全文获取类型
收费全文 | 5203篇 |
免费 | 791篇 |
国内免费 | 565篇 |
专业分类
化学 | 543篇 |
晶体学 | 59篇 |
力学 | 2180篇 |
综合类 | 74篇 |
数学 | 1132篇 |
物理学 | 2571篇 |
出版年
2024年 | 16篇 |
2023年 | 64篇 |
2022年 | 111篇 |
2021年 | 93篇 |
2020年 | 176篇 |
2019年 | 120篇 |
2018年 | 141篇 |
2017年 | 203篇 |
2016年 | 228篇 |
2015年 | 167篇 |
2014年 | 275篇 |
2013年 | 290篇 |
2012年 | 255篇 |
2011年 | 321篇 |
2010年 | 233篇 |
2009年 | 265篇 |
2008年 | 295篇 |
2007年 | 304篇 |
2006年 | 284篇 |
2005年 | 295篇 |
2004年 | 302篇 |
2003年 | 312篇 |
2002年 | 224篇 |
2001年 | 199篇 |
2000年 | 202篇 |
1999年 | 172篇 |
1998年 | 186篇 |
1997年 | 137篇 |
1996年 | 109篇 |
1995年 | 85篇 |
1994年 | 76篇 |
1993年 | 68篇 |
1992年 | 58篇 |
1991年 | 60篇 |
1990年 | 44篇 |
1989年 | 42篇 |
1988年 | 30篇 |
1987年 | 17篇 |
1986年 | 14篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1983年 | 3篇 |
1982年 | 15篇 |
1981年 | 5篇 |
1980年 | 4篇 |
1979年 | 8篇 |
1977年 | 4篇 |
1971年 | 5篇 |
1957年 | 8篇 |
1936年 | 1篇 |
排序方式: 共有6559条查询结果,搜索用时 531 毫秒
101.
Abdul -Latif K. Al-Jiburi Kisma H. Al-Niami Muthana Shanshal 《Theoretical chemistry accounts》1979,53(4):327-335
The harmonic force constants, vibrational frequencies and integrated intensity ratios of CH2, H2O, CH2O, C2H2, CO2, HCN, CH3, CH4, and C2H4 have been calculated using the MINDO—FORCES program and the Pulay method for the calculation of the molecular force constants. The results obtained are in general quite satisfactory when compared with available literature values. The results are, however, not as satisfactory in case of molecules containing heteroatoms, due to the neglect of some dipolar repulsion integrals for the heteroatoms by the MINDO/3 method. Calculated integrated intensities for CH3 and C2H4 agree well with experimental results. The calculated integrated intensities for other molecules are obtained for the first time and no comparison with published data is therefore possible.Part of the M.Sc. Thesis of K. H. A. 1978. 相似文献
102.
基于Lagrange原理和假设模态法建立了旋转输液管的动力学模型.通过降阶升维的方法求解系统的特征值问题,并分析了旋转输液管自由振动特性.得到了不同端部集中质量和转速下,系统特征值随流速升高的演变轨迹.揭示了临界流速随系统参数的变化规律.研究发现,内部流体的流动对旋转输液管动力学特性存在显著影响.在某些参数组合下,系统低阶模态能够形成不同形式的内共振关系.预示了旋转输液管模型蕴含丰富的动力学现象. 相似文献
103.
Effects of Cone Angles on Nonlinear Vibration Responses of Functionally Graded Shells北大核心CSCD 下载免费PDF全文
The nonlinear vibration responses of functionally graded materials (FGMs) shells with different cone angles under external loads were studied. Firstly, the Voigt model was employed to describe the physical properties along the thickness direction of FGMs conical shells. Then, the motion equations were derived based on the 1st-order shear deformation theory, the von Kármán geometric nonlinearity and Hamilton’s principle. Next, the Galerkin method was applied to discretize the motion equations and the governing equations were simplified into a 1DOF nonlinear vibration differential equation under Volmir’s assumption. Finally, the nonlinear motion equations were solved with the harmonic balance method and the Runge-Kutta method, and the amplitude frequency response characteristic curves of the FGMs conical shells were obtained. The effects of different material distribution functions and different ceramic volume fraction exponents on the amplitude frequency response curves of conical shells were discussed. The bifurcation diagrams of conical shells with different cone angles, as well as time process diagrams and phase diagrams for different excitation amplitudes, were described. The motion characteristics were characterized by Poincaré maps. The results show that, the FGMs conical shells present the nonlinear characteristics of hardening springs. The chaotic motions of the FGMs conical shells are restrained and not prone to motion instability with the increase of the cone angle. The FGMs conical shell present a process from the periodic motion to the multi-periodic motion and then to chaos with the increase of the excitation amplitude. © 2022 Editorial Office of Applied Mathematics and Mechanics. All rights reserved. 相似文献
104.
In this paper, we introduce a potential reduction method for harmonically convex programming. We show that, if the objective function and them constraint functions are allk-harmonically convex in the feasible set, then the number of iterations needed to find an -optimal solution is bounded by a polynomial inm, k, and log(1/). The method requires either the optimal objective value of the problem or an upper bound of the harmonic constantk as a working parameter. Moreover, we discuss the relation between the harmonic convexity condition used in this paper and some other convexity and smoothness conditions used in the literature.The authors like to thank Dr. Hans Nieuwenhuis for carefully reading this paper and the anonymous referees for the worthy suggestions. 相似文献
105.
M. N. Kotov V. F. Masterov V. V. Potapov A. V. Prichodko O. V. Smertin N. M. Shybanova 《International Journal of Infrared and Millimeter Waves》1993,14(8):1679-1687
Experimental investigations of superconductivity effects in single-phase and multiphase Bi2Sr2CaCu2O8 single-crystals have been carried out at 142 GHz frequency by means of the standing wave profile method [1]. Josephson harmonic generation has been observed to be responsible for the appearence of additional peaks on the standing wave profile of the open dielectric resonator loaded with a properly orientated multiphase high-Tc superconductor specimen. This leads to the conclusion that most of the Josephson junctions in multiphase crystals are located in certain crystallographic planes. The investigations of temperature dependencies showed that sharp resonant peaks of conductivity observed earlier [2] at 60 GHz could also be observed at 142 GHz. 相似文献
106.
Philippe Rukimbira 《Annals of Global Analysis and Geometry》1993,11(2):165-171
Let (M, ) be an R-contact manifold, then the set of periodic points of the characteristic vector field is a nonempty union of closed, totally geodesic odd-dimensional submanifolds. Moreover, the R-metric cannot have nonpositive sectional curvature. We also prove that no R-contact form can exist on any torus. 相似文献
107.
The vibration-rotational kinetic energy operators of four-particle system in various coordinates are derived using a new and simple angular momentum method. The operators are respectively suitable for studying the systems described by scattering coordinate, valence coordinate, Radau coordinate, Radau/Jacobi and Jacobi/valence hybrid coordinates and so on. Certain properties of these operators and their possible applications are discussed. 相似文献
108.
1 INTRODUCTION The bimetallic nanoclusters are of standing inte- rest since they can exhibit catalytic, electronic and optical properties distinct from those of corre- sponding pure nanoclusters[1~4]. Palladium and pla- tinum, well known for their catalytic properties, are often used as the catalyst in different fuel cells[5~8]. Several experimental results illustrate that for the oxygen reduction reaction (ORR), which is one of the primary reactions taking place in many fuel cells and… 相似文献
109.
110.