全文获取类型
收费全文 | 5799篇 |
免费 | 818篇 |
国内免费 | 629篇 |
专业分类
化学 | 1959篇 |
晶体学 | 85篇 |
力学 | 812篇 |
综合类 | 97篇 |
数学 | 1319篇 |
物理学 | 2974篇 |
出版年
2024年 | 16篇 |
2023年 | 62篇 |
2022年 | 112篇 |
2021年 | 139篇 |
2020年 | 121篇 |
2019年 | 127篇 |
2018年 | 110篇 |
2017年 | 174篇 |
2016年 | 236篇 |
2015年 | 184篇 |
2014年 | 300篇 |
2013年 | 438篇 |
2012年 | 317篇 |
2011年 | 425篇 |
2010年 | 358篇 |
2009年 | 374篇 |
2008年 | 384篇 |
2007年 | 426篇 |
2006年 | 352篇 |
2005年 | 348篇 |
2004年 | 269篇 |
2003年 | 258篇 |
2002年 | 247篇 |
2001年 | 178篇 |
2000年 | 174篇 |
1999年 | 147篇 |
1998年 | 114篇 |
1997年 | 110篇 |
1996年 | 91篇 |
1995年 | 86篇 |
1994年 | 87篇 |
1993年 | 82篇 |
1992年 | 56篇 |
1991年 | 54篇 |
1990年 | 37篇 |
1989年 | 32篇 |
1988年 | 27篇 |
1987年 | 18篇 |
1986年 | 19篇 |
1985年 | 23篇 |
1984年 | 18篇 |
1983年 | 8篇 |
1982年 | 27篇 |
1981年 | 14篇 |
1980年 | 18篇 |
1979年 | 6篇 |
1978年 | 7篇 |
1977年 | 4篇 |
1976年 | 11篇 |
1974年 | 4篇 |
排序方式: 共有7246条查询结果,搜索用时 15 毫秒
171.
《Surface and interface analysis : SIA》2003,35(5):429-431
Polycrystalline gold–nickel thin films are deposited on silicon (111) wafers by evaporation in a vacuum of 2 × 10?6 mbar. Concentration profiles of heat‐treated specimens are obtained by Auger electron depth profiling. The heat treatments are carried out in a vacuum furnace of 4 × 10?6 mbar in the temperature interval 473–773 K. The grain boundary diffusion coefficient is determined, using a modified Wipple model, to be (3 × 10?4 cm2 s?1) exp (?0.94 eV kT?1). It is concluded that interdiffusion in the investigated system is characterized by type B kinetics, and that grain boundary diffusion plays a dominant role in the mass transport process of such films. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
172.
173.
Nikos G. Tsierkezos 《Journal of solution chemistry》2007,36(10):1301-1310
The reduction of benzophenone (Bzph) in 3-pentanone (PEN), acetone (ACE), N,N-dimethylacetamide (DMA), N,N-dimethylformamide (DMF), tetrahydrofuran (THF), acetonitrile (ACN) and dimethyl sulfoxide (DMSO) with n-tetrabutylammonium hexafluorophosphate (TBAPF6) as background electrolyte was studied using the technique of cyclic voltammetry at the temperature of 263.15 K. The half-wave
potentials (E
1/2) were extracted. The reduction of Bzph occurs in two successive one-electron steps to produce first the free radical anion
Bzph− and then the dianion Bzph2−. The results indicated that the radical anion Bzph− is reoxidized to Bzph in all investigated solvent media whereas the dianion Bzph2− is reoxidized to Bzph− only in THF. The heterogeneous electron-transfer rate constants (k
s
) were evaluated by employing the electrochemical rate equation proposed by Nicholson. The rate of electron transfer for the
Bzph/Bzph− couple was found to be relatively slow in all investigated solvent media. Consequently, the electron-transfer processes can
be recognized as quasi-reversible. The diffusion coefficients (D) of Bzph in the investigated solvent media have been calculated using the modified Randles-Sevcik equation. The effect of
the physical and chemical properties of the solvent medium on the electrochemical behavior of Bzph has been examined. 相似文献
174.
The regularities of vapor-phase nitration of cellulose with HNO3 under conditions of natural convection and hindered heat removal in the absence of air were studied using the nonisothermal kinetic method. It was established that the nitration rate at the depth of conversion of 0.08 to 0.7 is described by the kinetic law d/dt =k
1
p/(1+), wherek
1 = 104.49±0.6 exp(–A/RT) s–1 atm–1, = 10–35.5±15.7exp(B/RT),A = 36.6±3.8 kl mol–1, andB = 203±88 kJ mol–1. The diffusion mechanism of vapor-phase nitration of cellulose, which explains the high value of activation energies, is discussed. The effective diffusion coefficient of HNO3 in cellulose at 25 °3.7 · 10–7 cm2 s–1) and the activation energy of diffusion (38.3±4.2 kJ mol–1) were estimated.For Part 1, see Ref. l.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 8, pp. 1981–1985, August, 1996. 相似文献
175.
在前人工作的基础上, 改进了液上空间气相色谱测活度系数的方法, 实验测定了TBP. 稀释剂和水形成的多个二元系、三元系和四元系的活度系数和密度. 选用的稀释剂有n-C_6H_(14)、n-C_7H_(16)、n-C_8H_(18)、C_6H_6、cy-C_6H_(12)、CCl_4和CHCl_3. 在Pierotti理论的基础上, 采用新的硬球作用表达式和径向分布函数, 并计及分子间的排斥能、色散能、取向能和诱导能, 建立了简单的活度系数模型, 并用于TBP和稀释剂体系的计算. 从二元系回归得到的分子参数较好地预测了三元系的活度系数. 相似文献
176.
Muñoz-Muriedas J Perspicace S Bech N Guccione S Orozco M Luque FJ 《Journal of computer-aided molecular design》2005,19(6):401-419
Summary The use of a recently proposed hydrophobic similarity index for the alignment of molecules and the prediction of their differences
in biological activity is described. The hydrophobic similarity index exploits atomic contributions to the octanol/water transfer
free energy, which are evaluated by means of the fractional partitioning scheme developed within the framework of the Miertus-Scrocco-Tomasi
continuum model. Those contributions are used to define global and local measures of hydrophobic similarity. The suitability of this computational strategy is examined for two series of compounds
(ACAT inhibitors and 5-HT3 receptor agonists), which are aligned to maximize the global hydrophobic similarity using a Monte Carlo-simulated protocol.
Indeed, the concept of local hydrophobic similarity is used to explore structure–activity relationships in a series of COX-2 inhibitors. Inspection of
the 3D distribution of hydrophobic/hydrophilic contributions in the aligned molecules is valuable to identify regions of very
similar hydrophobicity, which can define pharmacophoric recognition patterns. Moreover, low similar regions permit to identify
structural elements that modulate the differences in activity between molecules. Finally, the quantitative relationships found
between the pharmacological activity and the hydrophobic similarity index points out that not only the global hydrophobicity,
but its 3D distribution, is important to gain insight into the activity of molecules.
J.M.M. and S.P. have contributed equally to this study. 相似文献
177.
Hiltrud Breitenfelder-Manske Friedrich Kohler 《Monatshefte für Chemie / Chemical Monthly》1985,116(11):1247-1261
The second virial coefficients of homonuclear three-centerLennard-Jones molecules are calculated with various parameters of the isosceles triangle connecting the three sites. A special effort is made to establish the reducedBoyle temperaturesT
B and the values of the second virial coefficients atT/T
B=0.3 for the sake of comparison with one- and two-centerLennard-Jones molecules. It is shown that it is possible to find parameter values of the interaction potential of one- and two-centerLennard-Jones molecules which give very similar values of second virial coefficients forT/T
B0.3, and the equivalence conditions are established. These conditions might not only give a basis for a microscopic scaling of state variables, but also some restrictions for the validity of the group contribution concept.Presented in part at the DFG-Colloquium at Paderborn, 19th April 1982, and at the 5th Conference on Mixtures of Nonelectrolytes and Intermolecular Interactions, April 18–22, 1983, at Halle (GDR). 相似文献
178.
Abdelfetah Mounir Mohamed El Guendouzi Abderrahim Dinane 《Journal of solution chemistry》2002,31(10):793-799
Ternary aqueous solutions of MgSO4 and K2SO4 have been studied by the hygrometric method at 25°C. The relative humidity of this system is measured at total molalities from 0.35 mol-kg–1 to about saturation for three ionic-strength fractions (y = 0.25, 0.50, and 0.80 of MgSO4. The data allow calculation of water activities and osmotic coefficients. From these measurements, the Pitzer ionic mixing parameters are determined and used to predict the solute activity coefficients in the mixture. The results are used to calculate the excess Gibbs energy at total molalities for ionic-strength fraction y. 相似文献
179.
Thermogalvanic cells(also known as thermo-electrochemical cells) that convert waste heat energy to electricity are a new type of energy conversion device. However, the electron transfer kinetics and mass transfer of redox couples have not been thoroughly studied. Here, the ion reaction and charge transport in thermogalvanic cells are investigated by electrochemical impedance analysis. We first propose the detailed impedance model followed experimental verification on three types of electrode materials. Parameters including kinetic rate constants and ion diffusion coefficients for the electrodes are obtained by fitting the impedance data. Our study shows explicitly that impedance analysis can provide useful information on selecting suitable electrode materials for thermogalvanic cells. 相似文献
180.
Mathematical apparatus, which makes it possible to perform calculations of the current-voltage characteristics of cathodes of fuel cells with a solid polymer electrolyte in conditions where there are present extraneous diffusion restrictions is proposed. In so doing, the partial pressure of oxygen and the absolute pressure of gas in the gas chamber may assume any values. First of all presented are the results of calculations of the current-voltage characteristics intrinsic to active layers of the air and oxygen cathodes, which are performed under the assumption that the extraneous diffusion restrictions are absent altogether. Thereafter, in the same conditions (at the same parameters that characterize the active layer of a cathode), obtained are results of a calculation of the current-voltage characteristics inherent in the air and oxygen cathodes in the presence of extraneous diffusion restrictions. Afterward there is performed an analysis of the way a gas-diffusion layer restricts the process of generation of current in a cathode and of what measures should be taken in order for the extraneous diffusion restrictions to become less significant. 相似文献