全文获取类型
收费全文 | 2336篇 |
免费 | 206篇 |
国内免费 | 4篇 |
专业分类
化学 | 2045篇 |
晶体学 | 16篇 |
力学 | 38篇 |
数学 | 147篇 |
物理学 | 300篇 |
出版年
2023年 | 17篇 |
2022年 | 34篇 |
2021年 | 40篇 |
2020年 | 46篇 |
2019年 | 58篇 |
2018年 | 34篇 |
2017年 | 29篇 |
2016年 | 91篇 |
2015年 | 102篇 |
2014年 | 111篇 |
2013年 | 178篇 |
2012年 | 143篇 |
2011年 | 181篇 |
2010年 | 145篇 |
2009年 | 110篇 |
2008年 | 135篇 |
2007年 | 120篇 |
2006年 | 110篇 |
2005年 | 94篇 |
2004年 | 77篇 |
2003年 | 90篇 |
2002年 | 77篇 |
2001年 | 53篇 |
2000年 | 54篇 |
1999年 | 13篇 |
1998年 | 14篇 |
1997年 | 8篇 |
1996年 | 19篇 |
1995年 | 18篇 |
1994年 | 12篇 |
1993年 | 18篇 |
1992年 | 14篇 |
1990年 | 11篇 |
1989年 | 10篇 |
1988年 | 11篇 |
1987年 | 10篇 |
1985年 | 13篇 |
1984年 | 14篇 |
1983年 | 12篇 |
1982年 | 15篇 |
1981年 | 15篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 16篇 |
1977年 | 9篇 |
1976年 | 15篇 |
1975年 | 14篇 |
1974年 | 8篇 |
1958年 | 7篇 |
1957年 | 6篇 |
排序方式: 共有2546条查询结果,搜索用时 0 毫秒
51.
A. Michel T. Van Hoang B. Perrin Marie France Llauro 《Polymer Degradation and Stability》1981,3(2):107-119
It has been possible to explain the mechanisms of stabilisation and of the synergistic effects of the β-diketone derivatives claimed in a patent as new stabilisers for improving the efficiency of those recipes based upon zinc and calcium soaps in the prevention of the initial discoloration of poly(vinyl chloride).Using chlorohexene as a model compound for allylic chloride structures and benzoylacetone as a model compound for enolised β-diketone derivatives, it has been shown that the benzoylacetone can substitute allylic chlorine atoms through a C-alkylation reaction which takes place only in the presence of ZnCl2 as catalyst. This reaction drastically changes the percentage of the enol and causes the appearance of two bands at 1720 cm?1 and 1680 cm?1 in the infra-red spectrum due to the ketonic structures During the processing of the PVC on a rolling mill at 180°C in the presence of zinc and calcium stearates and benzoylacetone there is grafting of the ketone derivative through a C-alkylation reaction. There is a closed parallelism between the influence of the benzoylacetone on the dehydrochlorination of the chlorohexene and on the accumulation of chloride ions in the polymer matrix in the presence of zinc and calcium stearate. The synergistic effect of the benzoylacetone in the prevention of the initial discoloration is related to the substitution reaction through a C-alkylation which takes place only in the presence of zinc stearate which generates ZnCl2 which, in turn, acts as a catalyst for both the C-alkylation and dehydrochlorination. 相似文献
52.
M. Pealat D. Debarre J.-M. Marie J.-P.E. Taran A. Tramer C.B. Moore 《Chemical physics letters》1983,98(4):299-304
Coherent anti-Stokes Raman scattering (CARS) has been used to observe H2 from H2CO + hv H2 + CO. Photolysis of K = 1 (ortho) lines of the 2141 S1 ← S0 transition produced no para-H2 (<4% statistical); and demonstrated nuclear spin conservation. H2 states v = 1, 2 and 3 were comparably populated. 相似文献
53.
Sarah C. Hubbard 《Tetrahedron》2005,61(31):7425-7430
The preparation and investigation of triplet photosensitizers designed to be preferentially soluble in room-temperature ionic liquids are reported. Photosensitizers prepared by covalent attachment of 1-methylimidazole to aryl ketones are soluble in ionic liquids and remain in the ionic liquid layer when the solution is extracted with an organic solvent. The photosensitized isomerization of trans-β-ionol to cis-β-ionol was efficiently carried out in ionic liquid solution with the product ionol being extracted and the sensitizer/ionic liquid mixture being re-used in additional photosensitization reactions. The scope and utility of the sensitizers in sensitizing other reactions are discussed. 相似文献
54.
Ab initio SCF computations on the intrinsic preferences of the H+, CH
3
+
and C2H
5
+
cations towards the two principal sites of protonation or alkylation on cytosine, N3 or O2, show that this preference undergoes a continuous modification with the increase in size and complexity of the cation. N3 is the preferred site of fixation of H+, O2 the preferred site of C2H
5
+
, while CH
3
+
has no marked preference. The exchange repulsion term of the binding energy appears responsible for the preference of C2H
5
+
for O2.This work was supported by the Ligue Francaise contre le Cancer and the National Foundation for Cancer Research (USA) 相似文献
55.
Recently we characterised a novel 29 kDa endoplasmic reticulum protein that is widely expressed in rat tissues, and named it ERp29. Several ERp29-like gene products have been reported in human tissues but uncertainty surrounds their relationships with each other and rat ERp29. To clarify these issues, ERp29 was isolated from human liver and characterised by primary structural analysis and two-dimensional gel mapping. Comparisons with rat ERp29 revealed striking homologies both in sequence and physical properties. Characterisation of the isoelectric heterogeneity and anomalous mass on two-dimensional gels enabled two reported homologues (UL35 and ERp31) to be identified as ERp29. Resolution of a sequence discrepancy led to unequivocal correlation of human ERp29 with the cognate cDNA previously named ERp31 and ERp28. Consequent links established to human genome and proteome projects showed that ERp29 is encoded by a gene on chromosome 12 that is expressed universally in human tissues. Together, these findings unified various ERp29 homologues as products of a single gene orthologous to rat ERp29 and established ERp29 as the only known member of a new protein class. Investigations of ERp29 function in human health and disease should benefit from the integrated links between genome, proteome and murine model organisms established here. 相似文献
56.
Mihaylov M Chakarova K Hadjiivanov K Marie O Daturi M 《Langmuir : the ACS journal of surfaces and colloids》2005,21(25):11821-11828
Different carbonyls are formed after CO adsorption at ambient temperature on a Pt-Na-mordenite (Pt-Na-MOR) sample. Pt(3+)(CO)(2) dicarbonyls (nu(s) at 2205 cm(-1) and nu(as) at 2167 cm(-1)) are decomposed without formation of monocarbonyls. The respective mixed-ligand species, Pt(3+)((12)CO)((13)CO), formed after (12)CO-(13)CO coadsorption, display bands at 2192 and 2131 cm(-1), in excellent agreement with the theoretically calculated values. Pt(2+)-CO species absorb at 2145 cm(-1) and are not able to accept a second CO molecule. Pt(+)-CO carbonyls are characterized by a band at 2111 cm(-1). Under CO equilibrium pressure, these species are converted into dicarbonyls (nu(s) at 2135 cm(-1) and nu(as) at 2101 cm(-1)). The respective mixed-ligand species, Pt(+)((12)CO)((13)CO), manifest bands at 2123 and 2069 cm(-1), in good agreement again with the theory. Different carbonyls of metallic platinum are observed below 2100 cm(-)(1). In addition, weakly adsorbed CO was registered as Na(+)-CO complexes (2177 and 2165 cm(-1)) and Na(+)-OC-Na(+) species (2138 cm(-1)). It was found that during desorption of CO platinum was reduced, ultimately to metal. However, heating in a NO + O(2) mixture leads to reoxidation of the metal particles and restoration of the initial state of the sample. 相似文献
57.
Spyridon Divanis Tugce Kutlusoy Ida Marie Ingmer Boye Isabela Costinela Man Jan Rossmeisl 《Chemical science》2020,11(11):2943
Multiple strategies to overcome the intrinsic limitations of the oxygen evolution reaction (OER) have been proposed by numerous research groups. Despite the substantial efforts, the driving force required for water oxidation is largely making the reaction inefficient. In the present work, we collected published studies involving DFT calculations for the OER, with the purpose to understand why the progress made so far, for lowering the overpotential of the reaction, is relatively small. The data revealed that the universal scaling relationship between HO* and HOO* intermediates is still present and robust, despite the variety in methods and structures used for calculating the binding energies of the intermediates. On the other hand, the data did not show a clear trend line regarding the O* binding. Our analysis suggested that trends in doped semiconducting oxides behave very differently from those in other oxides. This points towards a computational challenge in describing doped oxides in a realistic manner. We propose a way to overcome these computational challenges, which can be applied to simulations corresponding to doped semiconductors in general.Multiple strategies to overcome the intrinsic limitations of the oxygen evolution reaction (OER) have been proposed by numerous research groups. 相似文献
58.
59.
Laurent Griffe Dr. Mary Poupot Dr. Patrice Marchand Dr. Alexandrine Maraval Dr. Cédric‐Olivier Turrin Dr. Olivier Rolland Pascal Métivier Dr. Gérard Bacquet Dr. Jean‐Jacques Fournié Dr. Anne‐Marie Caminade Dr. Rémy Poupot Dr. Jean‐Pierre Majoral Dr. 《Angewandte Chemie (International ed. in English)》2007,46(14):2334-2334
60.
The Iteraltive Extended Hückel Molecular Orbital method has been adapted to calculation of the properties of an electrode and compact layer. Predictions of the stablest orientations, on the Pt(111) surface of species such as H2O, Pt, OH?, H, and the halides, F?, Cl?, Br? and I?, based upon calculation of the total energy corresponding to various internuclear distances, are reported. The calculations correctly predict self-adsorption of Pt on the Pt(111) surface at the face-centered cubic closest-packing position. The H2O molecule is predicted to locate itself above three adjoining Pt atoms, with the O atom closest to the surface and the H atoms opposite the O. Similar results were obtained for OH? and the halides. Atomic H, however, is predicted to drop into the plane of centers of the Pt surface atoms, where it would lie between, three adjacent Pt atoms. Application of the method to electrode studies requires only modest amounts of computer time but produces surprisingly reliable qualitative predictions. Compulation of electrochemical quantities such as charge, differential capacitance, surface tension and potential energy as a function of electrode potential will be described in future work. 相似文献