全文获取类型
收费全文 | 55460篇 |
免费 | 8071篇 |
国内免费 | 5340篇 |
专业分类
化学 | 12050篇 |
晶体学 | 1103篇 |
力学 | 13228篇 |
综合类 | 772篇 |
数学 | 21116篇 |
物理学 | 20602篇 |
出版年
2024年 | 86篇 |
2023年 | 541篇 |
2022年 | 864篇 |
2021年 | 1147篇 |
2020年 | 1543篇 |
2019年 | 1348篇 |
2018年 | 1388篇 |
2017年 | 1894篇 |
2016年 | 2157篇 |
2015年 | 1652篇 |
2014年 | 2870篇 |
2013年 | 4175篇 |
2012年 | 3226篇 |
2011年 | 3876篇 |
2010年 | 3253篇 |
2009年 | 3680篇 |
2008年 | 3596篇 |
2007年 | 3628篇 |
2006年 | 3329篇 |
2005年 | 3082篇 |
2004年 | 2691篇 |
2003年 | 2441篇 |
2002年 | 2198篇 |
2001年 | 1883篇 |
2000年 | 1762篇 |
1999年 | 1605篇 |
1998年 | 1473篇 |
1997年 | 1213篇 |
1996年 | 988篇 |
1995年 | 837篇 |
1994年 | 748篇 |
1993年 | 588篇 |
1992年 | 609篇 |
1991年 | 478篇 |
1990年 | 378篇 |
1989年 | 293篇 |
1988年 | 226篇 |
1987年 | 182篇 |
1986年 | 130篇 |
1985年 | 165篇 |
1984年 | 137篇 |
1983年 | 83篇 |
1982年 | 93篇 |
1981年 | 70篇 |
1980年 | 39篇 |
1979年 | 60篇 |
1978年 | 39篇 |
1977年 | 44篇 |
1976年 | 19篇 |
1974年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
121.
Chemical bonding in isolated molecules and crystals of zwitterionic pentacoordinate silicon chelates
The electronic structures of a number of zwitterionic pentacoordinate silicon chelates were investigated using the results of X-ray diffraction studies and quantum-chemical calculatoins by the MPW1PW91/6-311G(d) method. The topological analysis of the electron density distribution function and the study in the framework of the natural bond orbital partitioning scheme showed that the character of chemical bonding in the axial fragments of the molecules under consideration changes from dative to three-center, four-electron as the silicon atom assumes a trigonal-bipyramidal coordination. 相似文献
122.
Mingxin Wang 《Journal of Mathematical Analysis and Applications》2002,274(1):424-436
This paper deals with positive solutions of degenerate and quasilinear parabolic systems not in divergence form: ut=up(Δu+av), vt=vq(Δv+bu), with null Dirichlet boundary conditions and positive initial conditions, where p, q, a and b are all positive constants. The local existence and uniqueness of classical solution are proved. Moreover, it will be proved that all solutions exist globally if and only if ab?λ12, where λ1 is the first eigenvalue of −Δ in Ω with homogeneous Dirichlet boundary condition. 相似文献
123.
124.
强流脉冲电子束在材料中的能量沉积剖面、能量沉积系数和束流传输系数受其入射角的影响很大,理论计算了0.5~2.0MeV的电子束以不同的入射角在Al材料中的能量沉积剖面和能量沉积系数,并且还计算了0.4~1.4MeV电子束以不同入射角穿透不同厚度C靶的束流传输系数。计算结果表明,随着入射角的增大,靶材表面层单位质量中沉积的能量增大,电子在靶材料中穿透深度减小,能量沉积系数减小,相应的束流传输系数也减小;能量为0.5~2.0MeV的电子束当入射角在60°~70°时在材料表面层单位质量中沉积的能量较大。 相似文献
125.
Based on the Collins formula and irradiance moments definition, the propagation of the kurtosis parameter of super-Gaussian beams through a spherically aberrated lens is studied. Detailed numerical results are given. It is shown that, as compared with aberration-free super-Gaussian beams, the profile of spherically aberrated super-Gaussian beams can be leptokurtic, mesokurtic or platykurtic, depending on the aberration and propagation distance. The results for the spherically aberrated Gaussian beams can be dealt with as a special case treated in this paper. Finally, the advantage of our method is pointed out. 相似文献
126.
A. D. Matveev 《Journal of Applied Mechanics and Technical Physics》2003,44(5):721-726
Modified three-dimensional formulations of bending problems of homogeneous elastic plates and beams are considered. Modification of the known three-dimensional formulations reduces to using additional constraints imposed on displacement functions. An advantage of the formulations proposed is that complex fixing conditions of plates and beams can be taken into account. 相似文献
127.
The propagation property of flattened Gaussian beams (FGBs) is studied based on the Collins formula, and analytical propagation equations for FGBs with and without an aperture are derived. It is shown that the closed-form expressions for the FGB of order N passing through paraxial ABCD optical systems without an aperture can be written as the combinations of Confluent Hyper-geometric Functions, and that the analytical equations for the FGB of order N passing through apertured paraxial ABCD optical systems may involve Error Functions. The proposed algorithms for the considered beams can save computing time, provide intuitive physical insights, and help to design optical systems. Our results are confirmed and illustrated with numerical examples. 相似文献
128.
The coordination polymer {[Co(phth)(phen)(H2O)]·(C2H5OH)}n (phth = phthalate, phen = 1,10-phenanthroline) with formula C22H20CoN2O6 (Mr = 467.33) was synthesized by the solvothermal method under low temperature, and its crystal structure was determined by singlecrystal X-ray diffraction. The complex belongs to monoclinic system, space group P21/n with a =12.7352(16), b = 8.8125(11), c = 18.677(2) (A), β = 102.633(2)°, V = 2045.3(4) (A)3, Z = 4, Dc =1.518 g/cm3,μ = 0.882 mm-1, F(000) = 964, S = 1.053, the final R = 0.0401 and wR = 0.0984 for 3135 observed reflections (I > 2σ(I)). Single-crystal X-ray analysis revealed that the Co(Ⅱ) ion is six-coordinated. The cobalt complex forms a one-dimensional chain structure via phthalate bridging ligand. In the cobalt complex, a 3D network structure is formed by π-π stacking interaction of 1,10-phenanthroline as well as intermolecular hydrogen bonding interactions. 相似文献
129.
By further generalizing the skew-symmetric triangular splitting iteration method studied by Krukier, Chikina and Belokon (Applied Numerical Mathematics, 41 (2002), pp. 89–105), in this paper, we present a new iteration scheme, called the modified skew-Hermitian triangular splitting iteration method, for solving the strongly non-Hermitian systems of linear equations with positive definite coefficient matrices. We discuss the convergence property and the optimal parameters of this new method in depth. Moreover, when it is applied to precondition the Krylov subspace methods like GMRES, the preconditioning property of the modified skew-Hermitian triangular splitting iteration is analyzed in detail. Numerical results show that, as both solver and preconditioner, the modified skew-Hermitian triangular splitting iteration method is very effective for solving large sparse positive definite systems of linear equations of strong skew-Hermitian parts. 相似文献
130.
介绍了车用压差式气体流量传感器的结构设计和电测电路,指出了传感器的使用范围,由实验测出了特性曲线.该仪器可检测车用发动机曲轴箱窜气量,适用于发动机不解体故障诊断. 相似文献