全文获取类型
收费全文 | 137689篇 |
免费 | 20727篇 |
国内免费 | 13815篇 |
专业分类
化学 | 72232篇 |
晶体学 | 1240篇 |
力学 | 6982篇 |
综合类 | 536篇 |
数学 | 12087篇 |
物理学 | 42070篇 |
无线电 | 37084篇 |
出版年
2024年 | 551篇 |
2023年 | 3295篇 |
2022年 | 4080篇 |
2021年 | 5345篇 |
2020年 | 5234篇 |
2019年 | 4921篇 |
2018年 | 4158篇 |
2017年 | 4109篇 |
2016年 | 6023篇 |
2015年 | 6202篇 |
2014年 | 7568篇 |
2013年 | 9705篇 |
2012年 | 11527篇 |
2011年 | 11781篇 |
2010年 | 8246篇 |
2009年 | 8194篇 |
2008年 | 8650篇 |
2007年 | 7850篇 |
2006年 | 7346篇 |
2005年 | 6408篇 |
2004年 | 4456篇 |
2003年 | 3723篇 |
2002年 | 3344篇 |
2001年 | 2795篇 |
2000年 | 2637篇 |
1999年 | 3051篇 |
1998年 | 2693篇 |
1997年 | 2347篇 |
1996年 | 2588篇 |
1995年 | 2055篇 |
1994年 | 1952篇 |
1993年 | 1567篇 |
1992年 | 1472篇 |
1991年 | 1296篇 |
1990年 | 1003篇 |
1989年 | 707篇 |
1988年 | 567篇 |
1987年 | 452篇 |
1986年 | 441篇 |
1985年 | 383篇 |
1984年 | 290篇 |
1983年 | 201篇 |
1982年 | 175篇 |
1981年 | 126篇 |
1980年 | 117篇 |
1979年 | 71篇 |
1978年 | 52篇 |
1977年 | 51篇 |
1976年 | 47篇 |
1974年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
151.
152.
The inhomogeneous Dirichlet problems concerning divergence form elliptic equations are studied. Optimal regularity requirements on the coefficients and domains for the W1,p theory, 1 < p < ∞, are obtained. The principal coefficients are supposed to be in the John‐Nirenberg space with small BMO seminorms. The domain is a Reifenberg domain. These conditions for the W1,p theory not only weaken the requirements on the coefficients but also lead to a more general geometric condition on the domains. In fact, these domains might have fractal dimensions. © 2004 Wiley Periodicals, Inc. 相似文献
153.
Wang Qing-Lun Zhao Bin Liao Dai-Zheng Yan Shi-Ping Cheng Peng Jiang Zong-Hui 《Transition Metal Chemistry》2003,28(3):326-330
The copper(II) complex Na2[Cu(pba)] · 6H2O has been synthesized and its structure determined. It consists of a novel two-dimensional network of [Cu(pba)]2– anions connected to each other through the sodium ion as a bridge. Spectroscopic and magnetic properties have been measured and a spin distribution calculation has been carried out with the GAUSSIAN-94. Theoretical calculations indicate the rather larger spin density on nitrogen atoms should be responsible for the satellite lines appearing beside the main hyperfine coupling signs of copper(II). The antiferromagnetic interaction may be due to the intermolecular interaction and/or different copper(II) ions through the Na+ bridge in the same layer. 相似文献
154.
家用吊扇的核心部分是单相异步电动机,此绕组如图1,有一主绕组(工作绕组)与一串联电容器C的副绕组(启动绕组)相并联.这样做可使二绕组的电流在相位上相差90°,从而形成旋转磁场.转子的转动过程与三相异步电动机相似. 相似文献
155.
Shiue-Lung Chen Shui-Jinn Wang Kai-Ming Uang Tron-Min Chen Wei-Chi Lee Bor-Wen Liou 《Photonics Technology Letters, IEEE》2007,19(6):351-353
Through the use of selective nickel (Ni) electroplating, patterned laser liftoff technique, and surface roughing of the top n-GaN epilayer, a novel process for the fabrication of vertical-structured metal-substrate GaN-based light-emitting diodes (VM-LEDs) to avoid difficulties in Ni substrate dicing and improve device yield was proposed and demonstrated. In conjunction with a sidewall passivation with SiO2 and keeping the size of epilayer smaller than that of Ni island, a considerable improvement in yield and device performance were shown. As compared to conventional lateral-structured GaN-based LEDs, VM-LEDs show an increase in light output power about 174% at 350 mA with a significant decrease in forward voltage from 3.5 to 3.17 V 相似文献
156.
Jung-Chieh Chen Yeong-Cheng Wang Jiunn-Tsair Chen 《Wireless Communications, IEEE Transactions on》2006,5(6):1241-1249
This paper presents a novel self-organizing distributed algorithm for finding a broadcasting schedule in a packet radio network via only local collaborative interactions among neighboring network stations. Inspired by the huge success of the low density parity check (LDPC) codes in the field of error control coding, we transform the broadcast scheduling problem (BSP) into an LDPC-like problem through a factor graph. In the proposed algorithm, the constraint rules of the BSP are divided into many simple local rules, each of which is enforced by a local processing unit in the factor graph. The soft-information, describing the probability that each station will transmit a data packet, is then efficiently exchanged among the local processing units by using the sum-product algorithm to iteratively optimize the broadcasting schedule. Simulation results indicate that the proposed algorithm performs better than the other existing central-processing algorithms in terms of the channel utilization and the average packet delay. This is true especially when the network scenario is very complex. Furthermore, the proposed algorithm is both low in complexity and completely distributed, which makes it suitable for implementation in practical network applications. 相似文献
157.
Nanofiber bundles of Ag2S, Ag2Se, and Ag have been successfully synthesized by making use of Ag2C2O4 template nanofiber bundles, utilizing both anion‐exchange and redox reactions. The obtained bundles were polycrystalline nanofibers composed of nanoparticles in which the precursor morphology was well‐preserved, indicating that Ag2C2O4 nanofiber bundles acted as a general sacrificial template for the synthesis of silver‐based semiconductor and metal nanofibers. Dispersing media and transforming reactants were found to be key factors influencing the chemical transformation in the system. In particular, separate single‐crystalline Ag nanofibers were obtained via a nontemplate route when ascorbic acid was used as a relatively weak reductant. An electrical transfer and switching device was built with the obtained Ag2S and Ag nanofiber bundles, utilizing the unique ion‐conductor nature of Ag2S and revealing their potential applications in electronics. 相似文献
158.
S. K. Chen A. Serquis G. Serrano K. A. Yates M. G. Blamire D. Guthrie J. Cooper H. Wang S. Margadonna J. L. MacManus‐Driscoll 《Advanced functional materials》2008,18(1):113-120
By applying a combination of characterisation tools, changes in structural and superconducting properties with nominal Mg non‐stoichiometry in MgxB2 are found. The non‐stoichiometry produces enhanced in‐field critical current densities (Jc's) and upper critical field / irreversibility field (Hc2/Hirr(T)) values. Upper critical fields of ~ 21 T (4.2 K) were obtained in nominal Mg‐deficient samples compared to ~ 17 T (4.2 K) for near‐stoichiometric samples. 相似文献
159.
This paper presents an investigation on a single-mode-multimode-single-mode fiber structure. A one-way guided-mode propagation analysis for the circular symmetry waveguide is employed to model the light propagation and the approximated formulations are derived and evaluated concerning the accuracy. Phase conjunction of the multimode interference within the fiber structure is revealed. A simple way to predict and analyze the spectral response of the structure is presented through the space to wavelength mapping with the derived approximated formulations. The prediction of spectral response is verified numerically and experimentally. 相似文献
160.