全文获取类型
收费全文 | 5706篇 |
免费 | 242篇 |
国内免费 | 466篇 |
专业分类
化学 | 2927篇 |
晶体学 | 246篇 |
力学 | 86篇 |
综合类 | 59篇 |
数学 | 2060篇 |
物理学 | 1036篇 |
出版年
2024年 | 15篇 |
2023年 | 60篇 |
2022年 | 113篇 |
2021年 | 105篇 |
2020年 | 101篇 |
2019年 | 117篇 |
2018年 | 95篇 |
2017年 | 105篇 |
2016年 | 120篇 |
2015年 | 121篇 |
2014年 | 154篇 |
2013年 | 366篇 |
2012年 | 206篇 |
2011年 | 393篇 |
2010年 | 374篇 |
2009年 | 402篇 |
2008年 | 392篇 |
2007年 | 393篇 |
2006年 | 310篇 |
2005年 | 269篇 |
2004年 | 253篇 |
2003年 | 212篇 |
2002年 | 351篇 |
2001年 | 142篇 |
2000年 | 140篇 |
1999年 | 130篇 |
1998年 | 108篇 |
1997年 | 76篇 |
1996年 | 96篇 |
1995年 | 79篇 |
1994年 | 68篇 |
1993年 | 44篇 |
1992年 | 49篇 |
1991年 | 29篇 |
1990年 | 17篇 |
1988年 | 12篇 |
1985年 | 38篇 |
1984年 | 19篇 |
1983年 | 23篇 |
1982年 | 23篇 |
1981年 | 44篇 |
1980年 | 29篇 |
1979年 | 13篇 |
1978年 | 24篇 |
1977年 | 30篇 |
1976年 | 26篇 |
1975年 | 22篇 |
1974年 | 28篇 |
1973年 | 21篇 |
1971年 | 16篇 |
排序方式: 共有6414条查询结果,搜索用时 9 毫秒
991.
EzzEldin Metwalli 《Journal of Non》2003,317(3):221-230
Copper oxidation states, structure and properties of xCuO · (50-x)PbO · 50B2O3 glasses were investigated. Both infrared (IR) and 11B magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopies were employed to determine the tetrahedral BO4 fraction in the glasses as a function of CuO content. IR study indicates that the replacement of Pb2+ by Cu2+ ions increases the BO3 units by converting BO4- containing groups into ring type metaborate groups. The oxidation states of copper ions in the glasses have been studied using both X-ray photoelectron spectroscopy (XPS) and the wet chemical method. For high CuO containing (?30 mol%) glasses, high Cu+ ion concentrations (Cu+/Cutot.>0.3) result in a relatively slow disproportionation of B4-containing groups because of the small coordination number of Cu+ compared to Cu2+ ions. Effects of both glass structure and redox states of copper ions on glass properties including density, Vickers’ hardness, coefficient of thermal expansion, and chemical durability have been discussed. 相似文献
992.
Transparent glasses composition of which can be expressed by the formula: (100−x) · (K2O · 2TiO2 · P2O5) · x(K2O · 2B2O3 · 7SiO2), where x=5, 10, 15 and 20 mol% (KTP-xKBS), were obtained by melt quenching technique. The structure and crystallization behavior of these glasses have been examined by Fourier transform infrared spectroscopy, differential thermal analysis and X-ray diffraction. In spite of their nominal composition, the studied glasses exhibit a similar oxygen polyhedra distribution. However, significant differences were found in the trigonal BO3 units amount. During DTA runs all the examined glasses devitrify in two steps. In the former, very small crystals of an unknown crystalline phase are produced. In KTP-5KBS and KTP-10KBS glasses anatase phase was also detected. Attempts were made in order to identify the unknown phase (UTP) for which a AB3(XO4)2(OH)6 Crandallite-type structure was proposed where the A, B and X sites were occupied by K, Ti and/or Al, and P, respectively. In the second devitrification step the crystallization of the KTiOPO4 phase occurs while the UTP phase previously formed disappears. Isothermal heat treatments performed at temperature just above Tg have allowed one to obtain transparent crystal-glass nanocomposites, formed by crystalline nanostructure of the UTP phase uniformly dispersed in the amorphous matrix. 相似文献
993.
Amorphous and polycrystalline (Pb0.76Ca0.24)TiO3 (PCT) thin films deposited on an Si(1 0 0) substrate have been prepared by a simple sol-gel process. The microstructure and surface morphologies of the thin films have been studied by X-ray diffraction (XRD) and atomic force microscopy (AFM). The polycrystalline PCT film on the Si(1 0 0) substrate has a tetragonal perovskite structure with grain size from 60 to 110 nm. AFM reveals smooth surfaces and root mean square (rms) roughness of 0.17 and 4.4 nm for amorphous and polycrystalline films, respectively. The refractive index n and extinction coefficient k of the amorphous and polycrystalline thin films was obtained by spectroscopic ellipsometry as a function of the photon energy in the range from 2.0 to 5.4 eV. The maximum n and direct bandgap energies of amorphous and polycrystalline thin films were 2.66 and 4.11 eV, 2.64 and 3.84 eV, respectively. 相似文献
994.
An edge e of a k-connected graph G is said to be a removable edge if G?e is still k-connected. A k-connected graph G is said to be a quasi (k+1)-connected if G has no nontrivial k-separator. The existence of removable edges of 3-connected and 4-connected graphs and some properties of quasi k-connected graphs have been investigated [D.A. Holton, B. Jackson, A. Saito, N.C. Wormale, Removable edges in 3-connected graphs, J. Graph Theory 14(4) (1990) 465-473; H. Jiang, J. Su, Minimum degree of minimally quasi (k+1)-connected graphs, J. Math. Study 35 (2002) 187-193; T. Politof, A. Satyanarayana, Minors of quasi 4-connected graphs, Discrete Math. 126 (1994) 245-256; T. Politof, A. Satyanarayana, The structure of quasi 4-connected graphs, Discrete Math. 161 (1996) 217-228; J. Su, The number of removable edges in 3-connected graphs, J. Combin. Theory Ser. B 75(1) (1999) 74-87; J. Yin, Removable edges and constructions of 4-connected graphs, J. Systems Sci. Math. Sci. 19(4) (1999) 434-438]. In this paper, we first investigate the relation between quasi connectivity and removable edges. Based on the relation, the existence of removable edges in k-connected graphs (k?5) is investigated. It is proved that a 5-connected graph has no removable edge if and only if it is isomorphic to K6. For a k-connected graph G such that end vertices of any edge of G have at most k-3 common adjacent vertices, it is also proved that G has a removable edge. Consequently, a recursive construction method of 5-connected graphs is established, that is, any 5-connected graph can be obtained from K6 by a number of θ+-operations. We conjecture that, if k is even, a k-connected graph G without removable edge is isomorphic to either Kk+1 or the graph Hk/2+1 obtained from Kk+2 by removing k/2+1 disjoint edges, and, if k is odd, G is isomorphic to Kk+1. 相似文献
995.
Brualdi et al. [Codes with a poset metric, Discrete Math. 147 (1995) 57-72] introduced the concept of poset codes, and gave an example of poset structure which admits the extended binary Golay code to be a 4-error-correcting perfect P-code. In this paper we classify all of the poset structures which admit the extended binary Golay code to be a 4-error-correcting perfect P-code, and show that there are no posets which admit the extended binary Golay code to be a 5-error-correcting perfect P-code. 相似文献
996.
Néstor E. Aguilera 《Discrete Mathematics》2008,308(11):2167-2174
Using results by McKee and Woodall on binary matroids, we show that the set of postman sets has odd cardinality, generalizing a result by Toida on the cardinality of cycles in Eulerian graphs. We study the relationship between T-joins and blocks of the underlying graph, obtaining a decomposition of postman sets in terms of blocks. We conclude by giving several characterizations of T-joins which are postman sets. 相似文献
997.
Imed Zaguia 《Discrete Mathematics》2008,308(23):5727-5734
We prove that a prime interval order, not necessarily finite, has no prime upper cover if and only if it has width two. On the other hand, we prove that every finite prime N-free order has at least one prime upper cover. 相似文献
998.
Tuomas Hytönen 《Journal of Functional Analysis》2008,254(3):675-726
Let L be an elliptic differential operator with bounded measurable coefficients, acting in Bochner spaces Lp(Rn;X) of X -valued functions on Rn. We characterize Kato's square root estimates and the H∞-functional calculus of L in terms of R-boundedness properties of the resolvent of L, when X is a Banach function lattice with the UMD property, or a noncommutative Lp space. To do so, we develop various vector-valued analogues of classical objects in Harmonic Analysis, including a maximal function for Bochner spaces. In the special case X=C, we get a new approach to the Lp theory of square roots of elliptic operators, as well as an Lp version of Carleson's inequality. 相似文献
999.
Turdebek N. Bekjan 《Journal of Functional Analysis》2008,254(9):2401-2418
Let M be a von Neumann algebra equipped with a normal semifinite faithful trace τ. Let T be a positive linear contraction on M such that τ○T?τ and such that the numerical range of T as an operator on L2(M) is contained in a Stoltz region with vertex 1. We show that Junge and Xu's noncommutative Stein maximal ergodic inequality holds for the powers of T on Lp(M), 1<p?∞. We apply this result to obtain the noncommutative analogue of a recent result of Cohen concerning the iterates of the product of a finite number of conditional expectations. 相似文献
1000.
Jung-Rye Lee 《Journal of Mathematical Analysis and Applications》2008,339(1):372-383
Let X and Y be Banach spaces and f:X→Y an odd mapping. We solve the following generalized additive functional equation