查看更多>>摘要：? 2022 The AuthorsCobalt-free ?LiNi0.5Mn1.5O4 (Lithium Nickel Manganese Oxide; LNMO) has garnered considerable interest as a cathode material due to its high working voltage, lower cost, and environmental friendliness. However, LNMO cathodes currently exhibit low cyclability and capacity deterioration, severely restricting their use on a broader scale. To this end, microwave-assisted chemical co-precipitation was used to produce spherical aggregated nanoparticles of LiNi0.5Mn1.5O4 (LNMO) coated with CeO2 (LNMO-Ce) and wrapped in graphene (LNMO-Ce-GO). Structural analysis demonstrates that the ceria coating along with the graphene wrapping prevents unwanted phases from forming and altering the morphology of the LNMO microspheres. LNMO-Ce-GO exhibits a discharge capacity of 132.4 mAhg?1 at the C/10 rate with a capacity retention of 95.3 % after 100 cycles, compared to LNMO-Ce and bare LNMO samples that provide a capacity retention of 91.6 % and 84.7 % respectively. DSC analysis elucidate that the ceria coating helps to suppress the adverse reactions at the electrode/electrolyte interface and reduce the Mn3+ dissolution due to the Jahn Teller effect, increasing cell cyclability. The graphene wrapping reduces material aggregation and provides conductive pathways that significantly improve the electrochemical performance of the LNMO cathode. This innovative material design strategy can be efficiently expanded to other classes of lithium-ion battery cathode materials to enhance their electrochemical performance.
查看更多>>摘要：? 2022 Elsevier B.V.The diffusion of nitrogen through γ(Ni)/γ'(L12-Ni3Al) interface in Ni-based superalloys was investigated based on first-principles calculations and the different nitriding behaviors of γ matrix and γ' precipitate were determined. At the interface, nitrogen atoms prefer to occupy the 6-Ni octahedral interstitial site with a large physical volume rather than the octahedral site constituted by Ni and Al with good chemical affinity. This indicates that γ matrix can provide more trapping sites for nitrogen atom than γ' precipitate. Moreover, the migration energy barrier of N crossing the (002)γ/γ' coherent plane is higher than that in γ phase. Nitrogen atoms can move into γ' phase unless the diffusion front has higher nitrogen concentration and O4 * (5Ni-Al octahedral site) is occupied. Inside the γ' matrix, nitrogen atoms are also easily trapped by the 6-Ni octahedral interstitial site, but their escape rates are much lower. It is the potential barrier role of the (002)γ/γ’ coherent plane and the low diffusion ability of N in γ' that are responsible for the unfavorable nitriding behavior of γ' phase in Ni-based superalloys.
查看更多>>摘要：? 2022 Elsevier B.V.In the present work, a series of Nb-Ta-W and Nb-Ta-W-Hf refractory multi-principal element alloy thin films were fabricated by multi-target direct current magnetron co-sputtering. The ternary Nb-Ta-W thin films displayed a single BCC solid solution structure with the flake-like surface morphologies and a double-layer structure for cross sections with a columnar upper layer and an amorphous layer near the substrate. The average grain size was 16–30 nm estimated by Williamson-Hall mode and the surface roughness Ra value was 5–7 nm. These ternary thin films had excellent mechanical properties with the hardness values of 25–29 GPa and elastic modulus of 294–325 GPa. After Hf was introduced into the equiatomic NbTaW thin film, the plane view morphology of the NbTaWHf0.6 thin film changed into a fine granular structure, but with a small amount of round particles on the surface. With the further increase of Hf content, the surfaces of the thin films kept being granular structure, but round particles disappeared on the surface of NbTaWHf film, and appeared again on the surfaces of high Hf content films with the increasing number. The cross sections of all Hf-contained thin films were smooth, dense and featureless. Meanwhile, the Ra of Nb-Ta-W-Hf thin films was significantly reduced, ranging from 0.92 to 1.47 nm. All Hf-contained thin films formed amorphous structure. The hardness of Nb-Ta-W-Hf thin films exceeded 19 GPa, and the NbTaWHf thin film had the highest hardness of 22.1 ± 1.3 GPa and elastic modulus of 239.3 ± 6.5 GPa, respectively. Due to the amorphous structure, Nb-Ta-W-Hf thin films exhibited excellent corrosion resistance. The corrosion current density was about two orders of magnitude lower than that of 304 stainless steel, and the polarization resistance was much higher than that of 304 stainless steel.
查看更多>>摘要：? 2022 Elsevier B.V.The NaCu0.2Fe0.8?xMnxO2 (x = 0.4; 0.5; 0.6; 0.7) samples were synthesized using conventional solid states method. Analyzing the DXR measurement through the use of FullProf allowed us to prove the purity of these samples and to corroborate that they crystallize in hexagonal system in the R-3 m space group. The refinement of the crystalline structure revealed the O3 structure type for compounds whose x = 0.4 and x = 0.5 and the P3 structure type for compounds whose x = 0.6 and x = 0.7. It is to be noted that the impedance spectroscopy results expressed in terms of a Nyquist diagram demonstrated three contributions which are grain, grain boundary and electrode effects. The conduction mechanism deduced from the study of ac conductivity goes in good agreement with the overlapping large polaron (OLPT) to compounds whose x = 0.4 and 0.5. However, for the sample whose x = 0.6 and 0.7, the conduction process was determined through the correlated barrier hopping (CBH) model. Furthermore, we examined the influence of Mn substitution on structural and electrical properties. Therefore, by increasing the Mn amount, we inferred that the structure changed from O3 to P3, which affected the electrical properties and enhanced the grain conductivity owing to the decreasing volume of the sodium site.
查看更多>>摘要：? 2022 Elsevier B.V.In the present study, a novel titanium matrix composite with a quasi-network structure was successfully fabricated via the hot-press sintering of the TC4 powder and Ti2AlC precursor at 1200 °C and 25 MPa. To explore the effect of the Ti2AlC content on the microstructure and properties of the prepared composites, 1–8 vol% Ti2AlC was selected as the representative samples. Microstructural analysis showed that the initial Ti2AlC was transformed into TiCx, and Al atoms diffused into the TC4 matrix to form a Ti(Al) solid solution. The α→β transformation temperature of Ti decreased based on thermal analysis. After increasing the amount of Ti2AlC, the α phase gradually shifted to equiaxed grains, and the grain size decreased, while the α/β ratio changed. When the content of Ti2AlC was 3 vol%, the composite's strength significantly improved, with smaller ductility loss and a tensile yield strength of 939 MPa, tensile ultimate strength of 1007 MPa, compressive yield strength of 1292 MPa, and compressive ultimate strength of 2041 MPa. This strength improvement was attributed to fine-grain strengthening, solid-solution strengthening and second-phase strengthening. Moreover, theoretical calculations indicated that the solid-solution strengthening dominated the improvement efficiency.
查看更多>>摘要：? 2022 Elsevier B.V.Herein, ultrathin MnO2 nanosheets consisting of crystalline/amorphous phases are prepared by a simple interfacial reaction of surfactant assistance in water-bath with high specific surface area of 317.56 m2 g?1, which can accelerate ionic transport and provide more active sites during electrochemical reaction. The as-synthesized MnO2 nanosheets electrode exhibits remarkable specific capacitance of 163.3 F g?1 at 1 A g?1 in a three-electrode system using 1 M NaSO4 as electrolyte, and long-term stability with a capacity retention of 89.3% at 3 A g?1 after 10,000 cycles, which can be ascribed to a structural transition from crystalline to amorphous phase during cycling. Meanwhile, the flexible all-solid-state MnO2 symmetry micro-supercapacitors (MSCs) using sodium alginate bio-hydrogel as electrolyte, the voltage window of the MSCs can be extended to 1.4 V. The MSCs reveals a high areal capacitance of 129.29 mF cm?2 at a current density of 0.2 mA cm?2. The capacity retention rate up to 94.5% after 4000 cycles, and higher energy density of 35.19 μWh cm?2 at 0.07 mW cm?2, as well as there is no significant capacitance degradation under different bending states, exhibiting a remarkable electrochemical performance and mechanical flexibility. The crystalline/amorphous ultrathin MnO2 sheets with high specific surface area were prepared through simple preparation process and found the mechanism of the transformation of MnO2 from crystalline phase to amorphous phase during the cycle providing a new perspective for the design of high-performance MnO2 electrodes.
查看更多>>摘要：? 2022Core-shell composites Fe-N @ SiO2 with 500 nm average sizes of cores have been successfully prepared by a combination of hydrothermal, gaseous nitriding and St?ber methods. The Fe-N cores are composed of Fe4N as major phase and FeN as minority. The thickness of SiO2 shell increases continuously with increase of the tetraethyl orthosilicate weight. Relative to the natures of Fe-N, Fe-N @ SiO2 composites show slightly weaker ferromagnetic capacity, but much higher oxidation resistance. The Fe-N sample exhibits extremely high relative permittivity. With filling loading of 50 wt%, it has a moderate microwave absorption performance with a maximum reflection loss about ?10 dB. Owing to low conductivity, non-magnetic property and superior anti-oxidation ability of the SiO2 shell, the core-shell structures have considerably degraded permittivity, and nearly unchanged permeability. These changes bring a much better impedance matching between the permittivity and permeability, and further bring a greatly enhanced microwave absorption performance. All of the Fe-N @ SiO2 composites have strong absorption abilities. They are quite stable with variation of shell thickness. The composite of 52 nm shell thickness has maximum reflection loss of ?23.1 dB at 17.2 GHz, and effective absorption bandwidth (<?10 dB) of 2.4 GHz with absorber thickness 5.5 mm. The present study gives an insight on the roles of silica material influencing the iron-based absorber's absorption ability.
查看更多>>摘要：? 2022Transition metal dichalcogenides have gained renewed attention as promising thermoelectric materials primarily because of their unique structures and superior electronic properties. In this study, we investigate the electrical transport properties of a series of Hf(Se,Te)2 samples (Hf(SexTe1-x)2, x = 0, 0.025, 0.25, 0.375, 0.5, and 1). All the samples form a single phase of a complete solid solution, displaying a wide variety of electrical properties based on the evolution of the composition. HfTe2 exhibits metallic conduction with very high electrical conductivity of ~1600 S/cm, whereas HfSe2 exhibits semiconducting conduction with very low electrical conductivity of 0.33 S/cm at room temperature. In contrast, the Seebeck coefficient of the HfTe2 sample is as low as 17 μV/K, whereas that of HfSe2 approaches 730 μV/K. Therefore, the power factor for HfTe2 represents a much higher value of 0.24 mW/mK2 when compared with 0.01 mW/mK2 for HfSe2 at 600 K. The superior electrical transport properties of HfTe2 are primarily caused by its high carrier concentration of 1021 cm?3 and high density-of-state effective mass of 0.91m0. A comparison of the electronic band dispersion between HfSe2 and HfTe2 calculated using the density functional theory indicates that the dispersions of the conduction and valence bands seem to remain unchanged: HfSe2 exhibits a finite band gap, whereas HfTe2 exhibits no or a very small band gap.
查看更多>>摘要：? 2022 Elsevier B.V.Alloying Co and Ni in traditional Fe-based nanocrystalline alloys always brings about crystals refinement and magnetic softness improvement, while we found opposite phenomena in Fe81.3Si4B13Cu1.7 nanocrystalline alloy with pre-existing α-Fe nanocrystals in present work and clarified the related mechanism by investigating the structure evolution of melt-spun (Fe, Co, Ni)81.3Si4B13Cu1.7 alloys. The Co and Ni alloying lowers the number density (Nd) of pre-existing α-Fe nanocrystals in the as-spun alloys by suppressing the formation of Cu-enriched clusters. The effect of Ni alloying is more distinct than that of Co and adding 10 at.% Ni completely eliminates the nanocrystals. The decreased Nd leads to the weakened competitive growth between the nanocrystals, combined with the absence of diffusion-suppression, resulting in the slightly crystals coarsening and corresponding magnetic softness deterioration in the annealed Co-containing alloys and much more distinct ones in the annealed Ni-containing alloys.
查看更多>>摘要：? 2022 Elsevier B.V.Materials with tunable dielectric properties were expected to be applied in electronic devices such as phased array antennas, tunable filters and phase shifters. The dielectric tunability of material greatly depends on the dielectric constant and breakdown strength. Herein, two kinds of Ba0.6Sr0.4TiO3/PVDF (BST/PVDF) composite materials with different dual-gradient structures were designed to obtain the serialized relative permittivity and improved breakdown strength. The numerical simulation based on Yamada theory confirmed that the multilayer dual-gradient structure design was beneficial to improve the dielectric constant, and the enhancement of breakdown strength was further identified by finite element simulation. The optimal composition has a high dielectric tunability of 78.27 % which is 45 % higher than the ungraded composite due to the high permittivity of 33.8 and high breakdown strength of 1900 kV/cm. The dielectric theoretical model of multilayer structure was developed by introducing the effect factor of layer number m into the Yamada model, and the calculated results are consistent with the experimental results especially m= 1.6–1.7. This work provides a new strategy to effectively improve the dielectric tunability of ceramic/polymer composite materials.
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