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To investigate the shock response of cyclotetramethylene tetranitramine (HMX) single crystals at elevated temperatures (below the phase transition point), plate impact experiments at elevated temperatures were designed and conducted. The HMX/window interface particle velocities at temperatures of 300 K, 373 K, and 423 K were measured by the velocity interferometry system for any reflector (VISAR) technique. To further analyze the related mesoscale deformation mechanisms, a nonlinear thermoelastic-viscoplastic model was developed, which considers thermal activation and phonon drag dislocation slip mechanisms. The proposed model could well reproduce the measured thermal hardening behavior of Hugoniot elastic limit (HEL) of HMX single crystals. At elevated temperatures, the reduced dislocation mobility was observed, which stems from both phonon scattering and radiative damping effects. Comparatively speaking, radiative damping contributes less than phonon scattering to thermal hardening behavior. The calibrated model was further used to predict shock response of HMX single crystals with different thicknesses at different initial temperatures. Both the stress relaxation and elastic precursor decrease with thickness are mainly due to the rapid dislocation generation. These insights shed light on the interplay between dislocation motion and dislocation generation in thermal hardening behavior, stress relaxation, and elastic precursor decay, which serves to reveal the mesoscale deformation mechanisms at elevated temperatures.

High-frequency induction brazing (HFIB) technology is one of the commonly used methods in brazing super abrasive grinding wheels. However, it’s difficult to obtain a uniform temperature distribution on the profiled wheel surface during HFIB, which is caused by the proximity and edge effects during high-frequency induction heating. In this study, the temperature uniformity of the profiled wheel surface and the optimization design of the coil were investigated by finite element simulation based on Box-Behnken design of response surface methodology. The mathematical model of average temperature and temperature uniformity was defined to evaluate the temperature distribution on the profiled surface of the grinding wheel. The regression equations of the response value average temperature and temperature uniformity were built based on the Box-Behnken design of simulation results. The two regression equations could well predict the results for the additional simulations. An optimization coil was designed for the brazing of profiled grinding wheel according to these regression equations. The experimental results showed good agreement with the simulation results within 4.62% errors, which verified the finite element model and the regression equations were all reliable. High-frequency induction brazing of profiled cBN grinding wheel showed that a consistent quality was obtained on the brazed wheel surface.

In order to reveal the microscopic mechanism of the titanium alloy die-sinking mixed-gas atomization discharge ablation process (MA-DAP), a single-pulse discharge test was performed using a needle electrode discharge device. According to the step-like characteristic of the single discharge waveform, the micro-erosion process could be divided into four stages: preparation for oxidation ablation; oxidation occurrence and oxide layer generation; oxidation-ablation dynamic balance and oxide layer prototype formation; and oxidation-ablation termination and integral oxide layer formation. Theoretical analysis showed that the violent exothermic oxidation reaction between the oxygen medium and the high-temperature melting material introduced a large quantity of external chemical energy into the sparking region. Fast vaporization and explosion of the atomized medium after heating gave MA-DAP high particle removal power. The technological experimental results showed MA-DAP provides high material removal efficiency and strong deep-hole machining ability. The depth-to-diameter ratio of the specially shaped titanium alloy MA-DAF blind hole was 11 times that from traditional electrical discharge machining, while the cross-sectional hardness can be reduced by 45%.

The chemical and phase compositions and the surface layer structure of a chromium- and aluminum-alloyed nickel-based alloy after quenching, aging, and nitriding in a glow discharge are determined using the results of X-ray diffraction, metallographic, and electron microscopic studies. The sequence of the transformations that occur during nitriding in the precipitation hardening alloy is found on the basis of the data obtained. Boundary conditions for mathematical models of the external (with the formation of a continuous surface layer of nitrides) and internal (when disperse nitride particles form in the diffusion layer) nitriding are developed. A comparison with the experimental data shows the adequacy of the proposed mathematical models.

The microstructure and the wear resistance of precision nickel alloy 40KhNYu-VI used for the production of paramagnetic bearings are studied after glow discharge nitriding. It is shown that the continuous layer of CrN nitride formed under the nitriding provides substantial elevation of the wear resistance of the surface of the parts. The wear intensity after the nitriding decreases by a factor of 5 – 8, which shows the possibility of application of glow discharge nitriding to parts operating under dry and boundary friction.

Precision nickel alloy 40KhNYu-VI used for fabricating nonmagnetic precision bearings is studied after nitriding in glow discharge in an experimental facility at a high temperature (730 and 900°C) and in commercial equipment at 540°C. The effect of the temperature and duration of nitriding and of the mode of heat treatment on the structure and hardness of the surface layer obtained is determined.

A mathematical model is developed for internal anisotropy of a chromium-nickel alloy alloyed with aluminum, adequate to the available experimental data. An increase in alloy yield strength is predicted by a factor of 1.5 after nitriding. A mathematical model is recommended for developing a production regime for nitriding non-magnetic bearing rings.

Influence of three different rolling routes on mechanical anisotropy and formability of commercially pure titanium sheet was investigated. Route A and Route B are unidirectional rolling (UR) where the rolling direction is along initial rolling direction (RD) and transverse direction (TD), respectively. Route C is cross rolling (CR) where the rolling direction is changed by 90° after each rolling pass. The microstructure and texture, tensile mechanical properties including strength and elongation, and also the anisotropy of the UR and CR sheets were investigated at room temperature. The XRD results indicate that the texture intensity of rolled samples gradually weakens from Route A to Route C. Compared with Route A and Route B rolled samples, the Route C rolled samples show a smaller planar anisotropy. The deep drawing tests reveal that cross rolling can avoid the occurrence of earing. Erichsen tests indicate that rolling routes have an effect on stretch formability of pure titanium sheet.

To study the influence of blank holder type on the drawability of 5182-O aluminum sheet at room temperature, the flat blank holder and curved blank holder were employed during the deep drawing process. The microstructures were characterized by optical microscopy (OM). The results reveal that the limiting drawing ratio (LDR) of 5182-O aluminum alloy sheet is 1.7 using the flat blank holder. The drawn cup have severe earring. Compared with using flat blank holder, the LDR of 5182-O aluminum alloy sheet is enhanced to 2.0 using curved blank holder. In addition, the earring ratio also reduces and flange wrinkling is prevented when the curved blank holder is used. These are due to a more uniform sheet flow in different directions with curved blank holder.

In this study, an Mg-air battery based on air cathode with different content of Na2SO4 was prepared to study the effect of Na2SO4 on the performance of Mg-air battery. The electrochemical performance of the air cathode was studied by potentiodynamic polarization and electrochemical impedance spectroscopy. The results indicated that the electrochemical activity of the electrode enhanced with the increasing Na2SO4. The discharge performance of the battery was investigated by constant-current discharge test, and the results showed that the discharge potential of the battery also improved with the increasing of Na2SO4. However, the forming of air cathode became difficult gradually with the increasing content of Na2SO4. What is worse, the oversize pore produced in the moisture barrier when the content of Na2SO4 was too high and the moisture barrier would lose resistance to water. So the content of Na2SO4 should not be too high.

Symmetric electrochemical capacitors (ECs) have been fabricated using graphene nanosheets (GNs) modified with carbon nanotubes (CNTs) as electrode materials and LiClO4/propylene carbonate as organic electrolyte. An efficient homogenizing method is employed to insert CNTs into the GNs, forming three-dimensional CNT/GN framework. The electrochemical properties of as-prepared Li-ion ECs are well characterized by cyclic voltammetry, galvanostatic charge–discharge cycling, and ac impedance spectroscopy. The CNT/GN-based EC delivers high specific capacitance of 107 F g−1, low equivalent resistance of 33.0 Ω, and high Li+ diffusion coefficient of 1.09 × 10−13 cm2 s−1, showing an obvious enhancement as compared to GN- and CNT-based ones. This improved performance can be attributed to the fact that the presence of CNTs facilitates not only more electro-active sites on the graphene surface but also better Li+ diffusion accessibility and electrolyte wetting in the carbon framework. The energy density of CNT/GN-based EC can reach as high as approximately 18 Wh kg−1 at a power density of 3000 W kg−1, analyzed by the Ragone plot. Accordingly, the CNT/GN hybrid can be a promising electrode material for Li-ion ECs owing to its low cost, high performance, and easy fabrication.

Mg batteries have received increasing attention mainly because of their high volumetric capacity (3832 mAhcm−3). In order to form type NO.5 cell packing for Magnesium battery the finite element simulation by Deform 3D was carried out. Then backward extrusion was conducted on an AZ31 magnesium alloy at 300°C. The results show that battery cell packing with the wall of 0.35 mm can be formed through backward extrusion with an AZ31 Mg alloys. A significant grain size refining was resulted from hot BE, however, the microstructure in different positions of the Mg cell packing was inhomogeneous. At bottom of the packing, the microstructure was formed by equiaxial and relatively coarse grains. The wall of the Mg cell packing was made of much finer grains.

In the present work, ultrasonic assisted grinding (UAG) and conventional grinding (CG, without ultrasonic) tests of Carbon fiber reinforced silicon carbide matrix (C/SiC) composites were conducted. In addition, analysis was done by comparing the machining quality, grinding force, and specific grinding energy between the two processes. The results showed that material removal mode of carbon fiber both in CG and UAG were brittle fracture, and fracture size had no obvious difference. Compared with CG, brittle fracture area of SiC increased during UAG. In comparison with CG, the normal grinding force and tangential grinding force for UAG were reduced maximally by 45%, 39% respectively of those for CG. Accordingly, specific grinding energy was also reduced by UAG. Therefore, UAG can improve the grinding performance of C/SiC composites significantly.

It is recognized that grinding efficiency and ground surface quality are determined by grinding wheel performance. Additionally, the investigation on wear behavior is essential for evaluating the grinding wheel performance. There is a lack of research on the tool wear behavior during ultrasonic vibration-assisted grinding (UAG), whose grain motion trajectory differs from that in conventional grinding (CG). In the present work, CG and UAG tests of silicon carbide (SiC) were conducted in order to investigate the effects of ultrasonic vibration on the tool wear through tracking observation of grains. Meanwhile, the grinding forces and ground surface roughness correlated to the tool wear stages were studied. The results demonstrated that the main wear types during UAG were micro-fracture and macro-fracture which caused the wheel sharpening, while during CG, the main wear type was attritious wear that made the wheel blunt. As a result, UAG obtained lower and more stable grinding forces while slightly rougher ground surface in comparison with CG.

Optical refrigeration of semiconductors is encountering efficiency difficulties caused by nonradiative recombination and luminescence trapping. A commonly used approach for enhancing luminescence efficiency of a semiconductor device is coupling a lens with the device. We quantitatively study the effects of a coupling lens on optical refrigeration based on rate equations and photon recycling, and calculated cooling efficiencies of different coupling mechanisms and of different lens materials. A GaAs/GaInP heterostructure coupled with a homo-epitaxial GaInP hemispherical lens is recommended.

Based on energy dispersion relation of the deformed single-walled carbon nanotubes （SWCNTs）, various curves of the electronic velocity and effective mass of the lowest conduction band versus deformation parameters for SWCNTs are computed. It is found that only when a deformation occurs in the special direction（s） and is limited to a region of low deformation （the deformation parameter ε≤002）, the variations of the electronic average velocity vmean and average effective mass m*mean of different types of SWCNTs are very small （the relative variation≤2%）, which suggests that in these cases the electronic transport properties for SWCNTs under a low bias are basically stable. Whereas in other cases the electronic average velocity, or electronic average effective mass, or both of them vary significantly, even change in a dramatic way. This means that the electronic transport properties for SWCNTs under a low bias are unstable, even extremely unstable.

Based on the π-electron energy dispersion relation of general compound lattices derived from the tight-binding model, and assuming that the transverse confinement potential of graphene nanoribbon is a infinite hard-wall potential，we obtain the energy dispersion relation of graphene nanoribbon and the conditions that determine whether it is metallic or semi-conducting. The results presented here show that the electronic structure of graphene nanoribbon is intimately related to its geometric structure （symmetry and width），so graphene nanoribbon can be modified as metallic or semi-conducting materials only by controlling its geometric configurations，which suggests that it is highly promising to use graphene nanoribbon to develop novel nano-scale devices.

WOS: 000450320600014, We report on the fabrication of high brightness AlGaN-based ultraviolet light emitting diodes (UV-LED) on patterned silicon. Using the lateral epitaxial overgrowth approach, we demonstrate the growth of a 6 mu m thick AIN layer of high crystalline quality. X-ray diffraction characterization showed a rocking curve with a full width at half maximum of 553 and 768 '' for the (00.2) and (10.2) planes, respectively. The low dislocation density of the AIN template enabled the growth of bright AlGaN/GaN quantum wells emitting at 336 nm. By appropriate flip-chip bonding and silicon substrate removal processing steps, the patterned AIN surface was exposed and efficient bottom-emission UV-LEDs were realized. Improvement of the AIN quality and the structure design allowed the optical output power to reach the milliwatt range under pulsed current, exceeding the previously reported maximum efficiency. Further investigations of the optical power at different pulsed currents and duty cycles show that thermal management in this device structure is still challenging, especially in continuous wave mode operation. The strategy presented here is of interest, since AIN crystalline quality improvement and optimization of the light extraction are the main issues inhibiting efficient UV emitter on silicon fabrication.

WOS: 000430037600030, We investigate the effect of InGaN stress compensation layer on the properties of light emitting diodes based on InGaN/GaN multiple quantum well (MQW) structures with step graded electron injectors. Insertion of an InGaN stress compensation layer between n-GaN and the step graded electron injector provides, among others, strain reduction in the MQW region and as a result improves epitaxial quality that can be observed by 15-fold decrease of V-pit density. We observed more uniform distribution of In between quantum wells in MQW region from results of electro- and photoluminescence measurement. These structural improvements lead to increasing of radiant intensity by a factor of 1.7-2.0 and enhancement of LED efficiency by 40%. (C) 2018 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey [113G042]; TUBITAK, Financial support for this work is provided by the Scientific and Technological Research Council of Turkey (Grant No: 113G042). Ismail Altuntas acknowledges the Ph.D. Grant support from TUBITAK. We thank UNAM-National Nanotechnology Research Center at Bilkent University for access to fabrication and characterization equipment.

WOS: 000425566100069, Enhancement of InGaN/GaN based light emitting diode performance with step graded electron injectors through a two-step passivation is reported. Perimeter passivation of LED dies with SiO2 immediately following ICP mesa etch in addition to conventional Si3N4 dielectric surface passivation leads to decrease in the reverse bias leakage current by a factor of two as well as a decrease in the shunt current under forward bias by an order of magnitude. Mitigation of the leakage currents owing to the two-step passivation leads to significant increase in the radiant intensity of LEDs by more than a factor of two compared to the conventional single step surface passivation. Further, micro-dome patterned surface of Si3N4 passivation layer allow enhanced light extraction from LEDs. (C) 2017 Elsevier Ltd. All rights reserved., Scientific and Technological Research Council of Turkey (TUBITAK) [113G042]; TUBITAK, Financial support for this project was provided by the Scientific and Technological Research Council of Turkey (TUBITAK) [Grant No: 113G042]. Ismail Altuntas, acknowledges the Ph.D. Grant support from TUBITAK. We thank to UNAM-National Nanotechnology Research Center at Bilkent University for the use of fabrication and characterization equipment.

WOS: 000415768800128, The effect of a transparent ITO current spreading layer on electrical and light output properties of blue InGaN/GaN light emitting diodes (LEDs) is discussed. When finite conductivity of ITO is taken into account, unlike in previous models, the topology of LED die and contacts are shown to significantly affect current spreading and light output characteristics in top emitting devices. We propose an approach for calculating the current transfer length describing current spreading. We show that an inter-digitated electrode configuration with distance between the contact pad and the edge of p-n junction equal to transfer length in the current spreading ITO layer allows one to increase the optical area of LED chip, as compared to the physical area of the die, light output power, and therefore, the LED efficiency for a given current density. A detailed study of unpassivated LEDs also shows that current transfer lengths longer than the distance between the contact pad and the edge of p-n junction leads to increasing surface leakage that can only be remedied with proper passivation. (C) 2017 Elsevier Ltd. All rights reserved., TUBITAK [113G042]; Scientific and Technological Research Council of Turkey (TUBITAK), This work has been performed with the support of TUBITAK (Grant no: 113G042). Ismail Altuntas, acknowledges the Ph.D. Grant support from the Scientific and Technological Research Council of Turkey (TUBITAK).