全文获取类型
收费全文 | 18816篇 |
免费 | 2462篇 |
国内免费 | 952篇 |
专业分类
化学 | 2285篇 |
晶体学 | 191篇 |
力学 | 6747篇 |
综合类 | 212篇 |
数学 | 5814篇 |
物理学 | 6981篇 |
出版年
2024年 | 35篇 |
2023年 | 123篇 |
2022年 | 239篇 |
2021年 | 273篇 |
2020年 | 394篇 |
2019年 | 357篇 |
2018年 | 320篇 |
2017年 | 473篇 |
2016年 | 666篇 |
2015年 | 515篇 |
2014年 | 691篇 |
2013年 | 1190篇 |
2012年 | 847篇 |
2011年 | 943篇 |
2010年 | 778篇 |
2009年 | 999篇 |
2008年 | 1088篇 |
2007年 | 1238篇 |
2006年 | 1162篇 |
2005年 | 1164篇 |
2004年 | 1024篇 |
2003年 | 1061篇 |
2002年 | 848篇 |
2001年 | 685篇 |
2000年 | 681篇 |
1999年 | 619篇 |
1998年 | 551篇 |
1997年 | 490篇 |
1996年 | 350篇 |
1995年 | 363篇 |
1994年 | 266篇 |
1993年 | 256篇 |
1992年 | 207篇 |
1991年 | 181篇 |
1990年 | 137篇 |
1989年 | 105篇 |
1988年 | 114篇 |
1987年 | 99篇 |
1986年 | 83篇 |
1985年 | 89篇 |
1984年 | 72篇 |
1983年 | 35篇 |
1982年 | 57篇 |
1981年 | 58篇 |
1980年 | 33篇 |
1979年 | 57篇 |
1978年 | 55篇 |
1977年 | 48篇 |
1976年 | 28篇 |
1971年 | 20篇 |
排序方式: 共有10000条查询结果,搜索用时 423 毫秒
51.
We derive a test problem for evaluating the ability of time-steppingmethods to preserve the statistical properties of systems inmolecular dynamics. We consider a family of deterministic systemsconsisting of a finite number of particles interacting on acompact interval. The particles are given random initial conditionsand interact through instantaneous energy- and momentum-conservingcollisions. As the number of particles, the particle density,and the mean particle speed go to infinity, the trajectory ofa tracer particle is shown to converge to a stationary Gaussianstochastic process. We approximate this system by one describedby a system of ordinary differential equations and provide numericalevidence that it converges to the same stochastic process. Wesimulate the latter system with a variety of numerical integrators,including the symplectic Euler method, a fourth-order Runge-Kuttamethod, and an energyconserving step-and-project method. Weassess the methods' ability to recapture the system's limitingstatistics and observe that symplectic Euler performs significantlybetter than the others for comparable computational expense. 相似文献
52.
研究非Chetaev型变质量非完整系统的Lie对称性与Noether对称性以及其间的 关系,给出Lie对称性导致Noether对称性以及Noether对称性导致Lie对称性的条件. 相似文献
53.
近年来,决定椭圆型方程系数反问题在地磁、地球物理、冶金和生物等实际问题上有着广泛的应用.本文讨论了二维的决定椭圆型方程系数反问题的数值求解方法.由误差平方和最小原则,这个反问题可化为一个变分问题,并进一步离散化为一个最优化问题,其目标函数依赖于要决定的方程系数.本文着重考察非线性共轭梯度法在此最优化问题数值计算中的表现,并与拟牛顿法作为对比.为了提高算法的效率我们适当选择加快收敛速度的预处理矩阵.同时还考察了线搜索方法的不同对优化算法的影响.数值实验的结果表明,非线性共轭梯度法在这类大规模优化问题中相对于拟牛顿法更有效. 相似文献
54.
55.
Yu. Satov B. Sharkov H. Haseroth Yu. Smakovski K. Makarov S. Kondrashev V. Roerich A. Stepanov H. Kugler R. Scrivens O. Camut A. Shumshurov A. Balabaev A. Charushin 《Journal of Russian Laser Research》2004,25(3):205-216
Experimental and numerical calculation results devoted to development of an optical system for an ion source based on a repetition rate CO2 laser are described. The laser chain consists of a master oscillator, gas absorber cells, and a four-pass amplifier. The optical system provides smooth laser pulses with variable duration and high spatial quality that ensures efficiency for plasma heating and ion generation. The parameters of the plasma ion component measured in the CERN laboratory are applied for a lead target illumination. 相似文献
56.
The reversible nonlinear conduction (RNC) in of high‐density polyethylene/acetylene carbon black composites with different degrees of crosslinking was studied above room temperature and below the melting point of high‐density polyethylene (HDPE). The experimental current density‐electric field strength curves can be overlapped onto a master curve, suggesting that the microscopic mechanisms for the appearance of RNC exist regardless of the ambient temperature and the crosslinking degree of the HDPE matrix. The relationship between the crossover current density and the linear conductivity can be explained in the framework of the dynamic random‐resistor‐network model. According to these results, two electron‐tunneling models are suggested to interpret the microscopic conduction behavior. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 1212–1217, 2004 相似文献
57.
Rachel L. McSwain Alison R. Markowitz Kenneth R. Shull 《Journal of Polymer Science.Polymer Physics》2004,42(20):3809-3821
To develop a greater understanding of interfacial interactions between a semicrystalline polymer and a glassy polymer, adhesion tests were performed on very thin layers of poly(ethylene oxide) (PEO) sandwiched between two layers of poly(tetramethyl bisphenol A polycarbonate) (TMPC). The tests were designed to provide intimate contact between the surfaces while they were heated above the melting point of the PEO and cooled back to room temperature. A contact mechanics approach, based on the Johnson, Kendall, and Roberts theory, was used to determine values of the energy release rate describing the energetic driving force for crack propagation within the interfacial region. The ability to measure crack propagation at large values of the energy release rate was limited by rupture of the silicone elastomer that was used to provide a sufficiently compliant matrix for the adhesion experiment. By cycling the tensile stress at relatively low loading levels, we were able to measure fatigue crack propagation at values of the energy release rate that did not result in failure of the elastomer. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 3809–3821, 2004 相似文献
58.
59.
对于超细长弹性杆静力学的Kirchhoff方程,用动力学的概念和方法研究其常值特解 和稳定性问题.计算了Kirchhoff方程相对固定坐标系、截面主轴坐标系以及中心线Frenet 坐标系的常值特解,进行了Kirchhoff动力学比拟,用一次近似理论分别讨论了它们的Lyapu nov稳定性,导出了若干稳定性判据,并在参数平面上绘出了稳定域.
关键词:
超细长弹性杆
Kirchhoff方程
常值特解
Lyapunov稳定性 相似文献
60.
Sufficient conditions are established for the asymptotic behavior of solutionsof nonlinear delay differential equations x′(t)+sum from i=1 to m(pi(t)x(t-т_i))=F(t,x_t),t≥0where 0<т_1<т_2<…<т_m≤r,pi∈C([0,∞)),i=1,2,…,m,F∈C([0,∞)×C_0,R).C_0=C([-r,0],R)equipped with the sup norm ||·|| forsome r>0. A new result is established, some known results are improved. 相似文献