全文获取类型
收费全文 | 3419篇 |
免费 | 109篇 |
国内免费 | 10篇 |
专业分类
化学 | 2262篇 |
晶体学 | 61篇 |
力学 | 72篇 |
数学 | 535篇 |
物理学 | 608篇 |
出版年
2023年 | 11篇 |
2022年 | 40篇 |
2021年 | 78篇 |
2020年 | 75篇 |
2019年 | 73篇 |
2018年 | 71篇 |
2017年 | 80篇 |
2016年 | 128篇 |
2015年 | 110篇 |
2014年 | 127篇 |
2013年 | 261篇 |
2012年 | 189篇 |
2011年 | 226篇 |
2010年 | 176篇 |
2009年 | 141篇 |
2008年 | 202篇 |
2007年 | 168篇 |
2006年 | 147篇 |
2005年 | 135篇 |
2004年 | 122篇 |
2003年 | 101篇 |
2002年 | 92篇 |
2001年 | 41篇 |
2000年 | 47篇 |
1999年 | 41篇 |
1998年 | 40篇 |
1997年 | 33篇 |
1996年 | 34篇 |
1995年 | 37篇 |
1994年 | 25篇 |
1993年 | 22篇 |
1992年 | 29篇 |
1991年 | 36篇 |
1990年 | 32篇 |
1989年 | 22篇 |
1988年 | 24篇 |
1987年 | 20篇 |
1986年 | 23篇 |
1985年 | 31篇 |
1984年 | 27篇 |
1983年 | 18篇 |
1982年 | 26篇 |
1981年 | 18篇 |
1980年 | 21篇 |
1979年 | 13篇 |
1978年 | 16篇 |
1977年 | 21篇 |
1976年 | 18篇 |
1975年 | 8篇 |
1969年 | 6篇 |
排序方式: 共有3538条查询结果,搜索用时 382 毫秒
31.
Bimetallic ruthenium-tin catalysts were prepared using the sol-gel method and the method of carrier impregnation. Two different
procedures were used for their reduction — reduction in hydrogen atmosphere under an increased temperature, and reduction
using an aqueous solution of sodium borohydride. Hydrogenation of 3-phenylpropenal (cinnamaldehyde) in tetrahydrofuran at
433 K and the total pressure 7 MPa was used as the model reaction for comparing the catalytic properties of the prepared catalysts.
It was demonstrated that a tin addition notably affected catalytic properties of ruthenium in the sense of a selectivity increment
related to the production of an unsaturated alcohol. 相似文献
32.
Marek Szymo
ski Jacek Ko
odziej Zbigniew Postawa Pawel Czuba Piotr. Piatkowski 《Progress in Surface Science》1995,48(1-4):83-96
Inelastic interactions of electrons with surfaces of ionic crystals result in emission of various particles such as ions, atoms and molecules. We will review such electron-stimulated desorption processes for the particular class of ionic crystals, namely for alkali halides. In this case, a dominant fraction of the emission is in the form of halogen and alkali atoms characterized by a thermal (Maxwellian) spectrum of translational energies. For several alkali halides (potassium and rubidium chlorides, bromides, and iodides), however, a significant part of the halogen atoms is ejected with nonthermal energies, i.e. energies of the order of 0.1 eV. The results of recent systematic studies of angular-resolved kinetic energy distributions of the emitted particles will be reported and current views on the electronic mechanisms of desorption will be described. In particular, it will be shown that the ESD mechanism can be understood in terms of the model involving a surface localisation of the so called “hot-holes” created by electron bombardment of alkali halides. A role of hot holes in ESD processes will further be discussed in relation to very recent experimental results obtained for the KBr crystals doped with In impurities which act as efficient hole traps. 相似文献
33.
Wagner-Czauderna Elżbieta Orlik Marek Zajączkowska Agnieszka Kalinowski Marek K. 《Transition Metal Chemistry》2004,29(3):276-279
Microsolvation of the [Ni(acac)(tmen)]+ complex by a series of aliphatic n-alcohols (Solv) has been studied in ClCH2CH2Cl solutions by spectrophotometry. Based on the changes in the electronic spectrum of the afore-mentioned complex, observed under the influence of any alcohol, the equilibrium constants for the formation of the [Ni(acac)(tmen)Solv]+ and [Ni(acac)(tmen)Solv2]+ species have been computed according to the algorithm presented in this work. It was found that, in all the systems studied, the stability of five-coordinated [Ni(acac)(tmen)Solv]+ is higher than that of octahedral [Ni(acac)(tmen)Solv2]+. The resulting values are discussed in terms of the Lewis basicity of alcohols. 相似文献
34.
Spectra of isobutylene with SnCl4 were studied at the temperature of 113°K in the dark and under illumination. Radical-cations of isobutylene are formed by irradiation of the isobutylene–SnCl4 mixture in n-heptane substantially more slowly than in the isobutylene–TiCl4 mixture. Polymerization of isobutylene occurs after increasing the temperature up to the melting point of this mixture. The yield of polymer depends on the duration of preirradiation, while the molecular weight of polyisobutylene (M η) does not depend on the preirradiation of reaction mixture. 相似文献
35.
Three synthetic routes to derivatives of 3,6 - diamino - 2,3,4,6 - tetradeoxy - DL - threo - hexopyranose were investigated. Addition of sodium azide in acetic acid to 6 - phthalimido - 5,6 - dihydro - 2 - pyrone gave 4-azido compound (7) of the erythro configuration. From methyl 2,4 - dideoxy - β -dl- erythro - hexopyranoside threo 4 - phthalimido - 6 - phthalimidomethyl - tetrahydro - 2 - pyrone (17) was obtained in three steps in low overall yield. Addition of sodium azide in acetic acid to butyl 6 - oxo - 2 - hydroxy - hex - 4 - enoate followed by methylation, amonolysis of the ester group, and reduction gave methyl 3,6 - diacetamido - 2,3,4,6 - tetradeoxy - α - DL - threo - hexopyranoside (26). 相似文献
36.
Summary Complexation equilibria of the Tl(I) ion with 18-crown-6 and dibenzo-18-crown-6 were studied polarographically in 10 nonaqueous solvents. The stability of the complexes is strongly influenced by the nature of solvents and varies with their Lewis basicities. It has been found that the logK
s value (K
s is the stability constant of the complex) can be well described by empirical relation logK
s=a DN+b, whereDN stands for the Gutmann donor number anda andb mark the regression coefficient. Addition of the second explanatory parameter, the acceptor number, is not statistically significant. This result is in agreement with the predominant role of Tl(I) ion solvation.
Elektrochemische Untersuchungen von Tl(I)-Kronenetherkomplexen in nichtwäßrigen Medien
Zusammenfassung Es wurden die Komplexierungsgleichgewichte des Tl(I)-Ions mit 18-Krone-6 und Dibenzo-18-krone-6 polarographisch in 10 nichtwäßrigen Lösungsmitteln untersucht. Die Stabilität der Komplexe wird sehr stark vom Solvens beeinflußt, wobei eine starke Abhängigkeit von der Lewis-Basizität beobachtet wird. Es wurde festgestellt, daß die logK s-Werte (K s ist die Komplexstabilitätskonstante) gut mit der empirischen Beziehung logK s=a DN+b beschrieben werden können, wobeiDN die Gutmann'sche Donorzahl unda undb die Regressionskonstanten bedeuten. Hinzunahme der Akzeptorzahl als zweiten Parameter bleibt statistisch insignifikant. Dieses Ergebnis stimmt mit dem dominierenden Einfluß der Tl(I)-Ionensolvatation überein.相似文献
37.
The overall stability constantsK
1 andK
2 of NdNO
3
2+
and Nd(NO3)
2
+
complexes were determined (K
1=1.77;K
2=1.28) using an extraction method with tri-n-butyl phosphate as the extractant. The ratio 1/2 of the stepwise stability constants is discussed. It was established that the Nd(NO3)
2
+
complex was an outer-sphere ion pair.
Die Assoziation von Nitrat-Ion mit Nd3+
Zusammenfassung Mittels einer Extraktionsmethode wurden die StabilitätskonstantenK 1 undK 2 von NdNO 3 2+ - und Nd(NO3)+-Komplexen bestimmt (K 1=1.77;K 2=1.28; Tri-n-butylphosphat als Extraktionsmittel). Das Verhältnis 1/2 der stufenweisen Stabilitätskonstanten wird diskutiert. Es stellte sich heraus, daß der Nd(NO3)+-Komplex als ein Outer-Shere-Ionenpaar vorliegt.相似文献
38.
Magdalena Biesaga Elzbieta Stolarczyk Krystyna Pyrzyńska Marek Trojanowicz 《Analytical sciences》2002,18(2):151-154
The silica-based Fe(III)-protoporphyrin and Zn-tetraphenylporphyrin stationary phases were examined for the HPLC separation of anions. The retention of nine common inorganic anions as well as benzoate anion (BA) and its hydroxy analogues (HBA) was examined using tartrate, acetate, and succinate eluents. The retention factors of inorganic anions on the FeProP stationary phase were in the order Cl- < NO3- < ClO4- < I- < SCN- and for organic anions benzoate < p-hydroxybenzoate < m-hydoxybenzoate < o-hydroxybenzoate. The retention factors of organic anions examined for a ZnTPP column were in the order p-HBA < m-HBA < BA < o-HBA. 相似文献
39.
M. Bokor T. Marek K. Süvegh K. Tompa A. Vértes Zs. Nemes-vetéssy K. Burger 《Journal of Radioanalytical and Nuclear Chemistry》1996,211(1):247-253
The complexes [Zn(1-propyltetraole)6](BF4)2 (Zn-ptz) and [Fe(methyltetrazole)6](BF4)2 (Fe-mtz) were investigated by positron annihilation and solid-state NMR spectroscopy. For Zn-ptz signs of structural rearrangements were found between 70 and 145 K. For the spin-crossover complex Fe-mtz a transition affecting the high-spin stat Fe2+ ions at lattice site B occurs below 15 K. 相似文献
40.
Using nano- and microsize extrusion, a simple synthetic procedure of preparing hollow monodispersed colloidal particles dispersed in an aqueous phase was developed. Hydrophobic styrene monomer containing 2-hydroxy-2-methyl propiophenone photoinitiator was forced into desired diameter membrane channels and stabilized by the hydrophobic regions of a liposome obtained from 1,2-dilauroyl-phosphocholine phospholipid in an aqueous phase. Such moieties exposed to 254-nm UV radiation polymerize monomers in the hydrophobic zone of the liposome, thus resulting in reinforced hollow vesicles. The size of such particles is controlled by the size of the membrane channels in the extruder and may vary from a few nanometers to micrometers, thus allowing the synthesis of monodisperse hollow colloidal spheres. 相似文献