全文获取类型
收费全文 | 28875篇 |
免费 | 4193篇 |
国内免费 | 2414篇 |
专业分类
化学 | 20240篇 |
晶体学 | 552篇 |
力学 | 1064篇 |
综合类 | 273篇 |
数学 | 957篇 |
物理学 | 12396篇 |
出版年
2024年 | 54篇 |
2023年 | 249篇 |
2022年 | 668篇 |
2021年 | 782篇 |
2020年 | 901篇 |
2019年 | 861篇 |
2018年 | 789篇 |
2017年 | 1086篇 |
2016年 | 1327篇 |
2015年 | 1273篇 |
2014年 | 1463篇 |
2013年 | 2732篇 |
2012年 | 1813篇 |
2011年 | 1722篇 |
2010年 | 1478篇 |
2009年 | 1638篇 |
2008年 | 1654篇 |
2007年 | 1658篇 |
2006年 | 1554篇 |
2005年 | 1431篇 |
2004年 | 1384篇 |
2003年 | 1113篇 |
2002年 | 1151篇 |
2001年 | 789篇 |
2000年 | 801篇 |
1999年 | 692篇 |
1998年 | 585篇 |
1997年 | 486篇 |
1996年 | 436篇 |
1995年 | 471篇 |
1994年 | 340篇 |
1993年 | 308篇 |
1992年 | 286篇 |
1991年 | 197篇 |
1990年 | 198篇 |
1989年 | 150篇 |
1988年 | 167篇 |
1987年 | 141篇 |
1986年 | 124篇 |
1985年 | 108篇 |
1984年 | 95篇 |
1983年 | 49篇 |
1982年 | 83篇 |
1981年 | 43篇 |
1980年 | 35篇 |
1979年 | 47篇 |
1978年 | 15篇 |
1977年 | 7篇 |
1976年 | 7篇 |
1973年 | 17篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
891.
Hyungrak Kim 《Journal of solid state chemistry》2005,178(6):1935-1939
The compound Ca14MnP11 crystallizes in the Ca14AlSb11 structure type with the tetragonal space group I41/acd (Z=8) and lattice parameters of , c=20.7565(9) at 90 K. The structure consists of MnP49− tetrahedron, P37− trimer, 4 P3− isolated anions and 14 Ca2+ cations. Similar to other compounds of this structure type containing phosphorous, the P37− trimer has a central P atom that is best modeled in the structure as being equally split between two sites. In addition, there is no additional distortion of the manganese-containing tetrahedron compared with the main group analog, Ca14GaP11, suggesting that the Mn oxidation state is Mn2+. Temperature-dependent magnetic susceptibility shows that the compound is paramagnetic over the entire temperature range measured (2-300 K). The data can be fit with a modified Curie-Weiss law and provide an effective magnetic moment of 5.80 (2) B.M. with a Weiss constant of −2.13(2) K and . This moment is significantly higher than those measured for any of the Mn-containing analogs and is consistent with Mn2+. This result will be discussed in light of the electron counting scheme for Mn compounds of the Ca14AlSb11 structure-type. 相似文献
892.
893.
The lipophilicity of some antiarrhythmic and antihypertensive active 1-[2-hydroxy- or 1-[2-acetoxy-3-(4-aryl-1-piperazinyl)propyl]pyrrolidin-2-one derivatives (1-12) has been investigated. Their lipophilicity (R(MO) and log k') was determined by reversed-phase thin-layer chromatography and reversed-phase high-performance liquid chromatography with mixtures of acetonitrile and Tris buffer as mobile phases. The partition coefficients of compounds 1-12 (log P(ScilogP)) were also calculated with the ScilogP program. Comparison of R(MO), log k' and calculated log D(7.0 ScilogP) values enabled calculation of clog D(7.0 TLC) and clog D(7.0 HPLC) values. Preliminary quantitative structure-activity relationship studies indicated that for active compounds there is a dependence between affinity for alpha(2)-adrenoceptors and their clog D(7.0 HPLC) values. 相似文献
894.
For Part II of our ongoing study, we present a strategy for stationary phase optimization for the capillary electrochromatographic (CEC) separation of the 12 methylated benzo[a]pyrene (MBAP) isomers. Utilizing the optimum mobile phase conditions from Part I of our study as a guide, seven commercially available stationary phases have been evaluated for their ability to separate highly hydrophobic MBAP isomers. Ranging in design from high-performance liquid chromatography (HPLC) to CEC application, each phase was slurry packed in house and tested for CEC suitability and performance. Several stationary phase parameters were investigated for their effects on MBAP separation including bonding type (monomeric or polymeric, % carbon loading, surface coverage), pore size, particle size, and type of alkyl substituent. In this manner, the present state of commercially available packings has been assessed in our laboratory. Utilizing the optimum polymeric C18-5 microm-100 A-PAH stationary phase, the effects of CEC packed bed length and capillary inside diameter (I.D.) were also evaluated. A 50 microm I.D. capillary, 25 cm packed bed length and 75% (v/v) acetonitrile, 12.5 mM Tris, pH 8.0, 20 degrees C at 30 kV, provided resolution of 11 out of 12 MBAP isomers thus showing the effectiveness of CEC for analysis of structurally similar methylated polyaromatic hydrocarbons. 相似文献
895.
Markus Håkansson Johanna Suomi Mauri Nauma Jouko Kankare Jarkko U. Eskola 《Analytica chimica acta》2006,556(2):450-454
Double insulating barrier tunnel emission electrodes were fabricated by adding a new pure aluminum layer upon oxidized aluminum electrodes by vacuum evaporation and thermally oxidizing the new aluminum layer in air at room temperature. Resulting Al/Al2O3/Al/Al2O3 electrodes allow the use of various aluminum alloys in the electrode body necessary for hardness or shaping ability of the electrode while obtaining the luminescence properties of pure aluminum oxide. During electrical excitation of luminescent labels by cathodic hot electron injection into aqueous electrolyte solution, the background noise is mainly based on high-field-induced solid-state electroluminescence and F-center luminescence of the outer aluminum oxide film. The more defect states and/or impurity centers the outer oxide film contains, the higher is the background emission intensity. The present electrode fabrication method provides a considerable improvement in signal-to-noise ratio for time-resolved electrochemiluminescence (TR-ECL) measurements when the original native oxide film of the electrode body contains luminescence centers displaying long-lived luminescence. The excellent performance of the present electrodes is demonstrated by extremely low-level detection of Tb(III) chelates, luminol, Pt(II) coproporphyrin and Tb(III) labels in an immunometric immunoassay by time-resolved electrochemiluminescence. 相似文献
896.
Both single crystals of VCl3(THF)3 as well as isotypic cocrystals of the composition MCl3(THF)3, M=Ti/V 1/3, undergo a topotactic reversible phase transition to a hitherto unknown low-temperature modification. The close relationship between this new structure and the room-temperature phase determined by Cottonet al. is discussed from the molecular and the intermolecular point of view: Both modifications are built up by conformationally very similar molecules which change their arrangement during the phase transition. Lattice energy calculations confirm that these two alternative arrangements correspond to minima of almost the same packing energy.Dedicated to Professor Dr. G. E. Herberich on the occasion of his 60th birthday. 相似文献
897.
Masaru Kojima 《Tetrahedron letters》2007,48(25):4431-4436
A new fluorous benzylidene acetal protecting group was regioselectively introduced into carbohydrates, deprotected under acidic conditions, and reused. Oligosaccharides were synthesized via regioselective conversion of the fluorous acetal group to the benzyl group by traditional reaction conditions. The fluorous compounds were easily separated from non-fluorous by-products by fluorous solid phase extraction. 相似文献
898.
Two ligand exchange chiral stationary phases (CSPs) based on (S)-leucinol derivative, sodium N-((S)-1-hydroxymethyl-3-methylbutyl)-N-undecylaminoacetate, and (R)-phenylglycinol derivative, sodium N-((R)-2-hydroxy-1-phenylethyl)-N-undecylaminoacetate, covalently bonded to silica gel have been successfully applied in the resolution of nine -hydroxycarboxylic acids. The latter was more effective than the former, the separation factors () being 1.05 to 2.12 while the resolution factors (RS) varying from 0.18 to 5.29 on the latter. The chromatographic resolution behaviors were dependent on the type and the content of organic modifier and the content of CuSO4 in aqueous mobile phase and the column temperature. A possible chiral recognition mechanism was also proposed based on the chromatographic resolution behaviors. 相似文献
899.
The method for the prediction of capacity factors in ternary mobile phases is presented. The adsorption mechanism of retention is considered. The simple theoretical equations are proposed for mobile phases for which the ratio of mole fractions of the weaker solvents remains fixed. The relations between parameters characterizing retention in ternary and binary mobile phases are discussed. The theoretical model is verified for numerous solutes and different mobile phases. 相似文献
900.
We describe a route for the polymer supported total synthesis of the cyclic bisbibenzyls of the isoplagiochin type found in liverworts. TentaGel® resins were used as solid support for a sequence involving Suzuki, Wittig and hydrogenation protocols. The polymer linked intermediates could be characterized by HR-MAS NMR. This route is to be extended to the synthesis of small libraries of differently halogenated derivatives. 相似文献