全文获取类型
收费全文 | 40255篇 |
免费 | 1096篇 |
国内免费 | 163篇 |
专业分类
化学 | 26006篇 |
晶体学 | 312篇 |
力学 | 667篇 |
综合类 | 4篇 |
数学 | 7065篇 |
物理学 | 7460篇 |
出版年
2023年 | 257篇 |
2022年 | 480篇 |
2021年 | 630篇 |
2020年 | 600篇 |
2019年 | 665篇 |
2018年 | 749篇 |
2017年 | 681篇 |
2016年 | 1368篇 |
2015年 | 1108篇 |
2014年 | 1164篇 |
2013年 | 2421篇 |
2012年 | 2426篇 |
2011年 | 2589篇 |
2010年 | 1642篇 |
2009年 | 1556篇 |
2008年 | 2308篇 |
2007年 | 2261篇 |
2006年 | 2071篇 |
2005年 | 1900篇 |
2004年 | 1627篇 |
2003年 | 1320篇 |
2002年 | 1131篇 |
2001年 | 864篇 |
2000年 | 774篇 |
1999年 | 606篇 |
1998年 | 457篇 |
1997年 | 441篇 |
1996年 | 548篇 |
1995年 | 404篇 |
1994年 | 430篇 |
1993年 | 388篇 |
1992年 | 371篇 |
1991年 | 334篇 |
1990年 | 312篇 |
1989年 | 259篇 |
1988年 | 247篇 |
1987年 | 245篇 |
1986年 | 245篇 |
1985年 | 319篇 |
1984年 | 327篇 |
1983年 | 219篇 |
1982年 | 238篇 |
1981年 | 237篇 |
1980年 | 232篇 |
1979年 | 167篇 |
1978年 | 160篇 |
1977年 | 164篇 |
1976年 | 143篇 |
1975年 | 129篇 |
1973年 | 130篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Cusmin P Salud J López DO de la Fuente MR Diez S Pérez-Jubindo MA Barrio M Tamarit JL 《The journal of physical chemistry. B》2006,110(51):26194-26203
The metastable phase diagram of the two-component system heptyloxycyanobiphenyl (7OCB)+nonyloxycyanobiphenyl (9OCB) was determined by means of modulated differential scanning calorimetry (MDSC) and optical microscopy measurements. It was experimentally established that the 7OCB+9OCB two-component system exhibits a monotropic re-entrant nematic behavior. A complete quantitative thermodynamic analysis, through Oonk's equal G analysis, was performed, allowing for the calculation of the monotropic re-entrant behavior and the prediction of two tricritical points, one of them experimentally accessible for the SmAd-to-N transition and the other non-experimentally accessible for the RN-to-SmAd transition. From specific-heat measurements, latent heats were obtained for those mixtures displaying a first-order SmAd-to-N transition. Additionally, for some mixtures, the specific-heat critical exponents (alpha), through the second-order SmAd-to-N transition, were obtained. Both batches of data enable us to access to the experimental tricritical temperature for the SmAd-to-N transition. 相似文献
952.
The 1,3 intramolecular hydrogen transfer reaction in free thymine and in Mg(II)-thymine have been studied at the density functional theory level. The mechanism of intramolecular proton transfer in these systems emerges from the analysis of the reaction force profile along the reaction path; it is rationalized in terms of structural and electronic reorganizations that take place during the chemical transformation. Results show that the presence of Mg(II) monocoordinated to thymine activates the hydrogenic motion by inducing structural and electronic changes in the molecular backbone. In the metallic complex, it is found that the hydrogen transfer is followed by a relaxation process that facilitates the metal cation migration to form a bicoordinated complex. 相似文献
953.
Tejedor D Santos-Expósito A García-Tellado F 《Chemical communications (Cambridge, England)》2006,(25):2667-2669
A novel ABB' 3 component reaction (3-CR) system based on the organocatalyzed homoaldolic condensation of alpha-ketoesters in the presence of terminal conjugated alkynoates is described. 相似文献
954.
Porras-Vázquez JM De la Torre AG Marrero-López D Losilla ER Aranda MA 《Dalton transactions (Cambridge, England : 2003)》2006,(22):2691-2697
Tricalcium oxy-silicates, Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01, have been prepared as crystalline single phases. Ca3(SiO4)O and Ca2.93Mg0.07(Si0.98Al0.02O4)O0.99 [square]0.01 have triclinic and monoclinic structures, respectively. The samples show oxide anion conductivity with a small p-type electronic contribution under oxidizing conditions. At 1023 K, the oxide transport numbers range between 0.97 and 0.85 from reducing (dry 5%-H2-Ar/air gradient) to oxidizing (O2/air gradient) conditions in the 1023-1173 K interval. The thermal analyses showed a large weight loss on heating due to the presence of water in the materials. The monoclinic compound has ionic conductivities higher than those of the triclinic stoichiometric oxy-silicate, as expected due to the introduction of oxide vacancies. Typical total conductivities for these un-optimised solids are 10(-5)-10(-4) S cm(-1) at 1100 K. These compounds may contain a small amount of water, approximately 0.05 H2O moles per chemical formula, and they display an important proton contribution under a humidified atmosphere. 相似文献
955.
Febles M Pérez-Hernandez N Pérez C Rodríguez ML Foces-Foces C Roux MV Morales EQ Buntkowsky G Limbach HH Martín JD 《Journal of the American Chemical Society》2006,128(31):10008-10009
Water molecules confined inside narrow pores are of great importance in understanding the structure, stability, and function of water channels. Here we report that besides the H-bonding water that structures the pore, the permanent presence of a significant, fast-moving fraction of incompletely H-bonded water molecules inside the pore should control the free entry and exit of water. This is achieved by means of complementary DSC and solid-state NMR studies. We also present compelling evidence from X-ray diffraction data that the cluster formed by six water molecules in the most stable cage-like structure is sufficiently hydrophobic to be stably adsorbed in a nonpolar environment. 相似文献
956.
The lambda > 300 nm photolysis of h4- or d4-pyruvic acid aqueous glasses at 77 K yields identical electron magnetic resonance (EMR) spectra arising from distant (r greater or similar 0.5 nm) triplet radical pairs. Spectra comprise: (1) well-resolved quartets, X, at g approximately ge, that closely match the powder spectra of spin pairs interacting across r approximately 1.0 nm with D approximately 3.0 mT, E approximately 0 mT zero field splittings (ZFS), and (2) broad signals, Y, centered at g approximately 2.07 that display marked g-anisotropy and g-strain, exclude D greater or similar 20.0 mT values (i.e., r less or similar 0.5 spin nm separations), and track the temperature dependence of related g approximately 4 features. These results imply that the n-pi excitation of pyruvic acid, PA, induces long-range electron transfer from the promoted carbonyl chromophore into neighboring carbonyl acceptors, rather than homolysis into contact radical pairs or concerted decarboxylation into a carbene. Since PA is associated into hydrogen-bonded dimers prior to vitrification, X signals arise from radical pairs ensuing intradimer electron transfer to a locked acceptor, while Y signals involve carbonyl groups attached to randomly arranged, disjoint monomers. The ultrafast decarboxylation of primary radical ion pairs, 3[PA+* PA-*], accounts for the release of CO2 under cryogenic conditions, the lack of thermal hysteresis displayed by magnetic signals between 10 and 160 K, and averted charge retrotransfer. All EMR signals disappear irreversibly above the onset of ice diffusivity at approximately 190 K. 相似文献
957.
A highly selective procedure is proposed for the determination of ultra-trace level concentrations of nickel in saline aqueous
matrices exploiting a micro-sequential injection Lab-On-Valve (μSI-LOV) sample pretreatment protocol comprising bead injection
separation/pre-concentration and detection by electrothermal atomic absorption spectrometry (ETAAS). Based on the dimethylglyoxime
(DMG) reaction used for nickel analysis, the sample, as contained in a pH 9.0 buffer, is, after on-line merging with the chelating
reagent, transported to a reaction coil attached to one of the external ports of the LOV to assure sufficient reaction time
for the formation of Ni(DMG)2 chelate. The non-ionic coordination compound is then collected in a renewable micro-column packed with a reversed-phase copolymeric
sorbent [namely, poly(divinylbenzene-co-N-vinylpyrrolidone)] containing a balanced ratio of hydrophilic and lipophilic monomers.
Following elution by a 50-μL methanol plug in an air-segmented modality, the nickel is finally quantified by ETAAS. Under
the optimized conditions and for a sample volume of 1.8 mL, a retention efficiency of 70 % and an enrichment factor of 25
were obtained. The proposed methodology showed a high tolerance to the commonly encountered alkaline earth matrix elements
in environmental waters, that is, calcium and magnesium, and was successfully applied for the determination of nickel in an
NIST standard reference material (NIST 1640-Trace elements in natural water), household tap water of high hardness and local
seawater. Satisfying recoveries were achieved for all spiked environmental water samples with maximum deviations of 6 %. The
experimental results for the standard reference material were not statistically different to the certified value at a significance
level of 0.05. 相似文献
958.
Aline G. Cunha Gloria Fernández-Lorente Melissa L. E. Gutarra Juliana V. Bevilaqua Rodrigo V. Almeida Lúcia M. C. Paiva Roberto Fernández-Lafuente Jose M. Guisán Denise M. G. Freire 《Applied biochemistry and biotechnology》2009,156(1-3):133-145
Lipases are an enzyme class of a great importance as biocatalysts applied to organic chemistry. However, it is still necessary to search for new enzymes with special characteristics such as good stability towards high temperatures, organic solvents, and high stereoselectivity presence. The present work’s aim was to immobilize the lipases pool produced by Penicillium simplissicimum, a filamentous fungi strain isolated from Brazilian babassu cake residue. P. simplissicimum lipases were separated into three different fractions using selective adsorption method on different hydrophobic supports (butyl-, phenyl-, and octyl-agarose) at low ionic strength. After immobilization, it was observed that these fractions’ hyperactivation is in the range of 131% to 1133%. This phenomenon probably occurs due to enzyme open form stabilization when immobilized onto hydrophobic supports. Those fractions showed different thermal stability, specificity, and enantioselectivity towards some substrates. Enantiomeric ratio for the hydrolysis of (R,S) 2-O-butyryl-2-phenylacetic acid ranged from 1 to 7.9 for different immobilized P. simplissicimum lipase fractions. Asymmetry factor for diethyl 2-phenylmalonate hydrolysis ranged from 11.8 to 16.4 according to the immobilized P. simplissicimum lipase fractions. Those results showed that sequential adsorption methodology was an efficient strategy to obtain new biocatalysts with different enantioselectivity degrees, thermostability, and specificity prepared with a crude extract produced by a simple and low-cost technology. 相似文献
959.
José M. Marín Emma Gracia-LorJuan V. Sancho Francisco J. LópezFélix Hernández 《Journal of chromatography. A》2009,1216(9):1410-1420
An ultra-high-pressure liquid chromatography–tandem mass spectrometry (UHPLC–MS/MS) method for the determination of 37 pesticides (herbicides, insecticides and fungicides) in environmental and wastewater has been developed. To efficiently combine UHPLC with MS/MS, a fast-acquisition triple quadrupole mass analyzer was used. This analyzer (minimum dwell time, 5 ms) allows acquiring up to three simultaneous transitions in the selected reaction monitoring mode for each compound assuring a reliable identification without resolution or sensitivity losses. A pre-concentration step based on solid-phase extraction using Waters Oasis HLB cartridges (0.2 g) was applied with a 100-fold pre-concentration factor along the whole analytical procedure. The method was validated based on European SANCO guidelines using surface, ground, drinking and treated water (from an urban solid residues treatment plant) spiked at two concentration levels (0.025 and 0.1 μg/L), the lowest having been established as the limit of quantification objective. The method showed excellent sensitivity, with instrumental limits of detection ranging from 0.1 to 7 pg. It was applied to environmental water samples (ground and surface water) as well as to samples of urban solid waste leachates (raw leachate and treated leachate after applying reversed osmosis) collected from a municipal treatment plant. Matrix effects have been studied in the different types of water samples analyzed, and several isotope-labelled internal standards have been evaluated as a way to compensate the signal suppression observed for most of the compounds studied, especially in wastewater samples. As a general remark, only those pesticides which response was corrected using their own isotope-labelled molecule, could be satisfactorily corrected in all type of samples, assuring in this way the accurate quantification in all matrix samples. 相似文献
960.
A new method for the electrophoretic separation of nine phenolic acids (derivatives of benzoic and cinnamic acids) with contactless conductometric detection is presented. Based on theoretical calculations, in which the mobility of the electrolyte co- and counterions and mobility of analytes are taken into consideration, the electrolyte composition and detection mode was selected. This approach was found to be especially valuable for optimization of the electrolyte composition for the separation of analytes having medium mobility. Indirect conductometric detection mode was superior to the direct mode as predicted theoretically. The best performance was achieved with 150 mM 2-amino-2-methylpropanol electrolyte at pH 11.6. The separation was carried out in a counter-electroosmotic mode and completed in less than 6 min. The LODs achieved were about 2.3-3.3 microM and could be further improved to 0.12-0.17 microM by using a sample stacking procedure. The method compares well to the UV-Vis detection. 相似文献