全文获取类型
收费全文 | 29043篇 |
免费 | 768篇 |
国内免费 | 174篇 |
专业分类
化学 | 18062篇 |
晶体学 | 217篇 |
力学 | 683篇 |
综合类 | 1篇 |
数学 | 4490篇 |
物理学 | 4252篇 |
无线电 | 2280篇 |
出版年
2021年 | 219篇 |
2020年 | 260篇 |
2019年 | 331篇 |
2018年 | 272篇 |
2017年 | 266篇 |
2016年 | 502篇 |
2015年 | 408篇 |
2014年 | 528篇 |
2013年 | 1431篇 |
2012年 | 1321篇 |
2011年 | 1631篇 |
2010年 | 875篇 |
2009年 | 743篇 |
2008年 | 1506篇 |
2007年 | 1572篇 |
2006年 | 1553篇 |
2005年 | 1537篇 |
2004年 | 1343篇 |
2003年 | 1131篇 |
2002年 | 1120篇 |
2001年 | 438篇 |
2000年 | 343篇 |
1999年 | 267篇 |
1998年 | 315篇 |
1997年 | 344篇 |
1996年 | 435篇 |
1995年 | 334篇 |
1994年 | 317篇 |
1993年 | 315篇 |
1992年 | 305篇 |
1991年 | 254篇 |
1990年 | 260篇 |
1989年 | 271篇 |
1988年 | 296篇 |
1987年 | 286篇 |
1986年 | 287篇 |
1985年 | 432篇 |
1984年 | 476篇 |
1983年 | 366篇 |
1982年 | 482篇 |
1981年 | 440篇 |
1980年 | 394篇 |
1979年 | 374篇 |
1978年 | 375篇 |
1977年 | 371篇 |
1976年 | 324篇 |
1975年 | 303篇 |
1974年 | 279篇 |
1973年 | 273篇 |
1972年 | 165篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Maria Ochsenkühn-Petropulu Petros Tarantilis John Tsarouchis Klaus Ochsenkühn George Parissakis 《Mikrochimica acta》1998,129(3-4):233-238
Large area coatings (>10cm2) of the high temperature superconductor YBa2Cu3O7–x (x = 0.1 – 0.2) (YBCO) have been prepared by scale up an electrophoretic deposition technique using silver sheets and Si-wafers coated with Ag or Au as substrates. Several parameters, like the kind of the solvent, the applied voltage, the distance between the electrodes, the initial concentration of the suspension and the temperature during the electrophoresis were investigated in order to attain high deposition rates, as well as uniform YBCO coatings with the proper stoichiometry. To obtain a strongly adherent and dense coating a subsequent appropriate sintering and annealing procedure has been developed. The coatings obtained were characterized for their stoichiometry and superconducting properties by X-ray diffraction (XRD), Raman spectroscopy and magnetic measurements. The homogeneity and thickness of the films and the average grain size of the deposited particles have been investigated by optical and scanning electron microscopy (SEM). 相似文献
992.
Jimmy W. Viers John C. Schug Michael D. Stovall Jeffrey I. Seeman 《Journal of computational chemistry》1984,5(6):598-605
MNDO molecular orbital calculations have been employed to investigate limited reaction pathways and potential energy surfaces for a series of SN2 reactions. Model calculations for X? + CH3X (X = H, F, OH, OCH3, and CN) indicate that the MNDO method gives qualitative agreement with ab initio studies except for the hydride–CH4 exchange. Studies involving alkylation of pyridine (Menschutkin reaction) were also carried out. For the reaction of pyridine with CH3Cl, which involves charge separation, our MNDO studies (which do not include solvation effects) do not produce a characteristic SN2 pathway. For the reaction of pyridine with trimethyloxonium cation [(CH3)3O+] as the alkylating agent, a well defined SN2 reaction pathway was obtained; this reaction involves charge transfer. A potential energy surface for the pyridine–trimethyloxonium cation reaction shows the presence of a saddle point transition state that resembles starting materials, in agreement with the Hammond postulate for this exothermic reaction. 相似文献
993.
The compounds previously reported2 to be the stable areneselenenic acids, -nitro-( and -benzoylbenzeneselenenic acid (), are shown actually to be the corresponding selenenic anhydrides (ArSeOSeAr). Solutions of the selenenic acids (ArSeOH), however, can be easily generated from the anhydrides byacid-catalyzed hydrolysis. 相似文献
994.
995.
997.
Investigation of the use of B3LYP zero-point energies and geometries in the calculation of enthalpies of formation 总被引:1,自引:0,他引:1
Larry A. Curtiss Krishnan Raghavachari Paul C. Redfern John A. Pople 《Chemical physics letters》1997,270(5-6):419-426
The use of B3LYP/6–31G* zero-point energies and geometries in the calculation of enthalpies of formation has been investigated for the enlarged G2 test set of 148 molecules [J. Chem. Phys. 106 (1997) 1063]. A scale factor of 0.96 for the B3LYP zero-point energies gives an average absolute deviation nearly the same as scaled HF/6–31G* zero-point energies for G2, G2(MP2), and B3LYP/6–311 + G(3df,2p) enthalpies. A scale factor of 0.98, which has been recommended in some studies, increases the average absolute deviation by about 0.2 kcal/mol. Geometries from B3LYP/6–31G* are found to do as well as MP2/6–31G* geometries in the calculation of the enthalpies of formation. 相似文献
998.
Yang J Fu X Jia Q Shen J Biggins JB Jiang J Zhao J Schmidt JJ Wang PG Thorson JS 《Organic letters》2003,5(13):2223-2226
In vitro glycorandomization (IVG) technology is dependent upon the ability to rapidly synthesize sugar phosphates. Compared with chemical synthesis, enzymatic (kinase) routes to sugar phosphates would be attractive for this application. This work focuses upon the development of a high-throughput colorimetric galactokinase (GalK) assay and its application toward probing the substrate specificity and kinetic parameters of Escherichia coli GalK. The demonstrated dinitrosalicylic assay should also be generally applicable to a variety of sugar-processing enzymes. [reaction: see text] 相似文献
999.
Iodinations of the ortho, meta, and para fluorous arenes (R(f8)CH(2)CH(2)CH(2))(2)C(6)H(4) (R(f8)=(CF(2))(7)CF(3)) with I(2)/H(5)IO(6) in AcOH/H(2)SO(4)/H(2)O give 3,4-(R(f8)CH(2)CH(2)CH(2))(2)C(6)H(3)I (5) and the analogous 2,4- (6) and 2,5- (7) isomers, respectively. Spectroscopic yields are >90 %, but 5 and 7 must be separated by chromatography from by-products (yields isolated: 70 %, 97 %, 61 %). Reaction of 1,3,5-(R(f8)CH(2)CH(2)CH(2))(3)C(6)H(3) with PhI(OAc)(2)/I(2) gives 2,4,6-(R(f8)CH(2)CH(2)CH(2))(3)C(6)H(2)I (8) on multigram scales in 97 % yield. The CF(3)C(6)F(11)/toluene partition coefficients of 5-8 (24 degrees C: 69.5:30.5 (5), 74.7:25.3 (6), 73.9:26.1 (7), 98.0:2.0 (8)) are lower than those of the precursors, but CF(3)C(6)F(11)/MeOH gives higher values (97.0:3.0 (5), 98.6:1.4 (6), 98.0:2.0 (7), >99.3:<0.3 (8)). Reactions of 5-8 with excess NaBO(3) in AcOH yield the corresponding ArI(OAc)(2) species 9-12 (9, 85 % as a 90:10 9/5 mixture; 10, 97 %; 11, 95 %; 12, 93 % as a 95:5 12/8 mixture). These rapidly oxidize 1,4-hydroquinones in MeOH. Subsequent additions of CF(3)C(6)F(11) give liquid biphase systems. Solvent removal from the CF(3)C(6)F(11) phases gives 5-8 in >99-98 % yields, and solvent removal from the MeOH phases gives the quinone products, normally in >99-95 % yields. The recovered compounds 5-8 are easily reoxidized to 9-12 and used again. 相似文献
1000.
The exchange-correlation energy of a many-electron system may be written as the electrostatic interaction between the electron density at position r and the density of the exchange-correlation hole at position r + u. If we average the hole over the entire system, we find that the energy is uniquely decomposed into contributions from various electronic separations u. We may also decompose the hole into contributions from parallel and antiparallel spins. We give several exact conditions which this system-averaged, spindecomposed exchange-correlation hole satisfies. Local spin density (LSD ) and generalized gradient approximations (GGAS ), are more appropriate for u → 0 than for large u and more trustworthy for antiparallel spins than for parallel spins. We illustrate how good LSD is as u = 0 with explicit examples, but also note that, contrary to expectation, LSD is not exact for u=0, except in certain limiting cases. We show that the dramatic failure of the second-order gradient expansion for large u can be cured by a real-space cutoff procedure which generates a nonempirical GGA, the Pw91 functional. We conclude with some thoughts about the search for greater accuracy in the next 30 years of density functional theory. © 1995 John Wiley & Sons, Inc. 相似文献