全文获取类型
收费全文 | 8744篇 |
免费 | 385篇 |
国内免费 | 57篇 |
专业分类
电工技术 | 234篇 |
综合类 | 23篇 |
化学工业 | 1933篇 |
金属工艺 | 183篇 |
机械仪表 | 210篇 |
建筑科学 | 255篇 |
矿业工程 | 9篇 |
能源动力 | 569篇 |
轻工业 | 875篇 |
水利工程 | 94篇 |
石油天然气 | 173篇 |
武器工业 | 4篇 |
无线电 | 1053篇 |
一般工业技术 | 1543篇 |
冶金工业 | 534篇 |
原子能技术 | 84篇 |
自动化技术 | 1410篇 |
出版年
2023年 | 191篇 |
2022年 | 443篇 |
2021年 | 561篇 |
2020年 | 398篇 |
2019年 | 405篇 |
2018年 | 523篇 |
2017年 | 362篇 |
2016年 | 417篇 |
2015年 | 263篇 |
2014年 | 386篇 |
2013年 | 667篇 |
2012年 | 435篇 |
2011年 | 501篇 |
2010年 | 333篇 |
2009年 | 278篇 |
2008年 | 251篇 |
2007年 | 249篇 |
2006年 | 205篇 |
2005年 | 176篇 |
2004年 | 138篇 |
2003年 | 120篇 |
2002年 | 137篇 |
2001年 | 88篇 |
2000年 | 80篇 |
1999年 | 91篇 |
1998年 | 160篇 |
1997年 | 119篇 |
1996年 | 95篇 |
1995年 | 95篇 |
1994年 | 63篇 |
1993年 | 58篇 |
1992年 | 52篇 |
1991年 | 41篇 |
1990年 | 37篇 |
1989年 | 53篇 |
1988年 | 59篇 |
1987年 | 41篇 |
1986年 | 44篇 |
1985年 | 60篇 |
1984年 | 58篇 |
1983年 | 53篇 |
1982年 | 41篇 |
1981年 | 36篇 |
1980年 | 40篇 |
1979年 | 33篇 |
1978年 | 25篇 |
1977年 | 33篇 |
1976年 | 38篇 |
1975年 | 23篇 |
1973年 | 22篇 |
排序方式: 共有9186条查询结果,搜索用时 0 毫秒
61.
Graft polymerization of methyl methacrylate (MMA) onto cotton cellulose using vanadium pentanitrate as initiator was studied under a variety of conditions. The graft yield increased with increasing initiator concentration up to 8 mmole/l. and then decreased upon further increase in initiator concentration. Increasing MMA concentration from 1 to 5% was accompanied by a significant increase in the degree of grafting. The latter was also affected by the kind and concentration of the acid incorporated in the polymerization medium. Based on graft yields, the efficiency of the acids follows the order H2SO4 > HNO3 > HClO4. Replacement of the acid with isopropyl alcohol was also examined. An isopropyl alcohol concentration of 10% constitutes the optimal concentration for grafting. Maximum graft yield depends upon the polymerization temperature; it follows the order 50°C ≥ 60°C > 40°C > 30°C > 70°C. Reaction mechanisms for grafting in the presence of acid as well as in the presence of isopropyl alcohol are proposed. 相似文献
62.
63.
Shoko Yoshikawa Toshitaka Ota Robert Newnham Ahmed Amin 《Journal of the American Ceramic Society》1990,73(2):263-267
Composite materials composed of randomly dispersed semiconducting ceramic particles in an insulating polymer matrix show a pronounced change in resistivity with pressure. Different amounts of iron oxide (Fe3 O4 ) powder and antimony-doped tin oxide (SnO2 :Sb) powder were dispersed in an epoxy polymer matrix to form pressure-sensitive composites. In each family of materials, an insulator-to-semiconductor transition is observed in agreement with percolation theory. Composites within a certain range of filler content showed substantial piezoresistive effect under both uniaxial and hydrostatic pressure in which sensitivity is controlled by the choice of filler material and the volume fraction. The effect of temperature on the piezoresistance effect was also examined. Piezoresistors made from Fe3 O4 composites showed larger temperature changes than those filled with Sb-doped SnO2 . 相似文献
64.
M. A. Ali M. A. Gafur M. S. Rahman G. M. Ahmed 《Journal of the American Oil Chemists' Society》1985,62(3):520-523
The kernels of 10 different mango varieties were extracted. The physico-chemical characteristics and lipid class composition
of fats were studied. The fat content of mango kernels grown under the soil and climatic conditions of Bangladesh varied from
7.1% to 10%, depending on the variety. The total lipid extracts were fractionated into lipid classes by a combination of column
and thin layer chromatography (TLC). The hydrocarbon and sterol esters varied from 0.3% to 0.7%, triglycerides from 55.6%
to 91.5%, partial glycerides from 2.3% to 4% and free sterol from 0.3% to 0.6%. Free fatty acids amounted to 3.0–37% as oleic;
glycolipids were 0.6–1.2% and phospholipids 0.11–0.8%. The fatty acid composition of triglyceride (TG) fractions was analyzed
by gas liquid chromatography (GLC). Palmitic acid varied from 7.9 molar % to 10.0 molar %, stearic from 38.2% to 40.2%, oleic
from 41.1% to 43.8%, linoleic from 6.0% to 7.6%, linolenic from 0.6% to 1.0% and arachidic acid from 1.7% to 2.6%. TLC revealed
the presence of lyso-phosphatidylcholine, phosphatidylcholine, phosphatidylinositol, phosphatidylethanolamine and phosphatidic
acid in the phospholipid fraction. 相似文献
65.
66.
Load balancing is a crucial factor in IPTV delivery networks. Load balancing aims at utilizing the resources efficiently, maximizing the throughput, and minimizing the request rejection rate. The peer-service area is the recent architecture for IPTV delivery networks that overcomes the flaws of the previous architectures. However, it still suffers from the load imbalance problem. This paper investigates the load imbalance problem, and tries to augment the peer-service area architecture to overcome this problem. To achieve the load balancing over the proposed architecture, we suggest a new load-balancing algorithm that considers both the expected and the current load of both contents and servers. The proposed load-balancing algorithm consists of two stages. The first stage is the contents replication according to their expected load, while the second stage is the content-aware request distribution. To test the effectiveness of the proposed algorithm, we have compared it with both the traditional Round Robin algorithm and Cho algorithm. The experimental results depict that the proposed algorithm outperforms the two other algorithms in terms of load balance, throughput, and request rejection rate. 相似文献
67.
Yasmin AlNoamany Ahmed AlSum Michele C. Weigle Michael L. Nelson 《International Journal on Digital Libraries》2014,14(3-4):101-115
The Internet Archive’s (IA) Wayback Machine is the largest and oldest public Web archive and has become a significant repository of our recent history and cultural heritage. Despite its importance, there has been little research about how it is discovered and used. Based on Web access logs, we analyze what users are looking for, why they come to IA, where they come from, and how pages link to IA. We find that users request English pages the most, followed by the European languages. Most human users come to Web archives because they do not find the requested pages on the live Web. About 65 % of the requested archived pages no longer exist on the live Web. We find that more than 82 % of human sessions connect to the Wayback Machine via referrals from other Web sites, while only 15 % of robots have referrers. Most of the links (86 %) from Websites are to individual archived pages at specific points in time, and of those 83 % no longer exist on the live Web. Finally, we find that users who come from search engines browse more pages than users who come from external Web sites. 相似文献
68.
A detailed study was made of the influence of the sulphate bath constituents: 0.3 - 0.0125 NiSO4·7H2O (I), 0.05 – 0.23 (NH4)2SO4 (II), 0.1 – 0.4 H3BO3 (III) and 0.07 – 0.35 mol l?1 Na2SO4·10H2O (IV) on the electrodeposition of nickel powder. The cathodic polarization, current efficiency, growth morphology, crystallite size and catalytic activity of the electrodeposited nickel powders were affected to different extents by the bath constituents. A highly pure nickel powder characterized by a small crystallite size (776 Å) and moderate catalytic activity was obtained from a bath containing: 0.0125 (I), 0.23 (II), 0.1 (III) and 0.07 mol l?1 (IV) at a current density of 10 A dm?2 and electrolysis time 60 min. at 25 °C. Structural studies with a scanning electron microscope are given and a reaction mechanism for the electrolytic powder deposition is discussed. 相似文献
69.
Ahmed A. Elkhateeb 《Nexus Network Journal》2014,16(1):219-232
This work presents four mathematical remarks concluded from the mathematical analysis for the interrelationships between the dependent and independent variables that control the measures: perimeter, floor area, walls surface area and total surface area in the regular forms that have a given volume. Such forms include prismatic and pyramidal forms. The work consists of four parts, of which this first part presents the remarks of the isosceles triangular right prism. The first remark examines the effect of θ, the angle of the triangular base, on the total surface area. The second remark calculates the minimum total surface area in two cases, depending on whether angle θ is constant or variable. The third remark calculates the walls ratio and the critical walls ratio. The last remark studies the required conditions for the numerical equality in two cases, where the perimeter is equal to the area, and where the total surface area is equal to the volume. 相似文献
70.
Cotton cellulose was graft copolymerized with poly(acrylonitrile) to different levels. The copolymers so obtained together with the nongrafted cellulose were oxidized at different pH's (4–10) and temperatures (50–70°C) with potassium persulphate. The oxidation reaction was studied with respect to oxygen consumption, mass loss, and changes in copper number and carboxyl content of the cellulosic materials. It was found that the rates of oxidation at pH 4 for the copolymers are substantially higher than that of the nongrafted cellulose and the rate of oxidation is higher the higher the level of grafting. The reverse is the case at pH 10. The mass loss increases as the oxygen consumption increases irrespective of the substrate used and the pH employed. The magnitude of the mass loss is substantially lower in the case of grafted cellulose than in the case of nongrafted cellulose. The cellulosic copolymers with higher graft levels show lower mass loss than those having lower graft levels. However, the copper number and carboxyl content of the oxidized grafted cellulose are higher than those of the nongrafted cellulose at the same oxygen consumption. It is believed that the presence of poly(acrylonitrile) graft in the molecular structure of cellulose impedes cellulosic chain scission without necessarily preventing oxidation of cellulose hydroxyls. 相似文献