Unbalanced Strain Research Articles

Damage mechanisms of polycrystalline aluminate magnesium spinel (PAMS) under different loading conditions of indentation and micro-cutting tests

This paper studies the damage mechanisms of polycrystalline aluminate magnesium (PAMS) under different loading conditions of indentation tests and micro-cutting with various rake angles of the cutting tools. A decreasing trend is confirmed in the hardness values with growing loadings. The observed hardness value approaches a plateau at about 1406–1679 HV. The tardive cracks, which break out spontaneously within a delayed period of time following the end of indentation, are for the first time observed in the indentation experiment of this paper. The tardive crack originates from a crack system containing a lateral subsurface crack with two median cracks on its both sides. These results imply that PAMS can store the unbalanced strain energy induced by the applied loading within the vicinity of the loaded volume which will be released in accordance with the intergranular crack-growing path extending to the surface. In addition, the linear plunge-cutting tests coin the effect of tool rake angle: the tool with a zero rake angle produces poor surface quality with sever surface damages whilst the tools with negative rake angles can improve the ductility of PAMS to a certain extent. However, negative rake angles are proved to be the root cause of median cracks and large spalling fractures. On the contrary, a positive rake angle tool can be applied to reduce the severity of surface damage and confine the damage patterns mainly as small spallings fractures. Besides the experimental study, finite element method (FEM) models are developed to reveal the underlying mechanism of the rake-angle effect on the micro-cutting process. The simulation results visualise the different stress distributions in the uncut areas and prove that a tool with positive rake angle can transfer the stress to the uncut chip layer whereby the damage probability to the machined surface is significantly reduced.

AlyGa1−yAs Bound GazIn1−zP Strain Compensation for Optical Enhancement of In0.07GaAs/GaAs1−xPx 940-nm Light-Emitting Diodes

The use of an AlxGa1−xAs bound GazIn1−zP strain compensation structure for optimum strain in latticed mismatched In0.07GaAs/GaAsP0.06 multiple quantum wells (MQWs) and its effect on the output power of an infrared light-emitting diode at 940-nm were investigated. A Ga0.53InP tensile strain structure, which effectively compensate excessive compressive strain in the In0.07GaAs/GaAsP0.06 MQWs, was inserted between a quantum well and a quantum barrier. The Al0.2GaAs material was used as both a growth buffer and a balancing barrier for In0.07GaAs/AlxGa1–x As-bound Ga0.53InP/GaAsP0.06 MQWs. From photoluminescence (PL) measurements and X-ray diffraction (XRD) rocking curves, we verified that the Ga0.53InP tensile strain barrier could effectively compensate the compressive strain of the In0.07GaAs/GaAsP0.06 MQWs. In addition, a further increase in the PL intensity from the In0.07GaAs/AlyGa1−yAs-bound Ga0.53InP/GaAsP0.06 MQWs was found after having adjusted the Al0.2GaAs strain tuning barrier. This result was significantly supported by the stable balance of the energy bandgap structure in the developed MQWs. From fabricated IR-LEDs chips, the LED with an In0.07GaAs/GaAsP0.06 MQW employing the Al0.2GaAs-bound Ga0.53InP strain compensation structure displayed a 48% higher light output power as compared with a conventional LED. These results suggest that the use of an Al0.2GaAs-bound Ga0.53InP strain compensation structure effectively improved both the unbalanced strain and the unbalanced energy bandgap of lattice-mismatched In0.07GaAs/GaAsP0.06 MQWs for 940-nm IR-LEDs.

Balanced Strain Compensation of In0.07GaAs/Al0.2GaAs/GaAsP0.06 for Improving 940 nm Infrared Light-Emitting Diodes.

Optimum strain compensation structures for In0.07GaAs-based MQWs were investigated to obtain a higher output power for infrared lighting-emitting diodes (IR-LEDs) requiring a 940 nm wave-length. A GaAsP0.06 tensile strain material for compensating the compressive strain of In0.07GaAs quantum wells was used as a quantum barrier. To improve upon the excessive unbalance strain condition caused due to the In0.07GaAs quantum well and GaAsP0.06 quantum barrier, a conditioned Al0.2GaAs strain balancing barrier was also investigated. Through subsequent photoluminescence (PL) measurements, it was found that the GaAsP0.06 tensile strain barrier could effectively compensate for the compressive strain of In0.07GaAs quantum wells. Furthermore, the PL intensity of In0.07GaAs/GaAsP0.06 MQWs was observed to be markedly improved by using an Al0.2GaAs strain balancing barrier. A fabricated IR-LED chip, having In0.07GaAs/GaAsP0.06 MQWs with an Al0.2GaAs strain balancing barrier, showed a 60% higher light output power than conventional MQWs. These results subsequently suggest that using GaAsP0.06 and Al0.2GaAs barriers effectively improved unbalanced strain conditions of lattice-mismatched In0.07GaAs based MQWs requiring a 940 nm emitting wavelength.

The optimum condition of GaxIn1-xP/GaAs1-yPy strain compensation for excessive strained In0.15GaAs MQWs in 1000 nm infrared light-emitting diode

Optimum conditions of GaxIn1-xP/GaAs1-yPy strain compensation structure were investigated to improve excessive compressive strain in In0.15Ga0.85As based lighting-emitting diodes with 1000 nm emission wavelength. The GaAs1-yPy material, where only tensile strain is present, was used as a quantum barrier for compensating compressive strain of a In0.15Ga0.85As quantum well. The GaxIn1-xP strain material, which has both tensile and compressive strain, was used as a strain tuning barrier in order to improve unbalanced strain in In0.15GaAs/GaAsP0.09 MQWs. Subsequent photoluminescence (PL) measurements, indicated that the GaAsP0.09 tensile strain barrier was effective in compensating compressive strain of the In0.07GaAs quantum wells. Furthermore, noticeable PL intensity was observed from In0.15GaAs/GaAsP0.09 MQWs with the conditioned Ga0.53In0.47P strain tuning barrier. A fabricated IR-LED containing In0.15GaAs based MQWs with optimized Ga0.53In0.47P/GaAsP0.09 strain compensation barriers displayed 110% higher light output-power than a conventional LED. This result suggests that using of GaAsP0.09 and Ga0.53In0.47P as tensile strain barriers effectively compensates excessive compressive strain of In0.15GaAs based MQWs with a 1000 nm emission wavelength.

Lateral Abdominal Wall Reconstruction.

Lateral abdominal wall defects, while rare, present a more challenging problem than commonly encountered ventral defects due to the complexity of the anatomy, physiologic forces, and impact of muscle denervation. The lateral abdominal wall encompasses a large surface area ranging from the costal margin superiorly to the iliac crest inferiorly and from the linea semilunaris anteriorly to the paraspinous musculature posteriorly. The ratio of muscle to fascia/aponeurosis is much higher, which makes repair through muscle tissue versus fascia less secure. Furthermore, these defects are subject to asymmetric forces caused by the independent contraction of anterior and posterior muscle units, which lead to unbalanced strain and hernia progression. These features necessitate the use of wide underlay mesh load bearing repairs supported by the static pillars of the abdominal wall. Management can be further complicated when defects extend beyond the defined boundaries, requiring surgical repair to be adapted based on the border structures involved. Primary fascial coaptation may not be as easily accomplished, and therefore careful planning is important to ensure stable coverage of exposed mesh.

Temperature Dependent Characterization of Imbedded InAs Quantum Dots in GaAs Superlattice Solar Cell Structures by High Resolution X-ray Diffraction

ABSTRACTThe insertion of nanostructured materials (such as quantum wells, wires, and dots) into the intrinsic region of p-i-n solar cells introduces an intermediate band within the bandgap of the host material. It has been shown that the sub-bandgap conversion provided by the nanostructured materials, enhances the short circuit current as well as the overall efficiency of InAs quantum dots (QD) imbedded in GaAs superlattice (SL) solar cells [1]. As a contender for space applications, it is necessary to subject these solar cell structures to temperatures encountered in the Low Earth Orbit (LEO), probing for any material degradation. Herein, we focus on temperature dependent characterization using high resolution X-ray diffraction (HRXRD) of InAs QD enhanced GaAs solar cell structures with varying growth parameters. The structures characterized can be classified into three groups: (1) GaP strain compensation coverage, (2) GaAs barrier coverage, and (3) InAs coverage for QD formation. HRXRD rocking curves of each structure focusing around the GaAs peak are analyzed at a range of temperatures up to 200˚C. Although no noticeable shifts in the SL peaks are detected, interfacial diffusion decreased the resolution of fringes produced by reflections at the SL interfaces in test structures with varying InAs QD coverage. Unbalanced strain in the same structures shows a distortion in the GaAs peaks.

Operational aspects of a large PEFC stack under practical conditions

Measurement results obtained from single-cell experiments give insight of electrochemical processes and allow for their optimization. However, the operator of large fuel cell stacks is confronted by a different set of problems that do not arise in such small scale experiments. Typically in a fuel cell stack the reactants and the cooling medium are fed in parallel to the cells. This can lead to an uneven flow distribution in the flow channels and an uneven cell voltage distribution across the stack. Therefore, a cleverly devised control and monitoring system is required to ensure that no unbalanced strain is put on the stack. This paper investigates some aspects critical to the operation of large fuel cell stacks in automotive applications such as control issues in the supply system, stack failures, and the appropriate countermeasures as well as some procedures to increase the voltage stability.

Pressure vessel wall thinning detection using multiple pairs of fiber Bragg gratings for unbalanced strain measurements

This paper proposes a method for using a pair of identical fiber Bragg gratings to measure the unbalanced strain factor in an isothermal-mechanical system. Cascading two identical fiber Bragg gratings, the unbalanced strain factor caused by the disturbance can be interrogated from the peak difference of the two reflecting Bragg wavelengths. If the mechanical system is in an isothermal environment, the thermal variations in the Bragg grating detection system can be automatically compensated. To verify the feasibility of the unbalanced strain detection using a pair of fiber Bragg gratings, the wall-thinning monitor in a pressure vessel system was investigated. The proposed technique is easy to install, immune to electro-magnetic and thermal interference, and highly sensitive in-time to remote monitoring for fiber optic smart structures. A finite element model (FEM) is used to simulate the wall thinning in a pressure vessel system. The minimum detectable unbalanced strain and cross-talk between two reflecting Bragg wavelengths were investigated.

Veränderungen der Katalogisierungstätigkeit durch regionale Verbundsysteme

The co-operative cataloguing of the titles in stock of regional libraries is a matter of priority to certify the local, regional and national supply with literature. For example in 1992 the Hamburger Bibliotheksverbund (HBV) extended into Nord-deutscher Bibliotheksverbund (NBV). With this development an integrated system of highly efficient interstate libraries was formed. The efficiency of this combined system is dependent on the inevitable co-operation of the associated libraries. There are enormous changes for the single scientific library affiliated to the Norddeutscher Bibliotheksverbund concerning the contents and the process of the cataloguing work, especially if the use of the associated regional systems is connected to the newly installed use of electronic data processing and if parallel to this and at the same time there is a new regulation for cataloguing being introduced. The work of the cataloguer is more and more concentrated on the computer screen, so that solutions have to be found for an ergonomic design of the work station compensating this unbalanced strain. The introduction of mixed work stations for example may extend the cataloguer's former field of activity.