全文获取类型
收费全文 | 2705篇 |
免费 | 96篇 |
国内免费 | 17篇 |
专业分类
化学 | 1743篇 |
晶体学 | 11篇 |
力学 | 68篇 |
数学 | 518篇 |
物理学 | 478篇 |
出版年
2023年 | 23篇 |
2022年 | 19篇 |
2021年 | 61篇 |
2020年 | 72篇 |
2019年 | 60篇 |
2018年 | 56篇 |
2017年 | 61篇 |
2016年 | 107篇 |
2015年 | 85篇 |
2014年 | 91篇 |
2013年 | 212篇 |
2012年 | 225篇 |
2011年 | 225篇 |
2010年 | 139篇 |
2009年 | 117篇 |
2008年 | 173篇 |
2007年 | 178篇 |
2006年 | 157篇 |
2005年 | 123篇 |
2004年 | 111篇 |
2003年 | 75篇 |
2002年 | 73篇 |
2001年 | 34篇 |
2000年 | 34篇 |
1999年 | 27篇 |
1998年 | 22篇 |
1997年 | 12篇 |
1996年 | 15篇 |
1995年 | 26篇 |
1994年 | 16篇 |
1993年 | 14篇 |
1992年 | 14篇 |
1991年 | 17篇 |
1990年 | 7篇 |
1989年 | 13篇 |
1988年 | 5篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 13篇 |
1984年 | 19篇 |
1983年 | 13篇 |
1982年 | 7篇 |
1981年 | 9篇 |
1979年 | 5篇 |
1978年 | 7篇 |
1976年 | 3篇 |
1975年 | 5篇 |
1974年 | 8篇 |
1973年 | 3篇 |
1972年 | 2篇 |
排序方式: 共有2818条查询结果,搜索用时 23 毫秒
91.
92.
93.
We interpret the Lorentz force equation as a geodesic equation associated with a non-linear connection. Using a geometric averaging procedure, we prove that for narrow and smooth one-particle distribution functions whose supports are invariant under the flow of the Lorentz equation, a bunch of charged point particles can be described by a charged cold fluid model in the ultra-relativistic regime. The method used to prove this result does not require additional hypotheses on the higher moments of the distribution. This is accomplished by estimating the expressions that include the differential operators appearing in the charged cold fluid model equation. Under the specified conditions of narrowness and ultra-relativistic dynamics, it turns out that these differential expressions are close to zero, justifying the use of the charged cold fluid model. The method presented in this work can also be applied to justify the use of warm plasmas and other models. Finally, a possible relation with chromohydrodynamics is discussed. 相似文献
94.
A two‐step reaction scheme leading to singlet carbene species that can be detected under matrix conditions for the reaction of Zr(3F) with either CH3F or CH3CN 下载免费PDF全文
Ana E. Torres Guadalupe Castro Ricardo Pablo‐Pedro Fernando Colmenares 《Journal of computational chemistry》2014,35(11):883-890
The results obtained from CASSCF‐MRMP2 calculations are used to rationalize the singlet complexes detected under matrix‐isolation conditions for the reactions of laser‐ablated Zr(3F) atoms with the CH3F and CH3CN molecules, without invoking intersystem crossings between electronic states with different multiplicities. The reaction Zr(3F) + CH3F evolves to the radical products ZrF· + ·CH3. This radical asymptote is degenerate to that emerging from the singlet channel of the reactants Zr(1D) + CH3F because they both exhibit the same electronic configuration in the metal fragment. Hence, the caged radicals obtained under cryogenic‐matrix conditions can recombine through triplet and singlet paths. The recombination of the radical species along the low‐multiplicity channel produces the inserted structures H3C? Zr? F and H2C?ZrHF experimentally detected. For the Zr(3F) + CH3CN reaction, a similar two‐step reaction scheme involving the radical fragments ZrNC· + ·CH3 explains the presence of the singlet complexes H3C? Zr? NC and H2C?Zr(H)NC revealed in the IR‐matrix spectra upon UV irradiation. © 2014 Wiley Periodicals, Inc. 相似文献
95.
Renato D’Angelo Cristy Leonor Azanza Ricardo Alberto Mittiga Paolo Scardi Matteo Leoni 《Journal of Sol-Gel Science and Technology》2014,72(3):490-495
Zinc-rich/copper-poor Cu2-xZn1+xSnS4 (x = 0.2, CZTS) has been successfully produced in film and powder form using two non-aqueous solutions (of metal salts and thiourea) without the need for sulfurization during the annealing phase. A reaction route is proposed and the choices of the solvents (water, ethyleneglycol, ethanol, methanol) and of the tin source (tin chloride pentahydrate or anhydrous) discussed and justified. A pure and coarse-grained material is obtained with a mix of metal salts in methanol and thiourea in ethylene glycol. The tin pentahydrate salt seems a better alternative to the commonly used anhydrous chloride. 相似文献
96.
Beatriz B. Caravieri Pollyanna F. de Oliveira Ricardo A. Furtado Denise C. Tavares Eduardo J. Nassar Katia J. Ciuffi Eduardo F. Molina 《Journal of Sol-Gel Science and Technology》2014,72(3):627-636
Herein, we used tri-ureasil organic–inorganic hybrid material (tU5000) in order to enhance the solubility of nonsteroidal anti-inflammatory drugs and fine tuning the drug delivery profile. For the first time, we used tU5000 as a film-forming agent in order to provide an alternative vehicle for transdermal drug delivery systems which the cell viability of practically 100 % for the highest and the lowest tested concentrations of pure tU5000 indicated that the material was not cytotoxic. The physicochemical properties of the tU5000 drug carrier and drug-loaded hybrids were systematically studied using powder X-ray diffraction, differential scanning calorimetry, small-angle X-ray scattering, and Fourier-transform infrared spectroscopy. The structural changes of tU5000 as well as the relationships between the drug content and in vitro drug release behaviors were investigated. The results showed that the ibu molecules were homogeneously distributed in the tU5000 xerogels contributing to fine-tuning the drug delivery profile. Considering the ability to incorporated high drug content, simple and mild preparation procedure by one-pot sol–gel route, high stability of the materials, sustained-release property, this class of hybrid based on polymers and inorganic compounds may have potential applications in the design of pharmaceutical formulation as ophthalmic (contact lenses), transdermal (patches) and implantable (soft tissue) drug delivery systems. 相似文献
97.
Ricardo Molina Miguel Gómez Chi-Wai Kan Enric Bertran 《Cellulose (London, England)》2014,21(1):729-739
Materials with hydrophilic–oleophobic properties are of relevance due to their application to different fields such as self-cleaning coatings, liquid–liquid separation membranes and functional textiles for different technical applications. In this work, hydrophilic–oleophobic coatings have been deposited on cellulosic materials (filter paper and bleached cotton) by means of plasma assisted polymerization of acrylic acid solutions in water followed by cationic fluorosurfactant complexation. Chemical composition of the coatings on cellulosic materials was characterized by means of FTIR–ATR and XPS whereas their morphology was studied by SEM. Hydrophilic–oleophobic behavior was characterized by means of contact angle and wetting time. Additionally wetting properties of cationic, anionic and non-ionic surfactant solutions on the hydrophilic–oleophobic coatings were used to characterize the polyelectrolyte electrostatic forces upon the functionalized layer. 相似文献
98.
Wilney de Jesus Rodrigues Santos César Ricardo Teixeira Tarley Lauro Tatsuo Kubota 《Analytica chimica acta》2009,631(2):170-50
This work reports the preparation of a molecularly imprinted polymer (MIP) for selective catalytic detection of serotonin (5-hydroxytryptamine, 5-HT). The process is based on the synthesis of polymers with hemin introduced as the catalytic center to mimic the active site of peroxidase. The copolymer MIP, containing artificial recognition sites for 5-HT, has been prepared by bulk polymerization using methacrylic acid (MAA) and hemin as the functional monomers, and ethylene glycol dimethacrylate (EGDMA) as the cross-linker. For the determination of 5-HT, a flow injection analysis system coupled to an amperometric detector was optimized using multivariate analysis. The effects of different parameters, such as pH, buffer flow rate, buffer nature, peroxide concentration and sample volume were evaluated. After optimizing the experimental conditions, a linear response range from 1.0 up to 1000.0 μmol L−1 was obtained with a sensitivity of 0.4 nA/μmol L−1. The detection limit was found to be 0.30 μmol L−1, while the precision values (n = 6) evaluated by relative standard deviation (R.S.D.) were, respectively, 1.3 and 1.7% for solutions of 50 and 750 μmol L−1 of 5-HT. No interference was observed by structurally similar compounds (including epinephrine, dopamine and norepinephrine), thus validating the good performance of the imprinted polymer. The method was applied for the determination of 5-HT in spiked blood serum samples. 相似文献
99.
Antonio L. Braga Ricardo S. Schwab Syed M. Salman Juliano B. Azeredo 《Tetrahedron letters》2009,50(20):2309-1982
A set of chiral β-seleno amines were prepared by the ring-opening reaction of unprotected aziridines. Under acid conditions diaryl or dialkyl diselenides were reduced by zinc and treated with unprotected aziridines to produce the desired products in good yields. Chiral β-telluro amine was also obtained using this method. 相似文献
100.
Mireille Perec Ricardo Baggio 《Acta Crystallographica. Section C, Structural Chemistry》2009,65(8):m296-m298
In the mononuclear title compound, [Cu(C4H4O5)(C6H6N2O)(H2O)2], the CuII centre is bound to a chelating oxydiacetate ligand, a monodentate pyridine‐3‐carboxamide unit and two water molecules, defining an octahedral coordination where the first two ligands form the equatorial plane and the last two occupy the apical sites. The planar oxydiacetate ligand is slightly disordered at its central ether O atom. The availability of efficient donors and acceptors for hydrogen bonding results in a complex interaction scheme where each monomer links to six similar units to define a well connected three‐dimensional structure. A comparison is made with related structures in the literature, and the reasons for their differences are discussed. 相似文献