全文获取类型
收费全文 | 870篇 |
免费 | 30篇 |
国内免费 | 3篇 |
专业分类
化学 | 559篇 |
晶体学 | 67篇 |
力学 | 10篇 |
数学 | 85篇 |
物理学 | 182篇 |
出版年
2022年 | 16篇 |
2021年 | 14篇 |
2020年 | 7篇 |
2019年 | 20篇 |
2018年 | 14篇 |
2017年 | 17篇 |
2016年 | 25篇 |
2015年 | 15篇 |
2014年 | 24篇 |
2013年 | 75篇 |
2012年 | 35篇 |
2011年 | 49篇 |
2010年 | 37篇 |
2009年 | 22篇 |
2008年 | 36篇 |
2007年 | 33篇 |
2006年 | 38篇 |
2005年 | 28篇 |
2004年 | 38篇 |
2003年 | 19篇 |
2002年 | 17篇 |
2001年 | 13篇 |
2000年 | 8篇 |
1999年 | 8篇 |
1998年 | 7篇 |
1996年 | 14篇 |
1995年 | 8篇 |
1994年 | 10篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 7篇 |
1990年 | 17篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 12篇 |
1986年 | 8篇 |
1985年 | 9篇 |
1984年 | 13篇 |
1983年 | 10篇 |
1982年 | 5篇 |
1981年 | 13篇 |
1980年 | 10篇 |
1979年 | 14篇 |
1977年 | 8篇 |
1976年 | 5篇 |
1947年 | 5篇 |
1946年 | 6篇 |
1941年 | 7篇 |
1940年 | 6篇 |
1937年 | 7篇 |
排序方式: 共有903条查询结果,搜索用时 31 毫秒
1.
Rajiv Manohar G. Tripathi A.K. Srivastava A.K. Prajapati 《Journal of Physics and Chemistry of Solids》2006,67(11):2300-2304
Dielectric properties of polymer-liquid crystal mixture, having constituent polymer, poly-butyl methacrylate (PBMA) and liquid crystal, cholesteryl nonanoate, are reported as a function of frequency and temperature. The measurement has been done in a temperature range of 300-375 K and frequency range of 100 Hz-10 MHz. The dielectric permittivity and dielectric loss shows significant changes with the addition of polymer molecules in liquid crystal. The significant feature of composite formation is that the pure liquid crystal and polymer do not show dielectric relaxation in the frequency range covered, while the composite shows relaxation peak at a particular frequency. The optical transmittance of pure liquid crystal and composite has also been measured and compared. 相似文献
2.
3.
A variety of aryl nitriles were prepared in excellent yields from the palladium acetate catalyzed coupling of aryl halides with Zn(CN)2 using polymer-supported triphenyl phosphine as the ligand and dimethylformamide as solvent under microwave irradiation conditions. 相似文献
4.
To improve aircraft crash safety, conditions critical to occupants survival during a crash must be known. In view of the importance of this problem, studies of post-crash dynamic behavior of victims are necessary in order to reduce severe injuries. In this study, crash dynamics program SOM-LA/TA, incorporating a dynamic model of the human body with a finite element model of the seat structure was used. Modifications were performed in the program for reconstruction of an occupant's head impact with the interior walls or bulkhead. A viscoelastic-type contact force model was used to represent the compliance characteristics of the bulkhead. Correlated studies of analytical simulations with impact sled test results were accomplished. A parametric study of the coefficients in the contact force model was then performed in order to obtain the correlations between the coefficients and the Head Injury Criteria. A measure of optimal values for the bulkhead compliance and displacement requirements was thus achieved in order to keep the possibility of a head injury as little as possible. This information could in turn be used in the selection of suitable materials for the bulkhead, instrument panel, or interior walls of an aircraft. 相似文献
5.
J.D. Desai 《Applied Surface Science》2006,252(6):2251-2258
Iron oxide thin films have been obtained by spray pyrolysis using 100% methanolic and ethanolic solutions of iron tri-chloride. The films were deposited onto ITO-coated glass substrates. The preparative conditions have been optimized to obtain compact, pin-hole-free and smooth thin films which are adherent to the substrate. The structural, morphological and compositional characterizations have been carried out by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The films deposited using ethanolic solution results into pure hematite; α-Fe2O3 thin films, however, films deposited using methanolic solution consists of hematite and maghemite-c phases of iron oxide. The films are nanocrystalline with particle size of 30-40 nm. The optical absorbance of the film was of the order of 105 cm−1. The optical band gap of films was found to be 2.26 and 2.20 eV for the films deposited using methanolic and ethanolic solutions, respectively. 相似文献
6.
The influence of humic substances on sorption and methylation processes for inorganic- and organotin species is presented. Four sediment samples from different locations of the Rivers Elbe, Mulde and Spittelwasser, Germany, with different organotin and humic contents were selected to extract the humic and fulvic acids. The various fractions—the original sediment, the humic acid, the fulvic acid and the residual sediment—were analysed for their organotin content. The individual buyltin species show quite different distribution patterns. Monobutyltin is found mostly associated with humic acids. Dibutyltin shows a nonunique behaviour. At low total organotin content, dibutyltin is found bonded to humic and fulvic acids, whereas at high organotin content dibutyltin is distributed more with the residual sediment. Most of the tributyltin remains in the sediment unextracted; only small quantities of it are in the fulvic acid fraction. Tetrabutyltin is only in the humic acid fraction when it binds to humic matter; it mostly remains in the sediment. General observations indicate that ionic butyltin species bind to fulvic acids whereas the non-polar tetrabutyltin is not found in the fulvic acid fractions in any of the samples. The appearance of monomethyl- and dimethyl-tin species in the humic and fulvic acid fractions after the alkaline extraction was surprising. There is a correlation between the humic content of the sample and the formation of methyltin species. Evidence is provided by experiments that humic substances act as methylation agents. 相似文献
7.
A theoretical framework is provided for generalizing the inferences drawn from the results of earlier experimental studies of kinetics of crystallization in oriented poly(ethylene terephthalate). The framework is obtained by combining extensions of classical nucleation theories in polymers3,4 and a theory of crystal growth with anisotropic incorporation of segments into growing crystals.15 It is shown that, while a very strong dependence of rate of primary nucleation on orientation does exist, there is a only a much weaker dependence of rate of crystal growth on the orientation of the crystallizing polymer. The theoretical formulation provided here would allow qualitative estimates for comparison with experiments. 相似文献
8.
Chandra S Kumar R 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2005,61(3):437-446
A new series of copper(II) complexes have been synthesized with macrocyclic ligands having three different donating atoms in the macrocyclic ring. It has been shown that the stereochemistry of complexes is dependent on the coordinated anions. These complexes are characterized by various physicochemical techniques, viz. elemental analysis, molar conductance, magnetic susceptibility measurements, IR, electronic, 1H NMR and EPR spectral studies. Cyclic voltammetric behavior of the complexes has also been discussed. The observed anisotropic g-values indicate that the chloro and acetato complexes are six-coordinate tetragonal. Whereas the sulfato and nitrato complexes are found to have five-coordinate square-pyramidal and four-coordinate square-planar geometry, respectively. 相似文献
9.
The unique advantages of quantum dot (QD) bioconjugates have motivated their application in biological assays. However, physical characterization of bioconjugated QDs after surface modification has often been overlooked. Here, biotinylated antibodies (biotin-IgG) were attached to commercial streptavidin-conjugated quantum dots (strep-QDs) at different stoichiometric ratios, and these QD bioconjugates were characterized with atomic force microscopy and discontinuous sodium dodecyl sulfate agarose gel electrophoresis (SDS-AGE). The results from these complementary analytical techniques showed that the molar ratio determined the relative sizes, molecular weights and morphologies of the QD bioconjugates. Additionally, the novel discontinuous SDS-AGE analysis confirmed specific binding between biotin-IgG and strep-QDs. Researchers who design QD bioconjugates for cell-based assays should consider stoichiometry-dependent differences in the physical properties of their QD bioconjugates. 相似文献
10.
A rapid and easy route to formamides by microwave assisted N-formylation of primary and secondary amines is described. Using an insoluble polymer or an inorganic solid-supported reagent as a formylating agent, microwave irradiation furnished the corresponding formamides in high yields, with reduced reaction time and solvent volume over the conventional approach. 相似文献