查看更多>>摘要：The Mg~(-1)2Gd-2Er-0.4Zr (GE122K) is fabricated by double extrusion, and the effects of grain refinement and precipitate evolution on the mechanical performance of the extruded alloys are deeply analyzed. Scanning electron microscope results spectacle that some bulky precipitates in the single extruded alloy are fragmented to fine precipitates during double extrusion and distribute linearly along the extrusion direction (ED). The fine precipitates exhibit the strongest dispersion strengthening effect, whereas the bulky precipitates act as crack resources and decrease the elongation. After double extrusion, a refined grain structure with an average grain size of 2.7 urn is achieved, which is attributed to dynamic recrystallization (DRX). Moreover, double extrusion boosts the content of rare-earth elements into the matrix by dissolving the fine precipitates. During aging, extensive p' precipitates form in the double extruded alloy than in the single extrude alloy. As a result, refined grains and strong precipitation strengthening make a significant contribution to a high yield strength (YS) of 422 ± 1.59 MPa with an elongation (EL) of 6.5 ± 0.89% in the double extruded+peak-aged alloy (ACX_2). The possible strengthening mechanisms are discussed, and it is found that grain refinement and precipitate strengthening are the main contributors to the high strength of the GE122K alloy.
查看更多>>摘要：The present work focuses on developing a low-cost medium entropy alloy (MEA) with desirable mechanical properties according to the benchmark CoCrFeMnNi high entropy alloy (MEA). By adjusting the ratio of each component and adding silicon to the system, the Fe_(50)Mn_(17.5)Cr_(12.5)Co_(10)Ni_5Si_5 MEA with a single face-centered cubic (FCC) phase was developed. After homogenization, hot rolling, cold rolling, and annealing, fully re-crystallized MEA specimens with grain sizes ranging from 10 pm to 149 pm were used for tensile tests. The microstructure of the elongated MEAs showed a ε-martensite transformation from the FCC phase to the hexagonal close-packed (HCP) phase, indicating the stacking fault energy (SFE) of the MEA was significantly reduced. The room-temperature deformed MEA showed improved mechanical properties in yield strength and tensile strength than the CoCrFeMnNi MEA. Meanwhile, the volume fraction of the HCP phase in cryogenic-deformed MEA is much larger than that in room-temperature deformed MEA; its yield strength was increased by two times, while the tensile strength exceeded the level of 1 GPa.
查看更多>>摘要：Study of the magnetic properties of the FeTiF_6 - 6 H_2O single crystal has shown that this compound is a two-dimensional antiferromagnet with a N e el temperature of T_N = 8 K and its magnetic moment anisotropy attains 7000% at a temperature of T = 4.2 K. The Mossbauer spectroscopy data unambiguously indicate the paramagnetic state of iron cations in the temperature range of 4.2-300 K. However, a sharp change in the difference between the quadrupole doublet linewidths at 10 K has been observed, which is consistent with the temperature of magnetic ordering. It has been suggested that the long-range magnetic order is established in the crystal through the formation of the exchange coupling revealed by the electron spin resonance measurements on the oriented single crystals.
查看更多>>摘要：In this paper, we investigate the effects of a SiO_x interfacial layer on ON/OFF current ratios, endurance characteristics and read-disturb immunities of Ag/Cu_xO/SiO_x/n~+-Si write-once-read-many-times (WORM) memories. The Cu_xO active layers were prepared using a sol-gel process. After coating Cu_xO films on n~+-Si substrates, the Cu_xO/n~+-Si samples were annealed in air at 400 °C and 600 °C, respectively, to obtain a SiO_x interfacial layer at the Cu_xO/n + -Si interfaces. The 400 °C-annealed Cu_xO device shows an ON/OFF current ratio of 104. However, degradation of OFF state current (IOFF) with increasing read-pulse cycles and stress time is observed. For the 600 °C-annealed Cu_xO device, stable ON and OFF state currents can be observed both in an endurance test for over 1.2 × 104 read cycles and in a read-disturb test for 2 × 104 s. Moreover, a higher ON/OFF current ratio of 107 is obtained. A rigorous retention test executed at an elevated temperature of 85 °C indicates that the data retention time is expected to last for 10 years. The performance improvement of the 600 °C-annealed Cu_xO device is due to an increase in the thickness of the SiO_x interfacial oxide layer. The mechanism for the influence of the interfacial layer on memory performance is investigated and illustrated. The OFF state current of the memory is limited by hopping and trap-assisted tunneling transport in the SiO_x interfacial layer. In the ON state, conductive paths in the device cause that the carriers can easily migrate by Ohmic and space charge limited conduction.
查看更多>>摘要：Order parameter coupling associated with the first order, improper ferroelastic (Pm3m - Pbam) transition at -510 K in PbZrO_3 has been analysed from the perspective of strain and elasticity. Formal treatment of spontaneous strains using lattice parameter data from the literature reveals typical coupling with the order parameter for octahedral tilting, Q_R, and stronger coupling with the order parameter for antiferroelectric displacements, QΣ. These indicate that coupling between the two order parameters via common strains is not only biquadratic, λQ_r~2Q_Σ~2, but may also have contributions from a higher order term, λQ_r~2Q_Σ~4- Variations of elastic and anelastic properties obtained by resonant ultrasound spectroscopy (RUS) at frequencies in the vicinity of 1 MHz show softening as the transition point is approached from above, discontinuous stiffening at the transition point and a pattern of further stiffening in the stability field of the orthorhombic structure. Below the transition point, the pattern of stiffening resembles the evolution of Q_R~2 and Q_Σ~2, as is typical of coupling dominated by terms with the form λe~2Q~2, where e is a spontaneous shear strain. The absence of softening due to terms of the form λeQ~2 implies that the relaxation time for changes in the order parameters in response to an induced shear strain is slower than ~10~(-6) s. Also in contrast with measurements from the literature made at lower frequencies, no evidence for mobility of ferroelastic domain walls was observed at RUS frequencies. A peak in acoustic loss observed at the transition point and precursor softening in the stability field of the cubic phase are consistent with evidence for local dynamical polar clusters. Apart from some differences in relaxation times, the antiferroelectric transition in PbZrO_3 does not appear to be overtly different from ferroelectric transitions such as occur in BaTiO_3.
Kirill A. CherednichenkoVladimir A. MukhanovAleksandr Kalinko
8页
查看更多>>摘要：Two boron-rich chalcogenides B_(12)S and B_(12)Se isostructural to a-rhombohedral boron were synthesized by chemical reaction of the elements at high-pressure - high-temperature conditions. The crystal structures and stoichiometries of both compounds were confirmed by Rietveld refinement of synchrotron X-ray diffraction data and elemental analysis. The experimental Raman spectra of B_(12)S and B_(12)Se were investigated for the first time. All observed Raman bands have been attributed to the theoretically calculated phonon modes, and the mode assignment has been performed.
查看更多>>摘要：As one of the most significant rare-earth oxides, cerium dioxide (ceria, CeO_2) has an enormous potential in a variety of materials science applications due to many excellent characteristics. In this work, cubic-phase mesoporous CeO_2 (mCeO_2) nanospheres with nanocrystalline frameworks, high Ce~(3+) and oxygen vacancy contents, large porosity and specific surface area, were fabricated via a facile solvothermal method using inorganic precursors and without surfactant as a template. The resulting products were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV-vis diffuse reflectance spectroscopy (DRS), X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, photoluminescence (PL) spectroscopy, and N_2 adsorption-desorption analyses. Raman and XPS results confirmed the presence of Ce~(4+), Ce~(3+) and Zr~(4+) ions and oxygen vacancies. By comparison with the undoped ones, Zr-doping in mCeO_2 nanospheres contributed to increased concentrations of Ce~(3+) (45.25% vs 32.64%) and oxygen vacancy (37.63% vs 27.53%), as well as decreased band gap energies (2.9 vs 3.1 eV). The photocatalytic activities of the pure and Zr-doped mCeO_2 nanospheres were evaluated with the degradation of methylene blue (MB) under visible-light irradiation. The Zr-doped mCeO_2 products exhibited a superior photodegradation rate of ca. 94% after 120 min visible-light illumination. A kinetic study suggested that the reaction followed pseudo-first order rate equation. The optimizations on Zr-doped mCeO_2 nanospheres against structural, optical, and physicochemical properties might be responsible for the enhanced visible-light-driven photocatalytic performance. The plausible photodegradation mechanism over MB dyes of mCeO_2 nanospheres were also proposed. This work is expected to provide an insight view into the design and synthesis of low-cost and high-efficiency CeO_2-based photocatalysts with improved visible photocatalytic activity.
查看更多>>摘要：Herein, thin films of (Bi_2Se_3)_(1-x)(Bi_2Te_3)_x were synthesized by thermal evaporation in high vacuum using highly crystalline bulk samples of Bi_2Se_3 alloyed with Bi_2Te_3. Preparation and characterization together with probing the thermoelectric properties of thin (Bi_2Se_3)_(1-x)(Bi_2Te_3)_x films are discussed in this article. Crystal structure, surface morphology, roughness and lattice features of the deposited films were probed via XRD, SEM and HRTEM techniques, which confirmed the perfect crystallinity and the nano-scalability of the prepared thin films. Thermoelectric measurements were carried out for the as-deposited films within a temperature range of 300-473 K. Seebeck coefficient of the studied samples is about two times larger than that of the previously reported bulk samples. The highest power factor was recorded at 131 μW/m K~2 at 473 K. The high value of the power factor shows that the materials under the study are promising for applications such as resource recovery of waste and also as nanomaterials for environmental applications. Very low electronic thermal conductivity was obtained due to the small electrical conductivity and due to the scattering of carriers by the tiny grains constituting the prepared films.
查看更多>>摘要：Electromagnetic interference (EMI) shielding materials with great absorption effectiveness is highly required in certain application field such as stealth technology but is hard to be realized by conventional metallic shielding materials. In this work, a graphene nanosheets (GNSs) reinforced magnesium (Mg) matrix composite was fabricated by spray deposition and subsequent hot-press sintering. The micro-structure, mechanical properties and EMI shielding effectiveness of the GNSs/Mg composites were characterized systematically. The results showed that synergistic strengthening of mechanical properties and EMI shielding performance of the composites was realized. The strengthening mechanisms on the mechanical and EMI shielding performance of the GNSs/Mg composites were analyzed thoroughly. It was found that the incorporated GNSs reinforced the composites mainly by load transfer and thermal expansion mismatch strengthening. Moreover, the propagation path of electromagnetic waves in the composites is extended by the GNSs, and the GNSs induced laminar structure and thus resulting in an increased electromagnetic absorption effectiveness. The obtained results in this work revealed the synergistic strengthening mechanisms by the laminated structure and provided a novel method to fabricate GNSs/Mg composites with superior mechanical and EMI shielding performance.
查看更多>>摘要：Recently, sustainable energy harvesting technologies have received much attention for powering the internet of things architecture. Triboelectric nanogenerators (TENGs) have been regarded as the most promising energy harvesting technology because they can generate a large amount of electrical energy even with very weak mechanical energy and very simple structures. Although there has been much investigation, TENG devices continue to be associated with insufficient levels of converted electrical power. In this study, we report the enhancement of TENG performance outputs based on polyvinylidene fluoride (PVDF)/fuIl-erene (C_(60)) composite nanofibers (NFs). Structural and chemical investigations show that C_(60) is uniformly incorporated into PVDF NFs with C_(60) content levels of less than 0.3 mg. PVDF/C_(60) composite NFs exhibit a consistent p-phase ratio regardless of the C_(60) content. Here, as the C_(60) content was increased from 0 to 0.2 mg, the maximum output power of the PVDF/C_(60) composite NF-based TENG devices increased from 129 to 282 μW, while it decreased to 147 μW with further addition of C_(60) beyond 0.4 mg. The enhanced TENG performance realized with an increase in the C_(60) content originated from the increased trap charge ability and the high electronegativity caused by the incorporated C_(60) in the PVDF matrix. The output power from a PVDF/C_(60) composite NF-based TENG device was sufficient to operate an eco-friendly display device.
NSTL
Elsevier