全文获取类型
收费全文 | 1712篇 |
免费 | 73篇 |
国内免费 | 12篇 |
专业分类
化学 | 1183篇 |
晶体学 | 1篇 |
力学 | 55篇 |
数学 | 224篇 |
物理学 | 334篇 |
出版年
2024年 | 6篇 |
2023年 | 20篇 |
2022年 | 47篇 |
2021年 | 57篇 |
2020年 | 66篇 |
2019年 | 60篇 |
2018年 | 40篇 |
2017年 | 33篇 |
2016年 | 85篇 |
2015年 | 68篇 |
2014年 | 71篇 |
2013年 | 102篇 |
2012年 | 111篇 |
2011年 | 142篇 |
2010年 | 67篇 |
2009年 | 92篇 |
2008年 | 117篇 |
2007年 | 112篇 |
2006年 | 102篇 |
2005年 | 92篇 |
2004年 | 63篇 |
2003年 | 41篇 |
2002年 | 53篇 |
2001年 | 12篇 |
2000年 | 22篇 |
1999年 | 14篇 |
1998年 | 8篇 |
1997年 | 11篇 |
1996年 | 13篇 |
1995年 | 8篇 |
1994年 | 6篇 |
1993年 | 8篇 |
1992年 | 2篇 |
1991年 | 3篇 |
1990年 | 5篇 |
1989年 | 5篇 |
1988年 | 4篇 |
1987年 | 6篇 |
1986年 | 1篇 |
1985年 | 5篇 |
1984年 | 4篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1979年 | 2篇 |
1977年 | 1篇 |
1976年 | 3篇 |
1973年 | 1篇 |
1972年 | 1篇 |
1944年 | 1篇 |
排序方式: 共有1797条查询结果,搜索用时 0 毫秒
121.
122.
Lucas P. Tendela Alejandro FedericoGuillermo H. Kaufmann 《Optics and Lasers in Engineering》2011,49(2):281-284
This paper presents an interferometric measurement of the out-of-plane deflections produced by a piezoelectric transducer, manufactured by thick-film deposition of a ceramic paste over an alumina substrate, when is subjected to a DC electric voltage. It is shown that a digital speckle pattern interferometer with an incorporated phase-shifting facility allows the measurement of nanometer displacements generated by the piezoelectric device. These measurements are used to evaluate the effective piezoelectric charge constant along the polarization direction (d33)eff that characterizes the thick-film transducer. 相似文献
123.
Clemente-Juan JM Coronado E Gaita-Ariño A Giménez-Saiz C Chaboussant G Güdel HU Burriel R Mutka H 《Chemistry (Weinheim an der Bergstrasse, Germany)》2002,8(24):5701-5708
The ground-state properties of a Co3II moiety encapsulated in a polyoxometalate anion were investigated by combining measurements of specific heat, magnetic susceptibility, and low-temperature magnetization with a detailed inelastic neutron scattering (INS) study on a fully deuterated polycrystalline sample of Na12[Co3W(D2O)2(ZnW9O34)2].40D2O (Co3). The ferromagnetic Co3O14 cluster core consists of three octahedrally oxo-coordinated CoII ions. According to the single-ion anisotropy and spin-orbit coupling of the octahedral CoII ions, the appropriate exchange Hamiltonian to describe the ground-state properties of the Co3 spin cluster is anisotropic and is expressed as H = -2 sigma a = x,y,z (Ja12 S1a S2a + Ja23 S2a S3a), where Ja are the components of the exchange interactions between the CoII ions. To reproduce the INS data, different orientations of the two anisotropic J tensors must be considered, and the following conditions had to be introduced: Jx12 = Jy23, Jy12 = Jx23, Jz12 = Jz23. This result was correlated with the molecular symmetry of the complex. The following set of parameters was obtained: Jx12 = Jy23 = 1.37, Jy12 = Jx23 = 0.218, and Jz12 = Jz23 = 1.24 meV. This set also reproduces in a satisfactory manner the specific heat, susceptibility, and magnetization properties of Co3. 相似文献
124.
The nature of superacid electrophilic species in HF/SbF(5): a density functional theory study 总被引:1,自引:0,他引:1
A density functional theory study at the B3LYP/6-31++G** + RECP(Sb) level of the HF/SbF(5) superacid system was carried out. The geometries of possible electrophilic species, such as H(2)F(+).Sb(2)F(11)(-) and H(3)F(2)(+).Sb(2)F(11)(-), were calculated and correspond with available experimental results. Calculations of different equilibrium reactions involving HF and SbF(5) allowed the relative concentration of the most energetically favorable species present in 1:1 HF/SbF(5) solutions to be estimated. These species are H(+).Sb(2)F(11)(-), H(2)F(+).Sb(2)F(11)(-), H(3)F(2)(+).Sb(2)F(11)(-), and H(4)F(3)(+).Sb(2)F(11)(-), which correspond to 36.9, 16.8, 36.9, and 9.4%, respectively. Calculations of the acid strength of the electrophilic species were also performed and indicated that, for the same anion, the acid strength increases with the solvation degree. The entropic term also plays a significant role in proton-transfer reactions in superacid systems. 相似文献
125.
García-Herrero A Montero E Muñoz JL Espinosa JF Vián A García JL Asensio JL Cañada FJ Jiménez-Barbero J 《Journal of the American Chemical Society》2002,124(17):4804-4810
We show that the conformational features of the molecular complexes of E. coli beta-galactosidase and O-glycosides may differ from those formed with closely related compounds in their chemical nature, such as C- and S-glycosyl analogues. In the particular case presented here, NMR and ab initio quantum mechanical results show that the 3D-shapes of the ligand/inhibitor within the enzyme binding site depend on the chemical nature of the compounds. In fact, they depend on the relative size of the stereoelectronic barriers for chair deformation or for rotation around Phi glycosidic linkage. 相似文献
126.
Kolton AB Exartier R Cugliandolo LF Domínguez D Grønbech-Jensen N 《Physical review letters》2002,89(22):227001
We study numerically correlation and response functions in nonequilibrium driven vortex lattices with random pinning. From a generalized fluctuation-dissipation relation, we calculate an effective transverse temperature in the fluid moving phase. We find that the effective temperature decreases with increasing driving force and becomes equal to the equilibrium melting temperature when the dynamic transverse freezing occurs. We also discuss how the effective temperature can be measured experimentally from a generalized Kubo formula. 相似文献
127.
It has been known for several decades that electrical alternans occurs during myocardial ischemia in both clinical and experimental work. There are a few reports showing that this alternans can be triggered into existence by a premature ventricular contraction. Detriggering of alternans by a premature ventricular contraction, as well as pause-induced triggering and detriggering, have also been reported. We conduct a search for triggered alternans in an ionic model of ischemic ventricular muscle in which alternans has been described recently: a one-dimensional cable of length 3 cm, containing a central ischemic zone 1 cm long, with 1 cm segments of normal (i.e., nonischemic) tissue at each end. We use a modified form of the Luo-Rudy [Circ. Res. 68, 1501-1526 (1991)] ionic model to represent the ventricular tissue, modeling the effect of ischemia by raising the external potassium ion concentration ([K(+)](o)) in the central ischemic zone. As [K(+)](o) is increased at a fixed pacing cycle length of 400 ms, there is first a transition from 1:1 rhythm to alternans or 2:2 rhythm, and then a transition from 2:2 rhythm to 2:1 block. There is a range of [K(+)](o) over which there is coexistence of 1:1 and 2:2 rhythms, so that dropping a stimulus from the periodic drive train during 1:1 rhythm can result in the conversion of 1:1 to 2:2 rhythm. Within the bistable range, the reverse transition from 2:2 to 1:1 rhythm can be produced by injection of a well-timed extrastimulus. Using a stimulation protocol involving delivery of pre- and post-mature stimuli, we derive a one-dimensional map that captures the salient features of the results of the cable simulations, i.e., the {1:1-->2:2-->2:1} transitions with {1:1<-->2:2} bistability. This map uses a new index of the global activity in the cable, the normalized voltage integral. Finally, we put forth a simple piecewise linear map that replicates the {1:1<-->2:2} bistability observed in the cable simulations and in the normalized voltage integral map. (c) 2002 American Institute of Physics. 相似文献
128.
Alejandro M. Fracaroli O. Fernando Silva Alejandro M. Granados Rita H. de Rossi 《Journal of Physical Organic Chemistry》2008,21(11):1007-1013
The UV–Vis spectrum of 5‐(1‐butylthio)‐3H‐1,2‐dithiole‐3‐thione (1a) and that of the chromium pentacarbonyl complex of 5‐methyl‐3H‐1,2‐dithiole‐3‐thione (3) present significant changes with the solvent polarity. The two absorption bands shown by the compounds in the region above 300 nm were identified by theoretical calculations. For Compound 1a these are n→π* and →π* transitions and for Compound 3 the longest wavelength absorption corresponds to a charge transfer band and shows a remarkably negative solvatochromism. Not only has the wavelength of maximum absorption changed with the solvent but also the ratio of the absorbances at the two wavelengths. The effect of solvents was correlated with solvatochromic parameters such as π* and α. The spectrum of 5‐(1‐butylthio)‐3H‐1,2‐dithiole‐3‐one ( 2 ) was also measured in different solvents but in this case the changes observed are less significant than for the other two compounds. The spectra of 1a and 3 were also determined in the presence of anionic (SDS), cationic (CTAB), and neutral surfactants (Brig‐35) and it is shown that these compounds can be used as probes for the polarity of the binding sites of organized assemblies. Copyright © 2008 John Wiley & Sons, Ltd. 相似文献
129.
Joaquín Borrego-Díaz Alejandro Fernndez-Margarit Mario Prez-Jimnez 《Mathematical Logic Quarterly》1996,42(1):341-348
We study the theories I?n, L?n and overspill principles for ?n formulas. We show that IEn ? L?n ? I?n, but we do not know if I?n L?n. We introduce a new scheme, the growth scheme Crγ, and we prove that L?n ? Cr?n? I?n. Also, we analyse the utility of bounded collection axioms for the study of the above theories. Mathematics Subject Classification: 03F30, 03H15. 相似文献
130.
It has been suggested that chaotic motion inside the nucleus may significantly limit the accuracy with which nuclear masses can be calculated. Using a power spectrum analysis we show that the inclusion of additional physical contributions in mass calculations, through many-body interactions or local information, removes the chaotic signal in the discrepancies between calculated and measured masses. Furthermore, a systematic application of global mass formulas and of a set of relationships among neighboring nuclei to more than 2000 nuclear masses allows one to set an unambiguous upper bound for the average errors in calculated masses, which turn out to be almost an order of magnitude smaller than estimated chaotic components. 相似文献