全文获取类型
收费全文 | 3470篇 |
免费 | 137篇 |
国内免费 | 14篇 |
专业分类
化学 | 2721篇 |
晶体学 | 38篇 |
力学 | 33篇 |
数学 | 195篇 |
物理学 | 634篇 |
出版年
2023年 | 30篇 |
2022年 | 26篇 |
2021年 | 47篇 |
2020年 | 74篇 |
2019年 | 64篇 |
2018年 | 24篇 |
2017年 | 17篇 |
2016年 | 76篇 |
2015年 | 78篇 |
2014年 | 96篇 |
2013年 | 184篇 |
2012年 | 211篇 |
2011年 | 217篇 |
2010年 | 122篇 |
2009年 | 115篇 |
2008年 | 231篇 |
2007年 | 230篇 |
2006年 | 223篇 |
2005年 | 206篇 |
2004年 | 211篇 |
2003年 | 158篇 |
2002年 | 159篇 |
2001年 | 69篇 |
2000年 | 54篇 |
1999年 | 63篇 |
1998年 | 39篇 |
1997年 | 45篇 |
1996年 | 31篇 |
1995年 | 29篇 |
1994年 | 28篇 |
1993年 | 27篇 |
1992年 | 29篇 |
1991年 | 19篇 |
1990年 | 16篇 |
1989年 | 25篇 |
1988年 | 16篇 |
1987年 | 20篇 |
1986年 | 13篇 |
1985年 | 42篇 |
1984年 | 32篇 |
1983年 | 23篇 |
1982年 | 27篇 |
1981年 | 26篇 |
1980年 | 19篇 |
1979年 | 25篇 |
1978年 | 16篇 |
1977年 | 16篇 |
1976年 | 16篇 |
1974年 | 11篇 |
1973年 | 14篇 |
排序方式: 共有3621条查询结果,搜索用时 15 毫秒
991.
We have applied cavity ring-down spectroscopy to a kinetic study of the reaction of NO3 with CH3I in 20-200 Torr of N2 diluent at 298 K. The rate constant of the reaction of NO3 + CH3I was determined to be (4.1 +/- 0.2) x 10(-13) cm3 molecule(-1) s(-1) in 100 Torr of N2 diluent at 298 K and is pressure-independent. This reaction may significantly contribute to the formation of reactive iodine compounds in the atmosphere. 相似文献
992.
Yasuhiro Fuma Masahiro Ebihara 《Acta Crystallographica. Section C, Structural Chemistry》2006,62(7):m284-m289
In diaquatetra‐μ‐acetamidato‐κ4N:O;κ4O:N‐dirhodium(II,III) hexafluorophosphate, [Rh2(C2H4NO)4(H2O)2]PF6, and diaquatetra‐μ‐acetamidato‐κ4N:O;κ4O:N‐dirhodium(II,III)hexafluorophosphate dihydrate, [Rh2(C2H4NO)4(H2O)2]PF6·2H2O, the cations and anions lie on inversion centers. Diaquatetra‐μ‐propionamidato‐κ4N:O;κ4O:N‐dirhodium(II,III) hexafluorophosphate dihydrate, [Rh2(C3H6NO)4(H2O)2]PF6·2H2O, and diaquatetra‐μ‐butyramidato‐κ4N:O;κ4O:N‐dirhodium(II,III) hexafluorophosphate, [Rh2(C4H8NO)4(H2O)2]PF6, crystallize with two crystallographically independent complexes that lie on inversion centers. In all of the structures, the dirhodium units are hydrogen bonded to one another. The hydrogen‐bonded networks vary with the alkyl substituents. 相似文献
993.
Nakamura T Suzuki M Ishihara M Ohana T Tanaka A Koga Y 《Langmuir : the ACS journal of surfaces and colloids》2004,20(14):5846-5849
Photolysis of perfluoroazooctane with diamond films led the chemical modification of the surface to introduce perfluorooctyl functional groups, confirmed by means of Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and time-of-flight secondary ion mass spectrometry measurements. Diamond films modified with fluorine moieties showed improved frictional property and reduction of surface energy, as evaluated by contact angle to water, compared with a pristine diamond film. The contact angle and friction coefficient of chemically modified diamond film are 118 degrees and 0.1, respectively. The results of the value of the contact angle depending on irradiation times are consistent with those of the F/C ratio of fluorinated diamond films by monitoring with XPS. 相似文献
994.
Nobuyuki Tanaka Chie Okabe Kenji Sakota Tuyoshi Fukaminato Tsuyoshi Kawai Masahiro Irie Alexander Goldberg Shinichirou Nakamura Hiroshi Sekiya 《Journal of Molecular Structure》2002,616(1-3):113-118
The fluorescence excitation and dispersed fluorescence spectra of the open-ring isomer of 1,2-bis(3-methyl-2-thienyl)perfluorocyclopentene have been measured in a supersonic free jet. No vibronic structure has been observed in the excitation spectrum. The intensity of fluorescence gradually increases with the excitation energy in the 25,500–28,700 cm−1 region, indicating that the geometry of the molecule substantially changes upon photoexcitation. The dispersed fluorescence spectrum is anomaly Stokes-shifted with respect to the excitation energy, suggesting that the S2(1B) state is initially excited followed by rapid internal conversion from the S2(1B) to S1(2A) state. The fluorescence is due to the S1(2A)–S0(1A) transition. Density functional theory calculations at the B3LYP/6-31G** level have been carried out to investigate stable conformations responsible for the observed spectra. 相似文献
995.
Capillary electrochromatographic enantioseparations using a packed capillary with a 3 microm OD-type chiral packing 总被引:1,自引:0,他引:1
A technique for separating methyl esters of monounsaturated fatty acids by argentation chromatography using silver nitrate-impregnated TLC plates is described. Monounsaturated fatty acid methyl esters are separated from polyunsaturated and saturated fatty acid methyl esters and the monounsaturated fatty methyl esters are resolved according to chain length. cis isomers are well resolved from the corresponding trans isomers. R(F) values for individual monounsaturated fatty acids are very reproducible. The potential of the technique in metabolic studies is demonstrated in the chain elongation of [14C]-18:1(n-9) and delta-9 desaturation of [14C]-18:0 by human skin fibroblasts. Recoveries of individual [14C]-fatty acids for scintillation counting exceed 94%. 相似文献
996.
Kazuhisa Murata Megumu Inaba Masahiro Saito Isao Takahara Naoki Mimura 《Reaction Kinetics and Catalysis Letters》2003,80(1):39-44
The stability of an Fe/Al2O3 catalyst in the methane decomposition in the presence of O2/CO2 was found to be improved by the addition of Mg into the catalyst (Mg/Fe=1/1 wt ratio), probably due to suppression of carbon
deposition even under CH4/ O2/CO2 (80/10/5 vol. ratio) conditions.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
997.
A novel alkenylation reaction of pyridine is developed. Heating a cationic ruthenium vinylidene complex [CpRu(=C=CHR)(PPh(3))(2)]PF(6) in pyridine at 100-125 degrees C for 24 h affords (E)-2-alkenylpyridine. Initially, pyridine coordinates to ruthenium by displacement of one of the phosphine ligands. Then, [2 + 2] heterocycloaddition occurs to form a four-membered ruthenacyclic complex. Deprotonation of the beta-hydrogen affords a neutral pi-azaallyl complex. Protonolysis furnishes the product. As a result, a vinylidene group is inserted into the alpha C-H bond of pyridine. The alkenylation reaction is made catalytic in ruthenium by the use of (alkyn-1-yl)silane as the vinylidene source. Treatment of (alkyn-1-yl)trimethylsilane with pyridine in the presence of a cationic ruthenium complex [CpRu(PPh(3))(2)]PF(6) affords the corresponding (E)-2-alkenylpyridine in good yield in a regio- and stereoselective manner. 相似文献
998.
A newly designed anthryl dendron having perfluoroalkyl chains at terminals showed thermotropic liquid crystallinity, which was characterized using polarizing optical microscopy, differential scanning calorimetry, and X-ray diffraction. The anthryl dendron forms SB phase at room temperature below 53 °C, at which temperature a phase transition to Colrd takes place. In a fluorous solvent, regioselective photodimerization of the anthryl dendron occurred to give head-to-head photodimers, although photodimerization in the SB phase and chloroform gave both head-to-head and head-to-tail photodimers. 相似文献
999.
Nano-meter sized structural inhomogeneities of (CuI)0.52-(Cu2MoO4)0.48 superionic conducting glass were investigated by high resolution transmission electron microscopy. The as-quenched sample of CuI-Cu2MoO4 is a homogeneous glass, in which the CuI component is finely and uniformly dispersed among the oxyanions of Cu2MoO4 glassy matrix. A two-step crystallization, starting at 440 and 495 K, was observed in the glass. After the first step crystallization, precipitating nano-crystalline cubic CuI 2-3 nm in diameter, the electrical conductivity increases by about 50%. On the other hand, the electrical conductivity decreases with the second crystallization event forming crystalline phases of CuI, Cu2O and others 20-30 nm in diameter. 相似文献
1000.
Masahiro Teraguchi Toshio Masuda 《Journal of polymer science. Part A, Polymer chemistry》1999,37(24):4546-4553
Three disubstituted acetylenes with an adamantyl group—1-(p-adamantylphenyl)-2-chloroacetylene (ClpAdPA), 1-(p-adamantylphenyl)-1-propyne (pAdPP), and 1-(p-adamantylphenyl)-2-phenylacetylene (pAdDPA)—polymerized in good yields in the presence of MoCl5- or TaCl5-based catalysts. The highest weight-average molecular weights of poly(ClpAdPA), poly(pAdPP), and poly(pAdDPA) reached 3.6 × 105, 1.1 × 106, and 6.0 × 106, respectively. The polymers were yellow to white solids and completely soluble in toluene, chloroform, and so forth. These polymers thermally were fairly stable, and the onset temperatures of weight loss in air were over 360 °C. Poly(pAdPP) and poly(pAdDPA) provided free-standing films by solution casting, and their oxygen permeability coefficients (PO2) at 25 °C were 8.6 and 55 barrers [1 barrer = 1 × 10−10 cm3 · (STP) · cm/(cm2 · s · cm Hg)], respectively, which are relatively small compared to those of other substituted polyacetylenes. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 4546–4553, 1999 相似文献