全文获取类型
收费全文 | 2247篇 |
免费 | 178篇 |
国内免费 | 138篇 |
专业分类
化学 | 1484篇 |
晶体学 | 42篇 |
力学 | 80篇 |
综合类 | 12篇 |
数学 | 259篇 |
物理学 | 686篇 |
出版年
2023年 | 25篇 |
2022年 | 49篇 |
2021年 | 31篇 |
2020年 | 56篇 |
2019年 | 50篇 |
2018年 | 41篇 |
2017年 | 43篇 |
2016年 | 82篇 |
2015年 | 54篇 |
2014年 | 88篇 |
2013年 | 123篇 |
2012年 | 149篇 |
2011年 | 153篇 |
2010年 | 88篇 |
2009年 | 86篇 |
2008年 | 107篇 |
2007年 | 126篇 |
2006年 | 103篇 |
2005年 | 104篇 |
2004年 | 71篇 |
2003年 | 59篇 |
2002年 | 62篇 |
2001年 | 49篇 |
2000年 | 53篇 |
1999年 | 40篇 |
1998年 | 22篇 |
1997年 | 39篇 |
1996年 | 44篇 |
1995年 | 32篇 |
1994年 | 33篇 |
1993年 | 26篇 |
1992年 | 29篇 |
1991年 | 18篇 |
1990年 | 20篇 |
1989年 | 18篇 |
1988年 | 15篇 |
1987年 | 15篇 |
1986年 | 24篇 |
1985年 | 17篇 |
1983年 | 16篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 15篇 |
1979年 | 16篇 |
1978年 | 11篇 |
1977年 | 23篇 |
1976年 | 13篇 |
1975年 | 18篇 |
1972年 | 9篇 |
1971年 | 9篇 |
排序方式: 共有2563条查询结果,搜索用时 15 毫秒
931.
Buckling and vibration analysis of functionally graded magneto-electro-thermo-elastic (FGMETE) circular cylindrical shell are carried out in the present work. The Hamilton principle, higher order shear deformation theory, constitutive equation considering coupling effect between mechanical, electric, magnetic, thermal are considered to derive the equations of motion and distribution of electrical potential, magnetic potential along the thickness direction of FGMETE circular cylindrical shell. The influences of various external loads, such as axis force, temperature difference between the bottom and top surface of shell, surface electric voltage and magnetic voltage, on the buckling response of FGMETE circular cylindrical shell are investigated. The natural frequency obtained by present method is compared with results in open literature and a good agreement is obtained. 相似文献
932.
933.
S. Zitta K. Stoschitzky R. Zweiker T. Lang H. Holzer F. Mayer 《Mathematical and Computer Modelling of Dynamical Systems: Methods, Tools and Applications in Engineering and Related Sciences》2013,19(2):190-207
A method of adapting two-compartment models to dynamic marker concentration profiles for the determination of renal clearance and of its acute changes due to protein ingestion in patients with essential hypertension is described. In 9 healthy controls glomerular filtration rates (GFR) and effective renal plasma flows (ERPF) (ml/min/1.73 m2; means +/- sd) rose significantly tested pairwise from 118.2 +/- 13.9 to 139.5 +/- 30.9, p = 0.023 and from 503.2 +/- 75.6 to 558.3 +/- 96.2, p = 0.013, respectively. Four patients with mild hypertension and mean arterial pressure (MAP) of 106 +/- 3 mmHg (duration = 13.8 +/- 10.3 years) showed rises in GFR (73.9 +/- 14.7 to 83.6 +/- 17.4, p = 0.034) after stimulation, whereas 6 patients with a MAP of 119 +/- 3 mmHg (duration = 17.5 +/- 13.7 years) exhibited ‘paradoxical’ decreases in GFR (113.3 +/- 18.7 to 103.0 +/- 14.3, p = 0.037). The ERPFs showed nonsignificant changes in the first group of patients (277.8 +/- 52.6 to 323.9 +/- 42.8), whereas the second group revealed increases in ERPF (430.7 +/- 134.5 to 502.3 +/- 113.1, p = 0.013). All patients had normal serum creatinine levels. The study demonstrates that modern system identification of kinetic experiments, but not traditional techniques relying on steady-state data, allow one to detect such dynamic alterations as measures of renal functional reserve. 相似文献
934.
We have developed a surface-enhanced Raman scattering (SERS) nanosensor firstly for Ag ions and Ag nanoparticles detection based on 2-mercaptoisonicotinic acid (2MNA)-functionalized Au nanoparticles. Ag(+) can coordinate with 2MNA resutling in a variation of its SERS spectrum, which is used as a criterion to determine Ag(+) in a solution. This sensor exhibits a detection limit no more than 25 nM and has a high selectivity against other metal ions. More importantly, it can be directly applied in real sample detection. 相似文献
935.
Reduced-order models are applied to the laminar vortex-shedding flow around a circular cylinder. The models rely on approximations of the solutions in the space spanned by a set of POD or CVT reduced basis vectors, which are computed from snapshots of a numerical solution. To enable the computation of drag and lift forces, the modeling of the velocity and pressure is realized via a one-way coupling. A comparison of the results of the CVT and POD reduced-order models is presented. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
936.
We describe the adsorption of γ–mercaptopropyltrimethoxysilane (γ-MPS) on zinc under various experimental conditions, including the age of the siloxane solution (tag), its pH (7 or 4), and the mode of preparation of the surface (RCA treatment or in situ polishing). It is shown by XPS studies that the structure of the adsorbed monolayer varies dramatically with the pH of the solution. At the natural pH of the siloxane solution (pH 7) where no hydrolysis of the SiOCH3 group occurs, adsorption proceeds through the SH moiety and not through SiOCH3 groups. This preferential attachment through SH is found whatever the age of the solution and the treatment of the zinc. It is confirmed by the fact that n-propyltrimethoxysilane (PSi) does not interact with the surface in the case of very old solutions (adsorption is not observed when Zn is polished in situ and only occurs with RCA zinc treatment for tag > 40 min). With siloxane solutions at pH 4, adsorption of γ-MPS is more complex and the structure of the adsorbed layer depends mainly on the age of the solution. With a fresh solution, hydrolysis is not very advanced and, as mentioned previously, adsorption occurs through the SH group. With older solutions and as a consequence of the progressive hydrolysis of the SiOCH3 group to SiOH, the density of the grafted siloxane monolayer increases (6 min < tag < 10 min), followed by a mixed adsorption through SH and SiOH (10 min < tag < 40–50 min) revealed by the decrease in the normalised (Si2p/S2p)* intensity ratio. Finally, adsorption of dimers and oligomers is observed with still older siloxane solutions. In contrast to PSi whose adsorption on zinc is favoured by the RCA treatment, neither treatment of the surface changes the results significantly in the case of γ-MPS. Comparison with alkanethiols confirms the transition from monomer to dimer adsorption and IRRAS studies clearly indicate a condensation reaction between OH and SH groups. 相似文献
937.
D. Haskel E. Kravtsov Y. Choi J. C. Lang Z. Islam G. Srajer J. S. Jiang S. D. Bader P. C. Canfield 《The European physical journal. Special topics》2012,208(1):141-155
The element- and site-specificity of X-ray resonant magnetic scattering (XRMS) makes it an ideal tool for furthering our understanding
of complex magnetic systems. In the hard X-rays, XRMS is readily applied to most antiferromagnets where the relatively weak
resonant magnetic scattering (10−2–10−6
I
c
) is separated in reciprocal space from the stronger, Bragg charge scattered intensity, I
c
. In ferro(ferri)magnetic materials, however, such separation does not occur and measurements of resonant magnetic scattering
in the presence of strong charge scattering are quite challenging. We discuss the use of charge-magnetic interference resonant
scattering for studies of ferromagnetic (FM) crystals and layered films. We review the challenges and opportunities afforded
by this approach, particularly when using circularly polarized X-rays. We illustrate current capabilities at the Advanced
Photon Source with studies aimed at probing site-specific magnetism in ferromagnetic crystals, and interfacial magnetism in
films. 相似文献
938.
Lang T Graf E Kyritsakas N Hosseini MW 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(33):10419-10426
The synthesis of a series of molecular turnstiles that contained both H-bond-donor and -acceptor sites was achieved. Their structures were based on tetra-aryl X(2)Sn(IV) porphyrins (X=Cl or OH) as H-bond-acceptor sites that were equipped with a rotor that contained a pyridyldiamide moiety as a H-bond donor. In the solution phase, 1D and 2D NMR spectroscopic analysis showed that switching between the closed state, which resulted from the formation of intramolecular H-bonds, and the open state of the turnstile was achieved by using external H-bond-acceptor molecules, such as DMSO. The solid-state structure of the closed state of the turnstile was established by single-crystal X-ray diffraction. 相似文献
939.
Lang X Ma W Zhao Y Chen C Ji H Zhao J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(9):2624-2631
Imines are important intermediates for the synthesis of fine chemicals, pharmaceuticals, and agricultural chemicals. Selective oxidation of amines into their corresponding imines with dioxygen is one of the most-fundamental chemical transformations. Herein, we report the oxidation of a series of benzylic amines into their corresponding imines with atmospheric dioxygen as the oxidant on a surface of anatase TiO(2) under visible-light irradiation (λ>420 nm). The visible-light response of this system was caused by the formation of a surface complex through the adsorption of a benzylic amine onto the surface of TiO(2). From the analysis of products of specially designed benzylic amines, we demonstrated that a highly selective oxygenation reaction proceeds via an oxygen-transfer mechanism to afford the corresponding carbonyl compound, whose further condensation with an amine would generate the final imine product. We found that when primary benzylic amines (13 examples), were chosen as the substrates, moderate to excellent selectivities for the imine products were achieved (ca. 38-94%) in moderate to excellent conversion rates (ca. 44-95%). When secondary benzylic amines (15 examples) were chosen as the substrates, both the corresponding imines and aldehydes were detected as the main products with moderate to high conversion rates (ca. 18-100%) and lower selectivities for the imine products (ca. 14-69%). When tribenzylamine was chosen as the substrate, imine (27%), dibenzylamine (24%), and benzaldehyde products (39%) were obtained in a conversion of 50%. This report can be viewed as a prototypical system for the activation of C-H bonds adjacent to heteroatoms such as N, O, and S atoms, and oxofuctionalization with air or dioxygen as the terminal oxidant under visible-light irradiation using TiO(2) as the photocatalyst. 相似文献
940.
Sander LC Bedner M Tims MC Yen JH Duewer DL Porter B Christopher SJ Day RD Long SE Molloy JL Murphy KE Lang BE Lieberman R Wood LJ Payne MJ Roman MC Betz JM NguyenPho A Sharpless KE Wise SA 《Analytical and bioanalytical chemistry》2012,402(1):473-487
A suite of three green tea-containing Standard Reference Materials (SRMs) has been issued by the National Institute of Standards and Technology (NIST): SRM 3254 Camellia sinensis (Green Tea) Leaves, SRM 3255 Camellia sinensis (Green Tea) Extract, and SRM 3256 Green Tea-Containing Solid Oral Dosage Form. The materials are characterized for catechins, xanthine alkaloids, theanine, and toxic elements. As many as five methods were used in assigning certified and reference values to the constituents, with measurements carried out at NIST and at collaborating laboratories. The materials are intended for use in the development and validation of new analytical methods, and for use as control materials as a component in the support of claims of metrological traceability. 相似文献