全文获取类型
收费全文 | 404361篇 |
免费 | 4820篇 |
国内免费 | 1451篇 |
专业分类
化学 | 214813篇 |
晶体学 | 5791篇 |
力学 | 17194篇 |
综合类 | 20篇 |
数学 | 51956篇 |
物理学 | 120858篇 |
出版年
2021年 | 2530篇 |
2020年 | 2908篇 |
2019年 | 3002篇 |
2018年 | 4590篇 |
2017年 | 4858篇 |
2016年 | 6498篇 |
2015年 | 4646篇 |
2014年 | 6402篇 |
2013年 | 17384篇 |
2012年 | 15822篇 |
2011年 | 18056篇 |
2010年 | 11922篇 |
2009年 | 11733篇 |
2008年 | 15311篇 |
2007年 | 15488篇 |
2006年 | 14971篇 |
2005年 | 17105篇 |
2004年 | 15429篇 |
2003年 | 12650篇 |
2002年 | 10890篇 |
2001年 | 12075篇 |
2000年 | 9089篇 |
1999年 | 7225篇 |
1998年 | 5890篇 |
1997年 | 5639篇 |
1996年 | 5550篇 |
1995年 | 5080篇 |
1994年 | 4807篇 |
1993年 | 4656篇 |
1992年 | 5240篇 |
1991年 | 5150篇 |
1990年 | 4855篇 |
1989年 | 4639篇 |
1988年 | 4860篇 |
1987年 | 4492篇 |
1986年 | 4334篇 |
1985年 | 6064篇 |
1984年 | 6206篇 |
1983年 | 5083篇 |
1982年 | 5470篇 |
1981年 | 5400篇 |
1980年 | 5154篇 |
1979年 | 5349篇 |
1978年 | 5429篇 |
1977年 | 5428篇 |
1976年 | 5336篇 |
1975年 | 5182篇 |
1974年 | 5061篇 |
1973年 | 5185篇 |
1972年 | 3100篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
J. Escudi C. Couret H. Ranaivonjatovo J. Satge 《Coordination chemistry reviews》1994,130(1-2):427-480
During the last 10 years, several compounds of the type Ge=X (X = C, Ge, N, P, S) have been isolated as monomers. The stabilization of such derivatives, which are generally highly polymerizable, was achieved by using very bulky groups both on the germanium atom and on the heteroelement X. Conjugation (particularly in a few germenes and germaimines) and intramolecular or intermolecular coordination with oxygen or nitrogen, also contributes, in some cases, to the stabilization. The X-ray analyses of such compounds show a significant bond shortening of the double bond (8–10%) relative to the corresponding single bond and a planar or nearly planar germanium. These doubly bonded germanium derivatives are usually thermally stable but must be handled in an inert atmosphere because of their high sensitivity to oxygen and moisture; they are extremely reactive, much more than the corresponding carbon analogues. Nearly quantitative additions on the double bond have been observed with electrophiles and nucleophiles, and various types of cycloadditions also occur. Except in one case, a germylene behavior has not been observed, proving that such compounds retain their structural integrity in solution. 相似文献
192.
A. Hagfeldt S. Lunell H. O. G. Sieghbahn 《International journal of quantum chemistry》1994,49(2):97-104
The energy levels of small titanium oxide clusters [(TiO2)2, and (TiO2H)3, and (TiO2H)2] have been calculated using ab initio SCF methods. Both crystal and relaxed geometries have been considered. Systematic changes in the valencelevel structure resulting from geometry relaxation are found, which may be related to band-gap changes experimentally observed for small titanium oxide particles. In addition, a Ti? OH local surface state is found to be well described within a limited cluster model. © 1994 John Wiley & Sons, Inc. 相似文献
193.
The properties of a polymer-concrete composed of polyester matrix and locally available rock aggregate are investigated. The formula of the concrete is found by an experimental-calculation approach in such a way as to attain a closer packing of the aggregate particles on the one hand, and to ensure the needed processing characteristics (placeability) of the mix on the other. It is shown experimentally that the material obtained has a rather high compression strength. Under prolonged compression loads, the polymer-concrete exhibits a noticeable creep behavior with a linear relation between the creep strains and stresses. After the action of half the ultimate load over 3000 h, the total strains exceed the instantaneous ones by 2.0 to 2.2 times. The accumulation of irreversible strains is also observed; however, their contribution to the total strain is small. It is found that the stress-strain relation can be represented by the equation of linear hereditary creep theory.Institute of Polymer Mechanics, University of Latvia, Riga, LV-1006, Latvia. Translated from Mekhanika Kompozitnykh Materialov, Vol. 35, No. 2, pp. 147–162, March–April, 1999. 相似文献
194.
195.
196.
In electrodialysis using thin membranes and spacers, the compactness of the membrane cell-pairs leads to a small potential drop, and hence to energy saving. The spacer design itself has a great effect on the cost of the plants, since spacers act as turbulence promoters. A careful design, to increase the mass transfer coefficients, can reduce the membrane surface area required for a given application. Limiting current measurements, cell-pair resistance and pressure losses are presented for several thin spacers, for different flow-velocity values and feed water concentrations. It was possible to find an advantageous geometry of the separating mesh leading to substantial savings on investment and operation costs. 相似文献
197.
A. J. Maroulis P. D. Akrivos C. P. Hadjiantoniou-Maroulis 《Journal of heterocyclic chemistry》1993,30(4):913-916
Oxidation of the title compounds yields, besides the reported isoimides 3 and/or the amides 4 , also the imides 5 . The observed product dichotomy is considered as the result of an intramolecular nucleophilic attack on the aroyl group, of the pressumed zwitterionic intermediate 2 , by O or N present in the ambident N-aroylimine site of 2 . The results of AM1 calculations agree with the product studies and both permit the formulation of a set of rules correlating structure and selectivity. 相似文献
198.
199.
J.V.P. de OliveiraA.V. Cardona M.T. VilhenaR.C. Barros 《Journal of Quantitative Spectroscopy & Radiative Transfer》2002,73(1):55-62
We describe a semi-analytical numerical method for coherent isotropic scattering time-dependent radiative transfer problems in slab geometry. This numerical method is based on a combination of two classes of numerical methods: the spectral methods and the Laplace transform (LTSN) methods applied to the radiative transfer equation in the discrete ordinates (SN) formulation. The basic idea is to use the essence of the spectral methods and expand the intensity of radiation in a truncated series of Laguerre polynomials in the time variable and then solve recursively the resulting set of “time-independent” SN problems by using the LTSN method. We show some numerical experiments for a typical model problem. 相似文献
200.
The microhardness and content of carbon in electroplated gold coatings were studied as influenced by the operation time of citrate and citrate-phosphate gold-plating electrolytes. Such physicomechanical properties as porosity, microhardness, internal stress, plasticity, and microstructure of electroless-plated and electroplated nickel coatings were studied and analyzed. 相似文献