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A. Dittmar Ch. GuguschevC. Hartmann S. GolkaA. Kwasniewski J. WollweberR. Fornari 《Journal of the European Ceramic Society》2011,31(14):2733-2739
Fine powders of WC, TaC and TaC with W and WC additives were cold-isostatically pressed to ceramic discs and pressurelessly sintered at temperatures up to 2100 °C in a second step. Afterwards, the discs were tested as crucible lid under typical AlN growth conditions. The prepared discs should be gas tight and ensure a better alignment of the thermal expansion coefficients of TaC lid and AlN.Ceramic discs densified by this method reveal a relative density up to 97%. The TaC ceramic discs without additives show a microstructure with grain sizes in the range of 10-200 μm after sintering. The grain enlargement could be reduced by W and WC additives in the range of 1-5 wt.%. The results show that the AlN boules adhere only to WC lids and tungsten containing lids with W contents higher than or equal to 3 wt.%. 相似文献
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Zorb Ch Weisert A Stapelmann J Smolik G Carter DC Wright BS Brunner-Joos KD Wagner G 《Microgravity science and technology》2002,13(3):22-29
For the purpose of bio-electronics, bacteriorhodopsin was crystallized into two habits through liquid-liquid-diffusion, namely individual needles of up to 1.9 mm in length and needle bunch-like clusters of up to 4.9 mm in total length. In both the reduced gravity experiments performed, the morphology of the individual needles (crystal form A) had improved in terms of sharp needle edges and compact needle packing, compared to the parallel ground controls. For the long duration wide range low gravity condition in the "Diffusion-controlled Crystallization Apparatus for Microgravity (DCAM)" on Mir (STS-89 up), needle bunches on average were longer there than on the ground, while the compactness of the clusters, i.e. the average ratio of clustered length to clustered width was the reverse. Some exceptionally large individual needles were grown in DCAM. For the "Commercial Protein Crystallization Facility (CPCF)" in short duration high definition microgravity condition during a science mission of the Space Shuttle Discovery (STS-95), size and shape of the individual needles were homogeneous and superior to those of both the parallel ground controls and the results in DCAM. In CPCF, the average volume of the individual needles in suspension was increased by 50 % in microgravity compared to those in the ground control. 相似文献
136.
The large-angle convergent beam electron diffraction (LACBED) technique is used for determining the crystal polarity of GaP and GaAs single crystals from < 1 1 0 > cross-sectional samples. The method which is based on an earlier approach using convergent beam electron diffraction (CBED) evaluates the polarity-sensitive contrast of high odd-index Bragg-lines in [0 0 2] dark-field patterns. The polarity is determined by application of a simple contrast rule as well as by direct comparison with dynamical simulations. For the two materials the ranges of applicability are determined by a detailed analysis of the Bragg-line contrast as a function of the sample thickness. The coexistence of the Bragg-line pattern and the of shadow image of the defect in correct rotational relationship to each other makes the analysis straightforward and free from possible sources of errors. As an example, the crystal polarity of GaP is related to the morphology of facetted voids. The LACBED method is shown to be suitable for relating the analysis of extended crystal defects. The advantages and the disadvantages of the LACBED method are discussed in comparison with the corresponding CBED method and with a recent method based on the analysis of bend contours. 相似文献
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H. C. Leligou Ch. Linardakis K. Kanonakis J. D. Angelopoulos Th. Orphanoudakis 《International Journal of Communication Systems》2006,19(5):603-617
The steadily rising demand for multimedia and data services, the falling cost and omnipresence of Ethernet and the maturity of passive optical networks (PON) technology, promise to radically change the landscape in the local loop. The heart of a gigabit PON system (recently standardized by FSAN/ITU) is the medium access controller (MAC), which arbitrates access to the upstream link among users with fluctuating traffic demands and effects the multiplexing and concentration policy. At the same time, it has to safeguard the service quality and enforce the parameters agreed in the service level agreements (SLAs) between the users and the service provider. In this paper, a MAC protocol designed to serve any mix of services according to their quality of service (QoS) needs, employing four priority levels along with a high number of logically separate data queues is presented. The architecture and implementation in hardware of a MAC algorithm capable of allocating bandwidth down to a resolution of a byte with QoS differentiation is the focus of this paper. It employs the bandwidth arbitration tools of the FSAN/ITU G.984.3 standard and maps SLA parameters to GPON service parameters to create an efficient, fair and flexible residential access system. Copyright © 2005 John Wiley & Sons, Ltd. 相似文献
139.
The systolic processing offers the possibility of solving a large number of standard problems on multicellular computing devices
with autonomous cells (Processing Elements—PEs). The resulting systolic arrays exploit the underlying parallelism of many
computationally intensive problems and offer a vital and effective way of handling them. Advances in technology and especially
in VLSI and FPGA have an ongoing contribution to the evolution of systolic arrays. Herein, a FPGA-based Systolic array prototype
implementing the Factorization stage of the Quadrant Interlocking Factorization—QIF (Butterfly) method is presented and the
corresponding time-complexities achieved are discussed. 相似文献
140.
We address the problem of image similarity in the compressed domain, using a multivariate statistical test for comparing color distributions. Our approach is based on the multivariate Wald-Wolfowitz test, a nonparametric test that assesses the commonality between two different sets of multivariate observations. Using some pre-selected feature attributes, the similarity measure provides a comprehensive estimate of the match between different images based on graph theory and the notion of minimal spanning tree (MST). Feature extraction is directly provided from the JPEG discrete cosine transform (DCT) domain, without involving full decompression or inverse DCT. Based on the zig-zag scheme, a novel selection technique is introduced that guarantees image's enhanced invariance to geometric transformations. To demonstrate the performance of the proposed method, the application on a diverse collection of images has been systematically studied in a query-by-example image retrieval task. Experimental results show that a powerful measure of similarity between compressed images can emerge from the statistical comparison of their pattern representations. 相似文献