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   935篇
  免费   16篇
  国内免费   2篇
化学   682篇
晶体学   15篇
力学   12篇
数学   84篇
物理学   160篇
  2023年   6篇
  2022年   7篇
  2021年   13篇
  2020年   12篇
  2019年   11篇
  2018年   13篇
  2017年   16篇
  2016年   25篇
  2015年   18篇
  2014年   28篇
  2013年   53篇
  2012年   36篇
  2011年   56篇
  2010年   40篇
  2009年   34篇
  2008年   34篇
  2007年   45篇
  2006年   42篇
  2005年   36篇
  2004年   44篇
  2003年   31篇
  2002年   35篇
  2001年   14篇
  2000年   13篇
  1999年   12篇
  1998年   12篇
  1997年   15篇
  1996年   14篇
  1995年   17篇
  1994年   10篇
  1993年   12篇
  1992年   9篇
  1991年   11篇
  1990年   10篇
  1989年   10篇
  1988年   10篇
  1987年   10篇
  1985年   10篇
  1984年   9篇
  1983年   7篇
  1982年   9篇
  1981年   6篇
  1980年   11篇
  1979年   9篇
  1978年   10篇
  1977年   6篇
  1976年   7篇
  1975年   5篇
  1974年   6篇
  1972年   6篇
排序方式: 共有953条查询结果,搜索用时 15 毫秒
1.
Bed&#x  ich Ko&#x  ata  V  clav Kozmik  Ji&#x  í  Svoboda  Vladimí  ra Novotn    P&#x  emysl Van   k  Milada Glogarov 《Liquid crystals》2003,30(5):603-610
Four series of new [1]benzothieno[3,2- b ][1]benzothiophene derivatives have been synthesized. In the non-chiral series a SmA phase occurs, while the chiral series exhibits a rather wide antiferroelectric SmC * A phase just below the SmA phase. The SmA-SmC * A phase transition has been studied using DSC and dielectric spectroscopy. In the SmC * A phase the spontaneous quantities have been measured. The tilt angle shows a typical temperature dependence and the values of spontaneous polarization are rather moderate. The length of the helical pitch increases on increasing the length of the non-chiral alkyl chain.  相似文献   
2.
Summary Automated methods for the determination of ammonium and ammonia are reviewed, and techniques based on gas diffusion using a semi-permeable membrane were selected for the determination of both total ammonium-nitrogen and free ammonia in fermentation samples. A simple and robust instrument based on sequential injection analysis (SIA) consisting of a piston pump and two selector valves was used. Two different methods of ammonia detection, the Berthelot method and detection using pH-indicators, have been evaluated and compared. The indicator method showed higher reproducibility and the range of determination could easily be adjusted to match the concentrations of the samples. The range of determination could be adjusted from 0.05 mmol/l to 350 mmol/l, depending on choice of acceptor solution. This method has been evaluated with fermentation medium samples and tested on-line in a yeast fermentation process.Dedicated to Professor Dr. Wilhelm Fresenius on the occasion of his 80th birthday  相似文献   
3.
Cellulolytic enzyme production in aqueous two-phase systems withTrichoderma reesei Rutgers C30 has been investigated. The influ ence of different phase systems, as well as addition of media compo nents and substrate on enzyme production have been studied. Extractive enzyme production in fed-batch cultivations was per formed in a phase system composed of PEG 8000 5%-Dextran T500 7% with 1% Solka-Floc BW 200 as substrate. The cellulolytic enzyme system was intermittently withdrawn with the top phase. Addition of media components every 24 h and cellulose every 72 h gave an aver age enzyme activity in the withdrawn top phase of 2.2 FPU/mL dur ing 170 h cultivation. The corresponding productivity was 18 FPU/lh. The productivity was increased to 24 FPU/l.h when media compo nents and cellulose were added every 72 h. The average enzyme con centration was then 1.6 FPU/mL. The results are discussed in relation to methods for cellulolytic enzyme production involving immobiliza tion and cell recycling.  相似文献   
4.
Port cities where marine cargo terminals are located are generally near urban areas characterized by high-congestion road traffic. Changes in cargo traffic volumes into a marine terminal, or in the surrounding traffic arteries, which carry this traffic, can significantly affect the terminal's operations. Conversely, activity at the terminal can have an impact on the traffic levels and congestion for a considerable distance from the terminal. This paper demonstrates a methodology useful for studying the impact of road traffic flows on marine container terminals located in highly congested areas. This model was developed at the request of the Virginia Center for World Trade and was used to answer three planning questions in the port of Hampton Roads, Virginia, USA-what would be the impact of: opening a new section of interstate highway, a projected doubling of container traffic at one terminal, and a daily unit train in the vicinity of another terminal. The problem was made more challenging by limited data-collection funds. None the less, the model was deemed valid by a panel of traffic experts and officials from several major state and private agencies involved in marine traffic management. The model results were subsequently a factor in two major decisions related to terminal management.  相似文献   
5.
Solutions of Cr(III) salts containing ammonium hydroxide and ammonium salts can be determined after proper pretreatment by inverse voltammetry at carbon paste and glassy carbon electrodes. The determination may be performed, too, with organic solutions of the Cr(III) oxinate. Sensitivity is 5 × 10?7 M (= 26 ng Cr/ml), standard deviation is in the range of 4–6%.  相似文献   
6.
The intercalates of Naphthol Yellow S, Tropaeolin 000, and Tropaeolin 00 were prepared by heating [Zn0.67Al0.33(OH)2](CO3)0.165 · 0.5H2O with acidic forms of the dye solutions in an open reaction vessel. The intercalates were characterized by chemical and thermal analysis, X-ray powder diffraction and UV–VIS spectroscopy. A possible arrangement of the dye molecules in the intercalates was suggested on the basis of their chemical compositions and interlayer distances, by taking into account van der Waals dimensions of the guest molecules and by assuming that the structure of the host layers is not changed during the intercalation process.  相似文献   
7.
The syntheses of several new CpTiCl2(OR) (R = alkyl, aryl) complexes are described. It was possible to isolate pure product when the R group is substituted such as to cause steric crowding at the metal centre; for example, particularly good yields of the phenolate complexes were obtained when there were isopropyl substituents in the 2 and 6 positions of the phenolate. Electrochemical studies of the complexes in dry THF show that the TiIII complexes are relatively stable, but only a diol complex could be reduced further to a TiII species. In general, the TiIV complexes undergo a reversible 1e reduction reaction. The chemistry is more complex if the electrolyte contains added water: both the TiIV and TiIII complexes can react with water, the OR group being replaced by OH. The reaction is particularly rapid for the TiIII alkoxide complexes.  相似文献   
8.
Intercalation compounds of α-Zr(HPO4)2 · H2O with 1,2-alkanediols (from C3 to C16) have been prepared by replacing 1-propanol in α-Zr(HPO4)2 · 2C3H7OH with the desired 1,2-alkanediols by a treatment in a microwave field. It was found that the intercalates contain 1.5 molecules of diol per formula unit. The diol molecules are placed between the host layers in a bimolecular way with their aliphatic chains tilted at an angle of 51°. The diol molecules are anchored in the interlayer space by H-bonds. A mixed intercalate, containing 1,2-butanediol and 1,2-decanediol in a roughly equimolar ratio, is formed when the α-Zr(HPO4)2 · 2C3H7OH intercalate, suspended in a mixture of 1,2-butanediol and 1,2-decanediol, is exposed to microwave radiation. No new phase containing both types of the guest molecules was observed when the 1-propanol intercalate, suspended in a mixture of 1-propanol and 1,2-octanediol, is exposed to microwave radiation.  相似文献   
9.
[structure: see text] A highly efficient and practical synthesis of 4,4-Disubstituted-2-Imidazolidinones utilizing a "self-reproduction of the center of chirality" strategy is described.  相似文献   
10.
For the preparation of proteins for proteome analysis, precipitation is frequently used to concentrate proteins and to remove interfering compounds. Various methods for protein precipitation are applied, which rely on different chemical principles. This study compares the changes in the protein composition of human blood platelet extracts after precipitation with ethanol (EtOH) or trichloroacetic acid (TCA). Both methods yielded the same amount of proteins from the platelet preparations. However, the EtOH-precipitated samples had to be dialyzed because of the considerable salt content. To characterize single platelet proteins, samples were analyzed by two-dimensional fluorescence differential gel electrophoresis. More than 90% of all the spots were equally present in the EtOH- and TCA-precipitated samples. However, both precipitation methods showed a smaller correlation with nonprecipitated samples (EtOH 74.9%, TCA 79.2%). Several proteins were either reduced or relatively enriched in the precipitated samples. The proteins varied randomly in molecular weight and isoelectric point. This study shows that protein precipitation leads to specific changes in the protein composition of proteomics samples. This depends more on the specific structure of the protein than on the precipitating agent used in the experiment.  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