全文获取类型
收费全文 | 482篇 |
免费 | 19篇 |
专业分类
化学 | 294篇 |
晶体学 | 4篇 |
力学 | 13篇 |
数学 | 60篇 |
物理学 | 130篇 |
出版年
2024年 | 2篇 |
2023年 | 3篇 |
2022年 | 14篇 |
2021年 | 13篇 |
2020年 | 8篇 |
2019年 | 19篇 |
2018年 | 16篇 |
2017年 | 21篇 |
2016年 | 22篇 |
2015年 | 21篇 |
2014年 | 20篇 |
2013年 | 47篇 |
2012年 | 39篇 |
2011年 | 45篇 |
2010年 | 25篇 |
2009年 | 21篇 |
2008年 | 32篇 |
2007年 | 29篇 |
2006年 | 22篇 |
2005年 | 24篇 |
2004年 | 18篇 |
2003年 | 10篇 |
2002年 | 9篇 |
2001年 | 3篇 |
2000年 | 1篇 |
1998年 | 1篇 |
1996年 | 6篇 |
1991年 | 2篇 |
1990年 | 1篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1985年 | 1篇 |
1983年 | 1篇 |
1980年 | 1篇 |
1971年 | 1篇 |
排序方式: 共有501条查询结果,搜索用时 31 毫秒
491.
DS Nesterov EN Chygorin VN Kokozay VV Bon R Boča YN Kozlov LS Shul'pina J Jezierska A Ozarowski AJ Pombeiro GB Shul'pin 《Inorganic chemistry》2012,51(16):9110-9122
The heterometallic complex [Co(4)Fe(2)OSae(8)]·4DMF·H(2)O (1) was synthesized by one-pot reaction of cobalt powder with iron chloride in a dimethylformamide solution of salicylidene-2-ethanolamine (H(2)Sae) and characterized by single crystal X-ray diffraction analysis, magnetic measurements, high frequency electron paramagnetic resonance (HF-EPR), and M?ssbauer spectroscopies. The exchange coupling in the Fe(III)-Fe(III) pair is of antiferromagnetic behavior with J/hc = -190 cm(-1). The HF-EPR spectra reveal an unusual pattern with a hardly detectable triplet signal of the Fe(III) dimer. The magnitude of D (ca. 13.9 cm(-1)) was found to be much larger than in related dimers. The catalytic investigations disclosed an outstanding activity of 1 toward oxidation of cycloalkanes with hydrogen peroxide, under mild conditions. The most efficient system showed a turnover number (TON) of 3.57 × 10(3) with the concomitant overall yield of 26% for cyclohexane, and 2.28 × 10(3)/46%, respectively, for cyclooctane. A remarkable turnover frequency (TOF) of 1.12 × 10(4) h(-1) (the highest initial rate W(0) = 3.5 × 10(-4) M s(-1)) was achieved in oxidation of cyclohexane. Kinetic experiments and selectivity parameters led to the conclusion that hydroxyl radicals are active (attacking C-H bonds) species. Kinetic and electrospray ionization mass spectrometry (ESI-MS) data allowed us to assume that the trinuclear heterometallic particle [Co(2)Fe(Sae)(4)](+), originated from 1 in solution, could be responsible for efficient generation of hydroxyl radicals from hydrogen peroxide. 相似文献
492.
A noninvasive, high-resolution optoacoustic technique is a promising alternative to currently used invasive methods of brain oxygenation monitoring. We present the results of our pilot clinical test of this technique in healthy volunteers. Multiwavelength optoacoustic measurements (with nanosecond optical parametric oscillator as a source of radiation) were performed on the area of the neck overlying the internal jugular vein, a deeply located large vein that drains blood from the brain and from extracranial tissues. Optoacoustic signals induced in venous blood were measured with high resolution and signal-to-noise ratio despite the presence of a thick layer of overlying tissue (up to 10 mm). The characteristic parameters of the signal at different wavelengths correlated well with the spectrum of the effective attenuation coefficient of blood. 相似文献
493.
Li ZJ Abramov Y Bordner J Leonard J Medek A Trask AV 《Journal of the American Chemical Society》2006,128(25):8199-8210
A cancer candidate, compound 1, is a weak base with two heterocyclic basic nitrogens and five hydrogen-bonding functional groups, and is sparingly soluble in water rendering it unsuitable for pharmaceutical development. The crystalline acid-base pairs of 1, collectively termed solid acid-base complexes, provide significant increases in the solubility and bioavailability compared to the free base, 1. Three dicarboxylic acid-base complexes, sesquisuccinate 2, dimalonate 3, and dimaleate 4, show the most favorable physicochemical profiles and are studied in greater detail. The structural analyses of the three complexes using crystal structure and solid-state NMR reveal that the proton-transfer behavior in these organic acid-base complexes vary successively correlating with Delta pKa. As a result, 2 is a neutral complex, 3 is a mixed ionic and zwitterionic complex and 4 is an ionic salt. The addition of the acidic components leads to maximized hydrogen bond interactions forming extended three-dimensional networks. Although structurally similar, the packing arrangements of the three complexes are considerably different due to the presence of multiple functional groups and the flexible backbone of 1. The findings in this study provide insight into the structural characteristics of complexes involving heterocyclic bases and carboxylic acids, and demonstrate that X-ray crystallography and 15N solid-state NMR are truly complementary in elucidating hydrogen bonding interactions and the degree of proton transfer of these complexes. 相似文献
494.
The hydrogen is contained in any material. Its concentration inside the materials leads to mechanical properties degradation. The two-continuum model of solid allows one to describe the influence of small concentration of hydrogen on the mechanical properties of materials in terms of changing the bonding energy of the second continuum, the latter being responsible for the hydrogen concentration. The application of this model to fatigue task give the hydrogen concentration that are critical for material destruction. Such fatigue destruction has a nature of parametric instability during the cyclic redistribution of the hydrogen under the cyclic loading. (© 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
495.
We consider the stable homotopy category S of polyhedra (finite cell complexes). We say that two polyhedra X,Y are in the same genus and write X ∼ Y if X
p
≅ Y
p
for all prime p, where X
p
denotes the image of Xin the localized category S
p
. We prove that it is equivalent to the stable isomorphism X∨B
0 ≅Y∨B
0, where B
0 is the wedge of all spheres S
n
such that π
n
S
(X) is infinite. We also prove that a stable isomorphism X ∨ X ≅ Y ∨ X implies a stable isomorphism X ≅ Y. 相似文献
496.
The axisymmetric time-fractional diffusion equation with mass absorption is studied in a circle under the time-harmonic Dirichlet boundary condition. The Caputo derivative of the order is used. The investigated equation can be considered as the time-fractional generalization of the bioheat equation and the Klein–Gordon equation. Different formulations of the problem for integer values of the time-derivatives and are also discussed. The integral transform technique is employed. The outcomes of numerical calculations are illustrated graphically for different values of the parameters. 相似文献
497.
Yuriy Azhniuk Vasyl Lopushansky Stepan Hasynets Volodymyr Kryshenik Alexander V. Gomonnai Dietrich R. T. Zahn 《Journal of Raman spectroscopy : JRS》2024,55(5):637-649
Raman spectra of (As1–xBix)2S3 glass samples with x ≤ 0.2 measured at the excitation with above-bandgap (532 nm) laser light at a relatively low power density (Pexc = 4 kW/cm2) clearly confirm the amorphous character, thereby markedly extending the known compositional interval of existence of the (As1–xBix)2S3 glass previously known (x ≤ 0.06). Spectra measured at an increased Pexc (40 kW/cm2) reveal a photostructural transformation in the illuminated area of the glass leading to an additional contribution of Bi–S bonds as well as to an increasing number of cage-type As4S4 units with homopolar As–As bonds. A number of new features in a broad range up to about 1,000 cm−1, which emerge in the Raman spectra of the (As1–xBix)2S3 glasses with high (x ≥ 0.14) Bi content and increase in intensity with the exposure time, are related to a photochemical transformation, namely, oxidation of arsenic and sulphur on the (As1–xBix)2S3 glass surface with formation of units containing arsenate AsO43− and sulphate SO42− ions. These processes are irreversible and occur only in the presence of a sufficient amount of bismuth. 相似文献
498.
Georgiy V. Malkov Alexey V. Shastin Yakov I. Estrin Elmira R. Badamshina Yuriy M. Mikhailov 《Macromolecular Symposia》2010,296(1):107-111
The results of the kinetic study of melt and solution polymerization at the 1,3-dipolar cycloaddition reaction of the AB2 monomer – 2-azido-4,6-bis(prop-2-yn-1-yloxy)-[1,3,5]-triazine (ABPOT) are presented in this work as well as the results of the 13C-NMR characterization of the obtained hyperbranched poly([1,2,3]-triazole-[1,3,5]-triazine)s. It is established, that the first-shell substitution effect during polyaddition process and unusual high degree of branching (up to 0.9) of polymers synthesized in melt are held. 相似文献
499.
500.