TEM/STEM Observation of ZrC-Coating Layer for Advanced High-Temperature Gas-Cooled Reactor Fuel

The Japan Atomic Energy Agency (JAEA) has started to study and develop ZrC-coated fuel particles for advanced high-temperature gas-cooled reactors. The ZrC-coating layer was fabricated using the bromide process at JAEA. In the early stage of the project, however, the deposition temperature was varied. This paper mainly focuses on the microstructures of the ZrC-coating layer developed in the early stage of the project. Some circumferential stripes were observed in the ZrC-coating layer on optical micrographs. It was found that the stripes were caused by the nonuniform distribution of the free carbon phase. It was also revealed by means of transmission electron microscope /scanning transmission electron microscope observations that crystal grains of the ZrC were small and columner in shape, and were not equiaxed especially near the surface. It appears that the oscillated deposition temperature results in the nonuniform distribution of the free carbon region. The structure of the free carbon region formed in the ZrC-coating layer appeared to be such that the c -plane was roughly parallel to its lengthened direction. The ZrC-coating layer appeared to be bound to the PyC layer. Fibrous carbon existing at the PyC/ZrC boundary was also observed.     

Strength of Pyrolytic SiC Coatings of Fuel Particles for High-Temperature Gas-Cooled Reactors

Methods for measuring the strength of a hemispherical shell were developed and applied to SiC coatings obtained from reactor fuel particles. These methods allow independent evaluation of the strengths of the inner and outer layers of the coatings. The applied loads are related to the maximum tensile stress using strain-analysis techniques. Coatings from standard SiC fuel particles have a median outer-surface strength of 620 MN m⁻² and a median inner-surface strength of 720 MN m⁻². These strengths relate only to a small region of the tested hemisphere, and statistical analysis is used to obtain strengths for complete spherical shells under internal pressure, the condition relevant to reactor performance. This procedure reduces the median working strength to 105 MN m⁻², with failure occurring preferentially from the outer surface. A second statistical analysis shows that the critical failure stress for 1 particle in 10⁴ (the reactor design requirement) is 22 MN m⁻². A fracture-mechanics analysis of strength indicates that failure occurs by the extension of inherent flaws at the surface. These flaws are associated with surface roughness. Strength was substantially improved by reducing this roughness by controlling the conditions of deposition of the coating and the topography of the underlying pyrocarbon layer. Greatly superior coatings were thus prepared; their median failure stress under internal pressure was 560 MN m⁻² and the 1-in-10⁴ failure stress 195 MN m⁻².     

Effect of Water Vapor on the Release of Fission Gases from Uranium Oxycarbide in High-Temperature, Gas-Cooled Reactor Coated Fuel Particles

The hydrolysis of uranium oxycarbide (UCO) was studied by injecting water vapor into a fuel element containing a known number of high-temperature, gas-cooled reactor (HTGR) coated particles with exposed fuel kernels. The experiments were conducted in the High Flux Isotope Reactor (HFIR) at temperatures between 700° and 1000°C, a system pressure of 200 kPa, and partial pressures of water vapor between 21 and 199 Pa. The general sequential response of the exposed fuel kernels to water vapor addition consisted of (1) a rapid release of stored fission gas with a concomitant increase in the steady-state release and (2) a period of constant steady-state release. Upon cessation of the addition of water vapor, a decline in the release to prehydrolysis values generally occurred. The release of stored fission gas was dependent on the square of the partial pressure of water vapor. The ratio of the constant steady-state release to the prehydrolysis value was independent of the partial pressure of water vapor. The time constant for the decline in the release was the same in all of the hydrolysis tests.     

碱性燃料电池产过氧化氢阴极C/PTFE的制备及性能表征

研究了碱性燃料电池合成过氧化氢阴极碳/聚四氟乙烯(C/PTFE)的制备方法,考察了制备过程中的主要影响因素,并通过伏安曲线和过氧化氢浓度曲线及交流阻抗图谱,对不同条件下制备的阴极性能进行了评价.结果表明,压力、整平层碳含量及聚四氟乙烯含量对阴极性能均有较大影响,其中压力为10.0 MPa、碳含量为0.2 g及聚四氟乙烯含量为50%条件下制备的阴极能获得最大电流,过氧化氢的产量最高,2.5 h后过氧化氢浓度达到56 mmol/L.     

Creep and Stress Rupture Behavior of an Advanced Silicon Nitride: Part II, Creep Rate Behavior

In Part I of this paper, experimental observations on creep testing of 74 tensile specimens of an advanced silicon nitride were presented. In this part, equations are developed for predicting creep rates in the primary and secondary regimes in the temperature range 1477–1673 K. The resulting model predicts creep strain rates to within a factor of two. The underlying phenomenological basis, which employs an activation energy approach, is discussed. The mechanisms that are likely to be responsible for the transiency of the primary creep regime and for the unique stress and temperature dependencies of the creep rates are explored.     

High-Temperature Centrifuge for Separation of the Liquid Phase from Refractories: II, Centrifuging Tests

A rotating-type induction furnace with graphite heater was used to demonstrate the application of the principles developed in Part I. Rotation at 1200 to 1300 r.p.m. for 15 minutes at an average temperature of 2560°F. (1405°C.) separated a low-melting slag from a silica brick which had been used in an open-hearth roof.     

Development of a microstructured reactor for heterogeneously catalyzed gas phase reactions: Part II. Reactor characterization and kinetic investigations

A new type of microstructured reactor was characterized and tested in the oxidative dehydrogenation of propane (ODP) with respect to reaction engineering aspects. Residence time behavior was measured using O₂/N₂ step injection experiments and theoretically analyzed by applying the axial dispersion model, resulting in Bodenstein numbers in the range of 70. Catalytic performance in the ODP was predicted on the basis of a kinetic model from the literature, showing good agreement for low degrees of propane conversion. In order to elucidate discrepancies between experimental and forecasted model data at higher degrees of propane conversion (i.e., >12.5%), possible sources of error were systematically investigated. Specifically, heat and mass transport limitations were excluded as well as possible inaccuracies of the applied kinetic model were examined. It could be shown that microstructured reactors are well suited to be applied for strongly exothermic heterogeneously catalyzed gas phase reactions since they allow isothermal reaction conditions over a wide range of concentrations and temperatures.     

Preparation, cross-linking and ceramization of AHPCS/Cp2ZrCl2 hybrid precursors for SiC/ZrC/C composites

SiC/ZrC/C composites were prepared via pyrolysis of a polymeric precursor, namely AHPCS/Cp₂ZrCl₂ hybrid precursor prepared by the blend of allylhydridopolycarbosilane (AHPCS) and bis(cyclopentadienyl) zirconium dichloride (Cp₂ZrCl₂). The cross-linking and polymer-to-ceramic conversion of as-synthesized AHPCS/Cp₂ZrCl₂ were characterized by means of FTIR, ¹³C NMR, TGA, EDS, Raman spectroscopy and XRD. It is suggested that dehydrocoupling, hydrosilylation and dehydrochlorication are involved in the cross-linking of the hybrid precursor, which is responsible for a relatively high ceramic yield of 75.5% at 1200 °C. The polymer-to-ceramic conversion is complete at 900 °C, and it gives an amorphous ceramic. Further heating at 1350 °C induces partial crystallization, and then the characteristic peaks of β-SiC and cubic ZrC appear at 1600 °C. The effect of the composition of the hybrid precursor is also studied in the work.     

Advanced Oxidation Treatment of Drinking Water: Part II. Turbidity,Particles and Organics Removal from Lake Huron Water

Pre-coagulation ozonation has been reported to be effective in drinking water treatment processes. Limited data are available on the impact of advanced oxidation processes (AOPs) on Lake Huron water which serves as a primary source of drinking water for many communities around the Great Lakes region. Impact of ozone/hydrogen peroxide based AOP on Lake Huron water was studied. The results show that AOPs can achieve higher particles removal in finished water and deliver improved filtered water turbidity compared to the conventional treatment process. Sharp decline in ultraviolet absorbance at 254 nm (UV₂₅₄) was observed immediately following AOP treatment while only minimal overall decrease in dissolved organic carbon (DOC) was achieved.     

Processing of ZrC–Mo Cermets for High-Temperature Applications, Part I: Chemical Interactions in the ZrC–Mo System

The chemical compatibility of ZrC and Mo was investigated in carburizing and carbon-free environments at temperatures from 1700° to 2200°C. Heating in the carburizing atmosphere resulted in the complete reaction of Mo with C, while the carbon-free atmosphere resulted in retained metallic phase with a maximum of 13.8 mol% Mo₂C formed. The presence of Mo₂C was not detected at 2100°C in the carbon-free atmosphere, confirming the existing phase equilibria in the Zr–Mo–C system. Heat treatments in the carbon-free atmosphere also showed liquid formation at 2200°C, as evident from microstructure analysis. Liquid formation was consistent with the interaction between Mo and Mo₂C. The liquid was found to comprise at least 7 vol% of the total component, based on a phase diagram for the Mo–C system. The formation of a liquid should allow for the processing of ZrC–Mo cermets by liquid-phase pressureless sintering.     

Thermodynamics of the Si-O-N System: II, Stability of Si2N2O(s) by High-Temperature Mass Spectrometric Vaporization

The Si₂N₂O(s) compound is vaporized in a multiple Knudsen-cell connected to a mass spectrometer. Comparison of SiO(g) and N₂(g) pressures between different samples in the Si-N-O system, as triphasic or diphasic mixtures from Si(s), Si₃N₄(s), and Si₂N₂O(s), allows the determination of the SiO(g) evaporation coefficient and consequently the calculation of the equilibrium pressure of SiO(g). Then the standard enthalpy of formation of Si₂N₂O(s) is determined: A_fH°_m(Si₂N₂O, s, 298.15 K) =−887.5 ± 10 kj.mol⁻¹.     

Stability Improvement of Rh/γ-Al2O3 Catalyst Layer by Ceria Doping for Steam Reforming in an Integrated Catalytic Membrane Reactor System

Significant improvement over the equilibrium methane conversion level was achieved by performing the reforming of methane in a catalytic membrane reactor, which was prepared by integrating a microporous silica membrane with a sandwiched-type Rh/γ-Al₂O₃ catalyst layer. However, the methane conversion activity decreased progressively owing to the deactivation of the intermediate catalyst layer under the reaction environments. On the other hand, addition of CeO₂ as a promoter for the Rh/γ-Al₂O₃ catalyst significantly improved the catalyst stability. The improvement was achieved probably by the kinetic and oxidative stabilization of the catalyst matrix with CeO₂. However, compared to the nonpromoted system, the ceria-promoted systems displayed lower catalytic activities based on the Rh/Ce ratios. The results led to the conclusion that a controlled interplay of the catalytic potentiality of Rh and the stabilization effect of Ce are essential to obtain an acceptable system. The membrane quality and its performance decreased especially with high Ce incorporation in the catalyst layer, possibly as a result of the observed microstructural variations in the catalyst layer with the Ce addition. Therefore, a consensus between the activity and stability of the material as a catalyst and the textural characteristics of the catalyst layer as a support layer for the silica membrane is considered to be an important factor that decides the success of the approach. A possible mechanism has been suggested to explain the role of ceria as a promoter in the Rh/γ-Al₂O₃ system.     

Numerical modeling of injection/compression molding for center-gated disk: Part II. Effect of compression stage

In the accompanying paper, Part I, we have presented a physical modeling and the associated numerical analysis of the injection molding process with a compressible viscoelastic fluid model. In Part II, effects of the compression stage in the injection/compression molding process are presented. Numerical results showed that the injection/compression molding process reduced birefringence as compared with the injection molding process. In this respect, the injection/compression molding process seems to be more suitable for manufacturing precise optical products of good optical quality than the injection molding process. Effects of the packing stage on the birefringence distribution in the injection/compression molding process were found to be similar to those in the injection molding process. Our numerical results show that the birefringence becomes smaller as the melt temperature gets higher and the closing velocity of the mold gets smaller with the flow rate and the mold temperature affecting the birefringence insignificantly. As far as the density distribution is concerned, the flow rate, melt temperature, and mold closing velocity have insignificant effects on the density distribution in comparison with the mold temperature.     

Conductive, Polypyrrole Coating on Mullite/Alumina Fibers for Electrophoretic Deposition of Oxide Matrices

Mullite/alumina fibers (Nextel™ 720) have been rendered conductive for electrophoretic deposition (EPD) by coating with polypyrrole (Ppy) from an aqueous solution of pyrrole (Py). The polymer coating was characterized by thermogravimetric (TG) analysis, scanning electron microscopy (SEM), and conductivity measurements. The Ppy coating thickness is ∼0.1 μm and the fiber resistance 1–2 (kΩ/cm)/fiber tow. A maximum conductivity of ∼48 S/cm was achieved from a 0.005 M pyrrole solution. A Nextel 720/Al₂O₃ composite was synthesized using Ppy-coated fibers as the cathode in an EPD cell. The green and sintered microstructures of the resultant composites are reported.     

Ultradispersed particles in heavy oil: Part II, sorption of H2S(g)

During steam assisted gravity drainage for heavy oil recovery aqua-thermolysis reactions take place, whereupon gaseous hydrogen sulfide, H₂S_(g), is produced. A method to capture H₂S_(g) and convert it into a chemically inactive species is deemed necessary for sustaining in-situ recovery and upgrading. Part I of the current study explored the formation and stabilization of colloidal FeOOH particles in heavy oil matrices. In this Part, we evaluate the H₂S_(g) sorption ability of these particles as well as other metal oxide/hydroxide particles. Furthermore, the effect of mixing and temperature on H₂S_(g) sorption was investigated. Results showed that the rate and capacity of H₂S_(g) sorption increased as the concentration of FeOOH increased. Mixing, on the other hand, had insignificant effect on the sorption capacity, however it improved the sorption kinetics. In addition, in-situ prepared colloidal particles showed better reactivity towards H₂S_(g) than commercial α-Fe₂O₃ nanoparticles. Temperature had an adverse effect on the H₂S_(g) sorption capacity of FeOOH. This was attributed to a change in chemical structure of FeOOH as the temperature increased. Nevertheless, in-situ prepared ZnO colloidal particles completely removed H₂S_(g) even at high temperatures.     

Synergistic effects of novel battery manufacturing processes for lead/acid batteries: Part II: Mechanistic studies

The mechanistic aspects of the beneficial effects of different manufacturing processes on the performance of lead/acid batteries have been studied using electrochemical impedance spectroscopy (EIS), X-ray diffraction (XRD) and scanning electron microscopy (SEM). The implications of grid etching, active material compression and spiking of active material with conductive additives, along with various combinations of these processes, on the PAM and positive electrode corrosion layer have shown that a combination of compression and conductive additives reduces the PbSO₄ content of both the positive active material (PAM) and the grid corrosion layer, leading to an enhancement in battery performance. Furthermore, the impact of the most promising battery manufacturing process on the kinetics of electrochemical processes occurring in the LAB, as studied using EIS, has shown that the charge transfer resistance of control cells in the fully discharged state increases with cycling, while the treated cells behaved like a near-perfect capacitor, yielding a negligible charge transfer resistance. EIS has shown that the kinetics of lead/acid battery processes in additive spiked and compressed cells is accelerated with respect to control cells.     

Ferroelectric Thin Films of Bismuth-Containing Layered Perovskites: Part II, PbBi2Nb2O9

Ferroelectric thin films of bismuth-containing layered perovskite PbBi₂Nb₂O₉ have been prepared by a metalorganic decomposition (MOD) method. Random and highly c-oriented films of the same starting composition have been obtained under different intermediate- and high-temperature heat treatments. A comparison of their crystallization and properties with those of Bi₄Ti₃O₁₂ films reveals similar trends that are common to bismuth-containing Aurivillius compounds. Heterogeneous nucleation of the perovskite phase either on the pyrochlore (444) plane because of lattice matching or on the substrate surface because of lower interfacial energy is proposed as the cause of orientation selection during crystallization. The different thickness of the pseudoperovskite subunits in these layered compounds may be responsible for the systematic difference in the anisotropic ferroelectric properties. Smaller polarization and higher coercive field are expected for PbBi₂Nb₂O₉, which has thinner pseudoperovskite units than Bi₄Ti₃O₁₂.     

Lipid methodology — chromatography and beyond. Part II. GC/MS,LC/MS and specific enzymic hydrolysis of glycerolipids

The combination of specific enzymic degradation with GC/MS or LC/MS identification and quantitation of enantiomeric diacylglycerols and reverse isomers has greatly improved the methods of structural analysis of triacylglycerols, so that in many instances complete characterization of both major and minor species is possible. The techniques described for the analysis of triacylglycerols and sn-1,2- and sn-2,3-diacylglycerols are also applicable to the X-1,3-diacylglycerols and X-1-monoacylglycerols following coversion to triacylglycerols by acylation with appropriate fatty acids. For many applications, however, a combination of specific enzymic hydrolysis with a GLC analysis of the products on polar capillary columns may be adequate for the identification and quantitation of the major molecular species of both triacylglycerols and diacylglycerols.     

Development of an environmentally friendly protective coating for the depleted uranium-0.75 wt.% titanium alloy: Part II: Coating formation and evaluation

Molybdenum oxide based coatings have been formed on the surface of the depleted uranium-0.75 wt.% titanium alloy. Surface activation prior to coating formation has been examined using fluorides and concentrated nitric acid. The electrochemical characteristics of the coating formation processes were studied using open circuit potential measurements. Residual fluoride from the activation process has been found to interfere with coating formation and surface activation by nitric acid yields a relatively thinner but more robust coating.The corrosion protection characteristics of the coatings were evaluated by potentiodynamic polarization testing in quiescent 0.05 M sodium chloride. The coatings have been studied using scanning electron microscopy, energy dispersive spectroscopy and optical microscopy.