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   4672篇
  免费   129篇
  国内免费   14篇
化学   3776篇
晶体学   37篇
力学   47篇
数学   218篇
物理学   737篇
  2023年   25篇
  2022年   27篇
  2021年   37篇
  2020年   54篇
  2019年   68篇
  2018年   40篇
  2017年   28篇
  2016年   74篇
  2015年   68篇
  2014年   112篇
  2013年   189篇
  2012年   193篇
  2011年   287篇
  2010年   112篇
  2009年   134篇
  2008年   248篇
  2007年   265篇
  2006年   256篇
  2005年   282篇
  2004年   241篇
  2003年   181篇
  2002年   208篇
  2001年   133篇
  2000年   154篇
  1999年   58篇
  1998年   46篇
  1997年   49篇
  1996年   73篇
  1995年   39篇
  1994年   61篇
  1993年   44篇
  1992年   62篇
  1991年   53篇
  1990年   51篇
  1989年   45篇
  1988年   60篇
  1987年   48篇
  1986年   67篇
  1985年   67篇
  1984年   71篇
  1983年   43篇
  1982年   48篇
  1981年   49篇
  1980年   38篇
  1979年   34篇
  1978年   34篇
  1977年   30篇
  1976年   27篇
  1975年   37篇
  1974年   29篇
排序方式: 共有4815条查询结果,搜索用时 15 毫秒
51.
An antiviral nucleoside or virazole was synthesized efficiently and regioselectively starting with benzyl cyanoformate.  相似文献   
52.
53.
The adsorbability of pertechnetate ion (TcO 4 ) reached 96% for an active carbon at the 0.1 g/50 ml concentration level, increasing with diminution of acid and its salt concentrations, and depending on the type of anions present. With constant anion concentration, TcO 4 adsorbability rose with decreasing pH in the acidic region, followed in the region around neutrality by a plateau, and in the basic region by an appreciable decline of TcO 4 adsorbability. In the acidic region, part of once-adsorbed TcO 4 appeared to be displaced by other anions. Reversibility was noted between the adsorption and desorption of TcO 4 .  相似文献   
54.
High-speed counter-current chromatography (HSCCC) was used to purify salidroside from an extract of Rhodiola crenulata with two steps using a two-phase solvent system composed of ethyl acetate-n-butanol-water (1:4:5, v/v) in the first run and chloroform-methanol-isopropanol-water (5:6:1:4) in the second run. The method yielded 21.9 mg of salidroside from 1.216 g of the crude sample at 98% purity determined by HPLC analyses. Identification was performed by 1H NMR, 13C NMR, and MS.  相似文献   
55.
Anatase nanocrystals were precipitated mainly at the surface of the silica-titania gel films with hot water treatment, whereas the addition of poly(ethylene glycol) (PEG) in the films led to the dispersion of anatase nanocrystals in the whole of the films after the treatment. Both films with and without PEG showed high photocatalytic activities for acetaldehyde, NO x and stearic acid in the gas-solid system, and for methylene blue and potassium iodide in the liquid-solid system. The addition of PEG improved the photocatalytic activities of the resultant films due to the smaller anatase crystallites and the porous film structure. The residual silica under-layer of the superficially anatase-precipitated films is expected to act as a protective one for an organic polymer substrate against the photocatalytic degradation.  相似文献   
56.
To sophisticate the nuclear fuel recycling processes, the transfer percentages for Pd, Mo, Te, and Sb should be determined. Each element solution containing NaNO3 or HNO3 was fed consistently into the thin film evaporator regulated in vac and at 50 °C. The analyte percentages in the inside of the lid, in the condenser, and in the distillate were 10-1%/m2, 10-3%/m2, and 10-3% (DF = 105), respectively. The Mo percentage in the condenser was lower by a factor of 10 than those of other elements investigated. The NO3 - percentages were nearly constant despite increasing HNO3 concentrations, however, the ratios decreased with increasing NaNO3 concentrations.  相似文献   
57.
Thiazolo[3,2-a]thieno[3,2-d]-, [3,4-d]- and [2,3-d]pyrimidin-5-one derivatives (6, 11 and 16), and polymethylene condensed thieno[3,2-d]-, [3,4-d]- and [2,3-d]pyrimidin-5-one derivatives (19-21), in which the oxygen atom of the oxazolidine moiety in 3 was replaced by a sulfur atom or methylene groups, were synthesized and evaluated for gastric antisecretory activity in pylorus-ligated rats. The structure-activity relationships of these compounds are discussed.  相似文献   
58.
Bromine and chlorine concentrations in some tissues (liver, spleen, thymus) of healthy mice (normal group) and lymphomatous mice (lymphoma group) were determined by neutron activation analysis. There were significant differences in the Br concentrations of the tissues between the normal and lymphomatous group, while the Cl concentrations were found relatively constant in all the tissues. As a result, the Br/Cl ratios in the tissues of the lymphomatous group were significantly higher than those of the normal group. It was difficult to conclude that Br was absorbed into the bodies of lymphomatous mice more easily than was Cl. It is suggested that Br excretion from the body is suppressed.  相似文献   
59.
-Cyclodextrin with two sodium anthranilate moieties (1) has been prepared as a sensor for detecting organic compounds including terpenoids and steroids. Compound1 shows a pure monomer fluorescence whose intensity is increased or decreased upon addition of the guest species examined. In this system, the sodium anthranilate moieties act either as a spacer, which enables the cyclodextrin to form a 1:1 host-guest complex by narrowing the -cyclodextrin cavity, or as a hydrophobic cap.1 recognizes steroids with much higher sensitivity than terpenoids, in which the appended moieties act as a hydrophobic cap for terpenoids and a spacer for steroids, respectively.  相似文献   
60.
Hepatic artery of dogs was embolized with cisplatin (CDDP) albumin microspheres containing chitin and chitosan to investigate the in vivo CDDP release kinetics from CDDP albumin microspheres, the CDDP cumulative characteristics in the liver, and the influence of microsphere administration on hepatic tissue. Results showed that changes in blood CDDP content were dependent on CDDP albumin microsphere type and that release kinetics were better sustained when chitin was added to the microspheres or when the microspheres were treated with chitosan. In particular, the administration of CDDP in the chitin-containing CDDP chitosan albumin microspheres showed a blood CDDP content of approximately 0.26 micrograms Pt/ml 14 d after administration. The administration of chitin-containing or chitosan treated CDDP microspheres showed a CDDP content in the hepatic tissue of 0.14 to 0.23 micrograms Pt/g 28 d after administration. They also showed better control of CDDP release than those without chitin or chitosan treatment. No CDDP influence on hepatic tissue was observed. We conclude that, even in vivo, chitin and chitosan are effective embolic materials.  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