全文获取类型
收费全文 | 3774篇 |
免费 | 129篇 |
国内免费 | 34篇 |
专业分类
化学 | 2748篇 |
晶体学 | 19篇 |
力学 | 160篇 |
数学 | 336篇 |
物理学 | 674篇 |
出版年
2024年 | 11篇 |
2023年 | 39篇 |
2022年 | 187篇 |
2021年 | 161篇 |
2020年 | 144篇 |
2019年 | 137篇 |
2018年 | 146篇 |
2017年 | 109篇 |
2016年 | 179篇 |
2015年 | 127篇 |
2014年 | 172篇 |
2013年 | 375篇 |
2012年 | 253篇 |
2011年 | 262篇 |
2010年 | 207篇 |
2009年 | 156篇 |
2008年 | 175篇 |
2007年 | 140篇 |
2006年 | 123篇 |
2005年 | 75篇 |
2004年 | 88篇 |
2003年 | 85篇 |
2002年 | 57篇 |
2001年 | 31篇 |
2000年 | 34篇 |
1999年 | 30篇 |
1998年 | 20篇 |
1997年 | 15篇 |
1996年 | 25篇 |
1995年 | 16篇 |
1994年 | 18篇 |
1993年 | 21篇 |
1992年 | 23篇 |
1991年 | 24篇 |
1990年 | 17篇 |
1989年 | 21篇 |
1988年 | 25篇 |
1987年 | 14篇 |
1986年 | 9篇 |
1985年 | 22篇 |
1984年 | 13篇 |
1982年 | 18篇 |
1981年 | 17篇 |
1980年 | 17篇 |
1979年 | 9篇 |
1978年 | 10篇 |
1976年 | 8篇 |
1975年 | 9篇 |
1974年 | 13篇 |
1973年 | 11篇 |
排序方式: 共有3937条查询结果,搜索用时 8 毫秒
51.
Hassan Amirhashchi Anirudh Pradhan Rekha Jaiswal 《International Journal of Theoretical Physics》2013,52(8):2735-2752
Some new exact solutions of Einstein’s field equations have come forth within the scope of a spatially homogeneous and anisotropic Bianchi type-III space-time filled with barotropic fluid and dark energy by considering a variable deceleration parameter. We consider the case when the dark energy is minimally coupled to the perfect fluid as well as direct interaction with it. Under the suitable condition, the anisotropic models approach to isotropic scenario. We also find that during the evolution of the universe, the equation of state (EoS) for dark energy ω (de), in both cases, tends to ?1 (cosmological constant, ω (de)=?1), by displaying various patterns as time increases, which is consistent with recent observations. The cosmic jerk parameter in our derived models are in good agreement with the recent data of astrophysical observations under appropriate condition. It is observed that the universe starts from an asymptotic Einstein static era and reaches to the ΛCDM model. So from recently developed Statefinder parameters, the behaviour of different stages of the universe has been studied. The physical and geometric properties of cosmological models are also discussed. 相似文献
52.
The effect of swelling in kerosene on the electrical conductance of 60, 80, and 100 part per hundred parts of rubber by weight of Fast Extrusion Furnace (FEF) carbon black-loaded styrene-butadiene rubber (SBR, 1502) was studied. It was found that there is a characteristic time of swelling after which a sudden decrease in conductivity appears. An ideal and simple model is suggested to calculate the carbon-carbon interspacing distance, D, in the carbon/rubber matrix. The dependence of D on the swelling time is also discussed. 相似文献
53.
Mohammad Reza Islami Zahra Hassani Hassan Sheibani 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2547-2554
Phthalimide or succinimide undergo a reaction with dialkyl acetylenedicarboxylates and triphenylphosphine to produce highly-functionalized stable heterocyclic phosphorus ylides in good yields in water-tetrahydrofuran (6:1). This methodology is of interest because of the use of water as solvent, thus minimizing the cost, operational hazards, and environmental pollution. 相似文献
54.
Abbas Shockravi Hashem Shargi Hassan Valizadeh Majid M. Heravi 《Phosphorus, sulfur, and silicon and the related elements》2013,188(11):2555-2559
An efficient synthesis of 3-substituted coumarins under solventless system is described. 相似文献
55.
Nashaat N. Nassar Azfar Hassan Pedro Pereira-Almao 《Journal of Thermal Analysis and Calorimetry》2012,110(3):1327-1332
This study investigated the catalytic effect of NiO, Co3O4 and Fe3O4 nanoparticles toward asphaltene thermal decomposition (pyrolysis) under inert conditions. Asphaltene adsorbed onto the selected nanoparticles were subjected to thermal decomposition up to 800?°C in a thermogravimetric analyzer. The presence of nanoparticles caused a significant decrease in the asphaltene decomposition temperature and activation energy. Activation energies for the process were calculated using the Ozawa?CFlynn?CWall method. All the selected metal oxide nanoparticles showed high catalytic activity toward asphaltene decomposition in the following order NiO?>?Co3O4?>?Fe3O4. This study confirms that metal oxide nanoparticles can significantly enhance the thermal decomposition of heavy hydrocarbons, like asphaltenes. 相似文献
56.
Amin Yoosefi Booshehri Ataallah Soltani Goharrizi Mohammad Hassan Fazaelipoor 《Research on Chemical Intermediates》2012,38(3-5):1021-1028
In this research the effect of silicone oil as a heavy organic solvent on the performance of a mixed tank bioreactor was investigated. In the first stage of the experiments, the response of a two-liquid-phase mixed tank bioreactor to periods of n-hexane starvation was compared with that of a control bioreactor. In the control bioreactor, after 3?days of starvation, approximately six days were needed to reach the removal efficiency it had before starvation. This figure was only 10?h for the silicone oil-containing bioreactor. The results confirmed that inclusion of a heavy organic solvent can increase the elimination capacity of a bioreactor and to help sustain high elimination capacity after starvation periods. In the second stage of the experiments, the effect of the amount of silicone oil on the performance of the bioreactor was investigated. Increasing the amount of oil from 5 to 10% (v/v) increased the maximum elimination capacity from 106 to 117?g/m3?h. 相似文献
57.
58.
Muhammad Imran ul‐haq Rajesh A. Shenoi Donald E. Brooks Jayachandran N. Kizhakkedathu 《Journal of polymer science. Part A, Polymer chemistry》2013,51(12):2614-2621
Hyperbranched polyglycerols (HPGs) are globular structures with a large number of functionalizable hydroxyl groups and have excellent in vitro and in vivo biocompatibility profiles comparable to polyethylene glycol. This work introduces a facile method for the synthesis of medium molecular weights (Mws) (50–300 kDa) HPGs, which has been difficult to synthesize with low polydispersity, with the assistance of solvents by ring opening polymerization. The influence of different solvents (1,4‐dioxane, tetrahydropyran (THP), ethylene glycol diethyl ether (EGDE) and decane), solvent to glycidol ratio, concentration of glycidol and the time of polymerization on Mw and polydispersity of HPGs has been studied. The Mw and polydispersity of HPGs are significantly affected by the nature of the polymerization phase (homogeneous or heterogeneous) and chemical structure of the solvent. The differences in the solvation of the potassium cations and change in the nucleophilicity of the alkoxide anion in various solvents may be responsible for the changes in Mw and PDI of the HPG. The Mw of the HPG decreases in the order 1,4‐dioxane > THP > EGDE >decane. The microstructure, solution and thermal properties of the HPG do not depend on the nature of solvent. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2013, 51, 2614–2621 相似文献
59.
Rapid direct and induced difference spectrophotometric methods for determination of pyrithioxin in single dosage forms (tablets and syrups) are reported. The direct methods depend upon measurement of the absorbance of pyrithioxin in different media at λmax i-e at 296 nm in 0.1 M hydrochloric acid, at 328 nm in citric acid-phosphate buffer of pH 7 and at 314 nm in 0.1 M sodium hydroxide. The mean percentage recovery of the authentic samples were 100.55±0.43, 101.21±0.58 and 100.29±0.64 respectively (P=0.05). The absorbance difference methods are based upon either measurement of the difference between the acid and the alkaline solutions i-e. Δ A (Alk-Acid) at 318 nm with an accuracy of 100.72±0.88 or the absorbance difference between the acid and neutral solutions i-e Δ A (pH 7-acid) at 328 nm with an accuracy of 100.31±0.68. 相似文献