全文获取类型
收费全文 | 16908篇 |
免费 | 1049篇 |
国内免费 | 1740篇 |
专业分类
林业 | 1271篇 |
农学 | 1131篇 |
基础科学 | 828篇 |
2032篇 | |
综合类 | 7652篇 |
农作物 | 1241篇 |
水产渔业 | 733篇 |
畜牧兽医 | 2832篇 |
园艺 | 1164篇 |
植物保护 | 813篇 |
出版年
2024年 | 145篇 |
2023年 | 394篇 |
2022年 | 901篇 |
2021年 | 918篇 |
2020年 | 806篇 |
2019年 | 758篇 |
2018年 | 545篇 |
2017年 | 890篇 |
2016年 | 575篇 |
2015年 | 854篇 |
2014年 | 884篇 |
2013年 | 1107篇 |
2012年 | 1521篇 |
2011年 | 1422篇 |
2010年 | 1305篇 |
2009年 | 1125篇 |
2008年 | 1065篇 |
2007年 | 1015篇 |
2006年 | 744篇 |
2005年 | 581篇 |
2004年 | 357篇 |
2003年 | 306篇 |
2002年 | 425篇 |
2001年 | 389篇 |
2000年 | 305篇 |
1999年 | 128篇 |
1998年 | 51篇 |
1997年 | 20篇 |
1996年 | 24篇 |
1995年 | 23篇 |
1994年 | 19篇 |
1993年 | 14篇 |
1992年 | 18篇 |
1991年 | 11篇 |
1990年 | 8篇 |
1989年 | 9篇 |
1988年 | 8篇 |
1987年 | 6篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 1篇 |
1962年 | 1篇 |
1956年 | 8篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
161.
A theoretical model is developed for the dropwise condensation heat transfer on the horizontal circular surface with radial gradient surface energy based on the heat transfer model of individual condensate drop and the size distribution model of condensate drop on homogeneous condensation surface.The effect of variation of contact angle on the gradient surface on condensation heat transfer and condensate drop size distribution is taken account of in this model.The theoretical calculation method was obtained to predict the dropwise condensation heat transfer coefficient on the horizontal circular surface with radial gradient surface energy under various wall subcooled temperature,contact angle profile on wall surface,and working fluid.The effects of surface energy gradient,wall subcooled temperature,and thermophysical properties of condensate on the condensation heat transfer are discussed respectively.The calculation results show that the condensation heat transfer coefficient increases slightly with the increase of wall subcooled temperature.As latent heat and surface tension increasing,the condensation heat transfer coefficient increases.A larger surface energy gradient induces a larger condensation heat transfer coefficient. 相似文献
162.
As the important method and instrument of agile manufacture,the technology of distributed collaborative design and manufacturing has become the hotspot of manufacturing. The basic concept and key technologies of IP_VPN are introduced in this paper,and a collaborative design network model based on IP_VPN is presented. Distributed collaborative design is a multi_user,multitask involved system, which is consisted of lots of collaborative processs,so the network security is the key of distributed collaborative design. We research on the security of collaborative design network based on IP_VPN, hence get a conclusion:we can construct an economical?practical and secure collaborative design network system by the application of data encryption,user authentication and access control based on the role,etc. 相似文献
163.
This paper expatiates on the bus structure and develop the bus mode Multi-Channel Analyzer (MCA). The nucleus of this MCA is the PC/104 embedded computer, and it introduces the bus communication to realize system and data communication. It may realize data collecting and showing in the field, because the bus mode is rapid. At first, on the hardware we introduce CMOS components, this reduces power, volume and weight. These make it adapts to use in the field; besides, on the software we adopt C and assembly language to program in order to improve conversion speed. At the same time, single-task DOS operating system is infroduced, which makes the software run more steadily, operate more simply. By means of testing on the spot it can normally work, so it suitable to work in the field. 相似文献
164.
Credit risk, the major risk of bank, is more and more intense during the period of economic restructuring in China. In view of the flaw of present risk measurement system, Conditional Value-at-Risk is used for the credit risk measurement which is better than Value-at-Risk. This paper creates the model ,while gives the method and procedure for solving it. So CVaRof credit combination is produced, which is just the early warning value of credit risk. At last, it is concluded that the CVaRrisk measurement is too difficult to use widely at present in china, then some advice is provided. 相似文献
165.
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
166.
With the rapid increase of the Internet user and widely application of Web,the current network technology is facing great challenge. More and more multimedia operations occupy plenty of bandwidth,and result in the network congestion. Therefore,people begin to use QoS to utilize the bandwidth effectively,and ensure the transport of the emergent operation data. The paper introduces the protocols and basic components of QoS,then expatiates the conception and the basic principles of policy control QoS, moreover,emphasizes the structure model of policy control QoS and how it runs. At last,it introduces the transport process of data packets under the control of policies in a policy control QoS system,and how to make the policies in the real world. 相似文献
167.
The agent - construction system, as a new public project management mode, has been practiced in some cities and attracted attention from the society. This paper firstly studies the features and operation modes of agent - construction system, and discusses the agent - construction system from the viewpoints of promoting competition, nourishing agent -construction enterprises, setting up a supervisory and encouraging mechanism, and its connection with the governmental project procurement system etc. 相似文献
168.
The tensile strength of high strength bolt is affected by many factors, including prying force. The prying force can result in the increment of bolt force, for which the connection failed ahead of time. Several methods are presented for calculating the prying force. In the China's "Code for Design of Steel Structure" (GB50017), there is no formula for calculation of prying force and a factor of 0.8 is considered in formula for tensile strength of bolt for taking the prying force into account. It is proposed that the design value of tensile resistance for high strength bolt should be increased. 相似文献
169.
170.