全文获取类型
收费全文 | 5971篇 |
免费 | 319篇 |
国内免费 | 362篇 |
专业分类
化学 | 1342篇 |
晶体学 | 9篇 |
力学 | 198篇 |
综合类 | 54篇 |
数学 | 3340篇 |
物理学 | 1709篇 |
出版年
2024年 | 5篇 |
2023年 | 36篇 |
2022年 | 109篇 |
2021年 | 78篇 |
2020年 | 91篇 |
2019年 | 98篇 |
2018年 | 106篇 |
2017年 | 114篇 |
2016年 | 105篇 |
2015年 | 92篇 |
2014年 | 162篇 |
2013年 | 426篇 |
2012年 | 223篇 |
2011年 | 434篇 |
2010年 | 337篇 |
2009年 | 596篇 |
2008年 | 571篇 |
2007年 | 480篇 |
2006年 | 499篇 |
2005年 | 362篇 |
2004年 | 206篇 |
2003年 | 165篇 |
2002年 | 268篇 |
2001年 | 207篇 |
2000年 | 179篇 |
1999年 | 121篇 |
1998年 | 129篇 |
1997年 | 75篇 |
1996年 | 83篇 |
1995年 | 50篇 |
1994年 | 42篇 |
1993年 | 20篇 |
1992年 | 19篇 |
1991年 | 15篇 |
1990年 | 9篇 |
1989年 | 13篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 8篇 |
1985年 | 10篇 |
1984年 | 12篇 |
1983年 | 5篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 12篇 |
1978年 | 10篇 |
1977年 | 6篇 |
1975年 | 3篇 |
1974年 | 3篇 |
1973年 | 6篇 |
排序方式: 共有6652条查询结果,搜索用时 15 毫秒
51.
调制不稳定性对级联放大光纤传输系统信噪比的影响 总被引:1,自引:0,他引:1
在考虑光纤损耗和级联放大器的情况下,推导了调制不稳定性的产生条件和增益的普适解析表达式,分析了调制不稳定性对信噪比的影响,给出了一个新的计算信噪比的表达式。 相似文献
52.
B. Hannoyer A. A. M. Prince M. Jean R. S. Liu G. X. Wang 《Hyperfine Interactions》2006,167(1-3):767-772
Crystalline LiFePO4 has been synthesized using solid-state, spray pyrolysis, and wet chemical methods. The crystal parameters were obtained from Rietveld’s refinement methods of the X-ray diffraction patterns. A detailed investigation of the Fe valency carried out using Mössbauer spectroscopy at room temperature indicates that Fe is predominantly present in its bivalent state. 相似文献
53.
A. Paesano Jr. S. C. Zanatta S. N. De Medeiros L. F. CÓtica J. B. M. Da Cunha 《Hyperfine Interactions》2005,161(1-4):211-220
We have investigated the mechanosynthesis of gadolinium and yttrium iron garnets by high-energy ball-milling of α-Fe2O3 and Gd2O3 or α-Fe2O3 and Y2O3, respectively, followed by short thermal annealings conducted at moderate temperatures. The samples were characterized by X-ray diffraction and Mössbauer spectroscopy, in order to determine the influence of the milling time and annealing conditions on the final products. For as-milled samples of each rare-earth system, the results revealed the formation of perovskite phases, in relative amounts that depend on the milling time. The formation of garnet phases was observed in as-annealed samples treated at 1000°C for 2 h or 1100°C for 3 h, i.e., at very modest annealing requirements when compared with ordinary solid-state-reaction processes performed without previous high-energy milling. Also, the occurrence was verified of a milling time for which the relative amount of garnet phases formed by annealing was maximized. This time depends on the rare-earth composing the garnet phase and on the annealing temperature. 相似文献
54.
Li Heng—pei 《大学数学》1993,(1)
<正> In this paper we'll prove a fundameutal property of the vector space by means of the ex-tension field,i.e.the numbers of the elements of a basis of the vector space V over the field Fequal to the d imensions(V:F). 相似文献
55.
E. Manova C. Estournès D. Paneva J.-L. Rehspringer T. Tsoncheva B. Kunev I. Mitov 《Hyperfine Interactions》2005,165(1-4):215-220
Iron–nickel spinel oxide NiFe2O4 nanoparticles have been prepared by the combination of chemical precipitation and subsequent mechanical milling. For comparison, their analogue obtained by thermal synthesis is also studied. Phase composition and structural properties of iron–nickel oxides are investigated by X-ray diffraction and Mössbauer spectroscopy. Their catalytic behavior in methanol decomposition to CO and methane is tested. An influence of the preparation method on the reduction and catalytic properties of iron–nickel samples is established. 相似文献
56.
I. Souza Azevedo E. Baggio Saitovitch M. Hücker B. Büchner H. Micklitz 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):313-315
57Fe M?ssbauer effect studies of La1.65Eu0.20Sr0.15CuO4 doped with 0.5 at% 57Fe performed in the temperature region 300 K >
T
> 4.2 K give an onset temperature for magnetic ordering of K. This temperature practically is the same as that found in Nd doped La2-xSrxCuO4. It indicates that the magnetic ordering temperature in the LTT phase of rare earth (RE) doped La2-xSrxCuO4 is independent of the RE moment. The direction of the 57Fe magnetic moment in the magnetically ordered state is within the CuO2 plane, while it has been found to be parallel to the c-axis in Nd doped La2-xSrxCuO4.
Received: 23 June 1998 / Accepted: 14 July 1998 相似文献
57.
Optimale Quantisierung 总被引:1,自引:0,他引:1
Peter M. Gruber 《Mathematische Semesterberichte》2002,49(2):227-251
Zusammenfassung. Optimale Quantisierungen oder – damit ?quivalent – minimale Summen von Momenten spielen in mehreren Zweigen der Mathematik
und ihrer Anwendungen eine Rolle. Ausgehend von der Fejes Tóth'schen Ungleichung für Summen von Momenten in der euklidischen
Ebene und einem zugeh?rigen Stabilit?tssatz, werden gewisse Erweiterungen auf normierte R?ume und riemannsche Mannigfaltigkeiten
h?herer Dimension besprochen. Die Ergebnisse werden dann auf Probleme aus folgenden Bereichen angewendet: (i) Datenübertragung,
(ii) Wahrscheinlichkeitstheorie, (iii) numerische Integration, (iv) Approximation konvexer K?rper und (v) isoperimetrische
Probleme.
Eingegangen am 29. Mai 2002 / Angenommen am 8. Juli 2002 相似文献
58.
A. Konjhodzic A. Adamczyk F. Vagizov Z. Hasan E E. Alp W. Sturhahn Jiyong Zhao J J. Carroll 《Hyperfine Interactions》2006,170(1-3):83-89
With the decrease in size of devices, rapid characterization of nano-devices is an inevitable necessity. It is shown that Mössbauer spectroscopy using synchrotron radiation from the advanced photon source provides such a tool of investigation. Results are presented and compared for conventional Mössbauer and Nuclear Forward Scattering for 151Eu-doped magnesium sulfide as an example, especially at low concentrations. 相似文献
59.
提出了一种非刚性点匹配的算法并把其运用于医学图像配准.该算法采用信号滤波的方法来获得点集间的匹配信息,并运用松弛标记法将各点邻域关系对模糊的匹配信息进行迭代获得明确的匹配关系.在此基础上,利用高斯径向基函数来描述点模式间的弹性形变,在基本的迭代框架下实现问题的求解.实验结果显示在形变程度为5%、出格点比率50%和噪声标准差为5%的情况下该算法的匹配误差能控制在0.13以下,表明了该算法的鲁棒性和有效性,较好地解决了医学非刚性形变的点匹配问题. 相似文献
60.
Alternatives based on polynomial expansions of gaussian basis set exponents are introduced and evaluated. The formulas presented here outperform methods based upon the even-tempered formula or combinations of it. They closely match the performance of other methods based upon larger polynomial expansions of the logarithm of the exponents using the same or one less parameter per orbital angular symmetry. 相似文献