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   58430篇
  免费   10005篇
  国内免费   1547篇
化学   55587篇
晶体学   559篇
力学   1331篇
综合类   1篇
数学   4083篇
物理学   8421篇
  2023年   141篇
  2022年   241篇
  2021年   615篇
  2020年   1595篇
  2019年   2893篇
  2018年   1281篇
  2017年   922篇
  2016年   3868篇
  2015年   3934篇
  2014年   3979篇
  2013年   5251篇
  2012年   4631篇
  2011年   4029篇
  2010年   3898篇
  2009年   3730篇
  2008年   3869篇
  2007年   3168篇
  2006年   2781篇
  2005年   2879篇
  2004年   2523篇
  2003年   2179篇
  2002年   2785篇
  2001年   1945篇
  2000年   1762篇
  1999年   664篇
  1998年   281篇
  1997年   315篇
  1996年   340篇
  1995年   259篇
  1994年   276篇
  1993年   280篇
  1992年   267篇
  1991年   208篇
  1990年   154篇
  1989年   143篇
  1988年   141篇
  1987年   118篇
  1986年   98篇
  1985年   169篇
  1984年   113篇
  1983年   98篇
  1982年   122篇
  1981年   90篇
  1980年   80篇
  1978年   82篇
  1977年   88篇
  1976年   96篇
  1975年   102篇
  1974年   81篇
  1973年   104篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
871.
The title complex, {[Cu2(C8H4O4)2(C3H4N2)4(H2O)]·H2O}n, is a three‐dimensional polymer formed through bridging by phthalate dianions of two different CuII cations and a network of O(N)—H⋯O hydrogen bonds. The Cu—O and Cu—N inter­action distances are in the ranges 2.0020 (16)–2.4835 (17) and 1.968 (2)–1.9855 (19) Å, respectively. The structure is composed of alternating polymer chains parallel to the c axis, with a shortest Cu⋯Cu distance of 6.3000 (5) Å.  相似文献   
872.
The title compound represents a new structure type, in which distorted VO6 octa­hedra are bridged by iodate groups to form infinite two‐dimensional [VO2(IO3)2] layers that are separated by octa­hedrally coordinated Li+ cations.  相似文献   
873.
Chronis N  Lee LP 《Lab on a chip》2004,4(2):125-130
A total internal reflection (TIR)-based biochip utilizing a polymer-filled cavity with a micromirror sidewall has been designed and fabricated. The implementation of the micromirror sidewall cavity facilitates precise alignment of the excitation light beam into the system. The incident angle of illumination can be easily modified by selecting polymers of different indices of refraction while optical losses are minimized. The design enables the hybrid, vertical integration of a laser diode and a CCD camera, resulting in a compact optical system. Brownian motion of fluorescent microspheres and real-time photobleaching of rhodamine 6G molecules is demonstrated. The proposed TIR-based chip simplifies current TIR optical configurations and could potentially be used as an optical-microfluidic platform for an integrated lab-on-a-chip microsystem.  相似文献   
874.
Using the mixture of cetyltrimethylammonium bromide (CTAB) and p‐Octyl polyethylene glycol phenyl ether (OP‐10) as templates, siliceous MCM‐48 materials can be synthesized with low molar ratio of CTAB to silica (0.139:1) and low concentration of mixed surfactants (ca.5%) and within a wide range of OP‐10/CTAB ratio (0.08?0.25). The materials were characterized by X‐ray powder diffraction, N2 adsorption/desorption isotherm, TEM, TG‐DSC and 29Si MAS NMR. Measurements indicated that the use of mixed surfactants allowed better condensation and higher ordering of the cubic mesostructure; at the same time, some properties of these materials were sensitive to the OP‐10/CTAB ratio. It was also found that the reduced pH of the gel which had been crystallized for a certain time gave a highly reproducible synthesis with a high silica yield (about 95%). Furthermore, the reaction mechanism of the synthesis is discussed in detail.  相似文献   
875.
The acetone-butanol production by simultaneous saccharification and extractive fermentation (SSEF) was investigated. In the SSEF employing cellulase enzymes andClostridium acetobutylicum, both glucan and xylan fractions of pretreated aspen are concurrently converted into acetone and butanol. Continuous removal of the fermentation products from the bioreactor by extraction was an important factor that allowed long-term fed-batch operation. The use of membrane extraction prevented the problems of phase separation and extractant loss. Increase in substrate feeding as well as reduction of nutrient supply was found to be beneficial in suppressing the acid production, thereby improving the solvent yield. Because of prolonged low growth conditions prevalent in the fed-batch operation, the butanol-to-acetone ratio in the product was significantly higher at 2.6–2.8 compared to the typical value of two.  相似文献   
876.
Five new nonalkaloid constituents, a neolignan, japonin A ( 1 ), a macrocyclic lactone, japonin B ( 2 ), a (phenylethyl)benzoquinone, japonin C ( 3 ), a phenanthraquinone, japonin D ( 4 ), as well as a dihydrostilbene, stilbostemin M ( 5 ), were isolated from the roots of Stemona japonica, together with eight known compounds. Their structures were established by spectroscopic analyses.  相似文献   
877.
878.
879.
The title compound, [Cu(ClO4)2(C4H9N3O2)2][Cu(C4H9N3O2)2(CH4O)2](ClO4)2·2CH3OH, comprises two independent CuII species lying on different inversion sites. In the Cu complexes, a distorted octa­hedral geometry arises (from basic square‐planar N4 coordination) from the weak coordination of two perchlorate ions (as Cu—O) in one species and two methanol mol­ecules in the other (also as Cu—O). Inter­actions between the O atoms of the perchlorate anions or methanol groups and the imide or amine NH groups afford an extensive inter­molecular hydrogen‐bonding network.  相似文献   
880.
5‐Sulfosalicylic acid (5‐SSA) and 3‐aminopyridine (3‐APy) crystallize in the same solvent system, resulting in two kinds of 1:1 proton‐transfer organic adduct, namely 3‐aminopyridinium 3‐carboxy‐4‐hydroxybenzenesulfonate monohydrate, C5H7N2+·C7H5O6S·H2O or 3‐APy·5‐SSA·H2O, (I), and the anhydrous adduct, C5H7N2+·C7H5O6S or 3‐APy·5‐SSA, (II). Both compounds have extensively hydrogen‐bonded three‐dimensional layered polymer structures, with interlayer homo‐ and heterogeneous π–π interactions in (I) and (II), respectively.  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