全文获取类型
收费全文 | 74824篇 |
免费 | 23437篇 |
国内免费 | 17134篇 |
专业分类
化学 | 46072篇 |
晶体学 | 946篇 |
力学 | 5071篇 |
综合类 | 38篇 |
数学 | 9576篇 |
物理学 | 53692篇 |
出版年
2024年 | 459篇 |
2023年 | 685篇 |
2022年 | 677篇 |
2021年 | 580篇 |
2020年 | 843篇 |
2019年 | 1371篇 |
2018年 | 1336篇 |
2017年 | 1977篇 |
2016年 | 2094篇 |
2015年 | 2304篇 |
2014年 | 2122篇 |
2013年 | 3882篇 |
2012年 | 4198篇 |
2011年 | 5942篇 |
2010年 | 9314篇 |
2009年 | 9488篇 |
2008年 | 3256篇 |
2007年 | 2830篇 |
2006年 | 2529篇 |
2005年 | 2900篇 |
2004年 | 3816篇 |
2003年 | 3127篇 |
2002年 | 2863篇 |
2001年 | 3109篇 |
2000年 | 2321篇 |
1999年 | 2523篇 |
1998年 | 2139篇 |
1997年 | 1932篇 |
1996年 | 2268篇 |
1995年 | 2724篇 |
1994年 | 2789篇 |
1993年 | 2854篇 |
1992年 | 2428篇 |
1991年 | 2089篇 |
1990年 | 1749篇 |
1989年 | 1844篇 |
1988年 | 1831篇 |
1987年 | 1126篇 |
1986年 | 1179篇 |
1985年 | 843篇 |
1984年 | 963篇 |
1982年 | 891篇 |
1981年 | 731篇 |
1980年 | 769篇 |
1979年 | 525篇 |
1978年 | 532篇 |
1977年 | 633篇 |
1976年 | 1042篇 |
1973年 | 438篇 |
1972年 | 533篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
61.
62.
Catalytic properties of silica-supported heteropolyacids (HPA) in a mechanical mixture with reduced Me-Ce oxides (Me = Ni,
Pd) in n-hexane isomerization are studied. The role of each component of the mixed oxides (Ce and, typically, Ni and Pd) and their
optimum content has been illuminated: cerium is not only beneficial for eliminating or preventing coke deposition but is also
effective for maintaining the Keggin structure of the highly-organized HPA during the reaction and probably allows a better
dispersion of the second metal species. Nickel and palladium, present as Ni0 and Pd0, reinforce the activation of the alkane, which is difficult to obtain by means of a direct attack by an acid, and, thus,
enhance noticeably the activity of the catalyst. The best mechanical mixtures are obtained with 30–70 wt % NiCeO-HPW/SiO2 and 50–50 wt % Pd0.1CeO-HPW/SiO2. These mixtures have the highest efficiency for a Ni/(Ni + W) atomic ratio of 0.66 and a Pd/(Pd + W) ratio of 0.40, respectively.
Finally, the conversion of n-hexane is in the order HPW > HSiW > HBW, which seems to be consistent with the order of their acid strength as per the literature,
but the isomerization selectivity appears to be slightly higher on HSiW.
Published in Russian in Kinetika i Kataliz, 2006, Vol. 47, No. 1, pp. 24–28.
The text was submitted by the authors in English. 相似文献
63.
We present all-atom molecular dynamics simulations ofn-hexane on the basal plane of graphite at monolayer and multilayer coverages. In keeping with experimental data, we find the
presence of ordered adsorbed layers both at single monolayer coverage and when the adsorbed layer coexists with excess liquid
adsorbate. Using a simulation method that does not impose any particular periodicity on the adsorbed layer, we quantitatively
compare our results to the results of neutron diffraction experiments and find a structural transition from a uniaxially incommensurate
lattice to a fully commensurate structure on increasing the coverage from a monolayer to a multilayer. The zig-zag backbone
planes of all the alkane molecules lie parallel to the graphite surface at the multilayer coverage, while a few molecules
are observed to attain the perpendicular orientation at monolayer coverage.
Dedicated to Professor C N R Rao on his 70th birthday 相似文献
64.
65.
Abstract. We propose a general approach to deal with nonlinear, nonconvex variational problems based on a reformulation of the problem
resulting in an optimization problem with linear cost functional and convex constraints. As a first step we explicitly explore
these ideas to some one-dimensional variational problems and obtain specific conclusions of an analytical and numerical nature. 相似文献
66.
In this paper, we give the following dominated theorem: Let φ(g) ∈ L1(G//K),φε(t)=ε> 0, and the least radical decreasing dominatedfunction φ(t) = sup |φ(y)| ∈L1(G//K). If shtφ(t) is monotonically decreasingon (0, ∞), then for any f∈L1loc(G//K) , the following inequality holds:sup |φε * f(x)| ≤ Cmf(x),where mf(x) is the Hardy-Littlewood maximal function of f, and C = ||φ||1.An application of this dominated theorem is also given. 相似文献
67.
Li Yingle Huang Jiying Gong Shuohong 《International Journal of Infrared and Millimeter Waves》2003,24(6):953-961
The dielectric constant for rain medium is investigated by utilizing the system identification method. The rain rate model and frequency model of permittivity in millimeter waves band for rain medium are presented. The results obtained with models are in very good agreement with references in calculating the attenuation of electromagnetic waves induced by rain, which Shows that the obtained models are valid and practicable. The cross-polar discrimination gotten with rain rate model is in agreement with references. 相似文献
68.
69.
R. Pizzoferrato L. Lagonigro T. Ziller A. Di Carlo R. Paolesse F. Mandoj A. Ricci C. Lo Sterzo 《Chemical physics》2004,300(1-3):217-225
We study the infrared emission at 1.54 μm of an organolanthanide complex, Er(III)-tetraphenylporphyrin [Er(TPP)acac], both as a result of direct optical excitation and via energy transfer from host π-conjugate polymers of type poly(arylene–ethynylene) [PAE]. In the first case, the emission of the neat complex is characterized in inert transparent materials and a value of the quantum yield at 1.54 μm φIR=4×10−4 is measured. Then, fluorescence resonance transfer is investigated in blends of Er(TPP)acac with PAEs by monitoring the quenching of the polymer fluorescence along with the enhancement of both the visible emission of the ligand and the near-infrared band of Er3+. These different procedures allow a detailed analysis of the transfer efficiency within a specific implementation of the Förster model for polymeric donors. The experimental values of the critical radius R0, ranging from 1.3 to 2.5 nm for the different blends, are in good agreement with theory for a wide interval of the physical and spectroscopic parameters. This suggests that other mechanisms for excitation transfer do not play a significant role in these materials. 相似文献
70.
Resistivity-temperature characteristics of sol gel YBa2Cu3Oy samples synthesized in flowing oxygen atmosphere 下载免费PDF全文
The relationship of resistivity versus synthesizing temperature of sol gel YBa_2Cu_3O_y samples was studied when prepared under flowing oxygen conditions. A set of high-temperature ρ-T curves was obtained for the whole process. After the sample finished the test measuring, its resistivity was ρ_{300}=9.83×10^{-3 }Ω·cm at room temperature. The ρ-T curve also showed that the orthorhombic-tetragonal phase transformation of sol-gel YBa_2Cu_3O_y sample occurred at 581℃ for the sample in the rising temperature process, but at 613℃ in the cooling process, lower than that of the samples made by using the conventional powder metallurgy methods. 相似文献