全文获取类型
收费全文 | 52397篇 |
免费 | 257篇 |
国内免费 | 546篇 |
专业分类
系统科学 | 1243篇 |
丛书文集 | 102篇 |
教育与普及 | 292篇 |
理论与方法论 | 531篇 |
现状及发展 | 30524篇 |
研究方法 | 590篇 |
综合类 | 17729篇 |
自然研究 | 2189篇 |
出版年
2013年 | 806篇 |
2012年 | 570篇 |
2011年 | 2622篇 |
2010年 | 430篇 |
2009年 | 614篇 |
2008年 | 781篇 |
2007年 | 846篇 |
2006年 | 955篇 |
2005年 | 1129篇 |
2004年 | 2281篇 |
2003年 | 1863篇 |
2002年 | 1527篇 |
2001年 | 1190篇 |
2000年 | 748篇 |
1999年 | 861篇 |
1998年 | 646篇 |
1997年 | 775篇 |
1996年 | 534篇 |
1994年 | 691篇 |
1993年 | 691篇 |
1992年 | 819篇 |
1991年 | 698篇 |
1990年 | 762篇 |
1989年 | 611篇 |
1988年 | 596篇 |
1987年 | 608篇 |
1986年 | 664篇 |
1985年 | 822篇 |
1984年 | 732篇 |
1983年 | 617篇 |
1982年 | 763篇 |
1981年 | 800篇 |
1980年 | 881篇 |
1979年 | 1258篇 |
1978年 | 1157篇 |
1977年 | 1144篇 |
1976年 | 1045篇 |
1975年 | 1021篇 |
1974年 | 849篇 |
1973年 | 1113篇 |
1972年 | 1157篇 |
1971年 | 1190篇 |
1970年 | 1286篇 |
1969年 | 1137篇 |
1968年 | 1091篇 |
1967年 | 963篇 |
1966年 | 855篇 |
1965年 | 671篇 |
1964年 | 490篇 |
1958年 | 503篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
91.
史济群 《华中科技大学学报(自然科学版)》1994,(3)
报道一种新颖的激光开槽、埋槽电极硅太阳电池的结构、工艺流程及其研制结果。在AM1.5,25℃,100mW/cm2的条件下,以面积为45cm3的36片硅片研制的硅太阳电池的输出参数的平均值为JSC=36.1mA/cm2,V∞=633mV,F.F.=0.798,η=18.23%,最后分析了种高性能硅太阳电池的设计特点。 相似文献
92.
Non-Minimum Phase Nonlinear System Predictive Control Based on Local Recurrent Neural Networks 总被引:3,自引:0,他引:3
Abstract: After a recursive multi-step-ahead predictor for nonlinear systems based on local recurrent neural networks isintroduced, an intelligent PID controller is adopted to correct the errors including identified model errors and accumulatederrors produced in the recursive process. Characterized by predictive control, this method can achieve a good controlaccuracy and has good robustness. A simulation study shows that this control algorithm is very effective. 相似文献
93.
Dynamic Characteristics of Inter-Satellite Links in LEO Networks 总被引:1,自引:0,他引:1
To establish an efficient inter-satellite link (ISL) in an LEO network, the effect of geometric characteristics of ISL on the ISLs and the devices on the LEO satellite should be examined. Because of the continuous movement of the LEO satellite, the time-varying behaviours of the ISL's geometric charactersistics continuously change with the changes of the satellite's position on the orbit. These dynamic geometric characteristics of the ISLs are important for ISL's performance analyzing and the design of the devices on the LEO satellite. This paper describes dynamic geometric characteristics of ISL, analyzes the impact of these regulations on the tracking system of the satellite's antenna and the power adjusting system of the satellite's transmitter, with the Iridium system as an example. 相似文献
94.
Qiangfu Zhao Zhong Wang Youan KeDept. of Electronic Eng. Beijing Institute of Technology P.O.Box Beijing China 《系统工程与电子技术(英文版)》1991,(2)
It has long been realized that the problem of radar imaging is a special case of image reconstruction in which the data are incomplete and noisy. In other fields, iterative reconstruction algorithms have been used successfully to improve the image quality. This paper studies the application of iterative algorithms in radar imaging. A discrete model is first derived, and the iterative algorithms are then adapted to radar imaging. Although such algorithms are usually time consuming, this paper shows that, if the algorithms are appropriately simplified, it is possible to realize them even in real time. The efficiency of iterative algorithms is shown through computer simulations. 相似文献
95.
M. Ishibashi M. Tsuda Y. Ohizumi T. Sasaki J. Kobayashi 《Cellular and molecular life sciences : CMLS》1991,47(3):299-300
Summary A new bromotyrosine-derived alkaloid with antileukemic activity, purealidin A (5), has been isolated from the Okinawan marine spongePsammaplysilla purea and its chemical structure elucidated on the basis of the spectroscopic data. 相似文献
96.
闫成新 《中国石油大学学报(自然科学版)》2002,26(2)
灵巧度是衡量机器人运动失真程度的重要参数。对机器人灵巧度评价指标进行了对比研究 ,结合大型喷浆机器人自由度较多的特点 ,用雅可比矩阵的条件数对喷浆机器人的灵巧度进行了分析。结果表明 ,大型喷浆机器人在不同形位下的条件数都很小 ,具有较好的运动灵巧度。 相似文献
97.
对目前的UBBE模型进行了适当的推广,使在解决估计问题时可以考虑那些可能是误差上界的数值,并在此基础上提出了一种方法,能够通过对估计精度和可靠性进行合理的权衡确定所需估计值,实际案例研究结果表明所提方法能够较好地解决估计精度和可靠性之间的矛盾。 相似文献
98.
Support vector machine (SVM), as a novel approach in pattern recognition, has demonstrated a success in face detection and face recognition. In this paper, a face recognition approach based on the SVM classifier with the nearest neighbor classifier (NNC) is proposed. The principal component analysis (PCA) is used to reduce the dimension and extract features. Then one-against-all stratedy is used to train the SVM classifiers. At the testing stage, we propose an al- 相似文献
99.
This article presents a case example involving the renaming of an organizational change process from BPR to outsourcing. The paper discusses the important ramifications of such a name change in that the recognition of outsourcing allowed an organizational commitment to counseling and termination advice. The paper discusses two ways of looking at the name change, one from a critical theory perspective and one from a realist perspective. It demonstrates how theory can provide useful yet markedly different interpretations of such organizational events. Critical theory operates from within what can be termed the transitive epistemological dimension, whereas critical realism tends to emphasize the importance of ontological issues. Each has important things to say about the situation and improves our understanding overall. 相似文献
100.
We develop in this paper an efficient way to select the best subset threshold autoregressive model. The proposed method uses a stochastic search idea. Differing from most conventional approaches, our method does not require us to fix the delay or the threshold parameters in advance. By adopting the Markov chain Monte Carlo techniques, we can identify the best subset model from a very large of number of possible models, and at the same time estimate the unknown parameters. A simulation experiment shows that the method is very effective. In its application to the US unemployment rate, the stochastic search method successfully selects lag one as the time delay and five best models from more than 4000 choices. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献