首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   257013篇
  免费   1115篇
  国内免费   636篇
化学   113499篇
晶体学   3414篇
力学   16075篇
综合类   7篇
数学   52720篇
物理学   73049篇
  2019年   2301篇
  2018年   13682篇
  2017年   13602篇
  2016年   10145篇
  2015年   2695篇
  2014年   3874篇
  2013年   7447篇
  2012年   9230篇
  2011年   16912篇
  2010年   11046篇
  2009年   11588篇
  2008年   12829篇
  2007年   14897篇
  2006年   5829篇
  2005年   6230篇
  2004年   6297篇
  2003年   6300篇
  2002年   5510篇
  2001年   5240篇
  2000年   3983篇
  1999年   2974篇
  1998年   2799篇
  1997年   2610篇
  1996年   2464篇
  1995年   2179篇
  1994年   2099篇
  1993年   2140篇
  1992年   2138篇
  1991年   2458篇
  1990年   2373篇
  1989年   2457篇
  1988年   2336篇
  1987年   2370篇
  1986年   2192篇
  1985年   2720篇
  1984年   2729篇
  1983年   2373篇
  1982年   2430篇
  1981年   2324篇
  1980年   2134篇
  1979年   2457篇
  1978年   2599篇
  1977年   2679篇
  1976年   2772篇
  1975年   2576篇
  1974年   2505篇
  1973年   2693篇
  1972年   2189篇
  1971年   2004篇
  1970年   1868篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
951.
Conclusions A method of synthesis of arylhydroxyacetic acids and ammo acids derived from ethinylphenols has been developed in view of possible physiological activity of these compounds.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 5, pp. 1170–1172, May, 1970.  相似文献   
952.
Conclusions The reactions of amides of arylphosphorous acids with acyl chlorides take place with replacement of the amide group and formation of amides of carboxylic acids and the corresponding aryl chlorophosphates.Translated from Izvestiya Akademii Nauk SSSR, Seriya Khimicheskaya, No. 9, pp. 2131–2133, September, 1968.  相似文献   
953.
2,3-Polymethylene-4-oxo-3,4-dihydrothieno[2,3-d]pyrimidine derivatives were obtained by reaction of esters of α-aminothiophene-β-carboxylic acids with lactame or α-aminothiophene-Β-carboxylic acids with lactims. Desulfuration of the thienopyrimidines obtained in this manner is a convenient method for the synthesis of pyrrolo-, pyrido-, and azepino[1,2-a]pyrimidine derivatives.  相似文献   
954.
In the essential oil of Asarum europaeum L. the four sesquiterpene alcohols α-eudesmol, β-eudesmol, selinane-7,11-diol and seline-6(7)en-11-ol have been isolated and identified. For limiting the possibilities of structure the dehydrogenating TAS method (sulphur dehydrogenation) has been employed. Good results have been obtained with unsaturated C15-alcohols. Saturated sesquiterpene alcohols with decalin structure could be better dehydrogenated by the selenium method. UV spectra have been recorded of the naphthalene derivatives formed directly from the thin-layer plate and employed for identification. By this combination of micro methods the basic structure of the alcohols could be determined unequivocally with μg amounts.  相似文献   
955.
The optical properties of pregnenes containing different substituents at C-17 (—SCN, —SAc, —SC(CH3)2O—, —Br, —OAc, —N3) have been investigated with the help of the CD spectra. It is shown that the sign and value of the Cotton effect of the carbonyl chromophore n → π* transition is determined by the nature of the substituent at C-17.  相似文献   
956.
2-R-Quinazolones have been synthesized (R=Me, Ph,-pyridyl,-furyl), and subsequently the 4-chlorides and the corresponding 2-R-4-hydrazinoquinazolines were obtained. By the reaction of orthoformic ester and nitric acid on the hydrazines, 5-R- [3, 4-c]-s-triazoloquinazolines and 5-R- [1, 5-c] tetrazoloquinazolines were prepared, respectively. The compounds in which R=Me differ considerably from compounds with other groups, both in color and stability of intermediate reaction products. 5-R- [1, 5-c] tetrazoloquinazolines are hydrolyzed by HCl into quinazol-4-ones.For Part IV see [1].  相似文献   
957.
To select a method for the synthesis of the previously difficulty accessible 5-bromo-1-phenyl-1,2,4-triazole (I) and 1-phenyl-1,2,4-triazoline-5-thione (II), a MO LCAO calculation has been carried out in Huckel's approximation of the energies of the radical, electrophilic, and nucleophilic localization on the various carbon atoms of 1-phenyl-1,2,4-triazole (III). In accordance with this calculation, the required compounds have been obtained in good yields by reactions taking place by a radical mechanism: I by bromination with bromosuccinimide and II by the direct thionation of III. The high reactivity of the bromine atom in position 5 has been noted.For part VIII, see [1].  相似文献   
958.
A series of novel mono-and binuclear copper(II) complexes with substituted salicylaldehyde acylhydrazones H2L of the formula CuL · xH2O (x = 0 and 1) and [Cu(HL)](ClO4)(CH3OH) were synthesized. The isolated dimeric complexes of copper acetate were found to exist as isomers with different bridging atoms. In dimers showing a superexchange between the paramagnetic centers through bridging phenoxide O atoms, the antiferromagnetic exchange couplings were much stronger than those in complexes with bridging O atoms of the a-oxyazine fragment.  相似文献   
959.
Centralized biogas plants in Denmark codigest mainly manure, together with other organic waste such as industrial organic waste, source sorted household waste, and sewage sludge. Today 22 large-scale centralized biogas plants are in operation in Denmark, and in 2001 they treated approx 1.2 million tons of manure as well as approx 300,000 of organic industrial waste. Besides the centralized biogas plants there are a large number of smaller farm-scale plants. The long-term energy plan objective is a 10-fold increase of the 1998 level of biogas production by the year 2020. This will help to achieve a target of 12-14% of the national energy consumption being provided by renewable energy by the year 2005 and 33% by the year 2030. A major part of this increase is expected to come from new centralized biogas plants. The annual potential for biogas production from biomass resources available in Denmark is estimated to be approx 30 Peta Joule (PJ). Manure comprises about 80% of this potential. Special emphasis has been paid to establishing good sanitation and pathogen reduction of the digested material, to avoid risk of spreading pathogens when applying the digested manure as fertilizer to agricultural soils.  相似文献   
960.
The electroreduction of gadolinium from a eutectic KCl–KF melt containing 0.210–1.233 wt % Gd2O3 on molybdenum and glassy carbon electrodes is studied at 1008 K. The solubility of gadolinium oxide is determined. It is shown that the reduction of a gadolinium oxyfluoride complex proceeds reversibly, with a subsequent reversible chemical reaction. The gadolinium reduction on an iron electrode in a eutectic KCl–KF melt purified of unwanted oxide ion impurities yields an iron-gadolinium alloy.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号