全文获取类型
收费全文 | 21130篇 |
免费 | 3753篇 |
国内免费 | 2376篇 |
专业分类
化学 | 15034篇 |
晶体学 | 236篇 |
力学 | 1305篇 |
综合类 | 120篇 |
数学 | 2207篇 |
物理学 | 8357篇 |
出版年
2024年 | 57篇 |
2023年 | 499篇 |
2022年 | 563篇 |
2021年 | 782篇 |
2020年 | 885篇 |
2019年 | 860篇 |
2018年 | 774篇 |
2017年 | 682篇 |
2016年 | 1053篇 |
2015年 | 1003篇 |
2014年 | 1199篇 |
2013年 | 1548篇 |
2012年 | 1930篇 |
2011年 | 1858篇 |
2010年 | 1288篇 |
2009年 | 1190篇 |
2008年 | 1317篇 |
2007年 | 1237篇 |
2006年 | 1141篇 |
2005年 | 1013篇 |
2004年 | 733篇 |
2003年 | 617篇 |
2002年 | 587篇 |
2001年 | 467篇 |
2000年 | 438篇 |
1999年 | 485篇 |
1998年 | 396篇 |
1997年 | 384篇 |
1996年 | 366篇 |
1995年 | 322篇 |
1994年 | 252篇 |
1993年 | 238篇 |
1992年 | 195篇 |
1991年 | 158篇 |
1990年 | 178篇 |
1989年 | 125篇 |
1988年 | 87篇 |
1987年 | 61篇 |
1986年 | 65篇 |
1985年 | 69篇 |
1984年 | 31篇 |
1983年 | 36篇 |
1982年 | 30篇 |
1981年 | 20篇 |
1980年 | 12篇 |
1979年 | 3篇 |
1977年 | 3篇 |
1957年 | 3篇 |
1930年 | 2篇 |
1916年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Shaoming Duan Edith R. Binkley Roger W. Binkley 《Journal of carbohydrate chemistry》2013,32(7):1029-1034
Abstract In 1968 Zen and coworkers reported that thep-tolylsulfonyl group could be removed from carbohydrate systems by photochemical reaction (eq 1).1 Since then other investigators have used this deprotection process in carbohydrate synthesis.2-10 Mechanistic studies11-16 have shown that tosylate photolysis is promoted by compounds (e.g., triethylamine) that donate an electron to an excitedp-toluenesulfonate to generate a radical anion (1). This intermediate then fragments to form the anion of the deprotected sugar (Scheme 1). Since generating a radical anion is the central element in this photochemical process, structural changes that impact radical anion formation should influence the reaction. Replacing the p - tolylsulfonyl group with the pentafluorophenylsulfonyl group generates a more stable radical anion (2) because the electronegative fluorine atoms can help stabilize the negative charge. Since we have a continuing interest in the photochemistry of sulfonic acid esters, we synthesized the pentafluorobenzenesulfonates (pentaflates) 3-6 and studied their photochemistry under electron transfer conditions. 相似文献
992.
Yaoyao Zhang Xiaoyan Wu Ye Lin Dan Wang Chunming Zhang Dannong He 《Journal of Sol-Gel Science and Technology》2013,68(2):169-174
LiNi1/3Co1/3Mn1/3O2 was prepared by a modified sol–gel method, selecting ethylene diamine tetraacetic acid and citric acid as the co-chelating agent. The mole ratios of ethylene diamine tetraacetic acid (EDTA) to metal ion (Mn+) were 0:1, 1:1 and 2:1. The obtained samples were characterized by XRD, BET and SEM. The XRD showed that LiNi1/3Co1/3Mn1/3O2 had good crystallinity and well-ordered layered structure. After calcined at 850 °C, the LiNi1/3Co1/3Mn1/3O2 particles exhibited a three-dimensional space network structure, which was greatly correlated with the ratio of EDTA to metal ion. The LiNi1/3Co1/3Mn1/3O2 obtained from a mole ratio of 1:1 (EDTA:Mn+) had the best electrochemical performance. The reversible capacities were reached 168 and 100 mAh/g at 1C and 10C discharge rate, respectively. The result of the cycling performance showed a high capacity maintenance ratio of 89.3 % at 1C and 25 °C after 50 cycles. The further electrochemical performance was evaluated by electrochemical impedance spectroscopy and cyclic voltammetry. 相似文献
993.
Pingting He Jie Tao Xianli Huang Jianjun Xue 《Journal of Sol-Gel Science and Technology》2013,68(2):213-218
Nitrogen doped TiO2 (N-TiO2) nanoparticles with about 30 nm in size were produced by a sol–gel method and characterized respectively by UV–vis, X-ray diffraction (XRD), Transmission electron microscopy, X-ray photoelectron spectroscopy (XPS). Their photocatalytic antibacterial properties were evaluated by the antibacterial ratio against Escherichia coli in dark and under simulated sunlight respectively. The XRD pattern showed that the doped nano-TiO2 was mainly composed of anatase phase. The XPS spectra of the N-TiO2 sample indicated that TiO2 was doped by nitrogen atom. The nitrogen doping created a new N 2p state slightly above the valence band top consists of O 2p state, and this pushes up the valence band top and decreased the band gap. Which leaded to the absorption edge was red-shifted to the visible light region of UV–vis spectra of nitrogen doped nano-TiO2 comparing with pure nano-TiO2. The antibacterial percentage of N-TiO2 against E. coli reached to 90 % under simulated sunlight for 2 h, which was much better than that in dark, also than that of pure nano-TiO2. The photo-catalytic antibacterial activity was activated under visible light. The structure and integrity of cell wall and cell membrane were destructed, and even caused the bacteria death. 相似文献
994.
Li Zhang Li Tong Zhang Jing He Zhi Jun Lin Yan Jiang Li Fu Chen 《Journal of Sol-Gel Science and Technology》2013,68(1):150-154
Aluminum nitride (AlN) fibers were prepared from alumina gel fibers and by heat-treatment in ammonia. The influence of silica on the formation of AlN was investigated. It was shown that phase transformation of alumina (γ-Al2O3 to α-Al2O3) and nitridation reaction took place above 1,100 °C for pure alumina fiber. The addition of a small amount of silica (3 wt%) suppressed the formation of α-Al2O3 and preserved the highly reactive metastable alumina, and nitridation rate was enhanced. Fine grain (~20 nm) AlN fibers were obtained for pyrolysis at 1,150–1,250 °C for 3 h in ammonia, and AlN was identified as the sole crystalline phase. 相似文献
995.
Y. J. Zhu X. R. Zhao L. B. Duan X. J. Bai H. N. Sun W. F. Duan 《Journal of Sol-Gel Science and Technology》2013,67(1):155-160
ZnO co-doped with 2 at.% Sm and different Li concentration (0–7 at.%) powders were fabricated by the sol–gel method with 700 °C annealing. The effect of Li doping concentration on the structure and photoluminescence (PL) of ZnO powders doped with 2 at.% Sm was investigated. Based on the balance of structure and valence, with the help of Li doping (1, 2 at.%) into ZnO powders doped with 2 at.% Sm, Sm3+ ions enter ZnO crystal lattice and induce the characteristic Sm3+ emission peaks by the ultra-violet (UV) excitation (278 nm). Especially, when the Li doping concentration is 2 at.%, the sample has the most efficient Sm characteristic emission line. However, Li will hinder the substitution of zinc location by Sm3+ when the Li doping concentration is above 3 at.%, which results in the disappearance of the characteristic samarium emission lines. 相似文献
996.
Li‐Wei Zhao Hui‐Min Shi Zhe An Jiu‐Zhao Wang Prof. Jing He 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(37):12350-12355
Effectively enhancing the enantioselectivity is a persistent challenge in heterogeneous asymmetric catalysis. Here, the validity of a layered double hydroxides (LDH) nanosheet as an efficient planar substituent to enhance the enantioselectivity has been investigated theoretically; first in vanadium‐catalyzed asymmetric epoxidation of allylic alcohols, and then in zinc‐catalyzed direct asymmetric aldol addition. The computational predication is further confirmed experimentally in zinc‐catalyzed direct asymmetric aldol addition by controlling the location of catalytic sites. 相似文献
997.
Yang Xu Ming Wu Yang Liu Prof. Dr. Xi‐Zeng Feng Prof. Dr. Xue‐Bo Yin Prof. Xi‐Wen He Prof. Yu‐Kui Zhang 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(7):2276-2283
Carbon dots (Cdots) are an important probe for imaging and sensing applications because of their fluorescence property, good biocompatibility, and low toxicity. However, complex procedures and strong acid treatment are often required and Cdots suffer from low photoluminescence (PL) emission. Herein, a facile and general strategy using carbonization of precursors and then extraction with solvents is proposed for the preparation of nitrogen‐doped Cdots (N‐Cdots) with 3‐(3,4‐dihydroxyphenyl)‐L ‐alanine (L ‐DOPA), L ‐histidine, and L ‐arginine as precursor models. After they are heated, the precursors become carbonized. Nitrogen‐doped Cdots are subsequently extracted into N,N′‐dimethylformamide (DMF) from the carbogenic solid. A core–shell structure of Cdots with a carbon core and the oxygen‐containing shell was observed. Nitrogen has different forms in N‐Cdots and oxidized N‐Cdots. The doped nitrogen and low oxidation level in N‐Cdots improve their emission significantly. The N‐Cdots show an emission with a nitrogen‐content‐dependent intensity and Cdot‐size‐dependent emission‐peak wavelength. Imaging of HeLa cells, a human cervical cancer cell line, and HepG2 cells, a human hepatocellular liver carcinoma line, was observed with high resolution using N‐Cdots as a probe and validates their use in imaging applications and their multicolor property in the living cell system. 相似文献
998.
Dr. Ming‐Hua Xie Xiu‐Li Yang Dr. Yabing He Prof. Dr. Jian Zhang Prof. Dr. Banglin Chen Prof. Dr. Chuan‐De Wu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(42):14316-14321
A simple strategy to rationally immobilize metalloporphyrin sites into porous mixed‐metal–organic framework (M′MOF) materials by a metalloligand approach has been developed to mimic cytochrome P450 monooxygenases in a biological system. The synthesized porous M′MOF of [Zn2(MnOH–TCPP)(DPNI)] ? 0.5 DMF ? EtOH ? 5.5 H2O ( CZJ‐1 ; CZJ=Chemistry Department of Zhejiang University; TCPP=tetrakis(4‐carboxyphenyl)porphyrin); DPNI=N,N′‐di(4‐pyridyl)‐1,4,5,8‐naphthalenetetracarboxydiimide) has the type of doubly interpenetrated cubic α‐Po topology in which the basic Zn2(COO)4 paddle‐wheel clusters are bridged by metalloporphyrin to form two‐dimensional sheets that are further bridged by the organic pillar linker DPNI to form a three‐dimensional porous structure. The porosity of CZJ‐1 has been established by both crystallographic studies and gas‐sorption isotherms. CZJ‐1 exhibits significantly high catalytic oxidation of cyclohexane with conversion of 94 % to the mixture of cyclohexanone (K) and cyclohexanol (A) (so‐called K–A oil) at room temperature. We also provided solid experimental evidence to verify the catalytic reaction that occurred in the pores of the M′MOF catalyst. 相似文献
999.
Dr. Qiu‐Feng Lü Jia‐Yin Zhang Jun Yang Zhi‐Wei He Prof. Dr. Chang‐Qing Fang Qilang Lin 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(33):10935-10944
Self‐assembled poly(N‐methylaniline)–lignosulfonate (PNMA–LS) composite spheres with reactive silver‐ion adsorbability were prepared from N‐methylaniline by using lignosulfonate (LS) as a dispersant. The results show that the PNMA–LS composite consisted of spheres with good size distribution and an average diameter of 1.03–1.27 μm, and the spheres were assembled by their final nanofibers with an average diameter of 19–34 nm. The PNMA–LS composite spheres exhibit excellent silver‐ion adsorption; the maximum adsorption capacity of silver ions is up to 2.16 g g?1 at an adsorption temperature of 308 K. TEM and wide‐angle X‐ray results of the PNMA–LS composite spheres after absorption of silver ions show that silver ions are reduced to silver nanoparticles with a mean diameter of about 11.2 nm through a redox reaction between the PNMA–LS composite and the silver ions. The main adsorption mechanism between the PNMA–LS composite and the silver ions is chelation and redox adsorption. In particular, a ternary PNMA–LS–Ag composite achieved by using the reducing reaction between PNMA–LS composite spheres and silver ions can be used as an antibacterial material with high bactericidal rate of 99.95 and 99.99 % for Escherichia coli and Staphylococcus aureus cells, respectively. 相似文献
1000.