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排序方式: 共有439条查询结果,搜索用时 15 毫秒
41.
Chi-Chao Hung Oates A.S. Horng-Chih Lin Yu-En Chang P. Jia-Lian Wang Cheng-Chung Huang You-Wen Yau 《Device and Materials Reliability, IEEE Transactions on》2007,7(3):462-467
This paper provides a new understanding of metal-insulator-metal-capacitor-degradation behavior under a wide range of constant-current-stress conditions. It was found that capacitance degrades with stress, but the behavior of the degradation strongly depends on the stress-current density. At high stress levels, the capacitance increases logarithmically as the injection charge increases until dielectric breakdown occurs. At lower stress conditions, the degradation rate is proportional to the stress current and reverses after a certain period of time. A metal-insulator interlayer is observed using cross-sectional transmission-electron-microscopy micrographs, which possibly explains this reversal phenomenon. 相似文献
42.
This paper reports the result of a study on the effect of aluminum pad surface morphology on the flip-chip solder bump reliability. The influence of the Al surface morphology on the electroless zinc/nickel/gold UBM is presented. The reliability of the solder bump as measured by ball shear force is reported. Al pad were produced using two RF sputtering systems: CVC-601 and Varian-3180. The Al targets used in CVC and Varian system were 99%Al–1%Si and 98.95%Al–1%Si–0.05%Ti respectively. The surface of the CVC sputtered Al samples were smooth while the surface of the Varian sputtered Al samples were rough. All the samples were subjected to the electroless zinc/nickel/gold plating. The results suggest that after plating, the smooth Al surface resulted in a fine nickel UBM surface while the rough Al surface formed a coarse nickel UBM surface. Ball shear test was conducted after the solder balls were bumped on the UBM. Result shows that the fine UBM surface samples have twice the shear strength compared to the samples with coarse UBM surface samples. The analysis of the results indicates that shear surface occurred at the UBM and the solder interface for samples with rough UBM surface leading to the lower shear strength. Nickel bump shear test result shows that pretreatment of Al pad surface by sodium hydroxide and nitric acid created more zinc seeds this led to better electroless nickel plating. Nickel bump shear tests also shows that double zincated bumps had higher shear strength than single zincated bumps. To obtain reliable flip-chip solder bumps, it is essential to maintain good Al pad surface morphology, pretreatment of the Al pad and undergo second zincation. 相似文献
43.
The recognition of odorants by olfactory receptors represents the first stage in odor discrimination. Here, we report the generation of an expression library containing a large and diverse repertoire of mouse olfactory receptor sequences in the transmembrane II-VII region. From this library, 80 chimeric receptors were tested against 26 odorants after transfection into HEK-293 cells. Three receptors were identified to respond to micromolecular concentrations of carvone, (-) citronellal, and limonene, respectively. We also found that the mouse I7 receptor, unlike the rat I7 receptor, prefers heptanal instead of octanal, as a result of a single valine-to-isoleucine substitution. This finding represents the beginning of a molecular understanding of odorant recognition. The identification, on a large scale, of cognate receptor-odorant interactions should provide insight into olfactory coding mechanisms. 相似文献
44.
A method for the design of MRI radiofrequency coils based on triangular and pulse basis functions. 总被引:1,自引:0,他引:1
This paper presents a numerical technique for the design of an RF coil for asymmetric magnetic resonance imaging (MRI) systems. The formulation is based on an inverse approach where the cylindrical surface currents are expressed in terms of a combination of sub-domain basis functions: triangular and pulse functions. With the homogeneous transverse magnetic field specified in a spherical region, a functional method is applied to obtain the unknown current coefficients. The current distribution is then transformed to a conductor pattern by use of a stream function technique. Preliminary MR images acquired using a prototype RF coil are presented and validate the design method. 相似文献
45.
Chlamydia trachomatis is an obligate intracellular pathogen, long recognized as an agent of blinding eye disease and more recently as a common sexually transmitted infection. Recently, two eukaryotic histone H1-like proteins, designated Hc1 and Hc2, have been identified in Chlamydia. Expression of Hc1 in recombinant Escherichia coli produces chromatin condensation similar to nucleoid condensation observed late in the parasite's own life cycle. In contrast, chromatin decondensation, observed during the early life cycle, accompanies down-regulation and nondetection of Hc1 and Hc2 among internalized organisms. We reasoned that the early upstream open reading frame (EUO) gene product might play a role in Hc1 degradation and nucleoid decondensation since it is expressed very early in the chlamydial life cycle. To explore this possibility, we fused the EUO coding region between amino acids 4 and 177 from C. trachomatis serovar Lz with glutathione S-transferase (GST) and examined the effects of fusion protein on Hc1 in vitro. The purified fusion protein was able to digest Hc1 completely within 1 h at 37 degrees C. However, GST alone exhibited no Hc1-specific proteolytic activity. The chlamydial EUO-GST gene product also cleaves very-lysine-rich calf thymus histone H1 and chicken erythrocyte histone H5 but displays no measurable activity towards core histones H2A, H2B, H3, and H4 or chlamydial RNA polymerase alpha-subunit. This proteolytic activity appears sensitive to the serine protease inhibitor 4-(2-aminoethyl)-benzenesulfonyl fluoride hydrochloride (AEBSF) and aspartic protease inhibitor pepstatin but resistant to high temperature and other broad-spectrum protease inhibitors. The proteolytic activity specified by the EUO-GST fusion product selectively digested the C-terminal portion of chlamydial Hc1, the domain involved in DNA binding, while leaving the N terminus intact. At a molar equivalent ratio of 1:1 between Hc1 and DNA, the EUO gene product cleaves Hc1 complexed to DNA and this cleavage appears sufficient to initiate dissociation of DNA-Hc1 complexes. However, at a higher molar equivalent ratio of Hc1/DNA (10:1), there is partial protection conferred upon Hc1 to an extent that prevents dissociation of DNA-Hc1 complexes. 相似文献
46.
Ever since the concept of estimation algebra was first introduced by Brockett and Mitter independently, it has been playing
a crucial role in the investigation of finite-dimensional nonlinear filters. Researchers have classified all finite-dimensional
estimation algebras of maximal rank with state space less than or equal to three. In this paper we study the structure of
quadratic forms in a finite-dimensional estimation algebra. In particular, we prove that if the estimation algebra is finite
dimensional and of maximal rank, then the Ω=(∂f
j
/∂x
i
−∂f
i
/∂x
j
)matrix, wheref denotes the drift term, is a linear matrix in the sense that all the entries in Ω are degree one polynomials. This theorem
plays a fundamental role in the classification of finite-dimensional estimation algebra of maximal rank.
This research was supported by Army Research Office Grants DAAH 04-93-0006 and DAAH 04-1-0530. 相似文献
47.
We analyze a recently proposed dynamic programming algorithm (REDP) for maximum likelihood (ML) parameter estimation of superimposed signals in noise. We show that it degrades gracefully with deviations from the key assumption of a limited interaction signal model (LISMO), providing exact estimates when the LISMO assumption holds exactly. In particular, we show that the deviations of the REDP estimates from the exact ML are continuous in the deviation of the signal model from the LISMO assumption. These deviations of the REDP estimates from the MLE are further quantified by a comparison to an ML algorithm with an exhaustive multidimensional search on a lattice in parameter space. We derive an explicit expression for the lattice spacing for which the two algorithms have equivalent optimization performance, which can be used to assess the robustness of REDP to deviations from the LISMO assumption. The values of this equivalent lattice spacing are found to be small for a classical example of superimposed complex exponentials in noise, confirming the robustness of REDP for this application 相似文献
48.
A very simple and convenient method of analysis for studying two-dimensional mixed-mode crack problems in rectilinear anisotropic solids is presented. The analysis is formulated on the basis of conservation laws of anisotropic elasticity and of fundamental relationships in anisotropic fracture mechanics. The problem is reduced to a system of linear algebraic equations in mixed-mode stress intensity factors. One of the salient features of the present approach is that it can determine directly the mixed-mode stress intensity solutions from the conservation integrals evaluated along a path removed from the crack-tip region without the need of solving the corresponding complex near-field boundary value problem. Several examples with solutions available in the literature are solved to ensure the accuracy of the current analysis. This method is further demonstrated to be superior to other approaches in its numerical simplicity and computational efficiency. Solutions of more complicated and practical engineering problems dealing with the crack emanating from a circular hole in composites are presented also to illustrate the capacity of this method.
Résumé On présente une méthode d'analyse simple et bien adaptée à l'étude de problèmes de fissuration bidimensionnelle de mode mixte dans des solides rectilinéaires anisotropes. L'analyse est formulée sur la base des lois de conservation de l'élasticité anisotrope et sur les relations fondamentales dans la mécanique de rupture anisotrope. Le problème est ramené à un système d'équations algébriques linéaires exprimant les facteurs d'intensité d'entaille pour un mode mixte. L'un des faits saillants de l'approche envisagée est qu'elle peut déterminer directement les solutions d'intensité de contrainte suivant un mode mixte à partir des intégrales de conservation évaluées le long d'un parcours éloigné par rapport à l'extrémité de la fissure, sans qu'il soit nécessaire de résoudre le problème correspondant de la valeur du champ proche complexe. Divers exemples, ainsi que des solutions disponibles dans la littérature, sont résolus et confrontés en vue de tester la fiabilité de l'analyse réalisée. La méthode s'avère supérieure aux autres approches de par sa simplicité numérique et son efficacité de calcul. Des solutions à des problèmes plus compliqués et pratiques rencontrés en ingénièrie où une fissure émane d'un trou circulaire dans des matières composites sont également présentées, en vue d'illustrer les capacités de cette méthode.相似文献
49.
50.