全文获取类型
收费全文 | 1421篇 |
免费 | 30篇 |
国内免费 | 8篇 |
专业分类
化学 | 931篇 |
晶体学 | 4篇 |
力学 | 94篇 |
数学 | 211篇 |
物理学 | 219篇 |
出版年
2022年 | 28篇 |
2021年 | 35篇 |
2020年 | 10篇 |
2019年 | 12篇 |
2018年 | 20篇 |
2017年 | 18篇 |
2016年 | 30篇 |
2015年 | 30篇 |
2014年 | 41篇 |
2013年 | 58篇 |
2012年 | 81篇 |
2011年 | 85篇 |
2010年 | 52篇 |
2009年 | 48篇 |
2008年 | 75篇 |
2007年 | 83篇 |
2006年 | 68篇 |
2005年 | 76篇 |
2004年 | 59篇 |
2003年 | 70篇 |
2002年 | 62篇 |
2001年 | 17篇 |
2000年 | 18篇 |
1999年 | 16篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 24篇 |
1995年 | 15篇 |
1994年 | 15篇 |
1993年 | 15篇 |
1992年 | 9篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 10篇 |
1986年 | 6篇 |
1985年 | 8篇 |
1984年 | 15篇 |
1983年 | 11篇 |
1982年 | 20篇 |
1981年 | 17篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1977年 | 12篇 |
1976年 | 10篇 |
1975年 | 7篇 |
1974年 | 9篇 |
1967年 | 7篇 |
1954年 | 8篇 |
排序方式: 共有1459条查询结果,搜索用时 15 毫秒
101.
Summary A nonlinear stability analysis is described for the Bénard problem for a chemically reacting fluid heated from above; the findings are compared with those of Wollkind & Frisch. Next an energy stability analysis is given for the model of Wollkind & Frisch and it is shown that instabilities may occur.
Sommario Si sviluppa un'analisi non lineare della stabilità per il problema di Bénard relativo ad un fluido chimicamente reagente e riscaldato dall'alto; i risultati sono confrontati con quelli trovati in precedenza da Wollkind & Frisch. Successivamente si elabora un'analisi non lineare della stabilità proprio per il modello di Wollkind & Frisch e si mostra che si possono presentare delle instabilità.相似文献
102.
Constitutive branching analysis of cylindrical bodies under in-plane equibiaxial dead-load tractions
Finite homogeneous deformations of hyperelastic cylindrical bodies subjected to in-plane equibiaxial dead-load tractions are analyzed. Four basic equilibrium problems are formulated considering incompressible and compressible isotropic bodies under plane stress and plane deformation condition. Depending on the form of the stored energy function, these plane problems, in addition to the obvious symmetric solutions, may admit asymmetric solutions. In other words, the body may assume an equilibrium configuration characterized by two unequal in-plane principal stretches corresponding to equal external forces. In this paper, a mathematical condition, in terms of the principal invariants, governing the global development of the asymmetric deformation branches is obtained and examined in detail with regard to different choices of the stored energy function. Moreover, explicit expressions for evaluating critical loads and bifurcation points are derived. With reference to neo-Hookean, Mooney-Rivlin and Ogden-Ball materials, a broad numerical analysis is performed and the qualitatively more interesting asymmetric equilibrium branches are shown. Finally, using the energy criterion, a number of considerations are put forward about the stability of the computed solutions. 相似文献
103.
Summary A general momentum balance for a mixture constituent is motivated from corpuscular considerations. The interpretations of stress and internal body force in the separate cases of ionic and non-ionic species are indicated. The Gurtin-Oliver-Williams paradox concerning the interpretation of partial stress is resolved, and relations between total and inner stress shown to be more complex than usually assumed. Comparison is made with kinetic theory.
Sommario Una relazione generale di bilancio della quantità di moto per un costituente di una mistura è ricavata sulla base di considerazioni particellari. Interpretazioni di stress e forza interna di volume sono fornite per costituenti sia carichi sia neutri. In tal modo il paradosso di Gurtin, Oliver e Williams sull'interpretazione degli stress parziali viene risolto e, inoltre, le relazioni tra stress totali e parziali risultano in generale più complesse di quanto si assuma solitamente. Infine si effettua un confronto con la teoria cinetica.相似文献
104.
Angelo Morro 《Meccanica》1977,12(4):194-207
Summary We consider a micropolar continuum, whose material points are endowed with magnetic moment, in the presence of a magnetic field within the approximation of the quasimagnetostatics. We deduce the balance equations by a suitable averaging technique and then we examine the compatibility of spatial interaction effects with thermodynamics. As a result, we show that such effects are confined to the constitutive equations for the dissipative stressDT, the dissipative couple stressDm and the heat flux q. Moreover, as a consequence of the reduced dissipation inequality, a linear theory would account only for a dependence ofDT,Dm, and q on the velocity gradient, the angular velocity, the gradient of the angular velocity, and on the temperature gradient.
This work was supported by the «Gruppo Nazionale per la Fisica Matematica» of CNR. 相似文献
Sommario Si considera un continuo micropolarc, i cui punti materiali sono dotati di momento magnetico, in presenza di un campo magnetico nell'approssimazione della quasimagnetostatica. Dopo aver dedotto le equazioni di bilancio mediante un opportuno procedimento di media, si esamina la compatibilità tra effetti di interazione spaziale e seconda legge della termodinamica e si mostra che tali effetti possono essere presenti solo nelle equazioni costitutive per lo stress dissipativo (di Cauchy)DT.lo stress dissipativo delle coppieDm e il flusso di calore q. Inoltre, in conseguenza della disuguaglianza di dissipazione ridotta, si osserva che una teoria lineare permetterebbe solo una dipendenza diDT,Dm, q dal gradiente di velocità, dalla velocità angolare, dal gradiente della velocità angolare e dal gradiente di temperatura.
This work was supported by the «Gruppo Nazionale per la Fisica Matematica» of CNR. 相似文献
105.
Giovanni Pellegrino Angelo Carotti Orazio Nicolotti Leonardo Pisani Angela Stefanachi Marco Catto 《Tetrahedron letters》2010,51(13):1702-7034
A small molecular library of 40 pyrimidine, pyrazole, and isoxazole derivatives, bearing structural features for a promising binding of therapeutically interesting enzymes, was designed and prepared. An efficient and straightforward solid phase synthesis was envisaged and carried out on a Rink amide resin. The assistance of microwave heating in any step reduced the reaction time, increased the reaction yields, and allowed an easy work-up and purification of the targeted compounds. 相似文献
106.
In this paper the analysis of a self-excited tower under turbulent wind flow is carried out. The structure is considered as a one dof nonlinear system, and the implications of this modeling are deeply discussed. The stationary wind is responsible for self-excitation, while the turbulent part provides both parametric and external excitations. The simultaneous presence of those excitations is taken into account in a specific resonance condition. The periodic and quasi-periodic solutions are studied by means of a perturbation method and the effects of the turbulence on the dynamics of the structure are analyzed. 相似文献
107.
Fast perturbation Monte Carlo method for photon migration in heterogeneous turbid media 总被引:1,自引:0,他引:1
Sassaroli A 《Optics letters》2011,36(11):2095-2097
We present a two-step Monte Carlo (MC) method that is used to solve the radiative transfer equation in heterogeneous turbid media. The method exploits the one-to-one correspondence between the seed value of a random number generator and the sequence of random numbers. In the first step, a full MC simulation is run for the initial distribution of the optical properties and the "good" seeds (the ones leading to detected photons) are stored in an array. In the second step, we run a new MC simulation with only the good seeds stored in the first step, i.e., we propagate only detected photons. The effect of a change in the optical properties is calculated in a short time by using two scaling relationships. By this method we can increase the speed of a simulation up to a factor of 1300 in typical situations found in near-IR tissue spectroscopy and diffuse optical tomography, with a minimal requirement for hard disk space. Potential applications of this method for imaging of turbid media and the inverse problem are discussed. 相似文献
108.
Gallo E Caselli A Ragaini F Fantauzzi S Masciocchi N Sironi A Cenini S 《Inorganic chemistry》2005,44(6):2039-2049
A reproducible synthesis of a competent epoxidation catalyst, [Ru(VI)(TPP)(O)2)] (TPP = tetraphenylporphyrin dianion), starting from [Ru(II)(TPP)(CO)L] (L = none or CH3OH), is described. The molecular structure of the complex was determined by using ab initio X-ray powder diffraction (XRPD) methods, and its solution behavior was in detail investigated by NMR techniques such as PGSE (pulsed field gradient spin-echo) measurements. [Ru(IV)(TPP)(OH)]2O, a reported byproduct in the synthesis of [Ru(VI)(TPP)(O)2], was synthesized in a pure form by oxidation of [Ru(II)(TPP)(CO)L] or by a coproportionation reaction of [Ru(VI)(TPP)(O)2] and [Ru(II)(TPP)(CO)L], and its molecular structure was then determined by XRPD analysis. [Ru(VI)(TPP)(O)2] can be reduced by dimethyl sulfoxide or by carbon monoxide to yield [Ru(II)(TPP)(S-DMSO)2] or [Ru(II)(TPP)(CO)(H2O)], respectively. These two species were characterized by conventional single-crystal X-ray diffraction analysis. 相似文献
109.
Fleury BG Figueiredo L Marcoli MI Teixeira VL Ferreira AB Pinto AC 《Natural product communications》2011,6(5):667-672
The distribution of fatty acids in 13 species of macroalgae (Chlorophyta, Ochrophyta and Rhodophyta) and 1 seagrass (Spartina sp), collected on the Rio de Janeiro state coast was determined. The results were evaluated in search of correlations between the taxonomic and phylogenetic position of these macrophytes. Statistical analyses showed the effectiveness as taxonomic and phylogenetic markers of the distribution of the methyl fatty acid esters in these macrophytes. 相似文献
110.
Angelo Kirchon Jialuo Li Fangqing Xia Gregory S. Day Benjamin Becker Wenmiao Chen Hung‐Jue Sue Yu Fang Hong‐Cai Zhou 《Angewandte Chemie (Weinheim an der Bergstrasse, Germany)》2019,131(36):12555-12560
Modulation and templating are two synthetic techniques that have garnered significant attention over the last several years for the preparation of hierarchically porous metal–organic frameworks (HP‐MOFs). In this study, by using fatty acids with different lengths and concentrations as dual‐functional modulators/templates, we were able to obtain HP‐MOFs with tunable mesopores that exhibit different pore diameters and locations. We found that the length and concentration of the fatty acids can determine if micelle formation occurs, which in turn dictates the porosity of the resulting HP‐MOFs. The HP‐MOFs with different mesopores differed in their performance in gas uptake and dye adsorption, and the structure–performance relationships were ascribed to the pore diameters and locations. This approach could provide a potentially universal method to efficiently introduce hierarchal mesopores into existing microporous MOF adsorbents with tunable properties. 相似文献