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   1154篇
  免费   27篇
  国内免费   4篇
化学   898篇
晶体学   5篇
力学   8篇
数学   85篇
物理学   189篇
  2021年   7篇
  2020年   8篇
  2019年   11篇
  2018年   9篇
  2017年   9篇
  2016年   22篇
  2015年   17篇
  2014年   21篇
  2013年   46篇
  2012年   55篇
  2011年   73篇
  2010年   36篇
  2009年   38篇
  2008年   56篇
  2007年   79篇
  2006年   78篇
  2005年   50篇
  2004年   59篇
  2003年   67篇
  2002年   63篇
  2001年   20篇
  2000年   18篇
  1999年   16篇
  1998年   15篇
  1997年   8篇
  1996年   22篇
  1995年   5篇
  1994年   20篇
  1993年   12篇
  1992年   16篇
  1991年   15篇
  1990年   10篇
  1989年   9篇
  1988年   13篇
  1987年   9篇
  1986年   11篇
  1985年   21篇
  1984年   18篇
  1983年   5篇
  1982年   14篇
  1981年   13篇
  1980年   11篇
  1979年   13篇
  1978年   15篇
  1977年   5篇
  1976年   5篇
  1973年   7篇
  1972年   4篇
  1969年   7篇
  1968年   4篇
排序方式: 共有1185条查询结果,搜索用时 15 毫秒
991.
We demonstrate memristive operations using gap-type Ag2S atomic switches, in which the growth and shrinkage of an Ag protrusion are controlled by using solid-electrochemical reactions. In addition to conventional memristive operations such as those proposed and demonstrated by resistive random-access memories (ReRAMs) using metal oxide compounds, gap-type Ag2S atomic switches also show new types of memristive operations by storing information from input signals without changing their output until a sufficient number of signals are inputted. The new types of memristive operations resemble the learning process seen in neuroplasticity, where changes occur in the organization of the human brain as a result of experience.  相似文献   
992.
993.
A sensitive and selective preconcentration method using solid-phase extraction (SPE) disk, namely multiwalled carbon nanotubes (MWCNTs) disk, is proposed for the determination of atrazine and simazine in water samples. Atrazine and simazine were extracted on MWCNTs disk and then determined by gas chromatography–mass spectrometry (GC/MS). Several parameters on the enrichment factor of the analytes were investigated. The experimental results showed that it was possible to obtain quantitative analysis when the solution pH was 5 using 200 mL of validation solution containing 0.1 μg of triazines and 5 mL of acetone as an eluent. The maximum enrichment factors for atrazine and simazine were 3900 ± 250 and 4000 ± 110, respectively when 200 mL of sample solution volume was used. Relative standard deviations for seven determinations were 6.9% (atrazine) and 3.0% (simazine) under optimum conditions. The linear range of calibration curves were 0.1 to 1 ng mL− 1 for each analyte with good correlation coefficients. The detection limits (3S/N) were 2.5 and 5.0 pg mL− 1 for atrazine and simazine, respectively. The proposed method was successfully applied to the determination of atrazine and simazine in environmental water samples with high precision and accuracy.  相似文献   
994.
We have developed an efficient total synthesis of (?)‐morphine in 5 % overall yield with the longest linear sequence consisting of 17 steps from 2‐cyclohexen‐1‐one. The cyclohexenol unit was prepared by means of an enzymatic resolution and a Suzuki–Miyaura coupling as key steps. Construction of the morphinan core features an intramolecular aldol reaction and an intramolecular 1,6‐addition. Furthermore, mild deprotection conditions to remove the 2,4‐dinitrobenzenesulfonyl (DNs) group enabled the facile construction of the morphinan skeleton. We have also established an efficient synthetic route to a cyclohexenol unit containing an N‐methyl‐DNs‐amide moiety.  相似文献   
995.
Isomeric oligosaccharides γ‐cyclodextrin (γ‐CD), glucosyl‐βCD (Glc1‐βCD) and maltosyl‐αCD (Glc2‐αCD) were analyzed by traveling‐wave ion mobility (twIM) mass spectrometry (MS). Their formation of multicharged multimers differed from each other. The ion mobility‐mass spectrometry was useful in the self‐assembling and complex formation analyses of CD isomers. The drift times of the isomers and their product ions with the same mass were almost the same in collision‐induced dissociation (CID) MS/MS. In contrast, the ion mobility peak widths were sensitive to structural differences of the isomeric product ions. The twIM peak width (ms ‐ µs) of the product ions [M ? Glcn + H]+ (n = 0 ~ 6) of γ‐CD correlated linearly with their masses (Da); the large and/or long chain product ions had wider peak widths, which were much wider than those from the general diffusion effect. This was a novel and useful ‘trend line’ to discriminate between the three isomers. Plots of [M ? Glc2 ~ 6 + H]+ of Glc1‐βCD and [M ? Glc3 ~ 6 + H]+ of Glc2‐αCD product ions' plots were on the same trend line as γ‐CD. The plots of [M ? Glc1 + H]+ of Glc1‐βCD and [M ? Glc1, 2 + H]+ of Glc2‐αCD strayed from the γ‐CD line; their peak widths were narrower than those of γ‐CD. These results indicated that product ions from the chemical species of Glc1‐β CD and Glc2‐αCD retained their CD structure. Analyses of the IM peak widths enable us to elucidate the structures of the product ions. Copyright © 2009 John Wiley & Sons, Ltd.  相似文献   
996.
A complex rotation : The solution conformation of spermidine (SPD) was elucidated by using diastereospecifically deuterated and 13C‐labeled derivatives to diagnose the orientation of seven conformationally relevant bonds. NMR coupling constants were determined for a complex formed from spermidine and adenosine triphosphate (ATP; see figure).

  相似文献   

997.
The synthesis of pipecolic acid and homopipecolic acid derivatives was developed from ω-(2-aminophenyl)-1-chloroalkyl p-tolyl sulfoxides by treatment with i-PrMgCl. An intramolecular nucleophilic substitution reaction of a magnesium carbenoid with an N-magnesio arylamine is the key step of this reaction. Proline and pipecolic acid derivatives were also synthesized from ω-(arylamino)-1-chloroalkyl p-tolyl sulfoxides by the same chemistry. Starting from enantiomerically pure (1S,RS)-1-chloro-3-[2-(N-methylamino)phenyl]propyl p-tolyl sulfoxide, enantiomerically pure (R)-pipecolic acid derivative was obtained. The intramolecular nucleophilic substitution reaction of the magnesium carbenoid with N-magnesio arylamine was proven to take place with inversion of the carbenoid carbon. The stereochemistry of these reactions is also discussed.  相似文献   
998.
Anion-exchange (AEX) columns were prepared by on-column polymerization of acrylates and methacrylates containing tertiary amino or quaternary ammonium groups on monolithic silica in a fused silica capillary modified with anchor groups. The columns provided a plate height (H) of less than 10 μm at optimum linear velocity (u) with keeping their high permeability (K = 9–12 × 10−14 m2). Among seven kinds of AEX columns, a monolithic silica column modified with poly(2-hydroxy-3-(4-methylpiperazin-1-yl)propyl methacrylates) (HMPMA) showed larger retentions and better selectivities for nucleotides and inorganic anions than the others. The HMPMA column of 410 mm length produced 42 000–55 000 theoretical plates (N) at a linear velocity of 0.97 mm/s with a backpressure of 3.8 MPa. The same column could be employed for a fast separation of inorganic anions in 1.8 min at a linear velocity of 5.3 mm/s with a backpressure of 20 MPa. In terms of van Deemter plot and separation impedance, the HMPMA column showed higher performance than a conventional particle-packed AEX column. The HMPMA column showed good recovery of a protein, trypsin inhibitor, and it was applied to the separation of proteins and tryptic digest of bovine serum albumin (BSA) in a gradient elution, to provide better separation compared to a conventional particle-packed AEX column.  相似文献   
999.
Fluorescence spectra of two fulvic acid (FA) samples, FA0 from underground water and FA1 from forest soil, were recorded in various surfactant solutions. Alkyltrimethylammonium ions with different alkyl chain lengths induced a decrease in the fluorescence intensity for both FAs at concentrations below the critical micelle concentration (cmc) and an enhancement above the cmc. The intensity minimum thus obtained at the cmc was deeper for surfactants with longer alkyl chains. This effect was attributable to the formation of insoluble FA–surfactant complexes below the cmc and to the solubilization of the complex into micelles above the cmc. Dodecylpyridinium chloride caused a monotonic decrease in the FA fluorescence even far above the cmc. This was attributable to the quenching of FA fluorescence by the positioning of the pyridinium head group near the FA fluorophore. Anionic and nonionic surfactants showed little to no effect on the FA fluorescence.  相似文献   
1000.
We developed a simple and easy method for solution mixing based on the heat-induced regulation of capillary action in thermo-responsive polymer-coated microchannels. The channels having two T-junctions were fabricated on a glass plate by a sand-blast technique and then coated with a poly(N-isopropylacrylamide) film. The polymer-coating was performed by the modification with allyltrimethoxysilane and the subsequent radical polymerization of N-isopropylacrylamide and N,N′-methylenebisacrylamide. When the channel was warmed by a Peltier device, a capillarity-based solution flow completely stopped because of the water-repellency of channel surfaces. On the other hand, the cooling of the channels allowed the restart of the solution flow through hydrophilic channels. Solution mixing downstream a T-junction was readily conducted by a Peltier device that had placed at the junction. The technique was applied to the fluorometric analysis of polyamines in saliva. The saliva sample was mixed with nickel(II) chloride solution at the first junction to mask amino acids and then mixed with o-phthalaldehyde solution at the second junction to form the fluorometric derivatives of polyamines. Blue fluorescence was observed downstream the second junction. Linear correlation was obtained between the emission intensity and the spermine concentration in the range of 20-100 μM. No mechanical pump or valve was required for the fluid manipulation.  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