全文获取类型
收费全文 | 29627篇 |
免费 | 4152篇 |
国内免费 | 3021篇 |
专业分类
化学 | 16085篇 |
晶体学 | 278篇 |
力学 | 1347篇 |
综合类 | 191篇 |
数学 | 2314篇 |
物理学 | 8272篇 |
无线电 | 8313篇 |
出版年
2024年 | 69篇 |
2023年 | 681篇 |
2022年 | 730篇 |
2021年 | 1096篇 |
2020年 | 1075篇 |
2019年 | 1010篇 |
2018年 | 872篇 |
2017年 | 910篇 |
2016年 | 1239篇 |
2015年 | 1280篇 |
2014年 | 1487篇 |
2013年 | 1977篇 |
2012年 | 2291篇 |
2011年 | 2398篇 |
2010年 | 1639篇 |
2009年 | 1680篇 |
2008年 | 1875篇 |
2007年 | 1764篇 |
2006年 | 1579篇 |
2005年 | 1311篇 |
2004年 | 1016篇 |
2003年 | 915篇 |
2002年 | 858篇 |
2001年 | 680篇 |
2000年 | 646篇 |
1999年 | 688篇 |
1998年 | 615篇 |
1997年 | 566篇 |
1996年 | 609篇 |
1995年 | 515篇 |
1994年 | 550篇 |
1993年 | 380篇 |
1992年 | 326篇 |
1991年 | 311篇 |
1990年 | 215篇 |
1989年 | 164篇 |
1988年 | 142篇 |
1987年 | 107篇 |
1986年 | 106篇 |
1985年 | 100篇 |
1984年 | 69篇 |
1983年 | 62篇 |
1982年 | 47篇 |
1981年 | 31篇 |
1980年 | 33篇 |
1979年 | 20篇 |
1978年 | 11篇 |
1977年 | 9篇 |
1974年 | 7篇 |
1973年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Dr. Qingquan Lu Steffen Greßies Felix J. R. Klauck Prof. Dr. Frank Glorius 《Angewandte Chemie (International ed. in English)》2017,56(23):6660-6664
A MnI-catalyzed regioselective C−H allenylation is reported that allows a broad range of 2-allenylindoles to be synthesized regioselectively on a gram scale under simple conditions. Notably, a highly efficient chirality transfer was observed (up to 93 % ee) in this transformation. This procedure was further found to allow, for the first time, the direct preparation of ketones by MnI-catalyzed C−H activation. Mechanistic investigations revealed that the precoordination of the oxygen atom to the manganese center as well as the congested tertiary carbon atom in the propargylic carbonates play a crucial role. 相似文献
992.
Wenxian Guo Dr. Meiqiong Chen Dr. Xiaoqing Liu Prof. Faliang Cheng Prof. Xihong Lu 《Chemistry (Weinheim an der Bergstrasse, Germany)》2021,27(13):4291-4296
A simple, cost-effective strategy was developed to effectively improve the electron transfer efficiency as well as the power output of microbial fuel cells (MFCs) by decorating the commercial carbon paper (CP) anode with an advanced Mo2C/reduced graphene oxide (Mo2C/RGO) composite. Benefiting from the synergistic effects of the superior electrocatalytic activity of Mo2C, the high surface area, and prominent conductivity of RGO, the MFC equipped with this Mo2C/RGO composite yielded a remarkable output power density of 1747±37.6 mW m−2, which was considerably higher than that of CP-MFC (926.8±6.3 mW m−2). Importantly, the composite also facilitated the formation of 3D hybrid biofilm and could effectively improve the bacteria–electrode interaction. These features resulted in an enhanced coulombic efficiency up 13.2 %, nearly one order of magnitude higher than that of the CP (1.2 %). 相似文献
993.
A series of sphere-rod shape amphiphiles were designed and synthesized by connecting the rod-like oligofluorenes with different lengths(OF_n) to the different positions of the spherical [60]fullerene(C_(60)) through a rigid linkage. The conjugates were characterized by ~1H-NMR, ~(13)C-NMR, FTIR, EA and MALDI-TOF mass spectrometry. The optical and electronic properties of the conjugates were studied by UV-Vis absorption spectroscopy, fluorescence spectrometry, and cyclic voltammetry. The results from UV-Vis absorption spectroscopy and cyclic voltammetry indicated that the energy profiles of C_(60) and OF_n remained unchanged when different lengths of OF_n were attached to C_(60). The electron affinities of the OF_n-C_(60) conjugates were close to that of C_(60), while slight electronic interaction was found between the two individual chromophores(C_(60) and OF_n) in their ground states. The fluorescence spectra exhibited a complete fluorescence quenching in the toluene solution, suggesting an effective energy transfer from OF_n to C_(60). It presents a systematic study on the selfassembly, structure-property relationship, and potential technical applications of the conjugates. 相似文献
994.
Turbulence modelling and role of compressibility on oil spilling from a damaged double hull tank
下载免费PDF全文
![点击此处可从《国际流体数值方法杂志》网站下载免费的PDF全文](/ch/ext_images/free.gif)
The viscosity plays an important role, and a multiphase solver is necessary to numerically simulate the oil spilling from a damaged double hull tank (DHT). However, it is uncertain whether turbulence modelling is necessary, which turbulence model is suitable; and what the role of compressibility of the fluids is. This paper presents experimental and numerical investigations to address these issues for various cases representing different scenarios of the oil spilling, including grounding and collision. In the numerical investigations, various approaches to model the turbulence, including the large eddy simulation (LES), direct numerical simulation and the Reynolds average Navier–Stokes equation (RANS) with different turbulence models, are employed. Based on the investigations, it is suggested that the effective Reynolds numbers corresponding to both oil outflow and water inflow shall be considered when classifying the significance of the turbulence and selecting the appropriate turbulence models. This is confirmed by new lab tests considering the axial offset between the internal and the external holes on two hulls of the DHT. The investigations conclude for numerically simulating oil spilling from a damaged DHT that when the effective Re is smaller the RANS approaches should not be used and LES modelling should be employed; while when the effective Reynolds numbers is large, the RANS models may be used as they can give similar results to LES in terms of the height of the mixture in the ballast tank and discharge but costing much less CPU time. The investigation on the role of the compressibility of the fluid reveals that the compressibility of the fluid may be considerable in a small temporal‐spatial scale but plays an insignificant role on macroscopic process of the oil spilling. Copyright © 2016 John Wiley & Sons, Ltd. 相似文献
995.
Quanfei Zou Yuan Gu Rong Lu Tiejun Zhang Guang‐Rong Zhao Changxiao Liu Duanyun Si 《Biomedical chromatography : BMC》2013,27(9):1123-1128
In this study, a simple and sensitive LC/MS/MS method was developed and validated for the determination of arctigenin in rat plasma. The MS detection was performed using multiple reaction monitoring at the transitions of m/z 373.2 → 137.3 for arctigenin and m/z 187.1 → 131.0 for psoralen (internal standard) with a Turbo IonSpray electrospray in positive mode. The calibration curves fitted a good linear relationship over the concentration range of 0.2–500 ng/mL. It was found that arctigenin is not stable enough at both room temperature and ?80 °C unless mixed with methanol before storage. The validated LC/MS/MS method was successfully applied for the pharmacokinetic study of arctigenin in rats. After intravenous injection of 0.3 mg/kg arctigenin injection to rats, the maximum concentration, half‐life and area under the concentration–time curve were 323 ± 65.2 ng/mL, 0.830 ± 0.166 and 81.0 ± 22.1 h ng/mL, respectively. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献
996.
997.
To improve the initial coulombic efficiency and bulk density of ordered mesoporous carbons, active Fe2O3 nanoparticles were introduced into tubular mesopore channels of CMK-5 carbon, which possesses high specific surface area (>1700 m 2 g-1 ) and large pore volume (>1.8 cm 3 g-1 ). Fine Fe2O3 nanoparticles with sizes in the range of 5 7 nm were highly and homogenously encapsulated into CMK-5 matrix through ammonia-treatment and subsequent pyrolysis method. The Fe2O3 loading was carefully tailored and designed to warrant a high Fe2O3 content and adequate buffer space for improving the electrochemical performance. In particular, such Fe2O3 and mesoporous carbon composite with 47 wt% loading exhibits a considerably stable cycle performance (683 mAh g-1 after 100 cycles, 99% capacity retention against that of the second cycle) as well as good rate capability. The fabrication strategy can effectively solve the drawback of single material, and achieve a high-performance lithium electrode material. 相似文献
998.
Yunxiang Deng Chaopan Peng Ping Liu Jiang Lu Longmei Zeng Lichang Wang 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(8):995-1004
Abstract The copolymerization products of α-pinene and styrene were prepared with the compound catalyst system SbCl3/AlCl3 by changing the Sb/Al ratio, feeding monomer ratio, solvent, and polymerization temperature. The compositions of the copolymerization products were quantitatively characterized by the method of the combination of micro-ozonization and thin-layer chromatography in terms of the contents of the homopolymers and the copolymers containing high or low mole fractions of α-pinene, the yields of pure copolymer, and the monomer unit ratios (F 1) of copolymers. The results show that it was easier to obtain the higher yield (up to 80%) of the pure copolymer with the complex catalyst system than with AlCl3 alone. The F 1 values could be controlled between 30 and 56% under the following polymerization conditions: Sb/Al 1/2, α-pinene/styrene 70%, and the conversion of styrene 90%. 相似文献
999.
The B3LYP/6-31G(d) method of density functional theory was used to study molecular geometry, electronic structure, infrared spectrum, and thermodynamic properties. Detonation properties were evaluated using Kamlet–Jacobs equations based on the calculated density and heat of formation. Thermal stability of 3,5,7,10,12,14,15,16-octanitro-3,5,7,10,12,14,15,16-octaaza-pentacyclo[7.5.1.12,8.04,13.06,11]hexadecane (cage-HMX) was investigated by calculating the bond dissociation energy at unrestricted B3LYP/6-31G(d) level. The calculated results show that the first step of pyrolysis is the rupture of the N–NO2 bond. The crystal structure obtained by molecular mechanics belongs to P21 space group, with lattice parameters a = 8.866 Å, b = 11.527 Å, c = 13.011 Å, Z = 4, and ρ = 2.219 g cm?3. Both the detonation velocity of 9.79 km s?1 and the detonation pressure of 45.45 GPa are better than those of CL-20. According to the quantitative standard of energetics and stability as a high-energy-density compound, cage-HMX essentially satisfies this requirement. These results provide basic information for molecular design of novel HEDCs. 相似文献
1000.
Chen Lai Liyuan Sheng Shibo Liao Tingfei Xi Zhixiong Zhang 《Surface and interface analysis : SIA》2013,45(11-12):1673-1679
In this study, we focused on the surface character of bacterial cellulose (BC) before and after oxidation mediated by 2,2,6,6‐tetramethylpiperidine‐1‐oxyl (TEMPO).Solid‐state 13C NMR, XPS, SEM, contact angle and surface free energy analyses were performed to investigate the effects of various parameters (reaction time and oxidant and catalyst concentrations) on the surface composition, morphology and polarity of the BC. The results provided by the combined use of these techniques showed that hydrogen bonds were disrupted on the BC surface after carboxylation occurred; therefore, the surface of oxidized BC was rougher than that of the original BC, and the surface free energy, especially the polar components, increased after oxidation. Copyright © 2013 John Wiley & Sons, Ltd. 相似文献