全文获取类型
收费全文 | 1631篇 |
免费 | 123篇 |
国内免费 | 459篇 |
专业分类
化学 | 1998篇 |
晶体学 | 41篇 |
力学 | 19篇 |
综合类 | 3篇 |
物理学 | 152篇 |
出版年
2024年 | 1篇 |
2023年 | 24篇 |
2022年 | 44篇 |
2021年 | 57篇 |
2020年 | 66篇 |
2019年 | 69篇 |
2018年 | 58篇 |
2017年 | 66篇 |
2016年 | 78篇 |
2015年 | 61篇 |
2014年 | 72篇 |
2013年 | 164篇 |
2012年 | 93篇 |
2011年 | 111篇 |
2010年 | 76篇 |
2009年 | 111篇 |
2008年 | 105篇 |
2007年 | 97篇 |
2006年 | 96篇 |
2005年 | 87篇 |
2004年 | 103篇 |
2003年 | 93篇 |
2002年 | 72篇 |
2001年 | 38篇 |
2000年 | 38篇 |
1999年 | 49篇 |
1998年 | 42篇 |
1997年 | 40篇 |
1996年 | 47篇 |
1995年 | 43篇 |
1994年 | 20篇 |
1993年 | 15篇 |
1992年 | 16篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 3篇 |
1986年 | 4篇 |
1985年 | 7篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1978年 | 1篇 |
1974年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有2213条查询结果,搜索用时 187 毫秒
91.
This work presents our data concerning the synthesis and characterization of some Cr(III), Fe(III) and Zr(IV) complexes with
substituted (2-hydroxy-4-methoxy-phenyl)-phenyl-methanone - C14H12O3, denoted by (L1). The synthesis of these complex compounds was performed using melted urea as reaction medium. The obtained
complexes have been studied by chemical analysis, IR spectroscopy, X-ray diffraction and thermogravimetric analysis. Based
on the data resulting from the thermal behaviour of the studied complex compounds, the kinetic parameters of the thermal decomposition
reactions have been determined.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
92.
IntroductionItiswellknownthatmetallocenesactivatedwithmethylaluminoxane (MAO)arehighlyactivehomogeneousZiegler Nattacatalyststhatproducepolyolefinwithcon trolledstereoregularityandnarrowmolecularweightdistri bution ,1 4 however,themolecularweightofthepolym… 相似文献
93.
Introduction Tremendous advances have been witnessed during the last decade in the syntheses and application of Group 4 metal complexes because of the motivity which comes from both academic and industrial field. Among them, studies have been dominated by the complexes supported by bis or mono cyclopentadienyl ligands, such as Cp2MX2 or half sandwich amide (CGC) com-plexes.1-6 At that time chemists paid a little attention to the non-cyclopentadienyl complexes, but in recent years, many chem… 相似文献
94.
A new method for the determination of triphenyltin hydroxide using capillary column gas chromatography with a tin-selective flame photometric detector has been developed. Triphenyltin hydroxide and its potential metabolites are converted to methyl derivatives and separated on glass capillary columns coated with OV-101. Derivatization of triphenyltin hydroxide, triphenyltin chloride, diphenyltin dichloride, phenyltin trichloride, and bis-triphenyltin oxide is nearly quantitative with a minimum of redistribution products. The selectivity of the flame photometric detector is cearly demonstrated by the comparison of chromatographic profiles obtained from using both the flame photometric and flame ionization detectors. The use of this chromatographic system in the analysis of triphenyltin hydroxide in a fortified water sample demonstrates the potential use of this system in organotin residue chemistry. 相似文献
95.
The crucial factor of the reaction of 2,6-di-tert-butylphenol with alkali hydroxides is temperature, depending on which two types of potassium or sodium 2,6-di-tert-butylphenoxides are formed. These types exhibit different catalytic activity in the alkylation of 2,6-di-tert-butylphenol with methyl acrylate. More active forms of 2,6-But
2C6H3OK or 2,6-But
2C6H3ONa are synthesized at temperatures higher than 160 °C and are predominantly the monomers, which dimerize on cooling. The
data of 1H NMR, electronic, and IR spectra for the corresponding forms of 2,6-But
2C6H3OK and 2,6-But
2C6H3ONa isolated in the individual state are in agreement with cyclohexadienone structure. In DMSO or DMF, the dimeric forms of
2,6-di-tert-butylphenoxides react with methyl acrylate to form methyl 3-(4-hydroxy-3,5-di-tert-butylphenyl)propionate in 64–92% yield.
Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 12, pp. 2138–2143, December, 2006. 相似文献
96.
A two-stage, no-mix anaerobic digester of 145 L capacity was used to investigate the effect of controlling the pH of the methanogenic
stage on the biogas production and the pollution potential reduction of acid cheese whey. The digester was operated at a 15-d
hydraulic retention time, and a temperature of 35°C. Controlling the pH of the methanogenic stage increased the biogas production
rate and methane yield by 77.77 and 289.00%, respectively. Reductions of up to 32.19, 44.44, and 35.86% in the COD, solids
and nitrogen were achieved. 相似文献
97.
Chenmou Zheng Xinmin Zhang Shan He Qun Fu Deming Lei 《Journal of solid state chemistry》2003,170(2):221-226
V2O3 nanopowder with spherical particles was prepared by reducing pyrolysis of the precursor, (NH4)5[(VO)6(CO3)4(OH)9]·10H2O, in H2 atmosphere. The thermolysis process of the precursor in a H2 flow was investigated by thermogravimetric analysis and differential thermal analysis. The results indicate that pure V2O3 forms at 620°C and crystallizes at 730°C. The effects of various reductive pyrolysis conditions on compositions of V2O3 products were studied. Scanning electron micrographs show that the particles of the V2O3 powder obtained at 650°C for 1 h are spherical about 30 nm in size with more homogeneous distribution. Experiments show that nanopowder has larger adsorption capacity to gases and is more easily reoxidized by air at room temperature than micropowder. Differential scanning calorimetry experiment indicates that the temperature of phase transition of nano-V2O3 powder is −119.5°C on cooling or −99.2°C on heating. The transition heats are −12.55 J g−1 on cooling and 11.42 J g−1 on heating, respectively. 相似文献
98.
All methionine aminopeptidases exhibit the same conserved metal binding site. The structure of this site with either Co2+ions or Zn2+ions was investigated using density functional theory. The calculations showed that the structure of the site was not influenced by the identity of the metal ions. This was the case for both of the systems studied; one based on the X-ray structure of the human methionine aminopeptidase type 2 (hMetAP-2) and the other based on the X-ray structure of the E. colimethionine aminopeptidase type 1 (eMetAP-1). Another important structural issue is the identity of the bridging oxygen, which is part of either a water molecule or a hydroxide ion. Within the site of hMetAP-2 the results strongly indicate that a hydroxide ion bridges the metal ions. By contrast, the nature of the oxygen bridging the metal ions within the metal binding site of eMetAP-1 cannot be determined based on the results here, due to the similar structural results obtained with a bridging water molecule and a bridging hydroxide ion. 相似文献
99.
D. P. Krutko M. V. Borzov E. N. Veksler A. V. Churakov 《Russian Chemical Bulletin》2005,54(1):117-123
The intracomplex conversion of (2-diphenylphosphanoethyl)cyclopentadienyl zirconium and titanium complexes into the corresponding 2-phosphinothioyl and 2-phosphinoyl derivatives, viz., (η5-C5H5)[η 5-C5H4CH2CH2P(S)Ph2]ZrCl2, [η5-C5H4CH2CH2P(S)Ph2]ZrCl3, [η5:η1C5H4CH2CH2P(O)Ph2]ZrCl3·THF, and [η5:η 1-C5H4CH2CH2P(O)Ph2]TiCl3 (7), was performed. The NMR spectroscopy data revealed the following order of the coordination ability of the functional groups with respect to the Zr center: Ph2P=O > Ph2P > Ph2P=S. An analogous order was found for the monodentate ligands (Ph3P=O > Ph3P > Ph3P=S) with respect to (η5-C5H5)ZrCl3. The molecular structure of complex 7 was established by X-ray diffraction analysis. Coordination of the Ph2P=O group to the titanium atom was found retained both in the crystalline state and solution.__________Published in Russian in Izvestiya Akademii Nauk. Seriya Khimicheskaya, No. 1, pp. 116–122, January, 2005. 相似文献
100.
Dietrich Mootz Reinhard Seidel 《Journal of inclusion phenomena and macrocyclic chemistry》1990,8(1-2):139-157
The tetramethylammonium hydroxide-water system has been studied by low-temperature differential thermal analysis and X-ray powder diffraction. The melting diagram was constructed for concentrations between 66.7 and 100 mol% H2O. It shows the existence and stability ranges of as many as eight crystalline hydrate phases:- and-Me4NOH·2H2O (phase transition at –85°C, decomposition atca. 105°C), Me4NOH·4 H2O (melting point 44°C, incongruent), and-Me4NOH·5 H2O (phase transition at 42°C, melting point 68°C, congruent),- and-Me4NOH·7.5 H2O (phase transition at 6°C, melting point 16°C, incongruent), and Me4NOH·10 H2O (melting point –20°C, incongruent). The structures of all these phases, except the already known one of-Me4NOH·5 H2O, were determined from single-crystal MoK diffractometer data. The decahydrate and the high-temperature forms of the 7.5-hydrate and the pentahydrate are genuinepolyhedral clathrate hydrates, the first ones reported of a strong base. Their mostly novel three-dimensional anionic host structures, formed by the hydrogen-bonded OH– ions and H2O molecules, arefour-connected throughout, in spite of their proton deficiency which is apparently leveled by disorder. Disorder also affects the enclosed cationic Me4N+ guest species. Like the low-temperature form of the pentahydrate, that of the 7.5-hydrate has a clathrate-related, but not fully polyhedral structure, some of the oxygen atoms being three-connected only. The tetrahydrate presents the rare case of both a hydrogen bond of the type OH–...OH2 and a (deprotonated) water-channel structure. This is fully ordered and apart from that can be derived from the polyhedral one of the-pentahydrate just by removing the appropriate number of water molecules from certain positions. The structures of- and-Me4NOH·2 H2O contain identical one-dimensionalspiro chains [HO–(HOH)/42] with the hydroxide protonnot participating in the hydrogen bonding. The Me4N+ ion is ordered in the and disordered in the phase.Supplementary Data relating to this article are deposited with the British Library as Supplementary Publication No. SUP 82076 (66 pages).Dedicated to Dr D. W. Davidson in honor of his great contributions to the sciences of inclusion phenomena. 相似文献