The effect of heat treatment on superconductivity of Zn‐doped YBa2Cu3‐xZnxO7‐y

YBa₂Cu_3‐xZn_xO_7‐y compounds with x = 0, 0.05, 0.15, and 0.30 have been synthesized by standard solid state reaction method. The crystal structure, lattice parameters, and oxygen content are not changed by the substitution of Zn for Cu since both valence state and ionic radius are almost identical for Zn and Cu elements in YBa₂Cu_3‐xZn_xO_7‐y. However, the superconducting transition temperature T_c decreases with the increase of Zn content, reflecting the T_c‐suppression effect of Zn substitution. Heat treatment experiments indicate that the heat treatment at low temperature is beneficial to improve the superconductivity of the sample. But T_c decreases with the increase of annealing temperature when the treatment temperature is above 300°C, and finally the superconductivity disappears at approximately 920°C, 700°C and 550°C for the samples with x = 0.0, 0.05 and 0.15, respectively. Our experiments indicate that the superconductivity of the sample with higher Zn content is more sensitive to the oxygen content, and a small decrease in the oxygen content can lead to a considerable decrease of T_c.     

Mixed Copper/Copper‐Oxide Anchored Mesoporous Fullerene Nanohybrids as Superior Electrocatalysts toward Oxygen Reduction Reaction

Developing a highly active, stable, and efficient non‐noble metal‐free functional electrocatalyst to supplant the benchmark Pt/C‐based catalysts in practical fuel cell applications remains a stupendous challenge. A rational strategy is developed to directly anchor highly active and dispersed copper (Cu) nanospecies on mesoporous fullerenes (referred to as Cu‐MFC₆₀) toward enhancing oxygen reduction reaction (ORR) electrocatalysis. The preparation of Cu‐MFC₆₀ involves i) the synthesis of ordered MFC₆₀ via the prevalent nanohard templating technique and ii) the postfunctionalization of MFC₆₀ with finely distributed Cu nanospecies through incipient wet impregnation. The concurrence of Cu and cuprous oxide nanoparticles in the as‐developed Cu‐MFC₆₀ samples through relevant material characterizations is affirmed. The optimized ORR catalyst, Cu(15%)‐MFC₆₀, exhibits superior electrocatalytic ORR characteristics with an onset potential of 0.860 vs reversible hydrogen electrode, diffusion‐limiting current density (?5.183 mA cm^?2), improved stability, and tolerance to methanol crossover along with a high selectivity (four‐electron transfer). This enhanced ORR performance can be attributed to the rapid mass transfer and abundant active sites owing to the synergistic coupling effects arising from the mixed copper nanospecies and the fullerene framework.     

Copper‐Vapor‐Assisted Growth and Defect‐Healing of Graphene on Copper Surfaces

Although there is significant progress in the chemical vapor deposition (CVD) of graphene on Cu surfaces, the industrial application of graphene is not realized yet. One of the most critical obstacles that limit the commercialization of graphene is that CVD graphene contains too many vacancies or sp³‐type defects. Therefore, further investigation of the growth mechanism is still required to control the defects of graphene. During the growth of graphene, sublimation of the Cu catalyst to produce Cu vapor occurs inevitably because the process temperature is close to the melting point of Cu. However, to date few studies have investigated the effects of Cu vapor on graphene growth. In this study, how the Cu vapor produced by sublimation affects the chemical vapor deposition of graphene on Cu surfaces is investigated. It is found that the presence of Cu vapor enlarges the graphene grains and enhances the efficiency of the defect‐healing of graphene by CH₄. It is elucidated that these effects are due to the removal by Cu vapor of carbon adatoms from the Cu surface and oxygen‐functionalized carbons from graphene. Finally, these insights are used to develop a method for the synthesis of uniform and high‐quality graphene.     

Einfluss der Al2Cu‐Phase auf die Superplastizität der AlCuMn Legierung

Influence of the Al₂Cu‐phase on the superplasticity of AlCuMn alloy High‐temperature creep‐resistant AlCuMn wrought alloy has been investigated and optimised with respect to their superplastic deformability; a maximal elongation ε of 850 per cent was thus attained at a deformation temperature of 530°C. Prerequisites for superplastic deformation behaviour and for the associated high elongation values of these aluminium alloys are an especially fine‐grained structure as well as a decrease in the amount of Al₂Cu phase and a uniform distribution of this phase in the structure. Superplastic deformation (SPD) results in a pronounced change in the shape of the large particles of the θ‐phase; the particles of this phase thereby form veins along the boundaries of adjacent grains. During deformation, the grains lose their equiaxial shape and elongate in the direction of tension as a result of pronounced intragranular sliding dislocation in the microstructure. Transmission electron micrographs of the deformed structure have revealed a pile‐up of dislocations in the grains of the aluminium alloy. The grain size of commercially available sheets of AlCuMn wrought alloys with a thickness of 1 mm is approximately 30 μm. After optimising, the grain size of the sheets produced by the new method was on 12 μm until 5 μm. The new technique differs only slightly from industrial manufacture.     

Classification of Lattice Defects in the Kesterite Cu2ZnSnS4 and Cu2ZnSnSe4 Earth‐Abundant Solar Cell Absorbers

The kesterite‐structured semiconductors Cu₂ZnSnS₄ and Cu₂ZnSnSe₄ are drawing considerable attention recently as the active layers in earth‐abundant low‐cost thin‐film solar cells. The additional number of elements in these quaternary compounds, relative to binary and ternary semiconductors, results in increased flexibility in the material properties. Conversely, a large variety of intrinsic lattice defects can also be formed, which have important influence on their optical and electrical properties, and hence their photovoltaic performance. Experimental identification of these defects is currently limited due to poor sample quality. Here recent theoretical research on defect formation and ionization in kesterite materials is reviewed based on new systematic calculations, and compared with the better studied chalcopyrite materials CuGaSe₂ and CuInSe₂. Four features are revealed and highlighted: (i) the strong phase‐competition between the kesterites and the coexisting secondary compounds; (ii) the intrinsic p‐type conductivity determined by the high population of acceptor Cu_Zn antisites and Cu vacancies, and their dependence on the Cu/(Zn+Sn) and Zn/Sn ratio; (iii) the role of charge‐compensated defect clusters such as [2Cu_Zn+Sn_Zn], [V_Cu+Zn_Cu] and [Zn_Sn+2Zn_Cu] and their contribution to non‐stoichiometry; (iv) the electron‐trapping effect of the abundant [2Cu_Zn+Sn_Zn] clusters, especially in Cu₂ZnSnS₄. The calculated properties explain the experimental observation that Cu poor and Zn rich conditions (Cu/(Zn+Sn) ≈ 0.8 and Zn/Sn ≈ 1.2) result in the highest solar cell efficiency, as well as suggesting an efficiency limitation in Cu₂ZnSn(S,Se)₄ cells when the S composition is high.     

Controlled Synthesis of Hollow Cu2‐xTe Nanocrystals Based on the Kirkendall Effect and Their Enhanced CO Gas‐Sensing Properties

This paper develops a facile solution‐based method to synthesize hollow Cu_2‐xTe nanocrystals (NCs) with tunable interior volume based on the Kirkendall effect. Transmission electron microscopy images and time‐dependent absorption spectra reveal the temporal growth process from solid copper nanoparticles to hollow Cu_2‐xTe NCs. Furthermore, the as‐prepared hollow Cu_2‐xTe NCs show enhanced sensitivity for the detection of carbon monoxide (CO), which is often referred to as the “silent killer”. The response and recovery time of the as‐prepared sensor for the detection of 100 ppm CO gas are estimated to be about 21 and 100 s, respectively, which are sufficient to render it a promising candidate for effective CO gas‐sensing applications. Such enhanced performance is achieved owing to the small grain size and large specific area of the hollow nanostructures. Therefore, the obtained hollow NCs based on the Kirkendall effect may have the potential as new functional blocks for high‐performance gas sensors.     

Recent Advances in Growth of Large‐Sized 2D Single Crystals on Cu Substrates

Large‐scale and high‐quality 2D materials have been an emerging and promising choice for use in modern chemistry and physics owing to their fascinating property profile. The past few years have witnessed inspiringly progressing development in controlled fabrication of large‐sized and single‐crystal 2D materials. Among those production methods, chemical vapor deposition (CVD) has drawn the most attention because of its fine control over size and quality of 2D materials by modulating the growth conditions. Meanwhile, Cu has been widely accepted as the most popular catalyst due to its significant merit in growing monolayer 2D materials in the CVD process. Herein, very recent advances in preparing large‐sized 2D single crystals on Cu substrates by CVD are presented. First, the unique features of Cu will be given in terms of ultralow precursor solubility and feasible surface engineering. Then, scaled growth of graphene and hexagonal boron nitride (h‐BN) crystals on Cu substrates is demonstrated, wherein different kinds of Cu surfaces have been employed. Furthermore, the growth mechanism for the growth of 2D single crystals is exhibited, offering a guideline to elucidate the in‐depth growth dynamics and kinetics. Finally, relevant issues for industrial‐scale mass production of 2D single crystals are discussed and a promising future is expected.     

Ultrafast Self‐Limited Growth of Strictly Monolayer WSe2 Crystals

The controllable synthesis of uniform tungsten diselenide (WSe₂) is crucial for its emerging applications due to the high sensitivity of its extraordinary physicochemical properties to its layer numbers. However, undesirable multilayer regions inevitably form during the fabrication of WSe₂ via the traditional chemical vapor deposition process resulted from the lack of significantly energetically favorable competition between layer accumulation and size expansion. This work innovatively introduces Cu to occupy the hexagonal site positioned at the center of the six membered ring of the WSe₂ surface, thus filtrates the undesired reaction path through precisely thermodynamical control and achieves self‐limited growth WSe₂ crystals. The as‐obtained WSe₂ crystals are characterized as strictly single‐layer over the entire wafer. Furthermore, the strictly self‐limited growth behavior can achieve the “win–win” cooperation with the synthesis efficiency. The fastest growth (≈15 times of the growth rate in the previous work) of strictly monolayer WSe₂ crystals thus far is realized due to the high‐efficiency simultaneous selenization process. The as‐proposed ultrafast Cu‐assisted self‐limited growth method opens a new avenue to fabricate strictly monolayer transition metal dichalcogenides crystals and further promotes their practical applications in the future industrial applications.     

Chemical Intercalation of Topological Insulator Grid Nanostructures for High‐Performance Transparent Electrodes

2D layered nanomaterials with strong covalent bonding within layers and weak van der Waals' interactions between layers have attracted tremendous interest in recent years. Layered Bi₂Se₃ is a representative topological insulator material in this family, which holds promise for exploration of the fundamental physics and practical applications such as transparent electrode. Here, a simultaneous enhancement of optical transmittancy and electrical conductivity in Bi₂Se₃ grid electrodes by copper‐atom intercalation is presented. These Cu‐intercalated 2D Bi₂Se₃ electrodes exhibit high uniformity over large area and excellent stabilities to environmental perturbations, such as UV light, thermal fluctuation, and mechanical distortion. Remarkably, by intercalating a high density of copper atoms, the electrical and optical performance of Bi₂Se₃ grid electrodes is greatly improved from 900 Ω sq^?1, 68% to 300 Ω sq^?1, 82% in the visible range; with better performance of 300 Ω sq^?1, 91% achieved in the near‐infrared region. These unique properties of Cu‐intercalated topological insulator grid nanostructures may boost their potential applications in high‐performance optoelectronics, especially for infrared optoelectronic devices.     

Nanoscale Behavior and Manipulation of the Phase Transition in Single‐Crystal Cu2Se

Phase transition is a fundamental physical phenomenon that has been widely studied both theoretically and experimentally. According to the Landau theory, the coexistence of high‐ and low‐temperature phases is thermodynamically impossible during a second‐order phase transition in a bulk single crystal. Here, the coexistence of two (α and β) phases in wedge‐shaped nanosized single‐crystal Cu₂Se over a large temperature range are demonstrated. By considering the surface free‐energy difference between the two phases and the shape effect, a thermodynamic model is established, which explicitly explains their coexistence. Intriguingly, it is found that with a precise control of the heating temperature, the phase boundary can be manipulated at atomic level. These discoveries extend the understanding of phase transitions to the nanoscale and shed light on rational manipulation of phase transitions in nanomaterials.     

Characterization of the dissolution of the Al2Cu phase in two Al–Si–Cu–Mg casting alloys using calorimetry

The dissolution of Al₂Cu in two Al–Si–Mg–Cu aluminum alloys was investigated by calorimetric analysis and metallographic measurements. Both alloys had similar compositions but very different microstructures. Alloy A was produced by gravity casting and alloy B by thixoforming. The main goal of this research was to analyze the possibility of monitoring the dissolution of the Al₂Cu phase through calorimetric analysis, relating the dissolution of this intermetallic phase with energy variations of the endothermic peaks during differential scanning calorimeter runs. The results obtained were in agreement with metallographic measurements.