Phonon Renormalization in Reconstructed MoS2 Moire Superlattices

In moiré crystals formed by stacking van der Waals (vdW) materials, surprisingly diverse correlated electronic phases and optical properties can be realized by a subtle change in the twist angle. Here, we discover that phonon spectra are also renormalized in MoS2 twisted bilayers, adding a new perspective to moiré physics. Over a range of small twist angles, the phonon spectra evolve rapidly due to ultra-strong coupling between different phonon modes and atomic reconstructions of the moiré pattern. We develop a new low-energy continuum model for phonons that overcomes the outstanding challenge of calculating properties of large moiré supercells and successfully captures essential experimental observations. Remarkably, simple optical spectroscopy experiments can provide information on strain and lattice distortions in moiré crystals with nanometer-size supercells. The newly developed theory promotes a comprehensive and unified understanding of structural, optical, and electronic properties of moiré superlattices.The spectroscopy experiments at UT-Austin (J.Q.) were primarily funded by the U.S. Department of Energy, Office of Basic Energy Sciences under grant DE-SC0019398 and a grant from the University of Texas. Material preparation was funded by the Welch Foundation via grant F-1662. The collaboration between the X.L., C.S., K.L., and M.A. groups is facilitated by the NSF-MRSEC under DMR- 1720595, which funded J.C. and J.E. partially. L.L. and F.L. acknowledge support by the TU-D doctoral program of TU Wien, as well as from the Austrian Science Fund (FWF), project I-3827. The authors ac- knowledge discussions with S. Reichardt and the use of facilities and instrumentation supported by the National Science Foundation through the Center for Dynamics and Control of Materials: an NSF MRSEC under Cooperative Agreement No. DMR-1720595. P.T. acknowledges support from the National Natural Science Foundation of China (Grant No.11874350) and CAS Key Research Program of Frontier Sciences (Grant No. ZDBS-LY-SLH004). M.L. acknowledge the support from the Project funded by China Post- doctoral Science Foundation (Grant No. 2019TQ0317). The PFM work (D.L. and K.L.) was supported by NSF DMR-2004536 and Welch Foundation Grant F-1814. K.W. and T.T. acknowledge support from the Elemental Strategy Initiative conducted by the MEXT, Japan, Grant Number JPMXP0112101001, JSPS KAKENHI Grant Numbers JP20H00354 and the CREST(JPMJCR15F3), JST.Center for Dynamics and Control of Material

Effect of Solution Treatment and Aging Treatment on Mechanical Properties of 2060 Al-Li Alloy and Process Multi-Objective Optimization

Solution and aging heat treatment are effective means to improve the mechanical properties of Al-Li alloys. The solution treatment and aging treatment tests of 2060 Al-Li alloy were carried out. The yield strength, tensile strength, elongation, Vickers hardness, and microstructure of specimens after heat treatment were obtained by the tensile test at room temperature, Vickers hardness test, SEM analysis, TEM analysis, and EDS analysis. The effects of solution and aging heat treatment parameters on mechanical properties of 2060 Al-Li alloy were analyzed by response surface model and test results. The results show that sufficient solution can make the Cu-rich second phase of the alloy continuously dissolves into the aluminum matrix, and consequently obtain the supersaturated solid solution, the insoluble second phase is mainly θ’ (Al2Cu) phase, T(Al2Cu2Mg3) phase, and S’ (Al2CuMg) phase. The strength and hardness of the alloy are improved, but the ductility worsens with the degree of solution treatment enhances. With the increase of aging temperature and aging time, the strength and hardness of the alloy increase, but the elongation decreases. Taking tensile strength, elongation, and Vickers hardness of the alloy as the optimization objectives, the NSGA-II multi-objective optimization algorithm was used to optimize the process parameters of solution and aging heat treatment, and the heat treatment experiment was carried out. The optimization results show that the best mechanical properties of the alloy with matching strength and toughness can be obtained under the solution treatment at 466 °C/60 min and aging treatment at 185 °C/13 h

Study of Top Dead Center Measurement and Correction Method in a Diesel Engine

Abstract: The thermal loss angle error analysis and maximum pressure determination method analysis were conducted first. Then the polytropic exponent method, the inflection point analysis, the loss function method and the symmetry method were utilized under different rotating speed, load and cooling water temperature, to calculate TDC in D6114 diesel engine and the results were compared with TDC position measured under the same condition with direct method of measurement. The study proved that (1) thermal loss angle of the diesel engine ranges from -1.0 ~ -0.6°CA; (2) Thermal loss angle is mainly affected by rotating speed and is reducing when rotate speed increases;(3) the symmetry method is generally the optimum for calculating the thermal loss angle of automotive diesel engines, with an error within 0.2°CA

Effect of Solution Treatment and Aging Treatment on Mechanical Properties of 2060 Al-Li Alloy and Process Multi-Objective Optimization

Solution and aging heat treatment are effective means to improve the mechanical properties of Al-Li alloys. The solution treatment and aging treatment tests of 2060 Al-Li alloy were carried out. The yield strength, tensile strength, elongation, Vickers hardness, and microstructure of specimens after heat treatment were obtained by the tensile test at room temperature, Vickers hardness test, SEM analysis, TEM analysis, and EDS analysis. The effects of solution and aging heat treatment parameters on mechanical properties of 2060 Al-Li alloy were analyzed by response surface model and test results. The results show that sufficient solution can make the Cu-rich second phase of the alloy continuously dissolves into the aluminum matrix, and consequently obtain the supersaturated solid solution, the insoluble second phase is mainly θ’ (Al2Cu) phase, T(Al2Cu2Mg3) phase, and S’ (Al2CuMg) phase. The strength and hardness of the alloy are improved, but the ductility worsens with the degree of solution treatment enhances. With the increase of aging temperature and aging time, the strength and hardness of the alloy increase, but the elongation decreases. Taking tensile strength, elongation, and Vickers hardness of the alloy as the optimization objectives, the NSGA-II multi-objective optimization algorithm was used to optimize the process parameters of solution and aging heat treatment, and the heat treatment experiment was carried out. The optimization results show that the best mechanical properties of the alloy with matching strength and toughness can be obtained under the solution treatment at 466 °C/60 min and aging treatment at 185 °C/13 h