全文获取类型
收费全文 | 857篇 |
免费 | 59篇 |
国内免费 | 3篇 |
专业分类
化学 | 513篇 |
晶体学 | 9篇 |
力学 | 19篇 |
综合类 | 1篇 |
数学 | 128篇 |
物理学 | 249篇 |
出版年
2023年 | 5篇 |
2022年 | 21篇 |
2021年 | 28篇 |
2020年 | 13篇 |
2019年 | 18篇 |
2018年 | 15篇 |
2017年 | 14篇 |
2016年 | 38篇 |
2015年 | 20篇 |
2014年 | 34篇 |
2013年 | 52篇 |
2012年 | 65篇 |
2011年 | 65篇 |
2010年 | 30篇 |
2009年 | 35篇 |
2008年 | 48篇 |
2007年 | 55篇 |
2006年 | 40篇 |
2005年 | 42篇 |
2004年 | 44篇 |
2003年 | 23篇 |
2002年 | 21篇 |
2001年 | 10篇 |
2000年 | 27篇 |
1999年 | 16篇 |
1998年 | 16篇 |
1997年 | 10篇 |
1996年 | 11篇 |
1995年 | 17篇 |
1994年 | 5篇 |
1993年 | 17篇 |
1992年 | 11篇 |
1991年 | 3篇 |
1990年 | 6篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1985年 | 1篇 |
1984年 | 5篇 |
1982年 | 3篇 |
1981年 | 11篇 |
1980年 | 2篇 |
1979年 | 3篇 |
1978年 | 4篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1936年 | 1篇 |
排序方式: 共有919条查询结果,搜索用时 234 毫秒
111.
Machado NF Ruano C Castro JL Marques MP Otero JC 《Physical chemistry chemical physics : PCCP》2011,13(3):1012-1018
A SERS study of chromone 3-carboxylic acid adsorbed on silver colloids was undertaken, in order to assess the ability of this compound to accept electrons in charge transfer (CT) processes. Theoretical SERS intensities under photoinduced CT resonant conditions have been carried out for both the neutral and the deprotonated species allowing to conclude, by comparison with the experimental data, that the recorded SER corresponds to the anionic form of the acid linked to the metal. It was shown that the SERS-CT mechanism predominates for this particular compound, thus explaining the strong enhancement of the band at ca. 1600 cm(-1) assigned to the 8a ring stretching mode. The identification of CT processes is of the utmost importance for understanding the mechanism through which these benzopyranes may act as antioxidants. 相似文献
112.
Consider a compact Riemannian manifold M of dimension n whose boundary ?M is totally geodesic and is isometric to the standard sphere S n?1. A natural conjecture of Min-Oo asserts that if the scalar curvature of M is at least n(n?1), then M is isometric to the hemisphere $S_{+}^{n}$ equipped with its standard metric. This conjecture is inspired by the positive mass theorem in general relativity, and has been verified in many special cases. In this paper, we construct counterexamples to Min-Oo??s Conjecture in dimension n??3. 相似文献
113.
Filipe J. Marques M. Theodor Loots Andriëtte Bekker 《Mathematical Methods in the Applied Sciences》2019,42(17):5718-5735
Series representations for several density functions are obtained as mixtures of generalized gamma distributions with discrete mass probability weights, by using the exponential expansion and the binomial theorem. Based on these results, approximations based on mixtures of generalized gamma distributions are proposed to approximate the distribution of the sum of independent random variables, which may not be identically distributed. The applicability of the proposed approximations are illustrated for the sum of independent Rayleigh random variables, the sum of independent gamma random variables, and the sum of independent Weibull random variables. Numerical studies are presented to assess the precision of these approximations. 相似文献
114.
Summary In the recent years various and interesting superposition states of the quantized radiation field were investigated in the
literature, such as that involving two number states by Wodkiewiczet al.; that involving two coherent states by Hillery and Gerryet al.; that involving two squeezed states by Xinet al., etc. Here we study a general one-parameter superposition state which unifies those states and others in the literature,
all becoming a particularization of ours. General expressions characterizing the physical properties of the field are obtained.
The role played by the present superposition state as an alternative interpolating state is also discussed.
The authors of this paper have agreed to not receive the proofs for correction. 相似文献
115.
S. Marques Pinto M. Teresa Oliveira‐Martins M. Cu Pinto 《Mathematical Logic Quarterly》2006,52(2):134-150
The main purpose of this work is to introduce the class of the monadic dynamic algebras (dynamic algebras with one quantifier). Similarly to a theorem of Kozen we establish that every separable monadic dynamic algebra is isomorphic to a monadic (possibly non‐standard) Kripke structure. We also classify the simple (monadic) dynamic algebras. Moreover, in the dynamic duality theory, we analyze the conditions under which a hemimorphism of a dynamic algebra into itself defines a quantifier. (© 2006 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
116.
117.
Ana M Amado Juan Carlos Otero M Paula M Marques Luis A E Batista de Carvalho 《Chemphyschem》2004,5(12):1837-1847
A structural study of [H3N(CH2)2NH3)]2+.2Cl-, the smallest element of the homologous series of the alpha,omega-diamine dihydrochlorides, was carried out by means of Raman and FTIR spectroscopy coupled to ab initio molecular orbital (MO) calculations. As a primary concern, an adequate molecular model for the representation of these solid amine salts was chosen. Thus, several models, varying in the number and position of the counterions as well as in the number of diamine units, were considered. It was found that the best molecular system (i.e., that yielding the best compromise between accuracy and computational requirements) consists of one ethylenediamine cation surrounded by six chloride ions in an arrangement based on the crystal structure reported in the literature for [H3N(CH2)2NH3)]2+.2Cl-. This conclusion will hopefully allow for a better understanding of the conformational preferences, in the solid state, of these biologically relevant linear polyamines. 相似文献
118.
A.M. Rao P.C. Eklund U.D. Venkateswaran J. Tucker M.A. Duncan G.M. Bendele P.W. Stephens J.-L. Hodeau L. Marques M. Núñez-Regueiro I.O. Bashkin E.G. Ponyatovsky A.P. Morovsky 《Applied Physics A: Materials Science & Processing》1997,64(3):231-2239
60 polymers. Pure and mixed phase polymeric samples were synthesized by simultaneously subjecting microcrystalline C60 powder or pellets to various pressures () and temperatures (). The optical spectra of the orthorhombic, tetragonal, and rhombohedral C60 polymer phases are observed to be quite distinct and rich. These spectra exhibit numerous lines and an overall downshift
in frequency relative to C60 is observed, consistent with a loss of double bonds from the fullerene cage. The LDMS spectra of a sample synthesized at
under hydrostatic conditions and , exhibited a succession of clear peaks at mass numbers corresponding to , similar to the LDMS data on the C60 photopolymer. This is taken as further evidence for interfullerene bonds in these high-pressure polymers. The XRD pattern
of this sample indicates the presence of a strong texture in the sample.
Received: 14 November 1996/Accepted: 8 January 1997 相似文献
119.
M.F. Carrasco S.K. Mendiratta L. Marques 《Applied Physics A: Materials Science & Processing》2005,80(2):361-367
High-energy ball milling is a relatively new process for the synthesis of technologically useful compounds that require high temperatures in the conventional solid-state reaction method. It is thought that high-impact energy transferred to microscopic amounts of powder momentarily trapped between two colliding balls leads to a local transient of high pressure and temperature that drives the reaction at the surface of the trapped particles. The technique of ball milling has been successfully used to produce nanocrystals of the sillenite phase of bismuth silicate (BSO), and its titanate (BTO) and germanate (BGO) homologues. Our aim in this paper is to see, experimentally, whether passage through an intermediate phase occurs en route to the final product. The results of our study using various analytical techniques show that indeed, in the case of BSO and BTO, an intermediate phase is formed temporarily along the grinding route. The SEM pictures of the powders in the various stages of the grinding process also indicate that a process of phase transformation takes place at the surface of the crystals. We have not been able to identify the precise structure of the intermediate phase, but show that it is an unstable phase that transforms with heat treatment preferentially into the desired sillenite phase in the case of BTO and to the reactants in the case of BSO. PACS 81.20.Ev 相似文献
120.
The nonohmic electrical features of (Ca1/4,Cu3/4)TiO3 perovskite ceramics, which have very strong gigantic dielectric is believed originate from potential barriers at the grain boundaries. In the present study, we used the admittance and impedance spectroscopy technique to investigate (Ca1/4,Cu3/4)TiO3 perovskite ceramics with low nonohmic electrical properties. The study was conducted under two different conditions: on as-sintered ceramics and on ceramics thermally treated in an oxygen-rich atmosphere. The results confirm that thermal treatment in oxygen-rich atmospheres influence the nonohmic properties. Annealing at oxygen-rich atmospheres improve the nonohmic behavior and annealing at oxygen-poor atmospheres decrease the nonohmic properties, a behavior already reported for common metal oxide nonohmic devices and here firstly evidenced for the (Ca1/4,Cu3/4)TiO3 perovskite related materials. The results show that oxygen also influences the capacitance values at low frequencies, a behavior that is indicative of the Schottky-type nature of the potential barrier. 相似文献