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   133408篇
  免费   25864篇
  国内免费   11156篇
化学   105446篇
晶体学   1484篇
力学   7224篇
综合类   534篇
数学   13763篇
物理学   41977篇
  2024年   235篇
  2023年   2070篇
  2022年   2629篇
  2021年   3707篇
  2020年   5144篇
  2019年   6133篇
  2018年   4254篇
  2017年   3730篇
  2016年   7918篇
  2015年   7775篇
  2014年   8762篇
  2013年   11073篇
  2012年   11778篇
  2011年   11579篇
  2010年   8925篇
  2009年   8633篇
  2008年   8613篇
  2007年   7501篇
  2006年   6725篇
  2005年   5980篇
  2004年   4623篇
  2003年   3827篇
  2002年   4230篇
  2001年   3208篇
  2000年   2929篇
  1999年   2399篇
  1998年   1882篇
  1997年   1755篇
  1996年   1905篇
  1995年   1510篇
  1994年   1471篇
  1993年   1184篇
  1992年   1081篇
  1991年   995篇
  1990年   803篇
  1989年   569篇
  1988年   472篇
  1987年   393篇
  1986年   381篇
  1985年   327篇
  1984年   254篇
  1983年   159篇
  1982年   151篇
  1981年   112篇
  1980年   92篇
  1979年   54篇
  1977年   42篇
  1976年   48篇
  1975年   42篇
  1974年   55篇
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
991.
Tumor cells transduced with herpes simplex virus thymidine kinase gene have been intensively applied to the field of positron emission tomography via imaging of its substrate. As a pilot synthesis approach, a facile preparation of 5‐[125I] iodoarabinosyl uridine starting from commercially available uridine is reported herein.  相似文献   
992.
In the crystal structures of two cyclic trihydroxamic acid derivatives containing the same substructure unit, viz. 1,3,5‐trihydroxy‐1,3,5‐triazinane‐2,4,6‐trione dihydrate, C3H3N3O6·2H2O, (I), and 1,3,5‐benz­yloxy‐1,3,5‐triazinane‐2,4,6‐trione, C24H21N3O6, (II), there is no significant difference in the geometric parameters. In (I), there are 11 hydrogen bonds of the O—H⋯O type inter­connecting the mol­ecules in a three‐dimensional network, while in (II) there are only two weak C—H⋯O hydrogen bonds. The results of IR spectroscopic analysis are in good agreement with the crystallographic study.  相似文献   
993.
Qiang Wang  Qing Dai 《Journal of Non》2007,353(4):354-365
A room temperature method for the encapsulation of pyrene in SiO2 nanoparticles is described. The relation between alkoxysilane surfactant chain length, reactant molar ratios and the uptake of dye, sample morphology, photophysical properties, and the ability of the silicate matrix to protect the encapsulated dye was examined. The synthesis can easily be adapted for the encapsulation of other hydrophobic and thermolabile substances, and used in the development of nanostructured optically active coatings, films and monoliths.  相似文献   
994.
995.
A new polymorph of tetraphenyldiboroxane [or oxybis(diphenylborane)], C24H20B2O, (Ia), has been found. It is monoclinic, like the already known form, (Ib), and can be refined in the same space group, namely P21/c, or in the equivalent setting P21/n. The molecular conformations of the two polymorphs differ in the rotations of two of the phenyl rings about the B—C bonds, leading to markedly different packing patterns and cell dimensions.  相似文献   
996.
997.
In the title compound, [Cd(C7H6NO2)2]n, the CdII ion has a distorted octahedral geometry. The 6‐methylpyridine‐2‐carboxylate anions are perpendicular to one another and act as bidentate and bridging ligands. Two carboxylate O atoms bridge the CdII ions, forming centrosymmetric dinuclear units. These units are further connected via carboxylate O atoms into a one‐dimensional polymeric chain which extends in the [100] direction.  相似文献   
998.
999.
1000.
Usually, Sm2+ ions could be reduced by heating the materials in reducing atmospheres. Exposure to ionizing radiations is also known to cause Sm3+→Sm2+ conversion. In this work, BaBPO5 doped with the samarium ion was prepared by high temperature solid-state reaction. Sm2+ ions were obtained by two different reduction methods, i.e., heating in H2 reduced atmosphere and X-ray irradiation. The measurements of X-ray diffraction (XRD), and scanning electron microscope (SEM) were investigated. It is found that the conversion of Sm3+→Sm2+ is very efficient in BaBPO5 hosts after X-ray irradiation. Sm2+ ions under these two reduction methods exhibit different characteristics that were studied by measurements of luminescence and decay. The results showed that the luminescence properties of Sm2+ ions in BaBPO5 were highly dependent on the sample preparation conditions.  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