全文获取类型
收费全文 | 22734篇 |
免费 | 3482篇 |
国内免费 | 2442篇 |
专业分类
化学 | 16070篇 |
晶体学 | 276篇 |
力学 | 1353篇 |
综合类 | 187篇 |
数学 | 2297篇 |
物理学 | 8475篇 |
出版年
2024年 | 67篇 |
2023年 | 423篇 |
2022年 | 739篇 |
2021年 | 810篇 |
2020年 | 849篇 |
2019年 | 833篇 |
2018年 | 711篇 |
2017年 | 736篇 |
2016年 | 1015篇 |
2015年 | 1035篇 |
2014年 | 1171篇 |
2013年 | 1606篇 |
2012年 | 1866篇 |
2011年 | 1989篇 |
2010年 | 1297篇 |
2009年 | 1266篇 |
2008年 | 1463篇 |
2007年 | 1349篇 |
2006年 | 1217篇 |
2005年 | 970篇 |
2004年 | 774篇 |
2003年 | 658篇 |
2002年 | 632篇 |
2001年 | 461篇 |
2000年 | 456篇 |
1999年 | 481篇 |
1998年 | 422篇 |
1997年 | 423篇 |
1996年 | 463篇 |
1995年 | 365篇 |
1994年 | 381篇 |
1993年 | 272篇 |
1992年 | 254篇 |
1991年 | 234篇 |
1990年 | 165篇 |
1989年 | 129篇 |
1988年 | 117篇 |
1987年 | 97篇 |
1986年 | 95篇 |
1985年 | 85篇 |
1984年 | 58篇 |
1983年 | 49篇 |
1982年 | 38篇 |
1981年 | 28篇 |
1980年 | 31篇 |
1979年 | 18篇 |
1978年 | 10篇 |
1977年 | 8篇 |
1975年 | 7篇 |
1973年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
921.
介绍了光纤光栅传感原理,依据传感机理制作了两种结构材料的光纤光栅传感器,对它们进行了试验,将试验后的传感数据进行了拟合分析和比对,并对它们在低温环境下产生的啁啾现象进行了分析,验证了采用预拉工艺对解决啁啾现象的可行性。 相似文献
922.
Chia-Wen Lu Yan Cao Hui Li Cathleen Webb Wei-Ping Pan 《Journal of Thermal Analysis and Calorimetry》2014,116(3):1241-1248
Titanium dioxide (TiO2), especially in its anatase form, is an effective photocatalyst under ultraviolet (UV) light. The particle size of TiO2 is a critical factor to determine its photoactivity based on its quantum effectiveness under light irradiations. Thus, nanocrystalline TiO2 has been widely accepted to significantly enhance this effect. The sol–gel method is generally used to synthesize the anatase form of nanocrystalline TiO2. In this study, we expanded the synthesis method of TiO2 to high pressures under direct heating (hydrothermal method) and indirect heating (microwave-assisted method). It was found that pH value is one of the major factors to control nano-sizes of TiO2 particles, and the neutral condition in all methods is preferable for controlling the sizes of the prepared TiO2 particles. The microwave-assisted method further improves quality of synthesized nano-size TiO2 below 10 nm. These results have been confirmed by both the direct size measurement using TEM images and indirect determination using XRD peaks. The collected samples are further analyzed using UV–Vis spectroscopy to identify the particle size-dependent photoreactivity and to confirm the effectiveness of microwave-assisting under neutral conditions. DSC is also a powerful tool to identify the crystalline transition of TiO2. 相似文献
923.
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
924.
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
925.
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
926.
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
927.
928.
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
929.
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
930.
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