全文获取类型
收费全文 | 169064篇 |
免费 | 1450篇 |
国内免费 | 450篇 |
专业分类
化学 | 89364篇 |
晶体学 | 2760篇 |
力学 | 6975篇 |
综合类 | 15篇 |
数学 | 16975篇 |
物理学 | 54875篇 |
出版年
2021年 | 1360篇 |
2020年 | 1523篇 |
2019年 | 1763篇 |
2018年 | 2525篇 |
2017年 | 2675篇 |
2016年 | 3740篇 |
2015年 | 2101篇 |
2014年 | 3380篇 |
2013年 | 7528篇 |
2012年 | 6245篇 |
2011年 | 7371篇 |
2010年 | 5797篇 |
2009年 | 5776篇 |
2008年 | 7251篇 |
2007年 | 6975篇 |
2006年 | 6640篇 |
2005年 | 5753篇 |
2004年 | 5020篇 |
2003年 | 4433篇 |
2002年 | 4321篇 |
2001年 | 5838篇 |
2000年 | 4533篇 |
1999年 | 3407篇 |
1998年 | 2523篇 |
1997年 | 2612篇 |
1996年 | 2184篇 |
1995年 | 1955篇 |
1994年 | 1950篇 |
1993年 | 1736篇 |
1992年 | 2228篇 |
1991年 | 2320篇 |
1990年 | 2157篇 |
1989年 | 2107篇 |
1988年 | 2095篇 |
1987年 | 2103篇 |
1986年 | 1921篇 |
1985年 | 2394篇 |
1984年 | 2407篇 |
1983年 | 1921篇 |
1982年 | 1923篇 |
1981年 | 1768篇 |
1980年 | 1743篇 |
1979年 | 2035篇 |
1978年 | 2108篇 |
1977年 | 2058篇 |
1976年 | 1978篇 |
1975年 | 1863篇 |
1974年 | 1852篇 |
1973年 | 1880篇 |
1972年 | 1238篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Semi-empirical molecular orbital calculations were carried out for the compounds (C2H5)3As, (C2H5)3Ga and RAsH2 (R = C2H5, i-C3H7, i-C4H9, and t-C4H9) by using the CNDO/2-U program, and their capability of β-elimination reaction is compared on the basis of the torsion energy to the transition state, electrostatic interactions and orbital overlapping between the central atom and the β-hydrogen, and bond order of the metal-carbon, and carbon-hydrogen bond. In the comparison of (C2H5)3As with (C2H5)3Ga, we found that the β-elimination of (C2H5)3As could hardly be expected to take place in the thermal decomposition. The capability of β-elimination would be smaller in C2H5AsH2 than that in (C2H5)3As. Moreover when the ethyl group is replaced by a t-butyl group in RAsH2, the β-elimination reaction appears to become more difficult and a large possibility for a radical process is suggested. 相似文献
32.
Thermoelastic stress analysis was used to document the effect of composite damage on the stress distribution in three ceramic matrix composites. Composite damage was found to significantly alter the thermoelastic response of each material, with the greatest effect noted in SiC/CAS. Thermoelastic imaging of these materials affords a more complete picture of how the various damage mechanisms affect the stress distribution. In particular, a stress concentration factor computed from thermoelastic images, serves as an indicator of stress redistribution. The stress concentration factors were computed by comparing notch root to far field temperatures, and monitored after the introduction of various amounts of damage. In each material, the stress concentration factor diminished as the damaging load approached the ultimate stress. Reduction in the stress concentration is associated with local changes in modulus, mechanistically arising from combinations of fiber, matrix and interface fracture. Stress redistribution occurs as a consequence of modulus changes, leading to lower notch sensitivity in each of the tested composites. 相似文献
33.
Q. Gong R. N tzel P.J. van Veldhoven T.J. Eijkemans J.H. Wolter 《Journal of Crystal Growth》2005,280(3-4):413-418
We report on the shape transition from InAs quantum dashes to quantum dots (QDs) on lattice-matched GaInAsP on InP(3 1 1)A substrates. InAs quantum dashes develop during chemical-beam epitaxy of 3.2 monolayers InAs, which transform into round InAs QDs by introducing a growth interruption without arsenic flux after InAs deposition. The shape transition is solely attributed to surface properties, i.e., increase of the surface energy and symmetry under arsenic deficient conditions. The round QD shape is maintained during subsequent GaInAsP overgrowth because the reversed shape transition from dot to dash is kinetically hindered by the decreased ad-atom diffusion under arsenic flux. 相似文献
34.
We theoretically investigated the mass dependence of the sympathetic cooling rate of gas-phase ions trapped in a linear radio-frequency-quadrupole
ion trap. Using an a priori molecular dynamical calculation, tracing numerically with Newtonian equations of motion, we found
that ions with a mass greater than 0.54±0.04 times that of the laser-cooled ions are sympathetically cooled; otherwise, they
are heated. To understand the mass dependence obtained using the molecular-dynamical calculation, we made a heat-exchange
model of sympathetic cooling, which shows that the factor of 0.54±0.04 is a consequence of absence of micro-motion along the
axis of the linear ion trap.
Received: 10 December 2001 / Revised version: 28 January 2002 / Published online: 14 March 2002 相似文献
35.
36.
T. V. Maslova 《Russian Journal of Mathematical Physics》2006,13(4):425-430
The results obtained in V. P. Maslov’s article [1] can automatically be carried over to arbitrary semiotic systems if the
string of signs is sufficiently long. We shall show in this article, by using arguments based on the ideas of Kolmogorov complexity
and by applying the theorem of V. P. Maslov from [2], that one can obtain similar formulas for the distribution of various
semiotic objects. In order to do this, we must introduce notions generalizing the occurrence frequency of words in corpora
of texts. We also interpret a formula due to Maslov (appearing in [3]) from the point of view of the general semiotic notions
introduced here. 相似文献
37.
John T. Herron 《国际化学动力学杂志》1987,19(2):129-142
The gas phase chemical kinetics of SF4, SF5, and S2F10 are reviewed with particular emphasis on relevance to the general problem of the dielectric breakdown of SF6. Specific reaction systems treated are SF4 + F2, SF5 + SF5, and the pyrolysis of S2F10. Computer modeling calculations were carried out to arrive at the best estimates of rate parameters. Based on the results of these calculations, sets of recommended rate parameters are provided. The major discrepancies and problems in establishing the kinetic data base are described. Thermochemical consequences of different model calculations are given. 相似文献
38.
V. A. Zayats A. N. Nikolaevskii T. A. Filippenko O. P. Kniga G. A. Tikhonova 《Russian Journal of Applied Chemistry》2006,79(10):1651-1654
Specific features of the initiated oxidation of polyethylene and its low-molecular-weight analogue tridecane, associated with the occurrence of the process in the short-chain mode, were studied. 相似文献
39.
T. Yu. Gumerov A. F. Dobrynina R. A. Yusupov V. P. Barabanov 《Russian Journal of Applied Chemistry》2006,79(12):1935-1939
The behavior of real disperse systems with organic impurities was examined under conditions of destabilization by inorganic coagulants. A number of physicochemical methods of analysis and a simulation method were used to identify the character and type of the complexes formed and to calculate their stability constants. 相似文献
40.
L. Villa P. D' Agati C. Mansueto C. Pellerito M. Scopelliti T. Fiore L. Nagy L. Pellerito 《应用有机金属化学》2003,17(2):106-112
Ascidia malaca gametes before fertilization incubated in 10?5 or 10?7 M solutions of tributyltin(IV) chloride, TBTCl, for 3 h appear highly damaged under transmission electron microscopy observation. Also, the fertilization process is affected by the compound: the damaged spermatozoa are present in the vitelline coat and the egg does not cleave. An increase of microbodies, structurally similar to peroxisomes, have been detected in the egg peripheral cytoplasm, probably in relation to their role in alleviating damage to some cellular components. The results have shown that the reproduction of ascidians under unfavourable environmental conditions is prevented. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献