全文获取类型
收费全文 | 41198篇 |
免费 | 1753篇 |
国内免费 | 1462篇 |
专业分类
化学 | 29295篇 |
晶体学 | 310篇 |
力学 | 823篇 |
综合类 | 10篇 |
数学 | 5954篇 |
物理学 | 8021篇 |
出版年
2023年 | 302篇 |
2022年 | 225篇 |
2021年 | 701篇 |
2020年 | 706篇 |
2019年 | 706篇 |
2018年 | 805篇 |
2017年 | 793篇 |
2016年 | 1534篇 |
2015年 | 1237篇 |
2014年 | 1326篇 |
2013年 | 2858篇 |
2012年 | 3100篇 |
2011年 | 3344篇 |
2010年 | 1919篇 |
2009年 | 1941篇 |
2008年 | 2573篇 |
2007年 | 2502篇 |
2006年 | 2201篇 |
2005年 | 2058篇 |
2004年 | 2154篇 |
2003年 | 1557篇 |
2002年 | 1431篇 |
2001年 | 883篇 |
2000年 | 744篇 |
1999年 | 587篇 |
1998年 | 446篇 |
1997年 | 347篇 |
1996年 | 433篇 |
1995年 | 338篇 |
1994年 | 297篇 |
1993年 | 346篇 |
1992年 | 296篇 |
1991年 | 206篇 |
1990年 | 198篇 |
1989年 | 171篇 |
1988年 | 164篇 |
1987年 | 155篇 |
1986年 | 151篇 |
1985年 | 243篇 |
1984年 | 216篇 |
1983年 | 172篇 |
1982年 | 148篇 |
1981年 | 153篇 |
1980年 | 110篇 |
1979年 | 123篇 |
1978年 | 105篇 |
1977年 | 98篇 |
1976年 | 93篇 |
1975年 | 93篇 |
1973年 | 93篇 |
排序方式: 共有10000条查询结果,搜索用时 265 毫秒
91.
In the present work, the effect of the oxidizing treatment with nitric acid on three activated carbon samples has been studied. The influence of the acid treatment on the surface groups of the different samples has been investigated by means of FT-IR spectroscopy. The pore size distributions of the different samples were determined by means of the HK and DFT methods. The HK method points out a moderate increment of the microporosity due to the action of the nitric acid, whereas the DFT method shows an increase in the microporosity range above 17 Å. Finally, the values of the fractal dimension reveal that the treatment of the samples with nitric acid leads to chemical reactions of a limited extent. 相似文献
92.
Rafael R. del Río Castillo 《Communications in Mathematical Physics》1991,142(2):421-431
In this work we study the behavior of embedded eigenvalues of Sturm-Liouville problems in the half axis under local perturbations. When the derivative of the spectral function is strictly positive, we prove that the embedded eigenvalues either disappear or remain fixed. In this case we show that local perturbations cannot add eigenvalues in the continuous spectrum. If the condition on the spectral function is removed then a local perturbation can add infinitely many eigenvalues. 相似文献
93.
Summary We study the integration theory for general integral metrics when restricted to upper integrals q, finding improvements in the relation between the classes of the q-integrable and the ql-integrable functions. We give new results and notions which lead to the desirable characterizations of q-integrable functions as ql-integrable f with q(|f|) < ∞, and of ql-integrable functions via the integrability of their upper truncations, under natural conditions which are fulfilled in most
finitely additive integration theories. 相似文献
94.
95.
96.
We study the spectral properties of a large class of compact flat Riemannian manifolds of dimension 4, namely, those whose
corresponding Bieberbach groups have the canonical lattice as translation lattice. By using the explicit expression of the
heat trace of the Laplacian acting on p-forms, we determine all p-isospectral and L-isospectral pairs and we show that in this class of manifolds, isospectrality on functions and isospectrality on p-forms for all values of p are equivalent to each other. The list shows for any p, 1 ≤ p ≤ 3, many p-isospectral pairs that are not isospectral on functions and have different lengths of closed geodesics. We also determine
all length isospectral pairs (i.e. with the same length multiplicities), showing that there are two weak length isospectral
pairs that are not length isospectral, and many pairs, p-isospectral for all p and not length isospectral.
Mathematics Subject Classifications (2000): 58J53, 58C22, 20H15. 相似文献
97.
J. J. EGOZCUE J. L. DIAZ-BARRERO V. PAWLOWSKY-GLAHN 《数学学报(英文版)》2006,22(4):1175-1182
The set of probability functions is a convex subset of L1 and it does not have a linear space structure when using ordinary sum and multiplication by real constants. Moreover, difficulties arise when dealing with distances between densities. The crucial point is that usual distances are not invariant under relevant transformations of densities. To overcome these limitations, Aitchison's ideas on compositional data analysis are used, generalizing perturbation and power transformation, as well as the Aitchison inner product, to operations on probability density functions with support on a finite interval. With these operations at hand, it is shown that the set of bounded probability density functions on finite intervals is a pre-Hilbert space. A Hilbert space of densities, whose logarithm is square-integrable, is obtained as the natural completion of the pre-Hilbert space. 相似文献
98.
Generally, in homotopy theory a cylinder object (or, its dual, a path object) is used to define homotopy between morphisms,
and a cone object is used to build exact sequences of homotopy groups. Here, an axiomatic theory based on a cone functor is
given. Suspension objects are associated to based objects and cofibrations, obtaining homotopy groups referred to an object
and relative to a cofibration, respectively. Exact sequences of these groups are built. Algebraic and particular examples
are given. We point out that the main results of this paper were already stated in [3], and the purpose of this article is
to give full details of the foregoing. 相似文献
99.
E. Liénard G. Ban J. Blieck D. Durand F. Duval X. Fléchard M. Herbane M. Labalme Y. Lemière F. Mauger A. Méry O. Naviliat-Cuncic D. Rodríguez J. C. Thomas 《Hyperfine Interactions》2006,172(1-3):29-33
We report on the status of the LPCTrap experiment, devoted to measure the β–ν angular correlation in the pure Gamow–Teller
decay of 6He. This measurement is motivated by the search for the presence of tensor type contributions to the weak interaction. The
6He ions are confined in a novel transparent Paul trap. The β particles and the recoil ions are detected in coincidence to
deduce the angular correlation parameter. The commissioning run performed in 2005 has given the proof of principle of this
experiment. Up to 105 coincidences were recorded during a second run in 2006. 相似文献
100.
Summary The gas chromatographic behaviour of crosslinked macroporous 2-hydroxyethyl methacrylate grafted with 2-methyloxazoline was
investigated and the range of applicability of the copolymer was assessed. Retention index values were measured on this copolymer
and compared with those obtained on the terpolymer 2-hydroxyethyl methacrylate-ethylene dimethacrylate-acrylonitrile. Rohrschneider
constants and efficiency data for selected sorbates are given. The use of this type of polymer in the separation of various
types of compounds is documented by various examples. 相似文献