Influence of colloidal Au on the growth of ZnO nanostructures

Vapor-liquid-solid processes allow growing high-quality nanowires from a catalyst. An alternative to the conventional use of catalyst thin films, colloidal nanoparticles offer advantages not only in terms of cost, but also in terms of controlling the location, size, density, and morphology of the grown nanowires. In this work, we report on the influence of different parameters of a colloidal Au nanoparticle suspension on the catalyst-assisted growth of ZnO nanostructures by a vapor-transport method. Modifying colloid parameters such as solvent and concentration, and growth parameters such as temperature, pressure, and Ar gas flow, ZnO nanowires, nanosheets, nanotubes and branched-nanowires can be grown over silica on silicon and alumina substrates. High-resolution transmission electron microscopy reveals the high-crystal quality of the ZnO nanostructures obtained. The photoluminescence results show a predominant emission in the ultraviolet range corresponding to the exciton peak, and a very broad emission band in the visible range related to different defect recombination processes. The growth parameters and mechanisms that control the shape of the ZnO nanostructures are here analyzed and discussed. The ZnO-branched nanowires were grown spontaneously through catalyst migration. Furthermore, the substrate is shown to play a significant role in determining the diameters of the ZnO nanowires by affecting the surface mobility of the metal nanoparticles

NbSe2 meets C2N: a 2D-2D heterostructure catalysts as multifunctional polysulfide mediator in ultra-long-life lithium–sulfur batteries

The shuttle effect and sluggish conversion kinetics of lithium polysulfides (LiPS) hamper the practical application of lithium–sulfur batteries (LSBs). Toward overcoming these limitations, herein an in situ grown C2N@NbSe2 heterostructure is presented with remarkable specific surface area, as a Li–S catalyst and LiPS absorber. Density functional theory (DFT) calculations and experimental results comprehensively demonstrate that C2N@NbSe2 is characterized by a suitable electronic structure and charge rearrangement that strongly accelerates the LiPS electrocatalytic conversion. In addition, heterostructured C2N@NbSe2 strongly interacts with LiPS species, confining them at the cathode. As a result, LSBs cathodes based on C2N@NbSe2/S exhibit a high initial capacity of 1545 mAh g-1 at 0.1 C. Even more excitingly, C2N@NbSe2/S cathodes are characterized by impressive cycling stability with only 0.012% capacity decay per cycle after 2000 cycles at 3 C. Even at a sulfur loading of 5.6 mg cm-2, a high areal capacity of 5.65 mAh cm-2 is delivered. These results demonstrate that C2N@NbSe2 heterostructures can act as multifunctional polysulfide mediators to chemically adsorb LiPS, accelerate Li-ion diffusion, chemically catalyze LiPS conversion, and lower the energy barrier for Li2S precipitation/decomposition, realizing the “adsorption-diffusion-conversion” of polysulfides.Award-winningPostprint (author's final draft

Influence of copper telluride nanodomains on the transport properties of n-type bismuth telluride

The high processing cost, poor mechanical properties and moderate performance of BiTe–based alloys used in thermoelectric devices limit the cost-effectiveness of this energy conversion technology. Towards solving these current challenges, in the present work, we detail a low temperature solution-based approach to produce BiTe-CuTe nanocomposites with improved thermoelectric performance. Our approach consists in combining proper ratios of colloidal nanoparticles and to consolidate the resulting mixture into nanocomposites using a hot press. The transport properties of the nanocomposites are characterized and compared with those of pure BiTe nanomaterials obtained following the same procedure. In contrast with most previous works, the presence of CuTe nanodomains does not result in a significant reduction of the lattice thermal conductivity of the reference BiTe nanomaterial, which is already very low. However, the introduction of CuTe yields a nearly threefold increase of the power factor associated to a simultaneous increase of the Seebeck coefficient and electrical conductivity at temperatures above 400 K. Taking into account the band alignment of the two materials, we rationalize this increase by considering that CuTe nanostructures, with a relatively low electron affinity, are able to inject electrons into BiTe, enhancing in this way its electrical conductivity. The simultaneous increase of the Seebeck coefficient is related to the energy filtering of charge carriers at energy barriers within BiTe domains associated with the accumulation of electrons in regions nearby a CuTe/BiTe heterojunction. Overall, with the incorporation of a proper amount of CuTe nanoparticles, we demonstrate a 250% improvement of the thermoelectric figure of merit of BiTeThis work was supported by the European Regional Development Funds and by the Generalitat de Catalunya through the project 2017SGR1246. Y.Z, C.X, M.L, K.X and X.H thank the China Scholarship Council for the scholarship support. MI acknowledges financial support from IST Austria. YL acknowledges funding from the European Union’s Horizon 2020 research and innovation program under the Marie Sklodowska-Curie grant agreement No. 754411. ICN2 acknowledges funding from Generalitat de Catalunya 2017 SGR 327 and the Spanish MINECO project ENE2017-85087-C3. ICN2 is supported by the Severo Ochoa program from the Spanish MINECO (grant no. SEV-2017-0706) and is funded by the CERCA Program/Generalitat de Catalunya. Part of the present work has been performed in the framework of Universitat Autònoma de Barcelona Materials Science PhD program

Theory and Practice of Using Pulsed Electromagnetic Processing of Metal Melts

In industrial practice, various methods of external influences on metal melts are used. For example, vibration processing, exposure to ultrasound, and other physical fields. The main purpose of such influences is purposeful grinding of the metal structure, which contributes to the improvement of mechanical characteristics. The article presents an overview of research on pulse processing of ferrous and non-ferrous melts: processing with pulsed current, electromagnetic pulses and pulsed magnetic fields. The results of the analysis showed that, despite the different methods and devices used for these treatments, their effect on the structure and properties of the cast metal is generally the same. The main effect is observed in the refinement of the macro and microstructure and a simultaneous increase in the strength properties and plasticity. The intensity of the observed effects depends on the characteristics of the equipment used to create the pulses. The main characteristics are: pulse duration, pulse frequency, current amplitude, and power

Synthesis and characterization of high entropy carbide-MAX two-phase composites

In the present work, the formation of high entropy MAX phase during mechanical alloying and subsequent spark plasma sintering (SPS) of two powder mixtures with (HfZrNbTiTa)3SiC2 and (HfZrNbTiTa)3(SiAl)1.1C1.95 stoichiometries is investigated. The results indicate that a high entropy carbide (HEC) with cubic HfC-prototype structure is formed for both compositions after 7 h of mechanical alloying. The microstructural observations indicate the nano-sized nature of the as-milled powders. For both specimens, consolidation at high temperatures leads to a phase transition from single HEC to HEC-MAX complex. Ultrahigh microhardness values of 11 ± 2 GPa and 8 ± 2 GPa were obtained for the consolidated (HfZrNbTiTa)3SiC2 and (HfZrNbTiTa)3(SiAl)1.1C1.95 specimens, respectively. The results also revealed that the (HfZrNbTiTa)3(SiAl)1.1C1.95 exhibits ferromagnetic behavior with low magnetization saturation of only 1.709 emu/g, while (HfZrNbTiTa)3SiC2 shows a ferromagnetic behavior with relatively low coercivity and magnetization saturation. Finally, the findings provide a new route to tailor mechanical properties of HEC-MAX complexes by controlling the evolution of MAX phase

Phase formation and thermoelectric properties of Zn1+xSb binary system

The phase formation and thermoelectric (TE) properties in the central region of the Zn−Sb phase diagram were analyzed through synthesizing a series of ZnSb (x=0, 0.05, 0.1, 0.15, 0.25, 0.3) materials by reacting Zn and Sb powders below the solidus line of the Zn−Sb binary phase diagram followed by furnace cooling. In this process, the nonstoichiometric powder blend crystallized in a combination of ZnSb and β-ZnSb phases. Then, the materials were ground and hot pressed to form dense ZnSb/β-ZnSb composites. No traces of Sb and Zn elements or other phases were revealed by X-ray diffraction, high resolution transmission electron microscopy and electron energy loss spectroscopy analyses. The thermoelectric properties of all materials could be rationalized as a combination of the thermoelectric behavior of ZnSb and β-ZnSb phases, which were dominated by the main phase in each sample. ZnSb composite exhibited the best thermoelectric performance. It was also found that Ge doping substantially increased the Seebeck coefficient of ZnSb and led to significantly higher power factor, up to 1.51 mW·m·K at 540 K. Overall, an exceptional and stable TE figure of merit (Z) of 1.17 at 650 K was obtained for ZnGeSb

Study of the Influence of V, Mo and Co Additives on the Carbide Formation and Microhardness during Thermal Diffusion Chrome Planting of X35CrNi2-3 Steel

Saturation diffusion with chromium has not been adequately studied among all the surface thermochemical treatment (STCT) processes of steels. Especially, the complex saturation behavior when several elements are added directly for chemical treatment needs to be systematically studied. This work aims at determining the effect of V, Mo, and Co on the parameters of chromium thermal saturation diffusion (thickness, phase composition, microstructure, and microhardness) of the surface layer in X35CrNi2-3 steel. The process was carried out at a temperature of 1000 °C for 24 h. The results showed that complex structural chromium plating together with the addition of strong carbide-forming elements (V and Mo) has a significant effect on the phase composition of the fabricated layer, where the formation of VC and Mo2C carbides significantly increases the microhardness of the samples to 2000 HV and 2500 HV, respectively. On the other hand, the addition of Co with a less carbide-forming affinity has little effect on the phase composition of the coating, and nitride compounds predominated in the microstructure similar to the single-element chromium plating. The results indicate the possibility of improving and accelerating the traditional thermal chromium plating processes and opening up new horizons for obtaining gradient coatings with superior tribological properties

Effect of Electromagnetic Pulses on the Microstructure and Abrasive Gas Wear Resistance of Al0.25CoCrFeNiV High Entropy Alloy

High entropy alloys (HEAs) are among the most promising materials, owing to their vast chemical composition window and unique properties. Segregation is a well-known phenomenon during the solidification of HEAs, which negatively affects their properties. The electromagnetic pulse (EMP) is a new technique for the processing of a metal melt that can hinder segregation during solidification. In this study, the effect of an EMP on the microstructure and surface properties of Al0.25CoCrFeNiV HEA is studied. An EMP, with an amplitude of 10 kV, a leading edge of 0.1 ns, a pulse duration of 1 ns, a frequency of 1 kHz, and pulse power of 4.5 MW, was employed for melt treatment. It was found that the microstructure of Al0.25CoCrFeNiV HEA changes significantly from dendritic, for an untreated sample, to lamellar “pearlite-like”, for an EMP treated sample. Moreover, EMPs triggered the formation of a needle-like σ-phase within the solid solution grains. Finally, these microstructural and compositional changes significantly increased the microhardness of Al0.25CoCrFeNiV HEA, from 343 ± 10 HV0.3 (without the EMP) to 553 ± 15 HV0.3 (after the EMP), and improved its resistance against gas-abrasive wear. Finally, an EMP is introduced as an effective route to modify the microstructure and phase formation of cast HEAs, which, in turn, opens up broad horizons for fabricating cast samples with tailorable microstructures and improved properties

Improving strength-ductility synergy of nano/ultrafine-structured Al/Brass composite by cross accumulative roll bonding process

Increasing the strength of metallic multilayered composites fabricated through accumulative roll bonding (ARB) is typically accompanied by a sacrifice in ductility. In the current work, we propose a strategy to achieve microstructural refinement and outstanding strength-ductility synergy in Al/Brass composites. Here, the aluminum matrix exhibits a bimodal grain distribution, consisting of fine equiaxed grains with an average size of ∼100 nm and ultrafine-elongated grains, in which the brass fragments were distributed uniformly. These microstructural features, introduced through cross accumulative roll bonding (CARB), provide synergistic strengthening effects. The CARB processed composite exhibits a mean misorientation angle of 43.16° and a fraction of high angle grain boundaries of 87%, compared to values of 38.02° and 79% for ARB processed specimen. The CARB processed composite demonstrates a major texture characterized by prominent Rotated Brass {110}, Rotated Goss {011}, and Rotated Cube {001} components. In contrast, the ARB processed specimen revealed strong Goss {011}, Rotated Goss {011}, Brass {011}, and S {123} components. The Copper {112} and S {123} components were nearly absent in the CARB processed composite, because both of them were unstable under the CARB regime. The CARB processed composite shows a tensile strength of 405 MPa and a remarkable elongation of 12.4% at ambient temperature, outperforming ARB processed specimen with a tensile strength of 335 MPa and elongation of 9.5%. These unique mechanical properties in the CARB processed composite are ascribed to the dislocation strengthening, bimodal grain size distribution, uniformity of the brass fragments, and quality of bonding at the interfaces

Ge-Doped ZnSb/β-Zn4Sb3 Nanocomposites with High Thermoelectric Performance

ZnSb/β-ZnSb nanocomposites are produced from Zn GeSb mixtures using a two-step process. First, proper amounts of the three elements are mixed, melted, and reacted at 800 K. During this process, the nonstoichiometric mixture is crystallized in a combination of ZnSb and β-ZnSb phases. Then, the material is ball milled and subsequently hot pressed. Through this process, a dense ZnSb/β-ZnSb composite, consisting of β-ZnSb nanoinclusions embedded within a ZnSb matrix, is formed. The particular phase distribution of the final ZnSb/β-ZnSb composites is a consequence of the harder and more brittle nature of ZnSb than ZnSb, which translates into a stronger reduction of the size of the ZnSb crystal domains during ball milling. This small particle size and the high temperature generated during ball milling result in the melting of the ZnSb phase and the posterior crystallization of the two phases in a ZnSb/β-ZnSb matrix/nanoinclusion-type phase distribution. This particular phase distribution and the presence of Ge result in excellent thermoelectric performances, with power factors up to 1.5 mW m K, lattice thermal conductivities down to 0.74 W m K, and a thermoelectric figures of merit, ZT, up to 1.2 at 650 K, which is among the highest ZT values reported for ZnSb