全文获取类型
收费全文 | 10483篇 |
免费 | 751篇 |
国内免费 | 255篇 |
专业分类
系统科学 | 1260篇 |
丛书文集 | 174篇 |
教育与普及 | 150篇 |
理论与方法论 | 288篇 |
现状及发展 | 884篇 |
研究方法 | 14篇 |
综合类 | 8719篇 |
出版年
2024年 | 24篇 |
2023年 | 62篇 |
2022年 | 112篇 |
2021年 | 128篇 |
2020年 | 96篇 |
2019年 | 45篇 |
2018年 | 790篇 |
2017年 | 809篇 |
2016年 | 503篇 |
2015年 | 181篇 |
2014年 | 220篇 |
2013年 | 211篇 |
2012年 | 489篇 |
2011年 | 1154篇 |
2010年 | 1044篇 |
2009年 | 688篇 |
2008年 | 777篇 |
2007年 | 986篇 |
2006年 | 205篇 |
2005年 | 209篇 |
2004年 | 258篇 |
2003年 | 296篇 |
2002年 | 244篇 |
2001年 | 182篇 |
2000年 | 138篇 |
1999年 | 214篇 |
1998年 | 174篇 |
1997年 | 184篇 |
1996年 | 178篇 |
1995年 | 171篇 |
1994年 | 136篇 |
1993年 | 109篇 |
1992年 | 86篇 |
1991年 | 87篇 |
1990年 | 75篇 |
1989年 | 77篇 |
1988年 | 73篇 |
1987年 | 38篇 |
1986年 | 18篇 |
1985年 | 8篇 |
1984年 | 2篇 |
1983年 | 1篇 |
1981年 | 2篇 |
1973年 | 2篇 |
1967年 | 1篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
Conclusion The structure of sputtered Ti and Zr deposited on substrate cooled by liquid nitrogen was investigated by X-ray diffraction
technique. Results show that deposited Ti and Zr are composed of nanocrystalline particles, and the average grain sizes are
respectively 7.2 and 6.3 nm. These nanocrystalline particles have different structures with common metal under the condition
of room temperature and atmospheric pressure. The sputtered nanocrystalline Ti has b. c. c. structure, and the sputtered nanocrystalline
Zr the ω-phase structure. Forming of the abnormal structure depends on the energy of sputtered particle and small grain size.
The energy carried by sputtered particles could have met the kinetic requirement for forming of abnormal structure, while
the nanometer grain size could make the abnormal structure have lower energy than normal structure. 相似文献
992.
Bernard BERCU 《系统科学与复杂性》2007,20(2):243-250
A wide range of literature concerning classical asymptotic properties for linear models with adaptive control is available, such as strong laws of large numbers or central limit theorems. Unfortunately, in contrast with the situation without control, it appears to be impossible to find sharp asymptotic or nonasymptotic properties such as large deviation principles or exponential inequalities. Our purpose is to provide a first step towards that direction by proving a very simple exponential inequality for the standard least squares estimator of the unknown parameter of Gaussian autoregressive process in adaptive tracking. 相似文献
993.
本文叙述一种双环自适应增量调制(TLADM)系统。它由一个内环和一个外环组成。内环是线性增量调制(LDM);外环是数字压扩增量调制(DCDM),具有量化阶自适应的功能。这种TLADM是专为移动通信研制的,具有数码率低、电路简单、单一电源、功耗低等特点。对所研制的系统进行了性能测试,在数码率为9.6KbPS时,对于400HZ正弦信号,最大的信号量化噪声比(SNR)_m是18.5dB,SNR>16dB的动态范围>25dB,800HZ正弦信号时的(SNR)_m是17.8dB,SNR>15dB的动态范围>20dB,平均单词可懂度为90%。数码率为8KbPS,400HZ正弦信号时的(SNR)_m为17.7dB,SNR>16dB的动态范围大于22dB,话音可懂。文中进行了理论分析,介绍了测试结果。 相似文献
994.
Cloning and expression profiles of 15 genes encoding WRKY transcription factor in wheat (Triticum aestivem L.) 总被引:2,自引:0,他引:2
WRKY proteins are involved in various physiological processes, including biotic and abiotic stress responses, hormone responses and development. However, no systematic identification, expression and function analysis of WRKY genes in wheat were reported. In this study, we isolated 15 wheat cDNAs with complete open reading frame (ORF) encoding putative WRKY proteins using in silico cloning. Phylogenetic analysis indicated that the 15 wheat WRKY genes belonged to three major WRKY groups. Expression analysis revealed that most genes expressed drastically in leaf, except TaWRKY10 which expressed in crown intensively. Four genes were strongly up-regulated with the senescence of leaves. Eight genes were responsive to low temperature, high temperature, NaCl or PEG treatment. Moreover, differential expression patterns were also observed between wheat hybrid and its parents, and some genes were more responsive to PEG treatment in the hybrid. These results demonstrated that wheat WRKY genes are involved in leaf senescing and abiotic stresses. And the changed expression of these WRKY genes in hybrid might contribute to the heterosis by improving the stress tolerance in hybrids. 相似文献
995.
996.
SU Yong ZHOU Jingli YU Shengsheng JIANG Minghua LIU Gang 《武汉大学学报:自然科学英文版》2007,12(2):249-254
This paper introduces agent-based methodology to build a distributed autonomic storage system infrastructure, and an effectively negotiation mechanism based on agent is applied for data location. We present Availability-based Data Allocation (ADA) algorithm as a data placement strategy to achieve high efficient utilization of storage resources by employing multiple distributed storage resources. We use Bloom filter in each storage device to track the location of data. We present the data lookup strategy that small size of read request is handled directly, and large size of read request is handled by cooperation with storage devices.The performance evaluation shows that the data location mechanism is high available and can work well for heterogeneous autonomic storage systems. 相似文献
997.
998.
LI Weifeng DU Jinyuan 《武汉大学学报:自然科学英文版》2007,12(6):985-991
We discuss the linear conjugate boundary value problems on the unit circle and the real axis. We obtain some Fredholm integral equations. Using thess equations we discuss the solvable conditions on these problems and we also give a direct method for the extension problems on the real axis. 相似文献
999.
Doug Walton 《Systemic Practice and Action Research》2007,20(5):369-386
Public discourse in Western democracies, particularly in the United States, is far from the Habermasian ideal of citizens
engaging in a rational discussion of public affairs. Rather than providing an arena for informed deliberation, the growing
expertise of the media, lobbyists, and politicians has sub-optimized the current system of discourse to focus on emotional
manipulation and the creation of polarized interests, each competing with each other for dominance. Avoiding a potentially
disastrous outcome from this unbalancing of public discourse requires a revitalization of the public sphere and a return of
the citizen voice in public decision making. To that end, systems thinkers have a tremendous opportunity and can play an important
role. This article will explore the current challenges facing public discourse and outline that potential role. 相似文献
1000.
XIA Yingqing TIAN Huamei DUAN Zhiguo WU Ling XU Penggen 《武汉大学学报:自然科学英文版》2007,12(3):481-484
This paper presents a method to analyze and calculate the angular glint of targets. By parting the target to very small size cells, using high-frequency approximation, the near-field radar cross section (RCS) is calculated based on the scattering from complex targets and environments (SCTE) system, and the angular glint is calculated by the phase grads. The results show that the angular glint can be calculated exactly in the SCTE system, and this method is correct and efficient. In the near-zone, the far-field theory is not applicable and the angular glint should be calculated by the near-field theory. 相似文献