Carbon aerogel spheres prepared via alcohol supercritical drying

We have developed a method that would allow for the fabrication of carbon aerogel (CA) spheres. The inverse phase suspension polymerization of resorcinol and formaldehyde monomers with Na₂CO₃ as a catalyst followed by supercritical drying was explored. The effects of the chemical formulation and processing procedures and the conditions of the structures of organic and related carbon aerogels were studied. The experimental results indicated that it was easy to avoid the accumulation of polymerization heat during gelation, and easy to take out the products from the reaction container, through this fabrication method. Sol-gel microspheres with diameters ranging from about 30-1000μm could be obtained. After drying the sol-gel spheres under alcohol supercritical drying conditions, aerogel spheres with a bulk density of 0.8-1.0 g/cm³were prepared, and by subsequently pyrolyzing them, CA spheres with surface areas of 250-650 m²/g were obtained. The resultant CA spheres could be used as the electrode materials of supercapacitors. The specific capacitance of the CA spheres was as high as 215 F/g, and the equivalent series resistance at 48 Hz was about 1 Ω.     

Theoretical consideration of the effect of porosity on thermal conductivity of porous materials

The thermal conductivity of porous materials is theoretically studied in connection with nanoporous materials used in recent semiconductor devices. The effects of porosity and pore size on the thermal conductivity are discussed. The thermal conductivity of insulating materials is determined by the heat capacity of phonons, the average phonon velocity and the phonon mean free path. We investigate the porosity dependence of these quantities, especially by taking into account phonon scatterings by pores, and present an expression for the thermal conductivity as a function of porosity. Our model consideration predicts that the thermal conductivity of nanoporous materials depends on the ratio of the pore size R_p to the phonon mean free path for zero-porosity, l₀. The thermal conductivity for l₀/R_p > 1 decreases steeply with increasing porosity because of effective phonon scatterings by pores. On the other hand, the thermal conductivity for l₀/R_p < 0.1 decreases moderately with increasing porosity because phonon scatterings by pores are no longer effective. On the basis of the present theoretical consideration, we discuss the principal factor dominating the porosity dependence of thermal conductivity in nanoporous materials. We also discuss how one can design nanoporous materials with lower or higher thermal conductivity.     

Preparation and characterization of mesoporous activated carbon from waste tires

Activated carbons were produced from waste tires and their characteristics were investigated. Rubber separated from waste tires was first carbonized at 500 °C in N₂ atmosphere. Next, the obtained chars were activated with steam at 850 °C. As a result, fairly mesoporous activated carbons with mesopore volumes and BET surface areas up to 1.09 cm³/g and 737 m²/g, respectively, were obtained. To further improve the porous properties of the activated carbons, the char was treated with 1 M HCl at room temperature for 1 day prior to steam activation. This treatment increased mesopore volumes and BET surface areas of the activated carbons up to 1.62 cm³/g and 1119 m²/g, respectively. Furthermore, adsorption characteristics of phenol and a dye, Black 5, on the activated carbon prepared via acid treatment were compared with those of a commercial activated carbon in the liquid phase. Although the prepared carbon had a larger micropore volume than the commercial carbon, it showed a slightly lower phenol adsorption capacity. On the other hand, the prepared carbon showed an obviously larger dye adsorption capacity than the commercial carbon, because of its larger mesopore volume.     

Characterization and modeling of diffusion process for mass transport through the Tournemire argillites (Aveyron, France)

Argillites are one of the geological formations studied by IRSN for their confining properties for isolation of radioactive wastes. One of the main objectives is the study of water transport through rocks with very low water content and very low hydraulic conductivity by modeling of natural tracer profiles. This paper presents the protocol developed for and applied to the acquisition of data for chloride content in interstitial water of the Toarcian argillites at the Tournemire site (Southern France). This protocol is based on laboratory diffusion experiments and on modeling. Experimental data obtained during the transient and steady-state parts of diffusion experiments enable, respectively, the assessment of the diffusion coefficient and the determination of Cl concentration in pore water. Using this protocol, profiles with depth for both of these data sets have been acquired along the geological sequence. Taking into account the present knowledge of the geological and hydrogeological history of the Tournemire massif, a conceptual model granting the main role for mass transport to diffusion has been proposed. According to this conceptual model, a one-dimensional numerical model was built for simulating the mass transport of chloride through the sedimentary column, over 53 Ma. The good agreement between experimental data and calculated values for both diffusion coefficients and concentrations of chloride confirms that diffusion is likely the main process for mass transport in the massif. This model was also tested with the deuterium content of interstitial water, applying variable concentrations at the aquifer system boundaries for reflecting the thermal dependency of isotopic composition in precipitation. These simulations also reveal the likely important role of heterogeneities, such as fractures, in the variability of tracer concentrations with regards to a simple diffusion profile.     

Sintering behaviour of La1−xSrxFeO3−δ mixed conductors

The sintering properties of La_1−xSr_xFeO_3−δ (x = 0.1, 0.25) mixed conductors have been investigated with particular emphasis on the effect of secondary phases due to cation non-stoichiometry (±5 mol% La excess and deficiency). Secondary phases, located at grain boundaries in cation non-stoichiometric materials, increased the sintering temperature compared to single-phase materials. Extensive swelling in final stage of sintering was observed in all materials, which resulted in micro-porous materials. The swelling was most pronounced in the phase pure and two-phase materials due to La-deficiency, while refractory secondary phases in La-excess materials inhibited both sintering, grain growth and swelling. In La-deficient materials, formation of molten secondary phases resulted in rapid swelling due to viscous flow. The present findings demonstrated the importance of controlling sintering temperature and time, as well as careful control of the cation stoichiometry of La_1−xSr_xFeO_3−δ in order to achieve fully dense and homogenous La_1−xSr_xFeO_3−δ ceramics.     

铝硅比对白云鄂博铁精矿烧结液相流动性的影响

运用红外线微型烧结法对白云鄂博铁精矿烧结液相流动特性进行研究。通过在烧结原料中添加纯化学试剂研究不同Al2O3与SiO2含量之比对烧结液相流动性的影响。结果表明,Al2O3含量的增加虽然促进了烧结液相量的增多,但同时提高了液相的粘度,随着铝硅比的增加烧结试样的液相流动性降低。通过对试样气孔大小的对比,揭示了烧结过程中气孔形成与烧结矿液相流动特性的关系,粘度是控制气孔形成的重要因素。气孔的形成过程伴随着液相在气孔壁上的物理化学反应,气孔直径减小使烧结试样液相流动性变差。     

A study on corrosion resistance characteristics of PVD Cr-N coated steels by electrochemical method

The corrosion behavior of Cr-N coated steels with different phases (-Cr, CrN and Cr₂N) deposited by cathodic arc deposition on AISI H13 steel was investigated in a 3.5% NaCl solution at ambient temperature. Potentiodynamic polarization tests, electrochemical impedance spectroscopy (EIS) and scanning electron microscopy (SEM) were the techniques applied to characterize the corrosion behavior. It was found that the CrN coating had a lower current density from potentiodynamic polarization tests than others. The porosity, corresponding to the ratio of the polarization resistance of the uncoated and the coated substrate, was higher in the Cr₂N coating than in the other Cr-N coated steels. EIS measurements showed, for most of the Cr-N coated steels, that the Bode plot presented two time constants. Also, the Cr₂N coating represented the characteristic of Warburg behavior after 72 h of immersion. The coating morphologies were examined in planar view and cross-section by SEM analysis and the results were compared with those of the electrochemical measurement. The CrN coating had a dense, columnar grain-sized microstructure with minor intergranular porosity. From the above results, it is concluded that the CrN coating provided a better corrosion protection than the other Cr-N coated steels.     

摩托车右曲轴箱盖渗漏分析及对策

根据铸件的结构和压铸工艺参数特点,结合其浇注系统,解剖和分析了125型摩托车右曲轴箱盖易产生渗漏的部位,发现渗漏的主要原因是凝固补缩不够,造成了最后凝固部位出现大量的缩松缩孔。为此,在原浇注系统上增加了分支浇道,优化了压铸工艺、精炼工艺参数,保证凝固终了时易渗漏部位有足够的补缩铝合金液,生产出了合乎要求的右曲轴箱盖,产品合格率达92%以上。     

Towards realistic characterisation of chemical reactors: An in-depth analysis of catalytic particle beds produced by sieving

Optimization of large-scale fixed particle bed catalytic reactors requires extensive insight into the multi-scale bed structure, even down to the micrometre scale. Theoretical studies of chemical reactors provide a time- and cost-effective means to supporting the optimisation process. However, they rely on simplified assumptions for the particles, e.g. homogeneous perfect spheres. In practise, the preparation of catalytic particles cannot attain this level of uniformity. Typical preparation techniques, such as sieving, are conducted with the aim of obtaining particle size distributions within a pre-defined range, governed by the sizes of the sieves. However, such methods offer limited control in the actual particle sizes and shapes. This paper evaluates the impact of sieving on the resulting particles and overall structural morphology of catalytic beds. The bed structure is quantified using micro-focus computed tomography (µ-CT), enabling the non-destructive examination and analysis of over 150 thousand particles, in terms of particle size, shape, uniformity, and interparticle porosity. Furthermore, the chemical performance of the resulting beds is compared. The detailed characterisation achieved paves the way for the evolution of more rigorous computational models coupling intricate, localised hydrodynamics with realistic chemical processes. Validation of such models at the lab-scale will accelerate the development of more accurate large-scale models.     

Pore segmentation of cement-based materials from backscattered electron images

A technique to segment pores from a normal backscattered electron (BSE) image of cement-based materials is presented. The upper threshold grey level for porosity is determined from the inflection point of the cumulative brightness histogram of the BSE image. This represents a critical point where a small incremental grey value will cause a sudden increase in thresholded area, a condition termed as overflow. The proposed technique was found to be more consistent and reliable than existing methods. Significantly fewer images are required to achieve a satisfactory level of statistical confidence for quantifying porosity.