全文获取类型
收费全文 | 14059篇 |
免费 | 869篇 |
国内免费 | 1333篇 |
专业分类
林业 | 1200篇 |
农学 | 1357篇 |
基础科学 | 677篇 |
1616篇 | |
综合类 | 6020篇 |
农作物 | 964篇 |
水产渔业 | 724篇 |
畜牧兽医 | 2004篇 |
园艺 | 935篇 |
植物保护 | 764篇 |
出版年
2024年 | 77篇 |
2023年 | 221篇 |
2022年 | 612篇 |
2021年 | 638篇 |
2020年 | 611篇 |
2019年 | 589篇 |
2018年 | 364篇 |
2017年 | 578篇 |
2016年 | 452篇 |
2015年 | 712篇 |
2014年 | 664篇 |
2013年 | 829篇 |
2012年 | 1119篇 |
2011年 | 1160篇 |
2010年 | 1024篇 |
2009年 | 904篇 |
2008年 | 965篇 |
2007年 | 895篇 |
2006年 | 811篇 |
2005年 | 649篇 |
2004年 | 347篇 |
2003年 | 228篇 |
2002年 | 272篇 |
2001年 | 280篇 |
2000年 | 248篇 |
1999年 | 187篇 |
1998年 | 130篇 |
1997年 | 108篇 |
1996年 | 87篇 |
1995年 | 98篇 |
1994年 | 72篇 |
1993年 | 64篇 |
1992年 | 51篇 |
1991年 | 50篇 |
1990年 | 45篇 |
1989年 | 37篇 |
1988年 | 31篇 |
1987年 | 15篇 |
1986年 | 11篇 |
1985年 | 4篇 |
1984年 | 5篇 |
1982年 | 3篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1964年 | 1篇 |
1962年 | 3篇 |
1956年 | 4篇 |
1955年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 296 毫秒
151.
This paper introduces the development of the electronic throttle control system (ETCS) , the structure and the principle of the system are also involved. After analyzing the nonlinearity problem existing in the mechanical structure of the system, a mathematical model of the ETCS is built up. Then three controllers (PID controller, Fuzzy controller, sliding mode controller) were designed and simulated in the matlab simulink. The result indicates that the PID controller has an overshoot,but the Fuzzy controller and the sliding mode controller can settle the nonlinearity problem better. 相似文献
152.
The cavity induced by top-blow gas jets impinging on the bath surface is one of most important parameters, which has overwhelming influence on the rate of slag formation and metallurgical performance. Water model experiments are conducted through a 1/10 scaled-down top-blown convertor. Effects of lance height and gas flow rate on penetration behavior are studied, and also the influence mechanism of liquid surface tension on penetration depth is addressed based on the experiments and theory analysis. An energy utilization index (EUI) is defined to reflect how much the impinging kinetic energy of gas jets are transferred into bath for formation of cavity, and an improved model is proposed to predict penetration depth of jets. The results show that penetration depth increases with lowering lance height and increasing gas flow rate, and impact diameter increases with increasing lance height, but is little affected by gas flow rate. Influence of liquid surface tension on penetration depth is enhanced with increasing surface tension and penetration depth. The EUI increases with increasing lance height, and a function relationship of it with lance height is obtained based on the experiments. 相似文献
153.
Zhang Wei Ren Zhen Qin Yihong Liao Qing 《保鲜与加工》1990,(1):56-62
A flexible mathematical model used for dynamic simulation studies of HVDC systems which is based on the stale variable method is presented, and the correspongding computer program has been implemented. Generality, easy to use, and suitable to run on microcomputers are the main features of the model and the computer program. Therefore the model is easy to be extended for application. An example is given in the paper to verily the coneclness of the model and the program. 相似文献
154.
Chen Ganping Xu Guoyu Qin Yihong 《保鲜与加工》1990,(1):43-48
A digital power system stabilizer ( DPSS ) and its design method are presented in this paper. The mathematical model of a single machine infinite bus power system is obtained using off-line parameter identifications, and on the base of the identified model the control system is synthesized by generalized minimum variance control principles. Through feedback combination of several signals this control system has double functions of damping low frequency oscillation and automatic voltage regulating. In order to avoid static errors the very effective way is to combine discrete generalized minimum variance control with continuous integral control. Besides the proposed control strategy is very simple in structures and very easy to realize. Simulation results under several cases have shown the correctness and effectiveness of the proposed control strategy. 相似文献
155.
156.
YANG Zhong-ren RAO Cheng ZOU Jian PENG Zhen-liane Ministry of Education Chongqing Chin 《保鲜与加工》2004,(2):32-35
Dong village is very valuable for research work because of its special regional and national character and rich cultural implication. In this paper, some information about the location, form and features of Dong village is given. Then, the relationship between the culture and the settlement is analyzed, which provides a clue for understanding Dong village and will be beneficial to creating modern human settlements. 相似文献
157.
Pushover analysis is a tool to study the inelastic behavior of structures under strong seismic action. The 3D-pushover analysis was performed to analyze the inelastic lateral-torsional coupled response of multistory asymmetric structures. The symmetric frame and frame-wall systems were studied in this paper, and the capability curve of the top displacement and base shear,the story drift and the position of rotation center were compared with reference to corresponding symmetric structures. The characters of inelastic response of multistory asymmetric structures were found. The conclusion provides reference for performance-based seismic design of asymmetric structures. 相似文献
158.
Sun Hongbo Xu Guoyu Qin Yihong 《保鲜与加工》1992,(4):80-86
This paper proposes a multi - objective fuzzy decision - making approach for power network planning under uncertainty ,which simutaneously considers not only the least investment cost,the minimum power loss,the maximum reliability,and the least environmental impacts, but also the uncertainty about the future load growth and the capital investment availability. The validity and effectiveness of the proposed approach is verified with numerical examples. 相似文献
159.
160.
Zhu Caichao Qin Datong Li Runfang Chen Zongyuan 《保鲜与加工》1997,(5):26-32
The epicyclic gear drive with internal tooth is analysed,a dynamic analysis model which is based on the compatibility conditions of deformation is set up.On the basis of this model,the analysis of dynamics and kinematics is made,and the influence of design parameters on dynamical characteristic is also studied. 相似文献