全文获取类型
收费全文 | 36677篇 |
免费 | 6284篇 |
国内免费 | 6997篇 |
专业分类
化学 | 17313篇 |
晶体学 | 711篇 |
力学 | 1623篇 |
综合类 | 814篇 |
数学 | 3293篇 |
物理学 | 10445篇 |
无线电 | 15759篇 |
出版年
2024年 | 195篇 |
2023年 | 571篇 |
2022年 | 1137篇 |
2021年 | 1266篇 |
2020年 | 1153篇 |
2019年 | 1126篇 |
2018年 | 1131篇 |
2017年 | 1366篇 |
2016年 | 1286篇 |
2015年 | 1677篇 |
2014年 | 2082篇 |
2013年 | 2699篇 |
2012年 | 2849篇 |
2011年 | 3089篇 |
2010年 | 2713篇 |
2009年 | 2912篇 |
2008年 | 3165篇 |
2007年 | 3039篇 |
2006年 | 2804篇 |
2005年 | 2335篇 |
2004年 | 1774篇 |
2003年 | 1318篇 |
2002年 | 1380篇 |
2001年 | 1261篇 |
2000年 | 1197篇 |
1999年 | 806篇 |
1998年 | 502篇 |
1997年 | 389篇 |
1996年 | 410篇 |
1995年 | 360篇 |
1994年 | 282篇 |
1993年 | 279篇 |
1992年 | 242篇 |
1991年 | 186篇 |
1990年 | 164篇 |
1989年 | 150篇 |
1988年 | 113篇 |
1987年 | 98篇 |
1986年 | 89篇 |
1985年 | 69篇 |
1984年 | 62篇 |
1983年 | 59篇 |
1982年 | 43篇 |
1981年 | 37篇 |
1980年 | 13篇 |
1979年 | 25篇 |
1976年 | 5篇 |
1974年 | 6篇 |
1971年 | 5篇 |
1959年 | 6篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
141.
A novel frequency offset estimation method for OFDM systems with large estimation range 总被引:4,自引:0,他引:4
Chunlin Yan Shaoqian Li Youxi Tang Xiao Luo Jiayi Fang 《Broadcasting, IEEE Transactions on》2006,52(1):58-61
In this paper, we proposed a novel frequency synchronization method which has larger estimation range than conventional method proposed by Tufvesson. A two steps frequency offset estimation method is performed in the new method. In the first step the received signal is correlated with local training sequence to eliminate the influence of training sequence which makes the estimation range of frequency offset is independent of period of the repeated data in the training sequence. The received signal is correlated with its delay in the second step to obtain the fine estimation of frequency offset. Precise estimation can be achieved in the proposed method than Tufvesson's method. The period of the repeated data in the training sequence determines the estimation range of the frequency offset in the conventional method, while in the new method they are irrelevant. Tens of subcarriers spacing can be estimated by the new method. The new method can be used in other cases as long as a known sequence is transmitted, which will introduce valuable flexibility in training sequence design. The validity of the algorithm is verified in AWGN channel and multipath fading channel. 相似文献
142.
本文介绍了高阶单比特∑△调制器在小数分频频率综合器中的应用。普通小数分频频率综合器容易产生很大的杂散频率,采用∑△调制器可以有效消除杂散频率降低相位噪声。由于多比特MASH结构的非线性,这里采用单比特高阶∑△调制器(CIFB),最后提出实现电路。 相似文献
143.
基于DSP的多通道高速数据采集系统 总被引:3,自引:0,他引:3
本文以TI公司的TMS320LF2407为核心处理器,CPLD为系统控制译码芯片提出了一种多通道高速MD数据采集系统,并介绍了基于本系统的对数据处理的一些软件和硬件的设计。 相似文献
144.
145.
Wenyu Fang Yiyu Feng Jian Gao Hui Wang Jing Ge Qingbin Yang Wei Feng 《Molecules (Basel, Switzerland)》2022,27(10)
Molecular photoswitches are considered to be important candidates in the field of solar energy storage due to their sensitive and reversible bidirectional optical response. Nevertheless, it is still a daunting challenge to design a molecular photoswitch to improve the low solar spectrum utilization and quantum yields while achieving charging and discharging of heat without solvent assistance. Herein, a series of visible-light-driven ethylene-bridged azobenzene (b-Azo) chromophores with different alkyne substituents which can undergo isomerization reactions promoted in both directions by visible light are reported. Their visible light responsiveness improves their solar spectrum utilization while also having high quantum yields. In addition, as the compounds are liquids, there is no need to dissolve the compounds in order to exploit this switching. The photoisomerization of b-Azo can be adjusted by alkyne-related substituents, and hexyne-substituted b-Azo is able to store and release photothermal energy with a high density of 106.1 J·g−1, and can achieve a temperature increase of 1.8 °C at a low temperature of −1 °C. 相似文献
146.
将位置敏感光电倍增管(Position sensitive photomultiplier tube,简称PSPMT)引入大气湍流引起的漂移效应的实验测量中.基于PSPMT的工作原理,分析了探测器的放置位置对输出信号的影响,通过实验方法对PSPMT进行了位置定标和增益定标,获得了入射光束在探测面上的位置坐标.测量结果显示:在入射光斑尺度达到厘米量级同时具有较高强度的情况下,一次采样无法正确反映入射位置.因此以若干次测量的统计平均值表示入射位置,并综合考虑采集卡的最高采样率、采样点和信号频率等因素,分析了在漂移测量中如何选择进行统计平均的数据点的个数. 相似文献
147.
Zhuwei Wang Zhicheng Liu Lihan Liu Chao Fang Meng Li Jingcheng Zhao 《Entropy (Basel, Switzerland)》2022,24(5)
With the rapid development of wireless sensor technology, recent progress in wireless sensor and actuator networks (WSANs) with energy harvesting provide the possibility for various real-time applications. Meanwhile, extensive research activities are carried out in the fields of efficient energy allocation and control strategy design. However, the joint design considering physical plant control, energy harvesting, and consumption is rarely concerned in existing works. In this paper, in order to enhance system control stability and promote quality of service for the WSAN energy efficiency, a novel three-step joint optimization algorithm is proposed through control strategy and energy management analysis. First, the optimal sampling interval can be obtained based on energy harvesting, consumption, and remaining conditions. Then, the control gain for each sampling interval is derived by using a backward iteration. Finally, the optimal control strategy is determined as a linear function of the current plant states and previous control strategies. The application of UAV formation flight system demonstrates that better system performance and control stability can be achieved by the proposed joint optimization design for all poor, sufficient, and general energy harvesting scenarios. 相似文献
148.
Heterometallic cluster-based framework materials are of interest in terms of both their porous structures and multi-metallic reactivity. However, such materials have not yet been extensively investigated because of difficulties in their synthesis and structural characterization. Herein, we reported the designable synthesis of atomically precise heterometallic cluster-based framework compounds and their application as catalysts in aldol reactions. By using the synergistic coordination protocol, we successfully isolated a broad range of compounds with the general formula, [Al4M4O4(L)12(DABCO)2] (L = carboxylates; DABCO = 1,4-diazabicyclo[2.2.2]-octane; M2+ = Co2+, Mn2+, Zn2+, Fe2+, Cd2+). The basic heterometallic building blocks contain unprecedented main-group γ-alumina moieties and surrounding unsaturated transition metal centers. Interestingly, the porosity and interpenetration of these frameworks can be rationally regulated through the unprecedented strategy of increment of the metal radius in addition to general introduction of sterically bulky groups on the ligand. Furthermore, these porous materials are effective catalysts for aldol reactions. This work provides a catalytic molecular model platform with accurate molecular bonding between the supporters and catalytically active metal ions.Mesoporous heterometallic architectures are designed by the incorporation of the Al4O4 cubane and interpenetration regulation. 相似文献
149.
150.
In this paper, by capturing the atomic information and reflecting the behaviour governed by the nonlinear potential function,
an analytical molecular mechanics approach is proposed. A constitutive relation for single-walled carbon nanotubes (SWCNT’s)
is established to describe the nonlinear stress-strain curve of SWCNT’s and to predict both the elastic properties and breaking
strain of SWCNT’s during tensile deformation. An analysis based on the virtual internal bond (VIB) model proposed by P. Zhang
et al. is also presented for comparison. The results indicate that the proposed molecular mechanics approach is indeed an
acceptable analytical method for analyzing the mechanical behavior of SWCNT’s.
The project supported by the National Natural Science Foundation of China (10121202, 90305015 and 10328203), the Key Grant
Project of Chinese Ministry of Education (0306) and the Research Grants Council of the Hong Kong Special Administrative Region,
China (HKU 7195/04E). 相似文献