全文获取类型
收费全文 | 206407篇 |
免费 | 2239篇 |
国内免费 | 539篇 |
专业分类
电工技术 | 3530篇 |
综合类 | 144篇 |
化学工业 | 31864篇 |
金属工艺 | 10273篇 |
机械仪表 | 7013篇 |
建筑科学 | 4010篇 |
矿业工程 | 2162篇 |
能源动力 | 4648篇 |
轻工业 | 12935篇 |
水利工程 | 2934篇 |
石油天然气 | 7749篇 |
武器工业 | 22篇 |
无线电 | 21531篇 |
一般工业技术 | 45221篇 |
冶金工业 | 34382篇 |
原子能技术 | 6626篇 |
自动化技术 | 14141篇 |
出版年
2021年 | 2109篇 |
2019年 | 1999篇 |
2018年 | 3679篇 |
2017年 | 3717篇 |
2016年 | 4001篇 |
2015年 | 2233篇 |
2014年 | 3783篇 |
2013年 | 8739篇 |
2012年 | 5680篇 |
2011年 | 7215篇 |
2010年 | 5855篇 |
2009年 | 6488篇 |
2008年 | 6609篇 |
2007年 | 6492篇 |
2006年 | 5569篇 |
2005年 | 5116篇 |
2004年 | 4630篇 |
2003年 | 4553篇 |
2002年 | 4366篇 |
2001年 | 4472篇 |
2000年 | 4184篇 |
1999年 | 4197篇 |
1998年 | 10183篇 |
1997年 | 7181篇 |
1996年 | 5500篇 |
1995年 | 4131篇 |
1994年 | 3597篇 |
1993年 | 3838篇 |
1992年 | 2983篇 |
1991年 | 2995篇 |
1990年 | 2889篇 |
1989年 | 2852篇 |
1988年 | 2864篇 |
1987年 | 2508篇 |
1986年 | 2568篇 |
1985年 | 2854篇 |
1984年 | 2687篇 |
1983年 | 2554篇 |
1982年 | 2301篇 |
1981年 | 2269篇 |
1980年 | 2348篇 |
1979年 | 2359篇 |
1978年 | 2382篇 |
1977年 | 2487篇 |
1976年 | 2876篇 |
1975年 | 2188篇 |
1974年 | 2068篇 |
1973年 | 2166篇 |
1972年 | 1940篇 |
1971年 | 1755篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
941.
942.
Summary The cyclopolymerization of 1,8-diethynylnaphthalene(DEN) and its copolymerization with diethyl dipropargylmalonate(DEDPM) were carried out by various transition metal catalysts. MoCl5-based catalyst was found to be very effective and gave almost a quantitative polymer yield. The resulting dark-brown homopolymer(PDEN) was partially soluble in chloroform, tetrahydrofuran, and dimethyl sulfoxide, whereas copolymer (1:1) with number avaerage molecular weight
of 2.1x104 by GPC showed good solubility in those solvents. The structure of the polymers was analysed by IR, NMR, and UV-VIS spectroscopies. From the spectral and solubility data, it is proposed that cyclized products with the conjugated system are formed. Room temperature conductivities of the I2-doped homo and copolymer were found to be about 7.5x10-3 and 5.1x10-3 S/cm, respectively. 相似文献
943.
944.
945.
946.
947.
948.
S. R. Ellis D. Pletcher W. N. Brooks K. P. Healy 《Journal of Applied Electrochemistry》1983,13(6):735-741
The anodic chlorination of naphthalene in a water/methylene chloride emulsion and using tetrabutylammonium ion as the phase transfer catalyst is demonstrated; in conditions where the aqueous phase is saturated NaCl, the organic yield of 1-chloronaphthalene is 56% and the current yield is 33% after the passage of 2.33 F mol–1 of naphthalene. It is shown, however, that when the aqueous phase also contains zinc chloride so that the species transferred is [(C4H9)4N+]2ZnCl
4
––
, the yields can be increased to 92% and 49% respectively. The mechanism of these chlorinations is discussed. 相似文献
949.
950.
Osvaldo Mercuri N. E. Carrazoni R. R. Brenner 《Journal of the American Oil Chemists' Society》1964,41(2):89-92
Methyl oct-cis-2-enoate was synthesized by selective hydrogenation of methyl oct-2-ynoate at atmospheric pressure with quinoline poisoned palladium as catalyst. The resulting product contained only 7% of thetrans form, as determined by GLC. The structure was confirmed by infrared spectra. Analysis of the infrared spectra is given. The cis andtrans forms of methyl oct-2-enoate were separable by GLC in polar or non-polar columns. GLC runs in polar and non-polar phases showed that the α position of the double bond of methyl oct-2-enoate so affects its properties, that practically no interaction was observed between the double bond and the polar phase. Consequently volatility was the main factor determining the retention times of cis andtrans methyl oct-2-enoate in the polar and non-polar phases studied. 相似文献