全文获取类型
收费全文 | 3857篇 |
免费 | 136篇 |
国内免费 | 4篇 |
专业分类
化学 | 2545篇 |
晶体学 | 29篇 |
力学 | 84篇 |
数学 | 285篇 |
物理学 | 1054篇 |
出版年
2023年 | 40篇 |
2022年 | 42篇 |
2021年 | 66篇 |
2020年 | 95篇 |
2019年 | 115篇 |
2018年 | 75篇 |
2017年 | 72篇 |
2016年 | 147篇 |
2015年 | 82篇 |
2014年 | 136篇 |
2013年 | 257篇 |
2012年 | 264篇 |
2011年 | 324篇 |
2010年 | 191篇 |
2009年 | 159篇 |
2008年 | 229篇 |
2007年 | 205篇 |
2006年 | 177篇 |
2005年 | 156篇 |
2004年 | 123篇 |
2003年 | 86篇 |
2002年 | 80篇 |
2001年 | 63篇 |
2000年 | 59篇 |
1999年 | 49篇 |
1998年 | 30篇 |
1997年 | 23篇 |
1996年 | 38篇 |
1995年 | 29篇 |
1994年 | 36篇 |
1993年 | 44篇 |
1992年 | 28篇 |
1991年 | 26篇 |
1989年 | 19篇 |
1988年 | 19篇 |
1987年 | 25篇 |
1986年 | 20篇 |
1985年 | 17篇 |
1984年 | 34篇 |
1983年 | 20篇 |
1982年 | 21篇 |
1981年 | 29篇 |
1980年 | 19篇 |
1979年 | 28篇 |
1978年 | 17篇 |
1977年 | 18篇 |
1976年 | 17篇 |
1975年 | 16篇 |
1973年 | 11篇 |
1957年 | 13篇 |
排序方式: 共有3997条查询结果,搜索用时 293 毫秒
71.
72.
73.
74.
We look at some one-dimensional semi-infinite superlattices with an underlying Hamiltonian that is of the nearest neighbour, tight binding type. A real space rescaling procedure which is exact in one dimension is applied to obtain the location of the subbands. It has been found that these subbands never overlap in 1D, and we interpret this as a band repulsion effect. Relevance in the case of a disordered system where this band repulsion crosses over to the well-known level repulsion is discussed. Then with a proper matching at the boundary we solve for the sets of denumerably infinite number of decaying solutions (the surface states) in the gaps. These types of states have been proposed quite some time ago. We look at detail theirexact analytical solutions in 1D and find that their decay lengths near the band edges diverge as |E–E
b|–v, wherev=1/2 andE
b is the nearest band edge. The decay lengths and their divergence exponent match extremely well with those obtained from transfer matrix method. Some recent experiments on quantum well structures seem to have observed such states. 相似文献
75.
76.
77.
Dulal C. Ghosh 《Journal of Chemical Sciences》1984,93(1):33-46
A cndo/2D study of the charge distribution obtained through Mulliken population analysis in the ground state of the title
compounds shows that the features of charge distribution found by severalab initio calculations are fairly well reproduced by this method. The one-particle density, the interference density at the mid-point
of the bond axis and the kinetic part of the interference energy calculated through the deorthogonalized density matrices
over a wide range of intermolecular separation between the donor and the acceptor show that the one-particle density and the
interference density steadily grow with decreasing internuclear separation, while the kinetic interference energy starts with
negative value at large distance, then decreases and passes through a minima near but above the equilibrium distance and then
increases rapidly below it conforming to the characteristic general behaviour of the kinetic component of Morse curve. The
orbital pairwise interference density and the corresponding kinetic energy components reveal that the orbitals involved in
the covalent binding are σ2p AO of B and 2S and σ2p AO of N and C atoms in H3B-NH3 and H3B-CO respectively. 相似文献
78.
Periodic first principle calculation correlates the role of metal substitution (Sr and Ba in place of Ca) on selective encaging of active O− radicals inside the microporous 12MO, 7Al2O3 crystal structure. We have exchanged Ca by Sr and Ba and as well extracted electron stepwise to monitor selectivity of different anion encaging inside the same structure type. Ca favors O2− encaging and shows no electron transition when neutral, whereas Sr shows no transition in absence of unpaired electron and can successfully trap O−, Ba though less active than the other metal substituents shows oxygen encaging at its zero and mono-positive state. 相似文献
79.
The synthesis and structure-activity-relationship (SAR) for a series of N-substituted piperazinyl carbamoyl 7-15 and piperazinyl acetyl 18-26 derivatives of tetrahydropapaverine have been carried out. The general synthetic methods of carbamoyl tetrahydropapaverine analogues involve N-substituted piperazines and carbamoyl imidazole tetrahydropapaverine as starting materials. Another route for synthesizing these compounds, involving the formation of carbamoyl imidazole piperazine has also been explored. Acylation of tetrahydropapaverine followed by substitution with various piperazinyl moities afforded the acetyl tetrahydropapaverine derivatives. Variously substituted piperazines have been used to monitor the effect of electron releasing and electron withdrawing substituents upon the antispasmodic activity of the molecules. Effect of varying electron densities on the antispasmodic activity, by altering the position of these groups on the benzene ring has also been monitored. Pharmacological methods involve the in vitro antispasmodic activity studies on a freshly removed guinea pig ileum using a force displacement transducer amplifier connected to a physiograph. Among the analogues synthesized in the present study, a promising compound 7, a potent muscle relaxant as compared to papaverine has been obtained. 相似文献
80.
In catalytic concentrations (10?5?10?4 mol l?1) sulphur dioxide induces polymerization of MMA, particularly on photoactivation. The effective initiating species appears to be the monomer-SO2 complex rather than free SO2. A mechanism involving biradical initiation by decomposition of the initiating species, linear propagation in two directions, and significant termination of growing chains by chain transfer with initiating species has been suggested. The initiator transfer constant is 1.6 at 40°. 相似文献