查看更多>>摘要：We study the effect of laser shock peening (LSP) without protective coating on the surface mechanical property of NiTi alloy. The Vickers microhardness and wear resistance are measured to determine the mechanical property of NiTi samples treated with different LSP parameters (3 J with 10 ns and 5 J with 20 ns). From the electron backscatter diffraction (EBSD) analysis, it can be found that the laser shock peening does not induce obvious grain refinement in the surface region of NiTi alloy. Both compressive and tensile residual stress in the top layer are determined using the hole drilling method. The results show that the LSP treatment without a protective coating increases the roughness and enhances the surface mechanical properties of NiTi alloy.
查看更多>>摘要：We fabricate the Ag/TaO_x/TiN device and confirm the structure of the element with transmission electron microscopy (TEM) and energy dispersive spectroscopy line scan (EDS). The device shows non-volatile bipolar resistive switching as well as volatile threshold switching. In order to identify the threshold switching, the first reset voltage is needed to activate the device and check how much the current level decreases by the reset voltage. The current gradually increases and drops at a relatively high compliance current (CC) with positive and negative values, respectively. To demonstrate the temporal learning in the volatile switching with short-term memory effect, the states of [1111], [1001], and [1000] 4-bits are controlled by applying different pulse streams, which outputs the letter "P" in the reservoir computing system. At the lowest CC, the abrupt threshold switching is obtained, and the relaxation time in transient measurement is investigated depending on the voltage amplitude.
查看更多>>摘要：Rotary swaging (RS), as a widely used industrial method for severe plastic deformation (SPD), is a near net forming process which uses high-speed pulse hammer of multi-hammer to process solid shaft parts or hollow variable cross-section forgings. Initially RS is mainly used to manufacture difficult-to-deform metals with poor plasticity (such as W and Mg alloys, etc.). Recently, because RS process can enable large hydrostatic stresses and facilitate the accumulation of large strain, it is also allowing for high-throughput mass production of different metals with enhanced mechanical properties. This paper first introduces the RS working principle and unique characteristics, then systematically summarized the microstructural characteristics (grain size, texture, etc.), mechanical properties (such as strength, ductility, etc.), physical and chemical properties of various ultrafine grained metals prepared by RS, finally compares the advantages and disadvantages of swaging with other deformation methods, and looks forward to the future development and application fields of rotary swaging.
查看更多>>摘要：In this study, the microstructural and mechanical properties of NiCoCrAlSi high entropy alloy (HEA) fabricated by mechanical alloying and spark plasma sintering (SPS) method was studied. Initially, the starting powders with the composition of Ni_(28)Co_(26)Cr_(26)Al_(10)Si_(10) were subjected to mechanical milling at different times of 5,10,15, 20 and 40 h. The 40 h milled powder was then sintered by SPS method at 1170 °C. The microstructural characterization, phase analysis and mechanical properties of this sample were evaluated by field emission scanning electron microscopy (FESEM), energy dispersive X-ray spectroscopy (EDS), X-ray diffraction (XRD), Rietveld analysis and tensile test. XRD results showed that by increasing the milling time the size of crystallites reduced and lattice strain increased. In addition, formation of FCC1 (F1) phase at final stages of mechanical alloying was observed. However, the peak of some of the remaining elements was still observed after 40 h of milling. After sintering, formation of FCC2 (F2) phase alongside Fl was observed. According to quantitative analysis, Fl phase with fraction of 89.3 wt% and F2 phase with fraction of 10.7 wt% is formed after SPS process. For this sample, a tensile strength of 1365 ± 141 MPa was obtained.
查看更多>>摘要：To explore the ductility and deformation nature, a new Mg-2.76Li-3Al-2.6Zn-0.39Y alloy has been successfully fabricated by decreasing-temperature multidirectional forging and hot rolling. The maximum elongation to failure of 223.0% was demonstrated in this alloy at a temperature of 633 K and a strain rate of 1.67 × 10~(-4) s~(-1). Flow stress curves and microstructural examination showed that continuous dynamic re-crystallization occurred in this alloy at different temperatures and strain rates in most cases, but the flow hardening phenomena appeared at a temperature of 603 K at a strain rate of 5× 10~(-4) s~(-1) and at 573,603 and 633 K at a strain rate of 1.67 × 10~(-4) s~(-1). A criterion of grain size changing rate was proposed to judge the occurrence of microstructural evolution mechanisms. A modified Johnson-Cook constitutive model established in this alloy was incorporated into dislocation models to realize the estimation of the dislocation density and the number of dislocations under specific conditions. A power-law constitutive equation was established in this alloy. The relationship between average grain size and Zener-Hollomon parameter was established. It was found that the stress exponent was 3.26, and the average experimental activation energy for deformation was 143.67 kj/mol; the dislocation density was 3.25 × 10~(13) m~(-2), and the number of dislocations was 59 at 633 K and 1.67 × 10~(-4) s~(-1). All of these results indicate that the predominant deformation mechanism of this alloy under this condition is dislocation glide creep controlled by lattice diffusion.
查看更多>>摘要：The cylindrical Sn: β-Ga_2O_3 crystal with high crystalline quality was successfully designed and grown by the innovative edge-defined film-fed growth (EFG) method equipped with a cylindrical Iridium die. The challenges for the growth of Sn: β-Ga_2O_3 crystals were overcome by optimizing the design of an afterheater. The growth morphology of cylindrical β-Ga_2O_3 crystal was studied using a theoretical model and the results from experimental crystal growth. The order of importance of growth conditions affecting β-Ga_2O_3 crystal growth morphology was examined, based on the morphological features of cylindrical β-Ga_2O_3 crystals obtained by the EFG and Czochralski methods. The iridium inclusions with three shapes were observed in bulk β-Ga_2O_3 crystal, and the formation mechanism was carefully discussed. The optical bandgap and valence band maximum (VBM) of Sn: β-Ga_2O_3 crystal were calculated to be 4.74 eV and 3.49 eV by absorption spectra and X-ray photoelectron spectroscopy (XPS), respectively. The corresponding surface barrier height (φ_(surf)) was 1.25 eV. The carrier concentration of 5.95× 10~(18) cm~(-3) was characterized by capacitance-voltage (C-V) measurement. By the Hall measurement, the carrier mobility and resistivity were estimated to be around 51 cm~2 V~(-2) s~(-1) and 3.55 ×10~(-2) Ω cm, respectively.
查看更多>>摘要：This work reports the high performance of NiCo-layered double hydroxide (NiCo-LDH) self-growing nanosheet arrays decorated with graphitic carbon nitride (g-C_3N_4) as a potential electrode material for supercapacitor. The composite is prepared by a convenient microwave method and its electrochemical behavior is evaluated. In three electrode system, the graphitic carbon nitride-NiCo-layered double hydroxide (CN-LDH) electrode exhibits high specific capacitance of 1936.36 F g~(-1) at 1 A g~(-1). The results show that adding a certain amount of g-C_3N_4 to LDH can significantly increase the specific capacitance of the electrode. In addition, the electrochemical performances of the electrode are further studied by Density functional theory (DFT) calculations and capacitance contribution fitting, which provides theoretical assistance for the elaborated electrochemical performance and reaction kinetics. Meanwhile, the possible reaction mechanisms are explained from two aspects of current density and scan rate, which revealed a reference idea for the future recombination of g-C_3N_4. The supercapacitor is assembled to verify the practical application of the CN-LDH electrode, CN-LDH//AC displays the maximum energy density of 50.63 Wh kg~(-1) at the power density of 0.80 kW kg~(-1), and successfully lights several LEDs. This confirms that the self-growing nanosheet arrays CN-LDH electrode possesses the potential value for supercapacitor applications.
查看更多>>摘要：Yttrium doped ZnO nanorod array films are hydrothermaliy grown on fluorine doped tin oxide substrates. The yttrium doped ZnO samples maintain the same wurtzite crystal structure and nanorod morphology as those of the undoped samples, but they shows much enhanced photoelectrochemical water splitting performance. It is found that yttrium doping not only enhances the light absorption of the ZnO, but also provides more donors for ZnO and thereby enhances the conductivity of the ZnO nanorods. Moreover, the charge carrier lifetime of ZnO becomes longer after yttrium doping. These changes in the electronic structure of ZnO caused by yttrium doping significantly improve its photoelectrochemical performance. Under AM 1.5 G illumination at 100 mW/cm~2 and a potential of 1.23 V vs. reversible hydrogen electrode, the yttrium doped ZnO samples can get a photocurrent density of - 0.50 mA/cm~2, while the undoped ZnO samples only show photocurrent densities of ~ 0.30 mA/cm~2 under the same conditions. Considering that ZnO has advantages of low cost, environmental friendliness and simple and mild preparation process, it is expected that the yttrium doped ZnO nanorod array films will provide valuable platforms for a wide photocatalytic applications, not just limited to photoelectrochemical water splitting.
查看更多>>摘要：To recover the remanence (or maximum energy product ((BH)_(max))) and squareness with high coercivity in diffusion-processed hot-deformed Nd-Fe-B magnets, new efforts have been made in this work. Annealing at a selected temperature of 800 °C under pressure was applied to the hot-deformed magnets diffused by the Nd_(70)Cu_(30) or Nd_(70)Cu_(15)Ga_(15) alloy. After this process, the thick powder boundary phase dissolved in the matrix and diffused into the intergranular regions. The c-axis misalignment of Nd_2Fe_(14)B grains near the diffusion surface was significantly improved, leading to increased remanence by 0.10-0.18 T and (BH)_(max) by 47-85 kj/m~3 in the two diffusion-processed magnets. The distribution of the powder boundary phase, the proportions of the intergranular Nd-rich phase, the c-axis alignment of grains and the grain sizes from the diffusion surface to the center of the magnets tend to be relatively uniform. The squareness factor of the demagnetization curve increased from 0.89 to 0.95 for the Nd_(70)Cu_(30)-diffused magnet and from 0.85 to 0.93 for the Nd_(70)Cu_(15)Ga_(15)-diffused magnet after the process, returning to the level of the as-deformed magnet. Microstructure analysis and micromagnetic simulations indicate that the improved squareness factor is due to the homogeneous magnetization reversal process caused by the homogeneous microstructure.
查看更多>>摘要：We report a novel mechanoluminescent phosphor Li_(1-x)Ca_xSi_xTa_(1-x)O_3:0.01Pr~(3+) (x = 0-0.04). The crystal structure, photoluminescence (PL), optical diffuse reflectance spectra, afterglow (AG), fluorescence decay curves, and mechanoluminescence (ML) properties were investigated. To clarify the defect types and the mechanoluminescence mechanisms, an X-ray photoelectron spectroscopy (XPS) and thermoluminescence curve (TL) were measured. The dopants Ca~(2+) and Si~(4+) ions occupied the Li+ and Ta~(5+) sites, respectively. An optimal sample Li_(0.98)Ca_(0.02)Si_(0.02)Ta_(0.98)O_3:Pr~(3+) was obtained with great enhancement in mechanoluminescence intensity, higher than that of non-doped sample LiTaO_3:Pr~(3+) by 262%. And the compression threshold, over which the ML intensity could not maintain the linear increase, has also been greatly improved. Furthermore, the stable output of Tribo-L was achieved, which was visible to the naked eye. A probable mechanism was proposed to illustrate the mechanoluminescent process in Lil-xCaxSixTal-xO3:0.01Pr3 + (x = 0-0.04). The study indicates that adjusting trap distribution by substituting host ions may be a novel way to develop high-quality mechanoluminescence.
NSTL
Elsevier