SiC半导体技术--即将腾飞的新一代半导体技术

电源设计者们仿佛正在目睹一个新的半导体技术的诞生。随之而来的新一代功率半导体器件远远优于现有的硅技术。SiC 材料可以让器件具有迄今为止设计工程师们梦寐以求却不能得到的出色特性。其最重要的优点包括以下几个方面：:工作结温高达225 ℃，而相应的漏电流只有适度的增加。由于本质上不会出现热偏移thermal run－away)，故可以在 很高的结温下可靠的工作。 没有正向或反向恢复，故即使在高温 下以高频工作时，也没有开关损耗。 甚至可以实现开关损耗极小、频率 高达1 M H z 的深度切换(h a rd 正向电阻具有…     

Impurity local phonon nonradiative quenching of Yb3+fluorescence in ytterbium-doped silicate glasses

We have studied the concentration quenching of Yb³⁺ ion fluorescence in Yb-doped silicate glasses containing up to 3.4×10 ²¹ cm^-3 Yb³⁺ ions. The absorption and fluorescence spectra are similar to those obtained for the Yb³⁺ ion in many different matrices, with a radiative lifetime of approximately 1400 μs. The fluorescence decay curves were different among samples, with strong dependence on the Yb concentration. The decay curves could always be resolved into two exponential components, indicating that the ions reside in two different sites, each of a different characteristic nonradiative decay mechanism. The fast decay times ranged between 6 and 300 μs, and the slow ones ranged between 190 and 1250 μs in different samples. The sites where ions exhibit the fast decay most probably consist of pairs of Yb³⁺ ions. The nonradiative decay probabilities for each site mere directly proportional to the Yb³⁺ concentration in the same site. We propose that the fluorescence quenching occurs by multiphonon nonradiative transitions involving polar local phonon bands created by the presence of the Yb³⁺ ion     

Iterative principal axes registration method for analysis of MR-PETbrain images

Computerized automatic registration of MR-PET images of the brain is of significant interest for multimodality brain image analysis. Here, the authors discuss the principal axes transformation for registration of three-dimensional MR and PET images. A new brain phantom designed to test MR-PET registration accuracy determines that the principal axes registration (PAR) method is accurate to within an average of 1.37 mm with a standard deviation of 0.78 mm. Often the PET scans are not complete in the sense that the PET volume does not match the respective MR volume. The authors have developed an iterative PAR (IPAR) algorithm for such cases. Partial volumes of PET can be accurately registered to the complete MR volume using the new iterative algorithm. The quantitative and qualitative analyses of MR-PET image registration are presented and discussed. Results show that the new PAR algorithm is accurate and practical in MR-PET correlation studies     

The relationship between dual mode cavity cross-coupling andwaveguide polarizers

Cross-coupling in dual-mode cavity filters may be obtained by introducing an asymmetry within the cavity cross section at an angle of 45° to the two orthogonal modes. This paper presents a novel formula relating the resulting cross coupling coefficient between the orthogonal resonances to the polarization of a waveguide polarizer. Previous theories for such polarizers may then be applied directly to the dual mode filter situation. Formulas enabling the dimensions of the asymmetries for required coupling coefficients are presented for square and circular waveguide cross sections     

坚不可催的病毒防护

我们都看过一部或两部关于僵尸的电影。某些医疗或化学事故创造出第一个，然后是一批，然后是成千上万几乎不可摧毁的无意识生物。除为剧情上的相似以外，大多数僵尸电影中常见的一个共同点是都有这样的情景。其中一个即将成为僵尸的受害者遇到一个僵尸，这个僵尸是以前一个亲爱的人变成的，因此这个人想“我们亲密的关系和信任所具有的力量可以保护我”，于是没有逃跑或进行防卫。结果是他被变成了另一具僵尸。     

Three-dimensional anatomical model-based segmentation of MR brainimages through principal axes registration

Model-based segmentation and analysis of brain images depends on anatomical knowledge which may be derived from conventional atlases. Classical anatomical atlases are based on the rigid spatial distribution provided by a single cadaver. Their use to segment internal anatomical brain structures in a high-resolution MR brain image does not provide any knowledge about the subject variability, and therefore they are not very efficient in analysis. The authors present a method to develop three-dimensional computerized composite models of brain structures to build a computerized anatomical atlas. The composite models are developed using the real MR brain images of human subjects which are registered through the principal axes transformation. The composite models provide probabilistic spatial distributions, which represent the variability of brain structures and can be easily updated for additional subjects. The authors demonstrate the use of such a composite model of ventricular structure to help segmentation of the ventricles and cerebrospinal fluid of MR brain images. Here, a composite model of ventricles using a set of 22 human subjects is developed and used in a model-based segmentation of ventricles, sulci, and white matter lesions. To illustrate the clinical usefulness, automatic volumetric measurements on ventricular size and cortical atrophy for an additional eight alcoholics and 10 normal subjects were made. The volumetric quantitative results indicated regional brain atrophy in chronic alcoholics     

Nightly@nbc.com: Audience Scope and the Perception of Interactivity in Viewer Mail on the Internet

A content analysis looked at 650 Internet mail messages sent to NBC Nightly News in response to an invitation during a series dealing with the impact of new technologies. Message indexes were created to reflect the scope of the audience they addressed. Macro-scopic messages were formal, dealt with political issues, and either did not mention technology or mentioned mass media. These messages resembled a letter to a newspaper editor. Mezzoscopic messages were positive and avoided political issues. They congratulated NBC as a team for their work on the technology series and tended to mention computers. Micro-scopic messages stood out for their authors' high perception of interactivity. These messages mentioned computer technology more than any other group and appear similar to face-to-face communication in many ways. Results are discussed in terms of how the relationship between media users' perceived sense of interactivity and the scope of the audience they address are related to message style and content.