全文获取类型
收费全文 | 320522篇 |
免费 | 3153篇 |
国内免费 | 1012篇 |
专业分类
化学 | 149317篇 |
晶体学 | 4805篇 |
力学 | 11553篇 |
综合类 | 7篇 |
数学 | 30899篇 |
物理学 | 87369篇 |
无线电 | 40737篇 |
出版年
2019年 | 2316篇 |
2018年 | 2759篇 |
2017年 | 2790篇 |
2016年 | 4661篇 |
2015年 | 2951篇 |
2014年 | 4671篇 |
2013年 | 12689篇 |
2012年 | 9185篇 |
2011年 | 11256篇 |
2010年 | 8058篇 |
2009年 | 8263篇 |
2008年 | 11215篇 |
2007年 | 11527篇 |
2006年 | 11144篇 |
2005年 | 10474篇 |
2004年 | 9687篇 |
2003年 | 8770篇 |
2002年 | 8618篇 |
2001年 | 9616篇 |
2000年 | 7634篇 |
1999年 | 6257篇 |
1998年 | 5474篇 |
1997年 | 5328篇 |
1996年 | 5098篇 |
1995年 | 4883篇 |
1994年 | 4639篇 |
1993年 | 4584篇 |
1992年 | 5018篇 |
1991年 | 4913篇 |
1990年 | 4732篇 |
1989年 | 4590篇 |
1988年 | 4490篇 |
1987年 | 4068篇 |
1986年 | 3786篇 |
1985年 | 4962篇 |
1984年 | 5149篇 |
1983年 | 4427篇 |
1982年 | 4772篇 |
1981年 | 4412篇 |
1980年 | 4462篇 |
1979年 | 4547篇 |
1978年 | 4668篇 |
1977年 | 4546篇 |
1976年 | 4617篇 |
1975年 | 4417篇 |
1974年 | 4271篇 |
1973年 | 4675篇 |
1972年 | 2981篇 |
1971年 | 2433篇 |
1968年 | 2370篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
Of all the products of electrical technology, none has made a more profound difference to our way of life than the humble light bulb. The author describes the history of the light bulb from the time it was introduced and started to replace gas lamps to the present day. In particular the author discusses the early developments in arc lighting and incandescent filament lamps. The author also discusses the introduction of mercury vapour lamps, fluorescent lamps, and evacuation of the lamps 相似文献
92.
Modeling and numerical simulations of the convective flows induced by the vibration of the monocrystal during crystal growth have been performed for two configurations simulating the Cz and FZ methods. This permitted to emphasize the role of different vibrational mechanisms in the formation of the average flows. It is shown that an appropriate combination of these mechanisms can be used to counteract the usual convective flows (buoyancy- and/or thermocapillary-driven) inherent to crystal growth processes from the liquid phase. While vibrational convection is rather complex due to these identified mechanisms, the new modeling used in the present paper opens up very promising perspectives to efficiently control heat and mass transfer during real industrial applications of crystal growth from the liquid phase. 相似文献
93.
94.
Punt J.B. Sparreboom D. Brouwer F. Prasad R. 《Vehicular Technology, IEEE Transactions on》1998,47(4):1302-1313
Dynamic channel selection (DCS) is an algorithm for flexible resource sharing in mobile radio systems. The digital enhanced cordless telecommunications (DECT) standard implements a version of DCS based on time as well as frequency multiplexing. In this paper, mathematical models are developed to evaluate the probabilities of channel availability, desired carrier power, and the carrier-to-interference ratio (CIR) for a constant traffic load. These results can be used to compute the call setup blocking probability. The models reported in this study are based on a decentralized DCS according to the DECT standard specifications. The results show that blocking due to availability of a channel (resource blocking), not interference blocking, generally is the most important factor in overall call blocking. Furthermore, it is seen that resource blocking is sensitive to the implementation of portable sets. Also, with homogeneous static traffic, this type of DCS ran be as good as fixed channel allocation (FCA) with respect to resource blocking 相似文献
95.
A. J. Kalkman H. P. M. Pellemans T. O. Klaassen W. Th. Wenckebach 《Journal of Infrared, Millimeter and Terahertz Waves》1996,17(3):569-577
The low temperature lifetime of electrons excited in the 2p?1 donor level of n-GaAs has been studied in a far-infrared pump-probe experiment. The measurement has been carried out using a pulsed far-infrared molecular gas laser working at a wavelength of 292µm, with the sample in a magnetic field of 5.1 T, resonant with the 1so?2p?1 transition. Two FIR pulses are sliced from one FIR-laser pulse by means of optical switching techniques using two Q-switched Nd:YAG lasers. The first pulse is used to saturate the transition, while the second pulse probes the return of the population in the excited state towards thermal equilibrium as a function of the time delay after the excitation pulse. The value of 350±50 ns found for the lifetime falls in line with CW saturation results on materials with other doping concentrations. 相似文献
96.
M. Trunk J. Popp I. Hartmann M. Lankers E. Urlaub W. Kiefer 《Fresenius' Journal of Analytical Chemistry》1996,355(3-4):354-356
In situ measurements of gas-liquid surface reactions of single aerosol microdroplets are presented. By means of optical levitation in combination with elastic (Mie) and inelastic (Raman) light scattering it is possible to get information on the chemistry of e.g. acid/base reactions as well as the physical behavior of single microparticles. 相似文献
97.
Arafa M. Fay P. Ismail K. Chu J.O. Meyerson B.S. Adesida I. 《Electron Device Letters, IEEE》1996,17(3):124-126
We report on the fabrication and characterization of high-speed p-type modulation-doped field-effect transistors (MODFETs) with 0.7-μm and 1-μm gate-lengths having unity current-gain cut-off frequencies (fT) of 9.5 GHz and 5.3 GHz, respectively. The devices were fabricated on a high hole mobility SiGe heterostructure grown by ultra-high-vacuum chemical vapor deposition (UHV-CVD). The dc maximum extrinsic transconductance (gm) is 105 mS/mm (205 mS/mm) at room temperature (77 K) for the 0.7-μm gate length devices. The fabricated devices show good pinch-off characteristics and have a very low gate leakage current of a few μA/mm at room temperature and a few nA/mm at 77 K 相似文献
98.
Yu. B. Orochko 《Ukrainian Mathematical Journal》1992,44(7):850-856
A sufficient Hartman-Ismagilov type condition for the essential self-adjointness of a one-parameter family of unbounded operators that arise in the solution of a Cauchy problem for the wave equation is established. An analog of this result is stated for unbounded integral operators.Translated from Ukrainskii Matematicheskii Zhurnal, Vol. 44, No. 7, pp. 940–948, July, 1992. 相似文献
99.
B. M. Gurevich 《Theoretical and Mathematical Physics》1992,90(3):289-312
The Liouville operator for an infinite-particle Hamiltonian dynamics corresponding to interaction potentialU is used to introduce the concept of a locally weakly invariant measure on the phase space and to show that if a Gibbs measure with potential of general form is locally weakly invariant then its Hamiltonian is asymptotically an additive integral of the motion of the particles with the interactionU.Moscow State University. Translated from Teoreticheskaya i Matematicheskaya Fizika, Vol. 90, No. 3, pp. 424–459, March, 1992. 相似文献
100.
B. Bavassano 《Il Nuovo Cimento C》1992,15(5):501-508
Summary A study has been promoted by NASA to evaluate the scientific return and the technological problems of a close-to-the-Sun mission
(Solar Probe). It has come out that using current technology it is possible to deliver at 4 solar radii from the Sun's centre
a scientific payload with such performances to allow a great improvement of our understanding of the coronal phenomena. The
mission concept currently under study uses planetary encounters to reach the required orbit. The final orbit period is 2.5
years, the estimated mission duration about 9 years. The orbit inclination is such as to give the maximum coverage in terms
of heliographic latitude. During the perihelion phases a conical structure shields the spacecraft from the intense solar radiation.
Paper presented at the V Cosmic Physics National Conference, S. Miniato, November 27–30, 1990. 相似文献