查看更多>>摘要：? 2022 Elsevier B.V.We investigated the carrier transport and space charge formation in the high purity semi-insulating 4H-SiC bulk single crystal. Using the Laser–induced Transient Current Technique we observed anomalous short current waveform oscillations that are caused by very short carrier lifetime ≤ 1 ns. We conclude from a detailed inspection of current waveform shape completed by the analysis of collected charge that the electron mobility decreases in an increasing electric field. The saturation value of electron drift velocity 8.7 × 106 cm2s-1 was evaluated. The Hall effect measurement revealed the n-type electrical conductivity and dominant donor energy of ED = 0.85 eV. We also observed the blocking character of prepared Au contacts, which induce the electron depletion and positive space charge formation in biased sample. The continuous increase of the space charge in a wide time interval of 10-1 s – 104 s, attested to testified on a presence of a dispersed impurity band localized below the Fermi energy. Based on these findings, we proposed a theoretical model describing the observed phenomena very well.
查看更多>>摘要：? 2022 Elsevier B.V.In-situ alloying is a flexible approach to develop novel alloy compositions via Laser Powder Bed Fusion (LPBF). Mixing dissimilar powders, however, turns out a heterogeneous alloying with elements segregation and unconsolidated powder particles. Therefore, the purpose of this work is twofold. First, this paper describes how the in-situ alloying of AlSi10Mg and Cu powders affects the distribution of mixed elements within the as-built alloy. On the other hand, a post-process homogenization heat-treatment was set up to increase the alloy homogeneity. The presence of Cu inhomogeneities was verified both in single scan tracks and bulk samples. Their origin was traced back to the different thermo-physical properties of starting powders. Consequently, due to the intrinsic hardening effect of Cu, nano-hardness spikes in correspondence with Cu segregations were measured. To homogenize the alloy properties across the material bulk, a homogenization heat-treatment at 515 °C was then conducted. After heat-treating, Cu inhomogeneities and intermetallic θ- Al2Cu phase were dissolved entirely into Al matrix leading to an effective alloy homogenization.
查看更多>>摘要：? 2022 Elsevier B.V.In the current study, the effect of in-situ alloying of AlSi10Mg with 5 wt% Ni (pseudoeutectic composition) through the LPBF additive manufacturing technique was investigated. Fabricated samples underwent supplementary annealing treatment at 300 °C for 15 and 120 min, eventually. During the thermal treatment, the fish-scale grains created after printing inside the melt pools after printing vanished gradually. In the as-built sample, molten Ni particles settled as big chunks of Al3Ni in the center and a thin strip of tiny Ni-rich masses in the borders of the melt pools. FIB/SEM and AFM images revealed that after 15 min annealing, silicon cellular dendrites fragment into fine Si particles, and during annealing for 120 min, the aluminum matrix expels out the supersaturated solute silicon atoms. Consequently, Si atoms leave the substrate and diffuse to pre-existing Si particles in cell walls and the triple junctions. Such a collective diffusion of Si atoms leads to the formation of coarse Si particles widespread in the Al matrix. Based on the XRD outputs, annealing for 15 min had not any major effect on Ni-rich phases. however after 120 min, a more brittle intermetallic shell of a Ni-rich phase formed on pre-exist coarse phases. The comparison of the mechanical properties of the Ni-reinforced AlSi10Mg alloy with those reinforced via the addition of other elements/compounds revealed that in a size range close to AlSi10Mg particles, Ni could not be an appropriate candidate for in-situ alloying through the LPBF method. Furthermore, spheroidization of the silicon particles and formation of fragile Ni-rich intermetallic shells having weak interfacial bonding to the Al matrix during the long-term annealing results in a significant reduction in mechanical strength of the specimens.
查看更多>>摘要：? 2022 Elsevier B.V.Mixed metal hydroxide (MMH) based supercapacitor electrodes, with their outstanding performance from the synergistic effect originating from mixed metallic elements, can effectively address the drawbacks of single metal hydroxide (SMH) based electrodes. Owing to their superior electrochemical properties stemming from their unique structural and chemical compositional design, MMH electroactive materials have attracted considerable attention for energy storage devices. Among those devices, a layered double hydroxide (LDH) structure has been emerged as an alternative electrode due to its large interlayer arrangement, high redox activity, and multiple oxidation states. Herein, we have successfully designed cobalt-iron LDH nanosheets (CoFe LDH NSs) on nickel fabric by a facile electrochemical deposition (ECD) method with a chronoamperometry voltage of ? 1.0 V for 100 s. Further, the unique as-fabricated structure that arises from the tunable chemical composition and wide variety of material properties was successfully manipulated to boost the electrochemical energy storage performance. Finally, the optimized electrode made of Co0.5Fe0.5 exhibited superior electrochemical performance due to its porous, surface-stimulated large electroactive area and high conductivity with great stability. By integrating a battery-type electrode of CoFe LDH and a capacitive-type electrode of activated carbon (AC), a hybrid supercapacitor (HSC) device was assembled. Besides, the high areal capacitance of 70 μF/cm2, it delivers high energy density (1.6 mWh/cm2) and power density (0.09 mW/cm2) values along with the excellent cycling stability (91% capacity retention). Moreover, the fabricated flexible device offers the significance as a power source in flexible electronic devices with its high degree of flexibility originated from flexible substrate. This work not only provides a promising electrode for supercapacitors, but also a unique structural and compositional design of electroactive materials for energy storage systems.
查看更多>>摘要：? 2022The rational design and construction of non-precious metal-based electrocatalysts with high activity and stability toward oxygen reduction reactions (ORR) are essential but challenging. In this work, a novel hybrid nanoporous structure of Co nanoparticles uniformly embedded in hollow porous carbon doped with nitrogen (CoNHPC) was constructed by a melt-evaporation-pyrolysis method, in which NaCl is utilized as an auxiliary template. The introduction to NaCl in this process served as the confining template to prevent nitrogen loss and promote graphitization, and the intercalating agent to form a mesoporous product. Due to the composite hierarchical porous structure, higher graphitization degree, and desirable nitrogen bonding type, the as-prepared CoNHPC-920 presents outstanding ORR electrocatalytic activity in terms of a half-wave potential of 0.87 V (vs. RHE) and onset potential of 0.97 V (vs. RHE). The composite material also shows higher selectivity and outstanding durability with most transition metal porous carbon materials reported. The strategy proposed in this study may provide ideas about metal nitrogen-carbon type catalysts enriched with carbon nanotubes and bare leaky hollow structures of the construction of advanced ORR electrocatalysts, opening up a new pathway.
查看更多>>摘要：? 2022 Elsevier B.V.The rational construction of highly efficient electrocatalysts comprising multiple components with distinctive bifunctionalities is still a challenge for the practical application due to their sluggish kinetics for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). Here, a series of cobalt-molybdenum alloy nanorods encapsulated in N-doped carbon shells (CoMo@NC) is synthesized via an in-situ carbonization-reduction method using CoMoO4 as the precursor. The high conductivity, strain-induced effect, and synergistic interactions between N-doped carbon and bimetallic cores endow the optimized catalyst with outperforming catalytic performances for HER and OER in alkaline solution with low overpotentials (98 mV for HER and 336 mV for OER at 10 mA cm?2), as well as high durability. The overall water splitting device using the CoMo@NC sample achieved at 800 ℃ treatment (CoMo@NC-800) as bifunctional catalyst could possess a low voltage of 1.67 V to drive a current density of 10 mA cm?2 and high durability. Furthermore, density functional theory calculations reveal that the pyridinic-N atoms of graphene anchored on CoMo alloy nanoparticles can efficiently modulate the electronic structure to generate optimal free energy of hydrogen adsorption (?0.029 eV), suggesting excellent HER intrinsic activity. This work may provide a facile avenue to achieve multiple metallic alloy-based nanomaterials for boosting electrochemical water splitting performance.
查看更多>>摘要：? 2022 Elsevier B.V.Cu/graphene oxide(GO)/Cu laminated composite foils possessed with simultaneously high thermal and mechanical properties were prepared by a novel fabricating method, including electro-deposition, ultrasonic spraying and vacuum hot-press sintering (VHP) processing. The effect of processing parameters and laminated construction on its microstructure, morphology and properties were studied. The results showed that the C-O-Cu covalent bonding was formed at the interface between electro-deposited layers of copper and ultrasonic sprayed GO films during the fabrication process. Under the optimum process parameters, the in-plane thermal conductivity of the composites reached up to 416.7 W·m-1·k-1, meanwhile, the tensile strength of the composites was 295.1 MPa with an elongation of 20.7%, which were increased by 24.4%, comparing with pure Cu foils. The strong graphene/copper interface bonding and laminated structure were responsible for the simultaneously enhanced thermal and mechanical properties.
查看更多>>摘要：? 2022 Elsevier B.V.A novel effort has been given to transform the incipient ferroelectrics behavior to real in SrTiO3 and to generate white-light emission by doping luminescent Eu3+ and Tb3+ ions. Ferroelectric study showed that upon doping Eu3+ ion, SrTiO3 became pure ferroelectric in nature and upon co-doping Tb3+ ion there is a significant enhancement of the saturation polarization. Photoluminescence study showed that both the Eu3+ doped and Eu3+& Tb3+ co-doped compounds are white-light-emitting materials due to perfect combination of blue light originated from defect centers present in the host with the green and red emissions from the dopant ions. Such white-light-emitting materials with controlled piezo potential may not need external excitation energy and have potential application in many flexible optoelectronic devices. Lifetime study confirmed the existence of two components for both the dopant ions and suggested that there is an asymmetric environment created due to distortion of geometry, which is linked to ferroelectric property. Finally, all the experimental results were supported by Hybrid functional based theoretical calculation. We have observed that the bond angle variance (BAV) parameter and distortion index (DI), which are directly linked with the distortion in the matrix, are responsible for ferroelectric behavior and they follow the order with the dopant ions as Eu3+&Tb3+>Tb3+>Eu3+. This corroborates well with the experimentally observed trend of the saturation polarization.
查看更多>>摘要：? 2022 Elsevier B.V.Multifunctional upconversion luminescent nanoheaters are realized based on NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals synthesized by surfactant-free solvothermal method. The structure, morphology and composition of the nanocrystals are analysed by XRD, SEM, EDS, TEM, HRTEM and SAED techniques. Under 980 nm irradiation, NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals emit blue luminescence with emission peaks located in the blue and near-infrared spectral regions. The electronic transition processes for the upconversion luminescence are discussed based on the excitation power-dependent upconversion luminescence, luminescent dynamics and Yb3+/Tm3+ energy levels. The nanocrystals show efficient photothermal conversion after irradiated by 980 nm laser. The photothermal conversion slope efficiency for NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals irradiated by 980 nm laser for 10 s duration is 17.43 °C/W for 10 mol% Yb3+ doping and 43.64 °C/W for 15 mol% Yb3+ doping. Multifunctional luminescent nanoheater NaGd(MoO4)2:Yb3+/Tm3+ nanocrystals exhibit both upconversion photoluminescence and photothermal conversion capabilities, and are promising candidates for temperature self-monitoring photothermal therapy or other biological applications.
查看更多>>摘要：? 2022 Elsevier B.V.This study experimentally investigates the metastable transformation behavior in a cast Ti-48Al-3Nb1.5Ta (at。%) alloy during continuous cooling by in-situ and ex-situ methods. A CCT diagram of the alloy was constructed. A small amount of Ta significantly promotes metastable transformations, e.g., Widmanst?tten, feathery, and massive γ transformations during continuous cooling at different rates (50–1200 °C min?1). In particular, Ta can reduce the cooling rates required for metastable transformations, decrease the undercooling required and enhance the volume fraction of metastable microstructures. Crystallographic and kinetics studies reveal that the Widmanst?tten transformation is a direct α→γ phase transformation that obeys the Σ11 CSL γ nucleation mechanism rather than {112?2}α twinning. Furthermore, the effect of Ta on metastable transformations is detailed by thermodynamic analyses. Ta may potentially reduce the energy of the CSL boundaries of the γ phase to promote the nucleation of metastable structures. Consequently, uniform refinement of the microstructure can be achieved in low-Ta containing TiAl-Nb alloys by heat treatment alone.
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