全文获取类型
收费全文 | 139846篇 |
免费 | 7937篇 |
国内免费 | 4278篇 |
专业分类
电工技术 | 5471篇 |
技术理论 | 3篇 |
综合类 | 7382篇 |
化学工业 | 19817篇 |
金属工艺 | 8083篇 |
机械仪表 | 6749篇 |
建筑科学 | 6842篇 |
矿业工程 | 2387篇 |
能源动力 | 2831篇 |
轻工业 | 9394篇 |
水利工程 | 2657篇 |
石油天然气 | 2822篇 |
武器工业 | 711篇 |
无线电 | 15983篇 |
一般工业技术 | 21602篇 |
冶金工业 | 4551篇 |
原子能技术 | 798篇 |
自动化技术 | 33978篇 |
出版年
2024年 | 371篇 |
2023年 | 1344篇 |
2022年 | 2742篇 |
2021年 | 3573篇 |
2020年 | 2451篇 |
2019年 | 1921篇 |
2018年 | 16342篇 |
2017年 | 15341篇 |
2016年 | 11692篇 |
2015年 | 3445篇 |
2014年 | 3604篇 |
2013年 | 3819篇 |
2012年 | 7639篇 |
2011年 | 13966篇 |
2010年 | 12207篇 |
2009年 | 9352篇 |
2008年 | 10388篇 |
2007年 | 11061篇 |
2006年 | 3039篇 |
2005年 | 3586篇 |
2004年 | 2908篇 |
2003年 | 2797篇 |
2002年 | 2346篇 |
2001年 | 1581篇 |
2000年 | 1134篇 |
1999年 | 720篇 |
1998年 | 423篇 |
1997年 | 347篇 |
1996年 | 287篇 |
1995年 | 226篇 |
1994年 | 187篇 |
1993年 | 149篇 |
1992年 | 112篇 |
1991年 | 94篇 |
1990年 | 53篇 |
1989年 | 34篇 |
1988年 | 45篇 |
1987年 | 27篇 |
1968年 | 43篇 |
1967年 | 33篇 |
1966年 | 42篇 |
1965年 | 44篇 |
1963年 | 28篇 |
1960年 | 30篇 |
1959年 | 37篇 |
1958年 | 37篇 |
1957年 | 36篇 |
1956年 | 34篇 |
1955年 | 63篇 |
1954年 | 68篇 |
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
81.
采用沉淀硫酸化法制备了复合固体超强酸催化剂SO2-4/Fe2O3-γ-Al2O3,确定了其最佳制备工艺条件:硫酸浸渍浓度为0.6 mol/L,浸渍时间4 h;焙烧温度550℃,焙烧时间3 h.并采用该催化剂合成丁酸丁酯,考察了物料配比、催化剂用量和反应时间对酯化反应的影响,确定了丁酸丁酯的最佳合成条件:丁醇与丁酸摩尔比为1.4:1,催化剂用量0.9%(以反应物质量计),回流条件下反应3.0 h,在此最佳合成条件下,酯化率可达95.6%. 相似文献
82.
83.
Atsushi Narumi Kosei Kawasaki Harumi Kaga Toshifumi Satoh Naoya Sugimoto Toyoji Kakuchi 《Polymer Bulletin》2003,49(6):405-410
Summary
The potato phosphorylase-catalyzed polymerization of α-D-glucose-1-phosphate (G-1-P) onto poly[styrene-block-(4-vinylbenzyl maltohexaoside)] (1) was performed at the molar ratios of [G-l-P]0 and [maltohexaose]0 of 35, 80, and 250. The product was found to be soluble in dimethyl sulfoxide, which was a good solvent for amylose, and
showed the complex-formation with iodine, indicating that the product was assignable to poly[styrene-block-(styrene-graft-amylose)] (2). The quantitative analysis of the liberated phosphoric acid gave the average degree of polymerization o f the glucose unit
(n) as 27, 5 1, and 180 for 2-I, 2-II, and 2-III, respectively.
Received: 29 November 2002/Accepted: 22 December 2002
Correspondence to Toyoji Kakuchi 相似文献
84.
超级电容器在汽车启动中的应用 总被引:2,自引:0,他引:2
根据超级电容器的结构特性,介绍在汽车启动过程中如何利用超级电容器减小对车内其他电子设备的干扰,改善汽车的启动性能,延长蓄电池使用寿命. 相似文献
85.
ZhaoYahong ZhangZhongpei WuWeiling 《电子科学学刊(英文版)》2003,20(3):177-182
Adaptive modulation and power allocation is introduced into the multicarrier DSCDMA system to improve the system performance and bandwidth efficiency.First,the system design appropriate for adaptive modulation and power allocation is given,then the algorithm of adaptive modulation and power allocation is applied.Simulation results demonstrate great performance improvement compared with the fixed modulated one. 相似文献
86.
As an aid towards improving the treatment of exchange and correlation effects in electronic structure calculations, it is
desirable to have a clear picture of the errors introduced by currently popular approximate exchange-correlation functionals.
We have performed ab initio density functional theory and density functional perturbation theory calculations to investigate
the thermal properties of bulk Cu, using both the local density approximation (LDA) and the generalized gradient approximation
(GGA). Thermal effects are treated within the quasiharmonic approximation. We find that the LDA and GGA errors for anharmonic
quantities are an order of magnitude smaller than for harmonic quantities; we argue that this might be a general feature.
We also obtain much closer agreement with experiment than earlier, more approximate calculations. 相似文献
87.
Effect of doping of carbon nanotubes by magnetic transition metal atoms has been considered in this paper. In the case of
semiconducting tubes, it was found that the system has zero magnetization, whereas in metallic tubes the valence electrons
of the tube screen the magnetization of the dopants: the coupling to the tube is usually antiferromagnetic (except for Cr). 相似文献
88.
We consider the problem of designing a network of optical cross-connects (OXCs) to provide end-to-end lightpath services to large numbers of label switched routers (LSRs). We present a set of heuristic algorithms to address the combined problem of physical topology design (i.e., determine the number of OXCs required and the fiber links among them) and logical topology design (i.e., determine the routing and wavelength assignment for the lightpaths among the LSRs). Unlike previous studies which were limited to small topologies with a handful of nodes and a few tens of lightpaths, we have applied our algorithms to networks with hundreds or thousands of LSRs and with a number of lightpaths that is an order of magnitude larger than the number of LSRs. In order to characterize the performance of our algorithms, we have developed lower bounds which can be computed efficiently. We present numerical results for up to 1000 LSRs and for a wide range of system parameters such as the number of wavelengths per fiber, the number of transceivers per LSR, and the number of ports per OXC. The results indicate that it is possible to build large-scale optical networks with rich connectivity in a cost-effective manner, using relatively few but properly dimensioned OXCs. 相似文献
89.
90.
鄂尔多斯盆地东部奥陶系盐下储层预测 总被引:1,自引:0,他引:1
鄂尔多斯盆地奥陶系的膏盐岩沉积面积达5×104km2,天然气的勘探程度很低。为此,利用测井技术和地震反演技术对盐下储层进行了预测。首先,通过对盐下沉积特征的综合分析,对盐下储集层段的发育层位和主要储集岩类型进行了描述;然后,采用ELAN测井技术对盐下储层的物性进行了分析,认为盐下储层的物性在普遍较差的背景上存在相对发育的层段和区带;最后,利用全局优化多道地震反演技术,对岩下储层进行了预测,认为盐下存在的局部隆起是圈闭和储层发育的有利区带。 相似文献