全文获取类型
收费全文 | 223998篇 |
免费 | 3422篇 |
国内免费 | 1566篇 |
专业分类
化学 | 125036篇 |
晶体学 | 3239篇 |
力学 | 9038篇 |
综合类 | 83篇 |
数学 | 24849篇 |
物理学 | 66741篇 |
出版年
2020年 | 1852篇 |
2019年 | 1987篇 |
2018年 | 2557篇 |
2017年 | 2593篇 |
2016年 | 3784篇 |
2015年 | 2539篇 |
2014年 | 3701篇 |
2013年 | 9116篇 |
2012年 | 7995篇 |
2011年 | 9687篇 |
2010年 | 6859篇 |
2009年 | 6620篇 |
2008年 | 9024篇 |
2007年 | 9080篇 |
2006年 | 8549篇 |
2005年 | 7853篇 |
2004年 | 7004篇 |
2003年 | 6188篇 |
2002年 | 6099篇 |
2001年 | 6783篇 |
2000年 | 5127篇 |
1999年 | 3731篇 |
1998年 | 3081篇 |
1997年 | 3060篇 |
1996年 | 3071篇 |
1995年 | 2660篇 |
1994年 | 2760篇 |
1993年 | 2616篇 |
1992年 | 2852篇 |
1991年 | 2912篇 |
1990年 | 2727篇 |
1989年 | 2628篇 |
1988年 | 2570篇 |
1987年 | 2510篇 |
1986年 | 2533篇 |
1985年 | 3315篇 |
1984年 | 3375篇 |
1983年 | 2816篇 |
1982年 | 3056篇 |
1981年 | 2817篇 |
1980年 | 2620篇 |
1979年 | 2802篇 |
1978年 | 3002篇 |
1977年 | 3034篇 |
1976年 | 3066篇 |
1975年 | 2798篇 |
1974年 | 2911篇 |
1973年 | 2957篇 |
1972年 | 2321篇 |
1971年 | 1848篇 |
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
991.
O. Kindel F. J. Herrmann L. Schmidt 《Journal of Radioanalytical and Nuclear Chemistry》1994,183(2):219-224
By simulation experiments with a 10–5 mol/l solution of iodododecane labeled with131I in n-dodecane the influence of various materials and conditions, which are possible in nuclear fuel reprocessing, has been investigated. The formation of decomposition products was detected via HPLC with a radioactivity monitor. By means of252Cf plasma-desorption mass spectrometry (PDMS) the decomposition products were identified. It was found that a temperature of 100°C favored the formation of iodoalkanes with chain lengths of C1 to C11. The presence of TBP(tri-n-butyl-phosphate) accelerated the decomposition of iodododecane. In pure TBP only iodobutane was formed as a decomposition product. 相似文献
992.
G. L. Silver 《Journal of Radioanalytical and Nuclear Chemistry》1994,182(2):291-294
The results of plutonium oxidation state distribution calculations do not depend on special attention to any oxidation state as has been incorrectly supposed in some literature sources. Experimentally determined alpha coefficients for natural water could make the distribution of environmental plutonium easy to estimate with a programmable pocket calculator.Mound is operated for the U.S. Department of Energy by EG&G Mound Applied Technologies, Inc., under Contract No. DE-AC04-88DP43495. 相似文献
993.
994.
In this work, the electro-optic response of a 6CB liquid crystal layer is studied using a sensitive differential technique. The layer is held at a temperature just above the nematic to isotropic phase transition. Transverse magnetic (p) polarized light incident on the cell is coupled to guided modes in the liquid crystal layer using prism coupling. The modes manifest themselves as sharp dips in the reflectivity as the angle of incidence is scanned. When a low frequency sinusoidal voltage is applied to the cell, the resonant mode shapes and excitation angles are altered at a frequency which is twice that of the applied field, resulting in a modulation of the reflectivity for a given angle of incidence. By synchronous observation of the modulated signal, a differential signal is recorded. Comparing the data with modelling generated from multilayer optics theory, two effects are then quantified. The first of these is an induced birefringence, varying quadratically with applied voltage, which is well understood and can be expressed in terms of Landau-de Gennes theory. The second is a field induced perturbation in the imaginary part of the optical permittivity, δεi, which implies a modification of the light scattering properties of the liquid crystal. The measurement of the latter effect is, as far as we know, a novel one, being only made possible by the remarkable sensitivity of the synchronous differential technique. 相似文献
995.
A kinetic model is presented to describe the high temperature (1800 K < T < 3000 K) surface oxidation of particulate boron in a hydrocarbon combustion environment. The model includes a homogeneous gas-phase B/O/H/C oxidation mechanism consisting of 19 chemical species and 58 forward and reverse elementary reactions, multi-component gas-phase diffusion, and a heterogeneous surface oxidation mechanism consisting of ‘elementary’ adsorption and desorption reaction steps. Thermochemical and kinetic parameters for the surface reactions are estimated from available experimental data and/or elementary transition state arguments. The kinetic processes are treated using a generalized kinetics code, with embedded sensitivity analysis, for the combustion of a one-dimensional (particle radius), spherical particle. Model results are presented for the oxidation of a 200 μm boron particle in a JP-4/air mixture at ambient temperatures of 1400 K and 2000 K. These results include temperature and gas-phase species profiles as a function of radial distance and particle burning rates. © 1994 John Wiley & Sons, Inc. 相似文献
996.
The gas-phase thermal decomposition rate of peroxy-n-butyryl nitrate (n-C3H7C(O)OONO2, PnBN) has been measured at ambient temperature (296 K) and 1 atm of air relative to that of peroxyacetyl nitrate (CH3C(O)OONO2, PAN) using mixtures of PAN (14–19 ppb), PnBN (22–46 ppb), and nitric oxide (1.35–1.90 ppm). The PnBN/PAN decomposition rate ratio was 0.773 ± 0.030. This ratio, together with a literature value of 3.0 × 10?4 s?1 for the thermal decomposition rate of PAN at 296 K, yields a PnBN thermal decomposition rate of (2.32 ± 0.09) × 10?4 s?1. The results are briefly discussed by comparison with data for other peroxyacyl nitrates and with respect to the atmospheric persistence of PnBN. © 1994 John Wiley & Sons, Inc. 相似文献
997.
Ll Abia X. L. Armesto M. Canle L. M. V. García M. Losada J. A. Santaballa 《国际化学动力学杂志》1994,26(10):1041-1053
This article analyzes the kinetics of the decomposition of N-Cl-Valine in aqueous solution, which is formed rapidly by chlorination of Valine with sodium hypochlorite. A general-base catalyzed process not yet described is reported. The experimental evidence shows two competitive decomposition paths: an unimolecular concerted fragmentation process (k = (1.8 ± 0.1) · 10?4 s?1 at 298 K) and the other one is an E2 elimination process whose importance increases with pH and depends on the nature and the concentration of the bases present in the medium. © 1994 John Wiley & Sons, Inc. 相似文献
998.
999.
P. G. Hjorth S. L. Kokkendorff S. Markvorsen 《Proceedings of the American Mathematical Society》2002,130(1):175-181
We study finite metric spaces with elements picked from, and distances consistent with, ambient Riemannian manifolds. The concepts of negative type and strictly negative type are reviewed, and the conjecture that hyperbolic spaces are of strictly negative type is settled, in the affirmative. The technique of the proof is subsequently applied to show that every compact manifold of negative type must have trivial fundamental group, and to obtain a necessary criterion for product manifolds to be of negative type.
1000.
We report the temperature dependence of electrical resistance (R) and thermopower (S) of clathrate Cs8Sn44 under high pressure up to 1.2 GPa. We observe a reversible gap widening, prominent relaxation effect of R, irreversible increase of |S| under high pressure. We also find that the power factor S2σ (σ: electrical conductivity) reaches a maximum at pressure of 0.3 GPa. Comparison of the experimental results with band structure calculations suggests that the intrinsic vacancy in the clathrate structure of Cs8Sn44 plays an important role in transport properties under high pressure. Measurements on Cs8Zn4Sn42, a clathrate which has defects other than vacancies, are compared with Cs8Sn44. The results indicate that replacing Sn by Zn has similar effect as the intrinsic vacancy on S. 相似文献