Hydrogen adsorption of O/N-rich hierarchical carbon scaffold decorated with Ni nanoparticles: Experimental and computational studies

Hierarchical carbon scaffold (HCS) with multi-porous structures, favoring hydrogen diffusion and physisorption is doped with 2–10 wt % Ni for storing hydrogen at ambient temperature. Due to N- and O-rich structure of melamine-formaldehyde resin used as carbon precursor, homogeneous distribution of heteroatoms (N and O) in HCS is achieved. 2 wt % Ni-doped HCS shows the highest hydrogen capacity up to 2.40 wt % H₂ (T = 298 K and p (H₂) = 100 bar) as well as excellent reversibility of 18.25 g H₂/L and 1.25 wt % H₂ (T = 298 K and p (H₂) = 50 bar) and electrical production from PEMFC stack up to 0.7 Wh upon eight cycles. Computations and experiments confirm strong interactions between Ni and heteroatoms, leading to uniform distribution small particles of Ni. This results in enhancing reactive surface area for hydrogen adsorption and preventing agglomeration of Ni nanoparticles upon cycling. Ni K-edge XANES spectra simulated from the optimized structure of Ni-doped N/O-rich carbon using DFT calculations are consistent with the experimental spectra and suggest electron transfer from Ni to hydrogen to form Ni–H bond upon adsorption. Considering low desorption temperature (323 K), not only chemisorbed hydrogen is involved in adsorption mechanisms but also physisorption and spillover of hydrogen.     

Synthesis, characterization, and magnetic properties of monodisperse CeO2 nanospheres prepared by PVP-assisted hydrothermal method

ABSTRACT: Ferromagnetism was observed at room-temperature in monodisperse CeO2 nanospheres synthesized by hydrothermal treatment of Ce(NO3)3.6H2O using polyvinylpyrrolidone (PVP) as a surfactant. The structure and morphology of the products were characterized by; X-ray diffraction (XRD), Raman spectroscopy, transmission electron microscopy (TEM), and field-emission scanning electron microscopy (FE-SEM). The optical properties of the nanospheres were determined using ultraviolet and visible spectroscopy (UV-Vis) and photoluminescence (PL). The valence states of Ce ions were also determined using X-ray absorption near edge spectroscopy (XANES). The XRD results indicated that the synthesized samples had a cubic structure with a crystallite size in the range of ~ 9-19 nm. FE-SEM micrographs showed that the samples had a spherical morphology with a particle size in the range of ~ 100-250 nm. The samples also showed a strong UV absorption and room temperature photoluminescence. The emission might be due to charge transfer transitions from the 4f band to the valence band of the oxide. The magnetic properties of the samples were studied by Vibrating Sample Magnetometer (VSM). The samples exhibited room temperature ferromagnetism (RT-FM) with a small magnetization of ~ 0.0026-0.016 emu/g at 10 kOe. Our results indicate that oxygen vacancies could be involved in the ferromagnetic exchange and the possible mechanism of formation was discussed based on the experimental results.     

Superplastic-like forming of non-superplastic AA5083 combined with mechanical pre-forming

Superplastic forming has been considered as an attractive process in the automotive and aerospace industries. However, the disadvantages of slow forming rate, high-temperature requirement, poor thickness distribution, and expensive base material have hindered its widespread use for high production volume. In this paper, the non-superplastic grade of 5083 aluminum alloy (AA5083) sheets with thickness of 3?mm was employed in a superplastic-like forming process, which is a combination of drawing (mechanical pre-forming) and superplastic forming (blow forming). Experimental trials were conducted to verify the possibility of improving the forming rate and lowering the process temperature. The blank was firstly pre-formed during the mechanical pre-forming phase. As a result, some part of material along the flange area was introduced inside the deformation cavity in advance of the blow forming phase. Secondly, argon gas was applied on the sheet, which would be deformed to come into contact with the inner die surface at the end of pressure cycle. It took only 8?min for the blow forming phase, and the process achieved an almost fully formed part at 400°C. The minimum thickness occurred at the inward corners, and the maximum thinning of the formed part was 54%. Grain growth and cavitation were found from the microstructure observations.     

On the characteristics of tubular materials for hydroforming—experimentation and analysis

Increasing acceptance and use of hydroforming technology within the automotive industry demands a comprehensive understanding of related issues such as material characteristics, tribology, part and tooling design. Among these issues, characterization and specification of tubular material properties under hydroforming conditions is the main concern of this paper. Analytical improvements and their comparison with experimental findings on measurement of material properties of tubes under hydraulic bulging conditions are explained. With these improvements, ‘on-line’ and continuous measurement of flow stress for tubular materials become possible, and are proven to be in good agreement with previous ‘off-line’ measurements presented by the authors.     

The influence of die geometry and workpiece mechanical properties in T-Shape friction test

In this study, T-Shape friction test was redesigned to make it more suitable for application to microforming processes. Workpiece with aspect ratio (length/diameter) of 5 was proposed in order to ease workpiece handling. The die geometry was also modified from the original test to improve friction sensitivity especially within the range of friction factors commonly observed in metal forming. Geometric deviation of the die was simulated using Deform-2D to establish the acceptable tolerance for the fabrication. The effect of variation in workpiece mechanical properties on the test behavior was also investigated through Deform-2D simulation. Based on simulations on a 1 mm diameter copper workpiece, a tolerance of 0.01 mm (1% of workpiece diameter) was found to be the most suitable for the die fabrication. In addition, it was shown that variations in workpiece mechanical properties of up to 10% do not significantly influence the friction test results. Ultimately, T-Shape test experiment was conducted using copper workpieces to examine how the test complied with the friction behavior observed in the experiment.     

Superplastic-like forming of Ti-6Al-4V alloy

Superplastic forming of titanium alloys is used for producing structural components, since it is an effective way to manufacture complex-shaped parts in a one-step operation. An optimized sheet-forming process has been designed incorporating a non-isothermal heating system to establish a fast forming process. This work sought to expand the advantages of the technology to the forming of Ti-6Al-4V alloy at 800 °C and shorter cycle time. The minimum thicknesses area was found at the outward corners, showing a maximum percent thinning of 54 %. In addition to stress variations, the cracks resulting from hot drawing and the oxidation on the sheet surface are the other reasons leading to thickness reduction. From the oxidization behavior of Ti-6Al-4V alloy, it was revealed that the decrease in forming temperature from 900 to 800 °C significantly reduced the formation rate of oxide film on the sheet surface. The study also showed that the main microstructure evolution of Ti-6Al-4V alloy under these conditions was recrystallization.