全文获取类型
收费全文 | 5687篇 |
免费 | 482篇 |
国内免费 | 333篇 |
专业分类
化学 | 2231篇 |
晶体学 | 63篇 |
力学 | 547篇 |
综合类 | 46篇 |
数学 | 1376篇 |
物理学 | 2239篇 |
出版年
2024年 | 13篇 |
2023年 | 49篇 |
2022年 | 114篇 |
2021年 | 115篇 |
2020年 | 124篇 |
2019年 | 146篇 |
2018年 | 127篇 |
2017年 | 139篇 |
2016年 | 174篇 |
2015年 | 144篇 |
2014年 | 241篇 |
2013年 | 461篇 |
2012年 | 297篇 |
2011年 | 365篇 |
2010年 | 313篇 |
2009年 | 359篇 |
2008年 | 405篇 |
2007年 | 368篇 |
2006年 | 307篇 |
2005年 | 266篇 |
2004年 | 234篇 |
2003年 | 229篇 |
2002年 | 159篇 |
2001年 | 155篇 |
2000年 | 154篇 |
1999年 | 172篇 |
1998年 | 128篇 |
1997年 | 93篇 |
1996年 | 66篇 |
1995年 | 88篇 |
1994年 | 60篇 |
1993年 | 61篇 |
1992年 | 64篇 |
1991年 | 40篇 |
1990年 | 32篇 |
1989年 | 22篇 |
1988年 | 35篇 |
1987年 | 21篇 |
1986年 | 20篇 |
1985年 | 21篇 |
1984年 | 22篇 |
1983年 | 9篇 |
1982年 | 14篇 |
1981年 | 17篇 |
1980年 | 14篇 |
1979年 | 6篇 |
1978年 | 8篇 |
1977年 | 14篇 |
1974年 | 4篇 |
1973年 | 5篇 |
排序方式: 共有6502条查询结果,搜索用时 9 毫秒
91.
固相微萃取参数选择及其对有机锡分析的影响 总被引:1,自引:0,他引:1
固相微萃取是一种新型的、不断发展和完善的样品前处理方法,它与其它技术联用可对多种样品基体中挥发、半挥发性有机化合物进行测定。目前,该技术在毒性金属有机化合物中的应用很少。本文分析参数选择对固相微萃取的影响的同时,还对其在有机锡化合物分析中的应用作了综述。 相似文献
92.
93.
分根法研究镧对水稻生长及其生理参数的影响 总被引:10,自引:1,他引:10
用分根营养液培养法研究了镧对水稻生长及其促进作用的生理机制。结果表明:低浓度镧(0 05~1 5mg·L-1)提高水稻产量,增加实粒数;随着镧浓度从0 05增加到0 75mg·L-1,镧降低第一片完全展开叶和根中过氧化氢酶(CAT)活性;当镧浓度为0 75~9mg·L-1时,镧显著降低叶和根中超氧化歧化酶(SOD)活性;0 75mg·L-1镧增加叶中赤霉素(GA)和吲哚乙酸(IAA)含量,0 75和3mg·L-1镧增加根中细胞分裂素(iPAs)和IAA含量,但降低脱落酸(ABA)含量;秧苗移栽后46d,0 25~6mg·L-1镧显著增加叶片气孔导度;秧苗移栽后59和83d,当镧浓度分别为15和6mg·L-1时,镧显著降低叶片气孔导度;镧对叶片叶绿素(Chl)含量、叶和根中蛋白质,脂质过氧化产物丙二醛(MDA)含量没有发现显著作用。还讨论了镧促进水稻生长和消除超氧阴离子(·O2-)的可能机制。 相似文献
94.
95.
咪唑类化合物-CuCl络合催化剂在甲醇氧化羰基化反应中的催化性能 总被引:21,自引:0,他引:21
研究了咪唑及其衍生物配合CuCl对甲醇液相氧化羰化合成碳酸二甲酯的催化性能。筛选出溶解性好、腐蚀性小且催化活性高的多功能助催剂。实验结果表明,反应体系中加入N-甲基咪唑后,CuCl可以完全溶解。当催化剂的浓度为0.2 mol/L, N-甲基咪唑与CuCl的量为4∶1,反应温度为120 ℃,反应压力为2.40 MPa,CO与O2的进气比2∶1,反应3 h的条件下甲醇的摩尔转化率为15.4%,选择性为98%以上。从腐蚀性试验结果看,50 ℃时,加入N-甲基咪唑化合物后,Q235钢在CuCl/CH3OH/H2O/CO/O2体系中的腐蚀速率为0.22mm/a,缓蚀效率为94.5%。动力学研究表明,反应近似为一级,加入N-甲基咪唑后,反应速率常数为0.15 min-1。 相似文献
96.
The generalized (reductive) criterion of solvent polarity was obtained by the method of multiparametric optimization of the Snyder index P", Hildebrand parameter
T
, permittivity
r
, and solvatochromism parameter E
T
(30). Possibilities of employing this criterion for estimation of the elution power of normal and reversed mobile phases used in high-performance liquid chromatography were considered. 相似文献
97.
E. Miyoshi T. Iura Y. Sakai H. Tochihara S. Tanaka H. Mori 《Journal of Molecular Structure》2003,630(1-3):225-232
We investigated the interactions between the Si(111) surface and the Na, Mg, and Al atoms using cluster model calculations. Calculations were performed at levels of complete-active-space self-consistent-field (CASSCF) and multi-reference singly and doubly excited configuration interaction (MRSDCI) calculations using the model core potential method. Our calculations revealed that the most favorable sites of Na, Mg, and Al adsorption on Si(111) are on top (T1), bridge (B2), and 3-fold filled (T4) sites, respectively. The nature of chemical bonds between these metal atoms and the dangling bonds of the surface Si atoms are found to be essentially covalent. 相似文献
98.
Summary. Two main types of binary systems with distinctive solubility behavior under sub- and supercritical conditions were used to
subdivide ternary water-salt systems into three classes. Characteristic features of solubility behavior and phase equilibria
in ternary water-salt systems of each class at temperatures above 200°C are discussed on the basis of available experimental
data and some conclusions obtained as a result of theoretical derivation of fluid and complete phase diagrams.
Corresponding author. E-mail: Valyashko@IGIC.RAS.RU
Received September 25, 2002; accepted (revised) November 28, 2002
Published online April 24, 2003
RID="a"
ID="a" Dedicated to Prof. Dr. H. Gamsjaeger on the occasion of his 70th birthday anniversary 相似文献
99.
Carlos Raimundo Andrade Lima Jefferson da Silva Martins Caroline Pinheiro Luis Fernando de Avila Roberto Rosas Pinho Maria Luiza Miranda Rocco Wilson de Souza Melo Fabio Zappa 《Journal of Polymer Science.Polymer Physics》2019,57(15):967-972
Photopolymerization is a phenomenon that is the basis of much of today's microfabrication technology and intense research is conducted to improve its control and the characteristics of end products for a variety of applications. The design of microscopic structures often relies on the accurate knowledge and modeling of photopolymer's behavior upon exposure, i.e. the Dill parameters, for each radiation species of interest and therefore the development of flexible characterization techniques is of great importance. SU‐8 is a popular compound that is representative of a whole class that relies on cationic polymerization, where an acid is obtained via photolysis of an onium salt during exposure. Here we report on the observation of SbF6? via laser desorption mass spectrometry on SU‐8 exposed to UV light at the wavelength of 365 nm and demonstrate that the yield of this counter‐anion as a function of exposure is consistent with the Dill C parameter value available in the literature. © 2019 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2019 , 57, 967–972 相似文献
100.
《Comptes Rendus Chimie》2019,22(5):386-392
Dimeric alkylammonium salts – gemini surfactants – due to their unusual very low critical micelle concentration and minimal inhibitory concentration are subject to intensive research as surface active and antimicrobial compounds. Thanks to the presence of two positively charged nitrogen atoms and a large molecular surface, gemini surfactants are also very efficient corrosion inhibitors. To strengthen the electrostatic adsorption of ammonium cations on a metal surface, which is a key parameter in the inhibition of corrosion, heteroatoms (O, S, N, or P) and π-electron systems can be introduced into the gemini surfactant structure to increase chemical adsorption. In this study, we investigated the relationship between the alkyl chain length and critical micelle concentration for gemini surfactants containing an oxygen atom in the spacer, that is, bis(N-alkyl-N,N-dimethylethylammonium)ether dibromides, for potential use as corrosion inhibitors. 相似文献