全文获取类型
收费全文 | 5856篇 |
免费 | 139篇 |
国内免费 | 34篇 |
专业分类
化学 | 4049篇 |
晶体学 | 88篇 |
力学 | 134篇 |
数学 | 986篇 |
物理学 | 772篇 |
出版年
2022年 | 36篇 |
2021年 | 42篇 |
2020年 | 41篇 |
2019年 | 62篇 |
2018年 | 58篇 |
2017年 | 48篇 |
2016年 | 113篇 |
2015年 | 113篇 |
2014年 | 113篇 |
2013年 | 314篇 |
2012年 | 263篇 |
2011年 | 321篇 |
2010年 | 148篇 |
2009年 | 145篇 |
2008年 | 287篇 |
2007年 | 313篇 |
2006年 | 264篇 |
2005年 | 302篇 |
2004年 | 251篇 |
2003年 | 282篇 |
2002年 | 235篇 |
2001年 | 103篇 |
2000年 | 88篇 |
1999年 | 84篇 |
1998年 | 63篇 |
1997年 | 86篇 |
1996年 | 96篇 |
1995年 | 93篇 |
1994年 | 86篇 |
1993年 | 67篇 |
1992年 | 55篇 |
1991年 | 62篇 |
1990年 | 51篇 |
1989年 | 64篇 |
1988年 | 73篇 |
1987年 | 55篇 |
1986年 | 52篇 |
1985年 | 78篇 |
1984年 | 99篇 |
1983年 | 67篇 |
1982年 | 93篇 |
1981年 | 96篇 |
1980年 | 90篇 |
1979年 | 74篇 |
1978年 | 78篇 |
1977年 | 73篇 |
1976年 | 68篇 |
1975年 | 58篇 |
1974年 | 48篇 |
1973年 | 36篇 |
排序方式: 共有6029条查询结果,搜索用时 0 毫秒
21.
22.
Richard Kotek Kyeong Pang Ben Schmidt Alan Tonelli 《Journal of Polymer Science.Polymer Physics》2004,42(23):4247-4254
Poly(ethylene isophthalate) (PEI) was synthesized for this research with essentially a condensation polymerization of isophthalic acid and ethylene glycol catalyzed by zinc acetate and antimony trioxide. Several samples were obtained, and their characteristics were observed and compared with poly(ethylene terephthalate) (PET). The synthesized PEI samples were chemically identified by 1H NMR. Thermal analysis with differential scanning calorimetry (DSC) yielded results that indicate the samples were primarily amorphous, with a glass‐transition temperature of 55–60 °C. Molecular weights of these PEI samples were also obtained through intrinsic viscosity measurements (Mark–Houwink equation). Molecular weights varied with conditions of the polymerization, and the highest molecular weight achieved was 21,000 g/mol. Finally, the diffusion coefficient, solubility, and permeability of CO2 gas in PEI were measured and found to be substantially lower than in PET, as anticipated from their isomeric chemical structures. This is because in PET the phenyl rings are substituted in the para (1,4) positions, which allows for their facile flipping, effectively permitting gases to pass through. However, the meta‐substituted phenyl rings in PEI do not permit such ring flipping, and thus PEI may be more suitable for barrier applications. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 4247–4254, 2004 相似文献
23.
Simon G. Bott Alan P. Marchand Dongxia Xing Rajesh Shukla Stephen James Obrey K. Venkatesan J. Narasimha Moorthy 《Journal of chemical crystallography》1997,27(11):661-665
NaBH4 reduction of a cage dione proceeds in a stereospecific fashion to give theendo,endo-diol. This reactivity is related to the crystal structure. 相似文献
24.
25.
This author wants to express his thanks to the School of Information Engineering at Teesside Polytechnic, Middlesbrough, England, for its support and hospitality during a visiting appointment of 3 months in 1989, when this paper was written. 相似文献
26.
27.
28.
Il Bong Jung Alan Lambert Jan Stochel 《Proceedings of the American Mathematical Society》2004,132(8):2291-2302
The concept of backward extension for subnormal weighted shifts is generalized to arbitrary subnormal operators. Several differences and similarities in these contexts are explored, with emphasis on the structure of the underlying measures.
29.
David Alan Bies 《Journal of sound and vibration》2004,278(3):581-587
Bies et al. (J. Sound Vib. 204(4) (1997) 631) investigated Curle's theory (Proc. R. Soc. Ser. A 231 (1955) 505) published in 1955 over a wide range of flow speeds from about 50-200 m/s and found only partial agreement with the experimental data. Here the experimental investigation has been repeated allowing the data to be recorded in a format amenable to analysis, which was not previously possible.Reintroduction of a term neglected by Curle has been found necessary as Curle's compact source condition ensures so low a radiation impedance that the effect cannot be detected in the jet background noise. The reintroduction of the term, which has been neglected, allows his analysis to include radiation from sources not compact but less than half a wavelength in characteristic dimension. It is shown that the power ratio defined as the measured sound power divided by Curle's amended prediction converges to about 3 whereas Curle predicts that the power ratio should converge to 1 as the wave number converges to zero. The introduction of the empirically determined constant 3 into Curle's prediction brings the measurements into very good agreement with prediction over the entire range of the non-dimensional wave numbers from about 0.4 to 3.2. 相似文献
30.