Numerical simulations of the current-matching effect and operation mechanisms on the performance of InGaN/Si tandem cells

Numerical simulations are conducted to study the current-matching effect and operation mechanisms in and to design the optimized device structure of InGaN/Si tandem cells. The characteristics of short circuit current density (J_sc), open circuit voltage (V_oc), fill factor (FF), and conversion efficiency (η) of InGaN/Si tandem cells are determined by the current-matching effect. The similar trend of η to that of J_sc shows that J_sc is a dominant factor in determining the performance of InGaN/Si tandem cells. In addition, the combined effects of the J_sc, V_oc, and FF lead to an optimized η in the medium-indium, xpn-InGaNInGaN‒to‒Si, InGaN/Si tandem cell. At xpn-InGaNInGaN‒to‒Si, the J_sc of the InGaN subcell is equal to that of the Si subcell such that an InGaN/Si tandem cell reaches the current matching condition to operate at the maximum power point. Similar to the J_sc and FF, the η for low- xpn-InGaN<xpn‒InGaNInGaN‒to‒Si and high-In xpn-InGaN>xpn-InGaNInGaN‒to‒Si InGaN/Si tandem cells are InGaN- and Si subcell-limited, respectively. Furthermore, the p- and n-layer thicknesses, indium content, and position of depletion region of InGaN subcell should be adjusted to reapportion the light between the two subcells and to achieve the maximum conversion efficiency. With appropriate thicknesses of p- and n-InGaN, In_0.5–0.6Ga_0.5–0.4 N/Si tandem cells can exhibit as high as approximately 34% to 36.5% conversion efficiency, demonstrating that a medium-indium InGaN/Si tandem cell results in a high-efficiency solar cell. Simulation results determine that the current-matching effect and operation mechanisms of InGaN/Si tandem cells can be utilized for efficiency enhancement through the optimized device structures.     

Glucocorticoids and aging

In this review we analyze the morphologic changes, hypothalamic-pituitary-adrenal (HPA) axis functions, glucocorticoid (GC) receptors, and steroidogenic enzyme activities in both animals and humans during aging. In rodent studies, older animals tend to show: 1) hypertrophy of adrenal zona fasciculata (ZF) cells; 2) neuronal loss in the hypothalamic area; 3) loss of GC receptors in the hippocampus; 4) raised circulating adrenocorticotropic hormone (ACTH) and GC levels, and increased release of corticotropin-releasing hormone from the hypothalamus; 5) reduced suppression of endogenous GC secretion after administration of dexamethasone; 6) decreased attenuation of response to chronic stress; and 7) increased activity of P450scc and 21-hydroxylase. According to the GC cascade hypothesis, stress and GCs facilitate the aging process in rats. Stress induces downregulation of GC receptors in the hippocampus, then impairs GC feedback on stress-induced HPA axis activation. Finally, an increase in the basal level of corticosterone and extended GC secretion following stress occurs. Because activation of the hippocampus decreases HPA axis function, the unrestrained elevation of GC concentration and the reduction in the level of GC receptors in the hippocampus may gradually weaken the feedback mechanisms and halt the response to stress. In humans, there are conflicting reports of HPA axis function during aging, so it is difficult to make a final conclusion regarding the relationship between aging and HPA axis function.     

Learning effectiveness in a Web-based virtual learning environment: a learner control perspective

Web-based technology has a dramatic impact on learning and teaching. A framework that delineates the relationships between learner control and learning effectiveness is absent. This study aims to fill this void. Our work focuses on the effectiveness of a technology-mediated virtual learning environment (TVLE) in the context of basic information technology skills training. Grounded in the technology-mediated learning literature, this study presents a framework that addresses the relationship between the learner control and learning effectiveness, which contains four categories: learning achievement, self-efficacy, satisfaction, and learning climate. In order to compare the learning effectiveness under traditional classroom and TVLE, we conducted a field experiment. Data were collected from a junior high school of Taiwan. A total of 210 usable responses were analysed. We identified four results from this study. (1) Students in the TVLE environment achieve better learning performance than their counterparts in the traditional environment; (2) Students in the TVLE environment report higher levels of computer self-efficacy than their counterparts in the traditional environment; (3) Students in the TVLE environment report higher levels of satisfaction than students in the traditional environment; and (4) Students in the TVLE environment report higher levels of learning climate than their counterparts in the traditional environment. The implications of this study are discussed, and further research directions are proposed.     

Electrical control of shape of laser beam using axially symmetric liquid crystal cells

This work demonstrates the electrical tuning of laser beam shape using an axially symmetric dye-dope liquid crystal (ASDDLC) device that is fabricated using a photo-alignment method. Various beam shapes can be obtained by linearly polarized Gaussian laser beams through an ASDDLC device under various applied voltages. The far-field intensity patterns generated by laser beams of selected shapes under various applied voltages are simulated, and the results are consistent with experiment. A rotatable petal-shaped beam is obtained by controlling the polarization of the output donut-shaped beam. The tenability of beam shape of light with a wavelength of 1064 nm, which is commonly used in biomedical applications, is also demonstrated.     

Unipolar resistive switching behavior of BiFeO3 thin films prepared by chemical solution deposition

Bismuth ferrite (BiFeO₃, BFO) thin films were spin-coated on Pt/Ti/SiO₂/Si substrates by a chemical solution deposition method. The ferroelectric BFO films annealed at 500 °C and 550 °C were found to possess unipolar resistive switching behaviors. The resistance ratio of the high resistance state (HRS) to the low resistance state (LRS) of the unipolar resistance switching is about 10³ for the ferroelectric BFO films. The conduction mechanisms are concluded to be space charge-limited conduction for the initial state and Ohmic conduction for the LRS. As for the HRS, the Poole-Frenkel emission fits well in the whole voltage region. Traps composed of oxygen vacancies are considered to play a key role in forming conducting paths. The relaxation time of electronic carriers is much shorter than that of ionic oxygen vacancies; therefore, the resistance switching is considered more probably due to carrier injection and emission through the Poole-Frenkel model after forming.     

A multi-point simulated annealing heuristic for solving multiple objective unrelated parallel machine scheduling problems

This study considers the problem of job scheduling on unrelated parallel machines. A multi-objective multi-point simulated annealing (MOMSA) algorithm was proposed for solving this problem by simultaneously minimising makespan, total weighted completion time and total weighted tardiness. To assess the performance of the proposed heuristic and compare it with that of several benchmark heuristics, the obtained sets of non-dominated solutions were assessed using four multi-objective performance indicators. The computational results demonstrated that the proposed heuristic markedly outperformed the benchmark heuristics in terms of the four performance indicators. The proposed MOMSA algorithm can provide a new benchmark for future research related to the unrelated parallel machine scheduling problem addressed in this study.     

Throughput analysis of IEEE 802.11e EDCA under heterogeneous traffic

The amended version of IEEE 802.11e defines the enhanced distributed channel access (EDCA) for quality of service (QoS) connections. EDCA provides a priority scheme to classify different traffic categories by the arbitration inter-frame space (AIFS), and the initial and maximum contention window sizes. Most previous Markov chain analyses of EDCA have concentrated on the effect of contention window size, they neglected the effect of AIFS. In this paper, we consider of AIFS events among each back-off procedure and evaluate the saturation throughput of the IEEE 802.11e EDCA under heterogeneous traffic scenarios. The analytical model is based on the differentiated AIFS and uses the discrete time slot to analyses the external collision time. We take advantage of extensive simulation studies to validate the accuracy of the proposed model. Our models accurately estimate the effects of varied back-off window size and AIFS under heterogeneous traffic.     

A Halide-based Synergistic Computing Framework for Heterogeneous Systems

Journal of Signal Processing Systems - New programming models have been developed to embrace contemporary heterogeneous machines, each of which may contain several types of processors, e.g., CPUs,...     

A novel process for fabricating electrical contact SnO<Subscript>2</Subscript>/Ag composites by reciprocating extrusion

Reciprocating extrusion combined with simple powder compaction was used to produce electrical contact SnO₂/Ag composites from Ag and SnO₂ powders with improved properties. The SnO₂ particles were uniformly dispersed in the Ag matrix after 20 passes of extrusion. The hardness of the composites increased with extrusion passes and leveled off after ten passes, whereas the electrical conductivity decreased slowly after five passes. The variation of the coefficient of thermal expansion with temperature revealed that SnO₂/Ag interface strength increased with extrusion passes and was greatest after 20 passes. The SnO₂/Ag composites with 20 passes showed the best electrical contact properties in which there was no mass loss through mass transfer from cathode to anode during the make-break erosion test at both low and high current conditions. On the other hand, SnO₂/Ag composites with one pass and commercial SnO₂-In₂O₃/Ag specimens had an apparent mass loss under the same condition. Electrical contact composites produced by the present method are promising in electrical contact applications in consideration of cost and performance.     

Media hash-dependent image watermarking resilient against both geometric attacks and estimation attacks based on false positive-oriented detection

The major disadvantage of existing watermarking methods is their limited resistance to extensive geometric attacks. In addition, we have found that the weakness of multiple watermark embedding methods that were initially designed to resist geometric attacks is their inability to withstand the watermark-estimation attacks (WEAs), leading to reduce resistance to geometric attacks. In view of these facts, this paper proposes a robust image watermarking scheme that can withstand geometric distortions and WEAs simultaneously. Our scheme is mainly composed of three components: 1) robust mesh generation and mesh-based watermarking to resist geometric distortions; 2) construction of media hash-based content-dependent watermark to resist WEAs; and 3) a mechanism of false positive-oriented watermark detection, which can be used to determine the existence of a watermark so as to achieve a tradeoff between correct detection and false detection. Furthermore, extensive experimental results obtained using the standard benchmark (i.e., Stirmark) and WEAs, and comparisons with relevant watermarking methods confirm the excellent performance of our method in improving robustness. To our knowledge, such a thorough evaluation has not been reported in the literature before.