首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   360888篇
  免费   7336篇
  国内免费   3685篇
化学   179662篇
晶体学   5642篇
力学   18629篇
综合类   229篇
数学   49768篇
物理学   117979篇
  2021年   2270篇
  2020年   2492篇
  2019年   2421篇
  2018年   11584篇
  2017年   12401篇
  2016年   8277篇
  2015年   4243篇
  2014年   4666篇
  2013年   11775篇
  2012年   13382篇
  2011年   23398篇
  2010年   14786篇
  2009年   14729篇
  2008年   20142篇
  2007年   23429篇
  2006年   9006篇
  2005年   15150篇
  2004年   10712篇
  2003年   9656篇
  2002年   7435篇
  2001年   8347篇
  2000年   6535篇
  1999年   5178篇
  1998年   4062篇
  1997年   4039篇
  1996年   3971篇
  1995年   3634篇
  1994年   3376篇
  1993年   3170篇
  1992年   3770篇
  1991年   3649篇
  1990年   3494篇
  1989年   3411篇
  1988年   3307篇
  1987年   3324篇
  1986年   3036篇
  1985年   4088篇
  1984年   4060篇
  1983年   3284篇
  1982年   3465篇
  1981年   3378篇
  1980年   3236篇
  1979年   3484篇
  1978年   3774篇
  1977年   3482篇
  1976年   3427篇
  1975年   3188篇
  1974年   3163篇
  1973年   3156篇
  1967年   2283篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
172.
周华亮  高自友  李克平 《物理学报》2006,55(4):1706-1710
在NaSch模型的基础上,针对铁路交通的特点提出一种用于模拟准移动闭塞系统的元胞自动机模型.应用该模型模拟了准移动闭塞系统列车延迟传播的交通现象,分析了准移动闭塞系统中的轨道定位单元长度、发车时间间隔、初始延迟时间等因素对列车延迟传播的影响. 关键词: 元胞自动机 交通流 准移动闭塞  相似文献   
173.
周晓林  刘科  陈向荣  朱俊 《中国物理》2006,15(12):3014-3018
We employ a first-principles plane wave method with the relativistic analytic pseudopotential of Hartwigsen, Goedecker and Hutter (HGH) scheme in the frame of DFT to calculate the equilibrium lattice parameters and the thermodynamic properties of AlB2 compound with hcp structure. The obtained lattice parameters are in good agreement with the available experimental data and those calculated by others. Through the quasi-harmonic Debye model, obtained successfully are the dependences of the normalized lattice parameters a/a0 and c/c0 on pressure P, the normalized primitive cell volume V/V0 on pressure P, the variation of the thermal expansion α with pressure P and temperature T, as well as the Debye temperature \ThetaD and the heat capacity CV on pressure P and temperature T.  相似文献   
174.
Stability of a Model of Relativistic Quantum Electrodynamics   总被引:1,自引:0,他引:1  
The relativistic “no pair” model of quantum electrodynamics uses the Dirac operator, D(A) for the electron dynamics together with the usual self-energy of the quantized ultraviolet cutoff electromagnetic field A– in the Coulomb gauge. There are no positrons because the electron wave functions are constrained to lie in the positive spectral subspace of some Dirac operator, D, but the model is defined for any number, N, of electrons, and hence describes a true many-body system. In addition to the electrons there are a number, K, of fixed nuclei with charges ≤Z. If the fields are not quantized but are classical, it was shown earlier that such a model is always unstable (the ground state energy E=−∞) if one uses the customary D(0) to define the electron space, but is stable (E > − const.(N+K)) if one uses D(A) itself (provided the fine structure constant α and Z are not too large). This result is extended to quantized fields here, and stability is proved for α= 1/137 and Z≤ 42. This formulation of QED is somewhat unusual because it means that the electron Hilbert space is inextricably linked to the photon Fock space. But such a linkage appears to better describe the real world of photons and electrons. Received: 8 September 2001 / Accepted: 18 March 2002  相似文献   
175.
Formalization for problems of multicriteria decision making under uncertainty is constructed in terms of guaranteed and weak estimates. A relevant definition of the vector maximinimax value is given. Parameterization and approximation of maximum, minimax, and maximinimax values based on the inverse logical convolution are suggested. An application for multicommodity networks is considered. Received: December 13, 2000 / Accepted: August 21, 2001?Published online May 8, 2002  相似文献   
176.
A numerical procedure, with an exact strain-displacement relationship of twisted and curved cylindrical panels having variable thickness derived by considering the Green strain tensor on general shell theory, is presented using the principle of virtual work and the Rayleigh-Ritz method with algebraic polynomials as in-plane and transverse displacement functions. The accuracy and applicability of the procedure are verified by comparing the present results with previous experimental and theoretical results for several panels. The effects of variation ratio of thickness in chordwise and lengthwise directions, twist, and curvature both in two directions aforementioned on vibrations of cylindrical panels are studied in detail, and typical vibration mode shapes are plotted to demonstrate the effects.  相似文献   
177.
The Wheeler–DeWitt equation of arbitrary Hartle–Hawking factor ordering for several minisuperspace universe models, such as the pure gravity Friedmann–Robertson–Walker and Taub ones, is mapped onto the dynamics of corresponding classical oscillators. The latter ones are studied by the classical Ermakov invariant method, which is a natural approach in this context. For the more realistic case of a minimally coupled massive scalar field, one can study, within the same type of approach, the corresponding squeezing features as a possible means of describing cosmological evolution. Finally, we comment on the analogy with the accelerator physics.  相似文献   
178.
In this paper a new state called odd-excited binomial state (OEBS) is introduced. It interpolates between the odd number state and the odd-excited coherent state. We discuss some statistical properties, such as the Glauber second-order correlation function and squeezing phenomenon (normal and amplitude-squared squeezing) for this state. The quasiprobability distribution functions (Husimi Q-function and Wigner W-function) are also examined.  相似文献   
179.
N-Terminal tripeptide analogs of fibrin alpha-chain were synthesized and their inhibitory effect on fibrinogen/thrombin clotting was examined. A new water-soluble active ester, 3-pyridinium ester, was used for the synthesis. Among the synthetic peptides, H-Gly-Pro-Arg-hexamethyleneimine exhibited the highest inhibitory effect on fibrinogen-thrombin clotting.  相似文献   
180.
The mode of action of the anti-complementary acidic heteroglycans, AAF-IIb-2 and IIb-3 which consisted of rhamnogalacturonan core and arabinogalactan moieties, purified from the leaves of Artemisia princeps PAMP (Japanese name = Gaiyo) were investigated. The anti-complementary activities of AAF-IIb-2 and IIb-3 were reduced partially in the absence of Ca2+ ions. A marked consumption of C4 was observed to have occurred when serum was incubated with both polysaccharides in the presence of Ca2+ ions. AAF-IIb-2 showed more potent C4 consumption than IIb-3. After the incubation of the serum with AAF-IIb-2 in the absence of Ca2+ ions, a cleavage of C3 in the serum was detected by immunoelectrophoresis. AAF-IIb-2 showed more significant consumption of the complement than IIb-3 when rabbit erythrocytes were used in the assay system in the absence of Ca2+ ions. These results indicate that AAF-IIb-2 activates the complement via both the alternative and classical pathways, whereas IIb-3 mainly activates the complement via the classical pathway. The absorption of serum with Protein A-Sepharose results in a decrease of the activity of AAF-IIb-2 and IIb-3. However, the decrease of the activity was restored by the replacement of the immunoglobulin G (IgG) fraction after its recovery from the Protein A-Sepharose. These results suggest that IgG dependent mechanisms are both involved in the anti-complementary activity of AAF-IIb-2 and IIb-3.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号