全文获取类型
收费全文 | 380931篇 |
免费 | 4288篇 |
国内免费 | 1319篇 |
专业分类
化学 | 202117篇 |
晶体学 | 6148篇 |
力学 | 16100篇 |
综合类 | 5篇 |
数学 | 44774篇 |
物理学 | 117394篇 |
出版年
2020年 | 2947篇 |
2019年 | 3334篇 |
2018年 | 4124篇 |
2017年 | 4116篇 |
2016年 | 6502篇 |
2015年 | 4158篇 |
2014年 | 6456篇 |
2013年 | 17489篇 |
2012年 | 12720篇 |
2011年 | 15594篇 |
2010年 | 10683篇 |
2009年 | 10655篇 |
2008年 | 14257篇 |
2007年 | 14212篇 |
2006年 | 13315篇 |
2005年 | 12200篇 |
2004年 | 11096篇 |
2003年 | 9899篇 |
2002年 | 9728篇 |
2001年 | 11218篇 |
2000年 | 8425篇 |
1999年 | 6607篇 |
1998年 | 5537篇 |
1997年 | 5458篇 |
1996年 | 5143篇 |
1995年 | 4822篇 |
1994年 | 4628篇 |
1993年 | 4667篇 |
1992年 | 5170篇 |
1991年 | 5125篇 |
1990年 | 4881篇 |
1989年 | 4809篇 |
1988年 | 4861篇 |
1987年 | 4719篇 |
1986年 | 4521篇 |
1985年 | 6113篇 |
1984年 | 6296篇 |
1983年 | 5317篇 |
1982年 | 5732篇 |
1981年 | 5465篇 |
1980年 | 5345篇 |
1979年 | 5504篇 |
1978年 | 5768篇 |
1977年 | 5582篇 |
1976年 | 5790篇 |
1975年 | 5292篇 |
1974年 | 5424篇 |
1973年 | 5862篇 |
1972年 | 3707篇 |
1971年 | 2953篇 |
排序方式: 共有10000条查询结果,搜索用时 593 毫秒
931.
The object of the present investigation is to introduce a new trigonometric method of summation which is both regular and
Fourier effective and determine its status with reference to other methods of summation (see §2-§4) and also give an application
of this method to determine the degree of approximation in a new Banach space of functions conceived as a generalized Holder
metric (see §5). 相似文献
932.
By an elementary calculation we obtain the exact mean values of Minkowksi functionals for a standard model of percolating sets. In particular, a recurrence theorem for the mean Euler characteristic recently put forward is shown to be incorrect. Related previous mathematical work is mentioned. We also conjecture bounds for the threshold density of continuum percolation, which are associated with the Euler characteristic. 相似文献
933.
Rafael R. del Río Castillo 《Communications in Mathematical Physics》1991,142(2):421-431
In this work we study the behavior of embedded eigenvalues of Sturm-Liouville problems in the half axis under local perturbations. When the derivative of the spectral function is strictly positive, we prove that the embedded eigenvalues either disappear or remain fixed. In this case we show that local perturbations cannot add eigenvalues in the continuous spectrum. If the condition on the spectral function is removed then a local perturbation can add infinitely many eigenvalues. 相似文献
934.
The stationary and the time-dependent homogeneous ordered states in convection may both become unstable against localized perturbations. Defects are then created and they may contribute to the disorganization of the homogeneous state. We present an experimental study of defects in some homogeneous stationary structures as well as in the traveling-wave states of convection of a nematic liquid crystal. We show that the core of the defects is a germ of the unstable state and it can become unstable under the external stress. Then, either fully homogeneous states with the symmetry of the core, or complex disordered states can develop from the local instability of defects in processes quite similar to displacive transitions in solids. Some of the main features are qualitatively similar to numerical simulations of an appropriate Landau-Ginzburg equation. 相似文献
935.
R. Németh 《Journal of statistical physics》1991,63(1-2):419-424
We calculate the site occupation probabilities of one-dimensional lattice gas models within the canonical and grand canonical ensembles. The appearing differences do not vanish if we increase the system size keeping the site energies discrete. In this way one can explain the surprising numerical results of Barszczak and Kutner. This effect in the single-site occupation number disappears in higher dimensions. 相似文献
936.
B. A. Usievich D. Kh. Nurligareev V. A. Sychugov K. M. Golant 《Optics and Spectroscopy》2006,101(6):930-934
A system of tunnel-coupled rectilinear waveguides is studied. The dependence of the number of modes in this system on the number of waveguides N, the distance between the waveguides, and the number of modes in a separate waveguide is considered. It is shown that the modes of the m = N and N + 1 orders in such a system are Bragg modes; i.e., the angle between the direction of their propagation and the system axis is close to the Bragg angle. The effective refractive indices n * of these modes change stepwise. The step size Δn * is found to be dependent on the distance s between the waveguides and on the number of modes in a separate waveguide. A system of single-mode waveguides with the number of guided modes M = 34 < N = 50 is studied experimentally. It is shown that the Bragg modes of the system lie among the leakage modes of the system and have rather low losses. It is demonstrated that the localization of the Bragg modes among the leakage modes may be favorable for their selection upon light generation. 相似文献
937.
Water suppression by diffusive attenuation was used to measure Brix in intact cellular tissue of apple and strawberry. Given
the signal-to-noise ratio, the correlation for apple was established without repeated acquisition, so this protocol should
also be useful for rapid, on-line measurements at low spectrometer frequencies. Water suppression by theT
1-Null method fails with cellular tissue because of the considerable variation in the longitudinal relaxation times of vacuolar
and cytoplasmic water. 相似文献
938.
Y. Qu J.X. Zhang A. Uddin C.Y. Liu S. Yuan M.C.Y. Chan B. Bo G. Liu H. Jiang 《Applied Physics A: Materials Science & Processing》2006,82(2):305-308
Ridge-waveguide InGaAsN triple-quantum-well strain-compensated lasers grown by metal organic chemical vapor deposition were fabricated with pulsed anodic oxidation. The laser’s output power reached 145 mW in continuous-wave mode at room temperature for a 4-?m -stripe-width laser. Continuous-wave single longitudinal mode operation was maintained at a high injection current level with a wavelength of 1287.3 nm at room temperature. Single longitudinal mode operation at 1317.2 nm was achieved at twice the threshold current at 100 °C. The band gap of InGaAsN in the quantum wells at different temperatures was calculated and compared to the measured temperature-dependent laser wavelength. 相似文献
939.
The deformation surrounding Vickers indentations on InGaAsP/InP epilayers have been studied in detail. The surface topography was characterized by using atomic force microscopy (AFM). The material pile-up and sink-in regions around the indentation impression was observed for the quaternary InGaAsP/InP epilayers. The sectional analysis mode of the AFM shows the depth profile at the indented region. Microindentation studies were carried out for different atomic fraction of the quaternary InGaAsP/InP compound semiconductor alloys. The microhardness values of InGaAsP/InP epilayers were found to be in the range of 5.08 and 5.73 GPa. These results show that the hardness value of the quaternary alloy drastically increases as the composition of As was increased by 0.01 atomic fraction and when the phosphorous concentration decreases from 0.4 to 0.38. The reason may be that the increase in As concentration hardens the lattice when phosphorous concentration was less and hardness decreases when phosphorous was increased. 相似文献
940.
A. A. Lamberov E. V. Dement’eva Kh. Kh. Gil’manov S. R. Egorova N. V. Romanova 《Russian Journal of Applied Chemistry》2006,79(9):1453-1457
The effect of the paste molding pressure on the physicomechanical and texture characteristics of catalysts was studied. An indirect criterion is suggested to evaluate the molding pressure in industrial extruders, whose optimal value ensures good physicomechanical properties of the catalysts and kinetic control over the catalyzed reaction. The results obtained were verified in paste molding on various industrial extruders, and this enabled the optimal choice of the molding equipment. 相似文献