全文获取类型
收费全文 | 1838篇 |
免费 | 45篇 |
国内免费 | 17篇 |
专业分类
化学 | 1369篇 |
晶体学 | 13篇 |
力学 | 9篇 |
数学 | 225篇 |
物理学 | 284篇 |
出版年
2023年 | 14篇 |
2022年 | 12篇 |
2021年 | 13篇 |
2020年 | 23篇 |
2019年 | 20篇 |
2018年 | 14篇 |
2017年 | 10篇 |
2016年 | 22篇 |
2015年 | 26篇 |
2014年 | 41篇 |
2013年 | 108篇 |
2012年 | 73篇 |
2011年 | 100篇 |
2010年 | 51篇 |
2009年 | 58篇 |
2008年 | 101篇 |
2007年 | 102篇 |
2006年 | 91篇 |
2005年 | 102篇 |
2004年 | 105篇 |
2003年 | 104篇 |
2002年 | 90篇 |
2001年 | 36篇 |
2000年 | 36篇 |
1999年 | 30篇 |
1998年 | 21篇 |
1997年 | 39篇 |
1996年 | 33篇 |
1995年 | 15篇 |
1994年 | 16篇 |
1993年 | 18篇 |
1992年 | 17篇 |
1991年 | 19篇 |
1990年 | 17篇 |
1988年 | 19篇 |
1987年 | 10篇 |
1986年 | 12篇 |
1985年 | 19篇 |
1984年 | 27篇 |
1983年 | 15篇 |
1982年 | 25篇 |
1981年 | 21篇 |
1980年 | 24篇 |
1979年 | 23篇 |
1978年 | 11篇 |
1976年 | 10篇 |
1975年 | 10篇 |
1974年 | 28篇 |
1973年 | 10篇 |
1972年 | 9篇 |
排序方式: 共有1900条查询结果,搜索用时 17 毫秒
111.
The effect of the dilution of silane and nitrogen with hydrogen on the optical properties of hydrogenated amorphous silicon-nitrogen films prepared by plasma deposition has been investigated as functions of the gas-volume ratio γ (= ([SiH4] + [N2])/([SiH4] + [N2] + [H2]) and the substrate temperature. The prepared films are characterized by the values of the deposition rate, the optical gap, the Urbach energy, the defect density, the integrated infrared absorption intensity and the refractive index, and by correlations between these parameters and the type of hydrogen- and nitrogen-bonding configurations estimated from infrared absorption spectra. The hydrogen dilution effect is discussed in terms of the above and compared with that in hydrogenated amorphous silicon reported in a previous paper by the present authors. It is pointed out that nitrogen atoms incorporated into the silicon network cause more disorder than incorporated hydrogen atoms, from the γ dependence of the Urbach energy and the integrated infrared intensities associated with the hydrogen and nitrogen bondings. 相似文献
112.
Nobuhiro Takeda Norihiro Tokitoh Renji Okazaki 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1):633-636
The first rotational isomers of stable selenoaldehydes are synthesized by deselenation of cyclic polyselenides having an efficient steric protection group, 2, 4, 6-tris[bis(trimetylsilyl)methyl]phenyl (Tbt). Reactions of these selenoaldehydes with W(CO)5.THF gives the corresponding η1-selenoaldehyde tungsten complexes, the structures of which are established by X-ray crystallography. 相似文献
113.
Shigeru Sasaki Kohji Sasaki Mariko Watanabe Kazunobu Ogawa Noboru Morita Masaaki Yoshifuji 《Phosphorus, sulfur, and silicon and the related elements》2013,188(1-3):256-261
Abstract Synthesis, redox properties, and electronic spectra of the sterically crowded triarylphosphines conjugated with π-electron systems, especially electron acceptors such as carbonyl group, are briefly reviewed. The sterically crowded triarylphosphines conjugated with various π-electron systems were synthesized from the common synthetic intermediate, (bromoaryl)phosphine, by conventional manner. The sterically crowded triarylphosphines conjugated with the electron acceptors exhibit visible absorption and fluorescence with large Stokes shift. Large solvent effect, redshift in polar solvent, and good correlation with the difference of the oxidation potential of the phosphine moiety and the reduction potential of the acceptor moiety suggest polar excited state resulting from HOMO-LUMO transition. 相似文献
114.
Shigekazu Ito Masaaki Yoshifuji 《Phosphorus, sulfur, and silicon and the related elements》2013,188(6-7):1609-1612
( Z )-2-Chloro-3,3-diphenyl-1-(2,4,6-tri- t -butylphenyl)-1,3-diphosphapropene was derived from chlorodiphenylphosphine and 1-chloro-2-(2,4,6-tri- t -butylphenyl)-2-phosphaethenyllithium and utilized for complex formations as a ligand of the corresponding carbonyl-tungsten(0) complexes. 相似文献
115.
Radiation-induced polymerization and pressure-volume (P-V) measurements of acrylonitrile (AN) were studied up to 8000 kg/cm2 in the temperature range of 6–72°C. P-V isotherms of AN have several small breaks, A phase diagram of AN was obtained from the breaking pressures and temperatures. Liquid phases were named LI, LII, and LIII, from low to high pressure. The polymerization behavior and volume contraction on polymerization changed in LI, LII, and LIII. The difference in entropy between original and activated states decreased with increasing pressure at the same phase, but increased with phase change in LI to LII and LII to LIII. It was concluded from these results and from IR data on PAN that molecular packing of AN in liquid changed in LI, LII, and LIII. In LII and LIII, AN molecules aligned in a less suitable geometry for polymerization than in LI. 相似文献
116.
The formation of the water-soluble glycolchitosan-copper (II) complex was studied by ultraviolet and visible spectroscopy and by viscosity measurement. The structure of the polymer complex in question was found to change with pH value of the system. The glycolchitosan-copper (II) complex was active as a free-radical initiator for the polymerization of methyl methacrylate and acrylonitrile in the presence of carbon tetra- chloride. The maximum conversion was observed at neutral pH, and was influenced by the presence of neutral salts. Relation- ship between the activity of the polymer complex for initiating the polymerization and its structure was discussed. 相似文献
117.
118.
Background
BTBD10 binds to Akt and protein phosphatase 2A (PP2A) and inhibits the PP2A-mediated dephosphorylation of Akt, thereby keeping Akt activated. Previous studies have suggested that BTBD10 plays an important role in preventing motor neuronal death and accelerating the growth of pancreatic beta cells. Because levels of BTBD10 expression are much lower in many non-nervous tissues than nervous tissues, there may be a relative of BTBD10 that has BTBD10-like function in non-neuronal cells.Results
A 419-amino-acid BTBD10-like protein, named KCTD20 (potassium channel tetramerization protein domain containing 20), was to found to bind to all Akt isoforms and PP2A. Overexpression of KCTD20 increased Akt phosphorylation at Thr308, as BTBD10 did, which suggests that KCTD20 as well as BTBD10 positively regulates the function of Akt. KCTD20 was ubiquitously expressed in non-nervous as well as nervous tissues.Conclusions
KCTD20 is a positive regulator of Akt and may play an important role in regulating the death and growth of some non-nervous and nervous cells.119.
Atsushi Tarui Shoji Kondo Kazuyuki Sato Masaaki Omote Hideki Minami Yoshihisa Miwa Akira Ando 《Tetrahedron》2013,69(5):1559-1565
A highly diastereoselective cross-coupling reaction of an α-bromo-α-fluoro-β-lactam with a wide range of aryl Grignard reagents was catalyzed by Ni/bis(oxazoline) in yields of up to 98%. The product was obtained diastereoselectively as an anti-isomer. This is the first successful α-arylation of an α-fluoro-β-lactam to produce diverse α-aryl-α-fluoro-β-lactams. 相似文献
120.
Takumi Kusano Kenta Fujii Masaaki Tabata Mitsuhiro Shibayama 《Journal of solution chemistry》2013,42(10):1888-1901
Aggregation structures of 1-alkyl-3-methylimidazolium based ionic liquids (ILs) in aqueous solution were investigated by small-angle neutron scattering (SANS) from the viewpoint of alkyl chain length, n, and anions (Cl?, Br? and trifluoromethanesulfonate, $ {\text{CF}}_{3} {\text{SO}}_{3}^{ - } $ ). In [C4mIm+]-based IL systems, no noticeable SANS intensity was observed for all of the systems examined here, although aqueous [C4mIm+][ $ {\text{BF}}_{4}^{ - } $ ] solutions show a significant SANS profile originating from concentration fluctuations in the solution (Almasy et al. J Phys Chem B 112:2382–2387, 2008). This suggests that [C4mIm+][Cl?], [C4mIm+][Br?] and [C4mIm+][ $ {\text{CF}}_{3} {\text{SO}}_{3}^{ - } $ ] homogeneously mix with water, unlike the [C4mIm+][ $ {\text{BF}}_{4}^{ - } $ ] system, due to preferential hydration of the ions. In the case of the C n mIm cations with longer alkyl chain lengths (n = 8 and 12), SANS profiles were clearly observed in the aqueous solutions at IL concentrations of C IL > 230 and 20.0 mmol·dm?3, respectively. For aqueous [C8mIm+][Br?] solutions, the asymptotic behavior of the scattering function varied largely from I(q) ~ q ?2 to ~q ?4 with increasing C IL, indicating that the solution changes from an inhomogeneous mixing state to a nano-scale micelle state. Aqueous [C12mIm+][Br?] solutions show a typical SANS profile for micelle formation in solution. It was found from a model-fitting analysis that the structure of the [C12mIm+][Br?] micelle is ellipsoidal, not spherical, in solutions over the C IL range examined here. 相似文献