全文获取类型
收费全文 | 2995篇 |
免费 | 100篇 |
国内免费 | 16篇 |
专业分类
化学 | 1821篇 |
晶体学 | 6篇 |
力学 | 129篇 |
数学 | 618篇 |
物理学 | 537篇 |
出版年
2024年 | 3篇 |
2023年 | 33篇 |
2022年 | 91篇 |
2021年 | 127篇 |
2020年 | 73篇 |
2019年 | 91篇 |
2018年 | 68篇 |
2017年 | 52篇 |
2016年 | 115篇 |
2015年 | 110篇 |
2014年 | 126篇 |
2013年 | 191篇 |
2012年 | 227篇 |
2011年 | 249篇 |
2010年 | 146篇 |
2009年 | 109篇 |
2008年 | 202篇 |
2007年 | 172篇 |
2006年 | 160篇 |
2005年 | 164篇 |
2004年 | 114篇 |
2003年 | 105篇 |
2002年 | 94篇 |
2001年 | 38篇 |
2000年 | 31篇 |
1999年 | 25篇 |
1998年 | 15篇 |
1997年 | 28篇 |
1996年 | 22篇 |
1995年 | 17篇 |
1994年 | 16篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 2篇 |
1990年 | 8篇 |
1989年 | 7篇 |
1986年 | 4篇 |
1985年 | 6篇 |
1984年 | 7篇 |
1983年 | 8篇 |
1982年 | 3篇 |
1981年 | 5篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1937年 | 2篇 |
排序方式: 共有3111条查询结果,搜索用时 0 毫秒
861.
Dr. Luca Guideri Prof. Francesco De Sarlo Prof. Dr. Fabrizio Machetti 《Chemistry (Weinheim an der Bergstrasse, Germany)》2013,19(2):665-677
Nitroacetates and nitroacetamides react in water as in chloroform with electron‐deficient dipolarophiles to give condensation or conjugate addition products under base catalysis. In general, high selectivity towards condensation is observed in water, with shorter induction periods than in chloroform. In water, condensations slowly occur even without base; kinetic profiles evidence the catalytic effect of the base, which should be related to the conversion into the tautomer nitronic acid. Condensations in water provide convenient access to isoxazole derivatives bearing various functional groups including ammonium, carboxy, and carboxyamide. 相似文献
862.
863.
Granular materials such as sand may be viewed as continuous bodies composed of much smaller elastic bodies. The multiscale geometry of structured deformations captures the contribution at the macrolevel of the smooth deformation of each small body in the aggregate (deformation without disarrangements) as well as the contribution at the macrolevel of the non-smooth deformations such as slips and separations between the small bodies in the aggregate (deformation due to disarrangements). When the free energy response of the aggregate depends only upon the deformation without disarrangements, is isotropic, and possesses standard growth and semi-convexity properties, we establish (i) the existence of a compact phase in which every small elastic body deforms in the same way as the aggregate and, when the volume change of macroscopic deformation is sufficiently large, (ii) the existence of a loose phase in which every small elastic body expands and rotates to achieve a stress-free state with accompanying disarrangements in the aggregate. We show that a broad class of elastic aggregates can admit moving surfaces that transform material in the compact phase into the loose phase and vice versa and that such transformations entail drastic changes in the level of deformation of transforming material points. 相似文献
864.
Charbonnière LJ Ziessel R Montalti M Prodi L Zaccheroni N Boehme C Wipff G 《Journal of the American Chemical Society》2002,124(26):7779-7788
The Gd(3+), Tb(3+), and Eu(3+) complexes of a bis-bipyridine-phenylphosphine oxide ligand PhP(O)(bipy)(2) 1 (bipy for 6-methylene-6'-methyl-2,2'-bipyridine) have been synthesized. In acetonitrile solutions at room temperature, the Tb(3+) and Eu(3+) complexes show a metal-centered luminescence, indicative of an efficient energy transfer from the two bipy subunits to the Ln center. The photophysical properties drastically depend on the nature of the anions present in solution. In particular, addition of 2 equiv of nitrate anions to a solution containing the [Ln.1](OTf-)(3) leads to an 11-fold increase of the luminescence intensity for the Eu(3+) and a 7-fold increase for the Tb(3+) complexes. Similar effects are provided with Cl-, F-, and CH(3)COO- anions. UV-vis titration experiments were used to determine association constants for binding of, respectively, one, two, and three anions. Stepwise anion addition has also been investigated on the molecular level using quantum mechanical (QM) calculations for the Eu complexes. These calculations reproduce the experimental findings, especially if solvent molecule addition is taken into account. The X-ray crystal structure of the nitrate salt of the Tb complex, as well as QM calculation of a similar Eu complex, demonstrates the coordination of three nitrate anions in a bidentate mode and the step-by-step relegation of the bipy subunits in the second coordination sphere. These features give valuable insights into the mechanism of the overall light amplification process. 相似文献
865.
Ferri J. M. Motoc D. Luca Bou S. Ferrandiz Balart R. 《Journal of Thermal Analysis and Calorimetry》2019,138(4):2691-2702
Journal of Thermal Analysis and Calorimetry - The objective of this study was to investigate the thermal expansivities and degradation properties for several in vitro conditioned biodegradable... 相似文献
866.
Cinzia Cristiani Giovanni Dotelli Mario Mariani Renato Pelosato Luca Zampori 《Chemical Papers》2013,67(5):526-531
A very simple, cost-effective, chloride- and alkali-free, carbonate co-precipitation synthesis in aqueous medium was applied in the preparation of perovskite-type lanthanum manganese oxide-based powders, i.e. La0.70Sr0.30MnO3?δ (LSM) and La0.75Sr0.25Cr0.5Mn0.5O3?δ (LSCrM). The precursors so obtained yielded nano-structured perovskite oxides when treated at 900°C and 800°C, respectively. The measured BET surface areas were in the low-end range for high temperature oxides (4 m2 g?1 and 10 m2 g?1) but the X-ray crystallite size was as low as 50 nm for LSCrM and 90 nm for LSM. 相似文献
867.
Luca Cevenini Elisa Michelini Marcello D’Elia Massimo Guardigli Aldo Roda 《Analytical and bioanalytical chemistry》2013,405(2-3):1035-1045
Bioassays represent promising complementary techniques to conventional analytical approaches used in doping analysis to detect illicit drugs like anabolic-androgenic steroids (AAS). The fact that all AAS share a common mechanism of action via the human androgen receptor (hAR) enables the use of bioassays, relying on the activation of hAR as antidoping screening tools. Previously, we developed a dual-color bioreporter based on yeast cells engineered to express hAR and androgen response elements driving the expression of the bioluminescent (BL) reporter protein Photinus pyralis luciferase. A second reporter protein, the red-emitting luciferase PpyRE8, was introduced in the bioreporter as internal viability control. Here, we report the first forensic application of a straightforward, accurate, and cost-effective bioassay, relying on spectral resolution of the two BL signals, in 96-microwell format. The bioreporter responds to dihydrotestosterone as reference androgen in a concentration-dependent manner from 0.08 to 1,000 nM with intra- and inter-assay variation coefficients of 11.4 % and 13.1 %, respectively. We also demonstrated the suitability of this dual-color bioreporter to assess (anti)-androgenic activity of pure AAS, mixtures of AAS, and other illicit drugs provided by the Scientific Police. Significant anti-androgenic activity was observed in samples labeled as marijuana and hashish, containing Δ9-tetrahydrocannabinol as major constituent. Figure
Evaluation of (anti)-androgenic activity of seized drugs with the dual-color bioluminescent bioreporter 相似文献
868.
Paolo Oliveri Maria Chiara Casolino Monica Casale Luca Medini Francesca Mare Silvia Lanteri 《Analytica chimica acta》2013
Analytical methods for confirmation of food authenticity claims should be rapid, economic, non-destructive and should not require highly skilled personnel for their deployment. All such conditions are satisfied by spectroscopic techniques. In order to be extensively implemented in routine controls, an ideal method should also give a response independent of the particular equipment used. In the present study, near-infrared (NIR) spectroscopy was used for verifying authenticity of commercial olives in brine of cultivar Taggiasca. Samples were analysed in two laboratories with different NIR spectrometers and a mathematical spectral transfer correction – the boxcar signal transfer (BST) – was developed, allowing to minimise the systematic differences existing between signals recorded with the two instruments. Class models for the verification of olive authenticity were built by the unequal dispersed classes (UNEQ) method, after data compression by disjoint principal component analysis (PCA). Models were validated on an external test set. 相似文献
869.
Luca Campone Anna Lisa Piccinelli Rita Celano Mariateresa Russo Luca Rastrelli 《Analytical and bioanalytical chemistry》2013,405(26):8645-8652
A simple, rapid, and sensitive method based on simultaneous protein precipitation and extraction of aflatoxin M1 (AFM1) followed by dispersive liquid–liquid microextraction (DLLME) and ultrahigh pressure liquid chromatography tandem mass spectrometry (UHPLC-MS/MS) analysis was developed for the determination of AFM1 in milk samples. In order to precipitate the proteins and extract AFM1 from milk, a sample pretreatment using acetonitrile and NaCl as the extraction/denaturant solvent and salting-out agent, respectively, was optimised. Subsequently, the acetonitrile (upper) phase, containing AFM1, was used as the disperser solvent in DLLME, and extractant (chloroform) and water were added in turn to the extract to perform the DLLME process. The main parameters affecting the extraction efficiency of the whole analytical procedure, such as acetonitrile volume, amount of salt, type and volume of extractant and water volume, were carefully optimised by experimental design. Under optimum conditions, the developed method provides an enrichment factor of 33 and detection and quantification limits (0.6 and 2.0 ng kg?1, respectively) below the maximum levels imposed by current regulations for AFM1 in milk and infant milk formulae. Recoveries (61.3–75.3 %) and repeatability (RSD?<?10, n?=?3), tested in different types of milk at four AFM1 levels, met the performance criteria required by EC Regulation No. 401/2006. Moreover, the matrix effect on the signal intensity of the analyte was negligible. The proposed method provides a rapid extraction and an accurate determination of AFM1 in milk and formula milk using a simple and inexpensive sample preparation procedure. Figure
Rapid analysis of aflatoxin M1 in milk using DLLME 相似文献
870.
Guidotti S Camurati I Focante F Angellini L Moscardi G Resconi L Leardini R Nanni D Mercandelli P Sironi A Beringhelli T Maggioni D 《The Journal of organic chemistry》2003,68(14):5445-5465
The reaction of pyrroles and indoles with B(C(6)F(5))(3) and BCl(3) produces 1:1 B-N complexes containing highly acidic sp(3) carbons, for example, N-[tris(pentafluorophenyl)borane]-5H-pyrrole (1) and N-[tris(pentafluorophenyl)borane]-3H-indole (2), that are formed by a new formal N-to-C hydrogen shift, the mechanism of which is discussed. With some derivatives, restricted rotation around the B-N bond and/or the B-C bonds was observed by NMR techniques, and some rotational barriers were calculated from experimental data. The acidity of the sp(3) carbons in these complexes is shown by their ability to protonate NEt(3), with formation of pyrrolyl- and indolyl-borate ammonium salts. The driving force for this reaction is given by the restoration of the aromaticity of the heterocycle. 相似文献