全文获取类型
收费全文 | 261篇 |
免费 | 3篇 |
专业分类
化学 | 104篇 |
晶体学 | 7篇 |
力学 | 14篇 |
数学 | 92篇 |
物理学 | 47篇 |
出版年
2022年 | 2篇 |
2020年 | 2篇 |
2018年 | 7篇 |
2017年 | 7篇 |
2016年 | 4篇 |
2015年 | 3篇 |
2014年 | 8篇 |
2013年 | 19篇 |
2012年 | 12篇 |
2011年 | 11篇 |
2010年 | 6篇 |
2009年 | 12篇 |
2008年 | 11篇 |
2007年 | 10篇 |
2006年 | 8篇 |
2005年 | 3篇 |
2004年 | 6篇 |
2003年 | 8篇 |
2002年 | 3篇 |
2001年 | 4篇 |
2000年 | 6篇 |
1999年 | 3篇 |
1998年 | 2篇 |
1996年 | 4篇 |
1995年 | 4篇 |
1994年 | 6篇 |
1993年 | 2篇 |
1992年 | 6篇 |
1991年 | 9篇 |
1990年 | 9篇 |
1989年 | 8篇 |
1988年 | 7篇 |
1987年 | 6篇 |
1985年 | 3篇 |
1984年 | 4篇 |
1983年 | 4篇 |
1982年 | 1篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 1篇 |
1978年 | 3篇 |
1977年 | 1篇 |
1976年 | 2篇 |
1975年 | 2篇 |
1974年 | 3篇 |
1900年 | 1篇 |
1899年 | 1篇 |
1898年 | 1篇 |
1897年 | 2篇 |
1896年 | 9篇 |
排序方式: 共有264条查询结果,搜索用时 0 毫秒
131.
The reaction paths of [2+2] cycloaddition of the X=C=Y cumulenes were modeled at the MP2/aug-cc-pVDZ level. Cycloadditions of allene and CO2, CS2, or OCS lead in part to the same four-membered products as dimerizations of either ketene or thioketene or addition of ketene and thioketene, respectively. All the reactions studied are concerted and mostly asynchronous. The majority of the allene cycloadditions studied are endoergic and proceed with much higher activation barriers than do the alternative (thio)ketene additions. In comparison with the energy of the substrates, the four-membered cycles incorporating S-atoms are stabilized more than the analogous structures with O-atoms built into the rings. There are also some products that are thermodynamically disfavored, yet seem to be obtainable thanks to a relatively low barrier of the reaction. The AIM analysis of the electron density distribution in the transition state structures allowed distinguishing pericyclic from pseudopericyclic and nonplanar-pseudopericyclic types of reaction. 相似文献
132.
Adriana Czímerová Alexander Čeklovský Juraj Bujdák 《Central European Journal of Chemistry》2009,7(3):343-353
Layer charge is one of the key parameters used for the characterisation of expandable clay minerals, smectites. It determines
most significant properties of the material which are important from the industrial application point of view. This work is
related to a novel method introduced to characterize the layer charge of smectites, based on using cationic organic dyes as
molecular sensors. One xanthene and four phenothiazine cationic dyes were tested using reduced charge montmorillonites (RCMs)
and compared with methylene blue, which has been used most frequently. The characterization of the charge was based on the
formation of molecular assemblies (H- and J-aggregates) composed by dye cations, which were easily detectable using absorption spectroscopy in the UV/VIS spectrum. More
detailed characterization of the spectra required calculations of second-derivative curves. For all of the reaction systems
tested in this work, the molecular aggregation increased with the layer charge of RCMs. Slight to moderate differences in
the formation of dye assemblies related to the differences in the molecular structures of the individual dye cations. For
example, the molecular asymmetry of azure A brought about the formation of coexistent species of similar structures. The structure
of the heteroaromatic skeleton affected the extent of the aggregation and spectral changes with time. The presence of reactive,
non-substituted amino groups in thionine cations probably partially decomposed in the clay mineral colloids based on high-charge
RCMs. Any of the tested dyes could be used as molecular sensors for empirical characterization of the layer charge of clays
taking into account the differences mentioned above.
相似文献
133.
I. A. Smirnov B. I. Smirnov E. N. Mokhov Cz. Sulkowski H. Misiorek A. Jezowski A. R. de Arellano-Lopez J. Martinez-Fernandez 《Physics of the Solid State》2008,50(8):1407-1411
The thermopower coefficients of cubic bio-SiC, a high-porosity semiconductor with cellular pores prepared from the biocarbon template of white eucalyptus wood, and single-crystal β-SiC taken as a reference are measured in the temperature range 5–280 K. It is revealed that, in the low-temperature range, the samples are characterized by a thermopower contribution associated with the electron drag by phonons. The thermopower of the bio-SiC samples is measured both along and across the empty pore channels and is found to be anisotropic. Two models are proposed to account for the anisotropy of the thermopower in cubic bio-SiC. 相似文献
134.
For varieties, congruence modularity is equivalent to the tolerance intersection property, TIP in short. Based on TIP, it was proved in [5] that for an arbitrary lattice identity implying modularity (or at least congruence modularity) there exists a Mal’tsev condition such that the identity holds in congruence lattices of algebras of a variety if and only if the variety satisfies the corresponding Mal’tsev condition. However, the Mal’tsev condition constructed in [5] is not the simplest known one in general. Now we improve this result by constructing the best Mal’tsev condition and various related conditions. As an application, we give a particularly easy new proof of the result of Freese and Jónsson [11] stating that modular congruence varieties are Arguesian, and we strengthen this result by replacing “Arguesian” by “higher Arguesian” in the sense of Haiman [18]. We show that lattice terms for congruences of an arbitrary congruence modular variety can be computed in two steps: the first step mimics the use of congruence distributivity, while the second step corresponds to congruence permutability. Particular cases of this result were known; the present approach using TIP is even simpler than the proofs of the previous partial results.Dedicated to the memory of Ivan RivalReceived February 12, 2003; accepted in final form August 5, 2004.This revised version was published online in August 2005 with a corrected cover date. 相似文献
135.
The importance of NMR high pressure studies of ordered magnetic materials for the understanding of the ground and excited
states of magnetic conductors is discussed with examples from recent work on the manganese perovskites and the rare earth
compounds SmMn2Ge2 and CeIn3. A brief discussion is given of the difference between true pressure experiments and the “chemical pressure” introduced by
changing the composition of a material.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
136.
137.
Ratajska-Gadomska B Bialkowski B Gadomski W Radzewicz C 《The Journal of chemical physics》2007,126(18):184708
We report on the investigation of a short-time collective dynamics of water confined in the pores of the gelatin gel, using the femtosecond optical Kerr effect spectroscopy. The ultrafast responses of water molecules obtained in bulk liquid and in three concentrations of gelatin gels are explained theoretically, both in a long time and in a short time regime, taking into account all molecular motions. We prove that the contribution of molecules involved in tetrahedral, strongly H-bonded structures stabilizing the gel network increases with the gel concentration. On the other hand the long-time relaxation of water molecules is significantly slowed down in the gel pores. 相似文献
138.
A semimodular lattice L of finite length will be called an almost-geometric lattice if the order J(L) of its nonzero join-irreducible elements is a cardinal sum of at most two-element chains. We prove that each finite distributive
lattice is isomorphic to the lattice of congruences of a finite almost-geometric lattice. 相似文献
139.
Gábor Czédli 《Mathematica Slovaca》2011,61(6):859-870
For each of the relations “less than or equal to”, “less than”, “covered by”, and “covered by or equal to”, we characterize
finite orders (also called posets) with the property that the pair of Galois closure operators induced by the relation in
question coincides with the pair of closure operators introduced and applied in our previous paper in 2007. We also consider
the “less than or equal to” relation between the set of join-irreducible elements and the set of meet-irreducible elements,
and we show that the above-mentioned pairs of closure operators coincide for finite modular lattices. 相似文献
140.
Let Lat denote the variety of lattices. In 1982, the second author proved that Lat is strongly tolerance factorable, that is, the members of Lat have quotients in Lat modulo tolerances, although Lat has proper tolerances. We did not know any other nontrivial example of a strongly tolerance factorable variety. Now we prove that this property is preserved by forming independent joins (also called products) of varieties. This enables us to present infinitely many strongly tolerance factorable varieties with proper tolerances. Extending a recent result of G. Czédli and G. Grätzer, we show that if ${\mathcal{V}}$ is a strongly tolerance factorable variety, then the tolerances of ${\mathcal{V}}$ are exactly the homomorphic images of congruences of algebras in ${\mathcal{V}}$ . Our observation that (strong) tolerance factorability is not necessarily preserved when passing from a variety to an equivalent one leads to an open problem. 相似文献