Atmospheric-pressure chemical vapor deposition (CVD) is used to grow monolayer MoS2 two-dimensional crystals at elevated temperatures on silicon substrates with a 300 nm oxide layer. Our CVD reaction ...is hydrogen free, with the sulfur precursor placed in a furnace separate from the MoO3 precursor to individually control their heating profiles and provide greater flexibility in the growth recipe. We intentionally establish a sharp gradient of MoO3 precursor concentration on the growth substrate to explore its sensitivity to the resultant MoS2 domain growth within a relatively uniform temperature range. We find that the shape of MoS2 domains is highly dependent upon the spatial location on the silicon substrate, with variation from triangular to hexagonal geometries. The shape change of domains is attributed to local changes in the Mo:S ratio of precursors (1:>2, 1:2, and 1:<2) and its influence on the kinetic growth dynamics of edges. These results improve our understanding of the factors that influence the growth of MoS2 domains and their shape evolution.
In recent years, impressive advances in harvesting renewable energy have led to a pressing demand for the complimentary energy storage technology. Here, a high Coulombic efficiency (∼99.7%) Al ...battery is developed using earth-abundant aluminum as the anode, graphite as the cathode, and a cheap ionic liquid analog electrolyte made from a mixture of AlCl₃ and urea in a 1.3:1 molar ratio. The battery displays discharge voltage plateaus around 1.9 and 1.5 V (average discharge = 1.73 V) and yielded a specific cathode capacity of ∼73 mAh g−1 at a current density of 100 mA g−1 (∼1.4 C). High Coulombic efficiency over a range of charge–discharge rates and stability over ∼150–200 cycles was easily demonstrated. In situ Raman spectroscopy clearly showed chloroaluminate anion intercalation/deintercalation of graphite (positive electrode) during charge–discharge and suggested the formation of a stage 2 graphite intercalation compound when fully charged. Raman spectroscopy and NMR suggested the existence of AlCl₄⁻, Al₂Cl₇⁻ anions and AlCl₂·(urea)n⁺ cations in the AlCl₃/urea electrolyte when an excess of AlCl₃ was present. Aluminum deposition therefore proceeded through two pathways, one involving Al₂Cl₇⁻ anions and the other involving AlCl₂·(urea)n⁺ cations. This battery is a promising prospect for a future high-performance, low-cost energy storage device.
High speed and high reliability are important characteristics and are part of the inevitable development trend in many machining production lines such as those in the automotive industry and ...electronic manufacturing. In high-speed machining, the linear axe feed drive system is an important component that moves the cutting tool and workpiece to their desired positions for part production. Because of the long amount of time or the high power continuous machining, gradual wear of the ball screw easily occurs, which will deteriorate its performance. However, due to time-varying factors during the machining process, such as the feeding speed, cutting force and table position, condition monitoring and health assessment of the feed drive system in the long-term running status are complicated. To solve this problem, the statistical characteristics of the dynamics of the feed drive system are introduced in this paper to develop a method to long-term condition monitoring and life cycle assessment. In this method, the modal parameters are estimated from the free-vibration response excited by the inertial force of the feed drive system during its high-speed acceleration or deceleration movement. Then, the long-term statistical characteristics of the dynamics are analysed, and their effects on the machining process are further studied. The spindle current in the milling process is monitored by the current sensor and evaluated using the sparse feature vector. The results show that the variance of the modal parameter increases with the wear of the screw, which will worsen the machining process fluctuations and significantly accelerate the wear rate of the cutting tool. Therefore, the health condition of the feed drive system of the machine tool can be accurately monitored by both the statistical characteristics of modal parameters and the sparse vectors of the cutting current.
•Life cycle health assessment of the feed drive system is conducted using operational modal analysis.•Product consistency is monitored by sparse feature vector similarity of the cutting current.•Dynamics uncertainties can increase the fluctuations of the cutting process, and worsen the working condition.
Welding deformation and residual stress have negative influence on assembly accuracy and service performance. Thermal elastic plastic (TEP) and inherent strain finite element analysis (FEA) methods ...were used to study this challenge. Basic principle of these two methods was first introduced. The influence of welding process, constraints, solid phase transformation and multi-pass welding on deformation and residual stress was discussed, and computation accuracy and efficiency were summarised. Loading method of inherent strain in inherent strain FEA was analysed, interface element was introduced to simulate effects of the gap on deformation in assembly welding especially for large structures. The future work, including accurately multiscale TEP model, efficiently transient prediction method of large structures, and flexible evaluation software, was planned.
In this paper, T-joint of 316L steel is obtained by subsection laser welding method without jig, while a thermal elastic-plastic finite element model considering multi-linear yield stress curves and ...multi-point constraint equations is developed to estimate welding deformation and residual stress. A simplified heat source model with the actual bead geometry is proposed to reduce the number of finite element meshes. Prediction errors between the simulation and experimental measurement results of angular distortions of three reference lines are −11.0, 0.9, and 7.9 % respectively, and the maximum residual stress is 396 MPa. Meanwhile by microstructure analysis, austenite and ferrite are consisted in weld, segregation and dendritic structures are also observed, while the heat-affected zone is very narrow. In a word, weld line with large travel can be welded by subsection laser welding method without jig, while the finite element method proposed in this paper is effective to predict welding deformation and residual stress.
With the development of ultra-precision machining technology, the aerostatic bearings with high stiffness are needed urgently. This paper proposes a new aerostatic bearing structure with split ...design, which can passively compensate the load displacement caused by the change of film thickness through the pressure feedback of the shell. The static characteristics of the bearing are analyzed by computational fluid dynamics analysis and fluid–structure interaction simulation. The effectiveness of the structure is verified by comparative experiments. The results show that the static stiffness of the bearing is improved by 171% within a certain load range, the dynamic stiffness is increased by up to 30% under low frequency excitation and capacity is better than that of the conventional aerostatic bearing.
Thermoplastic polyurethane (TPU) films have been widely used as substrates in the rapidly developing fields of flexible electronics and soft actuators. Patterned high-quality cutting is of great ...significance to the promotion of applications. In order to clarify the mechanism of laser cutting, Raman and thermal analyses were performed. Raman tests verified that no new solid by-products were produced after laser cutting. Thermal cracking and thermogravimetric-infrared combined analysis were used to simulate the process of nanosecond laser cutting, and the results showed that the gas by-products released were mainly composed of organic substances including ketone and phenol with benzene ring as the main body, as well as low-chain hydrocarbons and alcohols. Moreover, a cutting quality evaluation standard consists of cutting kerf zone (CKZ) and heat-affected zone (HAZ) was established. And based on this standard, the better nanosecond laser cutting parameters are 10 kHz and 0.03 m/s, which was obtained from a 2-factors and 5 levels orthogonal experiment. Main effect and interaction analysis were performed to clarify the impact of laser parameters on cutting quality and each other.
Fusion interface stress is sharply produced by heating–cooling cycle in 10-kW level laser welding of stainless steel 316L with 10 mm thick, and it has a direct contribution to microcrack formation. ...This topics is investigated through proposing a macro–microcoupling model that consists of four submodels: ① hybrid double ellipsoidal and peak linearly-increased Gaussian cylinder heat source are combined to simulate thermal cycle and distribution; ② 3-D Voronoi tessellation is applied to establish the weld grain (average size 60 μm) structure; ③ solidification procedure is developed based on temperature-related solid-phase fraction to obtain shrinkage displacements of weld pool and fusion interface; ④ elastic–plastic mechanical analysis constitutive submodel is used to analyze transient stress on fusion surface and its potential effect on microcrack. Experimental microstructure is obtained to investigate solidification cracking. Relative error of weld size is 2.9% between experiment and simulation results. The shrinkage displacement is 5.5 μm along the parallel direction of weld section when tensile stress reaches 291.7 MPa, and microcrack occurs in the solidification weld. The solidification cracking of 316L 10 kW-level laser welding can be predicted based on the obtained peak value of tensile stress on fusion surface.
Synthetic 2D crystal films grown by chemical vapor deposition are typically polycrystalline, and determining grain size within domains and continuous films is crucial for determining their structure. ...Here we show that grain boundaries in the 2D transition metal dichalcogenide WS2, grown by CVD, can be preferentially oxidized by controlled heating in air. Under our developed conditions, preferential degradation at the grain boundaries causes an increase in their physical size due to oxidation. This increase in size enables their clear and rapid identification using a standard optical microscope. We demonstrate that similar treatments in an Ar environment do no show this effect, confirming that oxidation is the main role in the structural change. Statistical analysis of grain boundary (GB) angles shows dominant mirror formation. Electrical biasing across the GB is shown to lead to changes at the GB and their observation under an optical microscope. Our approach enables high-throughput screening of as-synthesized WS2 domains and continuous films to determine their crystallinity and should enable improvements in future CVD growth of these materials.
Tube-to-tubesheet welding is widely used in manufacturing of heat exchangers, steam turbines, condensers, boilers, etc. It has evolved from manual arc welding, automatic welding to digital welding. ...In the future, it will be toward intelligent welding. However, the key technologies related to tube-to-tubesheet welding are seldom summarized. This paper presents the sensing, positioning, and control methods of tube-to-tubesheet welding in recent decades. The fundamental difference between automatic and digital tube-to-tubesheet welding is whether the operator or the sensors interacts or transmits data with the welding system. The characteristics of tube are obvious and consistent so that vision-aided robotic positioning is the most promising. Moreover, arc length control which is important to tube-to-tubesheet welding has also been reviewed. Three typical sensing methods and five typical control methods are proposed to realize arc length control. The development direction of tubesheet welding is also predicted. This paper has an active influence on the development of pressure vessels and related industries.
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