全文获取类型
收费全文 | 1408篇 |
免费 | 44篇 |
国内免费 | 6篇 |
专业分类
化学 | 771篇 |
晶体学 | 8篇 |
力学 | 72篇 |
数学 | 296篇 |
物理学 | 311篇 |
出版年
2023年 | 9篇 |
2022年 | 29篇 |
2021年 | 35篇 |
2020年 | 29篇 |
2019年 | 40篇 |
2018年 | 33篇 |
2017年 | 34篇 |
2016年 | 58篇 |
2015年 | 34篇 |
2014年 | 52篇 |
2013年 | 92篇 |
2012年 | 96篇 |
2011年 | 112篇 |
2010年 | 72篇 |
2009年 | 60篇 |
2008年 | 89篇 |
2007年 | 67篇 |
2006年 | 74篇 |
2005年 | 71篇 |
2004年 | 52篇 |
2003年 | 53篇 |
2002年 | 37篇 |
2001年 | 10篇 |
2000年 | 16篇 |
1999年 | 22篇 |
1998年 | 19篇 |
1997年 | 18篇 |
1996年 | 17篇 |
1995年 | 15篇 |
1994年 | 14篇 |
1993年 | 5篇 |
1992年 | 5篇 |
1991年 | 12篇 |
1990年 | 7篇 |
1989年 | 6篇 |
1987年 | 4篇 |
1986年 | 3篇 |
1984年 | 6篇 |
1982年 | 3篇 |
1980年 | 2篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 2篇 |
1976年 | 3篇 |
1975年 | 2篇 |
1973年 | 2篇 |
1972年 | 2篇 |
1969年 | 2篇 |
1966年 | 2篇 |
1938年 | 7篇 |
排序方式: 共有1458条查询结果,搜索用时 15 毫秒
101.
Intrinsic properties of the space itself and quantum fluctuations of its geometry are sufficient to provide a mechanism for
the acceleration of cosmological expansion (dark energy effect). Applying Bogoliubov–Born–Green–Kirkwood–Yvon hierarchy approach
to self-consistent equations of one-loop quantum gravity, we found exact solutions that yield acceleration. The permanent
creation and annihilation of virtual gravitons is not in exact balance because of the expansion of the Universe. The excess
energy comes from the spontaneous process of graviton creation and is trapped by the background. It provides the macroscopic
quantum effect of cosmic acceleration. 相似文献
102.
103.
Combined theoretical and experimental investigations led to the discovery of a new polymorph of titanium dioxide with titanium nine-coordinated to oxygen in the cotunnite (PbCl 2 ) structure. Hardness measurements on the cotunnite-structured TiO 2 synthesized at pressures above 60 GPa and temperatures above 1000 K reveal that this material is the hardest oxide yet discovered. Furthermore, it is one of the least compressible (with a measured bulk modulus of 431 GPa) and hardest (with a microhardness of 38 GPa) polycrystalline materials studied thus far. 相似文献
104.
One way functions and pseudorandom generators 总被引:2,自引:0,他引:2
Leonid A. Levin 《Combinatorica》1987,7(4):357-363
Pseudorandom generators transform in polynomial time a short random “seed” into a long “pseudorandom” string. This string
cannot be random in the classical sense of [6], but testing that requires an unrealistic amount of time (say, exhaustive search
for the seed). Such pseudorandom generators were first discovered in [2] assuming that the function (a
x
modb) is one-way, i.e., easy to compute, but hard to invert on a noticeable fraction of instances. In [12] this assumption was
generalized to the existence of any one-way permutation. The permutation requirement is sufficient but still very strong.
It is unlikely to be proven necessary, unless something crucial, like P=NP, is discovered. Below, among other observations,
a weaker assumption about one-way functions is proposed, which is not only sufficient, but also necessary for the existence
of pseudorandom generators.
Supported by NSF grant #DCR-8304498, DCR-8607492. 相似文献
105.
Hui H. Chin Leonid V. Azroff Agya R. Saini Robert W. Lenz 《Journal of Polymer Science.Polymer Physics》1989,27(10):2001-2009
Semiflexible thermotropic aromatic polyesters with lateral groups were aligned magnetically and mechanically in monodomains after separation into high-molecular-weight (polymer) and low-molecular-weight (oligomer) fractions. Based on the x-ray intensity distributions recorded for a series of seven different substituents in the arylsulfonyl substituted group, a novel smectic structure was determined for H, F, Cl, OCH3, and CH3 in the para position while a normal nematic alignment prevailed when it contained Br or NO2. 相似文献
106.
Edward Bormashenko Alexander A. Fedorets Leonid A. Dombrovsky Michael Nosonovsky 《Entropy (Basel, Switzerland)》2021,23(2)
Many small biological objects, such as viruses, survive in a water environment and cannot remain active in dry air without condensation of water vapor. From a physical point of view, these objects belong to the mesoscale, where small thermal fluctuations with the characteristic kinetic energy of kBT (where kB is the Boltzmann’s constant and T is the absolute temperature) play a significant role. The self-assembly of viruses, including protein folding and the formation of a protein capsid and lipid bilayer membrane, is controlled by hydrophobic forces (i.e., the repulsing forces between hydrophobic particles and regions of molecules) in a water environment. Hydrophobic forces are entropic, and they are driven by a system’s tendency to attain the maximum disordered state. On the other hand, in information systems, entropic forces are responsible for erasing information, if the energy barrier between two states of a switch is on the order of kBT, which is referred to as Landauer’s principle. We treated hydrophobic interactions responsible for the self-assembly of viruses as an information-processing mechanism. We further showed a similarity of these submicron-scale processes with the self-assembly in colloidal crystals, droplet clusters, and liquid marbles. 相似文献
107.
Leonid K. Antanovskii 《Meccanica》1994,29(1):27-42
The shape evolution of a two-dimensional bubble, bounded by a simple closed curve, which is initially placed within a potential viscous flow, is analysed. It is assumed that the influence of gravity and inertia forces is negligible, so the quasi-steady approximation can be applied. Reformulating the problem for Stokes equations with relevant boundary conditions at the free surface in terms of the bianalytic stress-stream function, and using the time-dependent conformal mappingz(,t) of a unit disk onto an unbounded flow domain sought, an infinite system of ordinary differential equations for the Laurent coefficients ofz(,t) is derived. A class of exact solutions is found for the case when the principal part of the complex velocity of the dominant flow at infinity is a polynomial, and the problem of formation of a pointed bubble is discussed.
Sommario E analizzata l'evoluzione di una bolla bi-dimensionale, limitata da una curva chiusa semplice, inizialmente posta in un flusso potenziale viscoso. Si assume che l'influenza della gravità e delle forze inerziali sia trascurabile, cosicchè si può applicare l'approssimazione quasi-stazionaria. Riformulando il problema per le equazioni di Stokes con le opportune condizionial contorno sulla superficie libera in termini della funzione stress-stream bianalitica, e usando la tecnica delle trasformazioni conformi dipendenti dal tempoz(,t) di un disco unitario su un dominio di flusso non limitato incognito, viene derivato un sistema infinito di equazioni differenziali ordinarie per i coefficienti di Laurent diz(,t). Viene trovata una classe di soluzioni esatte per il caso in cui la parte principale della velocità complessa del flusso dominante all'infinito è una polinomiale, ed è discusso il problema della formazione di una bolla lenticolare.相似文献
108.
Lopes S Domanskaya AV Fausto R Räsänen M Khriachtchev L 《The Journal of chemical physics》2010,133(14):144507
Formic acid (HCOOH, FA) and acetic acid (CH(3)COOH, AA) are studied in a nitrogen matrix. The infrared (IR) spectra of cis and trans conformers of these carboxylic acids (and also of the HCOOD isotopologue of FA) are reported and analyzed. The higher-energy cis conformer of these molecules is produced by narrowband near-IR excitation of the more stable trans conformer, and the cis-to-trans tunneling decay is evaluated spectroscopically. The tunneling process in both molecules is found to be substantially slower in a nitrogen matrix than in rare-gas matrices, the cis-form decay constants being approximately 55 and 600 times smaller in a nitrogen matrix than in an argon matrix, for FA and AA respectively. The stabilization of the higher-energy cis conformer is discussed in terms of specific interactions with nitrogen molecule binding with the OH group of the carboxylic acid. This model is in agreement with the observed differences in the IR spectra in nitrogen and argon matrices, in particular, the relative frequencies of the νOH and τCOH modes and the relative intensities of the νOH and νC=O bands. 相似文献
109.
Yan Pan Leonid Brown Lars Konermann 《Journal of the American Society for Mass Spectrometry》2010,21(11):1947-1956
Exposure of the membrane protein bacteriorhodopsin (BR) to SDS induces partial breakdown of the native conformation. The exact
structural properties of this SDS state remain a matter of debate, despite its widespread use in BR folding experiments. The
current work employs hydroxyl radical (·OH) labeling in conjunction with mass spectrometry (MS)-based peptide mapping for
probing the solvent accessibility of individual BR segments in the presence of SDS. Previous work revealed methionine sulfoxide
formation to be the dominant oxidative pathway. Those data suggested extensive unfolding of helices A and D in SDS. Unfortunately,
the lack of Met residues in helices C and F implies that no direct information on the behavior of the latter two elements
could be obtained. Here, we address this problem by employing two variants with additional Met residues, L93M (helix C) and
V179M (helix F). The oxidation behavior of the resulting 11 methionines can be grouped into three categories: (1) extensively
labeled both in native BR and in SDS (loop residues M32, M68, and M163); (2) protected in the native state but not in SDS
(M20, M118); (3) always protected (M56, M60, M93, M145, M179, M209). These data show that a solvent-inaccessible core is retained
in SDS. This core consists of partially intact helices B, C, E, F, and G. The termini of these helices are highly dynamic
and/or unraveled, particularly on the cytoplasmic side. Overall, this work demonstrates how the use of engineered ·OH labeling
sites can provide insights into structural properties of membrane proteins. 相似文献
110.
Laia Vilà‐Nadal José Pedro Sarasa Dr. Antonio Rodríguez‐Fortea Dr. Joan Igual Dr. Leonid P. Kazansky Dr. Josep M. Poblet Prof. Dr. 《化学:亚洲杂志》2010,5(1):97-104
DFT calculations were carried out to study 183W NMR chemical shifts in the family of the Keggin anions with formula α‐[XW12O40]q? (X=B, Al, Si, P, Ga, Ge, As, Zn), in the β‐ and γ‐[SiW12O40]4? geometric isomers, in the derivative Dawson anion [P2W18O62]6?, and in the most symmetrical Lindqvist [W6O19]2? anion and its derivative [W10O32]4?. In this article, we show that the geometry employed in the calculation of NMR chemical shifts in polyoxotungstates is extremely important if we want to be quantitative. Using very large basis sets of QZ4P quality and taking into account the conductor‐like screening model (COSMO) to account for solvent effects (aqueous and organic solutions), good geometries were found for the polyoxoanions. From these optimal geometries the 183W NMR chemical shifts were computed with the more standard basis sets of TZP quality and including spin–orbit corrections inside the zero‐order regular approximation (ZORA) to describe the relativistic effects of the internal electrons. With this strategy the mean absolute error between experimental and theoretical values was found to be less than 10 ppm, which is similar to the experimental error. We also discuss how the geometry of the polyoxoanion influences on the shielding. 相似文献