全文获取类型
收费全文 | 997篇 |
免费 | 42篇 |
国内免费 | 3篇 |
专业分类
化学 | 586篇 |
晶体学 | 9篇 |
力学 | 60篇 |
数学 | 172篇 |
物理学 | 215篇 |
出版年
2024年 | 3篇 |
2023年 | 7篇 |
2022年 | 11篇 |
2021年 | 33篇 |
2020年 | 25篇 |
2019年 | 34篇 |
2018年 | 44篇 |
2017年 | 42篇 |
2016年 | 65篇 |
2015年 | 33篇 |
2014年 | 56篇 |
2013年 | 115篇 |
2012年 | 68篇 |
2011年 | 61篇 |
2010年 | 49篇 |
2009年 | 63篇 |
2008年 | 45篇 |
2007年 | 36篇 |
2006年 | 29篇 |
2005年 | 31篇 |
2004年 | 28篇 |
2003年 | 22篇 |
2002年 | 21篇 |
2001年 | 9篇 |
2000年 | 4篇 |
1999年 | 6篇 |
1998年 | 2篇 |
1997年 | 8篇 |
1996年 | 8篇 |
1995年 | 2篇 |
1994年 | 5篇 |
1993年 | 7篇 |
1992年 | 7篇 |
1990年 | 5篇 |
1989年 | 3篇 |
1988年 | 2篇 |
1987年 | 3篇 |
1986年 | 5篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1983年 | 3篇 |
1982年 | 3篇 |
1981年 | 8篇 |
1979年 | 2篇 |
1975年 | 2篇 |
1973年 | 5篇 |
1970年 | 2篇 |
1968年 | 2篇 |
1957年 | 1篇 |
1936年 | 1篇 |
排序方式: 共有1042条查询结果,搜索用时 15 毫秒
121.
122.
Human serum albumin consists of a single polypeptide of 585 amino acid residues with 1 Trp residue. In the present work, we
measured fluorescence lifetimes of the protein in both native and denatured states. The results indicate that Trp emission
occurs with three lifetimes in both states. Lifetimes values and contribution to the global emission decay differ between
the two states. Data are interpreted as the results of an emission occurring from three substructures of the tryptophan formed
in the excited state. Two of these substructures are already present for the tryptophan free in solution. The third lifetime
is the result of the interaction between the tryptophan residue and surrounding microenvironment. The populations of these
substructures characterized by the pre-exponential parameters of the fluorescence lifetimes are dependent on the fluorophore
microenvironment and on the global protein structure. 相似文献
123.
Quantum confinement effect on the energy levels of Eu2+ doped K2Ca2(SO4)3 nanoparticles has been observed. The broad photoluminescence (PL) emission band of Eu2+ doped K2Ca2(SO4)3 microcrystalline sample observed at ~436 nm is found to split into two narrow well resolved bands, located at 422 and 445 nm in the nanostructure form of this material. This has been attributed to the reduction in the crystal field strength of the nanomaterials, which results in widening the energy band gap and splitting the broad 4f65d energy level of Eu2+. Energy band gap values of the micro and nanocrystalline K2Ca2(SO4)3 samples were also determined by measuring the UV–visible absorption spectra. These values are 3.34 and 3.44 eV for the micro and nanocrystalline samples, respectively. These remarkable results suggest that activators having wide emission bands might be subjected to weak crystal strength via nanostructure materials to modify their electronic transitions. This might prove a powerful technique for producing new-advanced materials for use in the fields of solid state lasers and optoelectronic devises. 相似文献
124.
The hierarchies of both Lovelock gravity and power-Yang–Mills field are combined through gravity in a single theory. In static, spherically symmetric ansatz exact particular integrals are obtained in all higher dimensions. The advantage of such hierarchies is the possibility of choosing coefficients, which are arbitrary otherwise, to cast solutions into tractable forms. To our knowledge the solutions constitute the most general spherically symmetric metrics that incorporate complexities both of Lovelock and Yang–Mills hierarchies within the common context. A large portion of our general class of solutions concerns and addresses to black holes for which specific examples are given. Thermodynamical behaviors of the system is briefly discussed in particular dimensions. 相似文献
125.
Hamam H 《Optics letters》2010,35(24):4175-4177
A steganographic method offering a high hiding capacity is presented in which the techniques of digital holography are used to distribute information from a small secret image across the larger pixel field of a cover image. An iterative algorithm is used to design a phase-only or complex hologram from a padded version of the secret image, quantizing this data according to the carrier data bits that are available within the intended cover image. By introducing the hologram data only into low-order bits of larger amplitude cover pixels, the change in the cover image remains imperceptible to the casual observer, with a peak signal-to-noise ratio of >40 dB. 相似文献
126.
127.
The group of bisections of groupoids plays an important role in the study of Lie groupoids. In this paper another construction is introduced. Indeed, for a topological groupoid G, the set of all continuous self-maps f on G such that (x, f(x)) is a composable pair for every \(x\in G\), is denoted by \(S_G\). We show that \(S_G\) by a natural binary operation is a monoid. \(S_G(\alpha )\), the group of units in \(S_G\) precisely consists of those \(f\in S_G\) such that the map \(x\mapsto xf(x)\) is a bijection on G. Similar to the group of bisections, \(S_G(\alpha )\) acts on G from the right and on the space of continuous self-maps on G from the left. It is proved that \(S_G(\alpha )\) with the compact- open topology inherited from C(G, G) is a left topological group. For a compact Hausdorff groupoid G it is proved that the group of bisections of \(G^2\) is isomorphic to the group \(S_G(\alpha )\) and the group of transitive bisections of G, \(Bis_T(G)\), is embedded in \(S_G(\alpha )\), where \(G^2\) is the groupoid of all composable pairs. 相似文献
128.
Saeid Amiri Bertrand S. Clarke Jennifer L. Clarke 《Journal of computational and graphical statistics》2018,27(1):195-208
We present a technique for clustering categorical data by generating many dissimilarity matrices and combining them. We begin by demonstrating our technique on low-dimensional categorical data and comparing it to several other techniques that have been proposed. We show through simulations and examples that our method is both more accurate and more stable. Then we give conditions under which our method should yield good results in general. Our method extends to high-dimensional categorical data of equal lengths by ensembling over many choices of explanatory variables. In this context, we compare our method with two other methods. Finally, we extend our method to high-dimensional categorical data vectors of unequal length by using alignment techniques to equalize the lengths. We give an example to show that our method continues to provide useful results, in particular, providing a comparison with phylogenetic trees. Supplementary material for this article is available online. 相似文献
129.
Habib?Maagli Abdelwaheb?DhifliEmail author Abdulah?Khamis?Alzahrani 《Mediterranean Journal of Mathematics》2018,15(2):68
In this paper, we study the following fractional Navier boundary value problem where \(\alpha ,\beta \in (0,1]\) such that \(\alpha +\beta >1\), \(D^{\beta }\) and \(D^{\alpha }\) stand for the standard Riemann–Liouville fractional derivatives and a, b are nonnegative constants such that \(a+b>0\). The function g is a nonnegative continuous function in \([0,\infty )\) that is required to satisfy some suitable integrability condition. Using estimates on the Green’s function and a perturbation argument, we prove the existence of a unique positive continuous solution, which behaves like the unique solution of the homogeneous problem.
相似文献
$$\begin{aligned} \left\{ \begin{array}{lllc} D^{\beta }(D^{\alpha }u)(x)=u(x)g(u(x)),\quad x\in (0,1), \\ \displaystyle \lim _{x\longrightarrow 0}x^{1-\beta }D^{\alpha }u(x)=-a,\quad \,\,u(1)=b, \end{array} \right. \end{aligned}$$
130.
Noura Beji Bassem Jarboui Mansour Eddaly Habib Chabchoub 《Journal of computational science》2010,1(3):159-167
The Redundancy Allocation Problem generally involves the selection of components with multiple choices and redundancy levels that produce maximum system reliability given various system level constraints as cost and weight. In this paper we investigate the series–parallel redundant reliability problems, when a mixing of components was considered. In this type of problem both the number of redundancy components and the corresponding component reliability in each subsystem are to be decided simultaneously so as to maximise the reliability of system. A hybrid algorithm is based on particle swarm optimization and local search algorithm. In addition, we propose an adaptive penalty function which encourages our algorithm to explore within the feasible region and near feasible region, and discourage search beyond that threshold. The effectiveness of our proposed hybrid PSO algorithm is proved on numerous variations of three different problems and compared to Tabu Search and Multiple Weighted Objectives solutions. 相似文献