全文获取类型
收费全文 | 31396篇 |
免费 | 1362篇 |
国内免费 | 48篇 |
专业分类
电工技术 | 399篇 |
综合类 | 41篇 |
化学工业 | 6944篇 |
金属工艺 | 495篇 |
机械仪表 | 560篇 |
建筑科学 | 1635篇 |
矿业工程 | 114篇 |
能源动力 | 847篇 |
轻工业 | 3080篇 |
水利工程 | 279篇 |
石油天然气 | 82篇 |
无线电 | 2360篇 |
一般工业技术 | 5720篇 |
冶金工业 | 5229篇 |
原子能技术 | 232篇 |
自动化技术 | 4789篇 |
出版年
2023年 | 256篇 |
2022年 | 426篇 |
2021年 | 722篇 |
2020年 | 451篇 |
2019年 | 550篇 |
2018年 | 688篇 |
2017年 | 617篇 |
2016年 | 857篇 |
2015年 | 741篇 |
2014年 | 979篇 |
2013年 | 1839篇 |
2012年 | 1571篇 |
2011年 | 1949篇 |
2010年 | 1475篇 |
2009年 | 1373篇 |
2008年 | 1545篇 |
2007年 | 1416篇 |
2006年 | 1230篇 |
2005年 | 990篇 |
2004年 | 951篇 |
2003年 | 835篇 |
2002年 | 738篇 |
2001年 | 514篇 |
2000年 | 461篇 |
1999年 | 485篇 |
1998年 | 909篇 |
1997年 | 658篇 |
1996年 | 549篇 |
1995年 | 472篇 |
1994年 | 442篇 |
1993年 | 417篇 |
1992年 | 309篇 |
1991年 | 233篇 |
1990年 | 293篇 |
1989年 | 294篇 |
1988年 | 236篇 |
1987年 | 257篇 |
1986年 | 252篇 |
1985年 | 292篇 |
1984年 | 254篇 |
1983年 | 245篇 |
1982年 | 233篇 |
1981年 | 216篇 |
1980年 | 188篇 |
1979年 | 215篇 |
1978年 | 174篇 |
1977年 | 207篇 |
1976年 | 260篇 |
1975年 | 167篇 |
1974年 | 131篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
52.
53.
54.
55.
The expression patterns of laminin A, B1, B2, and nidogen genes were identified by in situ hybridization in postimplantation mouse extraembryonic tissues and maternal decidua during the period when the chorioallantoic placenta is established. Laminin and nidogen genes were not coordinately expressed either in the decidua or in trophoblast cells, indicating that these genes are regulated independently in these cell types during the establishment of the placenta. Laminin A mRNA was absent from the decidua except in the outer layer of cells adjacent to the myometrium and in the central decidual zone adjacent to the remnant of the uterine epithelium on Day 9. At this stage laminin B1, B2, and nidogen genes were strongly expressed in these cells and also in other regions of the decidua. Laminin B1 mRNA was present at higher levels in the decidua capsularis than in the decidua basalis, while nidogen mRNA showed highest expression in the decidua basalis. Laminin B2 mRNA was produced uniformly throughout the decidua at very high levels, suggesting that laminin B2 chains may be an important component of the decidual matrix. By Day 11, the nidogen gene was expressed only in endothelial cells lining the maternal blood spaces within the decidua. Laminin B1 and nidogen mRNAs were found at high levels within trophoblast giant cells at all stages, while laminin A mRNA was detected in trophoblast giant cells at later stages and laminin B2 mRNA was not produced in high levels by these cells. The patterns of gene expression show a very high degree of regional specialization, suggesting that the extracellular matrices in different regions of the decidua and extraembryonic membranes are likely to be composed of quite different ratios of laminin and nidogen polypeptides.(ABSTRACT TRUNCATED AT 250 WORDS) 相似文献
56.
G.D. Wignall R.W. Hendricks W.C. Koehler J.S. Lin M.P. Wai E.L. Thomas R.S. Stein 《Polymer》1981,22(7):886-889
A series of small angle neutron scattering measurements on blends of normal polystyrene (PSH) and labelled (deuterated) polystyrene (PSD) have been made with concentrations of PSD from 5 to 50 mol %. It is shown that the single chain form factor of the polymer in bulk can be obtained from a single concentration measurement for any concentration of labelled molecules, providing the molecular weights of the parent and labelled molecules are the same and the molecular weight distributions are narrow. 相似文献
57.
In this paper we present a general strategy for finding efficient permutation routes in parallel networks. Among the popular parallel networks to which the strategy applies are mesh networks, hypercube networks, hypercube-derivative networks, ring networks, and star networks. The routes produced are generally congestion-free and take a number of routing steps that is within a small constant factor of the diameter of the network. Our basic strategy is derived from an algorithm that finds (in polynomial time) efficient permutation routes for aproduct network, G×H, given efficient permutation routes forG andH. We investigate the use of this algorithm for routingmultiple permutations and extend its applicability to a wide class of graphs, including several families ofCayley graphs. Finally, we show that our approach can be used to find efficient permutation routes among the remaining live nodes infaulty networks.This research was supported in part by a grant from the NSF, Grant No. CCR-88-12567. 相似文献
58.
59.
Mass transport in pulsating flow devices using either moving boundaries or oscillating imposed pressure drops are compared with each other by means of a calculation using a simple model. We conclude that there is no difference between the two configurations as long as one is interested only in the power required to move the fluid for the convective mass transport achieved. However, the boundary driven configuration is more efficient if the power is divided by the total mass transport where both the diffusive and convective parts are taken into account, the boundary driven configuration is more efficient. The amplitude of the piston stroke in the pressure driven configuration and the amplitude in the boundary driven case are assumed to be the same, and the inertia of the moving devices themselves are ignored in this calculation. 相似文献
60.