Analysis of spherical indentation of materials with plastically graded surface layer

AbstractIn the present work, a comprehensive parametric study for establishing contact mechanics of instrumented normal spherical indentation on homogeneous materials and materials with plastically graded surface layer (PGSL) was undertaken by dimensional analysis and finite element modeling. The spherical indentation response for homogeneous materials can be described only by two dimensionless parameters: strain hardening exponent and a unified parameter that can describe effects of both the normalized yield strength and the normalized indentation depth. The influences of these two parameters were investigated for a wide range of engineering materials, and the results may be used as an estimate of loading response and pile-up/sink-in behavior when the material properties are known. In the materials with PGSL, a linear gradient in yield strength, and no variation in elastic modulus and strain hardening exponent were explored. The indentation response of the materials with PGSL can be described only by three dimensionless parameters: the normalized indentation depth, the dimensionless strength gradient parameter, and the normalized PGSL thickness. The effects of these three parameters were studied systematically. The normalized pile-up/sink-in parameter is found to be an increasing function of the strength gradient parameter. The normalized pile-up/sink-in parameter increases (decreases) with increasing PGSL thickness for a fixed positive (negative) gradient case at large indentation depth. The results also indicate that the materials with positive PGSL can bear more loads and have significantly more resistance to contact crack formation

Highly selective oxidation of benzene to phenol with air at room temperature promoted by water

Phenol is one of the most important fine chemical intermediates in the synthesis of plastics and drugs with a market size of ca. $30b1 and the commercial production is via a two-step selective oxidation of benzene, requiring high energy input (high temperature and high pressure) in the presence of a corrosive acidic medium, and causing serious environmental issues2-5. Here we present a four-phase interface strategy with well-designed Pd@Cu nanoarchitecture decorated TiO2 as a catalyst in a suspension system. The optimised catalyst leads to a turnover number of 16,000-100,000 for phenol generation with respect to the active sites and an excellent selectivity of ca. 93%. Such unprecedented results are attributed to the efficient activation of benzene by the atomically Cu coated Pd nanoarchitecture, enhanced charge separation, and an oxidant-lean environment. The rational design of catalyst and reaction system provides a green pathway for the selective conversion of symmetric organic molecules

Bandgap engineering of organic semiconductors for highly efficient photocatalytic water splitting

The bandgap engineering of semiconductors, in particular low‐cost organic/polymeric photocatalysts could directly influence their behavior in visible photon harvesting. However, an effective and rational pathway to stepwise change of the bandgap of an organic/polymeric photocatalyst is still very challenging. An efficient strategy is demonstrated to tailor the bandgap from 2.7 eV to 1.9 eV of organic photocatalysts by carefully manipulating the linker/terminal atoms in the chains via innovatively designed polymerization. These polymers work in a stable and efficient manner for both H2 and O2 evolution at ambient conditions (420 nm < λ < 710 nm), exhibiting up to 18 times higher hydrogen evolution rate (HER) than a reference photocatalyst g‐C3N4 and leading to high apparent quantum yields (AQYs) of 8.6%/2.5% at 420/500 nm, respectively. For the oxygen evolution rate (OER), the optimal polymer shows 19 times higher activity compared to g‐C3N4 with excellent AQYs of 4.3%/1.0% at 420/500 nm. Both theoretical modeling and spectroscopic results indicate that such remarkable enhancement is due to the increased light harvesting and improved charge separation. This strategy thus paves a novel avenue to fabricate highly efficient organic/polymeric photocatalysts with precisely tunable operation windows and enhanced charge separation

Selective Partial Oxidation of Methane by Photocatalysis under Ambient Conditions

Photocatalytic partial oxidation of methane to high-value chemicals as an environment-friendly, cost-efficient and energy-efficient technology has a great potential to substitute the current energy-intensive multistage thermal catalytic pathway to activate methane for the chemical industry. The expected products include selective oxidised compounds (e.g. methanol, formic acid), long-chain products (e.g. ethylene, ethanol) and even cyclic compound (e.g. benzene). In the project, the feasibility of photocatalytic activation of methane was firstly investigated by TiO2 photocatalyst. Highly dispersed iron species on titanium dioxide was optimised to present a high selectivity for the transformation of methane in the presence of H2O2. It showed a methane conversion of 15% with an alcohol selectivity of over 97% (methanol selectivity over 90%) and a yield of 18 moles of alcohol per mole of iron active sites in just 3 hours, which was far better than the just reported benchmark results. Advanced characterisation data confirmed the function of the major iron-containing species—namely, FeOOH and Fe2O3, which enhanced charge transfer and separation, decreased the overpotential of the reduction reaction and improved the selectivity towards methanol over carbon dioxide production. However, the light absorption capability of TiO2 was limited by the relative wide bandgap of 3.2 eV and band positions for such metal oxides were hard to be manipulated by modification. Therefore, one group of polymer-based photocatalysts, covalent triazine-based framework, with tuneable band structure was then studied. The photocatalytic activity and band structure were first investigated by a similar but relatively easy process of water splitting half-reactions. By tuning the degree of conjugation and carbonisation, the capacity of charge transfer and separation was enhanced, resulting in an unprecedented activity for both oxygen and hydrogen evolution under visible light irradiation. The apparent quantum efficiencies (AQE) of 3.8% for O2 evolution and over 6% for H2 evolutional were achieved at 420 nm. The activity was near 20 times higher than the benchmark polymer photocatalyst g-C3N4 for oxygen evolution and 50 times higher for hydrogen evolution under visible light irradiation. Furthermore, the optimised covalent triazine-based framework (CTF)-1 was investigated for photocatalytic methane transformation. By using water-saturated air instead of expensive H2O2 as an oxidant, such approach could continuously convert methane to ethanol instead of methanol in a fluidic reactor with an extremely high selectively of ~ 80 % at one atmospheric pressure and room temperature, realising conversion and C-C coupling in one step. The reaction mechanism was proposed based on spectroscopic measurements and structural analysis. Isotopic labelling confirmed that water molecules enhanced the C-H cleavage of methane and calorimetry experiments suggested energetically favourable desorption of ethanol comparing to methanol on the polymer photocatalysts, making ethanol as the major product. Such catalysts presented no activity decay during 12-hour stability tests, resulting in ca. 250 μmol g-1 h-1 ethanol generation and ca. 7% apparent quantum efficiency at the applied single wavelength which was generated by the most economical LED light source

Numerical Simulation Study on Fatigue Life of Notched Specimens with Gradient Surface Strengthening Layer

A modified Tanaka-Mura model is carried out to derive the equivalent stress amplitude at stress ratio R=-1 of complex fatigue stress and the effect of compress residual stress on fatigue life. Then, the fatigue of notched specimens with gradient surface strengthening layer were investigated by means of numerical simulation. The results indicate that the fatigue initiation life and the initiation site of notched specimen are related with the thickness of the strengthening layer, the surface-to-internal hardness ratio and the residual stress. There is a critical thickness. If the strengthening layer thickness is less than the critical value, fatigue crack is initiated at the interface of the matrix and the strengthening layer, otherwise at the surface of the notch root. The critical thickness value is increased with the increase of surface-to-internal hardness ratio. Residual compress stress has little effect on the fatigue initiation life, but the residual tensile stress decreases the fatigue initiation life obviously

Modification of Covalent Triazine-Based Frameworks for Photocatalytic Hydrogen Generation

The conversion of solar energy and water to hydrogen via semiconductor photocatalysts is one of the efficient strategies to mitigate the energy and environmental crisis. Conjugated polymeric photocatalysts have advantages over their inorganic counterparts. Their molecular structures, band structures, and electronic properties are easily tunable through molecular engineering to extend their spectral response ranges, improve their quantum efficiencies, and enhance their hydrogen evolution rates. In particular, covalent triazine-based frameworks (CTFs) present a strong potential for solar-driven hydrogen generation due to their large continuous π-conjugated structure, high thermal and chemical stability, and efficient charge transfer and separation capability. Herein, synthesis strategies, functional optimization, and applications in the photocatalytic hydrogen evolution of CTFs since the first investigation are reviewed. Finally, the challenges of hydrogen generation for CTFs are summarized, and the direction of material modifications is proposed

Effects of Loading Frequency and Loading Type on High-Cycle and Very-High-Cycle Fatigue of a High-Strength Steel

High-cycle and very-high-cycle fatigue tests via rotary bending (52.5 Hz), electromagnetic resonance (120 Hz) axial cycling, and ultrasonic (20 kHz) axial cycling were performed for a high-strength steel with three heat treatment conditions, and the effects of loading frequency and loading type on fatigue strength and fatigue life were investigated. The results revealed that the loading frequency effect is caused by the combined response of strain rate increase and induced temperature rise. A parameter &eta; was proposed to judge the occurrence of loading frequency effect, and the calculated results were in agreement with the experimental data. In addition, a statistical method based on the control volume was used to reconcile the effect of loading type, and the predicted data were consistent with the experimental results

Investigation on the Fatigue Crack Propagation of Medium-Entropy Alloys with Heterogeneous Microstructures

The behavior and the mechanism of fatigue crack propagation in CrCoNi medium-entropy alloys (MEAs) with heterogeneous microstructures were investigated in this paper. After cold-rolling and recrystallization annealing at different temperatures and times, five sets of heterostructured specimens were acquired with different recrystallization levels. Then, the structure characterizations of these five sets of specimens were carried out by nanoindentation testing and electron back-scatter diffraction (EBSD) mapping. Finally, the fatigue crack propagation tests were conducted on single edge crack specimens of these different heterogeneous microstructures. The experimental results indicate that the crack propagation rates of specimens with partial recrystallization microstructures are higher than those with complete recrystallization microstructures, and the effect on fatigue crack thresholds of these specimens is the opposite. The fatigue cracks grow along the slip planes or twin boundaries in recrystallization grains (RGs), which induced crack deflections and the roughness-induced crack closure effect. For this reason, the area percentage of recrystallization and the grain size of RGs have a great effect on the value of the fatigue crack growth threshold