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   20519篇
  免费   137篇
  国内免费   89篇
化学   10387篇
晶体学   417篇
力学   603篇
综合类   7篇
数学   1713篇
物理学   7618篇
  2017年   155篇
  2016年   243篇
  2015年   166篇
  2014年   267篇
  2013年   840篇
  2012年   576篇
  2011年   635篇
  2010年   462篇
  2009年   472篇
  2008年   624篇
  2007年   674篇
  2006年   637篇
  2005年   582篇
  2004年   502篇
  2003年   472篇
  2002年   467篇
  2001年   691篇
  2000年   512篇
  1999年   430篇
  1998年   350篇
  1997年   332篇
  1996年   303篇
  1995年   380篇
  1994年   349篇
  1993年   319篇
  1992年   346篇
  1991年   389篇
  1990年   362篇
  1989年   364篇
  1988年   335篇
  1987年   334篇
  1986年   328篇
  1985年   407篇
  1984年   424篇
  1983年   318篇
  1982年   327篇
  1981年   350篇
  1980年   324篇
  1979年   336篇
  1978年   335篇
  1977年   314篇
  1976年   332篇
  1975年   290篇
  1974年   283篇
  1973年   299篇
  1972年   182篇
  1969年   159篇
  1968年   189篇
  1967年   334篇
  1966年   284篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
171.
172.
There is currently no market in Israel for the large amounts of waste from fish- and poultry-processing plants. Therefore, this waste is incinerated, as part of the measures to prevent the spread of pathogens. Anaerobic methanogenic thermophilic fermentation (AMTF) of wastes from the cattle-slaughtering industry was examined previously, as an effective system to treat pathogenic bacteria, and in this article, we discuss a combined method of digestion by thermophilic anaerobic bacteria and by flesh flies, as a means of waste treatment. The AMTF process was applied to the wastes on a laboratory scale, and digestion by rearing of flesh fly (Phaenicia sericata) and housefly (Musca domestica) larvae on the untreated raw material was done on a small scale and showed remarkable weight conversion to larvae. The yield from degradation of poultry waste by flesh fly was 22.47% (SD = 3.89) and that from fish waste degradation was 35.34% (SD = 12.42), which is significantly higher than that from rearing houseflies on a regular rearing medium. Bacterial contents before and after thermophilic anaerobic digestion, as well as the changes in the chemical composition of the components during the rearing of larvae, were also examined.  相似文献   
173.
The reaction of chloromethyl phenyl ether with butyllithium in olefins yields phenoxy-cyclopropanes. 1-Chloro-1-phenoxycyclopropanes can be prepared in a similar manner, though the yields are poor. Alkoxycyclopropanes are formed when dichloromethyl alkyl ethers are treated with methyllithium/lithium iodide in the presence of olefins. Cyclopropanols can be obtained in good yields by reaction of (β-chloroethoxy)cyclopropanes either with butyl- or ethyllithium or with bases. – As was shown by acetolysis experiments with cyclopropyl p-toluenesulfonates having a known steric configuration, the rearrangement of a cyclopropyl derivative into an allyl cation proceeds in accordance with the Woodward-Hoffmann-DePuy rule. The solvolysis of exo-bicyclo[n.1.0]alkyl p-toluene-sulfonates is assumed to proceed via “semi-open” intermediates, which are somewhere between an allyl cation and a cyclopropyl cation.  相似文献   
174.
Zusammenfassung Bei der Chromatographie auf Carboxylpapier herrschen definierte Ionenaustauschvorgänge vor, die sich theoretisch näherungsweise ableiten lassen. Durch Variation von Carboxylgehalt und austauschfähigem Kation im Papier, sowie vonph-Wert, Pufferkapazität und Kationenstärke in der entwickelnden Lösung ist es möglich, die für die chromatographische Trennung der drei basischen Aminosäuren günstigsten Bedingungen festzustellen. Es ist zu erwarten, daß sich auf Carboxylpapier weitere, kationenbildende Stoffe trennen lassen, die sich entweder in ihrer Affinität zur Oxycellulose oder in ihrer Basizität oder in beidem voneinander unterscheiden.Die Arbeit wurde durch wertvolle Kritik von Herrn Professor Dr. Th. Wieland gefördert. Wesentliche Anregungen stammen auch von Herrn Dr. W. Vollmer. Im Labor wurden die Chromatogramme sämtlich von Frau E. v. Dungen ausgeführt.  相似文献   
175.
13C ENDOR spectra of Cu(et2dtc)2 substituted into a single crystals of Zn(et2dtc)2 are reported. The symmetry of the incorporated guest molecule appears to be considerably influenced by the Zn(et2dtc)2 host lattice. The unexpectedly large isotropic 13C hfs coupling can be understood assuming a “transannular overlap” mechanism.  相似文献   
176.
On the basis of the S-matrix approach, a theory of perturbations in 1/Z is developed; the energy of a multielectron system is written in the form of a double series in powers of andZ and a method of determining the coefficients of this series in a j-j coupling scheme is given. The method is applied to configurations of the type , and numerical results are given for the wavelength of transitions between several states of the isoelectronic as sequence of carbon, illustrating the good agreement between experiment and theory.Translated from Izvestiya Vysshikh Uchebnikh Zavedenii, Fizika, No. 8, pp. 59–63, August, 1978.  相似文献   
177.
178.
The formation of needle shaped crystallites of V6O13 at low V concentrations on V2O5-anatase coated catalysts explains the low selectivity for phthalic anhydride during o-xylene oxidation. The (010) plane of V-oxide, most active for selective oxidation of o-xylene, is not accessible and the contact of this plane with the anatase faces promotes the anatase-rutile transformation and the incorporation and blocking of V4+ ions.
V6O13 V , V2O5-, -. (010) , -, , - V+4.
  相似文献   
179.
180.
Alkylation of 4-ethoxy-2-hydroxypyrimidines (10a, b), via their anions, provides a convenient procedure for preparing N1-substituted derivatives. Reaction of 10a, b with t-butyl 4-bromo-2-benzyloxycarbamidobutyrate (17) gives good yields of the N1-alkylated products 18a, b. The latter have been converted to uracil- (20a), thymine- (20b) and cytosine- (21) substituted -aminobutyric acids.  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