全文获取类型
收费全文 | 535541篇 |
免费 | 7151篇 |
国内免费 | 1788篇 |
专业分类
电工技术 | 9486篇 |
综合类 | 2678篇 |
化学工业 | 77617篇 |
金属工艺 | 25273篇 |
机械仪表 | 20368篇 |
建筑科学 | 11796篇 |
矿业工程 | 3241篇 |
能源动力 | 13411篇 |
轻工业 | 33084篇 |
水利工程 | 6041篇 |
石油天然气 | 9167篇 |
武器工业 | 17篇 |
无线电 | 62420篇 |
一般工业技术 | 111066篇 |
冶金工业 | 83215篇 |
原子能技术 | 10143篇 |
自动化技术 | 65457篇 |
出版年
2022年 | 2884篇 |
2021年 | 4396篇 |
2020年 | 3295篇 |
2019年 | 4166篇 |
2018年 | 24339篇 |
2017年 | 23735篇 |
2016年 | 18756篇 |
2015年 | 5578篇 |
2014年 | 7933篇 |
2013年 | 21230篇 |
2012年 | 16480篇 |
2011年 | 29495篇 |
2010年 | 24959篇 |
2009年 | 23587篇 |
2008年 | 24743篇 |
2007年 | 26595篇 |
2006年 | 13110篇 |
2005年 | 15064篇 |
2004年 | 13038篇 |
2003年 | 12809篇 |
2002年 | 11062篇 |
2001年 | 10054篇 |
2000年 | 9556篇 |
1999年 | 9803篇 |
1998年 | 24777篇 |
1997年 | 17012篇 |
1996年 | 13062篇 |
1995年 | 9745篇 |
1994年 | 8584篇 |
1993年 | 8588篇 |
1992年 | 6201篇 |
1991年 | 5818篇 |
1990年 | 5800篇 |
1989年 | 5442篇 |
1988年 | 5133篇 |
1987年 | 4533篇 |
1986年 | 4409篇 |
1985年 | 4929篇 |
1984年 | 4523篇 |
1983年 | 4078篇 |
1982年 | 3755篇 |
1981年 | 3828篇 |
1980年 | 3507篇 |
1979年 | 3402篇 |
1978年 | 3395篇 |
1977年 | 3841篇 |
1976年 | 4955篇 |
1975年 | 2903篇 |
1974年 | 2708篇 |
1973年 | 2771篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
931.
Less may not be more,but it still counts: The state of social capital in Yeoville,Johannesburg 总被引:2,自引:2,他引:0
Kirsten Harrison 《Urban Forum》2002,13(1):67-84
932.
The stability of the reactive interface during the solid-state displacement reaction, Cu2O+Co1−X
Fe
X
=2Cu+(Co1−X
Fe
X
)O, is studied as a function of Co-Fe alloy composition at 1223 K. For X≤0.03, the reaction zone has a layered structure, and the cation diffusion in (Co, Fe)O is the rate-limiting step. The interface
is unstable in the early stages of the reaction; the instability decreases with time as the oxide thickness increases, and
the interface becomes planar at long times. The time required for the attainment of interface planarity increases with the
value of X. The reaction kinetics are consistent with the available cation-diffusion data in (Co, Fe)O. For X≥0.045, the product zone is a composite of Cu+(Co, Fe)O, and the rate is limited by the oxygen transport in copper. The transition
to interface instability occurs when the oxide can support a cation flux that exceeds the maximum possible oxygen flux in
copper. During the reaction, composition gradients develop in (Co, Fe)O because of higher diffusion rates for iron than for
cobalt. 相似文献
933.
Chen H.H. Huang T.S. 《IEEE transactions on pattern analysis and machine intelligence》1991,13(9):872-878
The specific problem addressed by the authors is how to detect the true match of a fourth point from among candidate matches in a situation in which three points have already been matched. The two sets of points to be matched are both subject to measurement errors. The depth error is more dominant than errors in the other two coordinates; however, the exact statistical distribution of the measurement errors is not known. The authors present a new method for solving the problem. The method is based on the technique of motion analysis using orthographic views. It discards the noisy z (depth) coordinates and uses only the x and y coordinates of the points to verify the match. The effect of depth errors on the motion estimate is completely prevented. Results show that this method is substantially more effective than previous methods that use all three coordinates 相似文献
934.
On the existence and synthesis of multifinger positive grips 总被引:5,自引:0,他引:5
We study the criteria under which an object can be gripped by a multifingered dexterous hand, assuming no static friction between the object and the fingers; such grips are calledpositive grips. We study three cases in detail: (i) the body is at equilibrium, (ii) the body is under some constant external force/torque, and (iii) the body is under a varying external force/torque. In each case we obtain tight bounds on the number of fingers needed to obtain grip.We also present efficient algorithms to synthesize such positive grips for bounded polyhedral/polygonal objects; the number of fingers employed in the grips synthesized by our algorithms match the above bounds. The algorithms run in time linear in the number of faces/sides.The paper may be of independent interest for its presentation of algorithms arising in the study of positive linear spaces.Work on this paper has been supported by Office of Naval Research Grant N00014-82-K.-0381, National Science Foundation Grant No. NSF-DCR-83-20085, and by grants from the Digital Equipment Corporation, and the IBM Corporation. 相似文献
935.
A study is presented of the symptoms of the devices in a slice of PNP power transistors with a poor yield. The failure modes over the slice are analysed. In specific areas the electrical characteristics and optical appearance of the devices were studied and failure statistics are presented. The low yield is ascribed to cracks in the phosphosilicate glass used for the base diffusion. The effect of these cracks is just visible in the optical and scanning electron microscope. 相似文献
936.
On the basis of generalization of the results of investigation of the operation of rodlike elements of composite materials, we make a conclusion that the influence of various factors on the deformability of these elements is studied incompletely. The comparison of the stress-strain states for different models of concrete and reinforced-concrete elements makes it possible to describe the effect of longitudinal reinforcement (coefficient of reinforcement, strength and deformation characteristics) on the critical level of strains in the cross section. According to the results of our investigations, we give recommendations concerning the evaluation of the critical value of the coefficient of reinforcement above which one may observe the effect of reinforcement on the deformability of the analyzed sections. It is shown that the compression strength of the brittle base of the composite with inhomogeneous structure decreases as a result of the fracture processes. It is also demonstrated that the existing procedure used for the evaluation of the indicated decrease in strength requires significant improvement. The problem of evaluation of the ultimate value of the coefficient of reinforcement is also analyzed. 相似文献
937.
Using the SiO2 and Al2O3 components of the amorphous phase in coal fly ash (Fa), Fa was converted to Na-X zeolites in NaOH-NaAlO2 solutions by stirring at 35°C for 72 hr and then aging at 85°C for a given period. The molar ratio SiO2/Al2O3 of the starting materials was controlled from 2.0 to 13.2. The resulting materials were characterized by various means. Increasing the SiO2/Al2O3 ratio of the starting material increased the degree of crystallinity of faujasite, exhibiting a maximum at SiO2/Al2O3 = 8.0. The faujasite formed was identified as Na-X zeolite with Si/Al = 1.20. The amorphous phase in Fa was dissolved during the stirring to form a precursor of zeolite, such as amorphous aluminosilicate. The Na-X zeolite was formed by aging for 24 hr, and the degree of crystallinity of this material was increased with the increasing aging period. The cation exchange capacity and specific surface area were increased with the increasing degree of crystallinity of the Na-X zeolites. 相似文献
938.
The crystal structure of [NpO2{OC(NH2)2}5]ClO4·H2O and [NpO2Cl{OC(NH2)2}4] was studied by X-ray diffraction. The electronic absorption spectra of these complexes were recorded. The coordination polyhedron of Np in both complexes is a distorted pentagonal bipyramid. There is no cation-cation interaction of neptunyl cations in these complexes. 相似文献
939.
Numerical simulation of Zn coating solidification 总被引:2,自引:0,他引:2
A. Sémoroz Ph.D L. Strezov M. Rappaz 《Metallurgical and Materials Transactions A》2002,33(8):2685-2694
A numerical model, which simulates nucleation and growth of Zn grains, has been developed in order to describe quantitatively
the solidification of Zn coatings during the hot-dipping process. The inputs of the model are the nucleation distribution,
which has been measured by electron backscattered diffraction (EBSD), and the dendritic growth kinetics, calculated with an
analytical model of a parabolic dendrite tip modified to account for the interactions with the coating interfaces. The model
predicts the shapes of the grain envelopes as a function of the grain orientation and the texture induced by growth. Three
types of grain envelopes have been evidenced, depending on the angle between the c-axis and the normal to the coating plane.
Moreover, it has been shown that growth reinforces the already existing {00.1} nucleation texture, in good agreement with
experimental data. The model also predicts the cooling curve, including recalescence, and the grain size. Thus, it is used
to describe the effects of Pb additions on solidification. In particular, it has been shown that Pb increases the nucleation
undercooling and strongly decreases the density of active nuclei, thus resulting in a much larger grain size. 相似文献
940.
On the basis of the experimental data, we show that the difference between the macroscopic and microscopic fatigue crack growth rates in the second section of the kinetic diagram of fatigue fracture is caused by the effect of crack closure within the limits of its existence. We establish the relationships between the macroscopic and microscopic fatigue crack growth rates and the structure of the material in the second section of the diagram for various values of the load ratio with regard for the effect of crack closure and propose a procedure of examination of the fracture processes in structural materials based on the analysis of microscopic and macroscopic fatigue crack growth rates. 相似文献