全文获取类型
收费全文 | 379515篇 |
免费 | 4215篇 |
国内免费 | 1524篇 |
专业分类
化学 | 205181篇 |
晶体学 | 5664篇 |
力学 | 16250篇 |
综合类 | 8篇 |
数学 | 43007篇 |
物理学 | 115144篇 |
出版年
2021年 | 2473篇 |
2020年 | 2790篇 |
2019年 | 3099篇 |
2018年 | 3502篇 |
2017年 | 3462篇 |
2016年 | 5865篇 |
2015年 | 4263篇 |
2014年 | 6043篇 |
2013年 | 17717篇 |
2012年 | 13320篇 |
2011年 | 16533篇 |
2010年 | 10632篇 |
2009年 | 10696篇 |
2008年 | 14937篇 |
2007年 | 15006篇 |
2006年 | 13990篇 |
2005年 | 13017篇 |
2004年 | 11637篇 |
2003年 | 10258篇 |
2002年 | 10098篇 |
2001年 | 11638篇 |
2000年 | 8857篇 |
1999年 | 6889篇 |
1998年 | 5627篇 |
1997年 | 5578篇 |
1996年 | 5324篇 |
1995年 | 4922篇 |
1994年 | 4649篇 |
1993年 | 4622篇 |
1992年 | 5091篇 |
1991年 | 5098篇 |
1990年 | 4699篇 |
1989年 | 4674篇 |
1988年 | 4709篇 |
1987年 | 4557篇 |
1986年 | 4323篇 |
1985年 | 6176篇 |
1984年 | 6270篇 |
1983年 | 5141篇 |
1982年 | 5585篇 |
1981年 | 5383篇 |
1980年 | 5208篇 |
1979年 | 5243篇 |
1978年 | 5487篇 |
1977年 | 5296篇 |
1976年 | 5443篇 |
1975年 | 5021篇 |
1974年 | 5066篇 |
1973年 | 5363篇 |
1972年 | 3259篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
861.
C.S. Dimbylow I.J. McColm C.M.P. Barton N.N. Greenwood G.E. Turner 《Journal of solid state chemistry》1974,10(2):128-136
Crystal chemistry and phase relations for the bronze-forming region of the EuWO system have been investigated. A bronze EuxWO3 is stable up to 1000°C when x ? 0.125 and in the region 0.085 ? x ? 0.125 the symmetry is cubic. A tetragonal bronze exists at x = 0.05, and an orthorhombic bronze with a structure closely related to the orthorhombic form of WO3 exists below x = 0.01. Mössbauer spectra at room temperature and at 80 K indicate that in all these phases the europium is highly ionized as Eu(III) with no electron localization to give (EuII) even at low values for x. The decomposition products of the bronzes have been established, and the Mössbauer parameters for the highly nonstoichiometric tungstates EuxWO4 were determined. Both Eu(II) and Eu(III) resonances were obtained, and a cation vacancy model for EuxWO4 was found to fit the data best. In conformity with the foregoing data, a sample of composition “Eu2W2O7” was found not be be a pyrochlore but to comprise a mixture of Eu6WO12, EuxWO4, and W. The phase relationships for the europium bronze system EuxWO3 are compared with those of other ionic bronzes NaxWO3, LixWO3, and AlxWO3. 相似文献
862.
Polymorphic transformations of CoMoO4 were studied by means of high temperature X-ray measurements within the temperature range 25–1200°C. On heating phase a obtained from low temperature modification b a new modification a′ was discovered. Phase a obtained by thermal decomposition of solvated α-CoMoO4 shows different behaviour. At 700–930°C depending on the conditions of preparation it transforms irreversibly into still another modification a″. On cooling, a mixture of phases a + a″ is obtained, the presence of a″ being responsible for the explosionwise transition into b, observed around the room temperature. 相似文献
863.
A carbon-rod atomizer (CRA) fitted with a 'mini-Massmann' carbon rod was evaluated for routine analysis of petroleum and petroleum products for trace metal content by atomic-absorption spectroscopy. Aspects investigated included sensitivity, detection limit, effect of solvent type, and interferences. The results of analysis of oil samples with this technique were compared with those obtained by other techniques. Metals studied were silver, copper, iron, nickel, and lead. Sensitivity and detection limit values obtained with the CRA were similar to those obtained with the carbon-filament atomizer. Strong 'solvent effects' were observed as well as interference by cations. On the basis of this study, design changes for the CRA are suggested, with the object of minimizing 'solvent effects' and interferences, increasing the atomization efficiency, and increasing the residence time of the atomic vapour in the optical path of the instrumental system. 相似文献
864.
Friedel-Crafts reaction of benzylic or tertiary nitro compounds with benzene gave normal alkylation products. Similarly, 2-nitropropane (2-NP) gave cumene (1) which underwent further transformation affording a mixture whose behaviour with time was investigated : results were consistent with oxidative formation of the cumyl carbonium ion (7'). 相似文献
865.
Conjugated esters and amides react with aryldiazonium salts at room temperature (MeCN) to form the corresponding iminium hydrazone derivatives, which can be thermally cyclized to cinnoline-3-esters and cinnoline-3-amides. In general the intermediate iminium salts derived from the enamine amides cyclize faster than those from the enamine esters. Furthermore, the ease of cyclization depends upon the structure of the base-component of the enamine ester or the amide and the substituent in the aryl moiety of the diazonium salt. The configurational structure of the iminium hydrazones, studied by NMR spectroscopy, has been shown to involve H-bonding of the hydrazonyl N-H with the ester or the amide CO group. 相似文献
866.
Benzo[b] thiophene derivatives. XVIII. The sulfur isosteres of harmaline,harmine and related isomers
T. R. Bosin R. P. Maickel A. Dinner A. Snell E. Campaigne 《Journal of heterocyclic chemistry》1972,9(6):1265-1266
The sulfur analogs of harmaline, 7-methoxy-3,4-dihydro-1-methyl[1]benzothieno[2,3-c]-pyridine (Ib), harmine, 7-methoxy-1-methyl[1]benzothieno[2,3- c ]pyridine (IIb), and corresponding 6-methoxy isomers (Ic and IIc) have been synthesized for pharmacological evaluation as monoamine oxidase inhibitors. 相似文献
867.
Quantitative measurement of both bulk and localized oxygen concentrations is of importance in the study of oxidation kinetics.
We describe the combined application of two nuclear techniques, fast neutron activation analysis and (d, p) nuclear microprobe,
to the determination of oxygen concentrations and surface profiles for steam-oxidized Zircaloy-4 cladding specimens. Results
of measurements using these techniques are presented.
Work partially supported by Electric Power Research Institute Contract T.S.A. No. 20. 相似文献
868.
Reaction of a preformed lithium enolate and trimethyl orthoformate with added boron trifluoride leads to the corresponding α-dimethoxymethyl ketone. 相似文献
869.
The fast formation of cations of benzophenone, biphenyl, naphthalene, anthracene and phenanthrene in liquid cyclohexane was studied by nanosecond pulse radiolysis. Rate constants between 2.3 × 1010 M?1 s?1 (for naphthalene) and not below 2.5 × 1012 M?1 s?1 (for benzophenone) were derived and compared with the corresponding rate constants for anion formation. 相似文献
870.
Studies on Alkyl Metal Alkoxides of Aluminium, Gallium, and Indium. III. Properties and Vibrational Spectra of Dialkylgallium- and Indium Alkoxides The preparation and the properties of dialkyl metal alkoxides R2MOR′ (with R = CH3, CD3, C2H5; R′ = CH3, CD3 and M = Ga, In) are described. The vibrational data (IR and Raman) of the trimeric methyl derivatives point to puckered six-membered M3O3-ring systems. 相似文献