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Friction stir welding (FSW) has proved to be a successful joining technology for aluminum alloys and many other metallic materials. The severe plastic deformation of solid-state metal during FSW made it a fully coupled thermo-mech...
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Friction stir welding (FSW) has proved to be a successful joining technology for aluminum alloys and many other metallic materials. The severe plastic deformation of solid-state metal during FSW made it a fully coupled thermo-mechanical process. In order to quantitatively study both the total heat generation and the spatial distribution of the heat flux, a thermo-mechanical coupled model based on computational fluid dynamics was presented in this study. The heat generation, the temperature field and the material flow pattern were simulated in a fully coupled way. The simulated temperature distribution agreed well with the experimental results. The total heat generation was found to be proportional to the 0.75 power of the tool rotating speed. The spatial distribution of the heat flux around the FSW tool was almost axisymmetric about the tool axis. A radial distribution function was defined to describe the heat flux in different rotating rates. The radial distribution function in the shoulder region was fitted to a parabolic function.
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The starting processes of under-expanded free jets with nozzle pressure ratios of 2.15, 2.7, and 3.4 are systematically analyzed by large-eddy simulations, and the unified laws of the evolution of the screech frequency and the scr...
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The starting processes of under-expanded free jets with nozzle pressure ratios of 2.15, 2.7, and 3.4 are systematically analyzed by large-eddy simulations, and the unified laws of the evolution of the screech frequency and the screech mode in the starting jet are given. Through the development of vortices, the critical time points of the generations of screech tones are investigated. The wavenumber spectra and dispersion relations are employed, showing that the screech feedback loops in the different starting jets are all closed by the neutral waves excited by the interaction between the Kelvin-Helmholtz wavepacket and shock cells of different wavenumbers. The screech frequency prediction during the starting process is put forward for the first time, which is achieved by combining the neutral acoustic wave mode with wavenumber differences between shock cells and the Kelvin-Helmholtz wavepacket. Spectral proper orthogonal decomposition is applied to explain the reason for the change in the interaction mechanism during the starting process.
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A new model for evaporation/condensation is developed using a chemical kinetics framework. The unidirectional evaporation/condensation flux is calculated by the Arrhenius equation, in which the activation energy is assumed to be t...
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A new model for evaporation/condensation is developed using a chemical kinetics framework. The unidirectional evaporation/condensation flux is calculated by the Arrhenius equation, in which the activation energy is assumed to be the pseudo-chemical potential (PCP) barrier and the pre-factor is determined by the collision frequency of liquid/vapor atoms. The value of the PCP barrier is obtained from the equilibrium molecular dynamics (MD) simulations of a liquid-vapor coexisting argon system using umbrella sampling. Simulation results show that the PCP barrier from vapor to liquid is zero, and that from liquid to vapor is the PCP difference between the two phases. The collision frequency can be derived from the Maxwell-Boltzmann velocity distribution, which is established in vapor and liquid phases. A density expression of the model is further derived by estimating the PCP barrier with the densities of the liquid and vapor. The condensation fluxes calculated from the original and density expressions are consistent with the results from the MD simulations of argon condensation. Moreover, the density expression of the new model has been validated by comparing it with results from previous MD simulations for verification purposes.
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Purpose - The purpose of this paper is to determine the base position and the largest working area for mobile manipulators. The base position determines the workspace of the mobile manipulator, particularly when the operation mode...
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Purpose - The purpose of this paper is to determine the base position and the largest working area for mobile manipulators. The base position determines the workspace of the mobile manipulator, particularly when the operation mode is intermittent (i.e. the mobile platform stops when the manipulator conducts the task). When the base of the manipulator is in the intersection area of the Base's Workable Location Spaces (BWLSes), the end effector (EE) can reach all path points. In this study, the intersection line of BWLSes is calculated numerically, and the largest working area is determined using the BWLS concept. The performance of this method is validated with simulations on specific surface segments, such as plane, cylinder and conical surface segments. Design/methodology/approach - The BWLS is used to determine the largest working area and the base position in which the mobile manipulator can reach all path points with the objective of reducing off-line planning time. Findings - Without considering the orientation of the EE, the base position and the working area for the mobile manipulator are determined using the BWLS. Compared to other methods, the proposed algorithm is beneficial when the planning problem has six dimensions, ensuring the reachability and stability of the EE. Originality/value - The algorithm needs no manual configuration, and its performance is investigated for typical surfaces in practical applications.
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Se-containing precursor films with two different compositions were prepared by magnetron sputtering from Cu_(0.8)Ga_(0.2) and In_2Se_3 targets, and then were selenized using Se vapor. The effects of precursor composition and selen...
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Se-containing precursor films with two different compositions were prepared by magnetron sputtering from Cu_(0.8)Ga_(0.2) and In_2Se_3 targets, and then were selenized using Se vapor. The effects of precursor composition and selenization temperature on the film properties were investigated. The results show that Cu_(2-x)Se phase plays a critical role in film growth and electrical properties of CIGS films. The Cu-rich films exhibit different surface morphology and better crystallinity, as compared to the Cu-poor films. All the CIGS films exhibit p-type conductivity. The resistivity of the Cu-rich films is about three orders of magnitude lower than that of the Cu-poor films, which is attributed to the presence of p-type highly conductive Cu_(2-x)Se phase.
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Dynamic behavior of micro water droplet condensed on a lotus leaf with two-tier roughness is studied. Under laboratory environment, the contact angle of the micro droplet on single micro papilla increases smoothly from 80° to 160...
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Dynamic behavior of micro water droplet condensed on a lotus leaf with two-tier roughness is studied. Under laboratory environment, the contact angle of the micro droplet on single micro papilla increases smoothly from 80° to 160° during the growth of condensed water. The best-known "self-cleaning" phenomenon will be lost. A striking observation is the out-of-plane jumping relay of condensed droplets triggered by falling droplets, as well as its sustained speed obtained in continuous jumping relays. The underlying mechanism can be used to enhance the automatic removal of dropwise condensation without the help from any external force. The surface tension energy dissipation is the main reason controlling the critical size of jumping droplet and its onset velocity of rebounding.
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In this paper, nanosize titanium dioxide (TiO_2) deposited on pristine and acid treated carbon nanotubes (CNTs) were prepared by a modified sol-gel method. The nanoscale materials were extensively characterized by transmission ele...
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In this paper, nanosize titanium dioxide (TiO_2) deposited on pristine and acid treated carbon nanotubes (CNTs) were prepared by a modified sol-gel method. The nanoscale materials were extensively characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD), Fourier transform infrared spectrometer (FTIR) and Raman spectra. The results indicated that about 6.8 nm TiO_2 nanoparticles were successfully deposited on acid-treated CNTs surface homogeneously and densely, which was smaller than TiO_2 coated on pristine CNTs. The surface state of CNTs was a critical factor in obtaining a homogeneous distribution of nanoscale TiO_2 particles. Acid oxidization could etch the surface of CNTs and introduce functional groups, which were beneficial to controllable homogeneous deposition. The TiO_2 coated on acid-treated CNTs was used as photocatalyst for Reactive Brilliant Red X-3B dye degradation under UV irradiation, which showed higher efficiency than that of TiO_2 coated on pristine CNTs and commercial photocatalyst P25.
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摘要 :
Carbon nanotubes(CNTs)have excellent electronic and mechanical properties,which make them a competitive material in many applications,e.g.,in nanoelec-tronics or as reinforcing agents in composite materials.Aligned carbon nanotube...
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Carbon nanotubes(CNTs)have excellent electronic and mechanical properties,which make them a competitive material in many applications,e.g.,in nanoelec-tronics or as reinforcing agents in composite materials.Aligned carbon nanotube films(ACNTFs)represent an interesting architecture of nanotubes,where the quasi one-dimensional carbon cylinders are oriented perpendicular to a substrate.Due to the excellent electron emission properties of carbon nanotubes,ACNTFs may find applications in flat panel displays.It has also been proposed to use ACNTFs for highly efficient high-order harmonic generation.
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We propose herein a new optical sensor head designed in Littrow configuration for laser linear encoder which can directly measure X-direction and Z-direction displacement of a linear stage using one-dimensional (1-D) gratings. Giv...
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We propose herein a new optical sensor head designed in Littrow configuration for laser linear encoder which can directly measure X-direction and Z-direction displacement of a linear stage using one-dimensional (1-D) gratings. Given the benefits of Littrow configuration, the measuring range along Z-direction is extremely enlarged compared with traditional encoders. Finally, the optical configuration was successfully constructed and its measurement performance was evaluated and analysed. Under non-environmentally controlled conditions, the maximum positioning error in a movement of 2.0 mm was 0.261 μm with a standard deviation of 0.134μm in the X-direction and 0.211 μm with a standard deviation of 0.070 urn in the Z-direction.
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Existing compliance control methods take the force/moment precision as the only metric but do not explicitly guarantee the pose precision between assembly objects. In this paper, we first find significant time-variant and coupling...
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Existing compliance control methods take the force/moment precision as the only metric but do not explicitly guarantee the pose precision between assembly objects. In this paper, we first find significant time-variant and coupling characteristics in the process of modelling peg-in-hole assembly. Then, piecewise strategy and decoupling control (PSDC) method is proposed to explicitly improve the pose precision between the peg and hole. During designing PSDC controller, given the time-variant characteristic of assembly, piecewise strategy is utilized to improve the rapidity and stability of control, which is the basis of the pose precision. Given the coupling characteristic of assembly, an identification method of modelling error on the equation of output is proposed and the corresponding decoupling module is designed to avoid the system error on the pose between the peg and hole. Finally, the simulation and experiment results demonstrate that PSDC method achieves higher pose precision assembly in a force-guided compliance control framework.
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