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41.
Govindhan Dhanaraj Yi Chen Hui Chen Dang Cai Hui Zhang Michael Dudley 《Journal of Electronic Materials》2007,36(4):332-339
A chemical vapor deposition (CVD) system was designed and fabricated in our laboratory and SiC homo-epitaxial layers were
grown in the CVD process using silicon tetrachloride and propane precursors with hydrogen as a carrier gas. The temperature
field was generated using numerical modeling. Gas flow rates, temperature field, and the gradients are found to influence
the growth rates of the epitaxial layers. Growth rates were found to increase as the temperature increased at high carrier
gas flow rate, while at lower carrier gas flow rate, growth rates were observed to decrease as the temperature increased.
Based on the equilibrium model, “thermodynamically controlled growth” accounts for the growth rate reduction. The grown epitaxial
layers were characterized using various techniques. Reduction in the threading screw dislocation (SD) density in the epilayers
was observed. Suitable models were developed for explaining the reduction in the SD density as well as the conversion of basal
plane dislocations (BPDs) into threading edge dislocations (TEDs). 相似文献
42.
We describe a system for the automatic acquisition and processing of digital images in a high-resolution X-ray microscope, including the formation of large-field high-resolution image montages. A computer-controlled sample positioning stage provides approximate coordinates for each high-resolution subimage. Individual subimages are corrected to compensate for time-varying, non-uniform illumination and CCD-related artefacts. They are then automatically assembled into a montage. The montage assembly algorithm is designed to use the overlap between each subimage and multiple neighbours to improve the performance of the registration step and the fidelity of the result. This is accomplished by explicit use of recorded stage positions, optimized ordering of subimage insertion, and registration of subimages to the developing montage. Using this procedure registration errors are below the resolution limit of the microscope (43 nm). The image produced is a seamless, large-field montage at full resolution, assembled automatically without human intervention. Beyond this, it is also an accurate X-ray transmission map that allows the quantitative measurement of anatomical and chemical features of the sample. Applying these tools to a biological problem, we have conducted the largest X-ray microscopical study to date. 相似文献
43.
Alexander M. Korsunsky Xu Song Brian Abbey Thomas Connolley Robert C. Atwood Michael Drakopoulos 《Materials Letters》2010,64(15):1724-1727
Better understanding of the distribution of elastic and plastic strains in deformed polycrystalline, multiphase materials is important for structural engineering. The deformation response depends upon the interaction of grains of different orientations, and the anisotropy associated with each phase. Strain partitioning and tensile-compressive hardening asymmetry arises due to mismatches in modulus and ductility between grains and phases in alloys such as Ti-6Al-4V that displays both strong anisotropy within one phase and significant differences of properties between phases. Simple four-point bent beam samples were studied using the newly available energy-dispersive X-ray diffraction setup at the high energy white-beam synchrotron beamline I12 (JEEP) at Diamond Light Source. Diffraction patterns from the bent polycrystalline Ti6Al4V samples were collected using the new 23-cell “horseshoe” detector and interpreted using Pawley refinement to extract unit cell parameters, thus allowing elastic strain to be determined. The tensile-compressive hardening asymmetry was quantified for the grains oriented with the basal plane perpendicular to the loading direction. Initial evaluation of the performance of the new instrument is reported. 相似文献
44.
Dehydrogenation behavior of LiAlH4 (lithium alanate) admixed with multi-walled carbon nanotubes (MWCNTs) was investigated by using high-pressure thermal gravimetric analysis (HPTGA) and in-situ synchrotron X-ray diffraction (XRD) technique. HPTGA results indicated that MWCNTs play a catalytic role in the dehydrogenation of LiAlH4, subsequently decreasing the dehydrogenation temperature and improving the desorption kinetics. With the proper amount of MWCNT, the initial dehydrogenation temperature of LiAlH4 decreased by approximately 60 °C. Furthermore, in-situ synchrotron XRD analysis confirmed the dehydrogenation reaction products and paths of LiAlH4, with and without MWCNT addition. 相似文献
45.
The long‐term oxidation performance of an alloy is critically linked to the early‐stage oxidation behavior of high‐temperature alloys. This study investigates early‐stage oxidation behavior in terms of oxidation kinetics, scale evolution, and residual stresses developed within a scale of the commercially available cobalt‐rich alloys: HAYNES® 188, 6B, 25, and HR‐160® and a newly developed nitride‐dispersion strengthened NS‐163® alloy (HAYNES®, HR‐160®, NS‐163® are registered trademarks of Haynes International, Inc). Short‐term isothermal oxidation exposures were conducted in flowing air at 982 °C for durations of 1–50 h. Oxidation kinetics was assessed by weight‐change behavior, which showed that 188 alloy exhibited the lowest weight‐gain, while for similar times HR‐160 alloy underwent weight‐loss. SEM/EDS analysis was performed to characterize oxides formed in these alloys, while stresses developed in the oxides of different alloys were measured using synchrotron X‐ray radiation. The results in this paper clearly demonstrated the effects of alloy composition on the scale evolution and the amount of stresses developed in oxides. 相似文献
46.
WANG TongMin XU JingJing LI Jun HUANG WanXia LIU ShengChu & LI TingJu School of Materials Science Engineering Dalian University of Technology Dalian China 《中国科学:信息科学(英文版)》2010,(5)
This study was trying to observe the real-time dendrite growth of Sn-Bi and Sn-Pb binary alloys by a synchrotron radiation imaging technology.The imaging system includes an intense and high brightness synchrotron radiation source,a high-resolution and fast-readout charge coupled device camera,an alloy sample and a Bridgman solidification system.The imaging experiments were done at Beijing Synchrotron Radiation Facility with an updated synchrotron radiation imaging technique,diffraction-enhanced imaging,whic... 相似文献
47.
A. L pez-Rubio P. Hernandez-Mu oz R. Catala R. Gavara J. M. Lagar n 《Food Additives & Contaminants》2005,22(10):988-993
The objective was to demonstrate, as an example of an application, the potential of synchrotron X-ray analysis to detect morphological alterations that can occur in barrier packaging materials and structures. These changes can affect the packaging barrier characteristics when conventional food preservation treatments are applied to packaged food. The paper presents the results of a number of experiments where time-resolved combined wide-angle X-ray scattering and small-angle X-ray scattering analysis as a function of temperature and humidity were applied to ethylene-vinyl alcohol co-polymers (EVOH), polypropylene (PP)/EVOH/PP structures, aliphatic polyketone terpolymer (PK) and amorphous polyamide (aPA) materials. A comparison between conventional retorting and high-pressure processing treatments in terms of morphologic alterations are also presented for EVOH. The impact of retorting on the EVOH structure contrasts with the good behaviour of the PK during this treatment and with that of aPA. However, no significant structural changes were observed by wide-angle X-ray scattering in the EVOH structures after high-pressure processing treatment. These structural observations have also been correlated with oxygen permeability measurements that are of importance when guaranteeing the intended levels of safety and quality of packaged food. 相似文献
48.
Structural development of natural rubber during uniaxial stretching was examined by an in situ wide angle X-ray diffraction measurement using a synchrotron. During stretching, the amorphous part showed little change, i.e. an amorphous halo remained clear even at 500% strain. The fraction of induced crystals was very small, though a clear crystalline pattern was observed at 400% strain. Some polymer chains were oriented and crystallized, but most of the chains were not oriented at all in spite of large deformations of the specimen. Only a small amount of polymer chains contributes to the stress and hysteresis loss during elongation. 相似文献
49.
Mauro Perfetti Francesco Pineider Lorenzo Poggini Edwige Otero Matteo Mannini Lorenzo Sorace Claudio Sangregorio Andrea Cornia Roberta Sessoli 《Small (Weinheim an der Bergstrasse, Germany)》2014,10(2):323-329
The chemical synthesis and characterization of the first hybrid material composed by gold nanoparticles and single molecule magnets (SMMs) are described. Gold nanoparticles are functionalized via ligand exchange using a tetrairon(III) SMM containing two 1,2‐dithiolane end groups. The grafting is evidenced by the shift of the plasmon resonance peak recorded with a UV–vis spectrometer, by the suppression of nuclear magnetic resonance signals, by X‐ray photoemission spectroscopy peaks, and by transmission electron microscopy images. The latter evidence the formation of aggregates of nanoparticles as a consequence of the cross‐linking ability of Fe4 through the two 1,2‐dithiolane rings located on opposite sides of the metal core. The presence of intact Fe4 molecules is directly proven by synchrotron‐based X‐ray absorption spectroscopy and X‐ray magnetic circular dichroism spectroscopy, while a detailed magnetic characterization, obtained using electron paramagnetic resonance and alternating‐current susceptibility, confirms the persistence of SMM behavior in this new hybrid nanostructure. 相似文献
50.