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951.
SW Kim SS Cha HS Cho JS Kim NC Ha MJ Cho S Joo KK Kim KY Choi BH Oh 《Canadian Metallurgical Quarterly》1997,36(46):14030-14036
Bacterial Delta5-3-ketosteroid isomerase (KSI) catalyzes a stereospecific isomerization of steroid substrates at an extremely fast rate, overcoming a large disparity of pKa values between a catalytic residue and its target. The crystal structures of KSI from Pseudomonas putida and of the enzyme in complex with equilenin, an analogue of the reaction intermediate, have been determined at 1.9 and 2.5 A resolution, respectively. The structures reveal that the side chains of Tyr14 and Asp99 (a newly identified catalytic residue) form hydrogen bonds directly with the oxyanion of the bound inhibitor in a completely apolar milieu of the active site. No water molecule is found at the active site, and the access of bulk solvent is blocked by a layer of apolar residues. Asp99 is surrounded by six apolar residues, and consequently, its pKa appears to be elevated as high as 9.5 to be consistent with early studies. No interaction was found between the bound inhibitor and the residue 101 (phenylalanine in Pseudomonas testosteroni and methionine in P. putida KSI) which was suggested to contribute significantly to the rate enhancement based on mutational analysis. This observation excludes the residue 101 as a potential catalytic residue and requires that the rate enhancement should be explained solely by Tyr14 and Asp99. Kinetic analyses of Y14F and D99L mutant enzymes demonstrate that Tyr14 contributes much more significantly to the rate enhancement than Asp99. Previous studies and the structural analysis strongly suggest that the low-barrier hydrogen bond of Tyr14 (>7.1 kcal/mol), along with a moderate strength hydrogen bond of Asp99 ( approximately 4 kcal/mol), accounts for the required energy of 11 kcal/mol for the transition-state stabilization. 相似文献
952.
Byeong Yeon Moon Jong Hyun Choi Jae Gak Kim Jin Jang Dae Won Kim Sang Soon Bae Kyung Shik Yoon 《Solar Energy Materials & Solar Cells》1997,49(1-4)
We have studied the fabrication of amorphous silicon (a-Si : H) p-i-n solar cells using an ion shower doped n+-layer. The p-i-n cells with ion-doped n+-layer exhibited open-circuit voltage of > 0.8 V, fill factor of > 0.62 and conversion efficiency of > 8.4% when the ion acceleration voltage was between 3 and 7 kV. The a-Si : H p-i-n solar cell fabricated under an optimized ion-doping condition exhibited an open-circuit voltage of 0.84 V, a fill factor of 0.66 and a conversion efficiency of 9.9% which was very similar to those of conventional a-Si : H p-i-n cells fabricated in the same deposition chamber. Therefore, ion shower doping technique can be applied to fabricate large area, high performance a-Si : H p-i-n solar cells. 相似文献
953.
954.
The hydrodechlorination of 1,2‐dichloropropane (DCPA), a chlorinated organic waste which is produced in the epichlorohydrin
process, to propylene was carried out over Ni/SiO2 catalysts. The effects of Ni loading and calcination temperature on catalyst performance and catalyst deactivation of Ni/SiO2 were systematically investigated. The Ni/SiO2 catalysts efficiently converted DCPA into propylene in 95% selectivity or higher. The particle size of Ni on SiO2 was strongly related to the catalyst stability. In terms of the effect of Ni loading, the largest Ni particles on SiO2 showed the best durability against deactivation. A series of TPR and UV‐DRS measurements revealed that nickel hydrosilicate
was formed as the result of the interaction between Ni and SiO2. Nickel hydrosilicate was found to be responsible for the catalyst stability leading to low catalyst deactivation. HCl adsorption
on Ni/SiO2 was the main reason for catalyst deactivation. HCl modified the crystal structure of metallic Ni to NiCl2 and led to irreversible deactivation and metal sintering.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
955.
The low cycle fatigue tests of Ni-base superalloy Hastelloy X have been carried out in the temperature range of 650 - 870 °C with various total strain ranges. A change of slope in the Coffin-Manson (C-M) plot was found at 870 °C: the fatigue life significantly decreased at the total strain range less than 0.6%. The fatigue cracks initiated at the surface of the specimens and propagated transgranularly, regardless of test condition. However, the fatigue crack initiation site on the surface shifted from grain interior to grain boundary (GB) predominantly when the discontinuity of slope in the C-M plot began to occur. The fatigue crack tended to initiate preferentially at the oxidized GB rather than grain interior at 870 °C with total strain range below 0.6%. Under this condition, cyclic stability was pronounced, while cyclic hardening occurred at the rest of test conditions. The dislocation structures responsible for the cyclic stress response may partly account for the determination of major fatigue crack initiation site. 相似文献
956.
In this paper, low-velocity impact response and damage of composite laminates is analytically investigated. A modified displacement field of the plate considering initially loaded in-plane strain is proposed. From the displacement field, a finite element equation on the structural behavior of composite laminate is newly induced and a computational program is coded. Numerical results using the FEM code is compared with the numerical ones from reference. Additional numerical analysis is performed on another impact condition, and the effect of initial in-plane load is investigated. Potential delamination damage area in the first inter-ply surface from the bottom of the laminate is approximately estimated, and the effect of the initial in-plane load and the impact condition are also investigated. Consequently, it may be concluded that the initial in-plane load of the laminate does not affect so much on the impact damage area of the laminate. 相似文献
957.
Won-Yong Choi Sun-Mog Hong 《International Journal of Control, Automation and Systems》2009,7(6):922-929
The local search in tracking with a phased array radar can be characterized in terms of the events that can occur in the search
and the probabilities of those events. In predicting the event probabilities, it is required to evaluate the expectation of
a product of target detection probabilities for repeated beam-positioning in the local search. In this paper, we present a
model of the local search and derive an expression for the expectation of the detection-probability product. An expression
of angular measurement accuracy of target detection is also presented. Evaluation of the expressions requires numerical integration,
which is computationally expensive. Approximations to the expressions are presented in analytic form, which are fairly accurate. 相似文献
958.
A self-biasing magnetostrictive element for a magnetomechanical Electronic Article Surveillance (EAS) marker is fabricated
by annealing a ribbon of ferromagnetic amorphous material (Co47.4Fe31.6Si2Bi19) in a presence of magnetic field applied in a transverse direction relative to the ribbon’s longitudinal axis, and then annealing
in a presence of magnetic field applied in the direction of the longitudinal axis. The two-step annealed ribbon exhibits remanent
magnetization along the longitudinal axis and has plural magnetic domains situated along the longitudinal axis. The magnetic
orientation of each domain is canted by ± 03B8; < 90° from the ribbon axis with the direction of canting alternating from
domain to domain. To obtain such a canted anisotropy, theoretical background for the calculation of the field strength (H)
and canted angle (θ) is described. 相似文献
959.
A regular solution model for the difference of the chemical free energy between γ and ε phases during γ→ε martensitic transformation in the Fe-Mn binary system has been reexamined and partly modified based on many articles concerning
the M
s
and A
s
temperatures of Fe-Mn alloys. Using the regular solution model, the measured M
s
temperatures, and a thermodynamic model for the stacking fault energy (SFE) of austenite (γ), the driving force for γ→ε martensitic transformation, and the SFE of γ have been calculated. The driving force for γ→ε martensitic transformation increases linearly from − 68 to − 120 J/mole with increasing Mn content from 16 to 24 wt pct.
The SFE of γ decreases to approximately 13 at. pct Mn and then increases with increasing Mn content, which is in better agreement with
Schumann’s result rather than Volosevich et al.’s result. 相似文献
960.
Young Soon Chang In Su Choi Do Sun Bai 《Quality and Reliability Engineering International》2002,18(5):383-393
This paper proposes a new method of constructing process capability indices (PCIs) for skewed populations. It is based on a weighted standard deviation method which decomposes the standard deviation of a quality characteristic into upper and lower deviations and adjusts the value of the PCI using decomposed deviations in accordance with the skewness estimated from sample data. For symmetric populations, the proposed PCIs reduce to standard PCIs. The performance of the proposed PCIs is compared with those of standard and other PCIs, and finite sample properties of the estimates are investigated using Monte Carlo simulation. Numerical studies indicate that considerable improvements over existing methods can be achieved by the use of the weighted standard deviation method when the underlying distribution is skewed. Copyright © 2002 John Wiley & Sons, Ltd. 相似文献