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991.
Catalytic Enantioselective Synthesis of N,Cα,Cα‐Trisubstituted α‐Amino Acid Derivatives Using 1H‐Imidazol‐4(5H)‐ones as Key Templates 下载免费PDF全文
Julen Etxabe Joseba Izquierdo Dr. Aitor Landa Prof. Mikel Oiarbide Prof. Claudio Palomo 《Angewandte Chemie (International ed. in English)》2015,54(23):6883-6886
1H‐Imidazol‐4(5H)‐ones are introduced as novel nucleophilic α‐amino acid equivalents in asymmetric synthesis. These compounds not only allow highly efficient construction of tetrasubstituted stereogenic centers, but unlike hitherto known templates, provide direct access to N‐substituted (alkyl, allyl, aryl) α‐amino acid derivatives. 相似文献
992.
Dr. Chang Hyuck Choi Claudio Baldizzone Jan‐Philipp Grote Dr. Anna K. Schuppert Dr. Frédéric Jaouen Dr. Karl J. J. Mayrhofer 《Angewandte Chemie (International ed. in English)》2015,54(43):12753-12757
Fundamental understanding of non‐precious metal catalysts for the oxygen reduction reaction (ORR) is the nub for the successful replacement of noble Pt in fuel cells and, therefore, of central importance for a technological breakthrough. Herein, the degradation mechanisms of a model high‐performance Fe‐N‐C catalyst have been studied with online inductively coupled plasma mass spectrometry (ICP‐MS) and differential electrochemical mass spectroscopy (DEMS) coupled to a modified scanning flow cell (SFC) system. We demonstrate that Fe leaching from iron particles occurs at low potential (<0.7 V) without a direct adverse effect on the ORR activity, while carbon oxidation occurs at high potential (>0.9 V) with a destruction of active sites such as FeNxCy species. Operando techniques combined with identical location‐scanning transmission electron spectroscopy (IL‐STEM) identify that the latter mechanism leads to a major ORR activity decay, depending on the upper potential limit and electrolyte temperature. Stable operando potential windows and operational strategies are suggested for avoiding degradation of Fe‐N‐C catalysts in acidic medium. 相似文献
993.
Unraveling the Degradation Mechanism of Purine Nucleotides Photosensitized by Pterins: The Role of Charge‐Transfer Steps 下载免费PDF全文
Mariana P. Serrano Dr. Carolina Lorente Dr. Claudio D. Borsarelli Dr. Andrés H. Thomas 《Chemphyschem》2015,16(10):2244-2252
Photosensitized reactions contribute to the development of skin cancer and are used in many applications. Photosensitizers can act through different mechanisms. It is currently accepted that if the photosensitizer generates singlet molecular oxygen (1O2) upon irradiation, the target molecule can undergo oxidation by this reactive oxygen species and the reaction needs dissolved O2 to proceed, therefore the reaction is classified as 1O2‐mediated oxidation (type II mechanism). However, this assumption is not always correct, and as an example, a study on the degradation of 2′‐deoxyguanosine 5′‐monophosphate photosensitized by pterin is presented. A general mechanism is proposed to explain how the degradation of biological targets, such as nucleotides, photosensitized by pterins, naturally occurring 1O2 photosensitizers, takes place through an electron‐transfer‐initiated process (type I mechanism), whereas the contribution of the 1O2‐mediated oxidation is almost negligible. 相似文献
994.
We used a very simplified electrostatic model based on charge and polarizability of atoms and groups on an organic ligand around a lanthanide ion to predict the near‐infrared electronic circular dichroism (NIR ECD) spectra of Yb3+ (a monoelectronic ion). We tuned our method by using two widely different complexes. The first was the heterobimetallic species CsYb(hfbc)4 [hfbc=(?)‐3‐heptafluorobutyrylcamphorate], in which the ligand is a diketonate and, as such, is endowed with a chromophore with strong UV absorption (π–π*). Its oxygen atoms define a square antiprism, which provides a symmetric coordination polyhedron. The second system was Yb DOTMA [DOTMA=(1R,4R,7R,10R)‐α,α′,α′′,α′′′‐tetramethyl‐1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid], a chiral Yb analogue of Gd DOTA (DOTA=1,4,7,10‐tetraazacyclododecane‐1,4,7,10‐tetraacetic acid), in which the ligand lacks relevant electronic transitions and provides a dissymmetric cage. The relative weights of dynamic (ligand polarization) and static contributions to Yb NIR ECD were evaluated, and the spectra appear to have been well predicted by theory through the introduction of a heuristic weight factor. To validate the approach and to confirm the value of the weight factor, we applied it to two other compounds, namely, Na3Yb(BINOLate)3 and Yb(BINOLAM)3 [BINOLate=2,2′‐dihydroxy‐1,1′‐binaphthyl; BINOLAM=3,3′‐bis(diethylaminomethyl)‐1‐1′‐bi‐2‐naphthol]. 相似文献
995.
996.
Separation and concentration of sulfonylurea herbicides in milk by ionic‐liquid‐based foam flotation solid‐phase extraction 下载免费PDF全文
Liyuan Zhang Bocheng Cao Di Yao Runzhong Yu Changqing Yu Hanqi Zhang Aimin Yu 《Journal of separation science》2015,38(10):1733-1740
The ultrasound‐assisted ionic liquid foam flotation solid‐phase extraction of sulfonylurea herbicides in milk was developed and validated. The proteins and lipids were isolated from the sample matrix by adding salt and adjusting the pH value. The target analytes eluted from the solid‐phase extraction cartridge were determined by high‐performance liquid chromatography. Some experimental parameters, including the pH value of sample solution, amount of NaCl, ionic liquid type, extraction time, flow rate of carrier gas, flotation time, and solid‐phase extraction cartridge type were investigated and optimized. Under the optimized experimental conditions, the limits of detection for metsulfuron, pyrazosulfuron, chlorimuron‐ethyl, and nicosulfuron were 1.3, 0.6, 0.7, and 1.1 μg/L, respectively. When the present method was applied to the analysis of milk samples the recoveries of the analytes ranged from 84.3 to 105.2% and relative standard deviations were >5.7%. 相似文献
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Xiangrui Yang Shichao Wu Yang Li Yu Huang Jinyan Lin Di Chang Shefang Ye Liya Xie Yuan Jiang Zhenqing Hou 《Chemical science》2015,6(3):1650-1654
Delicate mesoscopic architectures, bearing complex forms with multiple hierarchy levels, lead to significant functions in biogenic minerals. Herein, a bio-inspired approach was developed to fabricate comet-shaped assemblies of an anti-tumor drug – 10-hydroxycamptothecin (HCPT). The anti-solvent co-precipitation of HCPT and the excipient – PEG-b-PLGA – within the emulsifier leads to the immediate nucleation of comet bundles, followed by a secondary nucleation to generate the comet head, which is an assembly of nanofibers aligned almost in parallel. The continuous manufacturing furnishes drug–excipient hybrid particles with high drug-loading and a sustained drug release profile. This simple and efficient bio-inspired approach led to a promising sustained local drug delivery system, and could be extended to the fabrication of other functional organic materials bearing mesoscopic structural units. 相似文献