全文获取类型
收费全文 | 49710篇 |
免费 | 3177篇 |
国内免费 | 3543篇 |
专业分类
化学 | 20201篇 |
晶体学 | 396篇 |
力学 | 3953篇 |
综合类 | 532篇 |
数学 | 11245篇 |
物理学 | 20103篇 |
出版年
2024年 | 74篇 |
2023年 | 456篇 |
2022年 | 1119篇 |
2021年 | 1092篇 |
2020年 | 1126篇 |
2019年 | 1145篇 |
2018年 | 999篇 |
2017年 | 1162篇 |
2016年 | 1279篇 |
2015年 | 1012篇 |
2014年 | 1636篇 |
2013年 | 3083篇 |
2012年 | 1907篇 |
2011年 | 2135篇 |
2010年 | 1724篇 |
2009年 | 2786篇 |
2008年 | 3052篇 |
2007年 | 3453篇 |
2006年 | 2926篇 |
2005年 | 2321篇 |
2004年 | 1903篇 |
2003年 | 2222篇 |
2002年 | 2621篇 |
2001年 | 1979篇 |
2000年 | 1889篇 |
1999年 | 1591篇 |
1998年 | 1543篇 |
1997年 | 897篇 |
1996年 | 857篇 |
1995年 | 731篇 |
1994年 | 787篇 |
1993年 | 571篇 |
1992年 | 649篇 |
1991年 | 427篇 |
1990年 | 411篇 |
1989年 | 320篇 |
1988年 | 281篇 |
1987年 | 279篇 |
1986年 | 244篇 |
1985年 | 226篇 |
1984年 | 232篇 |
1983年 | 142篇 |
1982年 | 189篇 |
1981年 | 182篇 |
1980年 | 120篇 |
1979年 | 149篇 |
1978年 | 126篇 |
1977年 | 118篇 |
1976年 | 74篇 |
1973年 | 57篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
91.
Dushmantha Kannangara Hailong Zhang Wei Shen 《Colloids and surfaces. A, Physicochemical and engineering aspects》2006,280(1-3):203-215
The spreading and recoiling of water drops on several flat and macroscopically smooth model surfaces and on sized paper surfaces were studied over a range of drop impaction velocities using a high-speed CCD camera. The water drop spreading and recoiling results on several model hydrophobic and hydrophilic surfaces were found to be in agreement with observations reported in the literature. The maximum drop spreading diameter for those model surfaces at impact was found to be dependent upon the initial drop kinetic energy and the degree of hydrophobicity/hydrophilicity of the surface. The extent of the maximum drop recoiling was found to be much weaker for hydrophilic substrates than for hydrophobic substrates. Sized papers, however, showed an interesting switch of behaviour in the process of water drop impaction. They behave like a hydrophobic substrate when a water drop impacts on it, but like a hydrophilic substrate when water drop recoils. Although the contact angle between water and hydrophilic or hydrophobic non-porous surfaces changes from advancing to receding as reported in literature, the change of contact angle during water impact on paper surface is unique in that the level of sizing was found to have a smaller than expected influence on the degree of recoil. Atomic force microscopy (AFM) was used to probe fibres on a sized filter paper surface under water. The AFM data showed that water interacted strongly with the fibre even though the paper was heavily sized. Implications of this phenomenon were discussed in the context of inkjet print quality and of the surface conditions of sized papers. Results of this study are very useful in the understanding of inkjet ink droplet impaction on paper surfaces which sets the initial condition for ink penetration into paper after impaction. 相似文献
92.
Changes in the hybrid state of atomic orbitals of nitrogen and p-character of LEP, which occur under the effect of saturated hydrocarbon radicals and polar substituents in aliphatic amines, differ substantially. Therefore, the effect of the both substituents on the basicity constants of amines cannot be described by the single formal type of interaction. The anomalous changes in the basicity in the series of primary, secondary, and tertiary alkylamines, which are discussed in the literature, and the correlations pK
BH+ = f(*) and G
B = f(*) are, in fact, imaginary, because the alkyl radicals at the N atom do not manifest the electron-donor properties. 相似文献
93.
J. Šesták D. Sedmidubsky G. Moiseev 《Journal of Thermal Analysis and Calorimetry》1997,48(5):1105-1122
Thermochemical and thermodynamical properties of HTSC phases are reviewed for the Y-Ba-Cu-O system and also presented for the newly calculated Bi-Sr-Cu-O system stressing out stoichiometric and phenomenological viewpoints. Simulated data are listed for (H
298
o
-H
o
o
, phase transformation temperatures, standard entropies, standard enthalpies of formation, heat capacities in crystalline phase, etc. Pseudobinary phase diagrams are treated showing the effect of oxygen partial pressure particularly illustrated on the (Sr, Bi, Ba)-Cu-O system.The work was carried out under the project No. A 2010532 supported by the Grant Agency of Academy of Sciences of the Czech Republic and the grant No. 104/97/0589 financed by the Grant Agency of the Czech Republic. 相似文献
94.
The structures of B32 and B32H2–32 with Ih symmetry have been investigated by means of ab initio calculations at STO-3G level. The relationship between molecular orbitals of them has been analyzed and their bonding properties have been discussed. Then the possibility of their existence, as well as the similarity and difference between B32 (B32H2–32) and C60 (C60H60) have been inferred. 相似文献
95.
N. A. Talzhanov D. T. Sadyrbekov F. M. Smagulova R. M. Mukanov V. A. Raldugin M. M. Shakirov A. V. Tkachev G. A. Atazhanova B. I. Tuleuov S. M. Adekenov 《Chemistry of Natural Compounds》2005,41(2):178-181
Chemical components from the aerial part of the Kazakhstan population of Artemisia pontica, from which the rarely encountered flavonoids 7-O-methyl- and 4′,7-di-O-methyl-esters of apigenin were isolated for the first time, were identified. The complete chemical composition of the essential oil was established using GC-MS.__________Translated from Khimiya Prirodnykh Soedinenii, No. 2, pp. 143–145, March–April, 2005. 相似文献
96.
The mercury(II)-catalysed replacement of cyanide in hexacyanoferrate(II) by p-nitrosodiphenylamine (p-NDA) in aqueous solution has been investigated spectrophotometrically by measuring the change in absorbance of the green complex ion, [Fe(CN)(5).p-NDA](3-), at 640 nm. Activation parameters of the catalysed and uncatalysed reactions have been calculated. The effects of the dielectric constant and water content of the medium on reaction rates are used to explain the formation of a polar activated complex and suggest an I(d) mechanism for the catalysed reaction. The varying catalytic activity of Hg(II) as a function of concentration has been discussed. 相似文献
97.
B. Korchowiec M. Puggelli G. Gabrielli M. Nocentini C. Focardi 《Colloid and polymer science》1997,275(9):860-868
The behavior of the single spreading monolayers of β-lacto-globulin (β-LG) and dioleyl-phosphatidylcholine (DOPC), as well
as their mixtures, has been studied on subphases containing Na+ or Ca++ ions. The results show an influence of temperature and subphase on the studied systems. The behavior of the areas as a function
of the weight fraction of the two components shows significative and prevalently positive deviations from the additivity and
their bidimensional miscibility. The variation of ΔG
ex, ΔH
ex and ΔS
ex calculated for the DOPC–β-LG mixture having maximum deviation on two different supports allows to deduce that the interactions
are prevalently repulsive.
FTIR–ATR spectra of transferred plurilayers show that DOPC has a surface orientation which can originate the miscibility between
the protein and DOPC.
Received: 3 February 1997 Accepted: 3 June 1997 相似文献
98.
A. V. Afonin 《Russian Chemical Bulletin》1996,45(5):1142-1144
The analysis of the effect of the solvents on the proton chemical shifts in1H NMR spectra of 2-vinyloxypyridine indicates that the C—H...N interaction of weak intramolecular hydrogen bond type hinders the formation of intermolecular hydrogen C—H...X and C-H... bonds. The protonating solvents reduce the intramolecular C—H...N interaction due to association with the N atom of the pyridine cycle.Translated fromIzvestiya Akademii Nauk. Seriya Khimieheskaya, No. S, pp. 1202–1204, May, 1996. 相似文献
99.
Summary The elution behavior of alkali and earth alkaline cations has been studied on a newly synthesized weak cationexchanger (WCX). Applying this stationary phase isocratic separation of these cations is possible. The dependence of retention on competing ion concentration is described via a simple model. The influence of crown ethers as well as organic eluent additives on selectivity and peak efficiency is demonstrated. With optimized separation such cations can be determined with detection limits of about 0.5 ppm using a conductivity detector without ion suppression. 相似文献
100.
M. E. M. Emam 《中国化学会会志》2002,49(3):319-323
The kinetic and thermodynamic parameters such as reaction order, activation energy, enthalpy, free energy and entropy change related to the thermal decomposition of Co(II), Ni(II) and Cu(II) hydrazone complexes of 4‐benzamido‐1‐(4‐actylpyridine)‐3‐thiosemicarbazone (BACPT) and 4‐benzamido‐1‐(pyridine‐3‐aldehye)‐3‐thiosemicarb‐azone (BAPT) were evaluated from differential thermal analysis (DTA) curves using Thomas‐Clarke's method. The thermal stability of these complexes are discussed in relation to the nature of the metalion and of the ligand present. A suitable mechanism for the thermal decomposition process of the complexes based on the data of the thermogravimetric analysis (TGA) was suggested. 相似文献