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Semiconductor quantum dot superlattices (QDSLs) have attracted much attention as key materials for realizing new optoelectronic devices such as solar cells with high conversion efficiency and thermoelectric elements with high electrical conductivity. To improve the charge transport properties of QDSL-based optoelectronic devices, it is important that the QD structures form minibands, which are the coupled electronic states between QDs. A shorter inter-QD distance and a periodic arrangement of QDs are the essential conditions for the formation of minibands. In this study, we use CdTe QDs capped with short ligands of N-acetyl-l cysteine (NAC) to fabricate three-dimensional QD assemblies by utilizing chemical bonding between NACs. Absorption spectra clearly display the quantum resonance phenomenon originating from the coupling of the wave functions between the adjacent QDs in CdTe QD assemblies. Furthermore, we demonstrate the formation of minibands in CdTe QD assemblies by examining both, the excitation energy dependence of photoluminescence (PL) spectra and the detection energy dependence of PL excitation spectra. The fabrication method of QD assemblies utilizing chemical bonding between NACs can be applied to all QDs capped with NAC as a ligand.

Quantum dot (QD) superlattices have the potential to provide new optical properties and functions based on interactions between adjacent QDs. Two types of interactions occur between the QDs: energy transfer (ET) from small-sized QDs to large QDs and resonant coupling between QDs with equal eigenenergies. Since ET and resonant coupling strongly depend on the distance between QDs, it is critical to precisely control the distance to understand the interaction mechanism. In this review, we describe that the distance between QDs can be controlled with an accuracy of 1 nm by a layer-by-layer method and further explain the mechanisms of ET and resonant coupling between adjacent QDs.

This work is concerned with the development of a new process map for wire drawing of pearlitic steel considering damage evolution. In this study, a ductile damage is defined as a porosity or void volume fraction and the porosity evolution model proposed by Lee and Dawson is adopted. Dilatational plastic deformation due to growth of micro voids is also considered. Correspondingly, an Eulerian finite element analysis coupled with damage evolution model is utilized in order to reflect the effects of dilatational plasticity due to growth of micro voids. Also, the accumulated damage in wire drawing could be evaluated. Finite element simulation for wire drawing of pearlitic steel are performed for various process conditions such as a half die angle and an area reduction ratio. Especially, the effects of process parameters on the deformation characteristic as well as damage evolution in wire drawing are carefully examined. Finally, a new process map is presented in terms of a half die angle and an area reduction ratio, which can identify the successful process conditions for wire drawing of pearlitic steel. Thus, it would be expected that this process map will help an engineer for the design of wire drawing of pearlitic steel.

In the present study, three-dimensional crystal plasticity finite element method (CPFEM) is used to simulate microstructure and texture evolution during channel die compression of a body-centered cubic (BCC) polycrystalline material of interstitial free (IF) steel. Microstructural heterogeneity and anisotropy were taken into account by assigning a crystal orientation to each integration point of the element and determining the stiffness matrix of the individual crystal. A numerical technique for direct data mapping of Euler angles and critical resolved shear stress is also suggested. Based on the experimental observation by electron backscatter diffraction (EBSD) measurement of an as-received IF steel sheet, representative volume element (RVE) of the initial microstructure was generated by using the Voronoi tessellation scheme. The microstructure and texture evolution during channel die compression of IF steel is investigated by comparing results numerically predicted by the CPFEM with the microstructure experimentally measured by the EBSD. From the contour maps and distributions of the rotation and misorientation angles, it was revealed that the rotation of crystals is apparently influenced by both the individual orientation itself and the interaction with neighboring crystals. Furthermore, the orientations, which exhibit small rotation and misorientation angles, are the ones initially oriented along the α- and γ-fibers and hardly change despite the increase in the plastic deformation.

Semiconductor ZnSe quantum dots (QDs) were hydrothermally prepared by the reaction of Zn2+ and NaHSe in the presence of N-acetyl-l-cysteine as a stabilizer. The absorption bandwidth depends strongly on the pH of the working solutions. The photoluminescence (PL) properties were dramatically improved by selecting appropriately the pH value of the precursor solution; the PL intensity in the ZnSe QDs prepared at pH 6 was strongest irrespective of the size of ZnSe QDs. The XRD pattern demonstrated the formation of a cubic zinc blende ZnSe lattice structure and crystallinity.

This study is concerned with a design process for a foldable container structure using a finite element analysis. A foldable container structure consists of frames, panels and hinge systems. The main structure of a foldable container carries all the loads while a hinge system is designed to provide its foldability. In this work, finite element structural analyses for the main foldable container structure are carried out based on the ISO standard regulation, whose results are then taken for the design of a hinge system. The finite element analysis with two types of hinge systems are also performed. It is found out that the main structure of a standard 20ft container could be used for the foldable container with the same capacity if the corner edge in the side assembly is strengthened. It is also concluded that the hinge systems proposed in this work could be successfully used in a foldable 20ft container.

This work is concerned with rolling of a Teflon-Cu-SPCC steel plate, which has been used as a base material for an oilless bearing or dry bearing. Materials for an oilless bearing require special characteristics such as high wear resistance, low friction, fast heat loss, as well as structural strength. Such superior mechanical properties could be obtained by rolling the multilayer plate composed of a mixture of Teflon-Cu powder and a SPCC steel. The main objective of this research is to investigate the deformation characteristics of a Teflon-Cu-SPCC steel plate during rolling process. Especially, focus is given to the thickness changes of the Teflon-Cu powder mixture layer. An Eulerian finite element method coupled with a contact profile correction algorithm is adopted for the simulation of rolling. Validity of the proposed finite element analysis is given. The important parameters chosen in this work are the reduction ratio, roll size, and the Cu content in a Teflon-Cu mixture. Effects of the above important parameters on the deformation behavior and on the final thickness of a Teflon-Cu mixture layer are carefully studied.

We have investigated the optical properties of ZnSe quantum dots (QDs) prepared by a hydrothermal method. The photoluminescence (PL)-decay profiles become slower with an increase in temperature up to 160 K, contrary to an ordinary behaviour due to thermal quenching. The temperature dependence of the PL-decay profile is explained by a three-state model consisting of a ground state and two excited states of the lower-lying bound-exciton and higher-lying dark-exciton states. The analysis of the temperature dependence of the decay time indicates that the dark-exciton state contributes to the PL-decay process in the ZnSe QDs.

It is experimentally well known that mechanical properties of the material depend on crystallographic texture distribution which varies depending on deformation history. When the material deforms at several stages, it is not easy to follow up the effect of prior deformation on the final texture of the material. In the present study, the effect of deformation history on texture evolution during equal channel angular pressing (ECAP) of FCC polycrystalline metal like AA1050 is investigated by the finite element method and polycrystal plasticity model based on full constraints Taylor model. The texture evolution during the multi-pass ECAP is simulated with the initial textures determined by three virtual specimens prepared by the fully annealed, extruded, and flat-rolled materials. By comparing the pole figures and orientation distribution functions, the effect of prior deformation histories on the texture evolution is numerically studied according to the routes A and C up to four passes across the thickness of the specimen. For the extruded specimen, it is not enough to wipe out the trace of the initial textures originated from the extrusion even after four passes. For the rolled specimen, strong development of the rotated simple shear texture readily occurred with the rotation about the transverse direction of the ECAP die. This study indicates that it is necessary to control the initial texture properly for achieving a desired mechanical property.

The main objective of this study is to simulate deformation induced anisotropy during the upsetting of commercially available pure aluminum AA1050 after being processed by the equal channel angular extrusion (ECAE) process including the friction effect. A Taylor-type polycrystalline constitutive model was adopted to investigate the effects of texture evolution and deformation heterogeneity during the ECAE and upsetting. In order to save computation time, a decoupled analysis between the crystal plasticity model and finite element method for the multi-pass ECAE and a fully coupled analysis for the subsequent upsetting process were conducted. The rigid-viscoplastic finite element analysis of the ECAE was carried out to determine the history of velocity gradient during the ECAE process in order to identify the effects of the processing routes A and C on texture evolution. The calculated history was applied to the crystal plasticity model to obtain the crystallographic texture distribution. Then, a finite element analysis based on the Taylor model was conducted for prediction of the anisotropic behavior of the three-pass ECAEed specimens which was obtained by applying the ECAE of the aluminum alloy AA1050 with routes A and C, respectively. For this analysis, algorithms for the data transfer of Euler angles and hardness were developed and implemented into the program. The good agreement between the measured and simulated deformed shapes indicates that the proposed simulation technique with the data transfer algorithms can be used effectively to simulate deformation induced heterogeneity during the bulk forming process.

Localized strain and crystallographic orientation distribution during rolling process have a significant effect on anisotropic flow behavior in sheet forming of aluminum alloy, resulting in local thinning. In this study, crystal plasticity finite element method (CPFEM), which incorporates a crystal plasticity constitutive law into the three-dimensional finite element method, was used to investigate strain localization and textural evolution during the flat rolling process of the face-centered-cubic material. A rate-dependent polycrystalline theory based on the Taylor model was fully implemented into an in-house program, CAMProll3D. The through-thickness texture evolution depending on the degree of draught was predicted by using the developed CPFEM program and compared well with the experimental data available in the literature. The orientation distributions not only in the thickness direction but also in the width direction of the flat rolled sheet were investigated depending on the amount of reduction during the multi-pass flat rolling in terms of pole figure, orientation distribution function and flow potential surface in the 3-plane. Finally, the effect of friction condition between the rolls and the material on rotation about the transverse direction was found to be important to determine the texture evolution at the surface of the rolled sheet. [doi:10.2320/matertrans.M2012346]

Effects of heat treatments on the on-line service life of a press die manufactured by W-EDM are studied. In this work, four manufacturing processes for a press die are considered: (1) milling and then grinding, (2) wire-cut electric discharge machining (W-EDM), (3) low temperature heat treatment after W-EDM, and (4) high temperature heat treatment after W-EDM. On-line punching experiments for an automobile part of BL646-chain are performed. The amount of wear of the die and punch, roll-over and burnish depth in the punched chain are measured every 1,000 strokes. Overall productivities are carefully compared. Finally, it is concluded that heat treatment after W-EDM for a press die can enhance its on-line service life. Especially, high temperature heat treatment after W-EDM is very attractive as a fast and cheap manufacturing method for a press die.

This paper is concerned with development of a process map for sequential compression-backward extrusion of bulk AZ31 Mg alloy at the warm temperatures. In experiments, metal flows and crack initiations are carefully investigated and formability is examined systematically for various forming conditions such as forming temperature, punch speed, and a gap between a specimen and die. Then, a process map for the sequential compression-backward extrusion of bulk AZ31 Mg alloy at the warm temperature is proposed. In order to further understand deformation behaviors and damage evolution during warm forming process, thermo mechanical finite element analyses coupled with damage evolutions are carried out. In general, finite element predictions support experimental observation. Finally, it is concluded that the process map, proposed for the sequential compression-backward extrusion of AZ31 Mg alloy, is valid.

Ultra-fine-grained bulk Al–Mg alloy was forged into cup shape product while its grain size and mechanical properties were conserved. Deformation characteristics of the fine-grained material in practical metal forming process were examined with backward can extrusion. The Al–7.5%Mg alloy produced by cryogenic milling and hot isostatic pressing was subjected to hot hydrostatic extrusion as a final consolidation to fabricate the ultra-fine-grained bulk material. The hydrostatic extrusion reduced porosity, and made fibrous structure along the extrusion direction. In the following backward can extrusion with the workpiece prepared by hydrostatic extrusion, severe plastic deformation was achieved successfully. Microstructure of the metal was stabilized and grain size remained under a few hundred nanometer scale during the processes. The material was developed into bimodal structure composed of fine grain matrix and coarse grain cluster inclusions. The coarse grain clusters were distorted according to metal flow and helped enhance formability. There was a slight grain growth during the can forging process in some region where material experienced high temperature provoked by work dissipation.

Grain boundary diffusion creep equations developed previously for nanocrystalline materials were reappraised in order to elicit further understanding of plastic deformation of these materials in relation to grain boundary diffusion. From a mechanistic viewpoint, the strain rate is inversely proportional to the second power of the grain size when the grain size is refined to the same order of the grain boundary thickness. The presence of the threshold stress appears to be inherent, as a relatively large volume fraction of the grain boundary region is associated with irregularities.

This paper is concerned with an Eulerian finite element analysis for the steady state forming of porous materials, such as nano-grained material manufactured via cryogenic milling. The constitutive relation for such porous materials is different from that for a fully dense matrix. The general form of the constitutive equation for a porous material is derived from the yield functions for the plastic deformation of a porous material, as proposed by Shima, Green, Doraivelu, Gurson, Kuhn, Park, and Lee. Then, that general form is utilized in the Eulerian finite element formulation for the strain hardening, dilatant, and viscoplastic deformation. Initial estimation of the porosity distribution in an Eulerian mesh is obtained from the velocity and scaled pressure fields computed by the Consistent Penalty finite element method for the incompressible viscoplastic deformation of the matrix. Applications of the proposed method to rolling and extrusion are given. The change of the porosity is predicted by integrating its evolution equation along a particle path constructed in an Eulerian domain. Comparisons of the predicted distributions of porosity to those by a Lagrangian finite element method and to those by experiments reported in the literature validate the proposed method.

Compression tests were performed on the well-annealed coarse grained (CG) and the near dislocation-free ultrafine grained (UFG) 5083 Al alloys at 298 K and 77 K. The mechanical behavior of the CG alloy was dominated by the stage III hardening at 298 K and the stage II hardening at 77 K. The microstructure of the CG alloy compressed at 298 K was characterized by well-developed dislocation cells and microbands embedded into those cells, but ill-defined dislocation cells were formed at 77 K. The UFG alloy exhibited the elastic-near perfect plastic behavior at both temperatures. The dislocation cell formation was unlikely to occur in the UFG alloy. Instead, localized shear bands were formed at the onset of plastic deformation at both temperatures as an alternative strain accommodation process. Therefore, the elastic-near perfect plastic behavior of the UFG alloy could be explained by the deformation mode change from the uniform mode associated with the cell formation to the localized mode associated with the shear band formation as the grain size was refined to the UFG regime.

The compression behaviors of well-annealed coarsegrained (CG) and ultrafine-grained (UFG) 5083 Al alloys at 77 and 298 K were compared. For the CG alloy, stage II and III strain hardening were dominant at 77 and 298 K, respectively, depending on the completeness of dislocation cell formation. The UFG alloy exhibited the elastic-near perfectly plastic behavior without distinctive dislocation cell formation at both temperatures. For both alloys, the flow stress at 77 K was much higher than that at 298 K.

Deformation texture with preferred orientation is developed by external disturbance applied to the grain during the deformation process such as rolling. The formation of deformation texture is strongly influencing the mechanical property of the product, and material anisotropy is observed from the deformation texture, macroscopically. Therefore, the proper consideration and analysis of deformation texture is required. In the present work, the method for prediction of deformation-induced anisotropy employing the phenomenological yield potential is proposed. The proposed algorithm is applied to the anisotropic evolution for simple deformation modes, such as uniaxial stress state and pure shear stress state in X – Y direction. In order to verify the effectiveness of the method, the result from the proposed algorithm is then compared with that from the crystallographic texture analysis.

Highly luminescent ZnSe-based quantum dots (QDs) were synthesized by a microwave-assisted hydrothermal method. The characteristics of the ZnSe precursor solution strongly influenced the photoluminescence (PL) quantum yields (QYs) of the QDs. The PL QY of ZnSe-core QDs synthesized under the optimum conditions reached 60%. Furthermore, the PL QY further increased to higher than 90% when a ZnS shell was applied to prepare ZnSe/ZnS-core/shell QDs.

A ductile-fracture criterion, which was proposed previously namely, ( Δ l/l 0 ) f at Δ σ m =( Δ l/l 0 ) f +(−1/ tan θ) Δ σ m (where (Δl/l0)f is the fracture elongation and Δσm is the mean stress variation) was made use of to study the working limit in axisymmetric extrusion. The present investigation is concerned with the application of theory on flow and fracture to the prediction of the workability of materials in axisymmetric bar extrusion, with special reference to central bursting. The influences of die geometry and manufacturing conditions on central bursting are predicted.

This paper is concerned with the mechanical property changes in drawing/extrusion operations of hardening viscoplastic materials with damage. The process model in this study includes two state variables, the hardness for strain hardening from slip dominated plastic distortion and the porosity for damage from growth of microvoids. The decrease in apparent density in strip drawing of aluminum for several die angles are computed and compared with that from experiments. Simulations of axisymmetric drawing/extrusion have shown that the accumulated porosity in drawing is much bigger than that in extrusion while the difference between the hardness distributions in these processes are insignificant. The effects of the process conditions, such as conical die angle, friction and drawing/extrusion speed, on the mechanical property changes are also examined.

In many forming processes, the mechanical properties of the workpiece material are deteriorated by the growth of voids during plastic deformation. In this study we present a new constitutive relation for void growth based on mechanical analyses of an isolated void growing in metallic media under various conditions of stress triaxiality. The incompressible, strain hardening, viscoplastic behavior of the matrix material is characterized by an isotropic state variable model. Systematic variations of the shear strength, mean stress, porosity and deviatoric deformation rate provided families of growth rate curves that were collapsed by appropriate scaling into two master curves. The forms of these curves and the scaling factors needed to obtain two master curves motivated the mathematical structure of the constitutive relation for void growth.

The decrease in apparent density that accompanies strip drawing of aluminum is computed using new equations for void growth (presented in Part I) in conjunction with a simulation model for steady-state forming processes. A novel feature of the void growth equations is the scaling of mean stress by the state variable for plastic flow resistance. The importance of this feature is demonstrated in the process simulations wherein some regions are deforming while others are not. Comparisons between trends in computed behavior and experimental data show that the model captures the magnitude of the porosity and its location within the strip.

An algorithm for integrating the constitutive equations for an elasto-viscoplastic cubic crystal is presented which is shown to be easily employed in a polycrystalline analysis. Anisotropic elastic behaviour is incorporated into the standard constitutive equations for ductile single crystals. The algorithm is shown to be efficient, robust and general. The primary advantage of this algorithm is that is provides an implicit integration of the plastic deformation gradient while including the elastic response. This permits taking large time steps while maintaining accuracy and stability. Several polycrystalline examples are presented to demonstrate the effect of the time step on the solution. Examples also are presented which compare the algorithm described herein to an algorithm which neglects the elastic part of the deformation. In addition, the effect of the anisotropic component of the elasticity is investigated by comparing the results with those obtained assuming isotropic elasticity.

A new algorithm for free surface corrections in Eulerian finite element computations is presented and applied to a steady state bar rolling process to predict lateral spread. A variety of rolling geometries are examined, including differing height to width ratios and reductions. The bulge profiles, as well as the maximum and mean bulge amounts, are compared in detail with experiments.

A poly crystal approach coupled with damage, one of the process model previously proposed for the simulation of milli‐forming, was applied to milli‐rolling. In order to obtain the proper initial values of field variables for the anisotropic, dilatant, viscoplastic flow problems when using an Eulerian analysis, special treatments are devised. Mutual effects of the texture development and damage evolution are carefully examined in terms of the distributions of strain components, accumulated damage, R‐value as well as yield surfaces.

3,3'-dichlorobenzidine (DCB) is suspected to be arcinogenic in experimental animal and human. Several studies have investigated excretion of metabolites in urine, hemoglobin adduction and cancer incidence among workers exposed occupationally to DCB. In these researches, metabolites of DCB had a very important role. The purpose of this study was to develop the urinary monitoring method of its metabolites from rats exposed with DCB, by easily synthesizing them in the laboratory. N,N'-diacetyl-DCB was easily synthesized with DCB in pyridine by adding sufficient acetyl chloride or acetic anhydride. N-acetyl-DCB was isolated from the supernatant, which made by adding 21/microl acetyl chloride (more 3 times than DCB in moles) to 32 mg DCB in 2 ml pyridine and 0.3 ml acetic acid. Gas chromatography/mass spectrometry (GC/MS) was used for the identification, gas chromatography nitrogen phosphorous detection (GC-NPD) for the quantification and gas chromatography flame ionization detection (GC-FID) for the determination of purity. The base peak of DCB, N-acetyl-DCB and N,N'-diacetyl-DCB was 252 m/z. The other main peaks were 294 m/z for N-acetyl-DCB, and 294 and 336 m/z for N,N'-diacetyl-DCB. The purities of N-acetyl-DCB and N,N'-diacetyl-DCB were identified as 98.82 and 98.72% by GC-FID, respectively. After treatment orally to rats with 20 mg DCB/kg body weight for 2 weeks, the urinary excretion amount of DCB was nearly constant at range of 0.11-0.18 mg/L for 2 weeks. But excretion of N-acetyl-DCB was continually increased from 1.30 mg/L on 1st day to 4.15 mg/L on 14th day. And level of N,N'-diacetyl-DCB in urine was sharply increased from 2.13 mg/L on 1st day to 11.00 mg/L on 14th day.

Rogowski coil is made having no ferromagnetic material in a core. So the coil cannot be driven into saturation. This result in that Rogowski coils may be calibrated at relatively low currents, and used with confidence at very high currents. However the lowest level of current that can be measured is limited by the sensitivity of the voltage measuring instrument and system noise. Therefore, geometrical effects were investigated in order to measure high sensitivity of low level current and the significant source of error was examined as well. In the results, the sources of error were associated with coil designs, i.e. shape, uniformity and a geometrical location of current source inside and outside of the Rogowski coil.

This work is concerned with the deformation behaviours at the Cu-Al contact surface during hydrostatic extrusion of a concentric Cu and Al bar. In general, the unavoidable oxidisation at the Cu-Al contact surface prevents the bonding at the Cu-Al contact. A ‘local extrusion’, defined as a material flow through a fractured and expanded oxidised layer, is necessary for the successful bonding at the Cu-Al contact in a post process. Experiments consisting of extrusion and sintering with a biaxial Cu-Al bar are performed and it is confirmed that a ‘local extrusion’ does occur. In finite element studies, focuses are given to the behaviours of the first principal strain and the stress component normal to the Cu-Al contact profile since they are essential parameters for the success of a ‘local extrusion’. In this work, half die angle and extrusion ratio are considered as the most important process parameters. Effects of such process parameters on the deformation characteristics at the Cu-Al contact surfaces during hydrostatic extrusion are carefully examined.

A methodology for computing residual stresses in forming operations is examined in which the elasticity is neglected during the loading phase of the operation. The elastic response is recovered on unloading through the analysis of an initially-stressed body. Two examples are presented which provide a quantitative assessment of the accuracy of the approach. The first is the axisymmetric expansion of a thick-walled tube. In this case the residual stresses are compared to those computed with an elastic-plastic analysis for both the loading and unloading phases. The second example is a ring upsetting application that has been analyzed using a finite element formulation and for which there are experimental data available for comparison.