全文获取类型
收费全文 | 211782篇 |
免费 | 2355篇 |
国内免费 | 662篇 |
专业分类
化学 | 108814篇 |
晶体学 | 3373篇 |
力学 | 9440篇 |
综合类 | 5篇 |
数学 | 23229篇 |
物理学 | 69938篇 |
出版年
2021年 | 1774篇 |
2020年 | 1971篇 |
2019年 | 2197篇 |
2018年 | 2809篇 |
2017年 | 2728篇 |
2016年 | 4015篇 |
2015年 | 2550篇 |
2014年 | 3863篇 |
2013年 | 9472篇 |
2012年 | 7813篇 |
2011年 | 9485篇 |
2010年 | 6403篇 |
2009年 | 6185篇 |
2008年 | 8502篇 |
2007年 | 8618篇 |
2006年 | 8277篇 |
2005年 | 7394篇 |
2004年 | 6875篇 |
2003年 | 6013篇 |
2002年 | 5830篇 |
2001年 | 6533篇 |
2000年 | 5005篇 |
1999年 | 3844篇 |
1998年 | 3094篇 |
1997年 | 3110篇 |
1996年 | 2871篇 |
1995年 | 2670篇 |
1994年 | 2493篇 |
1993年 | 2380篇 |
1992年 | 2724篇 |
1991年 | 2708篇 |
1990年 | 2624篇 |
1989年 | 2421篇 |
1988年 | 2464篇 |
1987年 | 2468篇 |
1986年 | 2295篇 |
1985年 | 3002篇 |
1984年 | 3241篇 |
1983年 | 2661篇 |
1982年 | 2838篇 |
1981年 | 2800篇 |
1980年 | 2661篇 |
1979年 | 2755篇 |
1978年 | 2837篇 |
1977年 | 2836篇 |
1976年 | 2906篇 |
1975年 | 2689篇 |
1974年 | 2657篇 |
1973年 | 2784篇 |
1972年 | 1767篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
A. A. Collyer D. W. Clegg M. Morris D. G. Parker G. W. Wheatley G. C. Corfield 《Journal of polymer science. Part A, Polymer chemistry》1991,29(2):193-200
High molecular weight alternating block copolymers of polyethesulphone (PES) and polydimethylsiloxane (PDMS) were prepared by the condensation of dimethylamino-terminated PDMS oligomers and hydroxy-terminated PES oligomers in 1,2-dichlorobenzene. Microphase separation of the block copolymers at exceptionally short block lengths was observed by differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The Si? O? C intersegment linkage in these materials appeared to display poor hydrolytic stability which is contrary to results obtained for other block copolymers. 相似文献
992.
In this paper we define the vertex-cover polynomial Ψ(G,τ) for a graph G. The coefficient of τr in this polynomial is the number of vertex covers V′ of G with |V′|=r. We develop a method to calculate Ψ(G,τ). Motivated by a problem in biological systematics, we also consider the mappings f from {1, 2,…,m} into the vertex set V(G) of a graph G, subject to f−1(x)f−1(y)≠ for every edge xy in G. Let F(G,m) be the number of such mappings f. We show that F(G,m) can be determined from Ψ(G,τ). 相似文献
993.
E. A. Sokolov S. D. Babenko N. P. Piven’ A. N. Ponomarev 《Russian Chemical Bulletin》1997,46(4):826-828
Experimental results related to the transition of spontaneous polymerization of acrylamide complexes with metal nitrates to
the “explosive” regime at room temperature are presented. It is suggested that the “explosion” has a thermal nature.
Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 4, pp. 859–861, April, 1997. 相似文献
994.
A. Jarid M. Aaid Y. Legoux J. Merini M. Loudet D. Gonbeau G. Pfister-Guillouzo 《Chemical physics》1991,150(3):353-360
The electronic and structural characteristics of CrF5, CrF4, RuF5 and RuF4 were studied. Ab initio (SCF-CI) calculations were performed with different structures and spin states for each complex. The favored conformation always corresponds to the highest multiplicity: doublet for CrF5 in D3h, triplet for CrF4 in Td, quadruplet for RuF5 in C4v and quintuplet for RuF4 in D4h symmetry. 相似文献
995.
A finite-element method (FEM) analysis has been carried out on the thermally-induced stresses generated when stainless steel
covers are laser-welded to Au-coated Kovar TO headers during the packaging of semiconductor lasers. In particular, the effect
of varying the Au coating thickness of these stresses was investigated. Maximum stresses were found to be reduced as the Au
thickness increased. This effect is attributed to the reduction in thermal gradient in the weld region provided by the increased
thermal conduction of thicker Au. The result suggests that adequate Au coating thickness is important for ensuring good reliability
of opto-electronic packages having laser-welded caps.
This revised version was published online in June 2006 with corrections to the Cover Date. 相似文献
996.
In this work, on the basis of a semiempirical model of bonding forces that allows for three-particle interaction and uses the pseudopotential method, the mechanical and electrical properties of barium chalcogenides are studied at hydrostatic pressure. Equations of state are formulated from hydrostatic compression, parameters of the type B1–B2 structural phase transitions are calculated, the change in the energy band spectrum under the influence of the pressures is studied, and an estimate is made of the critical amount of compression that induces a dielectric-metal phase transition. The results obtained are in agreement with the available data.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 11, pp. 44–52, November, 1991. 相似文献
997.
As acknowledged in almost all monographs on the fracture of composite materials, one of the major fracture mechanism in unidirectional fibrous composites under uniaxial compression along the reinforcing elements is the stability loss of the material structure (the structural instability). According to this mechanism, theoretical investigations of the fracture along the fibres are reduced to those of the stability loss in the material structure, and the value of external critical forces is accepted as the value of failure forces. At present, numerous theoretical investigations have been carried out in this field with the use of the three-dimensional linearized theory of stability in the framework of the piecewise-homogeneous body model. However, in all the investigations it is assumed that the matrix and the fibre material are isotropic. It is evident that in many cases it is necessary to take into account the anisotropy of the matrix material when investigating the stability loss of fibres. In view of the above, in the framework of the piecewise-homogeneous body model using the three-dimensional linearized theory of stability, the present paper considers the stability loss of the fibre in the anisotropic (transversally isotropic) matrix. The effect of the properties of the matrix material on the critical values of the external loading is examined.Submitted to the 10th International Conference of Mechanics of Composite Materials (Riga, April 20–23, 1998).Published in Mekhanika Kompozitnykh Materialov, Vol. 33, No. 5, pp. 603–611. September–October, 1997. 相似文献
998.
S. Coffa D. C. Jacobson J. M. Poate F. Priolo 《Applied Physics A: Materials Science & Processing》1992,54(6):481-484
We have investigated ion-beam-enhanced diffusion of Au in undoped and B doped amorphous Si. The diffusion coefficients depend linearly on ion flux and exibit an Arrhenius-like temperature dependence with an activation energy of 0.37 eV in the temperature range 200–350° C. Moreover the diffusivity is enhanced by a factor of 5 by B-doping at a concentration of 1×1020 atoms/cm3. A similar enhancement is observed in thermal diffusion of Au which has an activation energy of 1.5 eV. On the basis of these results a model for the ion-beam-enhanced diffusion of Au is proposed where the high density of defects present in amorphous Si act as traps for the fast moving interstitial Au atoms. The effectiveness of this trapping process can be changed by the high concentration of mobile defects generated by the beam and also by a change in the charge state of the traps induced by the presence of B. 相似文献
999.
1000.