全文获取类型
收费全文 | 3710篇 |
免费 | 108篇 |
国内免费 | 22篇 |
专业分类
化学 | 3129篇 |
晶体学 | 42篇 |
力学 | 40篇 |
数学 | 164篇 |
物理学 | 465篇 |
出版年
2023年 | 22篇 |
2022年 | 32篇 |
2021年 | 42篇 |
2020年 | 55篇 |
2019年 | 51篇 |
2018年 | 30篇 |
2017年 | 30篇 |
2016年 | 66篇 |
2015年 | 75篇 |
2014年 | 86篇 |
2013年 | 188篇 |
2012年 | 208篇 |
2011年 | 267篇 |
2010年 | 141篇 |
2009年 | 138篇 |
2008年 | 296篇 |
2007年 | 226篇 |
2006年 | 279篇 |
2005年 | 246篇 |
2004年 | 219篇 |
2003年 | 196篇 |
2002年 | 186篇 |
2001年 | 47篇 |
2000年 | 62篇 |
1999年 | 46篇 |
1998年 | 34篇 |
1997年 | 36篇 |
1996年 | 40篇 |
1995年 | 37篇 |
1994年 | 31篇 |
1993年 | 31篇 |
1992年 | 29篇 |
1991年 | 23篇 |
1990年 | 19篇 |
1989年 | 13篇 |
1988年 | 17篇 |
1987年 | 17篇 |
1986年 | 12篇 |
1985年 | 30篇 |
1984年 | 32篇 |
1983年 | 12篇 |
1982年 | 25篇 |
1981年 | 27篇 |
1980年 | 25篇 |
1979年 | 16篇 |
1978年 | 18篇 |
1977年 | 11篇 |
1976年 | 10篇 |
1975年 | 15篇 |
1974年 | 10篇 |
排序方式: 共有3840条查询结果,搜索用时 15 毫秒
991.
Intermolecular interaction and a concentration-quenching mechanism of phosphorescent Ir(III) complexes in a solid film 总被引:1,自引:0,他引:1
Solid-state self-quenching processes of highly efficient Ir(III) phosphorescent emitters are investigated by the measurement of thin film photoluminescence quantum efficiency and transient lifetime as a function of doping concentration in a host matrix. The radiative decay rate constant is found to be independent from the average distance between dopant molecules (R), and the concentration-quenching rate constant is shown to be dependent on R(-6). The quenching dependence on R strongly suggests that luminescent concentration quenching in a phosphorescent Ir(III) complex:host film is controlled by dipole-dipole deactivating interactions as described by the F?rster energy transfer model. 相似文献
992.
Ito M Ogino H Oshima H Shiraki N Shibamoto Y Kasai H Mase M Kawamura Y Miyati T 《Magnetic resonance imaging》2006,24(5):625-630
PURPOSE: A newly developed contrast material, CH3-DTPA-Gd (NMS60), a trimer containing 3 Gd(3+) atoms per molecule, has been shown to offer greater enhancement and longer vascular retention than gadopentetate dimeglumine (Gd-DTPA) in animals. We report on our early phase II study on NMS60 in brain tumor patients together with supplementary investigations. METHODS AND MATERIALS: The longitudinal relaxation rate (R(1)=1/T(1)) and the transverse relaxation rate (R(2)*=1/T(2)*) of NMS60 and Gd-DTPA were determined at 20 degrees C in water at 1.5 T. An NMS60 dose of 0.1 or 0.2 mmol (Gd)/kg was randomly assigned and administered to 10 patients (five women, five men; mean age: 49 years) with brain tumors. Safety and contrast-enhancing ability of NMS60 were evaluated. Dual dynamic contrast-enhanced T(1) and R(2)* studies (DUCE imaging) were also carried out in two patients. RESULTS: Regarding the relaxivity per Gd, R(1) and R(2)* of NMS60 were 9.5 and 11.0 (mmol/L x s)(-1), respectively, compared to 4.8 and 7.2 (mmol/L x s)(-1) for Gd-DTPA. Although a transient slight increase of alanine aminotransferase was observed in one case, no other adverse reactions were observed after administration of NMS60. Contrast enhancement by NMS60 was excellent at both concentrations, and when tumor detectability was assessed with a five-point scale, the diagnostic usefulness was 4 or higher in all cases. In DUCE imaging, NMS60 appeared to show high signal intensity, when compared with the data obtained separately for Gd-DTPA. CONCLUSION: NMS60 had a high contrasting effect and little toxicity, and is expected to be clinically useful. 相似文献
993.
Conformations and electronic states of Hückel-aromatic regular, singly, doubly, and triply N-confused [26]hexaphyrins were investigated using density functional theory (DFT) calculations. A comparison of the molecular energies of 754 structures in all revealed that the most stable conformers depend on the degree of confusion, where ring strain and intramolecular hydrogen bonding would play a critical role. Consequently, regular and singly N-confused hexaphyrins prefer a dumbbell conformation, doubly N-confused hexaphyrin prefers a rectangular conformation, and triply N-confused hexaphyrin prefers a triangular conformation. Introduction of N-confused pyrrole rings into the hexaphyrin framework causes narrower HOMO-LUMO energy gaps, while it does not affect the NICS values or aromaticity significantly. The steric repulsion imposed by meso-aryl substituents largely affects the relative energies among the conformers. 相似文献
994.
Ikeda S Toganoh M Easwaramoorthi S Lim JM Kim D Furuta H 《The Journal of organic chemistry》2010,75(24):8637-8649
A variety of N-fused porphyrin derivatives were prepared and their photophysical properties were investigated. Although intact N-fused tetraarylporphyrins showed almost no emission, introduction of electron-withdrawing groups such as a nitro group and a cyano group on the macrocycles caused significant refinements in their emission efficiency. Long emission wavelengths (900-1000 nm) as well as fairly large Stokes shifts (~1200 cm(-1)) are exceptionally unique photophysical properties among [18]annulenic compounds, which could be rationalized by the excited state intramolecular proton transfer (ESIPT) process. Relatively weak emission quantum yields (~5.0 × 10(-4)) and unusually short S(1) state lifetimes (~13.5 ps) are in good agreement with the ESIPT process. The solvent and substituent effects on the photophysical properties are also discussed in conjunction with the theoretical studies, where the mesityl groups at the meso-positions play a unique role. 相似文献
995.
The degradation of the mechanical properties of polyimide films was evaluated by means of tensile tests after exposure to a low earth orbit (LEO) environment. Polyimide films irradiated with atomic oxygen (AO), ultraviolet (UV) light, and electron beam (EB) rays using ground simulation facilities were also evaluated similarly and compared. In these experiments tensile stress (7.0 MPa or less) was applied to the samples in order to assess its effects on mechanical properties. The mechanical properties of the flight samples decreased concomitantly with increased exposure duration. The fracture surfaces exhibited characteristic radiated patterns initiating from the exposed surfaces which showed a rough texture. In the AO-irradiated samples the mechanical properties degraded and the surface texture developed as the AO fluence increased; similar fracture surfaces appeared in the flight samples. In contrast, UV and EB irradiation had little impact on mechanical properties. Based on these results, the eroded surfaces by AO irradiation served as the starting points of the rupture, resulting in degradation of mechanical properties of polyimide films exposed to a LEO environment. The tensile stress states induced no difference in evaluations. 相似文献
996.
Susumu Takabayashi Keishi Okamoto Hiroaki Motoyama Tatsuyuki Nakatani Hiroyuki Sakaue Takayuki Takahagi 《Surface and interface analysis : SIA》2010,42(2):77-87
Oxygen‐related surface functional groups on diamond‐like carbon (DLC) films were derivatized with fluorine‐ and nitrogen‐related groups by the gas‐phase chemical derivatization (GCD) method, and the groups were analyzed quantitatively by X‐ray photoelectron spectroscopy (XPS). It is desirable that a derivatization reaction is unique to the target group; however, it usually causes undesirable side reactions which affect other groups. This diversity of the reactions has complicated the analysis. In this report, we have overcome the problem by applying a mathematical treatment which takes the side reactions into account. This improved analysis shows that it is no longer necessary to have derivatization reactions unique to the target groups. As a result, it is demonstrated that the carbonyl (C?O) group is the dominant surface functional group on both the DLC and its wet‐oxidized films, the carboxyl (COOH) group plays a minor role, and the presence of the hydroxyl (OH) group is logically denied. Considering the oxidation steps of these oxygen‐related surface functional groups, it is suggested that the C?O group on the DLC films requires the cleavage of the carbon–carbon bond with a relatively high activation energy barrier to change into the COOH group. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
997.
Kazuma Gotoh Tetsuo Asaji Hiroyuki Ishida 《Acta Crystallographica. Section C, Structural Chemistry》2010,66(3):o114-o118
The crystal structures of two solid phases of the title compound, C4H5N2+·C6HCl2O4−·H2O, have been determined at 225 and 120 K. In the high‐temperature phase, stable above 198 K, the transition temperature of which has been determined by 35Cl nuclear quadrupole resonance and differential thermal analysis measurements, the three components are held together by O—H...O, N...H...O, C—H...O and C—H...Cl hydrogen bonds, forming a centrosymmetric 2+2+2 aggregate. In the N...H...O hydrogen bond formed between the pyrimidin‐1‐ium cation and the water molecule, the H atom is disordered over two positions, resulting in two states, viz. pyrimidin‐1‐ium–water and pyrimidine–oxonium. In the low‐temperature phase, the title compound crystallizes in the same monoclinic space group and has a similar molecular packing, but the 2+2+2 aggregate loses the centrosymmetry, resulting in a doubling of the unit cell and two crystallographically independent molecules for each component in the asymmetric unit. The H atom in one N...H...O hydrogen bond between the pyrimidin‐1‐ium cation and the water molecule is disordered, while the H atom in the other hydrogen bond is found to be ordered at the N‐atom site with a long N—H distance [1.10 (3) Å]. 相似文献
998.
Tamamura H Tsutsumi H Masuno H Mizokami S Hiramatsu K Wang Z Trent JO Nakashima H Yamamoto N Peiper SC Fujii N 《Organic & biomolecular chemistry》2006,4(12):2354-2357
A linear type of several low molecular weight CXCR4 antagonists were developed based on T140 analogs, which were previously found to be strong CXCR4 antagonists that block X4-HIV-1 entry and have inhibitory activities against cancer metastasis/progression and rheumatoid arthritis. 相似文献
999.
Yamada T Noto M Shirasaka K Kato HS Kawai M 《The journal of physical chemistry. B》2006,110(13):6740-6749
Ultraviolet photoassisted adsorption of terminally double-bonded molecules, allylamine (CH2=CH-CH2-NH2) and 1-butene (CH2=CH-CH2-CH3), on hydrogen-terminated silicon (111) surface was attempted to obtain adsorbates covalently terminating the surface Si atoms. The adsorption process was monitored by high-resolution electron energy loss spectroscopy, multiple internal infrared reflection-absorption spectroscopy, and Auger electron spectroscopy. Allylamine adsorbates emerged upon delivery of allylamine gas under ultraviolet irradiation. The N-H bonds in allylamine were evidenced to survive over the photoadsorption process by vibrational analysis and by the reaction with ketene. CH3- groups were detected at low coverage, indicating anchoring of the organic moieties by the secondary (sec-) type carbon atoms, which were taken over by the primary (n-) type with increasing coverage. C-D bonds were detected after deposition on deuterium-terminated Si(111) upon incorporation of Si-terminating H into the hydrocarbon part of adsorbates. In the case of 1-butene, not only the C=C end but also the CH3- end of a molecule might attach on Si, resulting in emergence of adsorbates composed of CH2 groups. The newly obtained adsorbates are prospective as a material applied for nanolithography, fine electrochemistry, and nano-biotechnology. 相似文献
1000.
Yamada T Shirasaka K Noto M Kato HS Kawai AM 《The journal of physical chemistry. B》2006,110(14):7357-7366
Grafting of unsaturated hydrocarbon moieties (-CH(2)-CH=CH(2), -CH=CH(2), -CH(2)-CH=CH-CH(3), and -CCH) by a C-Si covalent bond was attempted by the Grignard reaction on hydrogen-terminated Si(111) in tetrahydrofuran solutions. The product adsorbates were monitored by vibrational methods of high-resolution electron energy loss spectroscopy and multiple internal infrared reflection absorption spectroscopy, as well as Auger electron spectroscopy. The temperature and the period of reaction were adjusted so as to preserve the unsaturated carbon-carbon bonds. The -CH(2)-CH=CH(2) group was introduced by a mild reaction condition, with the reservation of the C=C double bond confirmed. The unsaturated bonds in -CH(2)-CH=CH-CH(3) and -CCH were also reserved. Only in the case of -CH=CH(2) was the reservation of the C=C double bond not realized. Unsaturated hydrocarbon moieties are applicable for further organic modification to introduce functional groups, and are prospective materials in nanofabrication and biological application on silicon wafer surfaces. 相似文献