全文获取类型
收费全文 | 42240篇 |
免费 | 614篇 |
国内免费 | 797篇 |
专业分类
电工技术 | 728篇 |
综合类 | 55篇 |
化学工业 | 6385篇 |
金属工艺 | 3110篇 |
机械仪表 | 1392篇 |
建筑科学 | 869篇 |
矿业工程 | 153篇 |
能源动力 | 1432篇 |
轻工业 | 2274篇 |
水利工程 | 358篇 |
石油天然气 | 1403篇 |
武器工业 | 1篇 |
无线电 | 3993篇 |
一般工业技术 | 10674篇 |
冶金工业 | 7219篇 |
原子能技术 | 792篇 |
自动化技术 | 2813篇 |
出版年
2021年 | 354篇 |
2020年 | 288篇 |
2019年 | 380篇 |
2018年 | 672篇 |
2017年 | 658篇 |
2016年 | 749篇 |
2015年 | 541篇 |
2014年 | 813篇 |
2013年 | 2390篇 |
2012年 | 1409篇 |
2011年 | 1890篇 |
2010年 | 1497篇 |
2009年 | 1786篇 |
2008年 | 1632篇 |
2007年 | 1776篇 |
2006年 | 1408篇 |
2005年 | 1230篇 |
2004年 | 1181篇 |
2003年 | 1084篇 |
2002年 | 1070篇 |
2001年 | 1120篇 |
2000年 | 965篇 |
1999年 | 1000篇 |
1998年 | 2118篇 |
1997年 | 1662篇 |
1996年 | 1443篇 |
1995年 | 985篇 |
1994年 | 758篇 |
1993年 | 775篇 |
1992年 | 597篇 |
1991年 | 572篇 |
1990年 | 524篇 |
1989年 | 485篇 |
1988年 | 369篇 |
1987年 | 402篇 |
1986年 | 364篇 |
1985年 | 381篇 |
1984年 | 305篇 |
1983年 | 300篇 |
1982年 | 301篇 |
1981年 | 296篇 |
1980年 | 349篇 |
1979年 | 341篇 |
1978年 | 294篇 |
1977年 | 415篇 |
1976年 | 629篇 |
1975年 | 308篇 |
1974年 | 292篇 |
1973年 | 311篇 |
1972年 | 272篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
992.
A display hologram of an object can be recorded and reconstructed in three primary colors if the angular selectivity of volume recording media is exploited. Three holograms are recorded in the same medium, each at a different primary color. These three holograms are reconstructed by simultaneous illumination of the hologram with the original reference beams. By proper choice of the angles that the reference beams make to the hologram, it is possible to suppress strongly cross talk between the different reconstructions (e.g., the red object reconstruction in green light). The technique exhibits high resolution, high diffraction efficiency, and vivid colors. Through the addition of three holographically recorded volume gratings it is possible to reconstruct the hologram with a beam of white light. The saturation and brightness of each primary color in the reconstruction can be adjusted by selection of an appropriate thickness for the corresponding grating. 相似文献
993.
We report a simple scheme for sensitive measurements of optical-noise spectra. Optical noise is separated from electronic noise when the output of an analog spectrum analyzer is real-time squared and then lock-in detected. This method directly yields the desired mean-square noise voltage, i.e., the power spectrum of the optical noise on a linear scale. To demonstrate this technique, the mean-square shot noise of a laser beam is measured and found to vary linearly with the laser power from several milliwatts down to one microwatt, in excellent quantitative agreement with predictions. 相似文献
994.
A scheme for the design of diffractive phase elements (DPE's) that integrates several optical functions is presented in a consistent sense based on the general theory of amplitude-phase retrieval and the Yang-Gu algorithm [Appl. Opt. 33, 209 (1994)]. We extend the original Yang-Gu algorithm to treat a system illuminated by a beam of incident light whose components are at different wavelengths, and a set of equations for determining the phase distribution of the DPE is derived. The profile of a surface-relief DPE can be designed with an iterative algorithm. Numerical simulations are carried out for the design of one-dimensional DPE's capable of both demultiplexing different wavelength components and focusing each partial wave at predetermined positions. The influence of the extension of sampling points in the DPE's from ideal geometric points to physical spots on design results is also investigated. The numerical simulation results show that the new algorithm can be used successfully to design the desired DPE's. It is therefore expected to be useful in the design of DPE's for micro-optical systems. 相似文献
995.
A polynomial expansion is suggested for achieving optical invariant pattern recognition. The expansion results in a real function and thus is theoretically able to be implemented under both coherent and spatially incoherent illumination. One obtains the expansion after applying the Gram-Schmidt algorithm on the Laurent's series in order to achieve orthonormality. The initial Laurent term with which we apply the Gram-Schmidt procedure is chosen according to the desired expansion order. The use of the polynomial expansion is demonstrated for shift- and one-dimensional scale-invariant pattern recognition as well as for shift-and two-dimensional scale-invariant recognition. 相似文献
996.
A coplanar refractive-diffractive doublet array employing surface-relief diffractive phase elements embedded within poly(methyl methacrylate) microlenses is introduced as an optomechanical building block for optoelectronic integrated systems. The design method, fabrication technology, and results are described. Coplanarity of the quadratic- and linear-phase elements constituting the doublet can reduce optomechanical complexity in applications to unguided optical interconnects. 相似文献
997.
The stress-plastic strain curve is regarded as a result of the simultaneous action of two processes: hardening occurring during instantaneous loading and softening which depends on temporal parameters. A physical interpretation of the piecewise-parabolic dependence for the stress—plastic strain curve is given. It was established experimentally that the instantaneous stress-plastic strain curve of a number of annealed polycrystals (copper, brass, duralumin, steel) regardless of the degree of plastic strain or type of deformation substructure can be described by a single parabolic function called the astructural stress-strain curve. The formation of the substructure provides realization of softening processes, the rate of which depends on the type of substructure. A criterion of transition from one parabolic segment of the stress-strain curve to another and, accordingly, change of the deformation substructures is formulated as the condition of maximum stress relaxation during deformation.Translated from Problemy Prochnosti, No. 10, pp. 18–33, October, 1995. 相似文献
998.
Cyclic deformations of annealed pure polycrystalline -iron with and without further mutually immiscible silver-ion implantation (90 keV, 6×1016 ions cm–2) were studied in a plastic strain-controlled tension-compression fatigue test (triangular loading waveform, frequency 0.02–0.3 Hz, and plastic strain range 3×10–3–1.2×10–2). The obtained plastic strain-life (p-N
f) curves showed that the iron specimens could survive for a greater number of cycles before failure when implanted. Comparison of the cyclic stress-strain curves suggested that the implanted specimens had maintained a relatively more stable microstructural change than those unimplanted ones which had undergone a violent cyclic hardening during cyclic deformation. This is proposed to be a strong indication that the fatigue ductility has been improved and the cross slip of screw dislocations, which leads to the evolution of the persistent slip bands for fatigue damage, was hindered to some extent after ion implantation. 相似文献
999.
Measurements have been made at room temperature of the strength and fracture toughness of a complex nickel silicide containing particles of a ductile phase. The matrix was either a single-phase L12 distorted Ni3Si (2) or a two-phase Ni3Si (1) and Ni31Si12 () mixture. The particles were a solid solution of Ni(Si) encased within a rim of 1; they contained a dispersion of 1 precipitates. The principal variable was the particle size, and this had little effect on the properties. The results show that the compressive yield strength ( 1200 MPa), the compressive ductility (2–10%) and the fracture toughness (17 ± 3 MPa m1/2) are relatively high. The toughness and ductility are attributed to the plastic deformation and ductile fracture of the crack-stopping Ni(Si) particles. 相似文献
1000.
Z. V. Djordjevic X. Feng Li Won Soo Shin S. L. Wunder G. R. Baran 《Journal of Materials Science》1995,30(11):2968-2980
The branching patterns formed as a result of crack growth in dimethacrylate resins below their glass transition temperatures looked similar to fractal trees. The skeletons of the patterns were analysed numerically for their topological and geometrical properties. The number of branches, N
i
, mean branch lengths, N
i
, and branch angles of a particular order, defined according to the Strahler and inverted Weibel schemes, followed exponential scaling behaviour: N
i
(R
b
)–i
and L
i
(R
l
)
i
. Using the relationship for the fractal dimension D=In R
B
/In R
L
, a value of D=1.4 was obtained for the fracture pattern. Fractal behaviour was also examined by the box-counting method which indicated a power-law dependence of the mass on the box size with fractal dimension exponent D=1.4 in the case of the fracture pattern. However, the mass-shell method for both the fracture pattern and the fractal trees gave an exponential increase of mass with distance from the origin, rather than the power-law behaviour expected for fractals. This was attributed to the fact that branches of different sizes were distributed in restricted regions of space closer to the periphery, rather than uniformly over the whole pattern. 相似文献