Degradation mechanism of concentrator solar receivers without protection layer

GaInP/GaInAs/Ge triple junction concentrator solar receivers without protection layer were conducted sequentially at 90, 110, 130 and 150 °C for 25, 30, 30 and 50 h, respectively. After the step stress accelerated degradation tests (SSADT), gradual degradation in the dark and light I–V characteristics were observed. Degradation of devices results in the decrease of open-circuit voltage (V_oc), fill factor (FF) and efficiency. The degradation mechanism of these triple junction concentrator solar receivers is attributed to the recombination current in the depletion region at the chip perimeter of solar cells. The mean time to failure (MTTF) of 380 h at 65 °C and active energy (E_a) of 0.16 eV were obtained by the statistical analysis with cumulative damage and exponential mode. Both of the obtained values of MTTF and E_a are low, and this results might be possibly due to our devices being without encapsulated protection layer.     

Comparative study of InP/InGaAs double heterojunction bipolar transistors with InGaAsP spacer at base-collector junction

In this article, the influence of InGaAsP spacers inserted at base-collector (B-C) junction in the InP/In_0.53Ga_0.47As double heterojunction bipolar transistors is demonstrated by two-dimensional semiconductor simulation. Due to the addition of an InGaAsP spacer layer, two small potential spikes are formed at B-C junction and the current blocking effect is reduced. The results exhibit that the maximum current gain increases from 30 to 374 (375) as the thickness of InGaAsP spacer layer varies from 0 to 100 Å (300 Å). On the other hand, the device with a thicker spacer layer (300 Å) could effectively improve the knee effect of the current-voltage curves as compared the other devices. In addition, the collector-emitter offset voltages less than 10 mV are observed in the three devices.     

Bionic soft crystalline lens materials for MEMs applications based on self-assembling amphiphilic block copolymer/nanoparticle hybrids

In this study, a biomimetic crystalline lens with properties that combine the softness of a hydrogel comparable to that of a human lens for adjustable focus and the property for image aberration correction with gradient refractive index (GRIN) was developed by the self-organization of an amphiphilic block copolymer blended with high refractive index titanium nanoparticles (TiO₂). The hydrogel lens was prepared by using a thermally responsive poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) (PPO-PEO-PPO) triblock copolymer (Poloxamer 407) with the lower-critical solution temperature (LCST) property that can be injectable into the eye capsule as a liquid below its LCST and solidifies as a soft gel above the LCST at the human body temperature. By chemical modification of the triblock copolymer with double bonds on its chain-ends (Poloxamer 407A), the soft lens network can be made by using UV crosslinking of the hydrogel. To further increase the refractive index (RI) of the hydrogel lens network and to match its RI to that of the human lens (=1.41), highly visible-light transparent titanium dioxide (TiO₂) nanoparticles were introduced into the hydrogel lens network. To create a biomimetic GRIN lens as in the human’s eyes, an electric field method was also used to induce an axial diffusion of TiO₂ nanoparticles from the center of the lens where TiO₂ nanoparticles were first injected to the edge. The above method demonstrates the potential to use the hydrogel/nanoparticle hybrid as an injectable replacement for a soft optical lens system with an adjustable focus for future MEMs application.     

Effect of repetition nanoindentation of GaN epilayers on a‐axis sapphire substrates

In this work, gallium nitride (GaN) epilayers were deposited on a‐axis sapphire substrate by means of metal organic chemical vapor deposition (MOCVD). Berkovich nanoindentation was used to explore the repetition pressure‐induced impairment of the GaN film. The observation of load‐displacement vs stress‐strain curves concludes that basal slip is implicated in the deformation on the A plane GaN. The increase in the hardness (H) and elastic modulus (E) was determined from cyclic nanoindentation, and resulted in a crack due to the formation of incipient slip bands and/or the to‐and‐fro motion of mobile dislocation. It is indicated that the generation of individual dislocation and residual deformation of the GaN films are showed by CL mapping analysis. From the morphological studies, it is revealed that the crack was found by means of atomic force microscope (AFM) technique at nine loading/reloading cycles even after the indentation beyond the critical depth on the residual indentation impression. Copyright © 2010 John Wiley & Sons, Ltd.     

Palladium-catalyzed coupling reactions of (ArCH2)Ti(O-i-Pr)3 with aromatic or heteroaromatic bromides

Three benzyltitanium compounds of (ArCH₂)Ti(O-i-Pr)₃ (Ar = Ph (1a), 4-MeOC₆H₄ (1b), 4-FC₆H₄ (1c)) were prepared and used as benzyl nucleophiles for coupling reactions with aromatic or heteroaromatic bromides. The simple catalytic system of 1 mol % Pd(OAc)₂ and 2 mol % PCy₃ worked efficiently for a wide variety of aromatic bromides, producing diarylmethanes in good to excellent yields of up to 96%. Coupling reactions of hindered aromatic bromides or aromatic bromides containing electron-withdrawing substituents were slower over longer reaction times of 3–6 h. Reactions of heteroaromatic bromides of bromopyridines, bromofurans, or bromothiophenes with benzyl reagents of 1a or 1b required either longer reaction times of 12–24 h or a higher reaction temperature of 80 °C, producing pyridyl-, furyl-, and thienyl-arylmethanes in moderate yields.     

Investigation of the Atmospheric Helium Dielectric Barrier Discharge Driven by a Realistic Distorted-Sinusoidal Voltage Power Source

The non-equilibrium atmospheric-pressure parallel-plate helium dielectric barrier discharge (DBD) driven by a realistic 20 kHz distorted-sinusoidal voltage waveform has been investigated by means of simulations and experiments. A self-consistent one-dimensional fluid modeling code considering the non-local electron energy balance was applied to simulate the helium DBD. The effect of selecting plasma chemistry was investigated by comparing simulations with experiments. The results show that the simulations, which include more excited helium, metastable helium and electron–ion-related reaction channels, can faithfully reproduce the measured discharged temporal current quantitatively. Based on the simulated discharge properties, we have found that there is complicated mode transition of discharges from the long Townsend-like to the “dark current”-like, then to the short primary Townsend-like and the short secondary Townsend-like for the helium DBD that is driven by a realistic distorted-sinusoidal voltage power source. Discharge properties in different periods of discharge are discussed in detail in the paper.     

Enhanced birefingence of MgF2 thin film at 193 nm by serial bideposition

Magnesium fluoride films were coated by serial bideposition with a varying number of sub-deposited layers. The deposition angle 70° remained constant throughout the process by a single electron-beam source. One, 4, 20, and 100 sub-deposition layers were prepared. The microstructure, residual stress, absorption of water, and linear birefringence of serial bideposition of anisotropic thin film at 193 nm were investigated and discussed. Microstructures of the films were observed by scanning electron microscopy. The residual stress was measured by phase-shifting interferometer. The water absorption bands of the thin films were measured by Fourier transform infrared (FTIR) spectroscopy, and refractive indices in two different directions (S- and P-polarized) were determined from a spectrometer equipped with a polarizer. The linear birefringence would increase and the stress would decrease when the sub-deposited layers were increased. When the number of layers was 100, the difference between the refractive indices for two different polarizations was able to reach 0.082.     

(1,3‐Diphenyl‐1H‐Pyrazol‐4‐yl)‐Methylamine Analogues as Inhibitors of Dipeptidyl Peptidases

A number of pyrazole compounds reported in literatures elicit anti‐hyperglycemic effects. By modifying the side chain of the heterocyclic skeleton, a new chemical class of DPP‐IV inhibitors structurally derived from the (pyrazol‐4‐yl)‐methylamine scaffold have been discovered and evaluated the biological activities of these inhibitors against DPP‐IV, DPP8, DPP‐II and FAP. The SAR studies showed the (1,3‐diphenyl‐1H‐pyrazol‐4‐yl)‐methylamines with 2,4‐dichloro substituents at the 3‐phenyl ring selectively preferred as DPP‐IV inhibitors, whereas with difluoro substituents at the 3‐phenyl ring selectively preferred as DPP8 inhibitors. The binding mode of representative compound 15h at the active site of DPP‐IV was predicted by computer model. In additional, 15h exhibited the ability to significantly decrease the glucose excursion in mice.     

Effect of Li2O Addition on the Preparation of (Y2‐yLiy)Ti2O7‐y

A new series of (Y_2‐yLi_y)Ti₂O_7‐y having an ordered pyrochlore phase was prepared by a solid state reaction method with a solid solution range of 0.05 ≥ y ≥ 0.10. Unit cell parameters obtained by the Rietveld refinement method shows that the a‐axis decreases linearly with increasing the amount of Li ion addition, indicating the successful incorporation of the Li ion into unit cell. The average x‐fractional coordinate of the O(1) site depends on the ionic radius ratio of r(A³+)/r(Ti⁴⁺) in the A₂Ti₂O₇ with a pyrochlore phase. The Ti K‐edge XANES spectra of the (Y_2‐yLi_y)Ti₂O_7‐y show that the valence of the Ti ions is slightly less than 4 so that Ti is in the mixed valence state. Average particle size increases with increasing the amount of extra Li ion addition, which acts as a flux to lower the melting point of the materials.