全文获取类型
收费全文 | 10815篇 |
免费 | 1559篇 |
国内免费 | 1371篇 |
专业分类
化学 | 5015篇 |
晶体学 | 173篇 |
力学 | 402篇 |
综合类 | 67篇 |
数学 | 715篇 |
物理学 | 2429篇 |
综合类 | 4944篇 |
出版年
2024年 | 72篇 |
2023年 | 291篇 |
2022年 | 532篇 |
2021年 | 486篇 |
2020年 | 496篇 |
2019年 | 445篇 |
2018年 | 388篇 |
2017年 | 363篇 |
2016年 | 433篇 |
2015年 | 541篇 |
2014年 | 625篇 |
2013年 | 742篇 |
2012年 | 864篇 |
2011年 | 816篇 |
2010年 | 619篇 |
2009年 | 620篇 |
2008年 | 680篇 |
2007年 | 622篇 |
2006年 | 502篇 |
2005年 | 483篇 |
2004年 | 369篇 |
2003年 | 336篇 |
2002年 | 370篇 |
2001年 | 334篇 |
2000年 | 302篇 |
1999年 | 234篇 |
1998年 | 166篇 |
1997年 | 147篇 |
1996年 | 163篇 |
1995年 | 125篇 |
1994年 | 113篇 |
1993年 | 93篇 |
1992年 | 73篇 |
1991年 | 62篇 |
1990年 | 57篇 |
1989年 | 52篇 |
1988年 | 30篇 |
1987年 | 22篇 |
1986年 | 14篇 |
1985年 | 12篇 |
1984年 | 10篇 |
1983年 | 14篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1979年 | 4篇 |
1978年 | 2篇 |
1973年 | 3篇 |
1957年 | 2篇 |
1955年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
952.
Yajuan Xu Tunhai Xu Yue Liu Shengxu Xie Yunshan Si Tonghua Liu Dongming Xu 《Chemistry of Natural Compounds》2010,46(2):242-245
The structures of five furostanol glycosides (1–5), of which the 26-O-β-D-glucopyranosyl-(25S),5α-furost20(22)-en-12-one-2α,3β,26-triol-3-O-β-D-glucopyranosyl-(1→4)-β-D-galactopyranoside (1) was new, from the leaves of Tribulus terrestris L. were established using chemical and NMR spectroscopic methods. 相似文献
953.
The study has investigated the feasibility of using synthetic siderite for F− removal from aqueous solution. Batch experiments were performed to test effects of adsorbent dosage, contact time, initial F− concentration, temperature, solution pH, and coexisting anions on F− removal. Results show that the kinetic rate of F− adsorption was high in the first 2 h, and thereafter significantly decreased. The kinetic data was better fitted to the pseudo-second order kinetic model than the pseudo-first order kinetic model. In comparison with Langmuir isotherm, both Freundlich and Redlich-Peterson isotherms better described the adsorption process, which indicates that the multilayer adsorption should be involved in the process of F− removal. Thermodynamic study manifests that F− adsorption on synthetic siderite was spontaneous and exothermic in nature. The synthetic siderite had high adsorption capacity for F− removal, which was up to 1.775 mg/g in the batch with an adsorbent dosage of 5 g/L and an initial F− concentration of 20 mg/L at 25 °C. The adsorption was relatively independent on solution pH between 4 and 9. The presence of Cl− and NO3− had less impact on F− adsorption, while PO43− significantly affected F− removal from aqueous solution. Results of X-ray diffraction (XRD) and scanning electron microscopy (SEM) suggest that the high adsorption capacity possibly arose from both coprecipitation of ferric hydroxide with F− and adsorption of F− on the fresh goethite. 相似文献
954.
A simple and efficient microwave-assisted extraction of polyphenols from industrial apple pomace was developed and optimized by the maximization of the yield using response surface methodology. A Box-Behnken design was used to monitor the effect of microwave power, extraction time, ethanol concentration and ratio of solvent to raw material (g/mL) on the polyphenols yield. The results showed that the optimal conditions were as follows: microwave power 650.4?W, extraction time 53.7?s, ethanol concentration 62.1% and ratio of solvent to raw material 22.9:1. Validation tests indicated that the actual yield of polyphenols was 62.68±0.35?mg gallic acid equivalents per 100?g dry apple pomace with RSD=0.86% (n=5) under the optimal conditions, which was in good agreement with the predicted yield and higher than those of reflux and ultrasonic-assisted extraction methods. HPLC analysis indicated that the major polyphenols of apple pomace consisted of chlorogenic acid, caffeic acid, syrigin, procyanidin B2, (-)-epicatechin, cinnamic acid, coumaric acid, phlorizin and quercetin, of which procyanidin B2 had the highest content of 219.4?mg/kg. 相似文献
955.
Yue Zhu Min Pu De-Cai Fang Yong-Qiang Ji Jing He David G. Evans 《Structural chemistry》2010,21(4):817-825
Abstract
The cis–trans isomerization pathways of 3,3′-azobenzene disulphonate in the S0 and T1 states are studied by DFT method at the B3LYP/6-31G(d,p) level. In the S0 state, the cis–trans isomerization concerns the complex pathway that is characterized by the inversion of one NNC angle combined with rotation around the NC bond, and the three sequential transition states are also found on the potential energy profile. Therefore, the cis–trans isomerization of 3,3′-azobenzene disulphonate can be understood in terms of a pathway involving successive rotation, inversion, and rotation processes. The energy barrier of the S0 state is 22.79 kcal mol−1. In the T1 state, the isomerization mainly concerns the rotational pathway around the NN double bond, and the two isomers are connected through only one transition state. The isomerization of the T1 state is related to a lower energy barrier, 5.02 kcal mol−1, but requires a change in spin-multiplicity. 相似文献956.
Ting-Fu Jiang Zhi-Hua Lv Yuan-Hong Wang Mei-E Yue Jia-Huan Peng 《Journal of Analytical Chemistry》2010,65(9):945-950
In this work, a simple, reproducible and sensitive micellar electrokinetic chromatography (MEKC) method was developed for the separation and determination of five triterpenoids, lupeol (1), 1β-hydroxy-lupeol (2), lup-3β,1α-diol
(3), lup-1β,3β,11α-triol (4) and 30-norlupan-3β,11α-diol-20-one (5) in traditional Chinese medicine of Salvia roborowskii Maxim. Field-enhanced sample injection with reverse migrating micelles (FESI-RMM) was used for on-line concentration of triterpenoids. The optimum buffer contained 50 mM H3PO4, 160 mM SDS, 20% acetonitrile and 15% 2-propanol and pH of buffer was 2.0. The sample solution was diluted with 10 mM H3PO4 (pH 2.5, containing 10 mM SDS) and injected for 15 s with −8 kV after injection of 4 s water plug. The effects of concentrations
of sodium dodecyl sulfate (SDS) and organic modifier, the sample matrix, the injection time of water plug, the injection voltage and injection time of sample
on the separation and stacking efficiency were investigated. Under the optimum conditions, the analytes were well separated
and by optimizing the stacking conditions, about 28–96-fold improvement in the detection sensitivity was obtained for triterpenoids.
The contents of five triterpenoids in Salvia roborowskii Maxim were successfully determined with satisfactory repeatability and recovery. 相似文献
957.
Ma Y Xu GF Yang X Li LC Tang J Yan SP Cheng P Liao DZ 《Chemical communications (Cambridge, England)》2010,46(43):8264-8266
Utilization of the strong electron-withdrawing ligand, hfac, leads to a novel dinuclear dysprosium single-molecule magnet featuring a pyrazine bridge with a large anisotropic barrier. 相似文献
958.
Two kinds of online preconcentration techniques including large-volume sample stacking-sweeping and selective-exhaustive injection-sweeping (SEI-S) were employed in CE to develop a rapid, simple and sensitive method for determination of melamine (MM) and its derivatives such as ammeline (AMN), ammelide (AMD) and cyanuric acid (CA) in liquid milk products. Though the sensitivity of large-volume sample stacking-sweeping is not particularly high, it can be used to detect all the four compounds simultaneously. However, it is very difficult to improve the sensitivity of these four compounds by using single SEI-S method directly, owing to their completely different extents of protonation or deprotonation. Grouping can solve this problem perfectly. After the four compounds are divided into two groups (MM/AMN and AMD/CA), cation-SEI-S or anion-SEI-S can be applied for their online stacking. In cation-SEI-S, the detection limits for MM and AMN were both 0.01 ng/mL based on the signal-to-noise ratio of 3. In anion-SEI-S, the detection limits were 0.05 and 0.02 ng/mL for CA and AMD, respectively. Results of this study show a great potential for the SEI-S method to be a useful tool for the rapid and sensitive determination of MM and its derivatives in liquid milk products. 相似文献
959.
Dual-functional materials with AIE behavior and mech-anoresponsive behavior have attracted considerable attention due to their promising applications in mechano... 相似文献
960.
Chuan Zhou Jia Yue Zhao Peng Fei Liu Jianfu Chen Sheng Dai Hua Gui Yang P. Hu Haifeng Wang 《Chemical science》2021,12(31):10634
Given a desired property, locating relevant materials is always highly desired but very challenging in a range of areas, including heterogeneous catalysis. Obviously, object-oriented design/screening is an ideal solution to this problem. Herein, we develop an inverse catalyst design workflow in Python (CATIDPy) that utilizes a genetic-algorithm-based global optimization method to guide on-the-fly density functional theory calculations, successfully realizing the highly accelerated location of active single-atom alloy (SAA) catalysts for the hydrogen evolution reaction (HER). 70 binary and 752 ternary SAA candidate catalysts are identified for the HER. Furthermore, via considering the segregation stability and cost of materials, we extracted 6 binary and 142 ternary SAA candidate catalysts that are recommended for experimental synthesis. Remarkably, guided by these theoretical identifications, homogeneously dispersed Ni-based bimetallic catalysts (e.g., NiMo, NiAl, Ni3Al, NiGa, and NiIn) were synthesized experimentally to test the reliability of the CATIDPy workflow, and they showed superior HER performance to bare Ni foam, indicating huge potential for use in real-world water electrolysis techniques. Perhaps more importantly, these results demonstrate the capacity of such a proposed approach for investigating unexplored chemical spaces to efficiently design promising catalysts without knowledge from the expert domain, which has far-reaching implications.An inverse catalyst design workflow in Python (CATIDPy) for discovering unexplored chemical spaces successfully realized the highly accelerated location of active single-atom alloy (SAA) catalysts for the hydrogen evolution reaction (HER). 相似文献