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   243篇
  免费   12篇
  国内免费   2篇
化学   199篇
数学   25篇
物理学   33篇
  2024年   1篇
  2023年   5篇
  2022年   16篇
  2021年   15篇
  2020年   10篇
  2019年   9篇
  2018年   4篇
  2017年   7篇
  2016年   14篇
  2015年   8篇
  2014年   5篇
  2013年   14篇
  2012年   14篇
  2011年   19篇
  2010年   12篇
  2009年   10篇
  2008年   24篇
  2007年   20篇
  2006年   15篇
  2005年   9篇
  2004年   6篇
  2003年   4篇
  2002年   1篇
  2001年   2篇
  2000年   3篇
  1999年   1篇
  1998年   3篇
  1997年   1篇
  1996年   2篇
  1994年   1篇
  1992年   2篇
排序方式: 共有257条查询结果,搜索用时 31 毫秒
121.
This paper presents a new multi-objective approach to a single machine scheduling problem in the presence of uncertainty. The uncertain parameters under consideration are due dates of jobs. They are modelled by fuzzy sets where membership degrees represent decision maker’s satisfaction grade with respect to the jobs’ completion times. The two objectives defined are to minimise the maximum and the average tardiness of the jobs. Due to fuzziness in the due dates, the two objectives become fuzzy too. In order to find a job schedule that maximises the aggregated satisfaction grade of the objectives, a hybrid algorithm that combines a multi-objective genetic algorithm with local search is developed. The algorithm is applied to solve a real-life problem of a manufacturing pottery company.  相似文献   
122.
123.
Mycosporine-like amino acids (MAAs) are ubiquitous photoprotective compounds in aquatic environments. MAAs are synthesized by a wide variety of organisms (i.e. bacteria, fungi and algae) and their production is photoinducible by ultraviolet radiation (UVR) (280–400 nm) and/or photosynthetically active radiation (400–750 nm). Most animals however, are unable to synthesize MAAs and must acquire these compounds through their diet or from symbiotic organisms. In this paper, we investigate the possible sources of MAAs and factors (temperature and initial MAA concentration) that may affect their bioaccumulation in freshwater copepods. We found that MAA accumulation may occur even if the copepods are cultured on a MAA-free diet. In addition, we found that the bacteriostatic antibiotic, chloramphenicol, inhibits the bioaccumulation of MAAs. These two pieces of evidence suggest that the source of MAAs in these copepods may be prokaryotic organisms in close association with the animals. The two factors investigated in this study, temperature and initial MAA concentrations, were found to affect the rates at which MAAs are accumulated. Temperature had positive effects on both uptake and elimination rates. On the other hand, the rate of uptake decreased at the highest assayed initial MAA concentration, probably because the concentration of MAAs was already close to saturation.  相似文献   
124.
The purpose of this study was to assess the stability of the polyfructan levan under different pH solution conditions by monitoring changes in the levan physicochemical properties, such as molar mass (M), root mean square radius (r rms ), hydrodynamic radius (r h ), structure factor (r rms /r h ), and aggregation state with respect to solution pH and hydrolysis time. A commercial levan produced from Z. Mobilis was characterized using asymmetric flow field-flow fractionation (AF4) in combination with online multiangle light scattering (MALS) and differential refractive index (dRI) detection. Under neutral pH solution conditions the levan was found to have a M ranging from 105 to 5?×?107 g/mol, a r rms ranging from ~25 to 100 nm and a r h from ~3 to 151 nm. Two populations were observed in the sample. One population with a M less than 106 g/mol which represented ~60 % of the sample and a second population with an ultrahigh M up to 5?×?107 g/mol, which comprised ~40 % of the sample. The measured r rms /r h structure factor decreased from 1.8 to 0.65 across the AF4 fractogram indicating that early eluting low M levan species had a random coil configuration and late eluting high M species had more homogeneous spherical structures. The measured apparent density values decreased from 80 to 10 kg/m3 across the elution profile and suggest that the observed second population also contains aggregates. The stability of levan in different pH conditions ranging from 1.3 to 8.5 was assessed by tracking changes in the average M and r h , and monitoring the formation of fructose over 1 week. The onset of levan acid hydrolysis was observed to occur sooner at lower pH conditions and no hydrolysis was observed for pH 5.5 and higher.  相似文献   
125.
Abstract

The aim of the present study was to perform a screening of extracts obtained from 15 medicinal plants using water (at 25 and 90?°C) or ethanol (at 25?°C), to bactericidal activity against cariogenic S. mutans ATCC 25175, as well as to carry out the preliminary phytochemical characterization of the extracts and HPLC/MS assay for selected extracts. The extractions were carried out for 5?h at 400?rpm. Only five from 45 tested extracts were selected based on their antibacterial activity. The IC50 varied from of 28?ppm for Quercus ilex up to 250?ppm for Jatropha dioica. Different polyphenolic and quinic acids, flavonoids, anthocyanin or tyrosol were identified by HPLC/MS in selected extracts from Rosa gallica L., Jatropha dioica Sessé, Mimosa tenuiflora (Willd.) Poir, Quercus ilex L., and Solanum nigrum. The obtained results confirm that selected extracts are good candidates to be used for cariogenic bacteria control.  相似文献   
126.
DNA Lattices carrying two cytosine residues bearing the N4‐[(tert‐butyldisulfanyl)ethyl] group at the apex of hairpin topological markers are prepared for first time. We show that these residues are important for the deposition of DNA lattices on gold surfaces.  相似文献   
127.
The electron impact mass spectral fragmentation pathways for a series of 1,3‐di‐ and 1,2,3‐trisubstituted imidazolidines were investigated. The data show remarkable differences depending on the location and identity of substituents. The atypical 1,2,2,3‐tetrasubstituted imidazolidine, 2‐cyano‐1‐methyl‐3‐(p‐nitro‐phenyl)‐2‐phenylimidazolidine, was also investigated.  相似文献   
128.
Improvement in the design of the source and the transport system in a multipurpose gamma irradiation plant is of fundamental importance. With this in mind we propose the implementation of a mathematical model permitting the improved design of gamma irradiators as a function of the efficiency of the facility and the parameters of the irradiation process. A linear relationship between the height of the container and the dose ratio can be deduced from the analysis of the results. Such relationship provides a useful and economic tool for calculating the most advantageous dimensions of the source and the volume of the material to be irradiated.  相似文献   
129.
Raman and surface-enhanced Raman spectra of new DNA/RNA-binding compounds consisting of three imidazole (Im) and three pyridine (Py) rings connected by tripodal polyaminomethylene linkages were obtained by the near-infrared excitation at 1064 nm. Study of interactions of Im and Py polyamines with single-stranded RNA polynucleotides (poly?A, poly?G, poly?C, poly?U), double-stranded DNA polynucleotides (poly?dAdT-poly?dAdT, poly?dGdC-poly?dGdC) and calf thymus DNA (ct-DNA) by surface-enhanced Raman spectroscopy (SERS) reveals unambiguous enhancement of the Raman scattering from the small molecules as well as appearance of new bands in spectra associated mainly with nucleobases. The SERS experiments point toward comparable interactions of Im and Py polyamines with single-stranded purine and pyrimidine polynucleotides. Furthermore, SERS experiments with double stranded polynucleotides reveal the base-pair dependent selectivity of Im and Py, whereby interactions within both, major and minor groove are indicated for poly?dAdT-poly?dAdT, at variance to preferred binding of Im and Py to only major groove of poly?dGdC-poly?dGdC. SERS spectra of Im and Py with ct-DNA imply that protonated amino groups of these compounds preferentially interact with N7 atoms (adenine, guanine) while nitrogen in aromatic rings of polyamines might be attracted to C6-NH(2) (adenine), all sites being located at the major groove of the DNA helix. Wavenumber downshift of the imidazole (Im) and pyridine (Py) ring vibrations supports aromatic stacking interactions of imidazole and pyridine aromatic moieties with DNA base-pairs.  相似文献   
130.
A protocol for stereochemical analysis, based on the systematic comparison between theoretical and experimental vicinal (1)H-(1)H NMR coupling constants, was developed and applied to a series of flexible compounds (1-8) derived from the 6-heptenyl-5,6-dihydro-2H-pyran-2-one framework. The method included a broad conformational search, followed by geometry optimization at the DFT B3LYP/DGDZVP level, calculation of the vibrational frequencies, thermochemical parameters, magnetic shielding tensors, and the total NMR spin-spin coupling constants. Three scaling factors, depending on the carbon atom hybridizations, were found for the (1)H-C-C-(1)H vicinal coupling constants: f((sp3)-(sp3)) = 0.910, f((sp3)-(sp2)) = 0.929, and f((sp2)-(sp2))= 0.977. A remarkable correlation between the theoretical (J(pre)) and experimental (1)H-(1)H NMR (J(exp)) coupling constants for spicigerolide (1), a cytotoxic natural product, and some of its synthetic stereoisomers (2-4) demonstrated the predictive value of this approach for the stereochemical assignment of highly flexible compounds containing multiple chiral centers. The stereochemistry of two natural 6-heptenyl-5,6-dihydro-2H-pyran-2-ones (14 and 15) containing diverse functional groups in the heptenyl side chain was also analyzed by application of this combined theoretical and experimental approach, confirming its reliability. Additionally, a geometrical analysis for the conformations of 1-8 revealed that weak hydrogen bonds substantially guide the conformational behavior of the tetraacyloxy-6-heptenyl-2H-pyran-2-ones.  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