全文获取类型
收费全文 | 203960篇 |
免费 | 21386篇 |
国内免费 | 25675篇 |
专业分类
化学 | 172213篇 |
晶体学 | 4161篇 |
力学 | 5334篇 |
综合类 | 1440篇 |
数学 | 26476篇 |
物理学 | 41397篇 |
出版年
2024年 | 233篇 |
2023年 | 1785篇 |
2022年 | 3641篇 |
2021年 | 4456篇 |
2020年 | 5152篇 |
2019年 | 6294篇 |
2018年 | 5584篇 |
2017年 | 7267篇 |
2016年 | 7660篇 |
2015年 | 8858篇 |
2014年 | 10373篇 |
2013年 | 17677篇 |
2012年 | 14601篇 |
2011年 | 13649篇 |
2010年 | 11415篇 |
2009年 | 13560篇 |
2008年 | 13923篇 |
2007年 | 13527篇 |
2006年 | 12290篇 |
2005年 | 10842篇 |
2004年 | 9874篇 |
2003年 | 8401篇 |
2002年 | 9133篇 |
2001年 | 5927篇 |
2000年 | 5560篇 |
1999年 | 4470篇 |
1998年 | 3540篇 |
1997年 | 2910篇 |
1996年 | 2526篇 |
1995年 | 2383篇 |
1994年 | 2189篇 |
1993年 | 1797篇 |
1992年 | 1618篇 |
1991年 | 1154篇 |
1990年 | 904篇 |
1989年 | 876篇 |
1988年 | 685篇 |
1987年 | 503篇 |
1986年 | 450篇 |
1985年 | 420篇 |
1984年 | 428篇 |
1983年 | 198篇 |
1982年 | 323篇 |
1981年 | 335篇 |
1980年 | 327篇 |
1979年 | 299篇 |
1978年 | 265篇 |
1977年 | 206篇 |
1976年 | 163篇 |
1973年 | 106篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
本文证明了d2k =δ2k =d2k ≥b2k,其中d2k ,δ2k , b2k分别表示A(BlMp)在lNq下的Kolmogorov,线性,Bernstein 2k-宽度,d2k 表示AT(BlN相似文献
992.
An interferometer in which an atom traverses two identical micromaser cavities in succession is proposed. Depending on the preparation of the cavity fields, the probability for finding the atom in a definite final state displays Ramsey fringes or not. If the initial cavity fields are such that the state of the atom between the cavities can be determined, then the Ramsey fringes disappear, as is required by the principle of complementarity. 相似文献
993.
在微波放电系统中,对NH_3-F-F_2-CF_3I体系进行研究,结果表明,向IF(X)传能的诸多媒介中,N_2(A)及N(~2D)起着主要作用,并且这一结论在经微波激发后的N_2与CF_3I的直接反应中得到了进一步证实。 相似文献
994.
三维射频加速腔程序包MAFIA的开发和应用 总被引:2,自引:1,他引:1
介绍了三维射频加速器计算机辅助设计(CAD)程序包MAFIA的特点、功能及其应用。用CAD的方法实现了射频加速器特别是非轴对称带耦合腔的加速器的三维腔形设计。同时介绍了节省内存、节约机时的模型方法,使得在中小型机器上运行大程序(八万条)成为可能。文章主要对L波段的加速腔进行了数值模拟,并和实验进行了比较,结果符合较好;通过三维数值模拟及图形模拟的方法比较了各种形状谐振腔的性能;计算分析了输入耦合腔及高阶模耦合器对加速腔电磁场分布等参量的影响;提出了加耦合器后谐振频率改变的补偿方法。 相似文献
995.
A model is reported to foresee ,for different mirror choices, the spectral properties of a high pressure, frequency tunable, CO2 laser, operating with a non-Littrow grating mounting. The calculations show a larger frequency selectivity for not collimated plane cavities. The model is experimentally verified in a suitable high pressure laser. In particular narrowed linewidths less than 300MHz ( FWHM) are obtained. 相似文献
996.
997.
998.
There is a high correlation between molecular surface area (TSA) of triorganotin and triorganolead compounds and their toxicity towards a bacterium (Escherichia coli) and an alga (Selenastrum capricornutum). Parallel attempts to correlate other Group IVA organometals incorporating silicon or germanium were unsuccessful. It was further demonstrated, however, that a high correlation was obtainable between certain series of compounds with the same organic substituent but different metal centers involving all Group IVA elements. In both instances, the inability to obtain a quantitative structure-activity relationship (QSAR) for all systems studied appears to be a function of the solubility of the compounds. While organotin TSA values have been found to correlate well with their toxicities toward various organisms, this study clearly suggests that this type of QSAR can be readily extended to include other organometal systems, provided that there is no solubility problem and the toxicity is a function of the hydrophobicity of the organometal compounds. 相似文献
999.
讨论了微分方程组 dx/dt=-y(1 -ax2 n) +bx-cx2 n+ 1,dy/dt=x(1 -ax2 n) ,并且给出了其极限环存在唯一的条件 . 相似文献
1000.
Hiroshi Ito Masaki Okazaki Dolores C. Miller 《Journal of polymer science. Part A, Polymer chemistry》2004,42(6):1478-1505
Radical copolymerizations of electron‐deficient 2‐trifluoromethylacrylic (TFMA) monomers, such as 2‐trifluoromethylacrylic acid and t‐butyl 2‐trifluoromethylacrylate (TBTFMA), with electron‐rich norbornene derivatives and vinyl ethers with 2,2′‐azobisisobutyronitrile as the initiator were investigated in detail through the analysis of the kinetics in situ with 1H NMR and through the determination of the monomer reactivity ratios. The norbornene derivatives used in this study included bicyclo[2.2.1]hept‐2‐ene (norbornene) and 5‐(2‐trifluoromethyl‐1,1,1‐trifluoro‐2‐hydroxylpropyl)‐2‐norbornene. The vinyl ether monomers were ethyl vinyl ether, t‐butyl vinyl ether, and 3,4‐dihydro‐2‐H‐pyran. Vinylene carbonate was found to copolymerize with TBTFMA. Although none of the monomers underwent radical homopolymerization under normal conditions, they copolymerized readily, producing a copolymer containing 60–70 mol % TFMA. The copolymerization of the TFMA monomer with norbornenes and vinyl ethers deviated from the terminal model and could be described by the penultimate model. The copolymers of TFMA reported in this article were evaluated as chemical amplification resist polymers for the emerging field of 157‐nm lithography. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 1478–1505, 2004 相似文献