全文获取类型
收费全文 | 1550篇 |
免费 | 58篇 |
国内免费 | 4篇 |
专业分类
化学 | 1041篇 |
晶体学 | 8篇 |
力学 | 45篇 |
数学 | 220篇 |
物理学 | 298篇 |
出版年
2023年 | 18篇 |
2022年 | 17篇 |
2021年 | 44篇 |
2020年 | 33篇 |
2019年 | 26篇 |
2018年 | 20篇 |
2017年 | 21篇 |
2016年 | 55篇 |
2015年 | 49篇 |
2014年 | 44篇 |
2013年 | 79篇 |
2012年 | 87篇 |
2011年 | 115篇 |
2010年 | 79篇 |
2009年 | 62篇 |
2008年 | 84篇 |
2007年 | 98篇 |
2006年 | 85篇 |
2005年 | 62篇 |
2004年 | 67篇 |
2003年 | 59篇 |
2002年 | 48篇 |
2001年 | 21篇 |
2000年 | 13篇 |
1999年 | 30篇 |
1998年 | 19篇 |
1997年 | 29篇 |
1996年 | 17篇 |
1995年 | 13篇 |
1994年 | 15篇 |
1993年 | 14篇 |
1992年 | 15篇 |
1991年 | 10篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 7篇 |
1987年 | 7篇 |
1986年 | 13篇 |
1985年 | 9篇 |
1984年 | 8篇 |
1983年 | 8篇 |
1982年 | 10篇 |
1981年 | 9篇 |
1980年 | 5篇 |
1979年 | 13篇 |
1978年 | 5篇 |
1976年 | 8篇 |
1974年 | 5篇 |
1973年 | 8篇 |
1970年 | 4篇 |
排序方式: 共有1612条查询结果,搜索用时 15 毫秒
991.
We investigated the 13CD3I isotopomer of methyl iodide as a source of TeraHertz laser radiation using the optical pumping technique. We used a pulsed
waveguide CO2 laser as the pump laser and an open Fabry–Pérot cavity for new laser line generation. We discovered 18 new laser lines with
wavelengths ranging from 308.4 to 1132.7 μm, plus two lines previously assigned to 12CD3I. All of the lines were characterized in wavelength, pump frequency offset, optimum pressure of operation, and relative polarization
and intensity. 相似文献
992.
Alexandros Kiriazis Gustav Boije af Gennäs Virpi TalmanElina Ekokoski Timo Ruotsalainen Irene KylänlahtiTobias Rüffer Gloria Wissel Henri XhaardHeinrich Lang Raimo K. TuominenJari Yli-Kauhaluoma 《Tetrahedron》2011,67(45):8665-8670
Protein kinase C (PKC) is a widely studied molecular target for the treatment of cancer and other diseases. We have approached the issue of modifying PKC function by targeting the C1 domain in the regulatory region of the enzyme. By using the X-ray crystal structure of the PKCδ C1b domain combined with molecular modeling, we discovered (3-aminodecahydro-1,4-methanonaphthalen-2-yl)methanol as a novel C1 domain ligand. The stereoselective synthesis of this tricyclic γ-amino alcohol was based on two successive Diels-Alder reactions to construct the six continuous stereocenters of the key intermediate. 相似文献
993.
Elgte Elmin B. de PaulaLorenzo C. Visentin Maria Irene YoshidaLuiz Fernando C. De Oliveira Flávia C. Machado 《Polyhedron》2011,30(2):213-220
Two new coordination polymers {[Mn2(BPP)2(C5O5)2(OH2)4]·3H2O}n (1) and {[Cu4(BPP)4(C5O5)4]·8H2O}n (2) [BPP = 1,3-bis(4-pyridyl)propane and C5O52− = croconate anion] were synthesized and characterized by thermal analysis, vibrational spectroscopy (Raman and infrared) and single crystal X-ray diffraction analysis. Compound 1 was obtained under solvothermal conditions and 2 by the diffusion method. Structure determination by single crystal X-ray diffraction analysis revealed that compound 1 presents two-dimensional polymeric nature due to the bridging coordination mode of both BPP ligand and croconate anion. In this case, the croconate anion acts in the unprecedented μ-1,2-bis(monodentate) coordination mode, forming interpenetrated grids of 63 topology. On the other hand, compound 2 is a three-dimensional 4-connected uninodal net of (42·84) topology, due to the presence of the flexible BPP ligand that bridges two Cu(II) sites connecting perpendicular one-dimensional polymeric chains, formed by croconate anions coordinated to the metal centers in a bis-bidentate coordination mode. 相似文献
994.
In order to establish which are the contribution of linear (total), hyperbolic (partial) or parabolic inhibitions by cellobiose,
and also a special case of substrate inhibition, the kinetics of cellobiohydrolase Cel7A obtained from Trichoderma reesei was investigated. Values of kinetic parameters were estimated employing integrated forms of Michaelis–Menten equations through
the use of non-linear regression, and criteria for selecting inhibition models are discussed. With cellobiose added at the
beginning of the reaction, it was found that cellulose hydrolysis follows a kinetic model, which takes into account a mixed
hyperbolic inhibition, by cellobiose with the following parameter values: K
m 5.0 mM, K
ic 0.029 mM, K
iu 1.1 mM, k
cat 3.6 h−1 and k
cat′ 0.2 h−1. Cellulose hydrolysis without initial cellobiose added also follows the same inhibition model with similar values (4.7, 0.029
and 1.5 mM and 3.2 and 0.2 h−1, respectively). According to Akaike information criterion, more complex models that take into account substrate and parabolic
inhibitions do not increase the modulation performance of cellulose hydrolysis. 相似文献
995.
Natali I Carretti E Angelova L Baglioni P Weiss RG Dei L 《Langmuir : the ACS journal of surfaces and colloids》2011,27(21):13226-13235
The preparation and structural characterization of a family of viscoelastic dispersions of borate cross-linked, 80% hydrolyzed poly(vinyl acetate) (80PVAc) in aqueous-organic liquids are presented. Correlations between mechanical properties (from rheological measurements) and the degree and nature of cross-linking (from (11)B NMR spectroscopy) are reported, and the results are used to assess their potential as low-impact cleaning agents for the surfaces of paintings. Because the dispersions can be prepared at room temperature by simple procedures from readily available materials and can contain up to 50% (w/w) of an organic liquid, they offer important advantages over previously described cleaning agents that are based on fully hydrolyzed PVAc (i.e., poly(vinyl alcohol). The mechanical properties of the various aqueous-organic dispersions, as determined quantitatively by rheological investigations and qualitatively by their ease of removal from a solid surface (i.e., the so-called "peel-off" ability) have been tuned systematically by varying the amount of organic liquid, its structure, and the concentrations of borax and 80PVAc. The (11)B NMR studies demonstrate that the concentration of borate ions actively participating in cross-linking increases significantly with the amount of organic liquid in the mixture. The degree of cross-linking remains constant when the 80PVAc and borax concentrations are varied, as long as their ratios are kept constant. Some of the 80PVAc-borax dispersions have been tested successfully as cleaning agents on the surface of a 16th-17th century oil-on-wood painting by Lodovico Cardi, "Il Cigoli", that was covered by a brown patina and on the surface of a Renaissance wall painting by Vecchietta in Santa Maria della Scala, Siena, Italy, that had a degraded polyacrylate coating from a previous conservation treatment. 相似文献
996.
Shu I Stewart JM Scian M Kier BL Andersen NH 《Journal of the American Chemical Society》2011,133(5):1196-1199
The (13)C chemical shifts measured for designed β-hairpins indicate that the structuring shifts (upfield for Cα and C', downfield for Cβ) previously reported as diagnostic for β-structuring in proteins appear only at the H-bonded strand residues. The resulting periodicity of structuring shift magnitudes is not, however, a consequence of H-bonding status; rather, it reflects a previously unrecognized alternation in the backbone torsion angles of β-strands. This feature of hairpins is also likely to be present in proteins. The study provides reference values for the expectation shifts for (13)C sites in β-structures that should prove useful in the characterization of the folding equilibria of β-sheet models. 相似文献
997.
A confirmatory method for the determination of residues of eleven coccidiostats including ionophore antibiotics: lasalocid, maduramycin, monensin, narasin, salinomycin, semduramycin and chemical coccidiostats: decoquinate, diclazuril, halofuginone, nicarbazin and robenidine in poultry eggs was developed and validated. The sample was extracted with acetonitrile, defatted with hexane and cleaned-up on a silica SPE cartridge. The analytes were identified and quantified by liquid chromatography-tandem mass spectrometry (LC-MS/MS). The method performance characteristics required by Commission Decision 2002/657/EC were estimated adopting a more flexible and simple validation design. In this alternative approach the experimental study focuses on a larger dynamic range with progressively increasing validation levels. Each of them presents equal concentrations of all the analytes. On the contrary the classical Decision plan investigates a restricted concentration range necessarily different for each analyte being determined by the individual permitted limit (0.5-1.5 times the permitted limit). As a consequence each validation level involves the simultaneous fortification with complex mixtures containing different concentrations of each analyte. Adopting this alternative strategy the validation of several substances with significantly different permitted limits is considerably simplified and a deeper knowledge of the method is achieved. The results proved that the method enables the confirmation of regulated coccidiostats in eggs at the levels required in the official control of residues (CCα in the range of 2.2-174 μg kg(-1), depending on the coccidiostat). The repeatability (CV(r) in the range of 1.1-19%) and within-laboratory reproducibility (CV(Rw) in the range of 1.8-30%) are also acceptable. The procedure was successfully verified in the proficiency test and implemented in the national residue control plan. 相似文献
998.
Petersen RK Christensen KB Assimopoulou AN Fretté X Papageorgiou VP Kristiansen K Kouskoumvekaki I 《Journal of computer-aided molecular design》2011,25(2):107-116
In a search for more effective and safe anti-diabetic compounds, we developed a pharmacophore model based on partial agonists
of PPARγ. The model was used for the virtual screening of the Chinese Natural Product Database (CNPD), a library of plant-derived
natural products primarily used in folk medicine. From the resulting hits, we selected methyl oleanonate, a compound found,
among others, in Pistacia lentiscus var. Chia oleoresin (Chios mastic gum). The acid of methyl oleanonate, oleanonic acid, was identified as a PPARγ agonist
through bioassay-guided chromatographic fractionations of Chios mastic gum fractions, whereas some other sub-fractions exhibited
also biological activity towards PPARγ. The results from the present work are two-fold: on the one hand we demonstrate that
the pharmacophore model we developed is able to select novel ligand scaffolds that act as PPARγ agonists; while at the same
time it manifests that natural products are highly relevant for use in virtual screening-based drug discovery. 相似文献
999.
Fabio?Lisi Davide?Carta Laura?Villanova Irene?Poli Dario?Buso Stefano?Costacurta Anita?J.?Hill Paolo?FalcaroEmail author 《Journal of Sol-Gel Science and Technology》2011,60(3):246-253
Functional coatings with amino groups are used in a wide range of biochemistry-related applications. A technological platform
that takes advantage of the affinity between amino-functionalized coatings and biomolecules is the DNA microarray methodology.
Reliability and reproducibility of the microarray data strongly depend on the quality of the substrate; therefore, a proper
awareness of how the storage conditions affect the amino-functionalized coatings is necessary. In this work we have studied
the influence of different relative humidity levels on amino-methyl-silane coatings prepared via sol–gel methodology. Drops
of a buffer solution containing a luminescent dye have been deposited (or spotted) on the coating; the dye molecules react with the amino-groups and leave circular luminescent marks (spots) on the substrate
surface. Shape and luminescence of the spots, as well as background signals, have been monitored using a microarray laser
scanner. With the proposed protocol we have measured the changes of these variables due to different storage conditions. FT-IR
measurements have been performed to investigate the related chemistry changes. 相似文献
1000.
Tsai IY Tomczyk N Eckmann JI Composto RJ Eckmann DM 《Colloids and surfaces. B, Biointerfaces》2011,84(1):241-252
Protein adsorption is fundamental to thrombosis and to the design of biocompatible materials. We report a two-dimensional electrophoresis and mass spectrometry study to characterize multiple human plasma proteins adsorbed onto four different types of model surfaces: silicon oxide, dextranized silicon, polyurethane and dextranized polyurethane. Dextran was grafted onto the surfaces of silicon and polyurethane to mimic the blood-contacting endothelial cell glycocalyx surface. Surface topography and hydrophobicity/hydrophilicity were determined and analyzed using atomic force microscopy and water contact angle measurements, respectively. Using two-dimensional electrophoresis, we show that, relative to the unmodified surfaces, dextranization significantly inhibits the adsorption of several human plasma proteins including IGHG1 protein, fibrinogen, haptoglobin, Apo A-IV, Apo A-I, immunoglobulin, serum retinal-binding protein and truncated serum albumin. We further demonstrate the selectivity of plasma protein adsorbed onto the different functionalized surfaces and the potential to control and manipulate proteins adsorption on the surfaces of medical devices, implants and microfluidic devices. This result shows that adsorption experiments using a single protein or a binary mixture of proteins are consistent with competitive protein adsorption studies. In summary, these studies indicate that coating blood-contacting biomedical applications with dextran is an effective route to reduce thrombo-inflammatory responses and to surface-direct biological activities. 相似文献