全文获取类型
收费全文 | 472353篇 |
免费 | 5360篇 |
国内免费 | 1436篇 |
专业分类
电工技术 | 9492篇 |
综合类 | 375篇 |
化学工业 | 66411篇 |
金属工艺 | 21253篇 |
机械仪表 | 17041篇 |
建筑科学 | 10595篇 |
矿业工程 | 2196篇 |
能源动力 | 11479篇 |
轻工业 | 33658篇 |
水利工程 | 4593篇 |
石油天然气 | 6915篇 |
武器工业 | 22篇 |
无线电 | 60419篇 |
一般工业技术 | 96288篇 |
冶金工业 | 86669篇 |
原子能技术 | 8747篇 |
自动化技术 | 42996篇 |
出版年
2021年 | 3178篇 |
2020年 | 2379篇 |
2019年 | 2985篇 |
2018年 | 12418篇 |
2017年 | 12888篇 |
2016年 | 9398篇 |
2015年 | 3977篇 |
2014年 | 6204篇 |
2013年 | 18156篇 |
2012年 | 12103篇 |
2011年 | 20883篇 |
2010年 | 17918篇 |
2009年 | 18548篇 |
2008年 | 19060篇 |
2007年 | 20776篇 |
2006年 | 11806篇 |
2005年 | 13887篇 |
2004年 | 12128篇 |
2003年 | 11927篇 |
2002年 | 10439篇 |
2001年 | 10583篇 |
2000年 | 9880篇 |
1999年 | 10460篇 |
1998年 | 27285篇 |
1997年 | 18827篇 |
1996年 | 14646篇 |
1995年 | 10950篇 |
1994年 | 9673篇 |
1993年 | 9592篇 |
1992年 | 6784篇 |
1991年 | 6600篇 |
1990年 | 6436篇 |
1989年 | 6086篇 |
1988年 | 5848篇 |
1987年 | 4835篇 |
1986年 | 4810篇 |
1985年 | 5359篇 |
1984年 | 4895篇 |
1983年 | 4588篇 |
1982年 | 4226篇 |
1981年 | 4257篇 |
1980年 | 3971篇 |
1979年 | 3707篇 |
1978年 | 3501篇 |
1977年 | 4272篇 |
1976年 | 5795篇 |
1975年 | 2981篇 |
1974年 | 2908篇 |
1973年 | 2830篇 |
1972年 | 2444篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Summary A group of new polyimides has been prepared by solution condensation of 2,6-bis[1-(p-dimethylaminophenylimino)ethyl] pyridine containing pendant NMe2 units and various dianhyrides in N-methylpyrrolidone (NMP). The tridentate (N-N′-N) pydim ligands were prepared by Schiff-base condensation of 2,6-diacetylpyridine and 4-dimethylaminoaniline in the presence of formic acid as catalyst. These polymers were investigated for their extraction capabilities for Zn(II), Mn(II), Cu(II), Cd (II) and Ni(II) and at different pH. Under different conditions enhanced selectivity was observed. Up to 98% quantitative recoveries were observed for all metals. 相似文献
992.
T.L. Phillips 《Polymer》2005,46(24):11035-11050
Results are presented from atomistic molecular dynamics simulations of the mobile pseudo-hexagonal phase of polyethylene, which occurs under conditions of elevated pressure and temperature. Three different types of model are considered, all of which employ periodic boundary conditions. The first model consists of n-alkane sequences (48×-C24H48-) that are bonded across the simulation box boundaries to produce chains that are effectively infinite in extent. On heating, at high pressure, this system displays a rotator phase, in which the chains retain an all-trans conformation, and rotate as semi-rigid units. A second model, consisting of finite n-alkanes (48×C24H50) displays the same behaviour at low temperatures, but at high temperature and pressure forms a conformationally disordered rotator phase, characterised by a large proportion of gauche defects and a significant lattice expansion. The final model considered contains long n-alkanes (24×C102H206) which contain jog defects and each pass twice through the simulation box. This model forms a conformationally disordered rotator phase at high temperature and ambient pressure. The behaviour of the three models, in terms of the variations in chain conformation and rotational and translational dynamics, are compared. The conformationally disordered phases provide useful representations of the experimentally observed mobile phase. 相似文献
993.
994.
The unlubricated sliding wear behavior of non-reinforced polyetheretherketone (PEEK) and a PEEK-carbon fiber (PEEK-Cf) composite was investigated under lightly loaded conditions against mild steel counterfaces. The results showed the occurrence of a relative minimum in wear for the non-reinforced PEEK-mild steel system, resulting from interfacial film formation. In addition, the wear particle morphology changed substantially as the counterface became rougher. For PEEK on abraded mild steel, attempts were made to correlate the wear rate with the average asperity radius model of Hollander and Lancaster. The PEEK-Cf composite, when compared under similar experimental conditions, also displayed a relative minimum in wear; however, it occurred against rougher counterfaces compared with those corresponding to the non-reinforced PEEK. In this case, the relative wear and transferred wear debris morphology reflected the addition of the carbon fibers. 相似文献
995.
The synthesis of thermal-shock-resistant materials from the system Ta2O5WO3 was investigated. Ta2WO8 had a very low unit-cell thermal expansion coefficient (+0.5 X 10–6° C–1). Ta30W2O81 also had a relatively low coefficient (+4.0 X 10–6 ° C–1) and a thermal durability over 1600° C. The thermal expansion curves of these polycrystalline ceramics were lowered because of microcracks caused by the large thermal expansion anisotropy of the crystal axes and were accompanied by hysteresis loops. The densification of Ta2WO8 ceramic was promoted by the addition of some metal oxides, and the strong ceramic of Ta30W2O81 was obtained by controlling grain growth. 相似文献
996.
Microbial sensor for preliminary screening of mutagens utilizing a phage induction test 总被引:1,自引:0,他引:1
For the preliminary screening of mutagens, a novel microbial sensor system was developed utilizing a phage induction test. Escherichia coli lysogenic strain GY5027 and nonlysogenic strain GY5026 were used in this study. The number of living cells was determined by measuring the respiration of cells immobilized onto an oxygen electrode. The injection of a mutagen, such as AF-2 and MNNG, caused the phage induction in the lysogenic strain, resulting in the decreased respiration of only the lysogenic strain immobilized onto the oxygen electrode but not of nonlysogenic strain. The rate of current increase correlated well with the concentration of mutagens. The sensor responses to the antibiotics and bactericides were definitely different from those of mutagens. Therefore, utilization of this microbial sensor system makes possible the estimation of a substrate's mutagenicity. 相似文献
997.
X. S. Ning K. Suganuma T. Okamoto A. Koreeda Y. Miyamoto 《Journal of Materials Science》1989,24(8):2879-2883
Two kinds of additive-free silicon nitride ceramics were brazed with aluminium; one was with as-ground faying surfaces and
the other was with faying surfaces heat-treated at 1073K for 1.8 ksec in air. The heat-treatment of the silicon nitride ceramics
formed a silicon oxynitride layer on the faying surfaces and increased the brazing strength of the joints. A silica-alumina
non-crystalline layer and a β′-sialon layer were formed successively from the aluminium side at the interface of the joints.
The heat-treatment which made the former layer thicker is a necessary process in making reliable, strong brazed joints. 相似文献
998.
999.
1000.