全文获取类型
收费全文 | 900篇 |
免费 | 25篇 |
国内免费 | 5篇 |
专业分类
化学 | 744篇 |
晶体学 | 6篇 |
力学 | 5篇 |
数学 | 95篇 |
物理学 | 80篇 |
出版年
2023年 | 4篇 |
2022年 | 13篇 |
2021年 | 9篇 |
2020年 | 20篇 |
2019年 | 17篇 |
2018年 | 8篇 |
2017年 | 8篇 |
2016年 | 22篇 |
2015年 | 17篇 |
2014年 | 13篇 |
2013年 | 33篇 |
2012年 | 72篇 |
2011年 | 74篇 |
2010年 | 42篇 |
2009年 | 49篇 |
2008年 | 63篇 |
2007年 | 64篇 |
2006年 | 66篇 |
2005年 | 51篇 |
2004年 | 31篇 |
2003年 | 44篇 |
2002年 | 49篇 |
2000年 | 12篇 |
1999年 | 10篇 |
1998年 | 9篇 |
1997年 | 8篇 |
1996年 | 14篇 |
1995年 | 9篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1991年 | 6篇 |
1988年 | 4篇 |
1987年 | 3篇 |
1986年 | 2篇 |
1985年 | 5篇 |
1984年 | 5篇 |
1983年 | 4篇 |
1982年 | 4篇 |
1980年 | 6篇 |
1979年 | 4篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1976年 | 4篇 |
1975年 | 2篇 |
1973年 | 3篇 |
1971年 | 2篇 |
1930年 | 2篇 |
1920年 | 2篇 |
1912年 | 2篇 |
1899年 | 4篇 |
排序方式: 共有930条查询结果,搜索用时 32 毫秒
1.
V. Boutou M. A. Lebeault-Dorget A. R. Allouche C. Bordas J. Chevaleyre 《Zeitschrift für Physik D Atoms, Molecules and Clusters》1997,40(1):448-453
In this article, we present results obtained on structures of small bare and oxygenated barium clusters produced by the laser vaporization technique. Contrary to the very weak Ba—Ba binding (0.202 eV), the Ba—O binding is rather strong (5.8 eV). As a consequence, extremely small amounts of oxygen in the cluster source are sufficient to induce the nucleation of oxygenated barium clusters. Surprisingly, at least for substoechiometric systems, BanOm clusters exhibit the same icosahedral structure as bare clusters do. Mass spectra seem to indicate that replacing a barium atom with a strongly polar BaO molecule does not affect the geometrical structure of the cluster. This aspect is more easily understandable if we assume that the oxygen atom takes place inside the structure of the Ban cluster without significant distortions. Ab-initio calculations have been undertaken in order to give a satisfactory account of this hypothesis. 相似文献
2.
Consider a portfolio containing heterogeneous risks. The premiums of the policyholders might not cover the amount of the payments which an insurance company pays the policyholders. When setting the premium, this risk has to be taken into consideration. On the other hand the premium that the insured pays has to be fair. This fairness is measured by a function of the difference between the risk and the premium paid—we call this function a distance function. For a given small probability of insolvency, we find the premium for each class, such that the distance function is minimized. Next we formulate and solve the dual problem, which is minimizing the insolvency probability under the constraint that the distance function does not exceed a given level. This paper generalizes a previous paper [Zaks, Y., Frostig, E., Levikson, B., 2006. Optimal pricing of a heterogeneous portfolio for a given risk level. Astin Bull. 36 (1), 161–185] where only a square distance function was considered. 相似文献
3.
A detailed analysis of one- and two-dimensional 1H and 13C NMR data for the endo and the exo adducts, obtained by Diels-Alder reaction of thuriferic and epithuriferic acids with cyclopentadiene is described. The unequivocal spectral data assignment of the endo and exo structures was complemented with molecular modelling studies and confirmed through X-ray diffraction studies. 相似文献
4.
García ME Riera V Ruiz MA Sáez D Vaissermann J Jeffery JC 《Journal of the American Chemical Society》2002,124(48):14304-14305
Protonation of [Mo2Cp2(mu-H)(mu-PHR*)(CO)4] (Cp = eta5-C5H5, R* = 2,4,6-C6H2tBu3) with HBF4.OEt2 gives the hydridophosphinidene complex [Mo2Cp2(mu-H)(mu-PR*)(CO)4]BF4, which is easily deprotonated with H2O to give the known phosphinidene complex [Mo2Cp2(mu-PR*)(CO)4] in 95% yield. Reaction of the latter with I2 gives the unsaturated phosphinidene complex [Mo2Cp2I2(mu-PR*)(CO)2], which exhibits an intermetallic distance of 2.960(2) A. Irradiation of solutions of [Mo2Cp2(mu-PR*)(CO)4] with UV light gives a mixture of the triply bonded [Mo2Cp2(mu-PR*)(mu-CO)2] and the hydridophosphido derivative [Mo2Cp2(mu-H){mu-P(CH2CMe2)C6H2tBu2}(CO)4] as major species. The latter complex results from an intramolecular C-H bond cleavage from a tBu group and has been characterized by spectroscopy and an X-ray study. Irradiation in the presence of HCC(p-tol) results in the insertion of the alkyne into the Mo-P bond to give [Mo2Cp2{mu-eta1:eta2,kappa-C(p-tol)CHPR*}(CO)4] structurally characterized through an X-ray study. 相似文献
5.
Cabrerizo FM Petroselli G Lorente C Capparelli AL Thomas AH Braun AM Oliveros E 《Photochemistry and photobiology》2005,81(5):1234-1240
Pterins are heterocyclic compounds with important biological functions, and most of them may exist in two acid-base forms in the pH range between 3 and 13 in aqueous solution. In this work, the photophysical properties of acid and basic forms of six compounds of the pterin family (6-hydroxymethylpterin [HPT], 6-methylpterin [MPT], 6,7-dimethylpterin [DPT], rhamnopterin [RPT], N-methylfolic acid [MFA], and pteroic acid [PA]) have been studied. The effects of the chemical nature of the substituents at position 6 of the pterin moiety and the effects of the pH on the absorption and emission properties are analyzed. The fluorescence characteristics (spectra, quantum yields, lifetimes) of these compounds have been investigated using the single-photon-counting technique. Results obtained for pterin derivatives containing small substituents with 1 carbon atom (HPT, MPT, DPT) and short hydrocarbon chain (4 carbon atoms) (RPT) are different from those found for pterin derivatives containing a p-aminobenzoic acid (PABA) moiety in the substituent (MFA and PA). Fluorescence quantum yields (Phi(F)) of the first group of compounds are relatively high (>/=0.4), whereas MFA and PA exhibit very small Phi(F) values (=0.01). 相似文献
6.
Payne RJ Toscano MD Bulloch EM Abell AD Abell C 《Organic & biomolecular chemistry》2005,3(12):2271-2281
Aromatic analogues of chorismate were synthesised as potential inhibitors of anthranilate synthase. Molecular modelling using GOLD2.1 showed that these analogues docked into the active site of Serratia marcescens anthranilate synthase in the same conformation as chorismate. Most compounds were found to be micromolar inhibitors of S. marcescens anthranilate synthase. The most potent analogue, 3-(1-carboxy-ethoxy)-4-hydroxybenzoate (K(I) 3 microM), included a lactyl ether side chain. This appears to be a good replacement for the enol-pyruvyl side chain of chorismate. 相似文献
7.
S. Bordas M. T. Clavaguera-Mora N. Clavaguera 《Journal of Thermal Analysis and Calorimetry》1988,33(3):719-725
For the alloy of eutectic composition in the quasibinary GeSe2-Sb2Se3 system the heat capacities of the glass, undercooled and stable liquid and crystalline phases have been measured using differential scanning calorimetry. Enthalpy, entropy and Gibbs free energy differences of vitreous and crystalline forms have been deduced for this alloy. The kinetic parameters of crystallization for glasses of nearby composition have been obtained by continuous heating experiments. The effective activation energy obtained has a value that accounts for the activation energy for nucleation calculated using simple nucleation theory formulae.
This research was supported by the Comisión Asesora de Investigatión Cientifica y Técnica (CAICYT) project no. 759-84 which is gratefully acknowledged. 相似文献
Zusammenfassung Die Wärmekapazitäten von Glas, unterkühlter und stabiler Flüssigkeit und kristallinen Phasen von Mischungen der eutektischen Zusammensetzung des quasibinären Systems GeSe2-Sb2Se3 werden mittels DSC gemessen. Aus den Ergebnissen werden Enthalpie, Entropie und freie Enthalpie der glasartigen und kristallinen Formen dieser Mischung abgeleitet. Die kinetischen Parameter der Kristallisation von Gläsern benachbarter Zusammensetzungen wurden durch temperaturprogrammierte Untersuchungen ermittelt. Die erhaltenen effektiven Aktivierungsenergien entsprechen dem für die Keimbildung nach einfachen theoretischen Formeln berechneten Wert.
, , GeSe2-Sb2Se3. , . . , .
This research was supported by the Comisión Asesora de Investigatión Cientifica y Técnica (CAICYT) project no. 759-84 which is gratefully acknowledged. 相似文献
8.
M. T. Clavaguera-Mora S. Suriñach M. D. Baró S. Bordas N. Clavaguera 《Journal of Thermal Analysis and Calorimetry》1991,37(6):1261-1268
It is shown that the study of the kinetics of crystallization of an amorphous alloy may be used to obtain the correct form of the low temperature part of the time-temperature-transformation (T-T-T) curves. A unified review of the current kinetics of crystallization studies is presented and the general kinetic equation which gives the reaction rate as a function of temperature and crystalline fraction is shown to contain information related to theT-T-T-curves. Finally examples of application to two metallic glasses, Nd3Fe77B20 and Fe67.5Co15Nb1.5B16, obtained by rapid solidification techniques are presented. The comparison between theory and experiment is satisfactory.
Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60th birthday
This work was supported by the Comision Interdepartamental de Cienca y Tecnologia, CICYT (project No. MAT88-0439) which are acknowledged. 相似文献
Zusammenfassung Es wird gezeigt, daß die Untersuchung der Kristallisationskinetik von amorphen Legierungen genutzt werden kann, die korrekte Form des Niedertemperaturbereiches der Zeit-Temperatur-Transformation (T-T-T) Kurven zu erhalten. Es wird ein einheitlicher Überblick über die jüngsten Untersuchungen von Kristallisationskinetiken gegeben und an der allgemeinen Gleichung der Reaktionsgeschwindigkeit als eine Funktion von Temperatur und Kristallanteil wird gezeigt, daß sie Informationen bezüglich der T-T-T-Kurven beinhaltet. Letztlich wird eine Anwendung an zwei, durch schnelles Erstarren gewonnenen metallischen Gläsern Nd3Fe77B20 und Fe67.5Co15Nb1.5B16 dargelegt. Zwischen Theorie und Praxis konnte eine ausreichende Übereinstimmung festgestellt werden.
Dedicated to Professor Dr. H. J. Seifert on the occasion of his 60th birthday
This work was supported by the Comision Interdepartamental de Cienca y Tecnologia, CICYT (project No. MAT88-0439) which are acknowledged. 相似文献
9.
J. Miguel Vadillo M. Esther Gonzalez Isabel Carretero J. Javier Laserna 《Mikrochimica acta》1995,118(3-4):273-282
Micellar liquid chromatography (MLC) and capillary zone electrophoresis (CZE) have been evaluated for the analysis of twelve banned drugs in sport including diuretics and -blockers. In MLC, a sodium dodecylsulphate aqueous solution has been used as mobile phase using an octadecylsilica column. In CZE, a pH 8 buffer solution and a silica capillary have been employed. Parameters of retention and efficiency have been compared. Limits of detection with UV detection at 254 nm and relative standard deviations for atenolol, furosemide, nadolol, spironolactone and triamterene were established and compared in both techniques. Examples of direct urine injection into the separation systems are presented. Drugs overlapping in MLC are well resolved in CZE, while the opposite is true for a limited number of drugs. Some interferences from urine may arise in CZE. The selectivity of analysis would be greatly enhanced by using both techniques, which require only filtration as pre-treatment. 相似文献
10.