全文获取类型
收费全文 | 2435篇 |
免费 | 86篇 |
国内免费 | 13篇 |
专业分类
化学 | 1953篇 |
晶体学 | 25篇 |
力学 | 23篇 |
数学 | 333篇 |
物理学 | 200篇 |
出版年
2023年 | 8篇 |
2022年 | 39篇 |
2021年 | 47篇 |
2020年 | 37篇 |
2019年 | 36篇 |
2018年 | 29篇 |
2017年 | 20篇 |
2016年 | 71篇 |
2015年 | 73篇 |
2014年 | 71篇 |
2013年 | 146篇 |
2012年 | 194篇 |
2011年 | 216篇 |
2010年 | 123篇 |
2009年 | 104篇 |
2008年 | 166篇 |
2007年 | 179篇 |
2006年 | 164篇 |
2005年 | 139篇 |
2004年 | 106篇 |
2003年 | 89篇 |
2002年 | 91篇 |
2001年 | 35篇 |
2000年 | 28篇 |
1999年 | 29篇 |
1998年 | 12篇 |
1997年 | 18篇 |
1996年 | 30篇 |
1995年 | 18篇 |
1994年 | 9篇 |
1993年 | 13篇 |
1992年 | 16篇 |
1991年 | 9篇 |
1990年 | 7篇 |
1989年 | 12篇 |
1988年 | 6篇 |
1987年 | 13篇 |
1986年 | 8篇 |
1985年 | 14篇 |
1984年 | 12篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 8篇 |
1980年 | 6篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 7篇 |
1976年 | 11篇 |
1973年 | 6篇 |
1888年 | 3篇 |
排序方式: 共有2534条查询结果,搜索用时 328 毫秒
221.
222.
Keiser T Höllriegl V Giussani A Oeh U 《Isotopes in environmental and health studies》2011,47(2):238-252
Thermal ionisation mass spectrometry (TIMS) method has been developed for the simultaneous detection of different cerium isotopes in biological samples (i.e., blood and urine) at very low concentrations. The work has been done in the frame of a biokinetic study, where different stable cerium isotopes have been administered orally and intravenously as tracers to the human body. In order to develop an appropriate detection method for the tracers in the biological samples, an optimum sample preparation technique has been set and adapted to the specific requirements of the analysis technique used, i.e., TIMS. For sample evaporation and ionisation, the double tantalum filament technique showed the best results. The ions produced were simultaneously collected on a secondary electron multiplier so that the isotopic ratios of the cerium isotopes in the biological samples could be measured. The technique has been optimised for the determination of cerium down to 1?ng loaded on the evaporation filament corresponding to cerium concentrations of down to 1?ng?ml(-1) in the blood or urine samples. It has been shown that the technique is reliable in application and enables studies on cerium metabolism and biokinetics in humans without employing radioactive tracers. 相似文献
223.
Parracino A Gajula GP di Gennaro AK Neves-Petersen MT Rafaelsen J Petersen SB 《Journal of fluorescence》2011,21(2):663-672
Medical interest in nanotechnology originates from a belief that nanoscale therapeutic devices can be constructed and directed
towards its target inside the human body. Such nanodevices can be engineered by coupling superparamagnetic nanoparticle to
biomedically active proteins. We hereby report the immobilization of a PhEst, a S-formylglutathione hydrolase from the psychrophilic
P. haloplanktis TAC125 onto the gold coated surface of modified superparamagnetic core-shell nanoparticles (Fe3O4@Au). The synthesis of the nanoparticles is also reported. S-formylglutathione hydrolases constitute a family of ubiquitous
enzymes which play a key role in formaldehyde detoxification both in prokaryotes and eukaryotes. PhEst was originally annotated
as a putative feruloyl esterase, an enzyme that releases ferulic acid (an antioxidant reactive towards free radicals such
as reactive oxygen species) from polysaccharides esters. Dynamic light scattering, scanning electron microscopy with energy
dispersive X-ray spectroscopy, UV–visible absorption spectroscopy, fluorescence spectroscopy, magnetic separation technique
and enzyme catalytic assay confirmed the chemical composition of the gold covered superparamagnetic nanoparticles, the binding
and activity of the enzyme onto the nanoparticles. Activity data in U/ml confirmed that the immobilized enzyme is approximately
2 times more active than the free enzyme in solution. Such particles can be directed with external magnetic fields for bio-separation
and focused towards a medical target for therapeutical as well as bio-sensor applications. 相似文献
224.
Random wall-pressure fluctuations due to the turbulent boundary layer (TBL) are a feature of the air flow over an aircraft fuselage under cruise conditions, creating undesirable effects such as cabin noise annoyance. In order to test potential solutions to reduce the TBL-induced noise, a cost-efficient alternative to in-flight or wind-tunnel measurements involves the laboratory simulation of the response of aircraft sidewalls to high-speed subsonic TBL excitation. Previously published work has shown that TBL simulation using a near-field array of loudspeakers is only feasible in the low frequency range due to the rapid decay of the spanwise correlation length with frequency. This paper demonstrates through theoretical criteria how the wavenumber filtering capabilities of the radiating panel reduces the number of sources required, thus dramatically enlarging the frequency range over which the response of the TBL-excited panel is accurately reproduced. Experimental synthesis of the panel response to high-speed TBL excitation is found to be feasible over the hydrodynamic coincidence frequency range using a reduced set of near-field loudspeakers driven by optimal signals. Effective methodologies are proposed for an accurate reproduction of the TBL-induced sound power radiated by the panel into a free-field and when coupled to a cavity. 相似文献
225.
A Pairwise Comparison Matrix (PCM) has been used to compute for relative priorities of elements and are integral components in widely applied decision making tools: the Analytic Hierarchy Process (AHP) and its generalized form, the Analytic Network Process (ANP). However, PCMs suffer from several issues limiting their applications to large-scale decision problems. These limitations can be attributed to the curse of dimensionality, that is, a large number of pairwise comparisons need to be elicited from a decision maker. This issue results to inconsistent preferences due to the limited cognitive powers of decision makers. To address these limitations, this research proposes a PCM decomposition methodology that reduces the elicited pairwise comparisons. A binary integer program is proposed to intelligently decompose a PCM into several smaller subsets using interdependence scores among elements. Since the subsets are disjoint, the most independent pivot element is identified to connect all subsets to derive the global weights of the elements from the original PCM. As a result, the number of pairwise comparison is reduced and consistency is of the comparisons is improved. The proposed decomposition methodology is applied to both AHP and ANP to demonstrate its advantages. 相似文献
226.
In this paper we obtain generalized Clarkson–McCarthy inequalities for spaces l q (S p ) of operators from Schatten ideals S p . We show that all Clarkson–McCarthy type inequalities are, in fact, some estimates on the norms of operators acting on the spaces l q (S p ) or from one such space into another. We also extend some inequalities for partitioned operators and for Cartesian decomposition of operators. 相似文献
227.
Farid Ammar Khodja Assia Benabdallah Manuel González-Burgos Luz de Teresa 《Comptes Rendus Mathematique》2014,352(5):391-396
We consider two parabolic equations coupled by a matrix A(x)=q(x)A0, where A0 is a Jordan block of order 1, and controlled by a single localized function, or by a single boundary control. The support of the coupling coefficient, q , and the control domain may be disjoint. We exhibit an explicit minimal time of null-controllability, T0(q)∈[0,+∞]. 相似文献
228.
Marino Badiale Vieri Benci Teresa D'Aprile 《Calculus of Variations and Partial Differential Equations》2001,12(3):223-258
In this paper we study existence, multiplicity and concentration of solutions for the following nonlinear field equation
where the potential V is positive and W is an appropriate singular function. Here is regarded as a small parameter. Under suitable conditions on V and W we find solutions exhibiting a concentration behaviour at an absolute minimum of V as Such solutions are obtained as local minima for the associated functional; the proofs of our results rely on a careful analysis
of the behaviour of minimizing sequences and use arguments inspired by the concentration-compactness principle.
Received July 21, 1999; Accepted April 9, 2000 / Published online September 14, 2000 相似文献
229.
Allyn T. Londregan David Bernhardson James Bradow Teresa M. Makowski Gregory Storer Joseph Warmus Ceshea Wooten Xiaojing Yang 《Tetrahedron: Asymmetry》2010,21(17):2072-2075
A number of approaches to the enantioselective synthesis of (R)- and (S)-3-amino-3,4-dihydro-1H-[1,8]naphthyridin-2-one were studied. A novel one-pot asymmetric reduction/lactamization provided the desired products in high yield and enantiomeric excess. 相似文献
230.
Magali A. Rodrigues Sérgio Brochsztain Teresa C. Barros Mauricio S. Baptista Mario J. Politi 《Photochemistry and photobiology》1999,70(1):35-39
Abstract— The effect of the protonation state of a novel aqueous soluble N,N'-di(2-phosphonoethyl)-l,4,5,8-naphthalene-diimide (DPN) on its spectroscopic properties is studied. In aqueous solution DPN depicts linear Lambert-Beer plots up to the solubility limits at pH 1.0, 7.1 and 10.1, in accordance with its solubilization as a monomer. A small increase in the extinction coefficients with pH is observed. Conversely a strong decrease in the fluorescence emission yields are observed with the increase in pH. These results are shown to originate from the distinct efficiencies of the intersystem crossing of DPN as a function of the net charge on the phosphono moieties. Accordingly, from the decrease in fluorescence emission, titration curves were obtained resulting in two pKa of –1.5 and 6.7 in agreement with the potentiometric analysis. In parallel monitoring the amount of heat deposited in the system shows an inverse pattern, that is, increase in the thermal tensing time-resolved signal with pH. Laser flash photolysis and thermal Iensing data in the absence and presence of oxygen show the interplay of an intersystem crossing process with the protonation state of the dye. The changes in protonation from neutral to two- and four-fold charged species are accompanied by an enhancing effect on the naphthalenediimide ring toward spin inversion (triplet formation) and consequently in different fluorescence yields 相似文献