全文获取类型
收费全文 | 207553篇 |
免费 | 2510篇 |
国内免费 | 687篇 |
专业分类
化学 | 109213篇 |
晶体学 | 2930篇 |
力学 | 9626篇 |
综合类 | 17篇 |
数学 | 20201篇 |
物理学 | 68763篇 |
出版年
2021年 | 1467篇 |
2020年 | 1572篇 |
2019年 | 1608篇 |
2018年 | 2017篇 |
2017年 | 1918篇 |
2016年 | 3361篇 |
2015年 | 2178篇 |
2014年 | 3418篇 |
2013年 | 8715篇 |
2012年 | 7195篇 |
2011年 | 9063篇 |
2010年 | 6220篇 |
2009年 | 6303篇 |
2008年 | 8293篇 |
2007年 | 8210篇 |
2006年 | 7697篇 |
2005年 | 7007篇 |
2004年 | 6257篇 |
2003年 | 5531篇 |
2002年 | 5417篇 |
2001年 | 7023篇 |
2000年 | 5267篇 |
1999年 | 3999篇 |
1998年 | 3240篇 |
1997年 | 3148篇 |
1996年 | 2834篇 |
1995年 | 2624篇 |
1994年 | 2540篇 |
1993年 | 2330篇 |
1992年 | 2882篇 |
1991年 | 2992篇 |
1990年 | 2688篇 |
1989年 | 2618篇 |
1988年 | 2532篇 |
1987年 | 2512篇 |
1986年 | 2300篇 |
1985年 | 3046篇 |
1984年 | 3097篇 |
1983年 | 2456篇 |
1982年 | 2591篇 |
1981年 | 2476篇 |
1980年 | 2363篇 |
1979年 | 2610篇 |
1978年 | 2828篇 |
1977年 | 2701篇 |
1976年 | 2652篇 |
1975年 | 2487篇 |
1974年 | 2584篇 |
1973年 | 2488篇 |
1972年 | 1573篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
91.
Uhm H.S. Lee P.H. Kim Y.I. Kim J.H. Chang H.Y. 《IEEE transactions on plasma science. IEEE Nuclear and Plasma Sciences Society》1995,23(4):628-635
A theory is developed for the density profile of low temperature plasmas confined by applied magnetic field and an experiment of the electron-cyclotron-resonance (ECR) plasma is conducted to compare the theoretical prediction and experimental measurements. Due to a large electron mobility along the magnetic field, electrons move quickly out of the system, leaving ions behind and building a space charge potential, which leads to the ambipolar diffusion of ions. In a steady-state condition, the plasma generation by ionization of neutral molecules is in balance with plasma loss due to the diffusion, leading to the electron temperature equation, which is expressed in terms of the plasma size, chamber pressure, and the ionization energy and cross section of neutrals. The power balance condition leads to the plasma density equation, which is also expressed in terms of the electron temperature, the input microwave power and the chamber pressure. It is shown that the plasma density increases, reaches its peak and decreases, as the chamber pressure increases from a small value (0.1 mTorr). These simple expressions of electron temperature and density provide a scaling law of ECR plasma in terms of system parameters. After carrying out an experimental observation, it is concluded that the theoretical predictions of the electron temperature and plasma density agree remarkably well with experimental data 相似文献
92.
NaY zeolite samples loaded with sodium metal by vapor phase deposition have been investigated using129Xe NMR spectroscopy. At low sodium concentration, the129Xe NMR spectrum showed three resonance lines which clearly indicate the existence of distinct domains in the zeolite sample. Such an observation suggests that the diffusion of the xenon atoms into each domain only occurs with respect to the NMR time scale (2.9 ms). As the sodium concentration increases, observation of a single broad line indicate a macroscopic homogenization of the system. The shift of this line is explained in part due to a paramagnetic interaction between the xenon atoms and the unpaired electrons of particles containing an odd number of sodium atoms. The linewidth is due to the distribution of the local magnetic fields partially averaged by the rapid motion of the xenon atoms and to the statistical distribution of the sodium particles in the supercage cavities. The paramagnetic interaction vanishes with the oxidation of the sample leading to a narrowing and a shift of the line to higher magnetic fields. 相似文献
93.
In a previous paper (Ref. 1), an exact solution of the optimal planar interception with fixed end conditions was derived in closed form. The optimal control was expressed as an explicit function of the state variables and two fixed parameters, obtained by solving a set of nonlinear algebraic equations involving elliptic integrals. In order to facilitate the optimal control implementation, the present paper derives a highly accurate simplified solution assuming that the ratio of the pursuer turning radius to the initial range is small. An asymptotic expansion further reduces the computational workload. Construction of a near-optimal open-loop control, based on the approximations, completes the present paper. 相似文献
94.
Ya. V. Fattakhov M. F. Galyautdinov T. N. L’vova I. B. Khaibullin 《Technical Physics》1997,42(12):1457-1459
The first results regarding the formation of a two-dimensional periodic structure of local melting regions on a silicon surface
upon pulsed light irradiation are presented. The conditions are established, and the mechanism of the formation of such structures
is discussed.
Zh. Tekh. Fiz. 67, 97–99 (December 1997) 相似文献
95.
F. A. van Goor S. V. Mitko V. N. Ochkin W. J. Witteman 《Journal of Russian Laser Research》1997,18(2):107-120
Experimental and theoretical results are presented on an XeF(C?A) blue-green laser driven by 5-kJ energy. The laser was pumped by a ferrite-induced discharge of 90 cm in length. The output energy of 0.22 J was obtained with a plane-parallel resonator. A program to simulate laser operation has been developed. Numerical results for a wide range of conditions are compared with experiments performed by us and by other authors. It is found that intracavity refractive losses limit laser operation for XeF pressures above 3 torr. The laser efficiency strongly depends on the discharge-to-cavity length ratio. Possible ways to increase the laser power and efficiency are discussed. 相似文献
96.
Translated from Khimiya Geterotsiklicheskikh Soedinenii. Vol. 30. No. 3, pp. 424–425, March, 1994. 相似文献
97.
V. D. Orlov Z. Kaluski N. P. Borob'eva E. Figas A. A. Tishchenko F. G. Yaremko 《Chemistry of Heterocyclic Compounds》1994,30(8):964-971
We have measured the PMR spectra of 5, 7-diaryl-1, 4-diazabicyclo[4. ].O]hept-4-enes. We have studied the structure of the 5-(4-nitrophenyl)-7-phenyl derivative by x-ray diffraction. We show that the six-membered heterocycle has the half-chair form. The 6-H and 7-H protons of the aziridine ring are trans, while the 7-aryl is exo relative to the six-membered ring. Introduction of substituents into the aromatic ring has practically no effect on the geometry of the bicycle.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 8, pp. 1117–1124, August, 1994.For Communication 11, see [1]. 相似文献
98.
99.
100.
A. N. Komissarenko T. P. Nadezhina N. F. Komissarenko 《Chemistry of Natural Compounds》1994,30(4):521-522
State Scientific Center for Drugs, Khar'kov. Ukrainian Pharmaceutical Academy, Khar'kov. Botanical Gardem, Russian Academy
of Sciences, St. Petersburg. Translated from Khimiya Prirodnykh Soedinenii, No. 4, pp. 564–565, July–August, 1994. 相似文献