ABSTRACT
At the initial rolling temperature of 400 °C, ZK60 magnesium alloy was hot rolled by three different rolling paths with different roll speed ratios (RSR) of 1:1.15, 1:1.2, and 1:1.5, respectively. The effects of different rolling processes on the microstructure and mechanical properties of the alloy were studied. The microstructure, plasticity, strength, hardness, and texture intensity of rolled samples were analyzed in this work. The results show that the microstructure uniformity of the alloy under multi-path asynchronous rolling (MAR) is significantly improved, which improves the mechanical properties of the material to a certain extent, and effectively weakens the texture intensity of the basal plane and reduces the anisotropy. The amount of randomly oriented grains of ZK60 magnesium alloy rolled by the C-1.5 (path C combined with the RSR of 1:1.5) process are significantly increased, which significantly weakens the basal texture and improves the ductility of the alloy, greatly enhancing the processing and formability of ZK60 magnesium alloy.
ABSTRACT
The electric field induces complex effects on the tribological properties of zinc oxide (ZnO) under lubricated conditions, particularly at the nanoscale, where the friction process and mechanism remain unclear. In this paper, the tribological behaviors of ZnO under the lubrication of poly α-olefins (PAO) were investigated by molecular dynamics (MD) simulations with reactive force field (ReaxFF). The results reveal a significant enhancement in the tribological performances of ZnO with the application of the electric field, resulting in a 58.6% reduction in the coefficient of friction (COF) from 0.193 at 0 V/Å to 0.080 at 0.1 V/Å. This improvement can be attributed to the weakening of interfacial interaction, evidenced by a reduction in the number of C-O covalent bonds under the influence of the electric field, along with the formation of an adsorption film due to applied load and shear effects. Notably, the effect of the electric field and applied load extends the impact of interface slip on the tribological performance of ZnO. Overall, this study provides a comprehensive understanding of the impact of the electric field on reducing the friction of ZnO-based structured models, shedding light on explaining their tribological properties and lubrication mechanisms.
ABSTRACT
High-entropy oxides (HEOs), a class of compounds that include five or more elemental species, have gained increasing attraction for their capability of optimizing the target properties. To date, even though some high-entropy oxides have been successfully prepared, their applications still need to be explored. In the present study, a lithium-manipulation strategy for constructing transition metal oxides (LTM) via a modified solid-state method was investigated. The as-synthesized LTM contained six highly dispersed metal species (Li, Fe, Co, Ni, Cu, Zn) and demonstrated a rocksalt-type structure. Besides, with the introduction of Li, more oxygen vacancies were produced which was also accompanied by shrinking of the lattice constant. When the molar ratio of Li was equal to the other TM cations (LTM16.7), the electrical conductivity was greatly enhanced by a factor of 10 times. Moreover, LTM16.7 achieved the best HER (η = 207 mV at 10 mA cm-2) and OER performances (η = 347 mV at 10 mA cm-2) with elevated electrical conductivity. To facilitate further design of this new kind of materials, we also conducted DFT calculations and elemental alternation experiments, which showed that Fe acted as electrocatalytic sites in this HEOs system. This Li-incorporation strategy opens a new way to understand and modify defect-related HEOs.
ABSTRACT
Design and synthesis of advanced electrode materials with fast and stable ion storage are of importance for energy storage applications. Herein, we propose that introducing the heterogeneous interface in layer-structured mesocrystals is an efficient way to greatly improve the rate capability and cycle stability of lithium-ion battery (LIB) devices. NH4TiOF3 mesocrystals were employed as a typical model system to demonstrate the idea. The NH4TiOF3 mesocrystals were obtained via the hydrothermal reaction, and the NH4TiOF3/TiO2 interfaces were generated through calcining at different temperatures under an argon atmosphere. Phase composition, microstructure, and chemical analyses show that the as-prepared NH4TiOF3 mesocrystals possess "tablet-like" morphology, and the formation of the NH4TiOF3/TiO2 interface can be controlled by the calcination temperature. When evaluated as the anode for LIBs, the optimized sample (NH4TiOF3 calcined at 250 °C, NTF-250) shows excellent, fast, and stable lithium storage properties. Specifically, the NTF-250 electrode holds a reversible capacity of 159.5 mA h g-1 after 200 cycles at 0.2 A g-1. At a high current density of 20 A g-1, the electrode still maintains a reversible capacity of 89.6 mA h g-1 and reaches a reversible capacity of 128.6 mA h g-1 at a current density of 1 A g-1 after 2000 cycles. Theoretical and experimental studies show that the synergistic effects of the heterogeneous NH4TiOF3/anatase TiO2 interface in the layer-structured NH4TiOF3 mesocrystals lead to the upgraded electrochemical properties. Especially, the local build-in electric field induced by the nonuniform distribution of charge across the NH4TiOF3/anatase TiO2 interface facilitates the charge transport during the charging and discharging cycling. The current electrode design strategy paves a new way in boosting stable ion storage and thus is of great interest in energy storage and conversion.
ABSTRACT
Hematite has been widely studied as one of the most promising photoanodes in the photoelectrochemical decomposition of water. At present, a prevailing strategy of coupling dopant (Ti or Sn) with oxygen vacancies has been proposed by experiment, and effectively improves the photocatalytic activity. In order to clarify the intrinsic reasons for the improvement of the photochemical activity, density functional theory is adopted to calculate the formation mechanism and electronic properties of hematite with doping ions and oxygen vacancies. The result shows that the doped atom is beneficial to the formation of oxygen vacancies in hematite, thus forming a stable structure containing doping ions and oxygen vacancies. Due to the synergistic effects of dopant and oxygen vacancies, the bandgap of hematite decreases, and donor levels are introduced into the bandgap, which lead to the increase of carrier concentration. In the system with doped Ti and oxygen vacancies, donor levels are introduced at 1.47 eV and 1.73 eV below the bottom of the conduction band, respectively. For the case containing Sn and oxygen vacancies, the donor level is introduced at 1.75 eV from the conduction band minimum. Our results elaborate the reasons for the enhancement of carrier densities in terms of electronic structure, and provide some guidance for the future modification of photocatalysts.
ABSTRACT
In this theoretical study, first-principles calculations were carried out to explore the photocatalytic activity of cation (Ti or Zr) and anion (N) compensated codoped hematite based on density functional theory (DFT). For (Ti/Zr,N) codoped hematite, the band edges of the conduction band and the valence band move close to each other, leading to an obvious bandgap reduction. Compared with the pure hematite, the optical absorption coefficient of codoped hematite is significantly enhanced in the visible light region. The charge distribution at the conduction band minimum (CBM) and valence band maximum (VBM) is spatially separated after codoping, which is beneficial for extending the carrier lifetime. More interestingly, the CBM becomes electronically delocalized in (Ti,N) doped hematite, which indicates better carrier transport properties in the bulk system. Due to these special features of (Ti/Zr,N) codoped hematite, an improved photocatalytic performance can be expected.
ABSTRACT
Two new ternary compounds in the TiO2-TiC system, Ti5C2O6 and Ti3C2O2, are reported for the first time based on ab initio evolutionary algorithm. Ti5C2O6 has a tube-structure in which sp(1) hybridized carbon chains run through the lattice along the b-axis; while in the Ti3C2O2 lattice, double TiO6 polyhedral are separated by the non-coplanar sp(2) hybridized hexagon graphite layers along the c-axis, forming a sandwich-like structure. At ambient conditions, the two compounds are found to be mechanically and dynamically stable and intrinsic transparent conductors with high hardness (about twice harder than the conventional transparent conducting oxides). These mechanical, electronic, and optical properties make Ti5C2O6 and Ti3C2O2 ternary compounds be promising robust, hard, transparent, and conductive materials.
ABSTRACT
The surface chemical activity is a critical factor affecting the photocatalytic efficiency of hematite. In this study, we investigate systematically the reaction kinetics of water heterolytic dissociation (H2O-OH(-) + H(+)) and hydrogen generation by water splitting on four kinds of hematite (0001) surfaces, namely perfect and defective O- and Fe-terminated surfaces, at the electronic level based on first-principles calculations. The simulation results illustrate that the chemical reaction rate for the dissociation and hydrogen generation is sensitive to the morphology of the hematite (0001) surface. For water heterolytic dissociation, the hydrogen atom is apt to drop from water molecules on the perfect O-terminated (0001) surface without energy consumption. However, the Fe-terminated (0001) perfect surface is a preferable candidate for hydrogen generation, on which the whole photoelectrochemical process needs to overcome a rate determined barrier of 2.77 eV. Our investigation shows that O- or Fe-vacancy on hematite (0001) surfaces is not conductive to hydrogen generation by water splitting.
