Heguang Liu

Electrochemistry of Selenium with Sodium and Lithium: Kinetics and Reaction Mechanism

There are economic and environmental advantages by replacing Li with Na in energy storage. However, sluggishness in the charge/discharge reaction and low capacity are among the major obstacles to development of high-power sodium-ion batteries. Among the electrode materials recently developed for sodium-ion batteries, selenium shows considerable promise because of its high capacity and good cycling ability. Herein, we have investigated the mechanism and kinetics of both sodiation and lithiation reactions with selenium nanotubes, using in situ transmission electron microscopy. Sodiation of a selenium nanotube exhibits a three-step reaction mechanism: (1) the selenium single crystal transforms into an amorphous phase Na0.5Se; (2) the Na0.5Se amorphous phase crystallizes to form a polycrystalline Na2Se2 phase; and (3) Na2Se2 transforms into the Na2Se phase. Under similar conditions, the lithiation of Se exhibits a one-step reaction mechanism, with phase transformation from single-crystalline Se to a Li2Se. Intriguingly, sodiation kinetics is generally about 4-5 times faster than that of lithiation, and the kinetics during the different stages of sodiation is different. Na-based intermediate phases are found to have improved electronic and ionic conductivity compared to those of Li compounds by first-principles density functional theory calculations.

NiSe2 pyramids deposited on N-doped graphene encapsulated Ni foam for high-performance water oxidation

The development of highly efficient water oxidation electrocatalysts made of low-cost and earth-abundant elements is a prerequisite. Sluggish kinetics in the reaction of water splitting is the major obstacle. Herein, we report the fabrication of a robust catalyst for the oxygen evolution reaction (OER) based on the hybrid of N-doped graphene coupled metallic NiSe2 pyramids (NG/NiSe2/NF). The reaction kinetics has greatly increased due to the synergistic effects of the two components providing enhanced electroconductibility and increased active sites. The NG/NiSe2/NF electrode exhibits superior water oxidation ability and cycle stability. This approach opens ways to design effective oxygen evolution electrodes.

Thermal Insulation Composite Prepared from Carbon Foam and Silica Aerogel Under Ambient Pressure

Carbon foam/silica aerogel composite as a promising thermal insulation material was prepared under ambient pressure successfully in the present work. Carbon foam was prepared by pretreatment, foaming, and carbonization process, while silica aerogel was synthesized by sol-gel method. The microstructure, morphology characteristics, compression strength, and thermal properties of composite were characterized by infrared spectroscopy, x-ray diffraction, scanning electron microscope, universal testing machine, and laser flash thermal detector, respectively. Results showed that silica aerogel was successfully synthesized in the surface foam cells of carbon foam due to the closed cell structure of carbon foam. Moreover, the compressive strength of the carbon foam was not affected by the silica aerogel in the cell structure of carbon foam, while its thermal insulation property at room temperature was improved.

Experimental Study of the Biaxial Cyclic Behavior of Thin-Wall Tubes of NiTi Shape Memory Alloys

Combined torsion-tension cycling experiments were performed on thin-wall tubes (with thickness/radius ratio of 1:20, similar to that found for stents) of nearly equiatomic NiTi shape memory alloys (SMAs). Experiments were controlled by axial displacement and torsional angle with step loading involving torsional loading to a maximum strain, followed by tensile loading, and reverse-order unloading. The superelasticity of the material is confirmed by pure torsion and tension experiments at the test temperature. The evolution of equivalent stress-strain curves as well as the separated tensile and torsional stress-strain curves during cycling is analyzed. Results show that the equivalent stress increases greatly with a small amount of applied axial strain, and the equivalent stress-strain curves have negative slopes in the phase transformation region. The shear stress drops when the torsional strain is maintained at its maximum value and the tensile strain is increased. The shear stress increases with decreasing tensile strain, but it cannot recover to the original value after the complete unloading of the tensile strain. Attention is also paid to dissipated energy density and characteristic stress evolutions during cycling.

Origin of Fracture-Resistance to Large Volume Change in Cu-Substituted Co3O4 Electrodes

The electrode materials conducive to conversion reactions undergo large volume change in cycles which restrict their further development. It has been demonstrated that incorporation of a third element into metal oxides can improve the cycling stability while the mechanism remains unknown. Here, an in situ and ex situ electron microscopy investigation of structural evolutions of Cu-substituted Co3 O4 supplemented by first-principles calculations is reported to reveal the mechanism. An interconnected framework of ultrathin metallic copper formed provides a high conductivity backbone and cohesive support to accommodate the volume change and has a cube-on-cube orientation relationship with Li2 O. In charge, a portion of Cu metal is oxidized to CuO, which maintains a cube-on-cube orientation relationship with Cu. The Co metal and oxides remain as nanoclusters (less than 5 nm) thus active in subsequent cycles. This adaptive architecture accommodates the formation of Li2 O in the discharge cycle and underpins the catalytic activity of Li2 O decomposition in the charge cycle.

Influences of ellipse cooling holes on creep crack initiation and surface morphology of nickel based superalloys

Abstract A creep damage model is developed with the consideration of cavitation and material microstructure degradation as damage factors. Five models are studied with different ratios between the length of ellipse hole axis parallel (denoted by y) and perpendicular to the loading direction (denoted by x), where y maintains constant. Two loading conditions are taken into account: constant external load and constant net section stress. The creep crack initiation time increases with increasing y/x ratio at both loading conditions. The stress distribution morphologies are quite similar for all the models at the start of creep where there are high stress strips between holes in the adjacent rows and low stress strips between holes in the same rows. During the creep, stress relaxation and redistribution induce some differences in the morphologies for different models at the crack initiation time. The damage distributions provide important information on crack initiation and propagation path.

A medium manganese steel designed for water quenching and partitioning

ABSTRACT An aluminium-containing medium manganese steel has been designed to undergo intercritical annealing followed by quenching in water and subsequent partitioning. Water quenching, replacing the quenching temperature (QT) between 150 and 300°C in conventional quenching and partitioning steels, is therefore adopted in QP alloys, in order to guarantee the precise QT in practice. The low intercritical annealing temperature of 750°C refines both ferrite and prior austenite grains into submicron size. The large fraction of ultra-fine ferrite, as well as the transformation-induced plasticity effect of retained austenite, improves the overall ductility of this water-quenched and partitioned steel. The alloy has achieved excellent mechanical properties of 1130 MPa ultimate tensile strength combined with 19.2% total elongation.

Kinetics of Sodium and Selenium Reactions in Sodium Ion Batteries

Selenium and sulfur, both as chalcogen elements, show similar volumetric capacity as cathode material for both lithium and sodium ion batteries.[1] Additionally selenium has notable higher electrical conductivity than sulfur.[2] In this work, we have investigated the kinetics of sodiation reaction in selenium nanotube as the cathode material for sodium ion battery. We have monitored the microstructure evolution and interface dynamics using in situ TEM during sodiation process. A three steps reaction mechanism appears to explain the sodiation process (Figure 1). In the first step, single crystalline selenium nanotube rapidly transforms to an amorphous NaxSe alloy phase. In the second step with continued charging, the amorphous phase recrystallizes to a polycrystal Na2Se2 phase. In the final step near full sodiation, polycrystalline Na2Se2 appears to completely transform into Na2Se phase with high content of Na. Intriguingly, the reaction front region movement is found to be quite different in the different sodiation stages. The solid-state amorphization process quickly finishes due to the high diffusion of sodium ions inside Se nanotube, with the highest nominal speed of ~2.8 nm/s, and the recrystallization processes has a speed of ~1.0 nm/s (Figure 2). Moreover the speed of solid-state amorphization process is nearly 10 times higher than lithation process when selenium nanotube were tested in lithiation reaction. Molecule Dynamics (MD) calculation shows all the intermediate phases produced in sodiation are good conductor of both electrons and ions. These observations can not only reveal the reaction mechanism and reaction process, but also to provide insights to design novel nanostructure of electrodes with excellent electrochemical performance.

The effect of elliptic type cooling holes on the creep behavior of nickel based single crystal alloys

Experimental results of our previous researches showed that cooling holes can increase the creep life of the plate specimens with round cooling holes. Finite element analysis showed the distances of the round cooling holes have influences on the creep characteristics of the specimen. In the present study, the influences of the elliptic type holes are investigated. The hole distances are chosen from the round holes with the longest creep life. Elliptic holes with different long/short axis ratios are studied with the minimum cross-sections of the calculated models maintaining the same. It could be found that elliptic hole type greatly affect the creep behavior of the alloys. The stress redistributions differ from that of the round holes. With the proper selection of long/short axis ratio, the creep life of the specimen can be greatly increased.