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：The valence electron structures, thermal properties, hardness and magnetic moments of single-phase high entropy alloys with the face-centered cubic structure have been first investigated with the empirical electron theory of solids and molecules. The theoretical bond lengths agree with the experimental ones well. The calculated melting points of CrFeCoNi, CrMnFeCoNi and CrFeCoNiCu alloys are consistent with their differential thermal analysis (DTA) measured ones. The calculated magnetic moments of ferromagnetic FeCoNi and FeCoNiX_(0.25) (X=A1 or Si) alloys agree with the observed ones well. The physical properties of these alloys are strongly associated with their valence electron structures. It is suggested that the melting point, cohesive energy, hardness and magnetic moments of the face-centered cubic high entropy alloys are mainly modulated by bonding factor/, covalent electrons per atom n_c/atom, covalence electron pair n_A and 3d magnetic electron, respectively.
查看更多>>摘要：Nanoporous silver (NPS) with a three-dimensional (3D) bicontinuous ligament/pore structure can be fabricated in different ways and with different treatments. Thermal treatments before or after dealloying can have a considerable impact on the dealloying process. High-temperature oxidation (HTO) has been researched as a method to tune the process of NPS fabrication. In this work, X-ray diffraction and X-ray photoelectron spectroscopy characterizations were used to identify the phase after HTO and dealloying. Copper oxide was found in the precursor alloy after HTO, which could be the key factor that would influence the mechanism of NPS fabrication. Scanning electron microscopy and transmission electron microscopy were executed to characterize the pathway of forming NPS and research the difference between HTO specimens and the as-cast sample. This demonstrated that, compared to the free corrosion pathway, the diffusion method of Ag atoms was different under HTO pretreatment, which was the predominant factor causing a larger 3D bicontinuous ligament/pore structure. Finally, the microstructure of the final NPS pretreated with HTO for 3 min was much more homogeneous than that of the others, and when the pretreatment time of HTO reached 5 min, the ligament/pore structure almost collapsed.
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：Fe-based nanocrystalline alloys with unique dual-nano-phase structure and superior magnetic softness have aroused tremendous interest, yet they generally suffer from the harsh annealing process due to the poor thermal stability. In this study, a high-entropy strategy was proposed to enhance the soft-magnetic property and nanostructure stability of Fe-based nanocrystalline alloys by adding P and C elements in the FeCuSiBPC alloy system. This alloying approach by complicating the composition will greatly increase the mixing entropy, which significantly increase the frequency factor for the formation of α-Fe grains and activation energy for the formation of compounds, which on one hand enhance the competition and soft-impingement effects due to the increased number density of a-Fe grains, and on the other hand hinder the precipitation of compounds in the intergranular amorphous interphase due to the improved crystallization resistance. These both contribute to a thermodynamically and kinetically stable dual-nano-phase structure with fine a-Fe grains embedded in amorphous matrix. Following this strategy of nanostructure stabilization by tuning the compositional complexity, FeCuSiBPC alloy with enlarged processing window and enhanced soft-magnetic properties were successfully obtained. This high-entropy strategy can be applied in a verity of alloy systems to develop high performance nanocrystalline alloys with excellent thermal stability suitable for large-scale industrial processing.
查看更多>>摘要：Prussian White (PW) layers were deposited on Au/Cr/Si substrates by electrodeposition and characterized by different techniques. Scanning electron microscopy (SEM) images and Raman mapping reveal a uniform and homogeneous deposit while scanning transmission electron microscopy (STEM) images disclose the grain boundary pattern and the thickness of 300 nm of the PW layer. Resistive switching (RS) effect with an ON/OFF ratio of about 10~2 was observed. The RS mechanism was investigated from the log-log current-voltage plots. Ionic conduction was observed with an activation energy of 0.4 eV that could be associated with potassium ions as possible charge carriers at the grain boundaries. The endurance characteristics were investigated and a stable abnormal RS was observed for consecutive 500 cycles. Moreover, the retention was also evaluated and the high resistive state (HRS) and low resistive state (LRS) were stable up to 1000 s.
查看更多>>摘要：The current work involves the synthesis of Mn_xCo_(0.5-x)Zn_(0.5)Fe_2O_4 (x = 0.0, 0.1, 0.2, 0.3, and 0.4) by adopting sol-gel auto combustion technique. For all the samples, the single-phase spinel structure with cubic symmetry was assured by the XRD studies. The crystallite size lies in the nanoscale range of 35.4-43.6 nm as estimated by utilizing Debye-Scherrer formula. FTIR spectroscopy affirms the formation of spinel structure due to the appearance of characteristic vibrational bands near 400 and 600 cm"1 which corresponds to the octahedral and tetrahedral sites respectively. FESEM micrographs reveal the presence of non-uniform grain growth which was agglomerated, inhomogenous in size and shape, and having porous morphology. VSM study exhibits soft ferromagnetic nature due to the low coercivity value. There is a decrement in the saturation magnetization with increasing Mn~(2+) concentration which is due to the decrease in the crystallite size and non-magnetic nature of the Mn~(2+).
查看更多>>摘要：Complex boron substructures lead to diversity properties for transition metal borides (TMBs), that provides them many application possibilities in numerous fields. To clarify the actual effect of boron substructures on mechanical, magnetic and electrical properties, we prepared polycrystalline β-FeB samples with zigzag boron chains by high pressure and high temperature. β-FeB exhibits high saturation magnetization (79.54 emu/g), good antioxidant capacity (> 800 K), high hardness (15.62 GPa) and low resistivity (3.4 × 10~(-6) Ω m); thus, it is a promising magnetic material for extreme environmental applications. Subsequently, we performed first-principle calculations combined with X-ray photoelectron spectroscopy analysis and found that the free electrons transferred from Fe atoms stabilize the zigzag boron chains. Spin selection occurs during electron transfer and bonding, with majority spin state electrons as the main participants. The zigzag boron chain substructure provides excellent mechanical properties, at the expense of electrical and magnetic properties. Therefore, we speculate that the spin-selective electrons transfer between the metal and boron substructure can effectively modulate the electrical, mechanical, and magnetic properties of TMBs. This study introduces an effective route for the design, preparation, and applications of high-hardness multifunctional TMBs.
查看更多>>摘要：The crystal structure, pressure composition isotherms, and electrochemical properties of Zr_(0.2)Ti_(0.2)Ni_(0.2+x)Cr_(0.2)Mn_(0.2) (x = 0, 0.025, 0.05, 0.075, and 0.1) high-entropy alloys (HEAs) were investigated. The crystal structures of all the HEAs consisted of two phases: a primary phase with a C14-type (Zr_(0.5)Ti_(0.5))Mn_2 hexagonal structure and a secondary phase with a B2-type Ti_(0.6)Zr_(0.4)Ni cubic structure. Rietveld analysis revealed that the secondary phase increased with an increase in the x value. The hydrogen storage capacity of the HEAs was lower than that of the alloy with x = 0 because of an increase in the B2-type Ti_(0.6)Zr_(0.4)Ni phase, whereas the change in enthalpy of hydride formation (|ΔH|) decreased with increasing x, leading to the instability of hydrides. The alkaline treatment was performed by immersing HEA powders or electrode in a 6 M KOH aqueous solution at 378 K for 2 h. The Zr_(0.2)Ti_(0.2)Ni_(0.2)Cr_(0.2)Mn_(0.2) alloy surface changed to a porous and rough structure, and a ZrO_2 passive thin layer on their surface, which is replaced by NiO or Ni(OH)_2 after the alkaline treatment. The charge-discharge tests conducted using the HEA negative electrodes for Ni-MH batteries depict that the discharge capacity increased with an increase in x, and the highest discharge capacity was 368 mAh g~(-1) at x = 0.075 after the alkaline treatment. On increasing the x value, the high-rate dischargeability and cycle performance were also enhanced.
查看更多>>摘要：In Al alloys, many experiments have shown the effectiveness of Al_3X precipitates in promoting the primary grain nucleation such as Al_3Ti, Al_3Er and Al_3Sc, preventing the α-Al grain recrystallization such as Al_3Cr and Al_3Zr, and strengthening the matrix such as Al_3Li and Al_3Ni. Almost all of those experiments have been measuring the stable structures, and none of them have been focusing on the intrinsic properties of Al_3X and their stabilities as a function of crystal structure and lattice constants due to the experimental limitations. In this study, the first-principle calculation is used to predict the metastablity of different Al_3X structures including L1_2, DO_(22), DO_(23), DO_(19), DO_(18), DO_(24), R3m and P6_3/mmc. The varitions of lattice constant have been achieved by changing external pressure from 0 to 50 GPa. It was found that the L1_2 structure not only had a smaller lattice mismatch with α-Al, but also had a higher symmetry than the other structures, and the stability of the L1_2 structure was enhanced when external pressure was applied, and some other structures may undergo a transition to the L1_2 structure, while the elastic modulus of the L1_2 structure was improved. In addition, the elastic modulus of the Al_3X precipitation phases of different structures was found to increase with decreasing distance between atomic pairs, and the distance between Al-X and X-X pairs had a much greater effect on the elastic modulus than the Al-Al pairs.
NSTL
Elsevier