J.C. Li

Hall–Petch relationship in nanometer size range

Abstract The effect of melting temperature on the Hall–Petch relationship has been studied. As grain size decreases, the melting temperature of the nano-structured crystals also decreases, therefore the Hall–Petch relationship has its limitation and is no longer sufficient when the grain size decreases to about the 15–30 nm range. When the yield strength or hardness is taken as a function of the reciprocal of the square root of the grain size, it has a numerical maximum whose location depends on the size of the bulk melting enthalpy of the crystals. Experimental results agree well with the modification induced by the size-dependence. In addition, the appropriate size range for the classic Hall–Petch relationship is discussed in terms of the modification of the relationship.

Hierarchical NiCoP nanocone arrays supported on Ni foam as an efficient and stable bifunctional electrocatalyst for overall water splitting

Design of cost-effective, highly efficient and stable bifunctional electrocatalysts for overall water splitting is necessary for renewable energy systems. In this study, NiCoP nanowire arrays grown on 3D Ni foam (NiCoP NWAs/NF) were successfully synthesized by a two-step method, which were developed as novel bifunctional electrocatalysts for evaluating in the hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Their special cone nanostructure and bifunctional crystal structure enable the electrocatalysts to display remarkable electrocatalytic performance and stability for OER and HER (maintained for 28 h in the long-term HER and OER stability test with slight attenuation). The electrodes have very low overpotentials of 197 mV and 370 mV for HER and OER in 1.0 M KOH at a high current density of 100 mA cm−2, respectively. All the merits can be attributed to several parameters: the inherent nature of transition metal phosphides, the presence of a bimetal synergetic effect, special morphology design, and the formation of “secondary” electrocatalysts on the surface of NiCoP. Meanwhile, the excellent bifunctional electrocatalysts can be developed as both anode and cathode of an alkaline electrolyzer (1.0 M KOH) which needs a cell voltage of 1.64 V to achieve 20 mA cm−2 current density.

Flexible MXene–graphene electrodes with high volumetric capacitance for integrated co-cathode energy conversion/storage devices

Developing self-powered integrated energy conversion and storage devices (ECSDs) is regarded as a promising approach to meet the quickly increasing demand for portable and wearable electronics. In this work, we describe a concept of using reduced graphene oxide (rGO) as a binder bridging electrochemically active conducting particles and enabling the manufacturing of flexible all-solid-state rGO/Ti3C2Tx (MXene) film-based supercapacitors. These supercapacitors were integrated with flexible thin-film solar cells by the co-cathode method to fabricate ECSDs. The porous rGO/Ti3C2Tx films were produced by vacuum-assisted filtration of the GO/Ti3C2Tx dispersion, followed by reduction at 300 °C under vacuum. This approach eliminated the need for the delamination of MXenes and allowed for the manufacturing of thick electrodes with good electrolyte accessibility. The volumetric and gravimetric capacitance of the rGO/Ti3C2Tx film in 6 M KOH reached 370 F cm−3 and 405 F g−1, respectively, which can be related to the synergistic effect of rGO and Ti3C2Tx. The assembled all-solid-state supercapacitors (SCs) using the polyvinyl alcohol (PVA)/KOH gel electrolyte exhibited a high energy density of 63 mW h cm−3 at a power density of 0.06 W cm−3. Their capacitance did not change after 10u2006000 charge/discharge cycles at the current density of 5 A g−1, ensuring stable performance of the ECSDs.

Coatings of FeAlCoCuNiV high entropy alloy

The coatings of FeAlCoCuNiV high entropy alloy were deposited by direct current magnetron sputtering. The microstructure and the mechanical and the electrochemical properties of the coatings are investigated. The coatings exhibit single FCC solid solution. The thickness of the coatings increases with the increasing deposited time. The hardness and Youngs modulus of the coatings are 19.7 and 189u2005GPa, respectively. All coatings exhibit better electrochemical corrosion resistance than the 201 stainless steel in acidic alkali and salt corrosion mediums.

Improving Levine model for dielectric constants of transition metal compounds

The density of states and optical properties of several transition metal compounds have been simulated using local density approximations+U. Results show that s electrons of nontransition elements in the compounds cannot be considered to have the same contribution on the number of valence electrons. In light of this observation, the parameter Γ in the Levine model is improved, which leads to better prediction for dielectric constants of transition metal compounds.

Flexible Supercapacitors Based on Polyaniline Arrays Coated Graphene Aerogel Electrodes

Flexible supercapacitors(SCs) made by reduced graphene oxide (rGO)-based aerogel usually suffer from the low energy density, short cycle life and bad flexibility. In this study, a new, synthetic strategy was developed for enhancing the electrochemical performances of rGO aerogel-based supercapacitor via electrodeposition polyaniline arrays on the prepared ultralight rGO aerogel. The novel hybrid composites with coated polyaniline (PANI) arrays growing on the rGO surface can take full advantage of the rich open-pore and excellent conductivity of the crosslinking framework structure of 3D rGO aerogel and high capacitance contribution from the PANI. The obtained hybrid composites exhibit excellent electrochemical performance with a specific capacitance of 432xa0Fxa0g-1 at the current density of 1 A g-1, robust cycling stability to maintain 85% after 10,000 charge/discharge cycles and high energy density of 25xa0Wxa0hxa0kg-1. Furthermore, the flexible all-solid-state supercapacitor have superior flexibility and outstanding stability under different bending states from the straight state to the 90° status. The high-performance flexible all-solid-state SCs together with the lighting tests demonstrate it possible for applications in portable electronics.

Microstructure and Properties of FeAlCrNiMo x High-Entropy Alloys

FeAlCrNiMox high-entropy alloys were prepared. The effect of Mo content on the microstructure and the properties of the alloys were investigated. When the Mo content was 0.1, the alloys were composed of single BCC solid solution; when Mo content reaches 0.25, the alloys were composed of BCC solid solution and ordered B2 solid solution. When Mo content is more than 0.75, some σ phases emerged. The volume fraction of the second phase increases with the increasing Mo content, and the crystal grains became coarsening. The yield strength, fracture strength, and hardness increase with the increasing Mo content and reach 2252, 2612xa0MPa, and 1006 Hv, respectively. The magnetic transformation undergoes from the ferromagnetism to paramagnetism with the increasing Mo content. The saturation intensity and remnant magnetism are decreased with the increasing Mo content.

Microstructure and Properties of FeAlCrNiMox High-Entropy Alloys

FeAlCrNiMox high-entropy alloys were prepared. The effect of Mo content on the microstructure and the properties of the alloys were investigated. When the Mo content was 0.1, the alloys were composed of single BCC solid solution; when Mo content reaches 0.25, the alloys were composed of BCC solid solution and ordered B2 solid solution. When Mo content is more than 0.75, some σ phases emerged. The volume fraction of the second phase increases with the increasing Mo content, and the crystal grains became coarsening. The yield strength, fracture strength, and hardness increase with the increasing Mo content and reach 2252, 2612xa0MPa, and 1006 Hv, respectively. The magnetic transformation undergoes from the ferromagnetism to paramagnetism with the increasing Mo content. The saturation intensity and remnant magnetism are decreased with the increasing Mo content.

Black aspergillus-derived highly porous carbon fibers for capacitive applications

The increasing demand for high-performance supercapacitors has aroused great interest in fabricating materials with excellent specific capacitance and cycling stability. Here we report a porous activated carbon (PAC) derived from the pyrolysis of the renewable filamentous fungi in an inert atmosphere. PAC exhibits a high specific surface area of 1800xa0m2xa0g−1 with hierarchical structures. The N doped PAC electrode shows a remarkable gravimetric capacitance of 298xa0Fxa0g−1, and excellent cycling stability with 100% capacitance retention after 10,000 cycles. In addition, the rate capability of this material is pretty good and better than that of previously reported graphene and activated carbon-based materials. Furthermore, the as-assembled symmetric supercapacitor device exhibits good gravimetric energy density of 10.32xa0Wxa0hxa0Kg−1. This work provides a new sustainable source of filamentous fungi for fabricating high performance porous carbon materials for supercapacitors.

Microstructure and Mechanical Properties of Multicomponent Aluminum Alloy by Rapid Solidification

Al90Ni2.5Ti2.5La2.5Mn2.5 alloy with multicomponent alloying elements was prepared by rapid solidification. The hardness and the compression strength of the alloy reached 285 HV and 712xa0Mpa, respectively. The alloy exhibited good wear resistance, which was three times that of the conventional A309 aluminum alloy. The high strength and wear resistance of the alloy were attributed to the second-phase strengthening and the solid solution strengthening mechanisms.