全文获取类型
收费全文 | 4855篇 |
免费 | 211篇 |
国内免费 | 1385篇 |
专业分类
林业 | 338篇 |
农学 | 355篇 |
基础科学 | 2469篇 |
1494篇 | |
综合类 | 1495篇 |
农作物 | 81篇 |
水产渔业 | 60篇 |
畜牧兽医 | 92篇 |
园艺 | 25篇 |
植物保护 | 42篇 |
出版年
2024年 | 63篇 |
2023年 | 135篇 |
2022年 | 204篇 |
2021年 | 196篇 |
2020年 | 223篇 |
2019年 | 214篇 |
2018年 | 149篇 |
2017年 | 200篇 |
2016年 | 336篇 |
2015年 | 300篇 |
2014年 | 371篇 |
2013年 | 327篇 |
2012年 | 540篇 |
2011年 | 418篇 |
2010年 | 272篇 |
2009年 | 316篇 |
2008年 | 251篇 |
2007年 | 394篇 |
2006年 | 298篇 |
2005年 | 237篇 |
2004年 | 173篇 |
2003年 | 188篇 |
2002年 | 104篇 |
2001年 | 73篇 |
2000年 | 55篇 |
1999年 | 50篇 |
1998年 | 47篇 |
1997年 | 47篇 |
1996年 | 38篇 |
1995年 | 37篇 |
1994年 | 37篇 |
1993年 | 31篇 |
1992年 | 32篇 |
1991年 | 24篇 |
1990年 | 18篇 |
1989年 | 20篇 |
1988年 | 19篇 |
1987年 | 11篇 |
1986年 | 2篇 |
1983年 | 1篇 |
排序方式: 共有6451条查询结果,搜索用时 15 毫秒
61.
Mathematical model of power frequency electric field is established based on Charge Simulation Method(CSM) to analyze power frequency electric field caused by EHV lines with overhead ground wires and its effect, which are entering the megalopolis. Calculative method on the electric field under the transmission lines is analyzed and validated. According to the criterion of environmental evaluation, the distributing characteristic of electric field is summarized, in case of various arrangement manners of lines. At last, some advice and methods of eliminating electric field under the transmission lines entering the megalopolis is proposed with regard to protecting environment. 相似文献
62.
This paper discuas the controlling principle of asynchronized turbogenerators,deduces their mathematical model,and studies their regulating characteristics of real and reactive power(or voltage)by means of simulation calculation of operating characteristics for a 200MW asychronlzed turbogenerator.The conclusion is that the asychronlzed tubogenerators not only can regulate real and reactive power independently but also has an excellent dynamic performance. 相似文献
63.
Li Jianzhong Wang Guanjie 《保鲜与加工》1993,(2):14-18
By comparing the techniges of the production and the logic predicate for knowledge representation, this paper presents a kind of constructure of combinatorial knowledge base applied to the expert system for switch operation in electric power system . This constructure of knowledge base to be expanded and revised easily,and it is cinvenitnt to build the general shell of the expert system. 相似文献
64.
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
65.
Liu Haijun Deng Renming 《保鲜与加工》1992,(5):51-56
This paper presents a new hierarchical and coordinative control method for reactive power optimization in large-scale interconnected power systems. It combines the interconnection forecast method for hierarchical control in large-scale steady state industrial systems with the incremental linear programming method for reactive power optimization in real time. The results of applying the method to power system examples show that the method not only solves the problem of high dimension in order, but also keeps the advantages of the incremental linear programming method. The proposed method is also suitable for the security analysis of reactive power correction in large-scale interconnected power systems. 相似文献
66.
Chang Baobo Sun Hongbo 《保鲜与加工》1997,(6):16-22
This paper has applied the var cost to the theory of spot price.An augmented Lagrange method is employed to develop the exact model of spot price of active power and reactive power,and the physical means of Lagrange multipliers are explained.The numerical examples are given to demonstrate the validity of the proposed model. 相似文献
67.
The feasible control problem is presented for the day-operation of the Three-Gorge Caseade Hydropower Station. The significance of the problem is explained. Basic propetties ofthe problem and the approaches to study it are discussed. Two methfods are given for solving theproblem. which are calculus of variation and linear programming. In the former. the Euler equationwith time-lag and the generalised two-point boundary conditions are obtained and the correspondingpractical implication is interpreted. In the latter. the rationality for the discrete model is exptainedand the computalional details are given for the implement software. some questions which should bepaid much attentfon and the corresponding propeels are prasented based on the numerical results andtheoretical analysis 相似文献
68.
LIU Zhongming 《保鲜与加工》2015,(1):133-139
Based on pitting strength, some approaches to enhance power density of gearboxes for wind turbines are discussed. The calculation load is greatly reduced by power division. Allowable stress numbers are effectively increased by improving property of quality materials, precision heat treatment or shot peening. Load capacities can be also obviously enhanced by improving the tooth shape of morphology optimization of tooth root, tooth correction or superfinishing of tooth surface. The center distance is effectively decreased by minishing minimum allowable safety factor or enlarging face width. The power density of gearbox can be further improved by means of structure optimization of planet carrier or uniform load of planetary gear transmission. 相似文献
69.
The method and principle of the parameter design for the power train of EV is explored and discussed,then the parametersof EV. based on SC7101 are reasonably selected.The method of calculating the SOC of Lead_acid battery is provided and the relation among the electromotive force,the interior resistance, the polarized resistance and the SOC are analyzed. The capacity is compensated according to the relation between the capacity and the temperature, which is the basis on which the energy management system is designed and improved. The dynamic model which reflects the dynamic characteristics of the power train is established,then the dynamic performance of the EV based on SC7101 is simulated, and the factors that affect the continuous mileage of EV are analyzed,the measures to increase the continuous mileage are provided.The result shows that the analysis of the electric characteristics of battery and the dynamic model are correct,and the dynamic performance perfectly meets the need of the design target. 相似文献
70.
Condition assessment on power transformer is an important approach to improve reliability of transformer and provide proper decision-making for condition-based maintenance. For long, static assessment method used in the past has already not adapted to the development of modem electricity transmission and distribution technology. So based on the deficiency of fuzzy synthetic assessment method, matter-element synthetic assessment method is introduced. The method can evaluate transformer condition not only in quality but also in quantity. Because each index weight can be given fairly and fast through improved analytic hierarchy process, which need not carry on consistency chec accuracy of assessment can be improved. Examples analysis has proved that assessment method of power transformer condition based on matter-element theory is rational and effective. 相似文献