Long-term Operation (long-term + operation)

Distribution by Scientific Domains


Selected Abstracts


Hydrogen- and carbon-related defects in heavily carbon-doped GaAs induced degradation under minority-carrier injection

ELECTRONICS & COMMUNICATIONS IN JAPAN, Issue 5 2010
Hiroshi Fushimi
Abstract GaAs/AlGaAs heterojunction bipolar transistors (HBTs) have attracted much attention because of their high-speed performance. However, long-term operation seriously degrades the device characteristics: the current gain decreases and the low-bias-leakage current increases. This degradation has long been an issue in GaAs-based devices operated under minority-carrier injection, such as laser diodes. The cause of degradation is thought to lie in the carbon-doped base, but this is not yet certain. In this paper the degradation of HBTs is described, especially that of GaAs/AlGaAs HBTs with a heavily carbon-doped base layer. Two types of device degradation are found, namely, hydrogen-related degradation and carbon-related degradation. The mechanisms governing the degradation are discussed in the framework of the recombination-enhanced defect reaction (REDR) and charge state effect (CSE). © 2010 Wiley Periodicals, Inc. Electron Comm Jpn, 93(5): 33,41, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/ecj.10208 [source]


Respiratory oscillations in yeast: mitochondrial reactive oxygen species, apoptosis and time; a hypothesis

FEMS YEAST RESEARCH, Issue 4 2003
David Lloyd
Abstract Oscillatory metabolic activities occur more widely than is generally realised; detectability requires observation over extended times of single yeast cells or synchrony of individuals to provide a coherent population. Where oscillations in intracellular metabolite concentrations are observed, the phenomenon has been ascribed to sloppy control, energetic optimisation, signalling, temporal compartmentation of incompatible reactions, or timekeeping functions. Here we emphasise the consequences of respiratory oscillations as a source of mitochondrially generated reactive O2 metabolites. Temporal co-ordination of intracellular activities necessitates a time base. This is provided by an ultradian clock, and one result of its long-term operation is cyclic energisation of mitochondria, and thereby the generation of deleterious free radical species. Our hypothesis is that unrepaired cellular constituents and components (especially mitochondria) eventually lead to cellular senescence and apoptosis when a finite number of respiratory cycles has occurred. [source]


Influence of Electric Field on Microstructures of Pentacene Thin-Films in Field-Effect Transistors,

ADVANCED FUNCTIONAL MATERIALS, Issue 2 2008
L. Cheng
Abstract We report on electric-field-induced irreversible structural modifications in pentacene thin films after long-term operation of organic field-effect transistor (OFET) devices. Micro-Raman spectroscopy allows for the analysis of the microstructural modifications of pentacene in the small active channel of OFET during device operation. The results suggest that the herringbone packing of pentacene molecules in a solid film is affected by an external electric field, particularly the source-to-drain field that parallels the a,b lattice plane. The analysis of vibrational frequency and Davydov splitting in the Raman spectra reveals a singular behavior suggesting a reduced separation distance between pentacene molecules after long-term operations and, thus, large intermolecular interactions. These results provide evidence for improved OFET performance after long-term operation, related to the microstructures of organic semiconductors. It is known that the application of large electric fields alters the semiconductor properties of the material owing to the generation of defects and the trapping of charges. However, we first suggest that large electric fields may alter the molecular geometry and further induce structural phase transitions in the pentacene films. These results provide a basis for understanding the improved electronic properties in test devices after long-term operations, including enhanced field-effect mobility, improved on/off current ratio, sharp sub-threshold swing, and a slower decay rate in the output drain current. In addition, the effects of source-to-drain electric field, gate electric field, current and charge carriers, and thermal annealing on the pentacene films during OFET operations are discussed. [source]


Cover Picture: Tuning the Dimensions of C60 -Based Needlelike Crystals in Blended Thin Films (Adv. Funct.

ADVANCED FUNCTIONAL MATERIALS, Issue 6 2006
Mater.
Abstract A new ordered structure of the C60 derivative PCBM is obtained in thin films based on the blend PCBM:P3HT, as detailed by Swinnen, Manca, and co-workers on p.,760. Needlelike crystalline PCBM structures, whose dimensions and spatial distribution ca be tuned by adjusting the blend ratio and annealing conditions, are formed. In typical solar-cell applications of these blended films, these results indicate that during long-term operation under normal conditions (50,70,°C) morphology changes and a decrease in cell performance could occur. A new ordered structure of the C60 derivative PCBM ([6-6]-phenyl C61 -butyric acid methyl ester) is obtained in thin films based on the blend PCBM:regioregular P3HT (poly(3-hexylthiophene)). Rapid formation of needlelike crystalline PCBM structures of a few micrometers up to 100,,m in size is demonstrated by submitting the blended thin films to an appropriate thermal treatment. These structures can grow out to a 2D network of PCBM needles and, in specific cases, to spectacular PCBM fans. Key parameters to tune the dimensions and spatial distribution of the PCBM needles are blend ratio and annealing conditions. The as-obtained blended films and crystals are probed using atomic force microscopy, transmission electron microscopy, selected area electron diffraction, optical microscopy, and confocal fluorescence microscopy. Based on the analytical results, the growth mechanism of the PCBM structures within the film is described in terms of diffusion of PCBM towards the PCBM crystals, leaving highly crystalline P3HT behind in the surrounding matrix. [source]


Cation distribution in spinel (Mn,Co,Cr)3O4 at room temperature

JOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 3 2010
A. Purwanto
As part of a study of the long-term operation of solid-oxide fuel cells, three (Mn,Co,Cr)3O4 samples have been synthesized and characterized. X-ray and neutron diffraction patterns from the powder samples at room temperature were measured and the data were co-refined. The neutron data were indispensible in locating Mn, Co and Cr within the crystallographic unit cell with their respective atomic occupancies. Two of these samples have been identified as cubic Mn0.76Co0.58Cr1.66O4 and Mn1.28Co1.72O4. The third is a two-phase sample containing cubic Mn1.66Co1.34O4 and tetragonal Mn2.05Co0.91O4 in a 59.1,(6):40.9,(6)% mass fraction ratio. Cr, which might be introduced from reaction with chromia during oxidation of interconnect materials, exhibits a preference for the octahedral site rather than the tetrahedral site. Without Cr, Mn dominates the octahedral site. [source]


Microbial composition and structure of a multispecies biofilm from a trickle-bed reactor used for the removal of volatile aromatic hydrocarbons from a waste gas

JOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 1 2004
Dariusch Hekmat
Abstract The microbial composition and structure of a multispecies biofilm of a laboratory-scale trickle-bed bioreactor for the treatment of waste gas was examined. The model pollutant was a volatile organic compound-mixture of polyalkylated benzenes called Solvesso 100®. Fluorescent in-situ hybridization (FISH) and confocal laser scanning microscopy (CLSM) were applied. Two new Solvesso 100® -degrading Pseudomonas sp strains were isolated from the multispecies biofilm. Corresponding isolate-specific oligonucleotide probes were designed and applied successfully. A major finding was that the fraction of Solvesso 100® -degrading bacteria in the biofilm was low (about 3,6% during long-term operation). The majority of the active cells were saprophytes which utilized intermediates and cell lysis products. The measured fraction of extracellular polymeric substances of the mature biofilm was 89,93% of the total biomass. The CLSM examinations of a 3-days-old approx 10 µm thick biofilm revealed highly heterogeneous structures with distinguished three-dimensional matrix-enclosed microcolony bodies spread across the substratum surface. The 28-days-old 80,960 µm thick biofilm exhibited voids, cell-free channels, and pores of variable sizes. In both cases, an even distribution of active cells and pollutant-degrading bacteria throughout the biofilm cross-section as well as through the biofilm depth was observed. Copyright © 2003 Society of Chemical Industry [source]


Double-deck aerated biofilm membrane bioreactor with sludge control for municipal wastewater treatment

AICHE JOURNAL, Issue 5 2009
Jirachote Phattaranawik
Abstract Alternative designs of an aerated moving-bed biofilm reactor and a flat-sheet membrane module for a biofilm membrane bioreactor process have been investigated to overcome a membrane clogging problem and to determine the performance of a new membrane module. Double-deck aerated biofilm reactor with integrated designs of sludge hopper, thickener, and velocity-zone concept for particle settlement was evaluated for the suspended solid control and removal. Hydrodynamics of bubbling, liquid, and solid particles were arranged in the bioreactor to obtain a particle settlement. New membrane modules used under low suspended solid environment having smaller membrane gaps were evaluated for filtration performance and clogging problems for long-term operation. The average suspended solids concentration in the bioreactor effluent was 44.6 mg/L. Relaxation applied with the membrane module provided the most optimum result for fouling control, and no clogging problems in the modules were observed in the system after continuous operation of 3 weeks. © 2009 American Institute of Chemical Engineers AIChE J, 2009 [source]


Low-temperature (9°C) AMD treatment in a sulfidogenic bioreactor dominated by a mesophilic Desulfomicrobium species

BIOTECHNOLOGY & BIOENGINEERING, Issue 4 2009
Hannele Auvinen
Abstract The possibilities for the treatment of low-temperature mine waste waters have not been widely studied. The amenability of low-temperature sulfate reduction for mine waste water treatment at 9°C was studied in a bench-scale fluidized-bed bioreactor (FBR). Formate was used as the electron and carbon source. The first influent for the FBR was acidic, synthetic waste water containing iron, nutrients, and sulfate, followed by diluted barren bioleaching solution (DBBS). The average sulfate reduction rates were 8,mmol,L,1,day,1 and 6,mmol,L,1,day,1 with synthetic waste water and DBBS, respectively. The corresponding specific activities were 2.4 and 1.6,mmol SO,g VSS,1 day,1, respectively. The composition of the microbial community and the active species of the FBR was analyzed by extracting the DNA and RNA, followed by PCR-DGGE with the universal bacterial 16S rRNA gene primers and dsrB -primers specific for sulfate-reducing bacteria. The FBR microbial community was simple and stable and the dominant and active species belonged to the genus Desulfomicrobium. In summary, long-term operation of a low-temperature bioreactor resulted in enrichment of formate-utilizing, psychrotolerant mesophilic sulfate reducing bacteria. Biotechnol. Bioeng. 2009; 104: 740,751 © 2009 Wiley Periodicals, Inc. [source]


Enhancement of Methanol Tolerance in DMFC Cathode: Addition of Chloride Ions

CHEMPHYSCHEM, Issue 10 2008
Sunghyun Uhm Dr.
Abstract In the operation of a direct methanol fuel cell, the modification by chloride ions on the surface of a Pt cathode can facilitate the extraordinary increase of power performance and long-term stability. Analyzing the results of cyclic voltammograms and electrochemical impedance spectroscopy, the positive shift of Pt oxidation onset potential and the depression of oxidation current are observed, which results from the role of chloride as surface inhibitor. In addition, O2 temperature-programmed desorption and X-ray photoelectron spectroscopy also reveal that the suppression of Pt surface oxide can be best understood in terms of lower binding of oxygen species by the alteration of electronic state of Pt atoms. Such a reduced surface oxide formation not only provides more efficient proton adsorption sites with high selectivity but also decreases the mixed potential by crossover methanol, resulting in higher performance and stability even under high voltage long-term operation. [source]