Boron carbide is a strategic material, finding applications in nuclear industry, armour for personnel and vehicle safety, rocket propellant, etc. Its high hardness makes it suitable for grinding and cutting tools, ceramic bearing, wire drawing dies, etc. Boron carbide is commercially produced either by carbothermic reduction of boric acid in electric furnaces or by magnesiothermy in presence of carbon. Since many specialty applications of boron carbide require dense bodies, its densification is of great importance. Hot pressing and hot isostatic pressing are the main processes employed for densification. In the recent past, various researchers have made attempts to improve the existing methods and also invent new processes for synthesis and consolidation of boron carbide. All the techniques on synthesis and consolidation of boron carbide are discussed in detail and critically reviewed.

Synthesis and consolidation of boron carbide: a review

Magnesium is a light metal, with a density two-thirds that of aluminium, is abundant on Earth and is biocompatible; it thus has the potential to improve energy efficiency and system performance in aerospace, automobile, defence, mobile electronics and biomedical applications. However, conventional synthesis and processing methods (alloying and thermomechanical processing) have reached certain limits in further improving the properties of magnesium and other metals. Ceramic particles have been introduced into metal matrices to improve the strength of the metals, but unfortunately, ceramic microparticles severely degrade the plasticity and machinability of metals, and nanoparticles, although they have the potential to improve strength while maintaining or even improving the plasticity of metals, are difficult to disperse uniformly in metal matrices. Here we show that a dense uniform dispersion of silicon carbide nanoparticles (14 per cent by volume) in magnesium can be achieved through a nanoparticle self-stabilization mechanism in molten metal. An enhancement of strength, stiffness, plasticity and high-temperature stability is simultaneously achieved, delivering a higher specific yield strength and higher specific modulus than almost all structural metals.

Processing and properties of magnesium containing a dense uniform dispersion of nanoparticles

Transition metal nitrides (TMNs), by virtue of their unique electronic structure, high electrical conductivity, superior chemical stability, and excellent mechanical robustness, have triggered tremendous research interest over the past decade, and showed great potential for electrochemical energy conversion and storage. However, bulk TMNs usually suffer from limited numbers of active sites and sluggish ionic kinetics, and eventually ordinary electrochemical performance. Designing nanostructured TMNs with tailored morphology and good dispersity has proved an effective strategy to address these issues, which provides a larger specific surface area, more abundant active sites, and shorter ion and mass transport distances over the bulk counterparts. Herein, the most up-to-date progress on TMN-based nanomaterials is comprehensively reviewed, focusing on geometric-structure design, electronic-structure engineering, and applications in electrochemical energy conversion and storage, including electrocatalysis, supercapacitors, and rechargeable batteries. Finally, we outline the future challenges of TMN-based nanomaterials and their possible research directions beyond electrochemical energy applications.

Transition metal nitrides for electrochemical energy applications.

Voltage fade is a major problem in battery applications for high-energy lithium- and manganese-rich (LMR) layered materials. As a result of the complexity of the LMR structure, the voltage fade mechanism is not well understood. Here we conduct both in situ and ex situ studies on a typical LMR material (Li1.2Ni0.15Co0.1Mn0.55O2) during charge–discharge cycling, using multi-length-scale X-ray spectroscopic and three-dimensional electron microscopic imaging techniques. Through probing from the surface to the bulk, and from individual to whole ensembles of particles, we show that the average valence state of each type of transition metal cation is continuously reduced, which is attributed to oxygen release from the LMR material. Such reductions activate the lower-voltage Mn3+/Mn4+ and Co2+/Co3+ redox couples in addition to the original redox couples including Ni2+/Ni3+, Ni3+/Ni4+ and O2−/O−, directly leading to the voltage fade. We also show that the oxygen release causes microstructural defects such as the formation of large pores within particles, which also contributes to the voltage fade. Surface coating and modification methods are suggested to be effective in suppressing the voltage fade through reducing the oxygen release.Voltage decay is a major problem in applications of high-energy Li- and Mn-rich layer-structured battery materials. Here, the authors report the evolution of redox couples as the origin of the voltage decay and discuss strategies to suppress the problem.

(Invited) Evolution of Redox Couples in Li- and Mn-Rich Cathode Materials and Mitigation of Voltage Fade by Reducing Oxygen Release

Evaporation of a Droplet: From physics to applications

Unprecedented catalytic performance in amine syntheses via Pd/g-C3N4 catalyst-assisted transfer hydrogenation

Manipulating the distribution of rare-earth activators in multiple cation lattices can achieve versatile color output for single-phased phosphor-converted white light-emitting diodes (LEDs). Howeve...

Preferential Neighboring Substitution-Triggered Full Visible Spectrum Emission in Single-Phased Ca10.5–xMgx(PO4)7:Eu2+ Phosphors for High Color-Rendering White LEDs

The wetting and evaporation behaviors of water–ethanol mixtures on polished poly(tetrafluoroethylene) (PTFE) surfaces were studied with an emphasis on the influence of concentration. Three representative stages: (i) constant contact diameter with decreasing contact angle and drop height; (ii) increase in contact angle and drop height with decreasing contact diameter; and (iii) simultaneous decrease in contact angle, drop height, and contact diameter, were identified in the wetting process under evaporation. In the initial stage, with increasing ethanol concentration, the wetting and evaporating behaviors of the mixture drops are close to those of pure ethanol, while in the final stage they are close to those of pure water, regardless of the concentration. The most significant difference between the pure substance and mixture drops is in the intermediate stage due to the different controlling mechanisms behind the de-pinning phenomenon. Moreover, the concrete wetting behavior is dependent on the composition of the mixture drops. For an evaporating sessile drop, the wetting should be evaluated using the combined parameters of contact angle, drop height, and contact diameter. Six representative modes are proposed to describe the wetting behaviors of the water–ethanol sessile drops on the PTFE surfaces under evaporation. Copyright © 2009 John Wiley & Sons, Ltd.

Dan Zhang

Papers

Unprecedented catalytic performance in amine syntheses via Pd/g-C3N4 catalyst-assisted transfer hydrogenation

Preferential Neighboring Substitution-Triggered Full Visible Spectrum Emission in Single-Phased Ca10.5–xMgx(PO4)7:Eu2+ Phosphors for High Color-Rendering White LEDs

Wetting and evaporation behaviors of water-ethanol sessile drops on PTFE surfaces

Wetting of polycrystalline B4C by molten Al at 1173–1473 K

Facile synthesis of Fe4N/Fe2O3/Fe/porous N-doped carbon nanosheet as high-performance anode for lithium-ion batteries