全文获取类型
收费全文 | 38349篇 |
免费 | 578篇 |
国内免费 | 397篇 |
专业分类
化学 | 17194篇 |
晶体学 | 348篇 |
力学 | 1869篇 |
综合类 | 15篇 |
数学 | 10153篇 |
物理学 | 9745篇 |
出版年
2023年 | 87篇 |
2022年 | 266篇 |
2021年 | 448篇 |
2020年 | 374篇 |
2019年 | 453篇 |
2018年 | 1620篇 |
2017年 | 1798篇 |
2016年 | 1289篇 |
2015年 | 953篇 |
2014年 | 1021篇 |
2013年 | 1688篇 |
2012年 | 3881篇 |
2011年 | 3020篇 |
2010年 | 2319篇 |
2009年 | 1957篇 |
2008年 | 1012篇 |
2007年 | 1004篇 |
2006年 | 875篇 |
2005年 | 4707篇 |
2004年 | 4068篇 |
2003年 | 2462篇 |
2002年 | 623篇 |
2001年 | 360篇 |
2000年 | 140篇 |
1999年 | 231篇 |
1998年 | 147篇 |
1997年 | 117篇 |
1996年 | 89篇 |
1995年 | 74篇 |
1994年 | 80篇 |
1993年 | 40篇 |
1992年 | 205篇 |
1991年 | 185篇 |
1990年 | 164篇 |
1989年 | 142篇 |
1988年 | 131篇 |
1987年 | 90篇 |
1986年 | 71篇 |
1985年 | 73篇 |
1984年 | 63篇 |
1983年 | 47篇 |
1982年 | 46篇 |
1979年 | 49篇 |
1978年 | 42篇 |
1976年 | 94篇 |
1975年 | 49篇 |
1974年 | 56篇 |
1973年 | 59篇 |
1972年 | 51篇 |
1966年 | 43篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
In this paper we construct a positive doubly power bounded operator
with a nonpositive inverse on an AL-space. 相似文献
92.
In this paper, it is shown that some of the results of torus actions on Poincaré duality spaces, Borel’s dimension formula
and topological splitting principle to local weights, hold if ‘torus’ is replaced by ‘pro-torus’. 相似文献
93.
?engül Dilem Yard?mc? 《Tetrahedron》2006,62(46):10633-10638
The photooxygenation of the 1-methyl-, 2,3-dimethyl-, and 1,4-dimethylcyclohexa-1,4-dienes, which are readily available through Birch reduction, yielded the corresponding ene-products. The formed endocyclic dienes were trapped by the addition of singlet oxygen to give the corresponding bicyclic endoperoxy hydroperoxides. In the case of 1-methylcyclohexa-1,4-diene and 1,4-dimethylcyclohexa-1,4,-diene, the cis-effect determined the product distribution. Photooxygenation of 2,3-dimethylcyclohexa-1,4-dienes gave mainly exocyclic olefin, which was attributed to the lowered rotational barrier of the methyl group and increased reactivity of the methyl groups. 相似文献
94.
Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr2O3 using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr2O3). A second phase, namely θ-(Al,Cr)2O3, was revealed by XRD in the sample containing 9.60 wt% Cr2O3. The structure of mullites was refined by the Rietveld method, location of Cr3+ was performed by the EPR spectroscopy. At low chromium doping level (Cr2O3 content less than ∼5 wt%) Cr3+ ions were substituted for Al3+ in the AlO6 octahedra of the mullite structure (M1 site). For higher doping level, Cr3+ ions were additionally substituted for Al3+ in the AlO6 octahedra of the second phase [θ-(Al,Cr)2O3 at 1400 °C, or α-(Al,Cr)2O3 at 1600 °C] which segregated in the system. Substitution of Cr3+ for Al3+ on M1 site in the mullite structure resulted in increase of average distances in (M1)O6 octahedron and decrease of average distances in T*O4 tetrahedron, while average distances in TO4 tetrahedron stayed almost constant. 相似文献
95.
I Cserpan M Kalman M L Tj?rnhammar A Simoncsits 《Acta chemica Scandinavica (Copenhagen, Denmark : 1989)》1991,45(3):265-272
A general method to convert single-stranded, chemically synthesized oligonucleotides into cloned duplexes is described. Oligonucleotides supplied with 3'-terminal extensions that are complementary to 3'-protruding ends obtained by certain restriction enzymes can be cloned either directly or with the help of an adapter molecule into double-stranded vectors. Two methods have also been developed for consecutive cloning applications. According to these methods, the synthetic oligonucleotides (and their enzymatically prepared complementary strands) are joined, one after the other, inside a cloning vector, each joining requiring one cloning step. Synthetic genes are thus built up from oligonucleotides corresponding to only one strand of the DNA. The sequential assembly of the cloned duplex takes place in the 5' to 3' direction. Each oligonucleotide is supplied with a four-nucleotide-long 3'-terminal extension, but this sequence is eliminated when the joining takes place, leaving no limiting sequence between the oligonucleotides. The two consecutive cloning methods, the adapter and the polycloning site methods, are illustrated by the assembly of short artificial genes. 相似文献
96.
Felix Ali Mehmeti 《Mathematical Methods in the Applied Sciences》1994,17(9):697-752
Consider two copies N1, N2 of the interval [0, ∞]. Consider Klein-Gordon equations with (different) constant coefficients on ? × Nj ( = time × space). Assume the coincidence of the values of the solution at the boundary points of the Nj for all times and a transmission condition relating its first (one-sided) space derivatives at these points. Under a symmetry condition, we extend the spatial part of the equation and the transmission conditions to a self-adjoint operator (by Friedrichs extension) and reformulate our problem in terms of an abstract wave equation in a suitable Hilbert space. We derive an expansion of the solution in generalized eigenfunctions of this self-adjoint extension and show, that the L∞-norms (in space) of the solution and its first k space derivatives at the time t decay for t → ∞ at least as const. t¼, if the initial conditions satisfy a compatibility condition of order k derived in this paper. The loss of decay rate in comparison with the full line case (const. t?½, cf. [28]) is caused by the tunnel effect. Further we show that an abstract wave equation in a Hilbert space with a Friedrichs extension as spatial part can always be derived from a stationarity principle for an associated action-type functional. This yields a physical legitimation of our model by the principle of stationary action and moreover a criterion for the physical interpretability of all models created by the linear interaction concept [4, 6, 8, 10], in particular for the coupling of media of different dimension (alternative to [13, 16] for similar models). 相似文献
97.
Joel M. Kauffman Ali Khalaj Peter T. Litak John A. Novinski Gurdip S. Bajwa 《Journal of heterocyclic chemistry》1994,31(4):957-965
To improve the solubility, photostability and fluorescence quantum yield (Φ) of 2-arylbenzimidazolium ions the N-1 hydrogen was replaced by an alkyl group before N-3 was quatemized; this substitution did not reduce Φ, proving the absence of steric inhibition to attainment of coplanarity in the excited state. A related symmetrical 2,2′-arylenebis(1,3-dimethyldibenzimidazolium ion) had Φ = 0.86. The related 2,5-bis(1-methyl-2-benzimidazolyl)phenol had Φ = 0.38 at 22° as well as a very large Stokes' shift due to proton-transfer. These findings supported a new insight into the electron distribution of the first excited singlet state of oligophenylenes. The compounds are of interest as laser dyes and scintillation fluors. 相似文献
98.
99.
Andrzej Wi?nicki 《Journal of Mathematical Analysis and Applications》2002,267(1):158-172
We use a newly introduced concept of neocompactness to study problems from metric fixed point theory. In particular, we give a sufficient condition for a superreflexive Banach space X to have the fixed point property and obtain shorter proofs of some well-known results in that theory. 相似文献
100.