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   2099篇
  免费   258篇
  国内免费   160篇
化学   1492篇
晶体学   22篇
力学   87篇
综合类   14篇
数学   163篇
物理学   739篇
  2024年   5篇
  2023年   36篇
  2022年   67篇
  2021年   61篇
  2020年   67篇
  2019年   61篇
  2018年   84篇
  2017年   60篇
  2016年   116篇
  2015年   107篇
  2014年   125篇
  2013年   187篇
  2012年   155篇
  2011年   147篇
  2010年   151篇
  2009年   140篇
  2008年   134篇
  2007年   125篇
  2006年   108篇
  2005年   83篇
  2004年   54篇
  2003年   71篇
  2002年   60篇
  2001年   39篇
  2000年   39篇
  1999年   28篇
  1998年   26篇
  1997年   20篇
  1996年   22篇
  1995年   9篇
  1994年   6篇
  1993年   11篇
  1992年   8篇
  1991年   9篇
  1990年   8篇
  1989年   11篇
  1988年   5篇
  1987年   9篇
  1986年   5篇
  1985年   6篇
  1984年   4篇
  1983年   5篇
  1982年   3篇
  1981年   6篇
  1980年   3篇
  1977年   6篇
  1976年   3篇
  1975年   3篇
  1972年   4篇
  1971年   3篇
排序方式: 共有2517条查询结果,搜索用时 15 毫秒
191.
The compounds diethyl-N-carbazolylmethylphosphonate 1 and diethyl-2-oxymethylphosphonate carbazole 2 have been synthesised and characterized by a range of techniques including NMR, absorption and emission spectroscopy, infrared, and mass spectroscopy. Compound 1 forms a 2:1 complex 3 with calcium nitrate and the single crystal X-ray diffraction structure of 3 is described. Titrations of 1 with Zn2+ and Ca2+ in ethanol reveal that the intrinsic fluorescence is only slightly perturbed in the presence of these metal ions in micromolar ethanol solutions. Compounds 1 and 2 are readily taken up and visualized in L929 fibrosarcoma and DRG (Dorsal root ganglia) cell lines.  相似文献   
192.
By means of collision models(CMs) where the environment is simulated by a collection of ancillas consisting of two entangled qubits, we investigate the effects of entanglement in the environment on the non-Markovianity of an open quantum system. Two CMs are considered in this study, in the first one the open quantum system S directly collides with the environment,while in the second one the system interacts with two intermediate qubits which, in turn, are coupled to the environment. We show that it is possible to enhance the non-Markovianity by environment entanglement in both models. In particular, in the second model, we show that the initial state of the auxiliary qubits can also affect the non-Markovianity of the system and there exists the optimal combination of the initial environmental state and the initial state of auxiliary qubits. In this case, the non-Markovianity can be greatly enhanced.  相似文献   
193.
Two metal–organic frameworks, [Co2(L)(H2O)2(4,4′-bipy)]·3CH3CN (1) and [Mn2(L)(1,10-phen)(H2O)]·H2O (2) (H4L = 5-[bis(4-carboxybenzyl)-amino]isophthalic acid; 4,4′-bipy = 4,4′-bipyridine, 1,10-phen = 1,10-phenanthroline), with two different N-donor ligands have been synthesized. The structures of both MOFs were determined using single-crystal X-ray diffraction technique. MOF 1 shows 3D uncommon (4,6,6)-c net with (4.53.62)2(57.66.82)(42.54.66.72.8) topology while in the case of 2, only L4? ligands link Mn(II) ions into a 2D layer structure with chelating 1,10-phen ligand. The results demonstrate that variation in the N-donor ligands plays a pivotal role in deciding the framework of the two MOFs. Both MOFs have been exploited as photocatalyst materials for the degradation of MV. The photocatalysis results indicate that the two MOFs are stable and are prospective candidates for degradation of methyl violet under UV light irradiation. Additionally, 2 displayed superior photocatalytic activity in comparison to 1. The probable photocatalytic activity mechanism for both 1 and 2 against MV has been proposed using density of states (DOS) calculations.  相似文献   
194.
Failure behavior of the delaminated stiffened composite plates under compression is studied by the finite element method, based on a Global-Local variational model. A virtual crack closure technique and a self-adaptive grid moving scheme are proposed to predict the delamination growth process. The contact effect along the delamination front is considered. The numerical results show that the influences of the distribution and location of the stiffeners, the configuration and size of the delamination, the boundary condition and the contact upon the failure behavior of the plates are significant. The project supported by the National Natural Science Foundation of China (59975014)  相似文献   
195.
在B-R缓冲介质中,十二烷基硫酸钠(SDS)对左氧氟沙星(LVFX)-Al3+体系有显著的增敏作用,以此为基础建立了利用SDS增敏分光光度法测定LVFX的新方法。研究表明,体系吸光度与LVFX浓度在0.2~10μg/mL范围内呈良好的线性关系,相关系数r=0.9993,检出限为0.096μg/mL。将该方法用于盐酸左氧氟沙星片剂的测定,结果满意。  相似文献   
196.
Two ammonium decavanadate(V) compounds, (H2tmen)3V10O28·6H2O (1) and (H2en)3V10O28·2H2O (2), where H2tmen and H2en represent the doubly protonated N,N,N′,N′-tetramethylethylenediammonium ion and doubly protonated ethylenediammonium ion, respectively, have been synthesized and structurally characterized by elemental analyses, spectroscopic (i.r., e.s.r.) studies, and single-crystal X-ray diffraction. Both crystal structures reveal the presence of the [V10O28]6− cluster anions, doubly protonated diammonium cations, and lattice water molecules. Hydrogen bonds assemble these two compounds to form three-dimensional networks. The in vitro anticancer activity against A549 and P388 tumor cells lines has also been determined by the MTT-based assay, and the results suggest that both compounds can inhibit proliferation of these two kinds of tumor cells.  相似文献   
197.
Ruthenium porphyrins (particularly [Ru(2,6‐Cl2tpp)CO]; tpp=tetraphenylporphinato) and RuCl3 can act as oxidation and/or Lewis acid catalysts for direct C‐3 alkylation of indoles, giving the desired products in high yields (up to 82 % based on 60–95 % substrate conversions). These ruthenium compounds catalyze oxidative coupling reactions of a wide variety of anilines and indoles bearing electron‐withdrawing or electron‐donating substituents with high regioselectivity when using tBuOOH as an oxidant, resulting in the alkylation of N‐arylindoles to 3‐{[(N‐aryl‐N‐alkyl)amino]methyl}indoles (yield: up to 82 %, conversion: up to 95 %) and the alkylation of N‐alkyl or N‐H indoles to 3‐[p‐(dialkylamino)benzyl]indoles (yield: up to 73 %, conversion: up to 92 %). A tentative reaction mechanism involving two pathways is proposed: an iminium ion intermediate may be generated by oxidation of an sp3 C? H bond of the alkylated aniline by an oxoruthenium species; this iminium ion could then either be trapped by an N‐arylindole (pathway A) or converted to formaldehyde, allowing a subsequent three‐component coupling reaction of the in situ generated formaldehyde with an N‐alkylindole and an aniline in the presence of a Lewis acid catalyst (pathway B). The results of deuterium‐labeling experiments are consistent with the alkylation of N‐alkylindoles via pathway B. The relative reaction rates of [Ru(2,6‐Cl2tpp)CO]‐catalyzed oxidative coupling reactions of 4‐X‐substituted N,N‐dimethylanilines with N‐phenylindole (using tBuOOH as oxidant), determined through competition experiments, correlate linearly with the substituent constants σ (R2=0.989), giving a ρ value of ?1.09. This ρ value and the magnitudes of the intra‐ and intermolecular deuterium isotope effects (kH/kD) suggest that electron transfer most likely occurs during the initial stage of the oxidation of 4‐X‐substituted N,N‐dimethylanilines. Ruthenium‐catalyzed three‐component reaction of N‐alkyl/N‐H indoles, paraformaldehyde, and anilines gave 3‐[p‐(dialkylamino)benzyl]indoles in up to 82 % yield (conversion: up to 95 %).  相似文献   
198.
A series of platinum(II) alkynyl‐based sensitizers has been synthesized and found to show light‐to‐electricity conversion properties. These dyes were developed as sensitizers for the application in nanocrystalline TiO2 dye‐sensitized solar cells (DSSCs). Their photophysical and electrochemical properties were studied. The excited‐state property was probed using nanosecond transient absorption spectroscopy, which showed the formation of a charge‐separated state that arises from the intramolecular photoinduced charge transfer from the platinum(II) alkynylbithienylbenzothiadiazole moiety (donor) to the polypyridyl ligand (acceptor). A lifetime of 3.4 μs was observed for the charge‐separated state. A dye‐sensitized solar cell based on one of the complexes showed a short‐circuit photocurrent of 7.12 mA cm?2, an open circuit voltage of 780 mV, and a fill factor of 0.65, thus giving an overall power conversion efficiency of 3.6 %.  相似文献   
199.
200.
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