全文获取类型
收费全文 | 203篇 |
免费 | 4篇 |
专业分类
化学 | 101篇 |
晶体学 | 8篇 |
力学 | 6篇 |
数学 | 39篇 |
物理学 | 53篇 |
出版年
2023年 | 2篇 |
2022年 | 6篇 |
2021年 | 6篇 |
2020年 | 2篇 |
2019年 | 4篇 |
2018年 | 3篇 |
2017年 | 2篇 |
2016年 | 4篇 |
2015年 | 8篇 |
2014年 | 5篇 |
2013年 | 19篇 |
2012年 | 13篇 |
2011年 | 16篇 |
2010年 | 7篇 |
2009年 | 9篇 |
2008年 | 11篇 |
2007年 | 13篇 |
2006年 | 10篇 |
2005年 | 5篇 |
2004年 | 9篇 |
2003年 | 1篇 |
2002年 | 4篇 |
2001年 | 1篇 |
2000年 | 3篇 |
1999年 | 1篇 |
1998年 | 3篇 |
1997年 | 2篇 |
1996年 | 1篇 |
1995年 | 1篇 |
1994年 | 3篇 |
1993年 | 2篇 |
1992年 | 5篇 |
1991年 | 2篇 |
1990年 | 2篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 1篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 1篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1966年 | 1篇 |
1965年 | 1篇 |
1963年 | 1篇 |
1937年 | 1篇 |
排序方式: 共有207条查询结果,搜索用时 15 毫秒
51.
The equilibrium of three liquid phases in solutions of two polystyrene homologues in cyclohexane has been studied experimentally for different values of the average molecular weights of the two polymer species. The compositions of the phases have been determined in some cases. The variation in the size of the three-phase region with change in the molecular weights is discussed, particularly in connection with the occurrence of a tricritical point. Some qualitative observations of the phase behavior in mixed solvent systems are mentioned. 相似文献
52.
53.
Nickel substitution at the cobalt site in Na0.75CoO2 induces an upturn in the resistivity on lowering the temperature, with the metal-to-insulator transition temperature (T
MIT) increasing with the Ni content. Low temperature far infrared measurements on polycrystalline samples of Na0.75CoO2 and Na0.75Co0.95Ni0.05O2, the latter having T
MIT ∼ 175 K, have been carried out. Dramatic changes in the Na mode frequencies, and relative intensities of the out-of-plane
modes corresponding to the two Na sites are observed, coincident with the MIT in Na0.75Co0.95Ni0.05O2. It is argued that these changes are associated with a charge ordering of the CoO2 layer, associated with the metal-insulator transition. 相似文献
54.
Ceria supported copper catalysts were synthesized by laser vaporization and controlled condensation method and characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), energy dispersive X-ray analysis (EDAX) and temperature programmed reduction (TPR). The catalytic activity of the nanopowders for CO oxidation reaction was tested in a fixed bed flow tube reactor in Ar–20%O2–4%CO mixture. Irrespective of the copper content, the catalytic activity of the nanopowders is similar in the initial CO test, and the catalytic activity improves (i.e. the light-off temperature decreases) during a subsequent run. The lowest light-off temperature during the second run is recorded in the material with 20% copper. TEM studies on 20%Cu–CeO2 sample in the as-prepared condition and after CO test exhibit two types of ceria particles namely, polygonal particles 3–5 nm in size and spherical particles of 15–20 nm in size. Rapid cooling of the nanoparticles formed during the laser ablation results in incorporation of a large amount of copper within the ceria as solid solution. Presence of solid solution of copper is confirmed by EDAX and electron diffraction analyses. In addition, copper-rich surface layer of Cu2O is found over the spherical particles. The cerium oxide components are essentially identical before and after CO test, except that the polygonal CeO2 particles contain newly formed fine crystals of CuO. TPR results reveal two reduction peaks, which further supports, the presence of two different copper species in the material. The shift in light-off temperature during the second run is attributed to the synergistic interaction between newly formed CuO crystals with the CeO2 matrix. 相似文献
55.
The set of problems we consider here are generalizations of square-free sequences [A. Thue, Über unendliche Zeichenreichen, Norske Vid Selsk. Skr. I. Mat. Nat. Kl. Christiana 7 (1906) 1-22]. A finite sequence a1a2…an of symbols from a set S is called square-free if it does not contain a sequence of the form ww=x1x2…xmx1x2…xm,xi∈S, as a subsequence of consecutive terms. Extending the above concept to graphs, a coloring of the edge set E in a graph G(V,E) is called square-free if the sequence of colors on any path in G is square-free. This was introduced by Alon et al. [N. Alon, J. Grytczuk, M. Ha?uszczak, O. Riordan, Nonrepetitive colorings of graphs, Random Struct. Algor. 21 (2002) 336-346] who proved bounds on the minimum number of colors needed for a square-free edge-coloring of G on the class of graphs with bounded maximum degree and trees. We discuss several variations of this problem and give a few new bounds. 相似文献
56.
LiCo0.2Fe0.8PO4 cathode material was prepared by succinic assisted soft combustion technique. This method of synthesis ensures homogeneity
of the precursor material, improved activity and crystal grain size. Calcinations at high temperature confirm the purity of
the cathode material. The aim of this work is to investigate the effects of cobalt substitution in LiFePO4. The structural characterization of the products was analysed by the X-ray powder diffraction. Conductivity measurements
were made to characterize the transport property of the cathode material. Fourier transform infrared spectroscopy and differential
scanning calorimeter and thermogravimetry analysis have been studied to investigate the functional group present and the phase
transition involved in the sample, respectively. 相似文献
57.
Optical absorption spectrum of VO2+ ion in sodium zinc sulphate tetrahydrate single crystal has been reported at room and liquid nitrogen temperatures. From the nature and position of the observed bands, they have been attributed to VO2+ ion in C4v symmetry. The crystal field and molecular orbital coefficients have been evaluated. 相似文献
58.
AC conductivity and dielectric properties of sulphate glasses have been studied as a function of temperature, frequency and
variation in interalkali concentration. AC conductivity at frequencies well beyond the dielectric loss peaks seems to arise
from local motion of alkali ions within the neighbouring potential wells. Activation energies for AC conductivity were found
to be very much lower than those for DC conductivity. Further, AC conductivity seems to be independent of interalkali variation,
whereas ε′ and tan δ show a mild degree of mixed alkali effect. The observations made here have been explained on the basis
of a structural model earlier proposed by us for these glasses.
Communication No. 167 from Solid State and Structural Chemistry Unit. 相似文献
59.
Birch reductio and reductive methylations of some substituted naphtholic acids have been examined. The factors influencing the mechanism of reduction process have been discussed. Some of the reduced naphthoic acids are useful synthons for synthesis. 相似文献
60.
Constant potential coulometry at ?0.1 V of decamethylferrocene in a 1.5/l aluminum chloride/1-butylpyridinium chloride melt gave a deep brown solution containing the decamethylferricinium dication. 相似文献