全文获取类型
收费全文 | 447095篇 |
免费 | 4613篇 |
国内免费 | 1259篇 |
专业分类
化学 | 229119篇 |
晶体学 | 6899篇 |
力学 | 20986篇 |
综合类 | 14篇 |
数学 | 52012篇 |
物理学 | 143937篇 |
出版年
2021年 | 3839篇 |
2020年 | 4150篇 |
2019年 | 4605篇 |
2018年 | 6220篇 |
2017年 | 6430篇 |
2016年 | 9134篇 |
2015年 | 5289篇 |
2014年 | 8723篇 |
2013年 | 20750篇 |
2012年 | 15940篇 |
2011年 | 19432篇 |
2010年 | 14119篇 |
2009年 | 13999篇 |
2008年 | 17441篇 |
2007年 | 17436篇 |
2006年 | 16073篇 |
2005年 | 14187篇 |
2004年 | 13060篇 |
2003年 | 11559篇 |
2002年 | 11577篇 |
2001年 | 13515篇 |
2000年 | 10125篇 |
1999年 | 7873篇 |
1998年 | 6646篇 |
1997年 | 6469篇 |
1996年 | 6031篇 |
1995年 | 5370篇 |
1994年 | 5310篇 |
1993年 | 5066篇 |
1992年 | 5684篇 |
1991年 | 5960篇 |
1990年 | 5552篇 |
1989年 | 5497篇 |
1988年 | 5243篇 |
1987年 | 5318篇 |
1986年 | 4991篇 |
1985年 | 6377篇 |
1984年 | 6532篇 |
1983年 | 5469篇 |
1982年 | 5646篇 |
1981年 | 5310篇 |
1980年 | 5213篇 |
1979年 | 5602篇 |
1978年 | 5731篇 |
1977年 | 5632篇 |
1976年 | 5649篇 |
1975年 | 5341篇 |
1974年 | 5345篇 |
1973年 | 5345篇 |
1972年 | 3727篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Torsional vibration of multi-step non-uniform rods with various concentrated elements 总被引:1,自引:0,他引:1
Q.S. Li 《Journal of sound and vibration》2003,260(4):637-651
An analytical approach and exact solutions for the torsional vibration of a multi-step non-uniform rod carrying an arbitrary number of concentrated elements such as rigid disks and with classical or non-classical boundary conditions is presented. The exact solutions for the free torsional vibration of non-uniform rods whose variations of cross-section are described by exponential functions and power functions are obtained. Then, the exact solutions for more general cases, non-uniform rods with arbitrary cross-section, are derived for the first time. In order to simplify the analysis for the title problem, the fundamental solutions and recurrence formulas are developed. The advantage of the proposed method is that the resulting frequency equation for torsional vibration of multi-step non-uniform rods with arbitrary number of concentrated elements can be conveniently determined from a homogeneous algebraic equation. As a consequence, the computational time required by the proposed method can be reduced significantly as compared with previously developed analytical procedures. A numerical example shows that the results obtained from the proposed method are in good agreement with those determined from the finite element method (FEM), but the proposed method takes less computational time than FEM, illustrating the present methods are efficient, convenient and accurate. 相似文献
992.
Impacts of structural-acoustic coupling on the performance of energy density-based active sound transmission control 总被引:2,自引:0,他引:2
S.K. Lau 《Journal of sound and vibration》2003,266(1):147-170
This paper investigates numerically the performance of the active sound transmission control into a rectangular cavity through a flexible panel under the energy density-based error-sensing algorithm. Full coupling between the sound transmitting panel and the enclosed space is considered. A pure vibration actuator, a pure acoustic source and a combined control source system are used as the secondary control source in the active control and their performances are studied. Formulae for the coupled eigenfrequencies of the cavity and the flexible panel are also derived. The strength of the structural-acoustic coupling, the ratio between the first eigenfrequencies of the cavity and the panel and the difference between the excitation frequency and the coupled eigenfrequencies, especially the latter, are found to have crucial impacts on the performance of the active control regardless the type of control source used. 相似文献
993.
994.
T. M. Shneidman G. G. Adamian N. V. Antonenko S. P. Ivanova R. V. Jolos W. Scheid 《Physics of Atomic Nuclei》2003,66(2):206-217
Based on the dinuclear system concept, the role of bending vibrations in creation of the angular momentum of primary fission fragments is investigated. For 252Cf spontaneous fission, the angular momenta of the fragments are calculated as a function of the neutron multiplicity and compared with available experimental data. Different cluster compositions of the 252Cf fission modes at the scission point are considered. 相似文献
995.
S. Schadmand 《The European Physical Journal A - Hadrons and Nuclei》2003,18(2-3):405-408
An overview over the experimental research programs with photon beams is given with the focus on the electromagnetic excitation of the nucleon and the subsequent decay via mesons. Photoproduction of mesons on the free proton and on nucleons embedded in nuclei up to 1 GeV incident-photon energy are discussed.Received: 30 September 2002, Published online: 22 October 2003PACS:
13.60.Le Meson production - 13.60.Rj Baryon production - 14.20.Gk Baryon resonances with S = 0 - 25.20.Lj Photoproduction reactions 相似文献
996.
Lasing and discharge in mixtures of SF6 with hydrogen and hydrocarbons are investigated. If the chemical reaction is initiated by a self-sustained discharge (a generator
with both an inductive and capacitive energy storage can be used), the maximum lasing efficiency is attained at high values
of the ratio E/p (where E is the electric field intensity and p is the pressure in the mixture) if shaped electrodes and preionization of the discharge gap are employed. For the first time,
the lasing efficiency obtained in a nonchain HF laser is as high as 10% of the energy deposit and 4.5% of the stored energy
for a specific radiation energy of ≈140 J/(l atm). At high efficiencies, the emission spectrum of the nonchain HF laser significantly
broadens and cascade lasing at P(3-2) → P(2-1) → P(1-0) vibrational transitions for several rotational lines is realized. 相似文献
997.
998.
Terminal alkynes undergo mild and efficient aminomethylation with aqueous formaldehyde and secondary amines under CuI catalysis. In most cases high to nearly quantitative yields of tertiary propargylamines are obtained in DMSO solution at room temperature. Aromatic, aliphatic and silylated acetylenes as well as alkynols can be used. Primary amines are less reactive and satisfactory yields of secondary propargylamines are obtained only with phenylacetylene. 相似文献
999.
Baidyshev V. S. Udodov V. N. Popov A. A. Potekaev A. I. 《Russian Physics Journal》2003,46(12):1193-1198
The paper offers a method for calculating phase diagrams of polytype transformations in close-packed crystals based on the generalized axial Ising's model of finite dimensions at non-zero temperatures. Using the Metropolis algorithm, the behavior of systems with polytype transitions is studied under the conditions of changing external shear stress and temperature. The longest polytype period considered here is taken to be 30 close-packed planes. 相似文献