Results 1 - 10 of 773
Results 1 - 10 of 773. Search took: 0.032 seconds
|Sort by: date | relevance|
[en] High-strength steels are favoured materials in the industry for production of safe and sustainable structures. The main technology used for joining the components of such steel is fusion welding. Steel alloy design concepts combined with advanced processing technologies have been extensively investigated during the development of High-Strength Low-Alloy (HSLA) steels. However, very few studies have addressed the issue of how various alloy designs, even with limited microalloy addition, can influence the properties of high-strength steel welds. In high-strength steel welding practices, the challenges regarding microstructure evolution and the resulting mechanical properties variation, are of great interest. The main focus is the debate regarding the role of microalloy elements on phase transformation and weld performance. Limited Heat Affected Zone (HAZ) softening and limited austenite grain coarsening are significant design essentials, but the primary goal is to ensure excellent toughness and tensile properties in the steel weld. To achieve this purpose, microalloy elements such as Ti, Nb, or V were intentionally added to modern high-strength steels. The focus of this work was to understand the mechanical properties of HSLA steels resulting from differences in alloy design after joining by modern welding processes. To begin, three microalloyed S690QL steels (Nb, Ti, and Ti+V addition) were investigated. Optical microscopy confirmed that similar mixtures of tempered bainite and martensite predominated the parent microstructure in the three steels, different types of coarse microalloy precipitates were also visible. These precipitates were analysed by using a thermodynamic-based software and then identified by Transmission Electron Microscopy (TEM). Results of mechanical testing revealed that all three steels performed above the standard toughness and tensile strength values, but with varied yielding phenomena. During the welding operation, each of the three steels was joined by using the same filler material. The fused weld metal was influenced by the high dilution of microalloyed elements in the base metal, this was significantly pronounced during the modified spray arc welding technique. As a result, the Nb-containing steel exhibited sufficient amounts of alloy pick-up to transition the microstructure in the weld metal from acicular ferrite to bainite as cooling rate was increased, leading to reduced toughness. This was not observed with the other two steels. A second focus was made on the microstructure Evolution and toughness properties of the coarse and fine grained HAZ as welding parameters changed. In order to characterise the microstructure and austenite grain growth behaviour, physical simulations were conducted. The microalloy precipitates were found to be a dominant factor restricting the austenite grain coarsening. The extent of Austenite coarsening in the HAZ is closely related to the type and volume fraction of each microalloy precipitate. Among the three steels, the Ti-containing HAZ exhibited the smallest extent of grain growth due to the sufficient amount of stable Ti-rich precipitates. Microalloy Addition also markedly influenced the subsequent phase transformation in the HAZ. The formation of intragranular acicular ferrite was promoted by Ti-rich precipitate, acting as favourable nucleation sites of ferrite. This structure enhanced the HAZ toughness owing to fine, high-angle boundaries of ferrite plates. The synergistic effect of Nb and Mo elements was beneficial to improve the HAZ toughness at fast cooling rates by promoting fine lower bainite formation. At high heat input, large upper bainite was formed which caused reduced toughness. The final set of experimental work was concentrated on understanding the HAZ softening mechanisms that influenced variations in the tensile properties of the welded joints. The tensile failure in the softened HAZ or base material depended on the welding parameters and the type of steel being joined. In Ti-containing steel, increased heat Input extended the softened zone width, which caused a significant decrease in hardness and then resulted in failure in this area. Therefore, limited heat Input was used to shift failure position to base material. But this was not observed in the other two steels. Hence, small differences in microalloy addition exhibited large variation in tensile properties. Among the three steels, Ti-containing welds were found to have the most pronounced softening, followed by Ti+V-containing welds and finally Nb-containing welds. This varied softening phenomenon was related to two significant processes supported by the results of additional dilatometry simulation: phase transformation and tempering behaviour. In the Ti-containing steel, the phase Transformation product ferrite was large-sized, as a consequence of initial large austenite grains. This led to the decreased corresponding hardness of the Ti-containing steel. Furthermore, lower tempering resistance in Ti-containing steel as compared to Nb-containing steel, resulted in additional softening effect in the softened HAZ. Therefore, steel alloy identification and heat Input during welding were critical, proven by the experimentation within the same S690QL steel grade. This work emphasised the influence of microalloy elements on weld microstructure and mechanical properties in welded joints. Knowledge of this delicate balance between steel alloy design and appropriate welding parameters is critical for the end product. Thus, this work provides specific recommendations and results to ensure proper welding practice and steel design of microalloyed high-strength steels.
[en] Rare charmless hadronic B decays are particularly interesting because of their importance in understanding the CP violation, which is essential to explain the matter-antimatter asymmetry in our universe, and of their roles in testing the ''effective'' theory of B physics. The study has been done with the BABAR experiment, which is mainly designed for the study of CP violation in the decays of neutral B mesons, and secondarily for rare processes that become accessible with the high luminosity of the PEP-II B Factory. In a sample of 89 million produced B(bar B) pairs on the BABAR experiment, we observed the decays B0 → ωK0 and B+ → ωρ+ for the first time, made more precise measurements for B+ → ωh+ and reported tighter upper limits for B → ωK* and B0 → ωρ0
[en] Kinetic and mass transport properties were investigated for the oxygen reduction reaction in Nafion 117 and a sulfonated poly (arylene ether sulfone) membrane (SPES-40, 40% sulfonated groups/repeat unit) under 1 atm oxygen pressure, 100% relative humidity in a temperature range of 303-343 K using a solid-state electrochemical cell. Kinetic parameters were obtained using slow-sweep voltammetry while mass transport parameters, the diffusion coefficient (D) and solubility (C), were obtained using chronoamperometry at a Pt (microelectrode)/proton exchange membrane (PEM) interface. Oxygen reduction kinetics was found to be similar for both Nafion [reg ] 117 and SPES-40 membrane at the Pt microelectrode interface. The temperature dependence of O2 permeation parameters showed same trends for both the membranes studied, there was an increase in D and a concomitant decrease in C. Despite lower equivalent weight and hence higher water content SPES-40 exhibited relatively close values of D with Nafion [reg ] 117. The results are discussed in the context of their different microstructures. Values of C showed a closer relationship to water content and the percent volume of aqueous phase in the respective membranes. The values of overall oxygen permeability were significantly higher in Nafion [reg ] 117, with a higher positive slope in its variation with temperature
[en] Lanthanum, yttrium, and neodymium doped uranium dioxide samples in the fluorite structure have been synthesized, characterized in terms of metal ratio and oxygen content, and their enthalpies of formation measured by high temperature oxide melt solution calorimetry. For oxides doped with 10–50 mol % rare earth (Ln) cations, the formation enthalpies from constituent oxides (LnO_1_._5, UO_2 and UO_3 in a reaction not involving oxidation or reduction) become increasingly exothermic with increasing rare earth content, while showing no significant dependence on the varying uranium oxidation state. The oxidation enthalpy of Ln_xU_1_−_xO_2_−_0_._5_x_+_y is similar to that of UO_2 to UO_3 for all three rare earth doped systems. Though this may suggest that the oxidized uranium in these systems is energetically similar to that in the hexavalent state, thermochemical data alone can not constrain whether the uranium is present as U"5"+, U"6"+, or a mixture of oxidation states. The formation enthalpies from elements calculated from the calorimetric data are generally consistent with those from free energy measurements. - Highlights: • We synthesize, characterize Ln_xU_1_−_xO_2_−_0_._5_x_+_y solid solutions (Ln = La, Y, Nd). • Formation enthalpies become more exothermic with increasing rare earth content. • Oxidation enthalpy of Ln_xU_1_−_xO_2_−_0_._5_x_+_y is similar to that of UO_2 to UO_3. • Direct calorimetric measurements are in good agreement with free energy data.
[en] Poly(2-mercaptobenzothiazole) (PMBT) modified glassy carbon electrode (PMBT/GCE) has been fabricated and employed for the selective and simultaneous determinations of dopamine (DA), uric acid (UA) and nitrite (NO2−) in 0.1 mol L−1 phosphate buffer solution (PBS, pH 6.0). The PMBT modified GCE not only exhibits electro-catalytic effects toward the electrochemical oxidations of DA, UA and NO2− with negatively shifted oxidation potentials and enhanced peak current responses, but also resolves the weak and overlapped voltammetric responses of DA, UA and NO2− into three strong and well-defined oxidation peaks using both cyclic voltammetry (CV) and differential pulse voltammetry (DPV) techniques, which can be applied for the selective and simultaneous determinations of DA, UA and NO2− in their mixture. The surface morphology of PMBT film has been investigated by using scanning electron microscope (SEM). The SEM image shows the formation of continuous PMBT film composed of nano-scaled particulates with the diameter varying in the range of 15-25 nm. Under the optimum conditions, the linear concentration dependences of DPV peak current responses are observed for DA, UA and NO2− in the concentration ranges of 0.8-45 μmol L−1, 1.0-165 μmol L−1 and 60-1000 μmol L−1, with the correlation coefficients of 0.9996, 0.9994 and 0.9979, respectively. The detection limits (S/N = 3) are 0.05 μmol L−1, 0.10 μmol L−1 and 0.30 μmol L−1 for measuring DA, UA and NO2−, respectively. The modified electrode has been used for simultaneous determinations of DA, UA and NO2− in human urine and serum samples using standard adding method with satisfactory results
[en] It introduced the principle of the gamma camera and portable gamma spectrometer. Several experiments are used to verify the two sets of instrument. The experimental results show that the gamma camera is able to accurately capture the hotspot locations of radioactive matter. It could supply the input parameters of the source-less calibration system by the hotspot size, distribution and other information for in-situ activity quantitative analysis. The source-less calibration results obtained from high energy nuclide activity were in good agreement with reference value. (author)
[en] The 2008 radioactive source survey results in Suqian City were presented in this paper. The existing problems are addressed in respects of less reasonable management regime, inadequate safety input, low level techniques and insufficient regulatory capability. The countermeasures with a view towards enhanced management of radioactive sources are raised form the prospectives of responsibility implementation, enhanced regulation and coordination, elaborated management measures, and classification-based emphasized management. (author)
[en] Highlights: •Unlike existing chemical modification, the liquid growth remains fiber’s strength. •ZnO nanowires array are grown on carbon fibers with controllable morphology. •ZnO nanowires array modified carbon fibers can reinforce the strength of ceramic matrix composite. •This research will provide a means to produce multifunctional composites. -- Abstract: The surface of carbon fibers was modified by ZnO nanowires using the liquid growth method. NiFe2O4 ceramic matrix composites reinforced by the modified carbon fibers were prepared by a high-temperature solid-state reaction method at 1300 °C for 5 h in N2 atmosphere. The influences of modified carbon fibers on the mechanical performances of NiFe2O4 composites were investigated. The crystal structure of modified carbon fibers and the morphology of modified carbon fibers surface and the NiFe2O4 composites fracture cross-section were observed by meaning of X-ray diffraction (XRD) and scanning electron microscope (SEM), respectively. The results showed that hexagonal wurtzite ZnO nanowires array grew from the surface of carbon fibers vertically, with nanowires diameters ranging from 170 nm to 380 nm and lengths up to 2.8 μm. Unlike existing chemical modification and high temperature oxidation method, the liquid growth allowed morphology control and maintained the single fiber tensile strength substantially unchanged under certain growth procedures. Compared to pure NiFe2O4 ceramic and bare carbon fibers reinforced NiFe2O4 composite, the bending strength of NiFe2O4 composite reinforced with ZnO nanowires surface modified carbon fibers was shown to increase by up to 70% and 45%, respectively. The development of an interphase offering control over the morphology will provide a means to produce multifunctional composites
[en] Highlights: • A novel sensor material based on ionic liquids@nickel cobaltate was constructed. • Various morphologies of NiCo_2O_4 were synthesized for electrocatalytic comparison. • ILs@NiCo_2O_4-P was used to detect cumulative toxic metals for the first time. • The sensor displayed well reproducibility, excellent selectivity and sensitivity. • The method was applied to detect practical samples with satisfactory results. - Abstract: The different morphologies of magnetic nickel cobaltate (NiCo_2O_4) electrocatalysts, consisting of nanoparticles (NiCo_2O_4-N), nanoplates (NiCo_2O_4-P) and microspheres (NiCo_2O_4-S) were fabricated. It was found that the electrocatalytic properties of the sensing materials were strongly dependent on morphology and specific surface area. The porous NiCo_2O_4 hexagonal nanoplates coupled with ILs as modified materials (ILs@NiCo_2O_4-P) for the simultaneous determination of thallium (Tl"+), lead (Pb"2"+) and copper (Cu"2"+), exhibited high sensitivity, long-time stability and good repeatability. The enhanced electrocatalytic activity was attributed to relatively large specific surface area, excellent electronic conductivity, and unique porous nanostructure. The analytical performance of the constructed electrode on detection of Tl"+, Pb"2"+ and Cu"2"+ was examined using differential pulse anodic stripping voltammetry (DPASV). Under optimal conditions, the electrode showed a good linear response to Tl"+, Pb"2"+and Cu"2"+ in the concentration range of 0.1–100.0, 0.1–100.0 and 0.05–100.0 μg/L, respectively. The detection limits (S/N = 3) were 0.046, 0.034 and 0.029 μg/L for Tl"+, Pb"2"+ and Cu"2"+, respectively. The fabricated sensor was successfully applied to detect trace Tl"+, Pb"2"+ and Cu"2"+ in various water and soil samples with satisfactory results. Hence, this work provided a promising material for electrochemical determination of cumulative toxic metals individually and simultaneously.
[en] A poly(2-amino-4-thiazoleacetic acid-co-3-amino-5-mercapto-1,2,4-triazole) (PATA-AMT) film modified glassy carbon electrode (GCE) has been fabricated by using electrochemical copolymerization of 2-amino-4-thiazoleacetic acid (ATA) and 3-amino-5-mercapto-1,2,4-triazole (AMT) and used for electro-catalytic oxidations of dopamine (DA), uric acid (UA) and nitrite ion (NO2−) in 0.1 mol L−1 phosphate buffer solution (PBS, pH 6.0). The surface morphology of the copolythiazole film has been investigated by using scanning electron microscope (SEM). The SEM images show the formation of a non-periodic nano-network structure with a diameter of the fibril varying over the range of 55–85 nm. The copolythiazole film (PATA-AMT) modified GCE (PATA-AMT/GCE) not only exhibits strong electro-catalytic activities toward oxidations of DA, UA and NO2− with negatively shifted oxidation overpotentials and enhanced peak current responses, but also can resolve the sluggish and overlapped voltammetric waves of DA, UA and NO2− into three sensitive and well-defined oxidation peaks by both cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which can be applied for the selective and simultaneous determinations of DA, UA and NO2− in a mixture. Under the optimum conditions, the linear concentration dependences of the DPV current responses are observed for DA, UA and NO2− in the concentration ranges of 0.85–390 μmol L−1, 1.95–1200 μmol L−1 and 2.0–1240 μmol L−1 with the correlation coefficients of 0.9993, 0.9993 and 0.9977, respectively. The detection limits are 0.2 μmol L−1, 0.25 μmol L−1 and 0.5 μmol L−1 for DA, UA and NO2−, respectively (S/N = 3). The different electrochemical behaviors of DA, UA and NO2− at various scan rates indicate that the electrode reaction of DA is an adsorption-controlled process, and those of UA and NO2− are diffusion-controlled processes at PATA-AMT/GCE. The practical application of the modified electrode has been investigated by the simultaneous determinations of DA, UA and NO2− in human urine and serum samples by using standard adding method with satisfactory results