全文获取类型
收费全文 | 305428篇 |
免费 | 2650篇 |
国内免费 | 883篇 |
专业分类
化学 | 152528篇 |
晶体学 | 5073篇 |
力学 | 14479篇 |
综合类 | 1篇 |
数学 | 35172篇 |
物理学 | 101708篇 |
出版年
2021年 | 2282篇 |
2020年 | 2594篇 |
2019年 | 2944篇 |
2018年 | 4755篇 |
2017年 | 4914篇 |
2016年 | 5988篇 |
2015年 | 3453篇 |
2014年 | 5500篇 |
2013年 | 12987篇 |
2012年 | 10590篇 |
2011年 | 12365篇 |
2010年 | 8993篇 |
2009年 | 8995篇 |
2008年 | 11139篇 |
2007年 | 11051篇 |
2006年 | 10498篇 |
2005年 | 11954篇 |
2004年 | 11226篇 |
2003年 | 9118篇 |
2002年 | 7774篇 |
2001年 | 8833篇 |
2000年 | 6651篇 |
1999年 | 5202篇 |
1998年 | 4430篇 |
1997年 | 4270篇 |
1996年 | 4027篇 |
1995年 | 3683篇 |
1994年 | 3608篇 |
1993年 | 3648篇 |
1992年 | 3815篇 |
1991年 | 4035篇 |
1990年 | 3778篇 |
1989年 | 3758篇 |
1988年 | 3705篇 |
1987年 | 3561篇 |
1986年 | 3370篇 |
1985年 | 4486篇 |
1984年 | 4750篇 |
1983年 | 3852篇 |
1982年 | 4083篇 |
1981年 | 3948篇 |
1980年 | 3792篇 |
1979年 | 3947篇 |
1978年 | 4145篇 |
1977年 | 4068篇 |
1976年 | 4086篇 |
1975年 | 3764篇 |
1974年 | 3792篇 |
1973年 | 3803篇 |
1972年 | 2696篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
Lanthanum modified lead titanate (PLT) thin films are one of the potential candidates for the pyroelectric and memory applications due to their excellent dielectric, pyroelectric and ferroelectric properties. PLT thin films with 25 at.% of La were deposited on platinum coated Si substrates by the laser ablation technique. The phase transition studies were done in the temperature range of −40 to 150 °C as a function of frequency and ac field. A diffused phase transition with the shifting of the maximum dielectric permittivity (?max) to higher temperatures with the increase of frequency and dielectric dispersion with frequency at the lower temperatures were observed. The variation of the temperature corresponding to maximum dielectric constant Tm, with frequency follows the Vogel-Fulcher relation, which is the characteristic of the relaxor-like behavior of the material. With the increase of ac drive, the Tmax was shifted to lower value. 相似文献
993.
It follows from the theory of trace identities developed by Procesi and Razmyslov that the trace cocharacters arising from the trace identities of the algebra Mr(F) of r×r matrices over a field F of characteristic zero are given by TCr,n=∑λΛr(n)χλχλ where χλχλ denotes the Kronecker product of the irreducible characters of the symmetric group associated with the partition λ with itself and Λr(n) denotes the set of partitions of n with r or fewer parts, i.e. the set of partitions λ=(λ1λk) with kr. We study the behavior of the sequence of trace cocharacters TCr,n. In particular, we study the behavior of the coefficient of χ(ν,n−m) in TCr,n as a function of n where ν=(ν1νk) is some fixed partition of m and n−mνk. Our main result shows that such coefficients always grow as a polynomial in n of degree r−1. 相似文献
994.
995.
996.
The periodic precipitation pattern formation in gelatinous media is interpreted as a moving boundary problem. The time law,
spacing law and width law are revisited on the basis of the new scenario. The explicit dependence of the geometric structure
on the initial concentrations of the reactants is derived. Matalon—Packter law, which relates the spacing coefficient with
the initial concentrations is reformulated removing many ambiguities and impractical parameters. Experimental results are
discussed to establish the significance of moving boundary concept in the diffusion controlled pattern forming systems 相似文献
997.
The vibronic spectra of charge transfer excitons (CTE) in a molecular one-component or alternatingly ordered two-component chain are treated in the framework of a dynamic approach (neglecting thermal excitations of the intramolecular vibrations). The model introduces two mechanisms of coupling between CTEs and vibrational quanta: (1) shift of the equilibrium positions of the nuclei in the ionized donor or acceptor; (2) change of the vibrational frequency in the ionized molecule. This model allows to generalize the simple CTE Hamiltonian and the vibronic Hamiltonian of Frenkel excitons. The linear optical susceptibility is calculated in the vibronic region (one CTE and one vibrational quantum). The double splitting of vibronics of CTEs was analyzed: (1) the splitting connected with the location of the intramolecular vibration on the donors or on the acceptors; (2) the splitting connected with the symmetry of the vibronic spectra (in the degenerate case). The general structure of the vibronic spectra of CTEs is established. It contains structureless absorption lines, which correspond to two-particle bands (the phonon is excited on a neutral molecule neighboring the donor or the acceptor) and Lorentz-type lines of one-particle states, which correspond to the bound propagation of the CTE and the phonon. 相似文献
998.
The rotational spectra of three low-energy conformers of thiodiglycol (TDG) (HOCH2CH2SCH2CH2OH) have been measured in a molecular beam using a pulsed-nozzle Fourier-transform microwave spectrometer. To determine the likely conformational structures with ab initio approach, conformational structures of 2-(ethylthio)ethanol (HOEES) (CH3CH2SCH2CH2OH) were used as starting points together with the consideration of possible intramolecular hydrogen bonding in TDG. Three lower-energy conformers have been found for TDG at the MP2=Full/6311G** level and ab initio results agree nicely with experimentally determined rotational constants. In addition, Stark measurements were performed for two of the three conformers for dipole moment determinations, adding to our confidence of the conformational structure matches between experimental observations and ab initio calculations. Of the three lower-energy conformers, one displays a compact folded-like structure with strong hydrogen bonding between the two hydroxyl groups and the central sulfide atom. Two other conformers have relatively open chain-like structures with hydrogen bonding between each of the hydroxyl groups to the central sulfur atom, of which one has pure b-type dipole moment according to the ab initio results. 相似文献
999.
The rotation-tunneling spectrum of the second most stable gGg′ conformer of ethylene glycol (1,2-ethanediol) in its ground vibrational state has been studied in selected regions between 77 and 579 GHz. Compared to the study of the more stable aGg′ conformer, a much larger frequency range was studied, resulting in a much extended frequency list covering similar quantum numbers, J?55 and Ka?19. While the input data were reproduced within experimental uncertainties up to moderately high values of J and Ka larger residuals remain at higher quantum numbers. The severe mixing of the states caused by the Coriolis interaction between the two tunneling substates is suggested to provide a considerable part of the explanation. In addition, a Coriolis interaction of the gGg′ ground vibrational state with an excited state of the aGg′ conformer may also contribute. Relative intensities of closely spaced lines have been investigated to determine the signs of the Coriolis constants between the two tunneling substates relative to the dipole moment components and to estimate the magnitudes of the dipole moment components and the energy difference between the gGg′ and the aGg′ conformers. Results of ab initio calculations on the total dipole moment and the vibrational spectrum were needed for these estimates. The current analysis is limited to transitions with quantum numbers J?40 and Ka?6 plus those having J?22 and Ka?17 which could be reproduced within experimental uncertainties. The results are aimed at aiding radioastronomers to search for gGg′ ethylene glycol in comets and in interstellar space. 相似文献
1000.
E. Kajari R. Walser W. P. Schleich A. Delgado 《General Relativity and Gravitation》2004,36(10):2289-2316
We present exact expressions for the Sagnac effect of Gödel's Universe. For this purpose we first derive a formula for the Sagnac time delay along a circular path in the presence of an arbitrary stationary metric in cylindrical coordinates. We then apply this result to Gödel's metric for two different experimental situations: First, the light source and the detector are at rest relative to the matter generating the gravitational field. In this case we find an expression that is formally equivalent to the familiar nonrelativistic Sagnac time delay. Second, the light source and the detector are rotating relative to the matter. Here we show that for a special rotation rate of the detector the Sagnac time delay vanishes. Finally we propose a formulation of the Sagnac time delay in terms of invariant physical quantities. We show that this result is very close to the analogous formula of the Sagnac time delay of a rotating coordinate system in Minkowski spacetime. 相似文献