全文获取类型
收费全文 | 169180篇 |
免费 | 4605篇 |
国内免费 | 2838篇 |
专业分类
化学 | 95905篇 |
晶体学 | 2645篇 |
力学 | 7388篇 |
综合类 | 190篇 |
数学 | 15842篇 |
物理学 | 54653篇 |
出版年
2022年 | 1497篇 |
2021年 | 1870篇 |
2020年 | 2017篇 |
2019年 | 2075篇 |
2018年 | 2341篇 |
2017年 | 2417篇 |
2016年 | 3548篇 |
2015年 | 2589篇 |
2014年 | 3503篇 |
2013年 | 7608篇 |
2012年 | 6677篇 |
2011年 | 7867篇 |
2010年 | 5453篇 |
2009年 | 5447篇 |
2008年 | 7150篇 |
2007年 | 6811篇 |
2006年 | 6540篇 |
2005年 | 5876篇 |
2004年 | 5188篇 |
2003年 | 4745篇 |
2002年 | 4619篇 |
2001年 | 5984篇 |
2000年 | 4405篇 |
1999年 | 3538篇 |
1998年 | 2508篇 |
1997年 | 2609篇 |
1996年 | 2521篇 |
1995年 | 2267篇 |
1994年 | 2271篇 |
1993年 | 1984篇 |
1992年 | 2458篇 |
1991年 | 2527篇 |
1990年 | 2296篇 |
1989年 | 2225篇 |
1988年 | 2187篇 |
1987年 | 2177篇 |
1986年 | 2003篇 |
1985年 | 2453篇 |
1984年 | 2443篇 |
1983年 | 1958篇 |
1982年 | 1947篇 |
1981年 | 1776篇 |
1980年 | 1766篇 |
1979年 | 2043篇 |
1978年 | 2110篇 |
1977年 | 2052篇 |
1976年 | 1980篇 |
1975年 | 1866篇 |
1974年 | 1847篇 |
1973年 | 1879篇 |
排序方式: 共有10000条查询结果,搜索用时 16 毫秒
51.
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
52.
Ana Luísa Daniel‐da‐Silva João Carlos Moura Bordado José Miguel Martín‐Martínez 《Journal of Polymer Science.Polymer Physics》2007,45(22):3034-3045
The degree of phase separation in several moisture‐cured poly(urethane urea)s (PUUs) was studied by FTIR spectroscopy, wide angle X‐ray diffraction (WAXD), and temperature‐modulated differential scanning calorimetry (TMDSC). This latter technique was shown to be particularly useful in analysing the degree of phase separation in PUU polymers. Both phase mixing and phase segregation coexisted in the PUUs and the degree of phase separation increased as the urea hard segment (HS) content in the PUU increased. The maximum solubility of urea HSs into the polyol soft segments (SSs) was achieved for 50 wt % urea HS content in diol‐based PUUs, whereas for triol‐based PUUs the highest solubility between HS and SS was reached for lower urea HS amount. Finally, the higher the urea HS content the higher the extent of phase separation in the PUU. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 3034–3045, 2007 相似文献
53.
The results of a numerical study are described in which the interactions of a primary shock wave with a secondary diaphragm
in expansion tubes are taken into account. The developing wave pattern in the interacting process of the shock with a secondary
diaphragm are visualized by many kinds of figures (e.g., the time-distance diagrams of the wave phenomena on the axis, the
acoustic impedance contours, and the time histories of the pitot pressure on the axis), and the influences of the shape and
rupture process of the diaphragm on the quality of the test gas are explored. 相似文献
54.
The electro-optic and complex dielectric behaviour of an antiferroelectric liquid crystal 4-(1-methylheptyloxycarbonyl)phenyl 4'-(n-butanoyloxyprop-1-oxy)biphenyl-4-carboxylate, having chiral SmCA* and hexatic smectic phases, have been investigated. Complex dielectric permittivities were measured as a function of frequency, d.c. bias field and temperature. Spontaneous polarization was measured by the current reversal technique; tilt angle was measured under a polarizing microscope using a low frequency electric field. The electro-optic properties and dielectric behaviour of the material are compared with results obtained by DSC and polarizing optical microscopy. Dielectric relaxation processes in SmCA* and hexatic smectic phases were determined. The dielectric strength at the SmCA* to hexatic smectic phase transition is discussed in terms of coupling between the long range bond orientational order and smectic C director. It seems from the results of spontaneous polarization and dielectric relaxation spectroscopy that the material might possess an additional phase between the SmCA* and hexatic smectic I* phases. 相似文献
55.
T. Banks 《General Relativity and Gravitation》2003,35(12):2075-2078
There are many theories of quantum gravity, depending on asymptotic boundary conditions, and the amount of supersymmetry. The cosmological constant is one of the fundamental parameters that characterizes different theories. If it is positive, supersymmetry must be broken. A heuristic calculation shows that a cosmological constant of the observed size predicts superpartners in the TeV range. This mechanism for SUSY breaking also puts important constraints on low energy particle physics models. 相似文献
56.
An upper bound for the best approximation of summable functions of several variables by trigonometric polynomials in the metric of L is determined in terms of Fourier coefficients. We consider functions representable by trigonometric series with certain symmetry of coefficients satisfying a multiple analog of the Sidon–Telyakovskii conditions. 相似文献
57.
Herein we report a new method to collect a qualified infrared spectrum of a solute in solution by two solvent cells with different thickness during background single-beam spectrum scanning. By collecting the background spectrum with two cells (two stages), we successfully achieved accurate solvent compensation between a sample and a reference, namely, the solvent amounts in the sample and background measurements could become congruent. Therefore, the solvent bands were thoroughly suppressed in the infrared spectrum and a qualified spectrum of the solute was obtained. 相似文献
58.
J. Song H. Jiang Z.J. Liu D.Y. Khang Y. Huang J.A. Rogers C. Lu C.G. Koh 《International Journal of Solids and Structures》2008,45(10):3107-3121
A finite-deformation theory is developed to study the mechanics of thin buckled films on compliant substrates. Perturbation analysis is performed for this highly nonlinear system to obtain the analytical solution. The results agree well with experiments and finite element analysis in wavelength and amplitude. In particular, it is found that the wavelength depends on the strain. Based on the accurate wavelength and amplitude, the membrane and peak strains in thin films, and stretchability and compressibility of the system are also obtained analytically. 相似文献
59.
60.
Local and nonlocal coercive estimates in Lebesgue, Hölder, and Morrey–Campanato spaces are proved for solutions of linear stationary systems of hydrodynamic type. For this purpose, the technique proposed by Italian mathematicians for the study of solutions to equations and systems of elliptic and parabolic types is developed. Bibliography: 14 titles. 相似文献