全文获取类型
收费全文 | 193834篇 |
免费 | 1010篇 |
国内免费 | 275篇 |
专业分类
化学 | 89573篇 |
晶体学 | 2635篇 |
力学 | 9377篇 |
综合类 | 7篇 |
数学 | 20912篇 |
物理学 | 58127篇 |
无线电 | 14488篇 |
出版年
2020年 | 2050篇 |
2019年 | 2566篇 |
2018年 | 3690篇 |
2017年 | 3799篇 |
2016年 | 4485篇 |
2015年 | 1951篇 |
2014年 | 3864篇 |
2013年 | 7453篇 |
2012年 | 5856篇 |
2011年 | 6979篇 |
2010年 | 5892篇 |
2009年 | 6144篇 |
2008年 | 6917篇 |
2007年 | 6993篇 |
2006年 | 6232篇 |
2005年 | 5566篇 |
2004年 | 5401篇 |
2003年 | 4938篇 |
2002年 | 5068篇 |
2001年 | 5508篇 |
2000年 | 4243篇 |
1999年 | 3305篇 |
1998年 | 3050篇 |
1997年 | 2874篇 |
1996年 | 2566篇 |
1995年 | 2347篇 |
1994年 | 2328篇 |
1993年 | 2333篇 |
1992年 | 2329篇 |
1991年 | 2625篇 |
1990年 | 2620篇 |
1989年 | 2638篇 |
1988年 | 2509篇 |
1987年 | 2438篇 |
1986年 | 2282篇 |
1985年 | 2802篇 |
1984年 | 2797篇 |
1983年 | 2424篇 |
1982年 | 2447篇 |
1981年 | 2355篇 |
1980年 | 2142篇 |
1979年 | 2455篇 |
1978年 | 2598篇 |
1977年 | 2687篇 |
1976年 | 2780篇 |
1975年 | 2592篇 |
1974年 | 2514篇 |
1973年 | 2682篇 |
1972年 | 2186篇 |
1971年 | 2004篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
971.
A new and simple compensated structure for a vertically installed planar (VIP) 3 dB directional coupler has been studied theoretically as well as experimentally by combining an improved 2-D-general finite difference (2-D-GFD) design procedure with a three-dimensional finite difference time domain (3-D-FDTD) method. The obtained full wave analysis results agree well with the measured ones. The investigations have shown that with this planar compensated structure, a better performance of the VIP coupler in the L-band can be realized by only using the same kind of dielectric substrate for its vertical and horizontal one 相似文献
972.
Polarisation insensitive semiconductor optical amplifier withintegrated electroabsorption modulators
Koren U. Miller B.I. Young M.G. Chien M. Raybon G. Brenner T. Ben-Michael R. Dreyer K. Capik R.J. 《Electronics letters》1996,32(2):111-112
The authors demonstrate a 1.55 μm wavelength multiquantum well semiconductor optical amplifier, integrated with bulk layer electroabsorption modulators and passive waveguide beam expanders at the input and output ports. The device has a fibre to fibre gain of 9 dB, an extinction ratio of 15 dB per modulator, a spectral range >35 nm, and polarisation sensitivity <1 dB 相似文献
973.
The problem with unknown boundaries for a first-order semilinear hyperbolic system is studied in the case where the curve of definition of the initial conditions degenerates to a point. An existence and uniqueness theorem for a classical solution of the problem is proved for small t. 相似文献
974.
Wireless ATM networks require efficient mobility management to cope with frequent mobile handoff and rerouting of connections. Although much attention has been given in the literature to network architecture design to support wide-area mobility in public ATM networks, little has been done to the important issue of user mobility estimation and prediction to improve the connection reliability and bandwidth efficiency of the underlying system architecture. This paper treats the problem by developing a hierarchical user mobility model that closely represents the movement behavior of a mobile user, and that, when used with appropriate pattern matching and Kalman filtering techniques, yields an accurate location prediction algorithm, HLP, or hierarchical location prediction, which provides necessary information for advance resource reservation and advance optimal route establishment in wireless ATM networks 相似文献
975.
The electromigration phenomenon has been one of the most intriguing physical problems in the semiconductor device reliability. The models to explain the phenomenon are here revised, together with the influence of materials and their microstructure. The various measuring techniques are described, including the design of special test patterns, and statistical data analysis is briefly reviewed. 相似文献
976.
V. P. Evtikhiev I. V. Kudryashov E. Yu. Kotel’nikov V. E. Tokranov A. N. Titkov I. S. Tarasov Zh. I. Alferov 《Semiconductors》1998,32(12):1323-1327
The electroluminescence and stimulated emission of lasers with one layer of InAs quantum dots (QD’s) grown in a single molecular-beam
epitaxial process on vicinal GaAs(001) surfaces misoriented in the direction [010] by 2, 4 and 6° are investigated. It is
discovered that an increase in the misorientation angle leads to a blue shift and a decrease in the full width at half maximum
(FWHM) of the electroluminescence spectrum. This effect is attributed to a decrease in the size of the quantum dots and improvement
in their size uniformity. A strong dependence of the threshold current density on the width of the spontaneous luminescence
spectrum is discovered. The room-temperature threshold current density of the lasers with one layer of quantum dots and the
spontaneous luminescence spectrum having the smallest FWHM (54 meV) equals 210 A/cm2.
Fiz. Tekh. Poluprovodn. 32, 1482–1486 (December 1998) 相似文献
977.
978.
A. Sachdev R. I. Masel J. B. Adams 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1993,26(1):310-312
The embedded atom method (EAM) was used to determine the equilibrium shapes of small platinum and palladium clusters (N=5–60 atoms). The stability of various polyhedral symmetries was performed at 0K. A search for the presence of any other lower energy structures was also carried out. Pt clusters reconstructed to lower energy clusters even at the magic numbers. Pd also reconstructed to lower energy structures except at the magic number sizes where the icosahedron was found to be the most stable. 相似文献
979.
This Review discusses the synthesis and characterization by our Group of new antibiotics belonging to the class of penicillins, cephalosporins and rifamycins with ferrocenyl and 1, 1′-ferrocenilene residues in the molecule. As reactants for 6-aminopenicillanic acid (6-APA) and 7-aminocephalosporanic acid (7-ACA) the following were used: 1, 1-bis(chlorocarbonyl)ferrocene, ferrocenyl sulfochloride, 1, 1′-ferrocenylenedisulfochloride and thioglycolic acids S-modified with ferrocene. In the synthesis of rifamycins, the hydrazides of the thioglycolic acids, S-modified with ferrocene, were employed as nucleophilic agents. The synthesized intermediates were characterized by elemental analysis, TLC, IR, UV and 1H NMR spectra. The characterization of new antibiotics was made by TLC, IR and UV spectral analysis. Biological activity was tested on Gram-negative and Gram-positive bacteria. Good activity is reported towards Gram-positive bacteria in the case of derivatives containing residues of thioglycolic acid S-modified with ferrocene, the antibacterial activity being similar to that of amoxicillin, carbenicillin and cephalothin. All compounds are inactive towards Gram-negative bacteria. 相似文献
980.
In this paper, we construct an integrator that converves volume in phase space. We compare the results obtained using this method and a symplectic integrator. The results of our experiments do not reveal any superiority of the symplectic over strictly volume-preserving integrators. We also investigate the effect of numerically conserving energy in a numerical process by rescaling velocities to keep energy constant at every step. Our results for Henon-Heiles problem show that keeping energy constant in this way destroys ergodicity and forces the solution onto a periodic orbit. 相似文献