全文获取类型
收费全文 | 76719篇 |
免费 | 4324篇 |
国内免费 | 5977篇 |
专业分类
林业 | 9088篇 |
农学 | 5707篇 |
基础科学 | 4596篇 |
10773篇 | |
综合类 | 38659篇 |
农作物 | 3225篇 |
水产渔业 | 2224篇 |
畜牧兽医 | 7004篇 |
园艺 | 1976篇 |
植物保护 | 3768篇 |
出版年
2024年 | 274篇 |
2023年 | 1096篇 |
2022年 | 1655篇 |
2021年 | 1842篇 |
2020年 | 1799篇 |
2019年 | 2181篇 |
2018年 | 1256篇 |
2017年 | 2295篇 |
2016年 | 2877篇 |
2015年 | 2604篇 |
2014年 | 3596篇 |
2013年 | 3692篇 |
2012年 | 5539篇 |
2011年 | 5871篇 |
2010年 | 4844篇 |
2009年 | 4785篇 |
2008年 | 4482篇 |
2007年 | 5461篇 |
2006年 | 4794篇 |
2005年 | 4049篇 |
2004年 | 3083篇 |
2003年 | 2673篇 |
2002年 | 2063篇 |
2001年 | 1807篇 |
2000年 | 1645篇 |
1999年 | 1302篇 |
1998年 | 1139篇 |
1997年 | 1026篇 |
1996年 | 1005篇 |
1995年 | 987篇 |
1994年 | 944篇 |
1993年 | 827篇 |
1992年 | 766篇 |
1991年 | 634篇 |
1990年 | 574篇 |
1989年 | 516篇 |
1988年 | 402篇 |
1987年 | 275篇 |
1986年 | 131篇 |
1985年 | 47篇 |
1984年 | 25篇 |
1983年 | 23篇 |
1982年 | 20篇 |
1981年 | 20篇 |
1980年 | 22篇 |
1979年 | 16篇 |
1978年 | 16篇 |
1977年 | 7篇 |
1976年 | 10篇 |
1962年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 109 毫秒
911.
912.
Additional time delay of arrival resulted from non-line of sight(NLOS)propagation and muti-path effect in(cellular) environment is the dominant factor resulting in TDOA location error,study on the statistical property of TDOA error is useful for improving the accuracy of TDOA location.Based on the exponential distribution property of NLOS propagation delay,and considering the system error resulted from the signal detection,the authors establish a statistical model of arrival time and a model of TDOA error distribution by statistics when non-light of sight and multi-path propagation exist.The models reflect the statistical property of arrival time and TDOA error in cell network,the simulation shows that the proposed models have a good accuracy and a satisfactory behavior. 相似文献
913.
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
914.
Yi Shuping Gai Xuchang Xu Xiangsui 《保鲜与加工》1992,(2):65-70
Based on Group Technology, part families of the norm of working hour are formed according to the similarity among components. With the progressive regerssion, the models of the norm of working hour are built for every part family. A computer aided norm model building program and a computer aided norm of working hour planning system are developed. The system is successfully integrated with CAD/CAPP system. Some examples of the model building and calculation results are illustrated as well. 相似文献
915.
Wang Yuanliang Li Shiqi 《保鲜与加工》1992,(2):116-121
Several aspects of the conditional equation derived from the accumulative equation are discussed in detail. The results indicate that the conditional equation not only is well consistent with the principles of electronegativity and absolute hardness equalization, but also satisfies the bonding of a full -prerequisite condition, e. g. two like electronic charge species repel and unlike attract each other'. In addition these principles are in the accumulative state. Mathematically, some other aspects of the conditional equation are worth studying based on the generalization of this equation. 相似文献
916.
Zhang Peiyuan Liu Song 《保鲜与加工》1989,(2):10-16
It's recommended that the term coupled in temperature and strain is nonlinear and is significant in thermoelastic theory. Using the solution of linear coupled and uncoupled theory and selecting appropriate general coordinates, authers approach the nonlinear coupled problems with a variational method. A solution to the problem in half space, is given. It is shown that the difference between the nonlinear coupled theory and the linear coupled or uncoupled theory would be notable when the amplitute of temperature is much larger than that of the reference slate. 相似文献
917.
Wang Bingle Song Liquan 《保鲜与加工》1988,(2)
This paper presents a new method by using mass and acceleration parameters for balancing inertial force of mechanism. Compared with other balance methods, this one is simple, clear and superior in regularity when the balancing equation of complex mechanism is established, and its solving procedure is also easy. The new methcd is convenient to learn and use as well as convenient to program. 相似文献
918.
Yang Yudin Liu Dejun 《保鲜与加工》1990,(1):110-118
In this paper, taking a D1201 spindle as an example, we discuss the dynamic analysis of spindle by using the theory of the complex model, we also analyse the factors which effect the dynamic characterics of spindle and give results which arc reference value to improving the structure of spindle, 相似文献
919.
920.
In this paper, the historical reasons of formation of the village inside the city are analyzed. It is indicated that such situation for the villages inside the city are the result of joint action of both the villagers and the government. The foundation of villages inside the city laid on the simple compensation for farmers in confiscating the rural land and lack of intelligence aids. In this paper, the ideas and measures for their reconstruction are put forward based on consideration of the reasons of their formation and difficulties in the reconstruction. 相似文献