Ti6A14V合金为基体,用微弧氧化法制得具有双相钙磷复合陶瓷膜层(BCP)的钛合金器件.通过控制工作液成分制得由不同比例的β-磷酸三钙(β-TCP)和羟基磷灰石(HA)组成的多孔性复合膜层.研究了电源占空比对BCP膜孔隙率和孔径大小的影响.采用能谱仪和X射线衍射仪分析了BCP膜层的...     

Mathematical modelling on rapid decompression in base natural gas mixtures under rupturing

The paper presents a one-dimensional mathematical model of transient compressible thermal multi-component gas mixture flows in pipes. The set of the mass, momentum and enthalpy conservation equations is solved for gas phase in the model. Thermo-physical properties of multi-component gas mixture are calculated by solving the Equation of State (EOS) model. The Soave–Redlich–Kwong (SRK–EOS) model is chosen. Gas mixture viscosity is calculated on the basis of the Lee–Gonzales–Eakin (LGE) correlation. Numerical analysis on rapid decompression process in a shock tube having base natural gases is performed by using the proposed mathematical model. The model is successfully validated on the experimental measurements of the decompression wave speed in base natural gas mixtures. The proposed mathematical model shows a very good agreement with the experiments in a wide range of pressure values and predicts the decompression in base natural gases much better than analytical and mathematical models, which are available from the open source literature.     

Numerical modelling of flow and transport processes in a calciner for cement production

Controlling the calcination process in industrial cement kilns is of particular importance because it affects fuel consumption, pollutant emission and the final cement quality. Therefore, understanding the mechanisms of flow and transport phenomena in the calciner is important for efficient cement production. The main physico-chemical processes taking place in the calciner are coal combustion and the strongly endothermic calcination reaction of the raw materials. In this paper a numerical model and a parametric study are presented of the flow and transport processes taking place in an industrial calciner. The numerical model is based on the solution of the Navier-Stokes equations for the gas flow, and on Lagrangean dynamics for the discrete particles. All necessary mathematical models were developed and incorporated into a computational fluid dynamics model with the influence of turbulence simulated by a two-equation (k-ε) model. Distributions of fluid velocities, temperatures and concentrations of the reactants and products as well as the trajectories of particles and their interaction with the gas phase are calculated. The results of the present parametric study allow estimations to be made and conclusions to be drawn that help in the optimization of a given calciner.     

Effect of gas feed flow and gas composition modulation on activated carbon performance in phenol wet air oxidation

Modulation of gas feed composition (air/N₂ cycling) and gas feed flow (on-off air cycling) was investigated in the catalytic wet air oxidation of phenol over activated carbon (AC). Fifty hours lasting experiments were conducted in a laboratory trickle bed reactor at 140-160 °C, 2 bar of oxygen partial pressure and different splits and periods to determine the set of cycling parameters that optimise the periodic reactor operation. To follow the dynamic behaviour of the phenol oxidation, temperature and conversion were continuously monitored by means of computerised data acquisition and automatic liquid sampling. Several long term tests over 144 h were also run using both periodic operating strategies to compare the activity and stability of AC with those obtained in a steady state operation at otherwise same conditions. The results show that, depending on the selection of split and period, modulation of the gas phase significantly improves the stability of AC compared to steady state operation, thereby performing a superior long term phenol conversion.     

Studies on a novel bioactive glass and composite coating with hydroxyapatite on titanium based alloys: Effect of γ-sterilization on coating

A novel silicate based bioactive glass coating composition containing B₂O₃ and TiO₂ having matching thermal properties with that of Ti₆Al₄ V implants was developed and characterized. A conventional vitreous enamelling technique was used for coating small flat surface and curved surface of small rods. Hydroxyapatite (HAp) micro and nano-crystalline particles were used to prepare bioactive glass-HAp composite coating. Scratch testing was used to study the coating adhesion and its fracture behaviour under simulated conditions. As observed from scratch testing results, adhesion strength of the coating improved from 21 N normal load to 27 N and 32 N on addition of micro-HAp and nano HAp powder, respectively, to bioactive glass matrix. Further, sterilization of the coated samples with 25 kGy gamma irradiation substantially enhanced the adhesion of glass coating and HAp-composite coating.     

Steady-state and transient gas—solids flow characteristics in vertical transport lines

The generalized application of the drift velocity approach to gas—solids flow is given, together with supporting data and suggested parameter evaluation techniques, to allow the prediction of the phase densities, phase flow rates and allowable operating conditions related to gas—solids flow in vertical pipe systems. Both steady-state and transient flow conditions are considered in co-current and countercurrent flow. Continuity wave velocities are employed to determine the stability of the flow and the presence of flooding. The method of analysis outlined here, previously used successfully in gas—liquid systems, is shown to also hold for gas—solid systems.     

基于阳极氧化的钛合金接骨板摩擦磨损特性研究

针对传统钛合金接骨板耐磨性能差的问题,提出用阳极氧化法对其进行改性,然后通过场发射扫描电镜和X射线衍射仪对改性后的结构进行表征,并对改性后钛合金接骨板摩擦磨损性能进行分析.结果表明:附着于接骨板Ti6Al4V基体表面的涂层主要为Ti、O两元素,分别占比34.06％和65.94％,证实涂层主要成分为TiO2;经过热处理后...     

A two-step reaction mechanism of oxygen release from Pd/CeO2: mathematical modelling based on step gas concentration experiments

Topics in Catalysis - A mathematical model has been developed to study the transient release of oxygen from a 1 wt% Pd/CeO2 catalyst in the 450–550 °C range based on alternate...     

Factors affecting the interfacial adsorption of stabilisers on to titanium dioxide particles (flow microcalorimetry, modelling, oxidation and FTIR studies): Nano versus pigmentary grades

A series of nano and micron particle size anatase and rutile titanium dioxide (TiO₂) were prepared with various densities of surface treatments in order to examine the influence of the particle size on the photoactivity of the titania particle surface and their degree and nature of interfacial interaction with polymer stabilisers namely, Irganox 1010 (Phenolic type) and Tinuvin 770 (hindered piperidine type). The surface characteristics of the synthesized powders were studied by Diffuse Reflectance Fourier Transform Infrared Spectroscopy (DRIFTS). The surface area was determined using the Brunauer Emmett Teller (N₂ BET) method, and particle size measurements using X-ray diffraction (XRD) and transmission electron microscopy (TEM). The photochemical activities of the titania particles have been examined by monitoring the oxygen consumption during photo-oxidation of 2-propanol. Surface activity of the titania with stabilisers has also been examined by flow microcalorimetry (FMC) and DRIFTS in order to determine the nature of the interfacial interactions with different polymer stabilisers. Photoactivity assessment verified the higher activity of the nanoparticles. Hydroxyl groups were also found to be accountable for the higher photoactivity of the nanoparticles. The rutile crystal form conferred an inherent photostabilising effect that was further improved by surface coating with alumina. FMC studies revealed that the calcination of nanoanatase increased adsorption activity of hindered phenol and hindered amine probes, with the latter being more strongly adsorbed due to the higher basicity of the amine functionality. DRIFTS indicated adsorption may also occur through the ester functionalities. Calcination of the titania causes a reduction in the surface concentration of Ti–OH and hence a reduction in the amount of strongly adsorbed water blocking the adsorption sites and possibly bridging the amorphous primary particles on the uncalcined sample. With the calcined samples the adsorption activity was proportional to surface area. The physical and chemical nature of these intermolecular forces are assessed and discussed in relation to the potential effects on polymer stabilisation processes.     

Study of oxygen exchange and transport in mixed conducting electroceramics

Oxygen diffusion is treated in a dense electronically conducting perovskite pellet blocked ionically on one surface, electronically on the other, and sealed on the periphery. Oxygen exchange at the electronically blocked surface is assigned first order reaction kinetics. An equivalent circuit model is suggested for the cell impedance by the Laplace transform of Fick’s second law. Asymptotic expressions are employed to extract the slope of the coulometric titration curve and the chemical diffusion coefficient from electrochemical-impedance-spectroscopy (EIS) data. NLLS fit with the theoretical model is performed to evaluate the surface oxygen exchange coefficient at the interface of the electrochemical cell. The methodology is applied to determine the chemical diffusion and surface exchange coefficients of oxygen in SrCo_0·5Fe_0·5O_3−δ, interfaced with a YSZ electrolyte. The experimental results are used to link the processes corresponding to the diffusion of oxygen vacancies to the ionic conductivity of the material. The data is applicable to solid oxide fuel cell cathodes, oxygen permeation membranes and related systems.     

磷酸盐-氢氧化钾溶液中镁合金微弧氧化膜层结构和性能研究

在磷酸盐-氢氧化钾电解液中,利用微弧氧化技术在AM60B镁合金表面获得了氧化膜层。截面光学显微形貌与扫描电镜照片显示,该氧化膜层为一连续的整体,氧化膜与基体存在清晰的界面,界面附近无孔洞、裂纹等缺陷,而在氧化膜表面则分布着大量的微孔,并有微裂纹存在;EDS成分分析表明,膜层主要由Mg、Al元素（来自于基体）和P、O元素（来自于电解液）组成;X-射线衍射实验证明了微弧氧化处理在镁合金表面形成了一层以MgO为主要组成的氧化膜,但未检测到P的存在,由此说明P以非晶态的形成存在于膜层中。膜层截面硬度分布曲线和耐蚀性实验表明,膜层硬度随着膜层与界面距离的增加起初增大,在距离界面10μm处达到最大值为580HV,之后逐渐降低;基体与微弧氧化膜层在w（NaCl）=3．5％NaCl溶液中的极化腐蚀电流密度分别为198A／cm^2和3．54A／cm^2,说明微弧氧化膜的生成使镁舍金的耐蚀性能明显增强。     

Effect of annealing temperature on mixed proton transport and charge transfer-controlled oxygen reduction in gas diffusion electrode

The effect of annealing temperature T_ann on mixed proton transport and charge transfer-controlled oxygen reduction in gas diffusion electrodes used in polymer electrolyte membrane fuel cells (PEMFCs) was investigated in 1 M H₂SO₄ solution using AC-impedance spectroscopy and potentiostatic current transient technique. For this purpose, the gas diffusion electrodes were annealed at different temperatures ranging from 140° to 180 °C in order to control the proton transport resistance distribution across the active catalyst layer (ACL). For the annealed gas diffusion electrodes with different proton transport resistance distributions, the measured impedance spectra exhibited a straight line inclined at a constant angle higher in absolute value than 45° to the real axis at high frequencies, followed by a depressed arc at low frequencies.From the quantitative analysis of the measured impedance spectra based upon the transmission line model (TLM) modified with the proton transport resistance distribution, it was found that as T_ann increased, the average proton transport resistance R_ave and the standard deviation σ of the proton transport resistance distribution increased as well. Furthermore, as T_ann rose, the charge transfer resistance R_ct increased and simultaneously the double layer capacitance C_dl decreased due to the smaller electrochemical active area A_ea. From the analysis of the cathodic current transients measured during nitrogen blowing, it was noted that as T_ann increased, the current decayed more rapidly with time, suggesting that the larger values of R_ave and σ kinetically impede proton transport through the Nafion membrane within the ACL due to the wider RC time constant distribution.