全文获取类型
收费全文 | 60791篇 |
免费 | 4185篇 |
国内免费 | 3958篇 |
专业分类
化学 | 36597篇 |
晶体学 | 947篇 |
力学 | 4109篇 |
综合类 | 779篇 |
数学 | 9567篇 |
物理学 | 16935篇 |
出版年
2024年 | 65篇 |
2023年 | 478篇 |
2022年 | 1153篇 |
2021年 | 1272篇 |
2020年 | 1338篇 |
2019年 | 1336篇 |
2018年 | 1222篇 |
2017年 | 1504篇 |
2016年 | 2055篇 |
2015年 | 1713篇 |
2014年 | 2357篇 |
2013年 | 4462篇 |
2012年 | 3449篇 |
2011年 | 3317篇 |
2010年 | 2921篇 |
2009年 | 3655篇 |
2008年 | 3604篇 |
2007年 | 3989篇 |
2006年 | 3432篇 |
2005年 | 2985篇 |
2004年 | 2935篇 |
2003年 | 2398篇 |
2002年 | 1896篇 |
2001年 | 1566篇 |
2000年 | 2285篇 |
1999年 | 1862篇 |
1998年 | 1738篇 |
1997年 | 1069篇 |
1996年 | 989篇 |
1995年 | 924篇 |
1994年 | 839篇 |
1993年 | 692篇 |
1992年 | 607篇 |
1991年 | 492篇 |
1990年 | 422篇 |
1989年 | 303篇 |
1988年 | 284篇 |
1987年 | 182篇 |
1986年 | 152篇 |
1985年 | 184篇 |
1984年 | 142篇 |
1983年 | 77篇 |
1982年 | 129篇 |
1981年 | 96篇 |
1980年 | 63篇 |
1979年 | 91篇 |
1978年 | 53篇 |
1977年 | 33篇 |
1976年 | 33篇 |
1973年 | 22篇 |
排序方式: 共有10000条查询结果,搜索用时 125 毫秒
31.
M. Georgelin A. Pocheau 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(2):169-174
The polycrystalline perovskitelike manganese oxides La1-xAxMnO (A = Na, and K, ) have been fabricated by sol-gel technique. For all the compositions explored in this work, the average manganese oxidation
state is practically constant, at for A = Na, and for A = K, respectively. A close relationship is confirmed to hold between the Curie temperature (Tc) and the bond distance of Mn-O. Results of magnetic measurements show that these materials can be utilized as suitable candidates
for magnetic refrigerants with wide applied temperature span, for their significant entropy change and the easily tuned Curie
temperature.
Received: 12 September 1997 / Revised: 18 December 1997 / Accepted: 21 January 1998 相似文献
32.
C. Waldtmann H.-U. Everts B. Bernu C. Lhuillier P. Sindzingre P. Lecheminant L. Pierre 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(4):501-507
We study the exact low energy spectra of the spin 1/2 Heisenberg antiferromagnet on small samples of the kagomé lattice of up to N=36 sites. In agreement with the conclusions of previous authors, we find that these low energy spectra contradict the hypothesis
of Néel type long range order. Certainly, the ground state of this system is a spin liquid, but its properties are rather
unusual. The magnetic () excitations are separated from the ground state by a gap. However, this gap is filled with nonmagnetic () excitations. In the thermodynamic limit the spectrum of these nonmagnetic excitations will presumably develop into a gapless
continuum adjacent to the ground state. Surprisingly, the eigenstates of samples with an odd number of sites, i.e. samples with an unsaturated spin, exhibit symmetries which could support long range chiral order. We do not know if these
states will be true thermodynamic states or only metastable ones. In any case, the low energy properties of the spin 1/2 Heisenberg
antiferromagnet on the kagomé lattice clearly distinguish this system from either a short range RVB spin liquid or a standard chiral spin liquid. Presumably
they are facets of a generically new state of frustrated two-dimensional quantum antiferromagnets.
Received: 27 November 1997 / Accepted: 29 January 1998 相似文献
33.
Matthew J. Katz 《Computational Geometry》1997,8(6):299-316
We present a new data structure for a set of n convex simply-shaped fat objects in the plane, and use it to obtain efficient and rather simple solutions to several problems including (i) vertical ray shooting—preprocess a set of n non-intersecting convex simply-shaped flat objects in 3-space, whose xy-projections are fat, for efficient vertical ray shooting queries, (ii) point enclosure—preprocess a set C of n convex simply-shaped fat objects in the plane, so that the k objects containing a query point p can be reported efficiently, (iii) bounded-size range searching— preprocess a set C of n convex fat polygons, so that the k objects intersecting a “not-too-large” query polygon can be reported efficiently, and (iv) bounded-size segment shooting—preprocess a set C as in (iii), so that the first object (if exists) hit by a “not-too-long” oriented query segment can be found efficiently. For the first three problems we construct data structures of size O(λs(n)log3n), where s is the maximum number of intersections between the boundaries of the (xy-projections) of any pair of objects, and λs(n) is the maximum length of (n, s) Davenport-Schinzel sequences. The data structure for the fourth problem is of size O(λs(n)log2n). The query time in the first problem is O(log4n), the query time in the second and third problems is O(log3n + klog2n), and the query time in the fourth problem is O(log3n).
We also present a simple algorithm for computing a depth order for a set as in (i), that is based on the solution to the vertical ray shooting problem. (A depth order for , if exists, is a linear order of , such that, if K1, K2 and K1 lies vertically above K2, then K1 precedes K2.) Unlike the algorithm of Agarwal et al. (1995) that might output a false order when a depth order does not exist, the new algorithm is able to determine whether such an order exists, and it is often more efficient in practical situations than the former algorithm. 相似文献
34.
35.
36.
本文给出了C^n中子空间之间最大和最小主角在矩阵逼近,投影算子的扰动分析,群逆以及Oraxin逆的扰动估计,条件数理论,Bott-Duffin系统扰动分析中一些应用。 相似文献
37.
XU Jun YAN Ai-Zhen XU Qin-Hua Department of Chemistry Nanjing University Nanjing Jiangsu China 《中国化学》1995,(2)
The strength of basic sites has been measured by pyrrole-IR on alkali metal cation exchanged β and X zeolites, as well as NaOH loaded Naβ. The influence of cation type and the structure of zeolites on their basicity has been studied. The acidic and basic properties of the samples were investigated by NH3-TPD and isopropanol reaction. It was shown that the strength of basic sites on samples could be characterized by the shift of vNH band in the pyrrole-IR spectra. The framework oxygen charges were calculated from the Sanderson electronegativity. The changes in basic properties with various alkali metal cation are consistent with the changes of local oxygen charges of the zeolite framework. 相似文献
38.
39.
固体力学有限元体系的结构拓扑变化理论 总被引:2,自引:1,他引:1
本文是文[1]的继续.文[1]提出了杆件系统的结构拓扑变化理论和拓扑变化法本文将这一理论和方法推进到连续体有限元体系;且在此基础上揭示出有限元体系的一个新性质,称为基本位移之梯度的正交性定理,从而给出一套设计敏度的显式表达式,可直接用于计算. 相似文献
40.