全文获取类型
收费全文 | 53243篇 |
免费 | 8501篇 |
国内免费 | 5660篇 |
专业分类
化学 | 37368篇 |
晶体学 | 641篇 |
力学 | 3075篇 |
综合类 | 328篇 |
数学 | 5955篇 |
物理学 | 20037篇 |
出版年
2024年 | 182篇 |
2023年 | 1120篇 |
2022年 | 2076篇 |
2021年 | 2214篇 |
2020年 | 2272篇 |
2019年 | 2141篇 |
2018年 | 1829篇 |
2017年 | 1776篇 |
2016年 | 2606篇 |
2015年 | 2638篇 |
2014年 | 3099篇 |
2013年 | 3953篇 |
2012年 | 4849篇 |
2011年 | 4785篇 |
2010年 | 3322篇 |
2009年 | 3112篇 |
2008年 | 3337篇 |
2007年 | 2955篇 |
2006年 | 2651篇 |
2005年 | 2276篇 |
2004年 | 1739篇 |
2003年 | 1388篇 |
2002年 | 1275篇 |
2001年 | 1018篇 |
2000年 | 895篇 |
1999年 | 1046篇 |
1998年 | 856篇 |
1997年 | 806篇 |
1996年 | 794篇 |
1995年 | 727篇 |
1994年 | 612篇 |
1993年 | 519篇 |
1992年 | 430篇 |
1991年 | 392篇 |
1990年 | 331篇 |
1989年 | 240篇 |
1988年 | 178篇 |
1987年 | 163篇 |
1986年 | 159篇 |
1985年 | 139篇 |
1984年 | 87篇 |
1983年 | 93篇 |
1982年 | 55篇 |
1981年 | 39篇 |
1980年 | 29篇 |
1979年 | 18篇 |
1977年 | 18篇 |
1976年 | 20篇 |
1975年 | 21篇 |
1972年 | 19篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
利用密度泛函理论(DFT)方法, 在B3LYP/6-311+G(d,p)水平下, 对质子化甜菜碱阳离子与双三氟甲磺酰亚胺阴离子形成的气态阴阳离子对([Hbet][Tf2N])进行理论研究, 通过几何结构优化和频率分析得到势能面上的六个稳定构型. 计算结果表明[Hbet]+和[Tf2N]-之间能够形成较强的氢键相互作用, 氢键相互作用的稳定化能主要来源于[Tf2N]-中O, N原子的孤对电子和[Hbet]+中参与形成氢键的O-H, C-H反键轨道之间的相互作用. 自然布局分析(NPA)给出气态[Hbet][Tf2N]离子对中阴阳离子间的电荷转移比传统离子液体中电荷转移的数值小. AIM (atoms in molecules)分析得到[Hbet]+和[Tf2N]-之间的氢键相互作用以静电作用为主. 通过实验和理论结果相比较, 初步探讨影响离子液体熔点及其对金属离子选择性溶解的结构因素. 相似文献
952.
953.
Dr. Wei Su Dr. Peiyuan Li Libei Yao Fang Yang Prof. Lifang Liang Juan Chen 《Chemphyschem》2011,12(6):1143-1147
We demonstrate a simple route to fabricating copper circuit patterns on the surface of polyimide film. The copper pattern can be obtained in three steps: 1) Formation of partially potassium hydroxide modified pattern via a screen‐printing process, 2) formation of macromolecular metal complex with copper, and 3) copper metallization by DMAB reduction. The morphologies of these copper patterns are determined by cross‐sectional transmission electronic microscopy (TEM), scanning electronic microscopy (SEM), and atomic force microscopy (AFM). Furthermore, the growing process of the metallic copper film is investigated. The direct patterning of copper patterns onto polyimide substrates is promising for use in electronics industry as a large‐area and low‐cost processing technique. 相似文献
954.
Mechanistic Insights into the Ni‐Catalyzed Reductive Carboxylation of C−O Bonds in Aromatic Esters with CO2: Understanding Remarkable Ligand and Traceless‐Directing‐Group Effects 下载免费PDF全文
Yan‐Li Han Bing‐Yuan Zhao Kun‐Yao Jiang Hui‐Min Yan Dr. Zhu‐Xia Zhang Dr. Wen‐Jing Yang Prof. Dr. Zhen Guo Prof. Dr. Yan‐Rong Li 《化学:亚洲杂志》2018,13(12):1570-1581
The mechanism of the Ni0‐catalyzed reductive carboxylation reaction of C(sp2)?O and C(sp3)?O bonds in aromatic esters with CO2 to access valuable carboxylic acids was comprehensively studied by using DFT calculations. Computational results revealed that this transformation was composed of several key steps: C?O bond cleavage, reductive elimination, and/or CO2 insertion. Of these steps, C?O bond cleavage was found to be rate‐determining, and it occurred through either oxidative addition to form a NiII intermediate, or a radical pathway that involved a bimetallic species to generate two NiI species through homolytic dissociation of the C?O bond. DFT calculations revealed that the oxidative addition step was preferred in the reductive carboxylation reactions of C(sp2)?O and C(sp3)?O bonds in substrates with extended π systems. In contrast, oxidative addition was highly disfavored when traceless directing groups were involved in the reductive coupling of substrates without extended π systems. In such cases, the presence of traceless directing groups allowed for docking of a second Ni0 catalyst, and the reactions proceed through a bimetallic radical pathway, rather than through concerted oxidative addition, to afford two NiI species both kinetically and thermodynamically. These theoretical mechanistic insights into the reductive carboxylation reactions of C?O bonds were also employed to investigate several experimentally observed phenomena, including ligand‐dependent reactivity and site‐selectivity. 相似文献
955.
A family of modified two-derivative Runge–Kutta (MTDRK) methods for the integration of the Schrödinger equation are obtained. Two new three-stage and fifth order TDRK methods are derived. The numerical results in the integration of the radial Schrödinger equation with the Woods–Saxon potential are reported to show the high efficiency of our new methods. The results of the error analysis are illustrated by the resonance problem. 相似文献
956.
XiaoYong Li Zhuqing Feng SiChuan Pu Yun Yang XingMin Shi Zhao Xu 《Plasma Chemistry and Plasma Processing》2018,38(5):919-936
Cold atmospheric pressure plasma is widely used in research for biomedical applications and is a promising therapy to selectively eradicate cancer cells. However, fundamental information related to the plasma modification of biomolecules in aqueous solution remains elusive. In this work, we studied the chemical basis of tryptophan (Trp) oxidation using an Ar plasma jet and investigated the effects of plasma-treated tryptophan (PTT) on cell viability. The results show that the main product of the Trp of plasma induced oxidation is a mixture of hydroxyl derivatives and hydroperoxides in aqueous form. The products result primarily from the hydroxyl radical (·OH) attacking the Trp, which can be explained by the interaction with both the aromatic and the pyrrole rings. We observed that the PTT has a different proliferation effect between the growth of the B16 melanoma cells and the L929 fibroblast cells. The experimental results indicated that the effect of the PTT is dose-dependent in the B16 cells, ranging from cell proliferation to cytotoxic damage with apoptosis. Furthermore, we examined the intracellular changes in hydrogen peroxide (H2O2) and superoxide radicals (\({\text{O}}_{2}^{{\bar{ \cdot }}}\)) following the Trp-derived treatment using the fluorescence probe method. Rigorous identification of the reactive oxygen species (ROS) produced by PTT in cells as \({\text{O}}_{2}^{{\bar{ \cdot }}}\) and H2O2 helped establish the cellular source of ROS. It is shown that the intracellular H2O2 might originate from the activation of NADPH. These results suggested that the amino acid and protein hydroperoxides may play a crucial role in the action of plasma on the biologic target. 相似文献
957.
Qian Sun Xueying Song Li Gao Wei Chen Yamin Li Liqun Mao Jing-He Yang 《Chemical Papers》2018,72(9):2203-2214
The series of graphene materials and N-doped graphene materials were successfully synthesized and improved by high-temperature treatment with trace iron oxide. XRD, Raman, FT-IR, TEM and XPS were employed for these catalysts. The catalytic performance of these catalysts was investigated in the selective oxidation of ethylbenzene with tert-butyl hydroperoxide as oxidant. The impacts of temperature, mass of catalysts, reaction time and oxidants on the selective oxidation of ethylbenzene were also investigated. The N-doped graphene materials exhibit greatly remarkable catalytic performance than others. The conversion of ethylbenzene is more than 90% and the selectivity of acetophenone is more than 95% at 353 K. Graphene can be used as catalyst owing to its unique structures and chemical properties. The characterization tests show that the doping of N atoms can create more defects and more active sites in the N-doped graphene materials which could greatly improve the catalytic performance. Furthermore, such cost-effective graphene-based catalysts possess good stability and could be reused at least five times without remarkable loss of the catalytic activity. 相似文献
958.
Dongyu Yang Chaosheng Yuan Kun Yang Chunguang Shao Yongqiang Wang Xuerui Cheng Lei Su 《Polymer Science Series A》2018,60(3):381-390
Isothermal crystallization behavior of Pluronic F127 blended with and without an ionic liquid (IL) was investigated by in situ polarized optical microscopy (POM) and Fourier transform infrared spectroscopy (FTIR). For the pure F127, the POM and FTIR results showed that the spherulite size and crystallinity of F127 increased with the melting temperature increasing to 60, 80, and 135°C. This could be explained by the flexibility of the polymer chain at high melting temperatures. For the F127 blended with IL, the POM results showed that the morphology of F127 evolved from spherulite to dendritic segregation and fibrous crystal with the increasing IL content. FTIR results indicated that hydrogen bonds were formed between F127 and IL, and the intensity of the hydrogen bonds became strengthened gradually with increasing IL content. The effect of hydrogen bonds on the morphology evolution of F127/IL is discussed. 相似文献
959.
Liyuan Han Caiyin You Qin Wang Shaohua Guo Ke Xu Weihua Zhang Rong Yang 《Russian Journal of Electrochemistry》2018,54(1):49-55
α-NaFeO2 layered LiNi1/3Co1/3Mn1/3O2 cathode materials were synthesized by mechanical milling accompanied by the solid phase sintering. The sample exhibited a good crystallinity and layered structure while sintered at 900°C, which can be further improved by adding a pre-sintering process at 500°C before high temperature sintering. The sample with a pre-sintering process presents an average particle size about 0.6 μm, and a hexagonal crystalline structure. The optimally fabricated sample showed a first charge capacity of 210.2 mA h/g, discharge capacity of 171.2 mA h/g with a current rate of 0.2 C within the voltage range of 2.7~4.5 V. With increasing the current rate to 1 C, the charge–discharge capacity faded quickly during the cycling process, which can be partially recovered while operated at a low current rate. However, the capacity fading at a current rate of 2 C was largely irreversible. The evolution of the surface chemical states was evaluated using X-ray photoelectron spectroscopy on the charged and discharged samples to understand the high rate capacity fading. 相似文献
960.
Several 8-8-coupled tetraarylfuran-type oligostilbenes were prepared through FeCl3·6H2O-catalyzed regioselective oxidative couplings of protected resveratrol precursors. The global deprotection of the coupling dimers in a one-pot reaction yielded (±)-pallidol instead of the targets (±)-restrytisol B and (±)-tricuspidatol A. 相似文献