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411.
An improved three-dimensional model for simulating cathodoluminescence (CL) in a semiconductor under electron-beam irradiation is described. The Monte Carlo method is used to simulate electron-beam-semiconductor interaction while F. Berz and H.K. Kuiken's (1976) formulation is used to obtain the excess carrier distribution. Optical losses of photons both within the semiconductor and at the semiconductor-air interface are also accounted for in this model. This model has been used to simulate the CL intensity as a function of electron-beam voltage, beam incidence angle, surface recombination velocity, diffusion length, absorption coefficient, and surface dead-layer thickness. The radiation patterns over the top face of a specimen with flat geometry are also simulated 相似文献
412.
Summary The crystallinity of six cellulose samples with different origin and treatment was determined using x-ray diffraction (XDR) and nuclear magnetic resonance with cross polarization and magic angle sample spinning (CP/MAS NMR) methods. The numerical results for crystallinity obtained by using curve fitting methods in both cases correlated very well. It was concluded that the values for the crystallinity can be determined from NMR spectra when CP times not exceeding 0.5 ms are used. The range of order of the samples was further characterized calculating the radial atomic density function from the x-ray diffraction patterns and determining the greatest distances with significant deviations from the average density.On leave from Institute of Chemical Physics and Biophysics, Estonian Academy of Sciences, SU-200105 Tallinn, USSR 相似文献
413.
Two plasmids are described which can be used to remove the "loxP-markerMX-loxP" cassettes in strains lacking the ura3 mutation. Both contain the Cre-recombinase under control of the GAL1 promoter and the natMX cassette with the dominant marker nat, which gives yeasts resistance to the antibiotic ClonNat. pNatCre contains ARSH and CEN6 for maintenance in Saccharomyces cerevisiae. pKlNatCre has a Kluyveromyces lactis replication origin and centromere in addition. 相似文献
414.
RM Dijkhuizen S Knollema HB van der Worp GJ Ter Horst DJ De Wildt JW Berkelbach van der Sprenkel KA Tulleken K Nicolay 《Canadian Metallurgical Quarterly》1998,29(3):695-704
BACKGROUND AND PURPOSE: Selective regional sensitivity and delayed damage in cerebral ischemia provide opportunities for directed and late therapy for stroke. Our aim was to characterize the spatial and temporal profile of ischemia-induced changes in cerebral perfusion and tissue status, with the use of noninvasive MRI techniques, to gain more insight in region-specific vulnerability and delayed damage. METHODS: Rats underwent 20 minutes of unilateral cerebral hypoxia-ischemia (HI). We performed combined repetitive quantitative diffusion-weighted, T2-weighted, and dynamic susceptibility contrast-enhanced MRI from before HI to 5 hours after HI. Data were correlated with parallel blood oxygenation level-dependent MRI and laser-Doppler flowmetry. Finally, MRI and histology were done 24 and 72 hours after HI. RESULTS: Severe hypoperfusion during HI caused acute reductions of the apparent diffusion coefficient (ADC) of tissue water in the ipsilateral hemisphere. Reperfusion resulted in dynamic perfusion alterations that varied spatially. The ADC recovered completely within 1 hour in the hippocampus (from 0.68 +/- 0.07 to 0.83 +/- 0.09 x 10[-3] mm2/s), cortex (from 0.56 +/- 0.06 to 0.77 +/- 0.07 x 10[-3] mm2/s), and caudate putamen (from 0.58 +/- 0.06 to 0.75 +/- 0.06 x 10[-3] mm2/s) but only partially or not at all in the thalamus (from 0.65 +/- 0.07 to 0.68 +/- 0.12 x 10[-3] mm2/s) and substantia nigra (from 0.80 +/- 0.08 to 0.76 +/- 0.10 x 10[-3] mm2/s). Secondary ADC reductions, accompanied by significant T2 elevations and histological damage, were observed after 24 hours. Initial and secondary ADC decreases were observed invariably in the hippocampus, cortex, and caudate putamen and in approximately 70% of the animals in the thalamus and substantia nigra. CONCLUSIONS: Region-specific responses and delayed ischemic damage after transient HI were demonstrated by MRI. Acute reperfusion-induced normalization of ADCs appeared to poorly predict ultimate tissue recovery since secondary, irreversible damage developed eventually. 相似文献
415.
A detailed theoretical study was made of the dissolution of precipitates. The kinetic analyses of the atomic mechanisms included
the concurrent effects of diffusion, interface reaction, and curvature, and considered the more complex problems of precipitate
arrays and nondepleted matrices. The general formalism thus obtained was used to interpret the results of an experimental
investigation of the technologically important problem of the dissolution of “reluctantly soluble” carbides in the Fe-C-V
system. It was found that vanadium carbide (and iron carbide as well) were quite easily dissolved while complex alloy sulfides,
silicates and oxides (possibly associated with the presence of vanadium) persisted, with little if any dissolution. Previous
views of the reasons for poor hardenability were reevaluated in light of these results.
Formerly with the Process Research Department, Scientific Research Staff, Ford Motor Co.
This paper is based on an invited talk presented at a symposium on Homogenization of Alloys, sponsored by the IMD Heat Treatment
Committee, and held on May 11, 1970, at the spring meeting of The Metallurgical Society of AIME, in Las Vegas, Nev. 相似文献
416.
Besides the generation of interface states and the associated positive trapped charge (N/sub tc1/), experimental results unambiguously show the generation of another positive trapped charge component (N/sub tc2/) during negative-bias temperature instability (NBTI) stressing of p-MOSFETs employing ultrathin silicon nitride gate dielectric. For a given gate stress voltage, N/sub tc2/ is generated at a much faster rate compared to N/sub tc1/. Under the pulsed gate condition studied, N/sub tc1/ could almost be completely annihilated, regardless of the NBTI stress voltage, whereas only partial annihilation of N/sub tc2/ is observed. This more resistant nature of N/sub tc2/ to post-stress relaxation has serious implications on the dynamic NBTI reliability of these p-MOSFETs. 相似文献
417.
Xiaobei Liu Chao Lu Tee Hiang Cheng Soo Ngee Koh 《Communications, IEEE Transactions on》2007,55(6):1103-1109
A simplified parallel step-by-step decoding algorithm is proposed for decoding Reed-Solomon (RS) codes. It uses new method to calculate the determinants of the temporarily changed syndrome matrices, based on the property of these matrices determined in this paper. By using the proposed method, the calculations of the determinants of the temporarily changed syndrome matrices become much simpler and thus the computational complexity of the step-by-step decoding algorithm is significantly reduced. 相似文献
418.
Condorelli G. Lombardo S. A. Palumbo F. Pey K.-L. Tung C. H. Tang L.-J. 《Device and Materials Reliability, IEEE Transactions on》2006,6(4):534-541
It has been shown that under accelerated stress below ap4 V, thin gate oxides are subject to progressive breakdown (BD), i.e., a gradual growth of the BD spot up to a destructive BD. This paper investigates the conduction mechanisms of the BD spot during the early stages of progressive BD through the measurement of the I-V characteristics using carrier separation. It is shown that a model with no free parameter based on the concept of cotunneling provides a good evaluation of the post-BD current. This model implies a physical microstructure, and its plausibility is compared to direct transmission electron microscopy (TEM) observations 相似文献
419.
DNA sequencing up to 1300 bases in two hours by capillary electrophoresis with mixed replaceable linear polyacrylamide solutions 总被引:6,自引:0,他引:6
Zhou H Miller AW Sosic Z Buchholz B Barron AE Kotler L Karger BL 《Analytical chemistry》2000,72(5):1045-1052
This paper presents results on ultralong read DNA sequencing with relatively short separation times using capillary electrophoresis with replaceable polymer matrixes. In previous work, the effectiveness of mixed replaceable solutions of linear polyacrylamide (LPA) was demonstrated, and 1000 bases were routinely obtained in less than 1 h. Substantially longer read lengths have now been achieved by a combination of improved formulation of LPA mixtures, optimization of temperature and electric field, adjustment of the sequencing reaction, and refinement of the base-caller. The average molar masses of LPA used as DNA separation matrixes were measured by gel permeation chromatography and multiangle laser light scattering. Newly formulated matrixes comprising 0.5% (w/w) 270 kDa and 2% (w/w) 10 or 17 MDa LPA raised the optimum column temperature from 60 to 70 degrees C, increasing the selectivity for large DNA fragments, while maintaining high selectivity for small fragments as well. This improved resolution was further enhanced by reducing the electric field strength from 200 to 125 V/cm. In addition, because sequencing accuracy beyond 1000 bases was diminished by the low signal from G-terminated fragments when the standard reaction protocol for a commercial dye primer kit was used, the amount of these fragments was doubled. Augmenting the base-calling expert system with rules specific for low peak resolution also had a significant effect, contributing slightly less than half of the total increase in read length. With full optimization, this read length reached up to 1300 bases (average 1250) with 98.5% accuracy in 2 h for a single-stranded M13 template. 相似文献
420.
Catalysis Letters - CeO2, V2O5 and CeVO4 were synthesised as bulk oxides, or deposited over activated carbon, characterized by XRD, HRTEM, CO2-TPO, C3H8-TPR, DRIFTS and Raman techniques and tested... 相似文献