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排序方式: 共有512条查询结果,搜索用时 15 毫秒
91.
92.
A problem that always annoys building employers and their consultants when drafting building contracts is whether it is more cost efficient to retain certain risks with themselves or to transfer the same to contractors, and it has long been a difficulty to accurately estimate the cost for transferring risks to the other contract party. It is a very common market practice to remove contractors’ entitlement to extension of time due to inclement weather, and in view of the significant impacts that inclement weather posed on construction progress, it would be helpful to assess the associated costs for such risk allocation exercises in order to ensure that it is a cost-effective transaction. The costs for these potential weather-caused delays are priced by contractors in their returned tenders. This risk-pricing behavior is affected by the contractor’s risk perception and risk attitude. However, merely working out contractors’ risk perception and risk attitude patterns is inadequate, as they may vary in accordance with different organizational sizes and project backgrounds. Therefore, further studies should be carried out on the implications of project information, such as project value and contract period, on different-sized contractors’ risk behaviors. In this research, a questionnaire survey was conducted to investigate the weather-risk-pricing behavioral patterns of various-sized contractors under different given project scenarios. The survey revealed that the impacts of project value and contract period (and, thus, the intensity of work and scale of exposure to weather risk) were significant on contractors’ risk behaviors, and these impacts varied in accordance with different organizational sizes. The medium-sized contractors’ pricing behaviors were rather constant under different project values and contract periods, while both small and large contractors behaved differently when the intensity of work and scale of risk exposure varied. Despite the fact that contractors of various sizes behaved differently in some given scenarios, the number of days of delaying cost allowed by them in returned tenders were generally less than the actual number of days of inclement weather affecting work. 相似文献
93.
The variations of the dimensions and locations of two stacked parasitic elements on the input impedance SWR bandwidth and radiation patterns of aperture-coupled rectangular microstrip antennas are analyzed. It is found that the SWR bandwidth can be improved by selecting appropriate sizes and locations of the parasitic patches. A 45% bandwidth of patch antennas was attained for SWR≤2.2. Experimental results are obtained to verify the theory. © 1998 John Wiley & Sons, Inc. Int J RF and Microwave CAE 8: 49–55, 1998. 相似文献
94.
Mingyu LI Hongjun JI Chunqing WANG Au Tai KUNG Han Sur BANG Hee Seon BANG 《材料科学技术学报》2006,22(4):483-486
The mechanism of ultrasonic wire wedge bonding, one of the die/chip interconnection methods, was investigated based on the characteristics of the ultrasonic wire bonding joints. The Al-1%Si wire of 25 μm in diameter was bonded on Au/Ni/Cu pad and the joint cross-section was analyzed by scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDX). The results indicated that it is irregular for the ultrasonic bond formation, non-welded at the centre but joining well at the periphery, especially at the heel and toe of the joint. Furthermore, the diffusion and/or reaction at the cross-section interface are not clear at C-zone, while there exists a strip layer microstructure at P-zone, and the composition is 78.96 at. pct Al and 14.88 at. pct Ni, close to the Al3Ni intermetallic compound. All these observations are tentatively ascribed to the plastic flow enhanced by ultrasonic vibration and repeated cold deformation driving interdiffusion between AI and Ni at bond interface. 相似文献
95.
Darko Vrečko Marko Nerat Damir Vrančić Gregor Dolanc Boštjan Dolenc Boštjan Pregelj Fabien Meyer Siu Fai Au Robert Makkus Đani Juričić 《International Journal of Hydrogen Energy》2018,43(12):6352-6363
One of the main challenges for wide-spread utilization of the solid oxide fuel cell (SOFC) power systems is how to achieve high electrical efficiency without increasing the degradation rate of the fuel cells. To run the SOFC power system at high efficiency over a long period of time, properly designed controllers are indispensable.Although a number of various approaches to control SOFC have been proposed so far, it seems that the design of control system, along with simple tuning procedure, has not been treated in a consistent manner. This issue is addressed in the present paper resulting in a feedforward-feedback control structure. The feedforward part is based on the stoichiometry of electro-oxidation, reforming and combustion reactions, which allow immediate response to variable current demand. The feedback part performs additional fine adjustment of fuel and air supply in order to minimize the undesired system temperatures variations. The selection of pairings of manipulated and controlled variables for control is based on physical knowledge of the system. Input/output pairing for single-loop feedback control is assessed by the relative gain analysis. An efficient procedure for tuning the parameters of the feedback controllers is suggested, relying on simple open-loop step responses of the system.The proposed low-level control is assessed on a detailed physical model of a 2.5 kW SOFC power system by simulating two nonstationary load regimes. Simulations show that the control provides a robust operation under large load variations while meeting the operating constraints. Due to its simplicity, the control is feasible for implementation on a real SOFC system. 相似文献
96.
Histology and the microscope have been used to identify Chinese herbal medicines for a long time. However, research on using the microscope for quantitative determination of identification characters is limited. A novel method which combines histological and microscopic analysis of laticifers by "blob" analysis is established to identify Wuzhimaotao, which is derived from species of Ficus (primarily F. hirta Vahl, but F. simplicissima Lour., F. hirta Vahl var. imberbis Gagnep., and F. esquiroliana Lévl. are also used). Results indicate that laticifers, which are stained orange-red by Sudan III, are mainly scattered in the phloem of Wuzhimaotao. The blob area of the laticifers is varied according to the species: F. hirta Vahl, F. simplicissima Lour., F. hirta Vahl var. imberbis Gagnep., and F. esquiroliana Lévl. showed 86,609 +/- 3,768 (mean +/- SD, n = 10), 48,582 +/- 2,603 (n = 10), 68,745 +/- 2,179 (n = 5), and 27,966 +/- 2,121 (n = 3) blob area, respectively. By directly measuring the blob area of laticifers in transverse sections, we could distinguish species of Wuzhimaotao in the same genus which were difficult to distinguish by microscopic examination of the dry roots, and we could provide objective data to describe and standardize the characters observed in microscopic images. This method is rapid, accurate, and inexpensive. 相似文献
97.
CH4/CO2 reforming over La2NiO4 and 10%NiO/CeO2–La2O3 catalysts under the condition of supersonic jet expansion was studied via direct monitoring of the reactants and products using the sensitive technique of cavity ring-down spectroscopy. Vibration–rotational absorption lines of CH4, H2O, CO2 and CO molecules were recorded in the near infrared spectral region. Our results indicated that La2NiO4 is superior to 10%NiO/CeO2–La2O3 in performance. In addition, we observed enhanced reverse-water-gas-shift reaction at augmented reaction temperature. The formation of reaction intermediates was also investigated by means of time-of-flight mass spectrometry and there was the detection of CHx+, OH+ and H+ species. 相似文献
98.
Wei Liu Suk Yin Lai Hongxing Dai Shuiju Wang Haizhen Sun Chak Tong Au 《Catalysis Today》2008,131(1-4):450-456
VOx catalysts supported on SBA-15 with and without MgO modification were prepared and characterized by N2 adsorption–desorption, XRD, HRTEM, H2-TPR, NH3-TPD and XPS. Compared to the VOx/SBA-15 catalyst, the VOx/MgO/SBA-15 ones exhibit much higher C4-olefins selectivity and yield in the oxidative dehydrogenation of n-butane. The enhanced performance can be attributed to the rise in VOx reducibility as well as to the relatively lower acidity of the MgO-modified SBA-15 materials. 相似文献
99.
Gintaras Baltrūnas Aušra Valiūnienė Justas Vienožinskis Ernestas Gaidamauskas Teofilis Jankauskas Žana Margarian 《Journal of Applied Electrochemistry》2008,38(11):1519-1526
Electrochemical gold deposition from sulfite solutions was studied by means of voltammetry, EIS and EQCM. A gold film electrode
was used for polyaniline layer formation by electrochemical oxidation of aniline. The standard electrochemical reduction potential
of the reaction [Au(SO3)2]3− + e− = Au + 2 SO32− was determined, and is equal to 0.116 V (vs. NHE). Both solution stirring and temperature increase accelerate the electrochemical
reduction of gold, when the electrode potential is below −0.55 V. When the potential is above −0.55 V the electrochemical
reduction proceeds via passive layer formation. Our study suggests that the passive layer consists of chemically adsorbed
sulfite ions and sulfur. The gold film deposited from sulfite solution is a high quality substrate suitable for conducting
polymer layer formation. This technique, where a polymer layer electrode is prepared by thin gold film deposition onto a metal
surface and by subsequent polymer layer formation, can be applied in sensor research and technology. 相似文献
100.
Three-dimension ordered macroporous (3D-OM) bismuth vanadates with a monoclinic crystal structure and high surface area (18-24 m(2) g(-1)) have been prepared using ascorbic acid (AA)- or citric acid (CA)-assisted poly(methyl methacrylate) (PMMA)-templating strategy with bismuth nitrate and ammonium metavanadate as the metal sources, HNO(3) as the pH adjuster and ethylene glycol and methanol as the solvent. The materials were characterized by a number of analytical techniques. The photocatalytic performance of the porous BiVO(4) samples was evaluated for the degradation of phenol in the presence of a small amount of H(2)O(2) under visible light illumination. The effects of the initial phenol concentration and the H(2)O(2) amount on the photocatalytic activity of the photocatalyst were examined. It is shown that the chelating agent, AA or CA, and the amount in which it is added had a significant impact on the quality of the 3D-OM structure, with a "(Bi + V):chelating agent" molar ratio of 2:1 being the most appropriate. Among the as-prepared BiVO(4) samples, the one with a surface area of ca. 24 m(2) g(-1) showed the best visible light-driven photocatalytic performance for phenol degradation (phenol conversion = ca. 94% at phenol concentration = 0.1 mmol L(-1) and in the presence of 0.6 mL H(2)O(2)). A higher phenol conversion could be achieved within the same reaction time if the phenol concentration in the aqueous solution was lowered, but an excess amount of H(2)O(2) was not a favorable factor for the enhancement of the catalytic activity. It is concluded that the excellent photocatalytic activity of 3D-OM BiVO(4) is due to the high quality 3D-OM structured BiVO(4) that has a high surface area and surface oxygen vacancy density. We are sure that the 3D-OM material is a promising photocatalyst for the removal of organics from wastewater under visible light illumination. 相似文献