Search

Uniaxial compression is widely employed to investigate the constitutive behavior and microstructural developments of metallic materials at elevated temperatures. However, the heat exchange between work-piece and anvils leads to temperature gradient and inhomogeneous deformation within the work-piece, which weakens the reliability of the testing method. The problem is more serious for titanium alloys due to the poor thermal conductivity and high sensitivity of deformation resistance on temperature. To quantify the effect of temperature gradient, a coupled thermo-electric-mechanical FE model is developed for hot compression of TA15 titanium alloy with a Gleeble thermal simulator. The precise temperature control is achieved via a negative feedback control algorithm. It is found that the temperature gradient prior to deformation is sensitive to the electrical contact conductance between specimen and anvils as well as the heat transfer between the anvil and anvil base. Bulging is sensitive to temperature gradient rather than interfacial friction. The measured flow stress is smaller than the true value at large strain for the titanium alloy, which is contrary to conventional perception. Thus, the traditional flow stress correction method degrades the accuracy of flow stress measurement. A correction procedure is developed based on inverse analysis of the FE simulation. The results can reduce the systematic error of flow stress measurement of titanium alloys with Gleeble system.

PurposeThe motivation for this work is to establish a model that not only includes the main factors resulting in macrosegregation but also retains simplicity and consistency for the sake of potential application in casting practice.Design/methodology/approachA mathematical model for the numerical simulation of thermosolutal convection and macrosegregation in the solidification of multicomponent alloys is developed, in which the coupled macroscopic mass, momentum, energy and species conservation equations are solved. The conservation equations are discretized by using the control volume‐based finite difference method, in which an up‐wind scheme is adopted to deal with the convection term. The alternative direction implicit procedure and a line‐by‐line solver, based on the tri‐diagonal matrix algorithm, are employed to iteratively solve the algebraic equations. The velocity‐pressure coupling is handled by using the SIMPLE algorithm.FindingsBased on the present study, the liquid flow near the dendritic front is believed to play an important role in large‐scale transport of the solute species. The numerical or experimental results in the literatures on the formation of channel segregation, especially those about the location of the initial flow as well as the morphology of the liquidus front, are well supported by the present investigation.Research limitations/implicationsThe modelling is limited to dealing with the thermosolutal convection of two‐dimensional cases. More complicated phenomena (e.g. crystal movement) and 3D geometry should be considered in future research.Practical implicationsThe present model can be used to analyze the effects of process parameters on macrosegregation and, with further development, could be applied as a useful tool in casting practice.Originality/valueThe numerical simulation demonstrates the capability of the model to simulate the thermosolutal convection and macrosegregation in alloy solidification. It also shows that the present model has good application potential in the prediction and control of channel segregation.

Large amounts of well-aligned carbon nanotubes (CNTs) with open tips have been produced by pyrolysis of iron(II) phthalocyanine. The aligned CNTs have an average length about 10 μm and diameters ranging from 92 to 229 nm. Some of produced CNTs showed Y-junction structure due to the self-joint growth of two neighboring CNTs. The well-aligned CNTs indicated a bamboo-shaped multiwalled structure and fairly good crystallinity. The aligned CNTs follow tip growth mechanism.

High-quality single-walled carbon nanotubes (SWNTs) with few defects and very small amount of amorphous carbon coating have been produced by catalytic decomposition of C2H2 over MgO supported Fe–Mo bimetallic catalyst at 800 °C. The diameters of SWNTs are in the range of 0.80–2.43 nm from Raman analysis. The use of MgO support facilitates a large-scale synthesis of SWNTs with a smaller diameter compared with Al2O3 support material. This work indicates that MgO is useful support material to realize the synthesis of high-quality SWNTs at high yield using C2H2 carbon source.

Well-aligned carbon nanotubes (CNTs) with high purity have been produced by pyrolysis of iron(II) phthalocyanine and acetylene at 800 °C. The synthesized CNTs have a length of 75 μm and diameters ranging from 20 to 60 nm. The CNTs have a bamboo-like structure and exhibit good crystallinity of graphite sheets. The growth rate of the CNTs was rapidly increased with adding C2H2. Our results demonstrate that the proposed growth method is suitable to large-scale synthesis of high-purity well-aligned CNTs on various substrates.

Single-walled carbon nanotubes (SWNTs) and double-walled carbon nanotubes (DWNTs) are simultaneously synthesized by catalytic decomposition of CH4 over Fe–Mo/Al2O3 catalyst. High-resolution transmission electron microscopy observation shows that produced carbon materials consist of about 70% SWNTs and about 30% DWNTs. The diameters of SWNTs are in the range of 0.8–1.5 nm while the outer and inner diameters of DWNTs are in the range of 1.75–3.1 and 0.95–2.3 nm, respectively. Raman analysis indicates that the synthesized SWNTs and DWNTs have high-quality graphite structure.

A large amount of carbon nanotubes with uniform diameter were produced from catalytic decomposition of C2H2 or CH4 over perovskite oxide LaCoO3 catalyst precersor. The result of XRD measurement indicated that after reduction of LaCoO3 at 700 ○C cobalt existed chiefly as Co0 particles supported and separated by La2O3. La2O3 can prevent Co0 from agglomeration. The size of Co0 nanoparticles determined the diameter of carbon nanotubes. The morphologies and microscopic structure of carbon nanotubes were examined by means of scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD) and Raman. Reduction of LaCoO3 in H2 and thermal oxidation of carbon nanotubes in air were made by thermogravimetric experiments (TG). XPS and Raman results revealed that the carbon nanotubes made from CH4 are probably more graphitized than those made from C2H2.

It is a great challenge to measure accurately the high values of thermal conductivity of diamond and diamond-like materials in the form of thin films. The objective of this study is to develop a technique based on the principle of holographic interferometry to determine these values relative to other well-known values of bulk materials such as aluminum, copper, or molybdenum. Our approach in this work was to measure directly the effective thermal diffusivity of diamond film/silicon substrate samples by first observing the change of the interference fringe pattern related to the deformation of the sample that was induced by a prescribed thermal excitation source. The thermal conductivity of diamond thin film was then determined from the known values of diamond density and specific heat at constant pressure based on an analytical model of a two-layer structure system developed for this study. In this paper, the efficacy of using this approach will be presented with a comprehensive result of the calculated thermal conductivity on a series of CVD diamond and diamond-like thin films on silicon substrate samples prepared by different CVD techniques including microwave CVD and hot-filament CVD.

The acceleration of protons in the interaction of a circularly polarized laser pulse with a thin foil is investigated. For circular-polarization laser pulses, the quasi-equilibrium for electrons is established due to the light pressure and the electrostatic field built up at the interacting front of the laser pulse. The protons located within the skin depth of the laser pulse can be synchronously accelerated and bunched like phase-stable acceleration in a conventional radio-frequency accelerator, so that a high-current proton beam with a low energy spread is generated. It has been proved by 1-D particle-in-cell simulations.

For colocated multiple-input multiple-output (MIMO) radar transmitting orthogonal polyphase coded waveform, a low range, angular and Doppler sidelobe is desirable. In this paper, we propose a MIMO radar sidelobe suppression method based on random space-time coding (STC) method. In detail, at each pulse repetition period, a random STC matrix, composed of a diagonal random phase matrix multiplied by a random permutation matrix, is modulated to waveforms before transmission. Such a kind of STC matrix can keep the constant modulus property of transmitting waveforms and can decrease range, angular and Doppler sidelobe significantly. (4 pages)

Hydrophobicity is a prominent characteristic of silicone rubber for polymer insulators. The accurate evaluation on the insulator hydrophobicity is increasingly required for the long-term performance of outdoor insulators in the smart grid. In this paper, based on dynamic dropping test (DDT), acoustic characteristic of surface discharges generated by the dynamic behavior of water droplet was investigated to evaluate the hydrophobic properties of silicone rubber insulator at different ageing levels. The specimens were prepared by using the method of corona degradation and determined by contact angle (CA). It is found that characteristic of discharge sound changing with the decrease of CA. Therefore, the results obtained indicate that the acoustic characteristic of discharge in DDT method is sensitive to the hydrophobic properties, which can be applied as a non-contact method for hydrophobicity evaluation of silicone rubber insulators.

By 3D particle-in-cell simulation and analysis, we propose a plasma lens to make high intensity, high contrast laser pulses with a steep front. When an intense, short Gaussian laser pulse of circular polarization propagates in near-critical plasma, it drives strong currents of relativistic electrons which magnetize the plasma. Three pulse shaping effects are synchronously observed when the laser passes through the plasma lens. The laser intensity is increased by more than 1 order of magnitude while the initial Gaussian profile undergoes self-modulation longitudinally and develops a steep front. Meanwhile, a nonrelativistic prepulse can be absorbed by the overcritical plasma lens, which can improve the laser contrast without affecting laser shaping of the main pulse. If the plasma skin length is properly chosen and kept fixed, the plasma lens can be used for varied laser intensity above 10(19) W/cm(2).

We investigated the three dimensional (3-D) deformation field around crack tips under stable growth in three different aluminum alloys with different hardening indices ( n = 3,3.5,7.7 ) using a new experimental technique combining in-plane Moire and a laser projected grating method. Through optical spatial filtering, contours of displacement components along x , y and z directions can be generated separately. Strain distributions are then calculated from the displacement information. The experimentally obtained singularity fields are compared with theoretical singularity form proposed by Gao, Hwang and Dai of mode I stable crack growth for strain hardening materials. And we map the region wherein the theory and experiment are in fairly good agreement. The experimentally obtained GHD singularity fields are larger than the fracture process zone and the finite strain zone around the crack tip. They increase as the load increases and the material hardening exponent decreases.

Biodegradable polymers are increasingly expected for substitute of present insulating materials in the electrical and electronic devices. However, their electrical properties are not systematically confirmed and need further investigation. In this paper, biodegradable polyethylene (BDPE) films are employed as the specimens and high density polyethylene (HDPE) films are as the reference. The breakdown endurance of BDPE film is examined using bias DC voltage under UV-irradiation environments. The images of breakdown craters are captured through applying a monocular-video-zoom microscope. Both breakdown area and box dimension of breakdown craters are obtained by an image-pattern-identification technique. The results reveal that for both BDPE and HDPE films, the breakdown voltage, breakdown area and box dimension all decrease with the increase of UV-irradiation level. Based on comparison of the results, it shows that under UV-irradiation environment, breakdown endurance of BDPE film is better than that of HDPE film.

A new ion acceleration method, namely, phase-stable acceleration, using circularly-polarized laser pulses is proposed. When the initial target density n(0) and thickness D satisfy a(L) approximately (n(0)/n(c))D/lambda(L) and Dl(s) with a(L), lambda(L), l(s), and n(c) the normalized laser amplitude, the laser wavelength in vacuum, the plasma skin depth, and the critical density of the incident laser pulse, respectively, a quasiequilibrium for the electrons is established by the light pressure and the space charge electrostatic field at the interacting front of the laser pulse. The ions within the skin depth of the laser pulse are synchronously accelerated and bunched by the electrostatic field, and thereby a high-intensity monoenergetic proton beam can be generated. The proton dynamics is investigated analytically and the results are verified by one- and two-dimensional particle-in-cell simulations.

Here we give our reply to the comment by Sibirtsev et al on our paper ``Mass and K-Lambda Coupling of the N*(1535).

This paper reports the results of a set of experiments that apply state-space search methods to explore efficient ways for online construction of an optimal quadtree. An optimal quadtree is defined as a quadtree that yields a minimum reconstruction error for a given compression rate. Although some researchers have proposed procedures for image compression with quadtrees most of these methods either refer to binary images or use an off-line method to fix the variance-threshold. The results of our experiments show that a very low distortion (measured in SNR) can be achieved in a reasonable computation effort. Comparison with the literature proves that in some cases, for a given compression, our quadtree construction algorithm yields less distortion than well known image compression methods such as DCT-based vector quantization, and wavelet-based vector quantization.

High-quality single-walled carbon nanotubes free of defects and amorphous carbon coating have been produced by catalytic decomposition of C2H2 over Fe-Mo/Al2O3 catalyst.

Strain concentration factors of circular cylinders with circular and square holes are analysed by a white light speckle method. We introduce a four-aperture arrangement for recording white light speckles and find that the resulting isothetics are better in quality.

The primary objective of this paper is to review critically the current state of research in the field of air pollution and its effect on vegetation and to derive, through existing data and documentation, a set of integrated economic damage functions of air pollution on various kinds of vegetation, for the purpose of prediction. Economic damage functions of vegetation were estimated via a stepwise least-squares regression method by relating economic losses of a variety of crops to air pollution and climatological variables. The crops and agricultural products selected included corn grain, soybean, cotton, root vegetables, other vegetables, nursery products, flowers, trees, field crops, and fruit and nuts. By utilizing the ‘average’ economic damage functions derived in this study, the changes in crop losses brought about by changes in the pollution or climatological variables can be predicted.

A SEN specimen made of AL 2024-0 is loaded to failure quasi-statically. A laser-optical method is used to map the deformation field surrounding the crack tip at different stages of loading. At the early stage of loading there exists a HRR field surrounding the crack tip. However, just before the arrest of crack growing the crack tip deformation is not characterizable by any of the known theoretical methods. When crack growth ensures, there seems to be significient experimental evidence to suggest the existence of a logarithmic singularity as proposed by Gao and Hwang (1983).

In this paper, deformed cross gratings around crack-tip are recorded. The derivative fields of displacements (∂u/∂x, ∂u/∂y, ∂v/∂x, ∂v/∂y, ∂w/∂x ∂w/∂y) are obtained by optical spatial filtering of two shifted deformed gratings. From which, strain contours around crack-tips can be obtained directly.