全文获取类型
收费全文 | 38524篇 |
免费 | 9689篇 |
国内免费 | 1444篇 |
专业分类
化学 | 43181篇 |
晶体学 | 347篇 |
力学 | 574篇 |
综合类 | 18篇 |
数学 | 2502篇 |
物理学 | 3035篇 |
出版年
2023年 | 41篇 |
2022年 | 24篇 |
2021年 | 211篇 |
2020年 | 1283篇 |
2019年 | 2634篇 |
2018年 | 1056篇 |
2017年 | 667篇 |
2016年 | 3430篇 |
2015年 | 3581篇 |
2014年 | 3493篇 |
2013年 | 4235篇 |
2012年 | 3167篇 |
2011年 | 2395篇 |
2010年 | 2931篇 |
2009年 | 2924篇 |
2008年 | 2572篇 |
2007年 | 1950篇 |
2006年 | 1648篇 |
2005年 | 1809篇 |
2004年 | 1593篇 |
2003年 | 1449篇 |
2002年 | 2091篇 |
2001年 | 1421篇 |
2000年 | 1357篇 |
1999年 | 409篇 |
1998年 | 105篇 |
1997年 | 75篇 |
1996年 | 97篇 |
1995年 | 63篇 |
1994年 | 64篇 |
1993年 | 55篇 |
1992年 | 50篇 |
1991年 | 41篇 |
1990年 | 43篇 |
1989年 | 31篇 |
1988年 | 36篇 |
1987年 | 33篇 |
1986年 | 24篇 |
1985年 | 67篇 |
1984年 | 41篇 |
1983年 | 42篇 |
1982年 | 37篇 |
1981年 | 53篇 |
1980年 | 43篇 |
1979年 | 33篇 |
1978年 | 35篇 |
1977年 | 35篇 |
1976年 | 26篇 |
1975年 | 29篇 |
1974年 | 42篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
971.
972.
973.
974.
M S Lie Ken Jie 《Journal of chromatography. A》1975,109(1):81-87
Five trimethylene-interrupted methyl octadecadiynoates, C18 delta-2a,-7a; delta-3a,-8a; delta-4a,-9a; delta-5a,-10a and delta-6a,-11a, and the corresponding cis,cis-octadecadienoates were synthesized, and their gas-liquid chromatographic properties were studied on Apiezon L, diethylene glycol succinate polyester and Silac 10C stationary phases. The equivalent chain lengths of these esters have been determined, and the separation of mixtures and the prediction of gas chromatographic behaviour of these isomers are discussed. 相似文献
975.
Background Pillar[n]arene(PA[n])contains a symmetrical structures,where para-bridge connection between the units looks like a column-like or pillar shape and polygonal structure from side and top views,respectively.The attached groups to the PA[n]are pointing towards the opposite directions along the columnar axis. 相似文献
976.
Philip S. H. Wong Barbara J. Antonio David V. Dearden 《Journal of the American Society for Mass Spectrometry》1994,5(7):632-637
Reactions of laser-desorbed Na+, K+, Rb+, and Cs+ with thermally vaporized valinomycin generate metal-ligand complexes in a Fourier transform ion cyclotron resonance trapping cell, proving that complexes can form via gas-phase ion-molecule reactions. Although desorption of intact pre-formed complexes cannot be ruled out, this route appears minor. Relative rate constants for the complexation reactions show strong dependence on the charge densities of the cations. Competition experiments between valinomycin and the synthetic ionophores 18-crown-6 (18C6) and [2.2.2]-cryptand ([2.2.2]) show that valinomycin has a higher intrinsic alkali metal cation affinity than either 18C6 or [2.2.2], in contrast to the complex formation constants observed in methanol, where K+ affinities are in the order [2.2.2] > 18C6 > valinomycin. 相似文献
977.
Ariana Posadaz Alicia Biasutti Csar Casale Jesús Sanz Francisco Amat‐Guerri Norman A. García 《Photochemistry and photobiology》2004,80(1):132-138
The Rose Bengal‐sensitized photooxidations of the dipeptides l ‐tryptophyl‐l ‐phenylalanine (Trp‐Phe), l ‐tryptophyl‐l ‐tyrosine (Trp‐Tyr) and l ‐tryptophyl‐l ‐tryptophan (Trp‐Trp) have been studied in pH 7 water solution using static photolysis and time‐resolved methods. Kinetic results indicate that the tryptophan (Trp) moiety interacts with singlet molecular oxygen (O2(1Δg)) both through chemical reaction and through physical quenching, and that the photooxidations can be compared with those of equimolecular mixtures of the corresponding free amino acids, with minimum, if any, influence of the peptide bond on the chemical reaction. This is not a common behavior in other di‐ and polypeptides of photooxidizable amino acids. The ratio between chemical (kr) and overall (kt) rate constants for the interaction O2(1Δg)‐dipeptide indicates that Trp‐Phe and Trp‐Trp are good candidates to suffer photodynamic action, with krlkt values of 0.72 and 0.60, respectively (0.65 for free Trp). In the case of Trp‐Tyr, a lower krlkt value (0.18) has been found, likely as a result of the high component of physical deactivation of O2(1Δg) by the tyrosine moiety. The analysis of the photooxidation products shows that the main target for O2(1Δg) attack is the Trp group and suggests a much lower accumulation of kynurenine‐type products, as compared with free Trp. This is possibly because of the occurrence of another accepted alternative pathway of oxidation that gives rise to 3a‐oxidized hydrogenated pyrrolo[2,3‐b]indoles. 相似文献
978.
A new fluorinated polystyrene bearing a p-sulbstiuted perfluoro[1-(2-fluorosulfonylethoxy)]ethyl group was synthesized via one-electron oxidation of polystyrene by perfluoro[2-(2-fluorosulfonylethoxy)]propionyl peroxide at different peroxide to polystyrene molar ratios.The yield of perfluoroalkylation decreases with the increase of the reactant molar ratio.The modified polymer has been characterized by various techniques:the ring pefluoro[1-(2-fluorosulfonylethoxy)]ethylation has been proved by FT-IR and ^19FNMR;the X-ray photoelectron spectra(XPS) show the maximum binding energy of F18,O18,C18(two kinds of carbon atoms,namely C-H and C-F)and S2p,respectively; desulfonylation of the fluorinated polystyrene appearing at 217℃ has been found by its thermogravimetric analysis (TGA).The determinations of contact angle,refractive index and glass transition temperature of the modified polymer have disclosed that when the contact angle increases with the increase of the molar ratio,the refractive index and glass transition temperature decrease.The polydispersity values indicate that the degradation of the polymer chains did not occur during the reaction. 相似文献
979.
The leaves of some plants, particularly among the Solanacea, contain crystals of calcium oxalate with a peculiar chiral pseudo‐tetrahedral morphology, even though the calcium oxalate crystal structure is centrosymmetric, hence achiral. We studied the morphology of these crystals extracted from the leaves of three Solanacea plants: the potato, the hot pepper, and a species of wild Solanum. The crystal morphology was the same in all three species. Based on the examination of more than 100 crystals from each plant, we showed that the crystal morphology is chiral with invariant chirality. We suggest that morphological chirality is induced by macromolecules during nucleation from a specific, genetically encoded crystal plane, and is further established during subsequent controlled crystal growth. This is one of few examples where it is possible to deduce a molecular mechanism for biologically induced breaking of morphological symmetry in organisms. A very high level of recognition is required by the macromolecules to allow them to distinguish between symmetry‐related crystal planes. It is also surprising that this finely controlled mechanism of crystal formation, including the chiral morphology, has been conserved during evolution. 相似文献
980.
Xiao‐Hong Fu 《Electroanalysis》2007,19(17):1831-1839
A new electrochemical immunosensor for the detection of carbohydrate antigen‐125 (CA125), a carcinoma antigen, was developed by immobilization CA125 antibody (anti‐CA125) on gold hollow microspheres and porous polythionine (PTH) modified glassy carbon electrodes (GCE). The gold hollow microspheres provided a biocompatible microenvironment for proteins, and greatly amplified the coverage of anti‐CA125 molecules on the electrode surface. The performance and factors influencing the immunosensor were investigated in detail. The detection is based on the current change before and after the antigen‐antibody interaction. Under optimal conditions, the amperometric changes were proportional to CA125 concentration ranging from 4.5 to 36.5 U/mL with a detection limit of 1.3 U/mL (at 3σ). The CA125 immunosensor exhibited good precision, high sensitivity, acceptable stability, accuracy and reproducibility. The as‐prepared immunosensors were used to analyze CA125 in human serum specimens. Analytical results suggest that the developed immunoassay has a promising alternative approach for detecting CA125 in the clinical diagnosis. 相似文献