关键词： Group IV hybrids   Thermal effect   Vacancy defects   Elastic parameters   Molecular dynamics simulations  

摘要： The mechanical properties of pristine and defective Si-based hybrid sheets are studied using molecular dynamics calculations for a temperature ranging from 100 to 800 K, in conjunction with a variable strain rate. When increasing temperature, the melting phase of the hybrids occurs from the solid to the liquid phase, while the increase in the strain rate enhances their elastic parameters. The absence of plastic stage reveals that the fracture pattern is brittle in these 2D materials. Under the uniaxial loading, the systems stretch, resulting in the failure of the crystalline skeletons that lose their rigidity with anisotropic behavior observed only for SiC. In defective hybrids, the point defects reduce the values of fracture strength and strain without affecting the brittle behavior of the sheets. The results impart that coupling high temperature to SiC material offers new possibilities for MEMS devices, whereas SiGe is a promising candidate for microelectronic devices.

关键词： Time division multiple access   Antennas   Optimization   Resource management   System performance   Radio frequency   Downlink   Wireless power transfer   intelligent reflecting surfaces  

摘要： In this letter, we propose a fairness-aware rate maximization scheme for a wireless powered communications network (WPCN) assisted by an intelligent reflecting surface (IRS). The proposed scheme combines user scheduling based on time division multiple access (TDMA) and (mechanical) angular displacement of the IRS. Each energy harvesting user (EHU) has dedicated time slots with optimized durations for energy harvesting and information transmission whereas, the phase matrix of the IRS is adjusted to focus its beam to a particular EHU. The proposed scheme exploits the fundamental dependence of the IRS channel path-loss on the angle between the IRS and the node's line-of-sight, which is often overlooked in the literature. Additionally, the network design can be optimized for large number of IRS unit cells, which is not the case with the computationally intensive state-of-the-art schemes. In fact, the EHUs can achieve significant rates at practical distances of several tens of meters to the base station (BS) only if the number of IRS unit cells is at least a few thousand.

关键词： YSn3   elastic properties   electronic properties   high-pressure   phonons  

摘要： A theoretical study of many physical properties, namely structural stability, electronic, mechanical stability, and phonon properties of the nonmagnetic intermetallic compound YSn3 is realized using the density functional theory (DFT) calculations. The structural properties are calculated in many phases, specifically, Cu3Au, Al3Ti, GdSn2.75, and AgSbTe2 structures. The results show that YSn3 crystallizes in the Cu3Au structure at ambient conditions, whereas the energy difference between the Cu3Au and Al3Ti structures is very weak and in the order of thermal energy at room temperature, which suggests the YSn3 may also coexist in this structure. The single-crystal elastic constants for different phases at zero pressure are computed, and main mechanical quantities are derived. The mechanical stability criteria predict that the crystal in the Cu3Au structure is stable when pressure is below 18.68 and 4.61 GPa for the Cu3Au and Al3Ti phases of YSn3. The phonon dispersion relations and their projected density of states are investigated. The titled compound is found to be dynamically and mechanically stable in the Cu3Au and Al3Ti prototype phases under ambient conditions. As far as the authors know, high-pressure features of YSn3 have not yet been explored neither experimentally nor theoretically.

关键词： 动力机械  

ISBN: (纸本)9787030696755

摘要： 本书讲述动力机械动力学、强度与可靠性设计方面的基本理论及方法。全书共五章，主要内容包括：内燃机运动学及动力学分析、动力机械振动、动力机械疲劳强度设计、动力机械强度可靠性设计、以及有限元法在动力机械强度与可靠性分析中的应用等。本书主要以车用内燃机为例展开有关动力学、振动、疲劳强度、可靠性和有限元法应用等方面的研究。 本书为动力机械及工程、能源与动力工程、工程机械、船舶工程和轮机工程等相关专业研究生（或本科生选择部分）的教材，也可供从事动力机械设计、制造和开发的工程技术人员参考。

关键词： biomechanics   fluorescence lifetime   hydrogel   peptide self-assembly   Thioflavin T  

摘要： The self-assembly of peptides is a key direction for fabrication of advanced materials. Novel approaches for fine tuning of macroscopic and microscopic properties of peptide self-assemblies are of a high demand for constructing biomaterials with desired properties. In this work, while studying the kinetics of the Fmoc-Diphenylalanine (Fmoc-FF) dipeptide self-assembly using the Thioflavin T (ThT) dye, we observed that the presence of ThT strongly modifies structural and mechanical properties of the Fmoc-FF hydrogel. Notably, the presence of ThT resulted in a tenfold increase of the gelation time and in the formation of short and dense fibers in the hydrogel. As a result of these morphological alteration higher thermal stability, and most important, tenfold increase of the hydrogel rigidity was achieved. Hence, ThT not only slowed the kinetics of the Fmoc-FF hydrogel formation, but also strongly enhanced its mechanical properties. In this study, we provide a detailed description of the ThT effect on the hydrogel properties and suggest the mechanisms for this phenomenon, paving the way for the novel approach to the control of the peptide hydrogels’ micro- and macroscale properties.

关键词： Dynamics  

摘要： Recently, palladium diselenide (PdSe2) has emerged as a promising material with potential applications in electronic and optoelectronic devices due to its intriguing electronic and optical properties. The performance of the device is strongly dependent on the charge-carrier dynamics and the related hot phonon behavior. Here, we investigate the photoexcited-carrier dynamics and coherent acoustic phonon (CAP) oscillations in mechanically exfoliated PdSe2 flakes with a thickness ranging from 10.6 nm to 54 nm using time-resolved non-degenerate pump-probe transient reflection (TR) spectroscopy. The results imply that the CAP frequency is thickness-dependent. Polarization-resolved transient reflection (PRTR) measurements reveal the isotropic charge-carrier relaxation dynamics and the CAP frequency in the 10.6 nm region. In addition, the deformation potential (DP) mechanism dominates the generation of the CAP. Moreover, a sound velocity of 6.78 x 10(3) m s(-1) is extracted from the variation of the oscillation period with the flake thickness and the delay time of the acoustic echo. These results provide insight into the ultrafast optical coherent acoustic phonon and optoelectronic properties of PdSe2 and may open new possibilities for PdSe2 applications in THz-frequency mechanical resonators.

关键词： Young’s modulus   Binding energy   Layers   Deformation   Molecular mechanics  

摘要： Because cystine kidney stones are a more serious challenge for health-related quality of life than other types of kidney stones, the search for a new treatment for cystinuria is considered the main goal of this study. To achieve the defined goal, classical molecular dynamics simulations and quantum mechanics calculations were implemented in this study. Three nanodrills with different stiffnesses (i.e., silicon, silica, and silicon carbide) were selected to find the efficient nanodrill to break the kidney stones into smaller pieces. The related nanodrills under various forces from 20 to 100 eV/angstrom inclusive were exerted on the cystine kidney stones to determine the effect of the force magnitude on the rate of destruction. The exerted forces were modeled via a hypothetical spring force. To bring this investigation closer to reality, the urinary tract and the bulk of cystine kidney stones were modeled by simulation of the real blockage of the kidney stones. The obtained results from quantum mechanics calculations reveal the strong interaction (chemisorption) between the cystine stone components. Moreover, the molecular dynamics simulations show that an increase in force does not necessarily lead to more destruction of cystine kidney stones. The maximum rate of cystine kidney stone destruction occurs under forces of 80, 70, and 60 eV/angstrom for SiO2, Si, and SiC nanodrills, which is about 19, 13, and 11%, respectively. In addition, the SiO2 nanodrill has more crossing time and z-direction deformation than other nanodrills due to the attractive interaction between SiO2 and stones, it shows less deformation during the process of kidney stone breaking because of repulsive interactions between the nanodrill and the kidney stone.

关键词： Asymmetric   Metallocene   Borate   Ethylene   Propylene   Copolymerization  

摘要： A set of ethylene-propylene (E-P) has been copolymerized by asymmetric metallocene catalysts i.e. silylene-bridged rac-Me2Si(2-Me-4-Ph-Ind)2ZrCl2 (SiPh) and ethylene-bridged rac-Et(Ind)2ZrCl2 (EBI) and cocatalyzed with triisobutylaluminum(TIBA)/ borate [Ph3C][B(C6F5)4] under similar conditions for differ-ent desire time. Quenching reaction of active center ([C*]/[Zr]) with 2-thiophenecarbonyl chloride (TPCC) for determining the " Sulfur "content. The obtained E/P copolymers were characterized with GPC, FT-IR, 13C-NMR and sulfur analyzer techniques. Changes in reactivity ratios "rE and rP" (reactivity ratio of ethylene (rE) and propylene (rP)), chain propagation rate Rp (chain propagation rate for ethylene ethy-lene (RpE) and chain propagation rate for propylene RpP respectively) molecular weight (Mw), molecular weight distribution (MWD), and chain propagation rate constant kp (propagation rate constant for E (kpE) and propagation rate constant for propylene (kpP) respectively) also discussed. The results of the kinetic constants, changing in the polymerization time (tP) exerted evident effects on [C*]/[Zr] and kp. At initial polymerization time lower [C *]/[Zr] with higher kp was observed in both metallocenes. Initial propaga-tion rate constant kpE and kpP were quite large and quickly declined with polymerization time (tP). The fast decline of kpE and kpP with (tP) greatly attributing to serious diffusion restriction produced by the growing polymer chains. When (SiPh) was used in E/P copolymerization presented higher steady stage [C *]/[Zr] levels as compared to EBI/borate system, and the effect of (SiPh) catalyst structure on active cen-ter [C *]/[Zr] of E/P copolymerization was considerably stronger. By taking sight into the time-dependent variations of active caters [C*]/[Zr], chain structure, molecular weight distribution, and late initiation of the centers with lesser intrinsic reactivity are suggested to be the basic reasons for the steep decline of chain prop

关键词： Characterization   Slow evaporation   Thermo gravimetric analysis   Second harmonic generation  

摘要： Semi organic nonlinear optical material of alanine adipate (AA) was grown by slow evaporation solution method using ethanol as a solvent. The synthesized material was subjected to various characterization techniques. The single crystal X-ray diffraction analysis was carried out and their lattice parameters were determined. The presence of various functional groups of AA and their vibrational modes were identified from Fourier transform infrared (FTIR). FT-Raman spectral study has been performed to confirm the functional groups. The optical transmittance of the material was identified from UV-VIS-NIR studies. and the optical energy gap of AA is determined as 4. 851 eV. Fluorescence spectrum was also carried out using spectrofluorometer and the emission peak was observed at 454. 48 nm confirming the blue emission of the prepared crystals. The mechanical property of the grown crystal was studied using Vicker's micro hardness tester and the mechanical parameter values are calculated. Mechanical measurement indicates that the grown crystal belongs to softer category. Thermal properties of AA were evaluated with thermogravimetric analysis, differential thermal analysis and differential scanning calorimetry. The second harmonic generation efficiency was found to be 1.06 higher than that of standard potassium dihydrogen orthophosphate (KDP). (C) 2021 Elsevier B.V. All rights reserved.

关键词： Microwave heating   Thermodynamics   Dynamics   Mechanical properties  

摘要： Al-4Cu alloy matrix composites reinforced with TiB2 particles are synthesized by microwave heating (MH). The effects of this heating method (as compared to conventional heating (CH)) and the volume fraction of reinforcements on the microstructure and mechanical properties of the composite samples were studied. Assisted by differential scanning calorimetry (DSC) analysis, the thermodynamics and kinetics of the two methods were discussed, respectively. The results showed that the activation energy for TiB2 formation was 33.8 kJ/mol by MH, against 113.8 kJ/mol by CH, suggesting that the microwave sintering facilitated the reaction process. Moreover, as compared to the CH samples, the ultimate tensile strength (UTS) of the composites prepared by MH method was greater and its value increased with TiB2 content at the expense of percent elongation. The UTS increases from 194.1 MPa to 329.1 MPa and the elongation decreases from 13.70% to 6.26%.