全文获取类型
收费全文 | 304545篇 |
免费 | 2005篇 |
国内免费 | 645篇 |
专业分类
化学 | 156861篇 |
晶体学 | 4654篇 |
力学 | 14925篇 |
综合类 | 10篇 |
数学 | 33575篇 |
物理学 | 97170篇 |
出版年
2020年 | 2805篇 |
2019年 | 3281篇 |
2018年 | 4644篇 |
2017年 | 4772篇 |
2016年 | 6219篇 |
2015年 | 3215篇 |
2014年 | 5510篇 |
2013年 | 12263篇 |
2012年 | 9730篇 |
2011年 | 11657篇 |
2010年 | 8972篇 |
2009年 | 9193篇 |
2008年 | 11187篇 |
2007年 | 10968篇 |
2006年 | 10256篇 |
2005年 | 9218篇 |
2004年 | 8660篇 |
2003年 | 7897篇 |
2002年 | 7938篇 |
2001年 | 9758篇 |
2000年 | 7186篇 |
1999年 | 5501篇 |
1998年 | 4452篇 |
1997年 | 4360篇 |
1996年 | 4057篇 |
1995年 | 3709篇 |
1994年 | 3657篇 |
1993年 | 3516篇 |
1992年 | 4008篇 |
1991年 | 4388篇 |
1990年 | 4149篇 |
1989年 | 4162篇 |
1988年 | 4051篇 |
1987年 | 4071篇 |
1986年 | 3750篇 |
1985年 | 4690篇 |
1984年 | 4728篇 |
1983年 | 3942篇 |
1982年 | 3994篇 |
1981年 | 3760篇 |
1980年 | 3566篇 |
1979年 | 4134篇 |
1978年 | 4318篇 |
1977年 | 4373篇 |
1976年 | 4423篇 |
1975年 | 4114篇 |
1974年 | 4029篇 |
1973年 | 4225篇 |
1972年 | 3152篇 |
1971年 | 2778篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
B. Y. Myung J. S. Kim J. J. Kim T. H. Yoon 《Journal of polymer science. Part A, Polymer chemistry》2003,41(21):3361-3374
2,2‐Bis[4(4‐aminophenoxy)phenyl]phthalein‐3′,5′‐bis(trifluoromethyl)anilide (6FADAP), containing fluorine and phthalimide moieties, was synthesized via the Williamson ether condensation reaction from 1‐chloro‐4‐nitrobenzene and phenolphthalein‐3′,5′‐bis(trifluoromethyl)anilide, which was followed by hydrogenation. Monomers such as 2,2‐bis[4(4‐aminophenoxy)phenyl]phthalein‐anilide containing phthalimide groups and 2,2‐bis[4(4‐aminophenoxy)phenyl]phthalein containing only phthalein moieties were also synthesized for comparison. The monomers were first characterized by Fourier transform infrared (FTIR), 1H NMR, 19F NMR, elemental analysis, and titration and were then used to prepare polyimides with 2,2‐bis(3,4‐dicarboxyphenyl)hexafluoropropane dianhydride. The polyimides were designed to have molecular weights of 20,000 g/mol via off‐stoichiometry and were characterized by FTIR, NMR, gel permeation chromatography (GPC), differential scanning calorimetry, and thermogravimetric analysis. Their solubility, water absorption, dielectric constant, and refractive index were also evaluated. The polyimides prepared with 6FADAP, containing fluorine and phthalimide moieties, had excellent solubility in N‐methylpyrrolidinone, N,N‐dimethylacetamide, tetrahydrofuran, CHCl3, tetrachloroethane, and acetone, and GPC analysis showed a molecular weight of 18,700 g/mol. The polyimides also exhibited a high glass‐transition temperature (290 °C), good thermal stability (~500 °C in air), low water absorption (1.9 wt %), a low dielectric constant (2.81), a low refractive index, and low birefringence (0.0041). © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 3361–3374, 2003 相似文献
992.
In this paper an automatic technique for handling discontinuous IVPs when they are solved by means of adaptive Runge–Kutta codes is proposed. This technique detects, accurately locates and passes the discontinuities in the solution of IVPs by using the information generated by the code along the numerical integration together with a continuous interpolant of the discrete solution. A remarkable feature is that it does not require additional information on the location of the discontinuities. Some numerical experiments are presented to illustrate the reliability and efficiency of the proposed algorithms. 相似文献
993.
994.
Stochastic models of the process of spontaneous ignition of coal particles are distinguished by a pronounced nonlinearity due to the Arrhenius approximation for the constant of the reaction rate and by a complex structure of the stochastic component (noise). We present models describing the dynamics of the distribution of this quantity. The models obtained consist of nonlinear ordinary differential equations, which are solved by numerical methods with the use of computer technologies. 相似文献
995.
996.
Abhimanyu O. Patil Stephen Zushma Robert T. Stibrany Steven P. Rucker Louise M. Wheeler 《Journal of polymer science. Part A, Polymer chemistry》2003,41(13):2095-2106
Novel nickel(II) bisbenzimidazole complexes were prepared via a three‐step synthetic procedure consisting of aniline/diacid condensation, ligand N‐alkylation, and metal complexation. The complexes were characterized by X‐ray crystallography and found to possess a pseudotetrahedral geometry. Upon activation with methylaluminoxane, these nickel bisbenzimidazoles did not polymerize simple olefins (e.g., ethylene, propylene, and 1‐butene) but were found to carry out the rapid and efficient polymerization of norbornene. The polynorbornene products were characterized by gel permeation chromatography/light scattering, 13C NMR, and IR, and their Mark–Houwink and dn/dc parameters were determined. The molecular weights of the polynorbornenes were very high (weight‐average molecular weight = 587,000–797,000 g/mol). 13C NMR suggested that the polymerization occurred via vinyl addition (i.e., a 2,3‐linked polymer); no ring‐opened product was observed. Thermogravimetric analysis indicated that the polynorbornenes were stable up to 400 °C under nitrogen. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2095–2106, 2003 相似文献
997.
J. Salgado E. Martinho I. F. Gonçalves 《Journal of Radioanalytical and Nuclear Chemistry》2004,260(2):317-320
The resonance neutron self-shielding factor, G
res, is required in neutron metrology and activation data analysis. In a previous paper, the authors have shown that a dimensionless
variable can be introduced which converts the dependence of G
res on the physical and nuclear properties of the material samples into an universal curve, valid for the isolated resonances
of any nuclide. This work presents a methodology based on the universal curve, which enables to calculate G
res for a group of isolated resonances by weighting its individual contributions. A good agreement was reached with results calculated
by the MCNP code and with experimental values for Mo foils and wires.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
998.
A consistent quantum theory of a two-pulse pump-probe experiment in the femto-and picosecond spectroscopy of diatomic molecules is developed. The theory aims at the use of weak quantum femtosecond light pulses to excite molecular coherence. It is suggested that the experiment be realized using a double-frequency excitation scheme that is feasible for Na2 molecules. On the basis of the theory developed, the luminescence signals are calculated at different temperatures and different optical densities of the medium for the time range from 100 fs to ~ 100 ps. The results obtained show the presence of a strong dependence of the time and frequency spectra of a luminescence signal on the optical density of the medium at temperatures from 50 to 300 K. The proposed approach makes it possible to use the quantum coherence properties of optically dense media for more detailed study of the vibronic dynamics of diatomic molecules, in particular, detection of weak optical transitions. 相似文献
999.
D. V. Isakov T. R. Volk L. I. Ivleva K. Betzler C. David A. Tunyagi M. Wöhlecke 《JETP Letters》2004,80(4):258-262
Measurements of 90°-scattering of weak laser light are used to investigate pulsed domain switching in ferroelectrics. The studies were performed on strontium-barium niobate (SBN) single crystals. A good agreement of the switching parameters estimated from the optical measurements with those obtained by means of conventional electrical methods proves the validity of the optical method for switching studies. Due to the limited scattering volume in all three spatial dimensions, the method facilitates local probing of the switching within the crystal bulk. In particular, local specialities of the domain density can be detected. Furthermore, the excellent time resolution inherent in optical probing techniques allows for a comprehensive study of the dynamics. 相似文献
1000.
C. Bourrely I. Caprini L. Micu 《The European Physical Journal C - Particles and Fields》2003,27(3):439-446
We derive dispersion relations for decay, using the Lehmann-Symanzik-Zimmermann formalism, which allows the analytic continuation of the amplitudes with respect
to the momenta of the external particles. No off-shell extrapolation of the field operators is assumed. We obtain generalized
Omnès representations, which incorporate the and S-wave phase shifts in the elastic region of the direct and crossed channels, according to the Watson theorem. The contribution
of the inelastic final-state and initial-state interactions is parameterized by the technique of conformal mappings. We compare
our results with previous dispersive treatments and indicate how the formalism can be combined with lattice calculations to
yield physical predictions.
Received: 14 October 2002 / Published online: 14 February 2003 相似文献