首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   94篇
  免费   6篇
化学   54篇
力学   5篇
数学   12篇
物理学   29篇
  2023年   2篇
  2022年   2篇
  2021年   2篇
  2020年   5篇
  2019年   4篇
  2018年   3篇
  2017年   3篇
  2016年   7篇
  2015年   6篇
  2014年   3篇
  2013年   5篇
  2012年   7篇
  2011年   9篇
  2010年   5篇
  2009年   3篇
  2008年   11篇
  2007年   5篇
  2006年   3篇
  2005年   1篇
  2004年   2篇
  2003年   1篇
  2002年   1篇
  2001年   1篇
  1999年   1篇
  1997年   1篇
  1996年   1篇
  1993年   1篇
  1991年   1篇
  1990年   1篇
  1978年   1篇
  1969年   1篇
  1968年   1篇
排序方式: 共有100条查询结果,搜索用时 0 毫秒
61.
Two new pseudoguaianolide‐type sesquiterpene lactones, named deacetyltetraneurin A ( 3 ) and hysterone E ( 4 ), were isolated from flowers of the plant Parthenium hysterophorus L., along with the seven known compounds parthenin ( 1 ), coronopilin, tetraneurin A, 8‐β‐hydroxycoronopilin, scopoletin, hysterone D, and conchasin A ( 2 ). The structures of all the compounds were deduced by means of elemental analysis and extensive spectroscopic (1D and 2D NMR) studies, and confirmed for 3 and 4 by X‐ray crystallographic analysis.  相似文献   
62.
We study the problem of allocating a limited quantity of a single manufacturing resource to produce a subset of possible part-types. Customer orders require one or more part-types. We assume that revenue is received for an order only if it is completely filled, and that set-up costs and order revenues dominate the variable costs of production. We present a heuristic for the solution of our problem, as well as families of cutting-planes for an integer programming formulation. Computational results on a set of random test problems indicate that the heuristic is quite effective in producing near optimal solutions. The cutting-planes appear to be quite useful in reducing the number of linear programming solutions required by branch-and-bound.  相似文献   
63.
Supply chain planning as one of the most important processes within the supply chain management concept, has a great impact on firms’ success or failure. This paper considers a supply chain planning problem of an agile manufacturing company operating in a build-to-order environment under various kinds of uncertainty. An integrated optimization approach of procurement, production and distribution costs associated with the supply chain members has been taken into account. A robust optimization scenario-based approach is used to absorb the influence of uncertain parameters and variables. The formulation is a robust optimization model with the objective of minimizing the expected total supply chain cost while maintaining customer service level. The developed multi-product, multi-period, multi-echelon robust mixed-integer linear programming model is then solved using the CPLEX optimization studio and guidance related to future areas of research is given.  相似文献   
64.
A copper-catalyzed condensation reaction of oxime acetates and α,β-unsaturated ketimines to give pyridine derivatives is reported. The reaction features mild conditions, high functional-group compatibility, and high regioselectivity with respect to unsymmetrical oxime acetates, thus allowing the preparation of a wide range of polysubstituted pyridines, many of which are not readily accessible by conventional condensation methods.  相似文献   
65.
66.
All the eight stereoisomers of 3-acetoxy-11,19-octacosadien-1-ol (1), the male sex pheromone (CH503) of Drosophila melanogaster, were synthesized from two acetylenic starting materials and the enantiomers of 3,4-epoxy-1-butanol PMB ether. Complete separation of the eight isomers of 1 by reversed phase HPLC at ?20 °C was achieved after their esterification with (1R,2R)-2-(2,3-anthracenedicarboximido)cyclohexanecarboxylic acid (27), and the natural CH503 was found to be (3R,11Z,19Z)-1.  相似文献   
67.
To date, the complex evaluation of physical and dispersive optical characteristics of the ZrON/Si film system has yet been reported. Hence, ZrON thin films have been formed on Si(100) substrates through oxidation/nitridation of sputtered metallic Zr in N2O environment at 500, 700, and 900 °C. Physical properties of the deposited films have been characterized by X-ray diffractometry (XRD), Fourier transform infrared (FTIR) spectroscopy, reflection high-energy electron diffraction (RHEED), and spectroscopic ellipsometry (SE). It has been shown that ZrON/Si thin films without optical absorption can be prepared by oxidation/nitridation reaction in N2O environment at 700–900 °C.  相似文献   
68.
69.
There are various concerns with current sunscreen actives in the sun care industry which pushes for a natural-based active that is sustainable as well as effective. Lignin is a natural polymer, capable of ultraviolet (UV) filtration and maintaining photostability. In this study, a variety of tuneable lignin-poly(ethylene glycol) (LP) nanodiamond (LP–ND) complexes were developed with varying ratios of LP to NDs. There are two fractions of LP–NDs, which use detonation NDs (LP–rNDs) and furnace NDs (LP–mNDs). Both complexes demonstrated favorable UV filtration, enhanced photostability, and uniform dispersion in water and cream. The LP–mND had maintained its particle size at <200 nm with an increasing LP ratio from 0.5 to 20 times as opposed to the increased particle size of the LP–rND. The LP–mND complex excelled in its photostability during solar radiation from 1 h to 6 h, demonstrating its effectiveness in absorbing 70% in the UV B region. Results had indicated enhanced synergistic effect of LP–rNDs and LP–mNDs with commercial sunscreen. The LP–rND complex had enhanced the sunscreen from 29 to 62, whereas the LP–mND had further boosted the sun protection factor to 89. In a nutshell, the lignin nanodiamond complexes offer a sustainable alternative to current actives in the industry.  相似文献   
70.
In this paper, the feasibility and advantage of employing FTIR spectroscopy and the corresponding second-derivative spectroscopy combined with 2D-IR spectroscopy for the analysis of water extracts of Ganoderma lucidum with different storage durations were investigated and demonstrated for the first time. In order to compare the second-derivative microscopic fingerprint spectra from the four periods of storage, the range from 1200 to 500 cm−1 has to be ignored due to the detection of starch in the samples and the standard as well. This is crucial because the main characteristic band of polysaccharide was assigned within this range. The addition of starch as outer constituent was considered spoilage of polysaccharide content investigation. In fact, polysaccharide content in G. lucidum plays a main role as anti-cancer properties. The ranges from 1480 to 1200 cm−1 and from 1700 to 1480 cm−1 were interpreted and directly compared. For the range (from 1800 to 400 cm−1), the 22 months sample was closest to the control, followed by 38 months, 13 months and 6 months product. The matching of the extract spectra with the control was not consistent and depended on which range of spectra had been chosen. Many factors can be considered which may have possibly affected inferences of the composition of the extract from different storage durations.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号