查看更多>>摘要：In this work, a series of (l-x)MgAl_2O_4-xMg_2TiO_4 (expressed as (l-x)MA- xMT) (0.0 < x < 0.9) ceramics were successfully prepared via a solid-state reaction, and the effect of Mg_2TiO_4 content on the phase compositions, microstructures and microwave dielectric properties of MgAl_2O_4 ceramics have been investigated. The MgAl_2O_4 could form a solid solution with Mg_2TiO_4, and the grain size of the (l-x)MA-xMT ceramics gradually increased with the increase of sintering temperature. The dielectric constants (ε_r) of (l-x)MA-xMT ceramics steadily increased with the increase of Mg_2TiO_4 content. The ε_r values were mainly affected by secondary phase and dielectric polarizability. The quality factor (Qf) of (l-x)MA-xMT ceramics was closely related to the relative density, secondary phase, cation vacancies and Ti valence; whereas the temperature coefficient of resonance frequency (τ_f) highly depended on the Al-site bond strength. For the sample at x = 0.9, the 0.1MA-0.9MT ceramics sintered at 1350 °C for 4 h exhibited the optimized microwave dielectric properties: ε_r = 12.36, Qf = 236,600 GHz and τ_f = -61 ppm/°C.
查看更多>>摘要：The analysis of the phase nucleation requires not only the in-situ images but also the accurate nucleation information. Thus, the nucleation of both the primary Ni_3Sn_2 and peritectic Ni_3Sn_4 phases were studied during peritectic solidification of Sn-Ni alloy using the high temperature confocal laser scanning microscope (HT-CLSM) and differential scanning calorimetry (DSC). The results by both methods confirm that the nucleation undercooling AT of the solid phases decrease with increasing cooling rates, which is demonstrated to be caused by higher concentration difference at larger cooling rates. Besides, the freezing range of the alloy is also found to decrease with the increase in cooling rate. However, the nucleation undercooling of the peritectic Ni_3Sn_4 phase determined by HT-CLSM is larger than that by DSC at each cooling rate. This is attributed to the slower capture of peritectic reaction when using HT-CLSM. Furthermore, the nucleation of peritectic Ni_3Sn_4 phase is confirmed to be controlled by the heterogeneous nucleation on primary Ni_3Sn_2 phase by comparison between the HT-CLSM observation and DSC result. In addition, the wetting angles for heterogeneous nucleation of both phases which are important parameters in analyzing the nucleation of solid phases were determined through the entropy of solidification data from the DSC results. The smaller wetting angle of peritectic Ni_3Sn_4 phase as compared with that of the primary Ni_3Sn_2 phase further proving the smaller nucleation undercooling of Ni_3Sn_4 phase during solidification.
查看更多>>摘要：Positron beam-based Doppler broadening results on He ion-implanted FeCrCoNi show distinct changes in defect-sensitive S-parameter corresponding to the formation of helium-vacancy complexes, nucleation/ growth of helium bubbles, and finally the helium bubble rupture, with isochronal annealing. Data analyses of the positron beam-based ratio curves provide clear information on the kinetics of bubble growth and rupture. The difference data in S-parameter (AS), which corresponds to the bubble growth, displays a maximum at 1123 K, with annealing temperature. Comparing the AS value to that reported for nickel, it is concluded that FeCrCoNi has higher bubble growth resistance than nickel.
查看更多>>摘要：The sluggish reaction kinetics, polysulfide shuttling and low-efficiently use of sulfur largely imped the rate performance and energy density of lithium-sulfur batteries. To overcome the bottleneck, we used 1D TEMPO-oxidized cellulose nanofibers (T-CNF) and 2D rGO nanosheets as "rebar" and "cement" to construct a tough framework, and the monolayer MXene (m-MXene) was uniformly wrapped on the framework forming a MXene-coated three-dimensional aerogel (MCG aerogel) with high ionic and electronic conductivity. It is worth mentioning that the negatively charged groups of MXene can not only accelerate the ion diffusion rate and the conversion process of polysulfides by anchoring polysulfides through strong adsorption sites, but also construct an ion selective layer to inhibit the shuttle effect of polysulfides. Moreover, the T-CNF-enhanced three-dimensional structure alleviates the negative impact of sulfur volume expansion during the charging and discharging process, making it possible to directly used the aerogel as self-supporting electrodes without any binder. Therefore, the T-CNF/rGO hybrid aerogels with monolayer MXene coating as cathode materials for Lithium-sulfur batteries exhibit high discharge capacity of 1470 mA h g~(-1) at 0.1 C and improved rate capability of 744 mA h g~(-1) at 5 C. The high-strength framework structure with uniform surface sulfur-anchor coating design strategy offers a new angle of regulating the diffusion kinetic rate of lithium-sulfur batteries.
查看更多>>摘要：Here, we report a robust method for fabricating ZnO nanorod arrays with arbitrary patterns on glass substrate for device integration applications. Photolithography patterned non-transparent Zn film is used as growth sites for ZnO nanorods in the hydrothermal reactions for facile and reproducible fabrication. Patterns including lines, dots, texts, and logos have been demonstrated and the dimension of ZnO nanorods are tunable. The semi-transparent ZnO nanorod/glass shows increased light transmission when immersed in water, which is an advantage when integrate into devices requiring optical imaging and sensing. For proof-of-concept purpose, surface-enhanced Raman scattering sensors based on Ag coated ZnO nanorods with back observation configuration are demonstrated and show high sensitivity.
查看更多>>摘要：As inspired by the surging demand for high performance energy storage systems, researchers have been designing and exploring advanced electrode materials exhibiting higher energy and power densities. In this study, a novel combination of nickel-cobalt sulfide and oxide was reported for high performance super-capacitors. The porous α-Ni(Co)S@NiCoO_2 composites with core-shell structure were derived from the well-designed precursor with Ni_2CoS_4 and ZIF-67 serving as the self-sacrificing templates through a two-step calcination process at elevated temperatures. Such a heterostructure exhibited an ultra-high specific capacity of 502.4 mA h g~(-1) at a current density of 1 A g~(-1), which was attributed to the unique porous core-shell structure and the ingenious combination of bimetallic sulfide and oxide. Furthermore, a hybrid supercapacitor based on the α-Ni(Co)S@NiCoO_2 composite and commercial activated carbon achieved a high energy density of 82.4 Wh kg~(-1) at a power density of 847.5 W kg~(-1). The prominent electrochemical performance exhibited by the α-Ni(Co)S@NiCoO_2 heterostructure in this study was demonstrated to be promising to practically serve as advanced electrode materials in supercapacitors.
查看更多>>摘要：In additive manufacturing especially in directed energy deposition, it is hard to control crystal growth pattern due to the high temperature gradient in AM. Essentially, the crystal grows epitaxially from the substrate and columnar crystals with strong texture are exhibited in AM components. Here, small spot diameter was used for DED-L of Ni-based alloys and various crystal growth patterns were obtained by changing power density. Samples processed under low power density were shown to exhibit wide and shallow spindle-like melt pool, along with apparently hierarchical planar-columnar-equiaxed micro-structure. While samples prepared under relatively high power density were shown to exhibit narrower and deeper melt pool with two sharp turning points, exhibiting inclined columnar grains and several discontinuous central axial columnar crystals. When highest power density and small spot diameter were applied, crystal growth with weak texture was achieved. Under this deep and narrow melt pool, the crystal growth can be separated into four regions: nearly-equiaxed grains with random grain orientations; horizontally symmetrically grown crystal grains; axial columnar in the center; columnar grains grew approximately vertical to the boundary of melt pool. Correlations were investigated between melt pool shape, solidification parameters and microstructure. The transformation of melt pool morphology was mainly attributed to the change of power density. Solidification parameters were shown to be different under different melt pool morphology. Diverse crystal growth patterns were achieved under different melt pool morphology controlled by power density, showing the feasibility of site-specific of microstructure control in DED-L according to the required mechanical properties.
查看更多>>摘要：The evolution of the crystallographic orientation and the microstructures in triangular adapters of grain continuators (GCs) with different flare angles and thicknesses prepared by a 3rd-generation single crystal (SX) superalloy were investigated under different withdrawal rates. The microstructures were observed by an optical microscope and the crystallographic orientation of the GCs was measured by electron backscatter diffraction (EBSD). The directional solidification process was simulated by a commercial simulation software ProCAST The experimental results showed that the [001] misorientation between both sides of the GCs at top cross section increased with the increasing of the flare angle, the thickness of specimens, and the withdrawal rate. These phenomena were attributed to the changing of the isotherm morphology in the mushy zones and the velocity of the solidification front. As for a low flare angle, the freckle defect formed at one side of specimens under the withdrawal rate of 1 mm/min. With the increasing of flare angle, the freckle defect cannot be found at the same position of the specimens. The formation mechanisms of freckle defect in the triangular adapters of GCs were further discussed.
查看更多>>摘要：In the present study, correlative corrosion testing and electron microscopy, including quasi-in-situ electron microscopy, are employed to investigate the corrosion behaviour of 2A97-T8 Al-Cu-Li alloy extrusion in 3.5 wt% NaCl solution. It is found that the corrosion behaviour of the alloy is influenced by synergetic effect of the plastic deformation introduced by the thermomechanical processing during alloy fabrication and the precipitation of T_1 phase during T8 treatment. Localized corrosion initiates and propagates preferentially within non-recrystallized regions of the partially recrystallized alloy. Corrosion initiated at coarse constituent intermetallic particles is confined to its periphery, which doesn't develop further when the particles are disconnected from alloy matrix.
查看更多>>摘要：Carbonate materials have been increasingly favoured in terms of the development of flame retardants because of the eternal subject of inorganic carbon sequestration. In this regard, anhydrous zinc carbonate (ZnCO_3) has been deemed as suitable candidate thanks to its excellent flame retardancy compared to traditional hydroxid flame retardant. However, the single crystals growth, accurate characterizations of structural and thermal properties are still not entirely clear from previous studies. With this in mind, ZnCO_3 single crystals were synthesized under high-pressure-temperature conditions (high P-T; 3 GPa and 973 K). The crystal structure of impurity-free ZnCO_3 was determined by means of single crystal X-ray diffraction (XRD). The symmetry was identified as R3c, while the unit cell parameters were a= 4.6463(3) A and c= 15.0015(11) A with a final R value of 0.0229. The quantitative analyses of Raman spectrum and infrared absorption indicate that the as-synthesized ZnCO_3 is anhydrous phase. Using Thermogravimetric (TG) / Differential Scanning Calorimeter (DSC) measurements, ZnCO_3 was decomposed in the temperature range of 593-773 K, whereas the heat capacity and the endothermic peak were determined. According to the single crystal XRD from 150 K to 383 K, the thermal expansion coefficients were quantified as α_a= 7.90 × 10~(-6) K~(-1) and α_c=22.8×10~6 K~(-1) as well as aVunit ceii=38.8× 10~(-6) K~(-1). These findings provide a precise characterisation and obtain important thermal parameters for the evaluation of its flame retardant properties.
NSTL
Elsevier