全文获取类型
收费全文 | 618858篇 |
免费 | 9017篇 |
国内免费 | 2457篇 |
专业分类
电工技术 | 11186篇 |
综合类 | 847篇 |
化学工业 | 92081篇 |
金属工艺 | 22836篇 |
机械仪表 | 18068篇 |
建筑科学 | 16133篇 |
矿业工程 | 2319篇 |
能源动力 | 16065篇 |
轻工业 | 57968篇 |
水利工程 | 5739篇 |
石油天然气 | 7372篇 |
武器工业 | 56篇 |
无线电 | 77492篇 |
一般工业技术 | 118622篇 |
冶金工业 | 115735篇 |
原子能技术 | 11856篇 |
自动化技术 | 55957篇 |
出版年
2021年 | 4363篇 |
2019年 | 4082篇 |
2018年 | 6711篇 |
2017年 | 6853篇 |
2016年 | 7307篇 |
2015年 | 5379篇 |
2014年 | 9013篇 |
2013年 | 28177篇 |
2012年 | 15301篇 |
2011年 | 21748篇 |
2010年 | 16795篇 |
2009年 | 19226篇 |
2008年 | 20333篇 |
2007年 | 20694篇 |
2006年 | 18260篇 |
2005年 | 16939篇 |
2004年 | 16686篇 |
2003年 | 16252篇 |
2002年 | 15683篇 |
2001年 | 15699篇 |
2000年 | 14770篇 |
1999年 | 15482篇 |
1998年 | 36195篇 |
1997年 | 26246篇 |
1996年 | 20371篇 |
1995年 | 15900篇 |
1994年 | 14102篇 |
1993年 | 13831篇 |
1992年 | 10220篇 |
1991年 | 9818篇 |
1990年 | 9367篇 |
1989年 | 9075篇 |
1988年 | 8597篇 |
1987年 | 7507篇 |
1986年 | 7500篇 |
1985年 | 8800篇 |
1984年 | 8196篇 |
1983年 | 7275篇 |
1982年 | 6829篇 |
1981年 | 6829篇 |
1980年 | 6561篇 |
1979年 | 6324篇 |
1978年 | 6054篇 |
1977年 | 7080篇 |
1976年 | 9514篇 |
1975年 | 5146篇 |
1974年 | 4969篇 |
1973年 | 4893篇 |
1972年 | 4051篇 |
1971年 | 3544篇 |
排序方式: 共有10000条查询结果,搜索用时 937 毫秒
991.
Paul J. Flory 《Polymer International》1985,17(2):96-102
The earliest investigations on rubber elasticity, commencing in the 19th century, were necessarily limited to phenomenological interpretations. The realisation that polymers consist of very long molecular chains. commencing c. 1930, gave impetus to the molecular theory of rubber elasticity (1932-). according to which the high deformability of an elastomer, and the elastic force generated by deformation, stem from the configurations accessible to long molecular chains. Theories of rubber elasticity put forward from 1934-1946 relied on the assumption that the junctions of the rubber network undergo displacements that are affine in macroscopic strain. The theory of James and Guth (1947) dispensed with this premise, and demonstrated instead that the mean positions of the junctions of a ‘phantom’ network consisting of Gaussian chains devoid of material properties are affine in the strain. The vital significance of the distinction between the actual distribution of chain vectors in a network and their distribution if the junctions would be fixed at their mean positions went unnoticed for nearly 30 years. Experimental investigations, commencing with the incisive work of Gee in 1946. revealed large departures from the relationship of stress to strain predicted by the theories cited. This discrepancy prompted extensive studies, theoretical and experimental, during succeeding years. Inquiry into the fundamentals of polymer networks, formed for example by interlinking very long polymer molecules, exposed the need to take account of network imperfections, typically consisting of chains attached at only one end to a network junction. Various means were advocated to make corrections for these imperfections. The cycle rank ζ of the network has been shown (1976) to be the fundamental measure of its connectivity, regardless of the junction functionality and pattern of imperfections. Often overlooked is the copious interpenetration of the chains comprising typical elastomeric networks. Theories that attempt to represent such networks on a lattice are incompatible with this universal feature. Moreover, the dense interpenetration of chains may limit the ability of junctions in real networks to accommodate the fluctuations envisaged in the theory of phantom networks. It was suggested in 1975 that departures from the form predicted for the elastic equation of state are due to constraints on the fluctuations of junctions whose effect diminishes with deformation and with dilation. Formulation of a self-consistent theory based on this suggestion required recognition of the non-affine connection between the chain vector distribution function and the macroscopic strain in a real network, which may partake of characteristics of a phantom network in some degree. Implementation of the idea was achieved through postulation of domains of constraint affecting the equilibrium distribution of fluctuations of network junctions from their mean positions. This led in due course to a theory that accounts for the relationship of stress to strain virtually throughout the ranges of strain accessible to measurement. The theory establishes connections between structure and elastic properties. This is achieved with utmost frugality in arbitrary parameters. 相似文献
992.
Nichols M.A. Siegel H.J. Dietz H.G. Quong R.W. Nation W.G. 《Parallel and Distributed Systems, IEEE Transactions on》1991,2(3):290-303
Efficient data layout is an important aspect of the compilation process. A model for the creation of perfect memory maps for large-scale parallel machines capable of user-controlled partitionable single-instruction-multiple data/single-program-multiple data (SIMD/SPMD) operation is developed. The term perfect implies that no memory fragmentation occurs and ensures that the memory map size is kept to a minimum. A major constraint on solving this problem is based on the single program nature of both the SIMD and SPMD modes of parallelism. It is assumed that all processors within the same submachine used identical addresses to access corresponding data items in each of their local memories. Necessary and sufficient conditions are derived for being able to create perfect memory maps, and results are applied to several partitionable interconnection networks 相似文献
993.
J M Gleason 《Computer methods and programs in biomedicine》1989,30(1):43-46
The widespread use of IBM personal computers in biomedical research and medical practice is evidenced by a variety of journal articles. Many of the application programs are written in the BASIC language, they model a stochastic process, and they depend on the Microsoft random number generator. Unfortunately, the Microsoft generator has exhibited a number of flaws including, for some seeds, a lack of uniformity of generated sequences of numbers, and serial correlation within such sequences. This paper provides evidence of these problems for several seeds, and provides suggestions for acceptable seeds which ensure that the generated sequences of numbers pass two fundamental statistical tests for randomness. 相似文献
994.
I. N. Polandov V. K. Novik O. K. Gulish B. P. Bogomolov V. B. Morozov 《Measurement Techniques》1989,32(9):888-890
Translated from Izmeritel'naya Tekhnika, No. 9, pp. 34–35, September, 1989. 相似文献
995.
996.
M. Polák M. Majoro F. Hanic J. Pitel M. Kedrová P. Kottman J. Talapa L. Vencel 《Journal of Superconductivity》1989,2(2):219-233
A method for contactless measurement of the shielding critical current density and its dependence on the external magnetic field is described and analyzed. The obtained values are compared with those measured resistively on two different samples. It is shown that the shielding critical current densityJ
cs
and the intergranular transport current densityJ
cr
are identical if the measurement conditions are similar. A degradation ofJ
cs
measured in the external field with AC ripple has been observed. 相似文献
997.
998.
Containment structures have several regions in which the continuity of the cylindrical pressure boundary is interrupted, e.g., shell penetrations, discontinuous stiffeners, and changes in the shell thickness. Significant strain concentrations can occur in these areas of discontinuity. The Sandia National Laboratories 1:8-scale steel containment equipment hatch was analyzed as an example of an eccentricity at a stiffener intersection.A portion of the as-built 1:8-scale model was modeled with the ANSYS general purpose finite element program using triangular, thin shell finite elements. The overall size of the model was determined from Saint-Venant type considerations of the stress field around the hatch. Shell elements were used to model the ring and formed stiffeners. Geometric and material nonlinear behavior were included. The model was loaded using discrete load steps up to a pressure of 165 psig. At this pressure, the maximum strain was 19.7 percent in the formed stiffener near its intersection with the ring stiffener. The finite element solution demonstrated the very localized nature of the strain field near the ring/formed stiffener intersection.In an attempt to reduce analysis costs, a small portion of the 1:8-scale model immediately surrounding the ring/formed stiffener intersection was selected for further analysis. Two smaller models, a ring/formed stiffener intersection and a ring/circular stiffener intersection, were studied. The models were significantly smaller than the regions used previously. A comparison of the two intersection models showed that the circular stiffener is a more efficient configuration. 相似文献
999.
The composition of the volatile oil of Mexican oregano (Lippia graveolens HBK) isolated by steam distillation was investigated by means of column chromatography, gas chromatography and mass spectrometry. A total of 33 components were identified including 22 hydrocarbons, 4 alcohols, 4 ethers, 2 phenols and 1 ketone. Of the 33 components observed in the present study 7 were identified in Mexican oregano for the first time. 相似文献
1000.
J. Ben Mariem M. A. Hamdi 《International journal for numerical methods in engineering》1987,24(7):1251-1267
In order to study problems on fluid–structure interaction, we have used a mixed formulation which couples the classical functional of the structure with a new variational formulation by integral equations for the fluid. This formulation has the advantage over the finite element methods of avoiding the discretization of the fluid domain. Furthermore, unlike collocation methods, the explicit calculation of the Hadamard finite part of the singular integrals is avoided. This leads after discretization by boundary finite elements to a small and symmetrical algebraic system. Typical examples are presented that demonstrate the efficiency of this variational formulation by studying the sound transmission through a baffled plane structure and through a flexible panel backed by a rigid cavity. These include the calculation of the transmission loss factor and the determination of which modes dominate the noise transmission. Good agreement is obtained between numerical results and analytical results found in the literature. 相似文献