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   18005篇
  免费   502篇
  国内免费   90篇
化学   11384篇
晶体学   278篇
力学   427篇
综合类   1篇
数学   2860篇
物理学   3647篇
  2023年   113篇
  2022年   236篇
  2021年   318篇
  2020年   304篇
  2019年   394篇
  2018年   292篇
  2017年   294篇
  2016年   609篇
  2015年   474篇
  2014年   564篇
  2013年   1186篇
  2012年   959篇
  2011年   1071篇
  2010年   793篇
  2009年   713篇
  2008年   932篇
  2007年   964篇
  2006年   785篇
  2005年   795篇
  2004年   669篇
  2003年   583篇
  2002年   518篇
  2001年   320篇
  2000年   285篇
  1999年   201篇
  1998年   215篇
  1997年   194篇
  1996年   193篇
  1995年   207篇
  1994年   149篇
  1993年   153篇
  1992年   149篇
  1991年   125篇
  1990年   146篇
  1989年   130篇
  1988年   116篇
  1987年   113篇
  1986年   138篇
  1985年   203篇
  1984年   217篇
  1983年   149篇
  1982年   136篇
  1981年   149篇
  1980年   152篇
  1979年   119篇
  1978年   130篇
  1977年   111篇
  1976年   102篇
  1975年   104篇
  1974年   99篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
We have obtained four new bispirane systems in which both spiroatoms are vicinal atoms in cyclopropane or cyclobutane rings and in which the heterocyclic component has the basic structure of 1,4-dioxep-2-ene. We have shown that from two possibilities (bispiran, propellan) the general reaction leads only to a bispiran system. To the bispiran2 exhibiting the highest hyperchromism we ascribed the bispirocoplanar conformation (Fig. 1).

Mit 3 Abbildungen  相似文献   
52.
The radiation-chemical processes in aqueous solutions of dichromates, isopolymolybdates and isopolytungstates have been studied. The radiation reduction of Cr(VI) to Cr(III), Mo(VI) to Mo(V) and W(VI) to W(V) takes place. Molybdenum and tungsten blues are the products of the last two processes in acid solutions. The yields of reduction decrease significantly with the increase of pH. The mechanisms of the processes have been discussed. The photochemical reduction of Mo(VI) and W(VI) for the production of corresponding 5-valent states have been used to the study of their electron scavenging efficiencies. It has been shown that these efficiencies decrease in the order: Mo(V)>W(V)>Cr(VI)>Mo(VI)>W(VI).  相似文献   
53.
Distribution coefficients of Pt, Ir, Pd and Rh in the systems Dowex 1 [Br?]?HBr plus 0.0035M bromine and Dowex 1 [Br?]?HBr plus 2% N2H4·HCl, were determined with the aid of radioactive tracers by batch equilibration for the resins of nominal cross-linking 2, 4 and 8, respectively. Column experiments showed that Dowex 1X4 should be used for separations because of the favourable dynamics of the column process. Elution conditions for the quantitative separation of both trace and milligram amounts of Ir?Pt, Rh?Pt, Pd?Pt and Ir?Pd?Pt mixtures are given. Some data on the effect of temperature upon the quality of separation are also presented.  相似文献   
54.
Aromatic compounds including phenols, carboxylic acids and aldehydes, can be rapidly separated on pellicular anion-exchange resins in the acetate form by elution with a sodium acetate solution. Like the aromatic carboxylic acids, hydroxybenzaldehydes are held as counter ions in the resin. The affinity of the resin for corresponding anions as well as that for sorbed non-electrolytes is largely influenced by the aromatic ring and non-polar aliphatic substituents. Formation of hydrogen bonds between the resin and phenolic groups is less important.  相似文献   
55.
A potentiometric reductimetric method for the determination of platinum (Pt(IV)Pt(II)) with a standard Fe(II) solution in an alkaline medium of mannitol is described. The method, the error of which does not exceed 2%, can be used in the presence of palladium, iridium, and rhodium.  相似文献   
56.
McSheehy S  Pohl P  Lobiński R  Szpunar J 《The Analyst》2001,126(7):1055-1062
Multidimensional (size-exclusion-anion-exchange-cation-exchange) liquid chromatography with ICP-MS detection was developed to produce a map of water-soluble species in an oyster test reference material. The presence of arsenobetaine, trimethyl(2-carboxyethyl)arsonium inner salt, arsenocholine, dimethylarsonic acid, tetramethylammonium ion, As(v) and two arsenosugars was demonstrated by ES-MS-MS. A previously unreported compound was isolated and identified by ES-MS-MS as 5-dimethylarsinoyl-beta-ribofuranose. Anion-exchange chromatography was optimized to produce a chromatographically pure peak of arsenobetaine (accounting for ca. 64% of all water-soluble As present) that was used to quantify this compound.  相似文献   
57.
Commercially available 1,4-dioxane, even of spectroscopic grade, contains various impurities that arise mostly from reaction of the solvent with oxygen. Aspects relevant to the spectroscopy and photochemistry of the dioxane/oxygen system are discussed. Methods of purification and storage of the solvent are presented.  相似文献   
58.
Substituted benzoic acid anions undergo decarboxylation in the medium-pressure region of an electrospray ion source yielding in most cases the correspondingly substituted phenide anions in high yield. The location of the anionic center is specified by the position of the carboxylic group. The only exceptions are compounds with substituents containing acidic hydrogen atoms, like OH and NH(2) groups. For such compounds, either an intra- or an intermolecular (mediated by the molecules of methanol or water) proton transfer from the more acidic position to the benzene ring is observed. The generated anions can be selected using the first quadrupole for studying their ion-molecule chemistry in the second quadrupole of a triple quadrupole mass spectrometer. Their reactions with CO(2), O(2), CH(3)COCH(3) and CCl(4) may serve as typical examples. The general applicability of this method for the generation of phenide anions has been confirmed on three different mass spectrometers. Experiments performed using carboxylic acids other then benzoic acid and its derivatives show that this method is not limited to phenide anions and can be used for the generation of a much wider range of carbanions in the gas phase.  相似文献   
59.
The photophysics and photochemistry of alpha-terthiophene (alphaT), compartmentalized in mixed nonionic/anionic micelles, have been investigated with focus on the influence of the micellar surface charge density on the formation of the radical coupling product alpha-hexathiophene (alphaH). By varying the ratio of nonionic-to-anionic surfactants, and assuming ideal mixing, the charge density of the mixed micelles was varied. From Poisson-Boltzmann calculations, performed using the cell model, the electrostatic potential and the counterion activity were estimated as a function of the distance from the micellar surface. Upon excitation, the triplet state of alphaT is formed, from which the alphaT radical cation can be formed by absorption of a second photon. The radical cation can form alphaH if it encounters another alphaT radical cation. Under the experimental conditions used, this implies that the alphaH formation only occurs if the compartmentalized radical cation is able to migrate from its host micelle to another micelle, either via the surrounding bulk or by fusion of two micelles followed by mixing of their contents before micellar fission. The formation yield of the radical cation depends on the charge density of the mixed micelle; a lower charge density, that is, an increased amount of nonionic surfactant, lowers the yield. The yield of the coupling product alphaH, however, does not follow the same trend. A maximum yield of alphaH is found at intermediate nonionic surfactant molar ratios. This behavior is understood in terms of the Poisson-Boltzmann simulation results and by comparing charge-density changes as a function of molar fraction with the changes in counterion activity. The alphaH yield is a result of the balance between an increased possibility of radical cation bulk migration and a lowered electrostatic stabilization of the radical.  相似文献   
60.
The generator is based on permeation of sulphur dioxide through silicone rubber tubes immersed in a buffered, thermostated solution of sodium hydrogensulphite. The generator contains three (or more) permeation tubes of different lengths immersed simultaneously in the generation solution. The tubes can be connected stepwise to the carrier gas source, so that three (or more) different concentrations of SO2 in the carrier gas can be obtained from a single solution. The tested generator produced 1.25–25.4 ng s?1 SO2, depending on the hydrogensulphite concentration in the solution. Long-term stability was tested for the production of 10.49 ng s?1 over 50 h; the relative standard deviation was 1.57%. The experimental conditions affecting the production of SO2 are discussed. An equation is derived for estimating the SO2 production for various solution compositions and surface areas of the permeation tubes.  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