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   248098篇
  免费   3156篇
  国内免费   1472篇
化学   122728篇
晶体学   3228篇
力学   8888篇
综合类   61篇
数学   24365篇
物理学   65672篇
无线电   27784篇
  2020年   1893篇
  2019年   2087篇
  2018年   2681篇
  2017年   2734篇
  2016年   3933篇
  2015年   2594篇
  2014年   3890篇
  2013年   9425篇
  2012年   8131篇
  2011年   9956篇
  2010年   7219篇
  2009年   7291篇
  2008年   9864篇
  2007年   10227篇
  2006年   9713篇
  2005年   8906篇
  2004年   8032篇
  2003年   7138篇
  2002年   7000篇
  2001年   7806篇
  2000年   6095篇
  1999年   4568篇
  1998年   3887篇
  1997年   3890篇
  1996年   3753篇
  1995年   3363篇
  1994年   3501篇
  1993年   3356篇
  1992年   3503篇
  1991年   3582篇
  1990年   3397篇
  1989年   3283篇
  1988年   3097篇
  1987年   2787篇
  1986年   2792篇
  1985年   3619篇
  1984年   3648篇
  1983年   3025篇
  1982年   3262篇
  1981年   3024篇
  1980年   2792篇
  1979年   2953篇
  1978年   3175篇
  1977年   3176篇
  1976年   3197篇
  1975年   2950篇
  1974年   3040篇
  1973年   3085篇
  1972年   2409篇
  1971年   1938篇
排序方式: 共有10000条查询结果,搜索用时 10 毫秒
941.
A tandem chromatographic procedure was used to isolate rapidly mouse IgM monoclonal antibodies produced by cultivation of hybridomas in vitro. Hybridoma culture supernatants containing mouse IgM monoclonal antibodies were first chromatographed on an anion-exchange Mono Q column connected to a fast protein liquid chromatography system. This anion-exchange step offers the advantage of obtaining IgM antibodies in a concentrated form. The IgM-rich fractions from the Mono Q column were then injected on a gel filtration Superose 6 column equilibrated with a low-ionic strength buffer and eluted with a high-ionic strength buffer. Assessment of the purity of isolated IgM monoclonal antibodies was performed by sodium dodecyl sulphate polyacrylamide gel electrophoresis together with a Coomassie Brillant Blue R 250 staining technique. Assessment of the immunoreactivity of isolated IgM monoclonal antibodies was performed by an enzyme linked immunosorbent assay using a solid phase adsorbed antigen against which IgM monoclonal antibodies were directed. The chromatographic procedures described allows the rapid isolation of mouse IgM monoclonal antibodies produced in vitro at a high degree of purity and in an immunoreactive state.  相似文献   
942.
Conclusions The partial hydrolysis of the pectin ofPanax ginseng C. A. Mey gives a polygalacturonide which has a linear carbohydrate chain consisting of residues of D-galacturonic acid in the pyranose form linked by -(1 4)-bonds.Khimiya Prirodnykh Soedinenii, Vol. 5, No. 4, pp. 201–203, 1969  相似文献   
943.
Non-K-region o-quinones of polycyclic aromatic hydrocarbons are prepared in four steps from cyclic ketones via dehydrogenation of tetrahydrodiols with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone.  相似文献   
944.
The redox reaction between the ferric complex of hydrophobic porphyrin and sodium dithionite in two different phases occurring at the interface between two immiscible liquids has been investigated by Volta potential measurements and spectroscopy. The reduction of the ferric complex of hydrophobic porphyrin adsorbed at the interface was found to be accompanied by a potential shift in the negative direction and to depend significantly on the nature of the anion and the ionic strength of the supporting electrolyte. Specifically adsorbed halogen anions inhibited the redox reaction in the sequence: Cl?, Br?, I?. Depending on pH, the ferric complex of hydrophobic porphyrin exists in the uncharged (FeP-O-PFe) form or in the cation FeP+ form. The interaction between the ferric complex hydrophobic porphyrin and water has also been investigated.  相似文献   
945.
Questions have been raised about the suitability of the “Boersma” type differential scanning calorimeter for the measurement of exothermic transitions such as the crystallization of a metallic glass. However, it is demonstrated that provided the mass of the sample is kept lower than a certain value, which depends on the transition being studied (typically about 10–15 mg at a heating rate of 20 K min? for a metallic glass), errors due to differences between actual sample temperature and that recorded by the instrument are negligible.  相似文献   
946.
Summary The problems associated with the preparation of glass open tubular (capillary) columns are well documented in the literature. These problems are even more evident if column production is done on a routine basis, in a production environment. On the other hand, from the point of the user who wants to rely on a supplier, the reproducible quality of the columns is crucial. Here quality does not only mean efficiency but also refers to the reproducibility of the average liquid film thickness (capacity ratio) and the relative retention, as well as the tube treatment influencing the polarity of the column tube. In a paper published one year ago [1] our investigation concerning the reproducibility of the column manufacturing process was reported. The present paper continues this investigation. Four different columns will be considered: two types of OV-101 columns with thin and thick liquid film as well as columns prepared with Carbowax 20M and DEGS liquid phases. The consistency of their characteristics and efficiency will be investigated. Column characteristics include the capacity ratio which is directly related to the thickness of the liquid phase film. Column efficiency is investigated with help of the HETP and peak resolution. The effective and theoretical plate numbers are compared and the incorrectness of the assumption that the former is better suited for column testing is demonstrated. Relative retention reproducibility of even difficult peak pairs shows the consistency of column wall treatment. Finally, the consistency and performance of shorter columns obtained by the breaking of long columns will be discussed.Paper presented at the Second International Symposium on Glass Capillary Chromatography, Hindelang, Allgäu, Federal Republic of Germany, May 2–6, 1977.  相似文献   
947.
Procedures for determining traces of calcium, titanium, vanadium, chromium, iron, copper, niobium, molybdenum and tungsten in tantalum are described utilizing 11 MeV proton activation. The instrumental method of analysis with the use of a Ge(Li) detector and the radiochemical separation of the corresponding fractions are described. The formula for the calculation of the concentration of traces with the use of thick target yields is given. Experimental data on the thick target yields of radioisotopes44Sc,52Mn,56Co,65Zn,93mMo,96Tc are presented. Based on the experimental data, the detection limits are estimated at 1–10 ppb for the above mentioned metal traces in tantalum. The successful solution of the task of the simultaneous determination of nine elements in tantalum confirms the wide applicability of proton activation analysis.  相似文献   
948.
The structures of three sesquiterpenoids, axisothiocyanate-2(4), axamide-1(5) and axamide-2(6), present in the marine sponge Axinella cannabina, have been determined on the basis of chemical and spectral evidences.  相似文献   
949.
3-Furylchromones     
-[2-(5-Methoxycarbonyl)furyl]-2,4-dihydroxy- and 2,4,6-trihydroxyacetophenones were obtained by the condensation of polyphenols with (2-methoxycarbonyl-5-furyl)acetonitrile. 7-Acetoxy- and 5,7-diacetoxy-2-methyl-3-[2-(5-methoxycarbonyl)furyl]chromones were obtained from them by reaction with acetic anhydride in the presence of triethylamine and were subsequently deacylated to 7-hydroxy and 5,7-dihydroxy compounds. 7-Hydroxy- and 5,7-dihydroxy-2-ethoxycarbonyl-3-[2-(5-methoxycarbonyl)furyl]chromones were obtained from the indicated acetophenones by reaction with ethoxalyl chloride.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1317–1320, October, 1972.  相似文献   
950.
9-Methylnaphtho[2,1-b][1,6]naphthyrldine and 2,3-dimethyl-1-azaphenanthrene were obtained on a K-16 dehydrogenating catalyst from N-(1,2,5-trimethyl-4-piperidylidene)--naphthylamine, while N-cyclohexylidene--naphthylamine yielded N-phenyl--naphthylamine and benzo[a]carbazole.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 10, pp. 1400–1403, October, 1972.  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