全文获取类型
收费全文 | 1931篇 |
免费 | 75篇 |
国内免费 | 6篇 |
专业分类
化学 | 1374篇 |
晶体学 | 16篇 |
力学 | 46篇 |
综合类 | 1篇 |
数学 | 200篇 |
物理学 | 375篇 |
出版年
2023年 | 13篇 |
2022年 | 20篇 |
2021年 | 28篇 |
2020年 | 36篇 |
2019年 | 34篇 |
2018年 | 29篇 |
2017年 | 20篇 |
2016年 | 42篇 |
2015年 | 50篇 |
2014年 | 37篇 |
2013年 | 80篇 |
2012年 | 116篇 |
2011年 | 132篇 |
2010年 | 59篇 |
2009年 | 49篇 |
2008年 | 96篇 |
2007年 | 122篇 |
2006年 | 134篇 |
2005年 | 99篇 |
2004年 | 109篇 |
2003年 | 73篇 |
2002年 | 79篇 |
2001年 | 35篇 |
2000年 | 19篇 |
1999年 | 15篇 |
1998年 | 12篇 |
1997年 | 16篇 |
1996年 | 19篇 |
1995年 | 35篇 |
1994年 | 27篇 |
1993年 | 15篇 |
1992年 | 22篇 |
1991年 | 18篇 |
1990年 | 20篇 |
1989年 | 13篇 |
1988年 | 14篇 |
1986年 | 14篇 |
1985年 | 14篇 |
1984年 | 15篇 |
1983年 | 12篇 |
1982年 | 19篇 |
1981年 | 18篇 |
1980年 | 17篇 |
1979年 | 14篇 |
1978年 | 19篇 |
1977年 | 15篇 |
1976年 | 12篇 |
1975年 | 17篇 |
1974年 | 11篇 |
1973年 | 14篇 |
排序方式: 共有2012条查询结果,搜索用时 15 毫秒
991.
Rotational motion of heated 72Ge is studied within the microscopic shell-model Monte Carlo approach. We investigate the angular momentum alignment and nuclear pairing correlations associated with J^{π} Cooper pairs as a function of the rotational frequency and temperature. The reentrance of pairing correlations with temperature is predicted at high rotational frequencies. It manifests itself through the anomalous behavior of specific heat and level density. 相似文献
992.
Ceria is an important component in three-way catalysts for the treatment of automobile exhaust gases owing to its ability to store and release oxygen, a property known as the oxygen storage capacity. Much effort has been focused on increasing the OSC of ceria, and one avenue of exploration is the ability to fabricate CeO(2)-based catalysts, which expose reactive surfaces. Here we show how models for a polycrystalline CeO(2) thin film, which expose the (111), (110), and dipolar (100) surfaces, can be synthesized. This is achieved by supporting the CeO(2) thin film on an yttrium-stabilized zirconia substrate using a simulated amorphization and recrystallization strategy. In particular, the methodology generates models which reveal the atomistic structures present on the surface of the reactive faces and provides details of the grain-boundary structures, defects (vacancies, substitutionals, and clustering), and epitaxial relationships. Such models are an important first step in understanding the active sites at the surface of a catalytic material. 相似文献
993.
Twenty-four individual geometric structures of the 20-carbon system are investigated using hybrid Hartree–Fock/density functional theory (DFT-B3LYP) in combination with the 6-31G and 6-311G* basis sets. These computations are carried out in order to evaluate the relative energies of the various C20 isomers utilizing both geometry optimization and single-point energy calculations. The relative energies of the three most widely studied C20 isomers (ring, bowl, and cage) are compared to the energies of other cyclic, bowl-like, and miscellaneous isomers. Although the ring and the bowl isomers are predicted to be the two most stable species, we have identified three to four additional cyclic structures that are predicted to be energetically competitive with the fullerene structure. 相似文献
994.
We review the recent literature on lattice simulations for few- and many-body systems. We focus on methods that combine the framework of effective field theory with computational lattice methods. Lattice effective field theory is discussed for cold atoms as well as low-energy nucleons with and without pions. A number of different lattice formulations and computational algorithms are considered, and an effort is made to show common themes in studies of cold atoms and low-energy nuclear physics as well as common themes in work by different collaborations. 相似文献
995.
对光折变全息记录特别是双中心全息记录中90°记录结构下较低的衍射效率进行了研究,采用局域衍射理论对90°记录结构的衍射进行了分析,表明在同样的折射率变化和2 mm的光束宽度的情况下,只有当折射率光栅振幅大于10^-4时,90°记录结构衍射效率才能够与小角度透射记录结构的衍射效率大致相当。针对环境干扰导致的干涉条纹振动影响光栅记录,提出了有效调制度概念,根据分析90°记录结构的干涉条纹间距很小,容易受外界环境干扰而导致低的折射率变化率,因此应采用主动条纹锁定系统。此外在双中心全息记录中,微观光学参量的优化对衍射效率影响也很重要。 相似文献
996.
Xinrong Tian Dominic P. Suarez William Hoi Hong Li Allison K. McSherry Robert M. Sanchez Michael L. Moore Jeffrey M. Axten 《Tetrahedron letters》2019,60(49):151312
We describe an efficient synthesis of 2,4-substituted pyrido[4,3-d]pyrimidin-5(6H)-ones, which involves the acid-promoted cyclization of cyano enamine 15 to afford 2,4-bis(methylthio)pyrido[4,3-d]pyrimidin-5(6H)-one 17 as a key intermediate. Selective displacement of the 4-methylthio group by a wide range of anilines followed by oxidation of the 2-methylthio group and subsequent substitution by amines enabled the synthesis of a variety of 2,4-disubstituted pyrido[4,3-d]pyrimidin-5(6H)-ones. 相似文献
997.
Selective donon-acceptor pair luminescence and luminescence excitation spectra reveal acceptor excitations to all excited states of principal quantum number 2, odd as well as even parity. These transition energies are independent of the D-A pair separation R for sufficiently large R and are then identical to those on isolated acceptors. However, they decrease significantly at small R and some splittings are revealed in our measurements on a bulk grown crystal in which the most common contaminant acceptor Zn is dominant. The p state transition energies are used together with the known ionization energy of the Zn acceptor to derive valence band parameters for InP according to the theory of Baldereschi and Lipari. We find γ1 = 5.6 ± 0.7, γ2 = 2.0 ± 0.3, γ3 = 2.4 ± 0.3, where γ1 is adjusted to fit the cyclotron resonance data of Leotin et al. Some reservations concerning the completeness of this theory are acknowledge. Information on the s acceptor excited state is consistent with that obtained from acceptor bound exciton “two-hole” satellites for this remarkable quality bulk crystal. 相似文献
998.
Brittany J. Allison Juan Canales Cádiz Nardrapee Karuna Tina Jeoh Christopher W. Simmons 《Applied biochemistry and biotechnology》2016,179(7):1227-1247
Tomato pomace is an abundant lignocellulosic waste stream from industrial tomato processing and therefore a potential feedstock for production of renewable biofuels. However, little research has been conducted to determine if pretreatment can enhance release of fermentable sugars from tomato pomace. Ionic liquids (ILs) are an emerging pretreatment technology for lignocellulosic biomass to increase enzymatic digestibility and biofuel yield while utilizing recyclable chemicals with low toxicity. In this study, pretreatment of tomato pomace with the ionic liquid 1-ethyl-3-methylimidazolium acetate ([C2mim][OAc]) was investigated. Changes in pomace enzymatic digestibility were affected by pretreatment time and temperature. Certain pretreatment conditions significantly improved reducing sugar yield and hydrolysis time compared to untreated pomace. Compositional analyses suggested that pretreatment primarily removed water-soluble compounds and enriched for lignocellulose in pomace, with only subtle changes to the composition of the lignocellulose. While tomato pomace was effectively pretreated with [C2mim][OAc] to improve enzymatic digestibility, as of yet, unknown factors in the pomace caused ionic liquid pretreatment to negatively affect anaerobic digestion of pretreated material. This result, which is unique compared to similar studies on IL pretreatment of grasses and woody biomass, highlights the need for additional research to determine how the unique chemical composition of tomato pomace and other lignocellulosic fruit residues may interact with ionic liquids to generate inhibitors for downstream fermentation to biofuels. 相似文献
999.
光横向传播的光折变体全息透镜设计及其衍射性质 总被引:2,自引:1,他引:2
提出了一种新型的可用于光束横向传播的光折变体全息透镜。该透镜是由入射方向相互垂直的一束平面波和一束球面波在双掺杂的铌酸锂晶体平板上记录形成的,它可以将垂直入射晶体平板的输入光束产生横向传输并聚焦,或对输入光点产生横传并准直输出。该透镜除了具有一般体全息透镜的稳定性好、可靠性高的特点外,特别具有光束横向传播的功能,因而具有空占比小、更易于整个光学透镜系统的微小化集成的优点。利用局域体全息的耦合波理论,计算了该透镜衍射光的振幅分布和衍射效率。计算了该透镜的焦面上的强度分布,并与理想球面波照射矩形孔径时焦面上的强度分布进行了比较。 相似文献
1000.
Abraham J Abreu P Aglietta M Aguirre C Allard D Allekotte I Allen J Allison P Alvarez-Muñiz J Ambrosio M Anchordoqui L Andringa S Anzalone A Aramo C Argirò S Arisaka K Armengaud E Arneodo F Arqueros F Asch T Asorey H Assis P Atulugama BS Aublin J Ave M Avila G Bäcker T Badagnani D Barbosa AF Barnhill D Barroso SL Baughman B Bauleo P Beatty JJ Beau T Becker BR Becker KH Bellido JA Benzvi S Berat C Bergmann T Bernardini P Bertou X Biermann PL Billoir P Blanch-Bigas O Blanco F Blasi P Bleve C 《Physical review letters》2008,101(6):061101
The energy spectrum of cosmic rays above 2.5 x 10;{18} eV, derived from 20,000 events recorded at the Pierre Auger Observatory, is described. The spectral index gamma of the particle flux, J proportional, variantE;{-gamma}, at energies between 4 x 10;{18} eV and 4 x 10;{19} eV is 2.69+/-0.02(stat)+/-0.06(syst), steepening to 4.2+/-0.4(stat)+/-0.06(syst) at higher energies. The hypothesis of a single power law is rejected with a significance greater than 6 standard deviations. The data are consistent with the prediction by Greisen and by Zatsepin and Kuz'min. 相似文献