全文获取类型
收费全文 | 170982篇 |
免费 | 1801篇 |
国内免费 | 506篇 |
专业分类
化学 | 88164篇 |
晶体学 | 3040篇 |
力学 | 7156篇 |
综合类 | 3篇 |
数学 | 19517篇 |
物理学 | 55409篇 |
出版年
2020年 | 1400篇 |
2019年 | 1552篇 |
2018年 | 1980篇 |
2017年 | 2066篇 |
2016年 | 3186篇 |
2015年 | 1894篇 |
2014年 | 3114篇 |
2013年 | 7665篇 |
2012年 | 5750篇 |
2011年 | 6889篇 |
2010年 | 5023篇 |
2009年 | 5006篇 |
2008年 | 6538篇 |
2007年 | 6432篇 |
2006年 | 6077篇 |
2005年 | 5503篇 |
2004年 | 5091篇 |
2003年 | 4594篇 |
2002年 | 4450篇 |
2001年 | 5218篇 |
2000年 | 3862篇 |
1999年 | 2967篇 |
1998年 | 2481篇 |
1997年 | 2385篇 |
1996年 | 2237篇 |
1995年 | 2113篇 |
1994年 | 2091篇 |
1993年 | 2000篇 |
1992年 | 2305篇 |
1991年 | 2244篇 |
1990年 | 2193篇 |
1989年 | 2121篇 |
1988年 | 2119篇 |
1987年 | 2122篇 |
1986年 | 2019篇 |
1985年 | 2629篇 |
1984年 | 2683篇 |
1983年 | 2381篇 |
1982年 | 2492篇 |
1981年 | 2307篇 |
1980年 | 2284篇 |
1979年 | 2426篇 |
1978年 | 2462篇 |
1977年 | 2388篇 |
1976年 | 2436篇 |
1975年 | 2360篇 |
1974年 | 2327篇 |
1973年 | 2504篇 |
1972年 | 1623篇 |
1971年 | 1337篇 |
排序方式: 共有10000条查询结果,搜索用时 718 毫秒
841.
842.
A differential equation is obtained that describes the change in the energy of a charged particle in the field of a slow wave (with phase velocity not equal to unity) as a function of the distance traversed. Analysis of this equation yields the amplitude and period of the change in the particle's energy.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 2, pp. 97–102, February, 1981. 相似文献
843.
844.
845.
A. A. Karabutov V. A. Larichev G. A. Maksimov I. M. Pelivanov N. B. Podymova 《Acoustical Physics》2006,52(5):582-588
The first experimental observation of the propagation dynamics of a short broadband acoustic pulse in a resonance medium with gas bubbles is carried out. The probing pulse is generated using the optoacoustic effect. It is shown that the theory of short pulse propagation in media with generalized resonance relaxation adequately and accurately describes the dynamics of short pulse dispersion. A possibility to determine the relaxation and resonance parameters of media by the pulsed testing technique is demonstrated. 相似文献
846.
B. V. Budaev 《Journal of Mathematical Sciences》1991,57(3):3087-3092
The problem of diffraction of a plane elastic wave by a free wedge is considered. A method of reduction of the problem to a singular integral equation is proposed.Translated from Zapiski Nauchnykh Seminarov Leningradskogo Otdeleniya Matematicheskogo Instituta im. V. A. Steklova, Vol. 179, pp. 37–45, 1989. 相似文献
847.
Despite its capability of high spatial resolution, simulation of turbulent flows with traditional Lagrangian (front tracking) scheme is often discouraged by numerical instability caused by clustering of marker nodes and topological changes of fronts. Contour advection surgery (CAS), being a robust front tracking scheme, can limit the growth of front complexity during simulation without jeopardizing accuracy or efficiency. This allows it to open up an advantage over traditional front-tracking schemes. It has already been demonstrated that CAS, with incorporation of the reaction sheet model, can accurately simulate the propagation and advection of a turbulent premixed V-shaped flame. In this study, it is further tested with 10 values of vortex circulation. A range of upstream turbulence levels of 1.8–19.8% was obtained. Results indicate that upstream turbulence increase the average flame length, flame zone area and the overall burning rate. Flame surface density Σ was also estimated. Maximum values of Σ obtained lie in the range 0.1–1.4 mm−1. Skewness towards the burnt region was observed in all profiles of Σ. Similar to results from laboratory experiments, it was found that Σ values decreases with upstream turbulence. From this study, the ability of CAS to cope with intense turbulence is demonstrated and a better quantitative understanding on the scheme has also been acquired. 相似文献
848.
It is shown that the small-scale Rossby waves in a shallow rotating fluid placed in a gravitational field can generate large-scale
zonal flows by means of a two-stream-like mechanism. This mechanism is revealed under the conditions when the Lighthill instability
criterion is not satisfied.
The text was submitted by the authors in English. 相似文献
849.
Cr-doped mullites were prepared from single-phase precursors containing up to 9.60 wt% Cr2O3 using a sol-gel technique followed by thermal treatment. Particle induced X-ray emission spectroscopy and X-ray powder diffraction were used to characterize the samples. Mullites were orthorhombic, space group Pbam. Cr doping caused the increase of unit-cell parameters. Strongest expansion was noticed along c-axis followed by a and b (Δc/c=0.089, Δa/a=0.061, Δb/b=0.045% per mole Cr2O3). A second phase, namely θ-(Al,Cr)2O3, was revealed by XRD in the sample containing 9.60 wt% Cr2O3. The structure of mullites was refined by the Rietveld method, location of Cr3+ was performed by the EPR spectroscopy. At low chromium doping level (Cr2O3 content less than ∼5 wt%) Cr3+ ions were substituted for Al3+ in the AlO6 octahedra of the mullite structure (M1 site). For higher doping level, Cr3+ ions were additionally substituted for Al3+ in the AlO6 octahedra of the second phase [θ-(Al,Cr)2O3 at 1400 °C, or α-(Al,Cr)2O3 at 1600 °C] which segregated in the system. Substitution of Cr3+ for Al3+ on M1 site in the mullite structure resulted in increase of average distances in (M1)O6 octahedron and decrease of average distances in T*O4 tetrahedron, while average distances in TO4 tetrahedron stayed almost constant. 相似文献
850.
B. M. Zhirnov 《International Applied Mechanics》2002,38(4):507-512
Within the framework of the first asymptotic approximation, a dynamic analysis is made of a frictional mechanical self-oscillating system with three degrees of freedom excited by a weak periodic force near the resonance. 相似文献