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981.
Given a graph G=(V,E) and a positive integer D , we consider the problem of finding a minimum number of new edges E' such that the augmented graph G'=(V,E\cup E') is biconnected and has diameter no greater than D. In this note we show that this problem is NP-hard for all fixed D , by employing a reduction from the DOMINATING SET problem. We prove that the problem remains NP-hard even for forests and
trees, but in this case we present approximation algorithms with worst-case bounds 3 (for even D ) and 6 (for odd D ). A closely related problem of finding a minimum number of edges such that the augmented graph has diameter no greater than
D has been shown to be NP-hard by Schoone et al. [21] when D=3 , and by Li et al. [17] when D=2.
Received April 19, 1999; revised June 5, 2001. 相似文献
982.
Numerical simulation of Zn coating solidification 总被引:2,自引:0,他引:2
A. Sémoroz Ph.D L. Strezov M. Rappaz 《Metallurgical and Materials Transactions A》2002,33(8):2685-2694
A numerical model, which simulates nucleation and growth of Zn grains, has been developed in order to describe quantitatively
the solidification of Zn coatings during the hot-dipping process. The inputs of the model are the nucleation distribution,
which has been measured by electron backscattered diffraction (EBSD), and the dendritic growth kinetics, calculated with an
analytical model of a parabolic dendrite tip modified to account for the interactions with the coating interfaces. The model
predicts the shapes of the grain envelopes as a function of the grain orientation and the texture induced by growth. Three
types of grain envelopes have been evidenced, depending on the angle between the c-axis and the normal to the coating plane.
Moreover, it has been shown that growth reinforces the already existing {00.1} nucleation texture, in good agreement with
experimental data. The model also predicts the cooling curve, including recalescence, and the grain size. Thus, it is used
to describe the effects of Pb additions on solidification. In particular, it has been shown that Pb increases the nucleation
undercooling and strongly decreases the density of active nuclei, thus resulting in a much larger grain size. 相似文献
983.
Incremental transmission loss analysis has been used for decades, but recent interest in its application to loss allocation calls for new in-depth results. This paper demonstrates that, for incremental methods to be applied correctly in loss allocation, it is first necessary to specify the load distribution and loss supply strategies. Incremental loss allocation among bus power injections is shown to be arbitrary and, therefore, open to challenge as discriminatory. Loss allocation is possible among incremental loads and/or generators, but the proportion of the total losses assigned to either one is arbitrary. Unique, nonarbitrary incremental loss allocations are however possible among the "equivalent" incremental bilateral exchanges between generators and loads. From these basic components it is possible then to calculate the allocation among generators or loads in any specified proportion. The main results, although developed initially for small increments, are extended to large variations. Finally, a general incremental loss allocation algorithm is developed and tested 相似文献
984.
ABSTRACT: The thermal stability of Bacillus licheniformis α-amylase at low moisture content was studied, based on isothermal experiments performed in a temperature range 113 to 125 °C. The thermal inactivation was monitored by measuring the decrease in thermal denaturation enthalpy and/or the decrease in enzymic activity on pnitrophenyl-α-D-maltoheptaoside, or on starch as a substrate. Based on enthalpy readings, an enzymic system with a z-value of 10.4 °C was observed when using a relative humidity of 81% at 4 °C. A theoretical study showed that this system could be used as a Time Temperature Integrator (TTI) to monitor the safety of sterilization processes of numerous food products. 相似文献
985.
Androstenedione labeled with tritium at positions 1 and 2 was prepared. The molar radioactivity of the sample prepared using homogeneous catalysts was as high as 1.4-1.5 PBq mol-
1. [1,2-3H]Androst-4-ene-3,17-dione can be converted into the [1-3H]steroid with the molar radioactivity of 0.9 PBq mol-
1. 相似文献
986.
987.
M. Ravi U. T. S. Pillai B. C. Pai A. D. Damodaran E. S. Dwarakadasa 《Metallurgical and Materials Transactions A》2002,33(2):391-400
The effect of Fe content (0.2 to 0.6 pct) on the microstructure and mechanical properties of a cast Al-7Si-0.3Mg (LM 25/356)
alloy has been investigated. Further, 1 pct mischmetal (MM) additions (a mixture of rare-earth (RE) elements) were made to
these alloys, and their mechanical properties at room and at elevated temperatures (up to 200 °C) were evaluated. A structure-property
correlation on this alloy was attempted using optical microstructure analysis, fractographs, X-ray diffraction, energy-dispersive
analysis of X-rays (EDX), and quantitative metallography by image analysis. An increase in Fe content increased the volume
percentage of Fe-bearing intermetallic compounds (β and π phases), contributing to the loweryield strength (YS), ultimate tensile strength (UTS), percentage elongation, and higher
hardness. An addition of 1 pct MM to the alloys containing 0.2 and 0.6 pct Fe was found to refine the microstructure; modify
the eutectic silicon and La, Ce, and Nd present in the MM; form different intermetallic compounds with Al, Si, Fe, and Mg;
and improve the mechanical properties of the alloys both at room and elevated temperatures. 相似文献
988.
Field R. D. Hartwig K. T. Necker C. T. Bingert J. F. Agnew S. R. 《Metallurgical and Materials Transactions A》2002,33(13):965-972
The equal-channel angular extrusion (ECAE) technique has been applied to a powder metallurgy (P/M) source Be alloy. Extrusions
have been successfully completed on Ni-canned billets of Be at approximately 425°C. No cracking was observed in the billets,
and significant grain refinement was achieved. In this article, microstructural features and dislocation structures are discussed
for a singlepass extrusion, including evidence of <c> and <c+a> dislocations. Significant crystallographic texture developed during ECAE, which is discussed in terms of this unique deformation
processing technique and the underlying physical processes which sustain the deformation.
S.R. AGNEW, formerly with the Oak Ridge National Laboratory, Oak Ridge, TN 37831-6115
This article is based on a presentation made in the symposium entitled “Defect Properties and Mechanical Behavior of HCP Metals
and Alloys” at the TMS Annual Meeting, February 11–15, 2001, in New Orleans, Louisiana, under the auspices of the following
ASM committees: Materials Science Critical Technology Sector, Structural Materials Division, Electronic, Magnetic & Photonic
Materials Division, Chemistry & Physics of Materials Committee, Joint Nuclear Materials Committee, and Titanium Committee. 相似文献
989.
It is essential in the simulation of power electronics applications to model magnetic components accurately. In addition to modeling the nonlinear hysteresis behavior, eddy currents and winding losses must be included to provide a realistic model. In practice the losses in magnetic components give rise to significant temperature increases which can lead to major changes in the component behavior. In this paper a model of magnetic components is presented which integrates a nonlinear model of hysteresis, electro-magnetic windings and thermal behavior in a single model for use in circuit simulation of power electronics systems. Measurements and simulations are presented which demonstrate the accuracy of the approach for the electrical, magnetic and thermal domains across a variety of operating conditions, including static thermal conditions and dynamic self heating 相似文献
990.
A quantum well (QW) in the simultaneous presence of a terahertz field polarized in the growth direction and an incident optical field near an excitonic resonance results in substantial frequency mixing between the terahertz and optical fields. In particular, a response at new frequencies given by the input optical frequency plus or minus multiples of the terahertz frequency occurs-the terahertz sidebands. In a symmetric QW, the dominant contribution to terahertz-sideband formation is the high-frequency modulation of the overlap integral of the relevant conduction- and valence-subband envelope functions that determine the strength of the interband dipole moment. terahertz-sideband generation is shown to be strongly enhanced in a high quality-factor optical microcavity. Numerical values of the reflected intensity into the first terahertz sideband normalized with respect to the reflected intensity at the fundamental as large as /spl sim/10% are estimated. This suggests that terahertz-sideband generation in semiconductor microcavities is a promising option worthy of exploration for wavelength conversion for wavelength-division multiplexing applications. 相似文献