全文获取类型
收费全文 | 17439篇 |
免费 | 733篇 |
国内免费 | 142篇 |
专业分类
化学 | 10970篇 |
晶体学 | 104篇 |
力学 | 441篇 |
数学 | 2798篇 |
物理学 | 2827篇 |
无线电 | 1174篇 |
出版年
2023年 | 161篇 |
2022年 | 260篇 |
2021年 | 379篇 |
2020年 | 457篇 |
2019年 | 426篇 |
2018年 | 290篇 |
2017年 | 263篇 |
2016年 | 653篇 |
2015年 | 530篇 |
2014年 | 617篇 |
2013年 | 937篇 |
2012年 | 1213篇 |
2011年 | 1390篇 |
2010年 | 718篇 |
2009年 | 637篇 |
2008年 | 1049篇 |
2007年 | 997篇 |
2006年 | 980篇 |
2005年 | 881篇 |
2004年 | 762篇 |
2003年 | 549篇 |
2002年 | 557篇 |
2001年 | 198篇 |
2000年 | 174篇 |
1999年 | 175篇 |
1998年 | 141篇 |
1997年 | 142篇 |
1996年 | 191篇 |
1995年 | 120篇 |
1994年 | 140篇 |
1993年 | 135篇 |
1992年 | 100篇 |
1991年 | 112篇 |
1990年 | 90篇 |
1989年 | 88篇 |
1988年 | 96篇 |
1987年 | 85篇 |
1986年 | 79篇 |
1985年 | 105篇 |
1984年 | 114篇 |
1982年 | 125篇 |
1981年 | 132篇 |
1980年 | 102篇 |
1979年 | 88篇 |
1978年 | 116篇 |
1977年 | 104篇 |
1976年 | 79篇 |
1975年 | 78篇 |
1974年 | 74篇 |
1973年 | 73篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
161.
162.
163.
Kennedy F. Rubert-Nason Liza M. Holeski John J. Couture Adam Gusse Daniel J. Undersander Richard L. Lindroth 《Analytical and bioanalytical chemistry》2013,405(4):1333-1344
Poplar (Populus) and birch (Betula) species are widely distributed throughout the northern hemisphere, where they are foundation species in forest ecosystems and serve as important sources of pulpwood. The ecology of these species is strongly linked to their foliar chemistry, creating demand for a rapid, inexpensive method to analyze phytochemistry. Our study demonstrates the feasibility of using near-infrared reflectance spectroscopy (NIRS) as an inexpensive, high-throughput tool for determining primary (e.g., nitrogen, sugars, starch) and secondary (e.g., tannins, phenolic glycosides) foliar chemistry of Populus and Betula species, and identifies conditions necessary for obtaining reliable quantitative data. We developed calibrations with high predictive power (residual predictive deviations?≤?7.4) by relating phytochemical concentrations determined with classical analytical methods (e.g., spectrophotometric assays, liquid chromatography) to NIR spectra, using modified partial least squares regression. We determine that NIRS, although less sensitive and precise than classical methods for some compounds, provides useful predictions in a much faster, less expensive manner than do classical methods. Graphical abstract
Near-infrared reflectance spectroscopy with calibrations based on modified partial least squares regression can provide quantitative measurements of foliar nitrogen, carbohydrate, tannin, and phenolic glycoside content in poplar and birch 相似文献
164.
Manuel Alberto Flores‐Hidalgo Daniel Glossman‐Mitnik D. H. Galvan Diana Barraza‐Jimenez 《中国化学会会志》2013,60(8):1082-1091
Density Functional Theory employing hybrid and M06 functionals in combination with three different basis sets is used to calculate the ground state of a cage like (ZnO)12 nanocluster which has been consistently reported as the more stable cluster for its particular size. B3LYP and B3PW91 hybrid functionals combined with 6‐31+G*, Lanl2dz and SDD basis sets are employed to treat the ZnO molecular system. Alternatively, three M06 functionals in combination with three basis sets are employed in the nanostructure calculations. Results obtained by treating ZnO sodalite cage nanocluster with M06 functionals demonstrated comparable quality to results obtained with hybrid functionals. Within this study, efficient theoretical DFT methods with the widely known hybrid and the recently created M06 meta‐hybrid functionals are employed to study nanostructured ZnO. Our resulting parameters provide a fresh approach performance wise on the different theoretical methods to treat transition metal nanostructures, particularly, ZnO nanoclusters geometry and electronic structure. 相似文献
165.
Chlorosulfonyl isocyanata (CSI) reacts with 2H-azirines 1 a-c at -78°C to form [2+2+2] cycloadducts 3a-c and 4a-c. The tricyclic aziridine derivatives 4a-c undergo CSI extrusion reactions and subsequent oxidation to the corresponding pyrazines 5a-c. Structural identifications of 3a-c and 4a-c are based on ir, nmr and mass spectral data. 相似文献
166.
Bromonitromethane, triethyl orthoformate and aniline react together, in a one-flask procedure, to afford the title compound. 相似文献
167.
Douglas M. Jackson Robert L. Ashley Callan B. Brownfield Daniel R. Morrison Richard W. Morrison 《合成通讯》2013,43(23):2691-2700
This article reports a new methodology taking advantage of superheated chemistry via either microwave or conventional heating for the facile decarboxylation of alpha amino acids using the recoverable organocatalyst, R-carvone. The decarboxylation of amino acids is an important synthetic route to biologically active amines, and traditional methods of amino acid decarboxylation are time consuming (taking up to several days in the case of L-histidine), are narrow in scope, and make use of toxic catalysts. Decarboxylations of amino acids including L-histidine occur in just minutes while replacing toxic catalysts with green catalyst, spearmint oil. Yields are comparable to or exceed previous methods and purification of product ammonium chloride salts is aided by an isomerization reaction of residual catalyst to phenolic carvacrol. The method has been shown to be effective for the decarboxylations of a range of natural, synthetic, and protected amino acids. 相似文献
168.
Synthesis of new fluorinated tertiary malonamides (F-malonamides) was accomplished, and their liquid/liquid (L/L) extraction properties with f-elements were investigated. These molecules are fluorinated analogues of well known extractants used in several processes designed towards the treatment of nuclear wastes, and the efficient separation of lanthanides from minor actinides; however, the synthesis of F-malonamides deserved a modification of the general synthetic route commonly employed to prepare H-malonamides. Extraction of neodymium from various aqueous media into both fluorous and classical solvents was studied, which revealed an opposite trend between F-malonamides and H-malonamides: L/L extraction ability is very sensitive to the nitrogen atoms substitution pattern, and the most efficient F-malonamide is compound 3 (R1 = Me), whereas the best H-malonamide is compound 5 (R1 = Bu, DMDBTDMA). 相似文献
169.
Hana Macková Daniel Horák Eduard Petrovský Jana Kovářová 《Colloid and polymer science》2013,291(1):205-213
To prepare functionalized magnetic polymer particles that are thermally responsive, inverse emulsion copolymerization of N-isopropylacrylamide, N,N′-methylenebisacrylamide and glycidyl acrylate (GA) was investigated in paraffin oil in the presence of γ-Fe2O3 nanoparticles dispersed in a water/glycerol mixture. The resulting polymer particles were characterized regarding the morphology, size, polydispersity, iron content, and the temperature-dependent phase transition using optical microscopy, transmission electron microscopy, scanning electron microscopy, atomic absorption spectroscopy, and differential scanning calorimetry. Magnetic properties were examined using hysteresis loop measurements and by analyzing the magnetic susceptibility with respect to temperature. We have also investigated the influence of the concentration of γ-Fe2O3 and GA in monomers on properties of the particles (morphology, size, and presence of oxirane groups). The particles possessed a hollow structure as a result of phase separation between water/glycerol hydrophilic solvents in the polymerization feed and the forming polymer. Depending on the concentration of γ-Fe2O3 in the monomer phase, the magnetic hollow particles contained 5–24 wt% iron. In water, the particles gradually collapsed when the temperature was raised to 40 °C because the elevated temperature weakened hydration and the PNIPAAm chains gradually became more hydrophobic. 相似文献
170.
Diamino‐ and diazidofuranoses represent useful precursors, for example, for the synthesis of substituted nucleosides and metal complexes, respectively. Known procedures for their synthesis lack the availability of cheap starting materials, adequate yields, and the access to all possible diastereomeres. Therefore, 3,5‐diazido‐3,5‐dideoxy‐ and ‐2,3,5‐trideoxyfuranoses both with ribo‐ and xylo‐configuration were prepared using different approaches. 相似文献