全文获取类型
收费全文 | 475047篇 |
免费 | 5435篇 |
国内免费 | 1452篇 |
专业分类
化学 | 251888篇 |
晶体学 | 7141篇 |
力学 | 21226篇 |
综合类 | 19篇 |
数学 | 53777篇 |
物理学 | 147883篇 |
出版年
2021年 | 4070篇 |
2020年 | 4485篇 |
2019年 | 4792篇 |
2018年 | 6216篇 |
2017年 | 6074篇 |
2016年 | 9436篇 |
2015年 | 6004篇 |
2014年 | 9190篇 |
2013年 | 21936篇 |
2012年 | 17234篇 |
2011年 | 21291篇 |
2010年 | 14990篇 |
2009年 | 14855篇 |
2008年 | 19422篇 |
2007年 | 19195篇 |
2006年 | 17866篇 |
2005年 | 15905篇 |
2004年 | 14777篇 |
2003年 | 13013篇 |
2002年 | 12794篇 |
2001年 | 15274篇 |
2000年 | 11449篇 |
1999年 | 8918篇 |
1998年 | 7307篇 |
1997年 | 7044篇 |
1996年 | 6692篇 |
1995年 | 6014篇 |
1994年 | 5855篇 |
1993年 | 5627篇 |
1992年 | 6421篇 |
1991年 | 6540篇 |
1990年 | 6197篇 |
1989年 | 5941篇 |
1988年 | 5941篇 |
1987年 | 5853篇 |
1986年 | 5497篇 |
1985年 | 7126篇 |
1984年 | 7250篇 |
1983年 | 5697篇 |
1982年 | 5761篇 |
1981年 | 5730篇 |
1980年 | 5306篇 |
1979年 | 5792篇 |
1978年 | 5942篇 |
1977年 | 5861篇 |
1976年 | 5730篇 |
1975年 | 5263篇 |
1974年 | 5266篇 |
1973年 | 5211篇 |
1972年 | 3638篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
111.
112.
The electro-optic response of ferroelectric smectic C* liquid crystals has been studied. Anomalous switching behaviour of such materials which possess a negative dielectric anisotropy has been reported. These materials show a minimum in response time at a sufficiently high field. We present results showing the dependency of this minimum upon spontaneous polarisation and the effect of AC bias. Calculations based upon the equation of motion of the director around the cone are presented which describe this effect and its dependence on the relative magnitudes of the spontaneous polarization and dielectric anisotropy of the material. Good agreement with the experimental results is found. 相似文献
113.
114.
R. Jayakumar Y.‐S. Lee S. Nanjundan 《Journal of polymer science. Part A, Polymer chemistry》2003,41(18):2865-2878
The calcium salt of mono(hydroxyethoxyethyl)phthalate [Ca(HEEP)2] was synthesized by the reaction of diethylene glycol, phthalic anhydride, and calcium acetate. Calcium‐containing poly(urethane ether)s (PUEs) were synthesized by the reaction of hexamethylene diisocyanate (HMDI) or tolylene 2,4‐diisocyanate (TDI) with a mixture of Ca(HEEP)2 and poly(ethylene glycol) (PEG300 or PEG400) with di‐n‐butyltin dilaurate as a catalyst. A series of calcium‐containing PUEs of different compositions were synthesized with Ca(HEEP)2/PEG300 (or PEG400)/diisocyanate (HMDI or TDI) molar ratios of 2:2:4, 3:1:4, and 1:3:4 so that the coating properties of the PUEs could be studied. Blank PUEs without calcium‐containing ionic diols were also prepared by the reaction of PEG300 or PEG400 with HMDI or TDI. The PUEs were well characterized by Fourier transform infrared, 1H and 13C NMR, solid‐state cross‐polarity/magic‐angle‐spinning 13C NMR, viscosity, solubility, and X‐ray diffraction studies. The thermal properties of the polymers were also studied with thermogravimetric analysis and differential scanning calorimetry. The PUEs were applied as top coats on acrylic‐coated leather, and their physicomechanical properties were also studied. The coating properties of PUEs, such as the tensile strength, elongation at break, tear strength, water vapor permeability, flexing endurance, cold crack resistance, abrasion resistance, color fastness, and adhesive strength, were better than the standard values. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2865–2878, 2003 相似文献
115.
A method for suppressing self-modulation in backward-wave tubes is proposed. Additional delay is introduced into the feedback
circuit, owing to which the output signal amplitude affects the electron beam current that enters into the interaction space.
Numerical simulations demonstrate that the operating current in the single-frequency oscillation mode may be increased roughly
twofold. 相似文献
116.
Salimgareeva V. N. Polevoi R. M. Kolesov S. V. Ostakhov S. S. Ponomareva V. A. Leplyanin G. V. 《Russian Journal of Applied Chemistry》2003,76(5):804-808
The luminescence and scintillation properties of poly(methyl methacrylate) containing 1,1,3-trimethyl-3-phenylindan were studied. 相似文献
117.
118.
L. Villa P. D' Agati C. Mansueto C. Pellerito M. Scopelliti T. Fiore L. Nagy L. Pellerito 《应用有机金属化学》2003,17(2):106-112
Ascidia malaca gametes before fertilization incubated in 10?5 or 10?7 M solutions of tributyltin(IV) chloride, TBTCl, for 3 h appear highly damaged under transmission electron microscopy observation. Also, the fertilization process is affected by the compound: the damaged spermatozoa are present in the vitelline coat and the egg does not cleave. An increase of microbodies, structurally similar to peroxisomes, have been detected in the egg peripheral cytoplasm, probably in relation to their role in alleviating damage to some cellular components. The results have shown that the reproduction of ascidians under unfavourable environmental conditions is prevented. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
119.
A new parallel storm surge model, the Parallel Environmental Model (PEM), is developed and tested by comparisons with analytic solutions. The PEM is a 2‐D vertically averaged, wetting and drying numerical model and can be operated in explicit, semi‐implicit and fully implicit modes. In the implicit mode, the propagation, Coriolis and bottom friction terms can all be treated implicitly. The advection and diffusion terms are solved with a parallel Eulerian–Lagrangian scheme developed for this study. The model is developed specifically for use on parallel computer systems and will function accordingly in either explicit of implicit modes. Storm boundary conditions are based on a simple exponential decay of pressure from the centre of a storm. The simulated flooding caused by a major Category 5 hurricane making landfall in the Indian River Lagoon, Florida is then presented as an example application of the PEM. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
120.
C. Vaca‐Garcia G. Gozzelino W. G. Glasser M. E. Borredon 《Journal of Polymer Science.Polymer Physics》2003,41(3):281-288
The main transitions of cellulose fatty esters with different degrees of substitution (DSs) were investigated with dynamic mechanical thermal analysis. Two distinct main relaxations were observed in partially substituted cellulose esters (PSCEs). They were attributed to the glass‐transition temperature and to the chain local motion of the aliphatic substituents. The temperatures of both transitions decreased when DS or the number of carbon atoms (n) of the acyl substituent increased. Conversely, all the transitions of fully substituted cellulose esters occurred within a narrow temperature range, and they did not vary significantly with n. This phenomenon was explained by the formation of a crystalline phase of the fatty substituents. The presence of few residual OH groups in PSCEs was responsible for a large increase in the storage bending modulus, and it eliminated the effect of n on damping. © 2002 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 41: 281–288, 2003 相似文献