全文获取类型
收费全文 | 6065篇 |
免费 | 348篇 |
国内免费 | 52篇 |
专业分类
化学 | 4577篇 |
晶体学 | 44篇 |
力学 | 112篇 |
数学 | 632篇 |
物理学 | 1100篇 |
出版年
2023年 | 24篇 |
2022年 | 66篇 |
2021年 | 116篇 |
2020年 | 94篇 |
2019年 | 110篇 |
2018年 | 96篇 |
2017年 | 90篇 |
2016年 | 199篇 |
2015年 | 187篇 |
2014年 | 218篇 |
2013年 | 373篇 |
2012年 | 428篇 |
2011年 | 503篇 |
2010年 | 307篇 |
2009年 | 239篇 |
2008年 | 402篇 |
2007年 | 352篇 |
2006年 | 359篇 |
2005年 | 305篇 |
2004年 | 284篇 |
2003年 | 241篇 |
2002年 | 214篇 |
2001年 | 108篇 |
2000年 | 101篇 |
1999年 | 73篇 |
1998年 | 53篇 |
1997年 | 51篇 |
1996年 | 61篇 |
1995年 | 52篇 |
1994年 | 49篇 |
1993年 | 32篇 |
1992年 | 32篇 |
1991年 | 26篇 |
1990年 | 38篇 |
1989年 | 36篇 |
1988年 | 29篇 |
1987年 | 23篇 |
1986年 | 19篇 |
1985年 | 42篇 |
1984年 | 29篇 |
1983年 | 22篇 |
1982年 | 26篇 |
1981年 | 28篇 |
1980年 | 20篇 |
1979年 | 30篇 |
1978年 | 19篇 |
1977年 | 25篇 |
1976年 | 22篇 |
1975年 | 18篇 |
1973年 | 27篇 |
排序方式: 共有6465条查询结果,搜索用时 281 毫秒
201.
202.
Park S Kwon OH Kim S Park S Choi MG Cha M Park SY Jang DJ 《Journal of the American Chemical Society》2005,127(28):10070-10074
We have synthesized a novel class of imidazole-based excited-state intramolecular proton-transfer (ESIPT) materials, i.e., hydroxy-substituted tetraphenylimidazole (HPI) and its derivative HPI-Ac, which formed large single crystals exhibiting intense blue fluorescence and amplified spontaneous emission (ASE). Transparent, clear, and well-defined fluorescent single crystals of HPI-Ac as large as 20 mm x 25 mm x 5 mm were easily grown from its dilute solution. From the X-ray crystallographic analysis and semiempirical molecular orbital calculation, it was deduced that the four phenyl groups substituted into the imidazole ring of HPI and HPI-Ac allowed the crystals free from concentration quenching of fluorescence by limiting the excessive tight-stacking responsible for intermolecular vibrational coupling and relevant nonradiative relaxation. Fluorescence spectral narrowing and efficient ASE were observed in the HPI-Ac single crystal even at low excitation levels attributed to the intrinsic four-level ESIPT photocycle. 相似文献
203.
Won Young Jung Byung Ho Noh Seung Hee Baek Gun-Dae Lee Seong Soo Park Seong-Soo Hong 《Reaction Kinetics and Catalysis Letters》2007,91(2):223-231
SBA-15 mesoporous material was prepared by microwave-hydrothermal method and was used as support in TiO2-loaded SBA-15 photocatalysts. The physical properties of these particles were investigated. We also examined the activity
of these samples as photocatalysts for the decomposition of orange II. Titania loaded on a silica matrix decreases the surface
area of the support as expected for TiO2 incorporation. For TiO2-loaded SBA-15 photocatalysts, the IR absorption at ∼960 cm−1 commonly accepted as the characteristic vibration of the Ti-O-Si bond. The photocatalytic activity increases with an increase
of the TiO2 loading. 相似文献
204.
Nak Bae Kim Young Suk Kim Joon Kon Kim Keung Shik Park 《Journal of Radioanalytical and Nuclear Chemistry》1988,125(2):403-417
A Peak Identification and Activity Determination (PIAD) computer program has been developed. A new concept of peak significance is introduced and a great change has been made to the currently used associated gamma lines technique in calculating the confidence index; confidence factor is also given to the gamma line of a nuclide that is not seen in the spectrum. The program is written in Fortran IV under the operating system of RSX-11M. 相似文献
205.
Attila R. Imre W. Alexander Van Hook Bong Ho Chang Young Chan Bae 《Monatshefte für Chemie / Chemical Monthly》2003,134(12):1529-1539
Summary. Upper critical solution temperatures (UCSTs) for liquid–liquid demixing in a set of mixtures of linear alkanes (pentane (N
1=5) to pentacontane (N
1=50)) with an oligostyrene (1241amu, N
2=12) are reported. We find strong correlation between the Hildebrand solubility parameters of the alkanes and the UCST. Correlations are developed which enable predictions concerning the miscibility of mixtures of compounds with longer chains. 相似文献
206.
Young Gun Ko Ung Su Choi Yong Sung Park Je Wan Woo 《Journal of polymer science. Part A, Polymer chemistry》2004,42(8):2010-2018
The role of hydrogen bonding in the chemistry of transition‐metal complexes remains a topic of intense scientific and technological interest. Poly(acrylo‐amidino diethylenediamine) was synthesized to study the effects of hydrogen bonding on complexes at different pHs. The polymer was synthesized through the coupling of diethylene triamine with polyacrylonitrile fiber in the presence of AlCl3 · 6H2O addition. The adsorption capacity of this polymer was 11.4 mequiv/g. The ions used for the adsorption test were CrO, PO, Cu2+, Ni2+, Fe2+, and Ag+. All experiments were confirmed with Fourier transform infrared. In the study of anion adsorption, at low pHs, only ionic bonds existed, whereas at high pHs, no bonds existed. However, in the middle pH region, both ionic bonds and hydrogen bonds formed between poly(acrylo‐amidino diethylenediamine) and the chromate ion or phosphate ion. When poly(acrylo‐amidino diethylenediamine) and metal ions (Cu2+, Ni2+, Fe2+, and Ag+) formed complexes, a hydrogen‐bonding effect was not observed with Fourier transform infrared. The quantity of metal ions adsorbed onto poly(acrylo‐amidino diethylenediamine) followed the order Ag+ > Cu2+ > Fe2+ > Ni2+. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 2010–2018, 2004 相似文献
207.
[reaction: see text] Palladium-catalyzed alpha-arylation of N-benzoyl sulfoximine ethyl ester with various aryl bromides leads to alpha-arylated products that can easily be hydrolyzed, giving the corresponding N-protected benzyl phenyl sulfoximines. In this manner, new sulfoximine derivatives are accessible that have so far been difficult to prepare in enantiopure form. 相似文献
208.
TRANSIENT INTERMEDIATES IN INTRAMOLECULARLY PHOTOSENSITIZED PYRIMIDINE DIMER SPLITTING BY INDOLE DERIVATIVES 总被引:1,自引:0,他引:1
Tish Young Sang-Tae Kim John R. Van Camp Rosemarie F. Hartman Seth D. Rose 《Photochemistry and photobiology》1988,48(5):635-641
Abstract— Intramolecularly photosensitized pyrimidine dimer splitting can serve as a model for some aspects of the monomerization of dimers in the enzyme-substrate complex composed of a photolyase and UV-damaged DNA. We studied compounds in which a pyrimidine dimer was covalently linked either to indole or to 5-methoxyindole. Laser flash photolysis studies revealed that the normally observed photoejection of electrons from the indole or the 5-methoxyindole to solvent was diminished by an order of magnitude for indoles with dimer attached (dimer-indole and dimer-methoxyindole). The fluorescence lifetime of dimer-indole in aqueous methanol was 0.85 ns, whereas that of the corresponding indole without attached dimer (tryptophol) was 9.7 ns. Similar results were obtained for the dimer-methoxyindole (0.53 ns) and 5-methoxytryptophol (4.6 ns). The quantum yield of dimer splitting for the dimer-methoxyindole (φ287 K7 = 0.08) was only slightly greater than the value found earlier for the dimer bearing the unsubstituted indole (4>2K7 = 0.04). Transient absorption spectroscopy also revealed lower yields of indole radical cations following laser flash photolysis of dimer-indole compared to the indole without attached dimer. Dimer-methoxyindole behaved similarly. These results are interpreted in terms of an enhanced rate of radiationless relaxation of the indole and methoxyindole excited singlet states in dimer-indoles. The possible quenching of the indole and methoxyindole excited states via electron abstraction by the covalently linked dimer is discussed. 相似文献
209.
A connection which is both Einstein and semisymmetric is called anES connection, and a generalizedn-dimensional Riemannian manifold on which the differential geometric structure is imposed by a unified field tensorg
through anES connection is called ann-dimensionalES manifold and denoted byESX
n
. We investigate some necessary and sufficient conditions for submanifolds ofESX
n
to be also Einstein and derive the generalized fundamental equations on various submanifolds ofESX
n
, such as generalized Gauss formulas, generalized Weingarten equations, and generalized Gauss-Codazzi equations. We employ the useful and powerful concept ofC-nonholonomic frame of reference, introduced in earlier work. 相似文献
210.