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Quality Comparable (quality + comparable)
Selected AbstractsHomoepitaxy of ZnTe on (100) oriented substrates: Technology issues and MOVPE growth aspectsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 10-11 2005M. Traversa Abstract The metalorganic vapour phase epitaxy of ZnTe on single crystal (100)ZnTe:P wafers is reported. The technological steps to prepare a substrate surface suitable for the high quality homoepitaxy of ZnTe are identified and optimised in terms of structural and morphological properties of overgrown epilayers. Removal of ,7 µm of material from the ZnTe:P wafers by chemical etching in 1% Br2 -methanol solution proved necessary to achieve a sufficiently smooth and homogeneous surface; in-situ H2 heat treatment of the wafers at 350°C immediately before growth ensures optimal desorption of residual oxides, allowing epilayer crystalline quality comparable to the substrate. However, the structure of epilayers degrades for growth temperatures (TG) above 350 °C due to the occurrence of stacking faults (SFs) within ,200-300 nm from the epilayer-substrate interface. Accordingly, the epilayer band-edge luminescence vanishes below 350 nm, indicating a worsening of the material radiative efficiency in very thin epilayers. The epilayer surface morphology is the result of a complex interplay between SF nucleation and Te:Zn ad-atom stoichiometry during growth. Almost featureless morphologies are obtained for growth at 350 °C, i.e. under nearly stoichiometric surface conditions. Pyramid-like hillocks develop instead for TG , 360 °C, corresponding to Te-rich surface conditions, their density rapidly increasing up to around 9 × 106 cm,2 at TG = 400 °C. Hillocks occur in close pairs on the epilayer surface, their nucleation being strongly reduced if a thin ZnTe buffer layer is grown at low (325 °C) temperature, i.e. if SFs do not occur at the epilayer-substrate interface. This demonstrates that hillocks form as a result of three-dimensional growth around partial dislocations pairs bounding SFs, the phenomenon being driven by Te ad-atoms experiencing a Schwoebel potential barrier at the surface step edges around the dislocations. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Call admission control for voice over IPINTERNATIONAL JOURNAL OF COMMUNICATION SYSTEMS, Issue 4 2006Huseyin Uzunalioglu Abstract Voice over Internet protocol (VoIP) is a technology that enables the transmission of voice over an IP network. Recent years have witnessed heavy investment in this area in the commercial world. For VoIP to replace Public Switched Telephone Network (PSTN), it should provide voice quality comparable to circuit-switched PSTN networks. This paper addresses the mechanisms to guarantee VoIP quality of service (QoS). The focus is given to the call admission control, which blocks voice calls when the required resources are not available to guarantee the QoS for the call. We review call admission control approaches that can be applied to VoIP, and describe the advantages and disadvantages of each approach. In the second part of the paper, we present a measurement-based admission control scheme that achieves QoS in an efficient and scalable manner. The scheme uses voice traffic load measurements at each router link to compute link-level blocking policies for new call attempts. Then, these policies are translated into path-level blocking policies, which are applied to new call set-up requests. The performance of the scheme is presented for single and multiple-priority voice calls. Copyright © 2006 John Wiley & Sons, Ltd. [source] Identification of some optimal parameters to achieve higher laminate quality through tape placement processADVANCES IN POLYMER TECHNOLOGY, Issue 2 2010Muhammad Amir Khan Abstract Investigations of the tape placement process are still underway to achieve the laminate quality comparable with other highly developed composite manufacturing methods, such as autoclave-based processes. The evaluation of the products and the mechanical properties of the material has revealed regions where some upgrading could bring significant enhancement in performance. This study is concerned with issues related to the characterization of material properties, interlaminar bond development, and ways of optimizing process parameters. These include heating, layup velocity, tool temperature, and consolidation force, which were studied to determine their effects on the mechanical strength of the composites produced. The bonding degree of the laminates was simulated taking into account a combination of process parameters, comparing the predictions with the actual test results. The thermal stability of the polyether ether ketone matrix, in relation to the processing conditions used in the manufacture, and their effects on the interlaminar bonding stability were also investigated and explained. © 2010 Wiley Periodicals, Inc. Adv Polym Techn 29:98,111, 2010; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/adv.20177 [source] Highly accelerated first-pass contrast-enhanced magnetic resonance angiography of the peripheral vasculature: Comparison of gadofosveset trisodium with gadopentetate dimeglumine contrast agentsJOURNAL OF MAGNETIC RESONANCE IMAGING, Issue 5 2009Jeffrey H. Maki MD Abstract Purpose: To investigate the blood pool agent gadofosveset trisodium for first-pass, dynamic peripheral contrast-enhanced magnetic resonance angiography (pMRA), and compare the results with a conventional gadolinium contrast agent. Materials and Methods: A total of 16 patients were imaged at 1.5T using a prototype peripheral vascular coil with high SENSE acceleration. Five received gadopentetate dimeglumine (,0.25 mmol/kg), and 11 received gadofosveset trisodium (five standard-dose 0.03 mmol/kg, six high-dose 0.05 mmol/kg). Quantitative contrast-enhancement and qualitative image quality evaluation was compared between agents and doses. Results: High-quality diagnostic images were uniformly obtained. The contrast ratio did not significantly differ between gadopentetate dimeglumine and high-dose gadofosveset trisodium, both of which were greater than standard-dose gadofosveset trisodium. High-dose gadofosveset trisodium was equivalent to gadopentetate dimeglumine in image quality and subjective vessel-to-background ratio, but significantly better for depicting small muscular arteries. Standard-dose gadofosveset trisodium showed equivalent image quality and small artery depiction with a slight but significant decrease in vessel-to-background ratio as compared to gadopentatate dimeglumine. Both gadofosveset trisodium doses trended toward more venous enhancement, but this was not a diagnostic problem. Conclusion: First-pass peripheral CE-MRA using gadofosveset trisodium is feasible, yielding image quality comparable to double to triple-dose gadopentetate dimeglumine. Increasing the gadofosveset trisodium dose to 0.05 mmol/kg yields further improvements. J. Magn. Reson. Imaging 2009;30:1085,1092. © 2009 Wiley-Liss, Inc. [source] Synchrotron white-beam X-ray topography of ribonuclease S crystalsACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2002W. M. Vetter With careful experimental design, indexed synchrotron white-beam X-ray topographs of ribonuclease S crystals at ambient temperature could be recorded with a definition and contrast comparable to that of monochromatic beam topographs of other proteins reported in the literature. By excluding wavelengths longer than 1,Å from the white beam with a filter, a radiation dose equivalent to that required to record about 18 topographs could be tolerated without appreciable radiation damage to the samples. Bragg angles of 0.5° or less were required to select low-index harmonically pure reflections with high intensities and extinction lengths only several times the sample's thickness. The resulting X-ray topographs in some cases showed topographic detail and in others showed the even featureless background that has been considered characteristic of a protein crystal of low mosaicity. The ribonuclease S crystals were well ordered single crystals of a quality comparable to other protein crystals that have been studied by X-ray topography. [source] Low radiation dose imaging of myocardial perfusion and coronary angiography with a hybrid PET/CT scannerCLINICAL PHYSIOLOGY AND FUNCTIONAL IMAGING, Issue 1 2009S. Kajander Summary Objectives:, To test the image quality and feasibility of a sequential low radiation dose protocol for hybrid cardiac PET/CT angiography (CTA). Background:, Multidetector computed tomography (MDCT) is a non-invasive method for coronary angiography. The negative predictive value of MDCT is high but perfusion imaging has a role in detecting functional significance of coronary lesions. This has encouraged combining these techniques. However, radiation dose is of concern. We report our first experiences with a low dose sequential CTA mode applicable to hybrid imaging. Methods:, In the first phase, 10 consecutive cardiac MDCT angiographies were performed with spiral acquisition and compared in terms of image quality and dose with the following 10 patients performed with a new sequential mode. In the second phase, feasibility and radiation dose of a combined 15O-water rest-stress PET perfusion/sequential CTA protocol were assessed in another group of 61 consecutive patients. Results:, Mean effective radiation dose was 60% lower in the sequential group than in the spiral group (19·3 versus 7·6 mSv, P<0·001). In the second phase, the new sequential hybrid protocol proved possible in 87% of the patients given the preconditions determined by the manufacturer. Mean effective dose of the CT acquisition was 7·6 mSv and total dose from the PET/CTA hybrid study 9·5 mSv. Conclusion:, Low dose PET/CT allows cardiac hybrid studies with <10 mSv. The protocol can be applied to almost nine out of 10 patients with CT image quality comparable to spiral acquisition. 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