全文获取类型
收费全文 | 18903篇 |
免费 | 859篇 |
国内免费 | 98篇 |
专业分类
化学 | 13028篇 |
晶体学 | 120篇 |
力学 | 420篇 |
数学 | 2894篇 |
物理学 | 3398篇 |
出版年
2023年 | 116篇 |
2022年 | 140篇 |
2021年 | 268篇 |
2020年 | 377篇 |
2019年 | 397篇 |
2018年 | 234篇 |
2017年 | 236篇 |
2016年 | 600篇 |
2015年 | 555篇 |
2014年 | 644篇 |
2013年 | 1024篇 |
2012年 | 1115篇 |
2011年 | 1248篇 |
2010年 | 744篇 |
2009年 | 659篇 |
2008年 | 1035篇 |
2007年 | 1020篇 |
2006年 | 949篇 |
2005年 | 880篇 |
2004年 | 771篇 |
2003年 | 637篇 |
2002年 | 605篇 |
2001年 | 308篇 |
2000年 | 290篇 |
1999年 | 273篇 |
1998年 | 241篇 |
1997年 | 231篇 |
1996年 | 264篇 |
1995年 | 247篇 |
1994年 | 217篇 |
1993年 | 211篇 |
1992年 | 199篇 |
1991年 | 174篇 |
1990年 | 174篇 |
1989年 | 174篇 |
1988年 | 149篇 |
1987年 | 125篇 |
1986年 | 114篇 |
1985年 | 166篇 |
1984年 | 161篇 |
1983年 | 126篇 |
1982年 | 153篇 |
1981年 | 157篇 |
1980年 | 132篇 |
1979年 | 140篇 |
1978年 | 158篇 |
1977年 | 115篇 |
1976年 | 100篇 |
1975年 | 117篇 |
1974年 | 90篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
911.
The development of electrocatalysts is crucial for renewable energy applications. Metal‐doped graphene hybrid materials have been explored for this purpose, however, with much focus on noble metals, which are limited by their low availability and high costs. Transition metals may serve as promising alternatives. Here, transition metal‐doped graphene hybrids were synthesized by a simple and scalable method. Metal‐doped graphite oxide precursors were thermally exfoliated in either hydrogen or nitrogen atmosphere; by changing exfoliation atmospheres from inert to reductive, we produced materials with different degrees of oxidation. Effects of the presence of metal nanoparticles and exfoliation atmosphere on the morphology and electrocatalytic activity of the hybrid materials were investigated using electron microscopy, energy‐dispersive X‐ray spectroscopy, X‐ray photoelectron spectroscopy, and cyclic voltammetry. Doping of graphene with transition metal nanoparticles of the 4th period significantly influenced the electrocatalysis of compounds important in energy production and storage applications, with hybrid materials exfoliated in nitrogen atmosphere displaying superior performance over those exfoliated in hydrogen atmosphere. Moreover, nickel‐doped graphene hybrids displayed outstanding electrocatalytic activities towards reduction of O2 when compared to bare graphenes. These findings may be exploited in the research field of renewable energy. 相似文献
912.
Martin Pisárčik Mája Polakovičová Ferdinand Devínsky 《Central European Journal of Chemistry》2013,11(4):619-628
The solubilisation of poorly soluble antineoplastic drug camptothecin by nonionic surfactants (polysorbates and octylphenol ethoxylates) and alkyldimethylamine oxide surfactants with the alkyl chain length 8 to 16 carbon atoms was investigated. The hydrophobicity of the solubilising agent turned out to be the primary structural parameter controlling the solubility efficiency of camptothecin in an aqueous solution. The quantitative parameter of solubilisation (drug loading coefficient) provided values in the range of 0.1–1.2% and 0.1–1.0% for alkyldimethylamine oxides and nonionic surfactants, respectively. The decreasing number of oxyethylene units and the extension of the hydrophobic part of nonionic surfactant molecule resulted in the increase of camptothecin solubility. From the dynamic light scattering measurements, the hydrodynamic diameter values of camptothecin-loaded alkyldimethylamine oxide and nonionic micelles were found in the range of 4–42 nm and 5–120 nm, respectively. The experimental values confirmed the increase in micellar size with the increasing alkyl chain length. The values of the packing parameter of camptothecin-loaded dodecyldimethylamine oxide micelles indicate their spherical shape at all the investigated surfactant concentrations. A simple computer model of camptothecin-loaded dodecyldimethylamine oxide micelle provided the diameter of the structure cross section which is consistent with the experimental values. 相似文献
913.
Peirong Chen Ly May Chew Aleksander Kostka Kunpeng Xie Martin Muhler Wei Xia 《Journal of Energy Chemistry》2013,22(2):312-320
Oxygen- and nitrogen-functionalized carbon nanotubes (OCNTs and NCNTs) were applied as metal-free catalysts in selective olefin hydrogenation. A series of NCNTs was synthesized by NH3 post-treatment of OCNTs. Temperature-programmed desorption, N2 physisorption, Raman spectroscopy, high-resolution transmission electron microscopy and X-ray photoelectron spectroscopy were employed to characterize the surface properties of OCNTs and NCNTs, aiming at a detailed analysis of the type and amount of oxygen- and nitrogen-containing groups as well as surface defects. The gas-phase treatments applied for oxygen and nitrogen functionalization at elevated temperatures up to 600 °C led to the increase of surface defects, but did not cause structural damages in the bulk. NCNTs showed a clearly higher activity than the pristine CNTs and OCNTs in the hydrogenation of 1,5-cyclooctadiene, and also the selectivity to cyclooctene was higher. The favorable catalytic properties are ascribed to the nitrogen-containing surface functional groups as well as surface defects related to nitrogen species. In contrast, oxygen-containing surface groups and the surface defects caused by oxygen species did not show clear contribution to the hydrogenation catalysis. 相似文献
914.
Charles E. Nwankire Gerard G. Donohoe Xin Zhang Jonathan Siegrist Martin Somers Dirk Kurzbuch Ruairi Monaghan Maria Kitsara Robert Burger Stephen Hearty Julie Murrell Christopher Martin Martha Rook Louise Barrett Stephen Daniels Colette McDonagh Richard O’Kennedy Jens Ducrée 《Analytica chimica acta》2013
In this paper we report a centrifugal microfluidic “lab-on-a-disc” system for at-line monitoring of human immunoglobulin G (hIgG) in a typical bioprocess environment. The novelty of this device is the combination of a heterogeneous sandwich immunoassay on a serial siphon-enabled microfluidic disc with automated sequential reagent delivery and surface-confined supercritical angle fluorescence (SAF)-based detection. The device, which is compact, easy-to-use and inexpensive, enables rapid detection of hIgG from a bioprocess sample. This was achieved with, an injection moulded SAF lens that was functionalized with aminopropyltriethoxysilane (APTES) using plasma enhanced chemical vapour deposition (PECVD) for the immobilization of protein A, and a hybrid integration with a microfluidic disc substrate. Advanced flow control, including the time-sequenced release of on-board liquid reagents, was implemented by serial siphoning with ancillary capillary stops. The concentration of surfactant in each assay reagent was optimized to ensure proper functioning of the siphon-based flow control. The entire automated microfluidic assay process is completed in less than 30 min. The developed prototype system was used to accurately measure industrial bioprocess samples that contained 10 mg mL−1 of hIgG. 相似文献
915.
An efficient synthesis of dicarbonyl(η5—cyclopentadienyl)iron—(II)—complexes of 2,3—dihydrofuran and 3,4—dihydro—2H—pyran from 3—bromo—2—methoxytetrahydrofuran and—pyran, respectively, is described. 相似文献
916.
A. Sri Hari Kumar V. N. Kalevaru A. Qiao A. Alshammari N. Lingaiah Ch. Sailu P. S. Sai Prasad A. Martin 《Kinetics and Catalysis》2013,54(5):615-619
Oxidative dehydrogenation of ethane (ODHE) to ethylene was investigated over a series of alumina supported molybdophosphoric acid (MPA) catalysts. The MPA was transformed into surface Mo oxides on Al2O3 when subjected to calcination at 600°C. The catalysts were characterized by N2-adsorption, XRD, FT-IR spectroscopy and TPR techniques. The results showed that MPA loading and the source of Mo precursor had a clear influence on the catalytic performance. The evaluation of the catalysts for ODHE at temperatures between 450 and 550°C revealed superior ethane conversion (X~24%) and ethylene selectivity (S = ca. 65%) over 20 wt % MPA/Al2O3 catalyst. The transformation of MPA into finely dispersed Mo oxides on Al2O3 appeared to be responsible for this improved performance. 相似文献
917.
Under stannic chloride catalysis, trans-2-diethylphosphoryloxy-1,3-pentadiene (1) undergoes Diels-Alder reaction regio- and stereoselectively with a variety of α,β-unsaturated ketones at low temperature. 相似文献
918.
A. Ebenezer Martin 《合成通讯》2013,43(11):1778-1783
A novel method for the synthesis of 6,12‐dihydro‐2‐methylindolo[2,3‐b]carbazol‐6‐ones was developed from 1‐oxo‐2,3,4,9‐tetrahydro‐1H‐carbazol‐1‐one through methyl 6‐methyl‐2‐(1‐oxo‐2,3,4,9‐tetrahydro‐1H‐carbazol‐2‐yl)oxoacetate in good yields. This method provides an alternative path for the synthesis of this product using 2‐hydroxy methylene‐2,3,4,9‐tetrahydro‐1H‐carbazol‐1‐one. 相似文献
919.
The title compounds were prepared from halobromobenzenes via a palladium catalysed coupling followed by cyclisation. 相似文献
920.
Susann Paul W. Bernd Schweizer Graham Rugg Hans Martin Senn Ryan Gilmour 《Tetrahedron》2013,69(27-28):5647-5659
Herein, we report the synthesis and X-ray structural analysis of a collection of fluorinated metal N-heterocyclic carbenes (Ag, Au, Pd, Rh, Ir) and their precursor salts. The common structural feature of these species is a flanking fluoroethyl group, which is either freely rotating or embedded within a bicyclic framework. Solid state analysis confirmed a gauche conformational preference in all cases with the fluorine adopting a syn clinal arrangement (?[NCCF]~60°) with respect to the triazolium nitrogen at the vicinal position of the NHC. A density functional theory analysis was employed to quantify these effects and evaluate the influence of electronic modulation of the carbenic carbon [(CN+); neutral carbene (C:); metal-bound carbene (CM)], on the relative gauche/anti preference, thus highlighting the potential of this conformational phenomenon as a useful molecular design strategy for controlling the topology of organometallic complexes. 相似文献