Zhigang Zak Fang

A lithium–oxygen battery based on lithium superoxide

Batteries based on sodium superoxide and on potassium superoxide have recently been reported. However, there have been no reports of a battery based on lithium superoxide (LiO2), despite much research into the lithium–oxygen (Li–O2) battery because of its potential high energy density. Several studies of Li–O2 batteries have found evidence of LiO2 being formed as one component of the discharge product along with lithium peroxide (Li2O2). In addition, theoretical calculations have indicated that some forms of LiO2 may have a long lifetime. These studies also suggest that it might be possible to form LiO2 alone for use in a battery. However, solid LiO2 has been difficult to synthesize in pure form because it is thermodynamically unstable with respect to disproportionation, giving Li2O2 (refs 19, 20). Here we show that crystalline LiO2 can be stabilized in a Li–O2 battery by using a suitable graphene-based cathode. Various characterization techniques reveal no evidence for the presence of Li2O2. A novel templating growth mechanism involving the use of iridium nanoparticles on the cathode surface may be responsible for the growth of crystalline LiO2. Our results demonstrate that the LiO2 formed in the Li–O2 battery is stable enough for the battery to be repeatedly charged and discharged with a very low charge potential (about 3.2 volts). We anticipate that this discovery will lead to methods of synthesizing and stabilizing LiO2, which could open the way to high-energy-density batteries based on LiO2 as well as to other possible uses of this compound, such as oxygen storage.

Hydrogen Storage Properties of Nanosized MgH2−0.1TiH2 Prepared by Ultrahigh-Energy−High-Pressure Milling

Magnesium hydride (MgH(2)) is an attractive candidate for solid-state hydrogen storage applications. To improve the kinetics and thermodynamic properties of MgH(2) during dehydrogenation-rehydrogenation cycles, a nanostructured MgH(2)-0.1TiH(2) material system prepared by ultrahigh-energy-high-pressure mechanical milling was investigated. High-resolution transmission electron microscope (TEM) and scanning TEM analysis showed that the grain size of the milled MgH(2)-0.1TiH(2) powder is approximately 5-10 nm with uniform distributions of TiH(2) among MgH(2) particles. Pressure-composition-temperature (PCT) analysis demonstrated that both the nanosize and the addition of TiH(2) contributed to the significant improvement of the kinetics of dehydrogenation and hydrogenation compared to commercial MgH(2). More importantly, PCT cycle analysis demonstrated that the MgH(2)-0.1TiH(2) material system showed excellent cycle stability. The results also showed that the DeltaH value for the dehydrogenation of nanostructured MgH(2)-0.1TiH(2) is significantly lower than that of commercial MgH(2). However, the DeltaS value of the reaction was also lower, which results in minimum net effects of the nanosize and the addition of TiH(2) on the equilibrium pressure of dehydrogenation reaction of MgH(2).

Densification and grain growth during sintering of nanosized particles

Abstract The sintering of nanosized particles is a scientific and technological topic that affects the manufacture of bulk nanocrystalline materials and the understanding of the stability of nanoparticles. Owing to their extremely small size and the high surface to volume ratio, nanoparticles during sintering exhibit a number of distinctively unique phenomena compared to the sintering of coarse powders. Particularly, it is generally found that the sintering temperatures of nanosized particles are dramatically lower than that of their micrometre or submicrometre sized counterparts. Research has also shown that the grain growth during nanosintering consists of an initial dynamic grain growth stage that occurs during heating up and the normal grain growth stage that occurs mostly during isothermal holding. For nanoparticles, the effect of the initial grain growth cannot be ignored because that it is sufficient to cause the material to lose nanocrystalline characteristics. This review aims to bring to focus the understanding of the fundamental issues of nanosinteirng, including the thermodynamic driving force of nanosintering, non-linear diffusion and the kinetics of nanosintering, and the relationships between agglomeration, densification and grain growth. This review will also examine the effects of microstructure and processing variables.

Hydrogenation of Nanocrystalline Mg at Room Temperature in the Presence of TiH2

Magnesium and magnesium-based alloys are considered attractive candidates as rechargeable hydrogen storage materials because of their high hydrogen storage capacities (theoretically up to 7.6 wt %), reversibility, and low cost. In this work, the hydrogenation of nanocrystalline magnesium at room temperature in the presence of TiH(2) was studied. The magnesium was derived by dehydrogenation of nanostructured MgH(2)-0.1TiH(2) prepared by using an ultra-high-energy and high-pressure planetary milling technique. Significant uptake of hydrogen by magnesium at room temperature was observed. The results demonstrate that the nanostructured MgH(2)-0.1TiH(2) system is superior to undoped nano- or micrometer-scaled MgH(2) with respect to the hydrogenation properties of magnesium at room temperature. This finding is potentially useful for a range of energy applications including mobile or stationary hydrogen fuel cells, cooling medium in electricity generation, and differential pressure compressors.

Mechanical properties of a hybrid cemented carbide composite

Microstructural effects on the mechanical properties of a hybrid metal matrix composite, double cemented (DC) carbide, have been investigated. DC carbide contains granules of WC/Co cemented carbide in a matrix of cobalt. Overall composite hardness increases with decreased granule cobalt content as well as with decreased intergranular matrix fraction of cobalt. High-stress abrasive wear resistance also increases with decreased granule cobalt content and matrix fraction. Fracture toughness of the composite increases with increased cobalt matrix fraction and to a lesser extent with increased granule cobalt content. Increased granule size increases both fracture toughness and wear resistance. DC carbide exhibits a superior combination of fracture toughness and high-stress wear resistance than conventional cemented carbide. The combination of toughness and wear resistance in the composite improves with increased granule hardness.

Thermodynamic and kinetic destabilization of magnesium hydride using Mg-In solid solution alloys.

Efforts to thermodynamically destabilize magnesium hydride (MgH2), so that it can be used for practical hydrogen storage applications, have been a difficult challenge that has eluded scientists for decades. This letter reports that MgH2 can indeed be destabilized by forming solid solution alloys of magnesium with group III and IVB elements, such as indium. Results of this research showed that the equilibrium hydrogen pressure of a Mg-0.1In alloy is 70% higher than that of pure MgH2. The temperature at 1 bar hydrogen pressure (T1bar) of Mg-0.1In alloy was reduced to 262.9 °C from 278.9 °C, which is the T1bar of pure MgH2. Furthermore, the kinetic rates of dehydrogenation of Mg-0.1In alloy hydride doped with a titanium intermetallic (TiMn2) catalyst were also significantly improved compared with those of MgH2.

Fracture Resistant Super Hard Materials and Hardmetals Composite with Functionally Designed Microstructure

Abstract Polycrystalline diamond and other hard materials are widely used in earth boring, mining, and construction tool applications. Chipping and fracture resistance is often improved by various means at the expense of hardness and wear resistance. This trade-off between wear resistance and chipping resistance hinders the development of hard and super hard materials for many industrial applications. A new approach, characterized as “hard materials composites with functionally designed microstructure” including polycrystalline diamond and cemented tungsten carbide, is discussed. The functionally designed microstructure offers enhanced chipping resistance and toughness without significantly sacrificing wear resistance.

Titanium and Titanium Alloy via Sintering of TiH2

Blended elemental (BE) powder metallurgy (PM) is a promising low cost approach for manufacture titanium and titanium alloy components. Conventional BE method relies on sintering of pure titanium metal powder, while the new approach examined in this investigation produces bulk titanium materials by sintering titanium hydride powder. Dehydrogenation and densification of TiH2 powders with different particle sizes and TiH2-6Al-4V alloy powder was studied using thermogravimetric and dilatometric techniques. The results show that the dehydrogenation of TiH2 leads to very rapid shrinkage of α-Ti during sintering. In contrast, densification TiH2-6Al-4V requires dissolution of alloy elements which occurs during sintering above its beta transus temperature.

Influence of particle size distribution on coarsening

Abstract This study has examined the evolution of the particle size distribution and the effect of the initial and transient distributions on coarsening kinetics and the path of evolution. A numerical procedure has been employed to simulate the coarsening process statistically. It was found that the distribution passes through a continuous range of forms from the initial distribution towards the asymptotic state, with initially narrow distributions widening and wide distributions narrowing. Experimental studies using liquid phase sintered Cu-20 Co alloy with different initial distribution widths agreed with the above simulation results qualitatively. The simulated coarsening rate was found to be related to the width and shape of particle size distribution. A rate constant has been derived relating the instantaneous coarsening rate and the transient moments of the distribution. The effect of the initial distribution on coarsening rate was found to be particularly significant in the early stage of coarsening when rapid distribution changes occur. After these early rapid transients but still far from the asymptotic state, the instantaneous coarsening rate was closely related to the instantaneous geometric standard deviation of the distribution.

A New, Energy-Efficient Chemical Pathway for Extracting Ti Metal from Ti Minerals

Titanium is the ninth most abundant element, fourth among common metals, in the Earths crust. Apart from some high-value applications in, e.g., the aerospace, biomedicine, and defense industries, the use of titanium in industrial or civilian applications has been extremely limited because of its high embodied energy and high cost. However, employing titanium would significantly reduce energy consumption of mechanical systems such as civilian transportation vehicles, which would have a profound impact on the sustainability of a global economy and the society of the future. The root cause of the high cost of titanium is its very strong affinity for oxygen. Conventional methods for Ti extraction involve several energy-intensive processes, including upgrading ilmenite ore to Ti-slag and then to synthetic rutile, high-temperature carbo-chlorination to produce TiCl4, and batch reduction of TiCl4 using Mg or Na (Kroll or Hunter process). This Communication describes a novel chemical pathway for extracting titanium metal from the upgraded titanium minerals (Ti-slag) with 60% less energy consumption than conventional methods. The new method involves direct reduction of Ti-slag using magnesium hydride, forming titanium hydride, which is subsequently purified by a series of chemical leaching steps. By directly reducing Ti-slag in the first step, Ti is chemically separated from impurities without using high-temperature processes.