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   1648篇
  免费   133篇
  国内免费   22篇
化学   1324篇
晶体学   9篇
力学   46篇
数学   171篇
物理学   253篇
  2023年   3篇
  2022年   17篇
  2021年   34篇
  2020年   38篇
  2019年   35篇
  2018年   35篇
  2017年   26篇
  2016年   58篇
  2015年   46篇
  2014年   75篇
  2013年   111篇
  2012年   119篇
  2011年   134篇
  2010年   88篇
  2009年   71篇
  2008年   119篇
  2007年   101篇
  2006年   110篇
  2005年   91篇
  2004年   82篇
  2003年   63篇
  2002年   51篇
  2001年   34篇
  2000年   30篇
  1999年   18篇
  1998年   13篇
  1997年   8篇
  1996年   9篇
  1995年   14篇
  1994年   16篇
  1993年   21篇
  1992年   15篇
  1991年   7篇
  1990年   8篇
  1989年   9篇
  1988年   7篇
  1987年   5篇
  1985年   13篇
  1984年   4篇
  1982年   7篇
  1980年   8篇
  1979年   3篇
  1978年   9篇
  1977年   3篇
  1976年   4篇
  1975年   4篇
  1974年   7篇
  1973年   3篇
  1970年   3篇
  1966年   2篇
排序方式: 共有1803条查询结果,搜索用时 46 毫秒
41.
42.
43.
44.
45.
Novel synthetic approaches toward 5,6-dihydro-pyrimido[4,5-b][1,4]oxazepines were reported that led to successful introduction of poorly reactive anilines and various 2-hydroxybenzaldehydes to this therapeutically relevant scaffold. More extensive SAR studies on this scaffold hence became possible.  相似文献   
46.
New diamino monomers IIa – IIg were synthesized in a two-step reaction sequence starting from p-acetamidobenzenesulfonyl chloride. Solution polymerization of these monomers in DMAC with terephthaloyl or isophthaloyl chloride resulted in the formation of a series of 14 poly(amide sulfonamide)s (PASAs) in excellent yield (> 95%). The polymers have in-trinsic viscosities of 0.32–1.11 dL g?1. Except for polymers IIIa ? p and IIId - p , all other PASAs were readily soluble in aprotic polar solvents including DMAC, DMF, and DMSO. Thermogravimetric analyses of the polymers showed moderate thermal stability with 10% weight loss being recorded in the range of 325–408°C. In addition, these polymers exhibit moderate chemical stabilities toward alkali, acidic, and chromic acid solution. © 1995 John Wiley & Sons, Inc.  相似文献   
47.
LacTek tests are competitive enzyme-linked immunosorbent assays intended for rapid detection of antimicrobial residues in bovine milk. In this study, the LacTek test protocol was modified for use with extracts of bovine tissue to detect beta-lactam, tetracycline, and sulfamethazine residues. Test performance characteristics--precision, accuracy, ruggedness, practicability, and analytical specificity and sensitivity--were investigated. Results suggest that LacTek tests can be easily adapted to detect antimicrobial residues in extracts of lean ground beef. However, positive samples may not contain residues at violative concentrations (i.e., Canadian maximum residue limits), and therefore, additional analysis would be required for final confirmation and quantitation (e.g., chromatography).  相似文献   
48.
Five methods of acid digestion of amniotic fluid phospholipids for inorganic phosphate release were evaluated at a temperature of 225 °C and a heating time of 30 min. The methods used the following digestants: (A (perchloric acid), B (perchloric acid containing molybdate), C (sulfuric acid, 30% hydrogen peroxide, 5% urea), D (sulfuric-perchloric acids), and E (sulfuric-perchloric acids with vanadium pentoxide). After digestion and dilution with water, a hydrogen ion concentration of about 2.5 mol/ liter was obtained that permitted the use of stannous chloride-hydrazine sulfate as the reducing agent for phosphate color development. Recoveries of the different amniotic fluid phospholipids and aqueous phosphate standards were quantitative for all methods. A comparative study with amniotic fluids showed similar results for all methods. Since all methods were found to be equivalent under the specified conditions described, the choice was dependent upon the user's preference.  相似文献   
49.
Adsorption of small amounts of D(2)O (ca. 0.01 L exposure) on CO(sat)/Pt(100) surfaces in ultrahigh vacuum at 105 K was found to split the asymmetric peak at 2100 cm(-1) in the infrared reflection absorption spectra attributed to the stretching of CO adsorbed on atop sites into two clearly defined features: an intense component, which shifted toward lower energies due to surface hydration of adsorbed CO (originally at 2100 cm(-1), peak a), and a smaller peak centered at 2094 cm(-1) (peak b), which remained fixed in position even after closing the D(2)O dosing valve. The energies of peaks a and b, as determined by statistical analyses, correlated very well with those reported in the literature for CO adsorbed at high coverages on Pt(100) originally in the (5 x 20) or (hex) reconstruction, and on the unreconstructed Pt(100)-(1 x 1), respectively, at 90 K. On these bases, the asymmetry of the peak observed for CO(sat)/Pt(100) (no D(2)O dosing) is ascribed to the presence of CO linearly adsorbed on these two different sites on the surface, for which the rate of hydration is larger for the (5 x 20) compared to the (1 x 1) phases.  相似文献   
50.
SKH-1 hairless mice repair UV-induced pyrimidine dimers in epidermal DNA   总被引:3,自引:0,他引:3  
The SKH-1 hairless mouse strain has been used extensively as a model for human photocarcinogenesis, photoimmunology and photoaging, but little is known about DNA repair in living mouse skin. Mice were irradiated with UV-B light at doses which produce mild to severe sunburn, and the frequency of pyrimidine dimers in epidermal DNA was measured immediately and 6 h after irradiation using T4 endonuclease V treatment and alkaline agarose gel electrophoresis. The results demonstrate significant removal of pyrimidine dimers in mouse skin in vivo, with a dimer half-life of 7.4 h. These findings are similar to the repair of dimers in human skin in vivo. The SKH-1 hairless mouse is thus a useful model for pyrimidine dimer repair in human skin.  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