全文获取类型
收费全文 | 1481篇 |
免费 | 63篇 |
国内免费 | 139篇 |
专业分类
林业 | 165篇 |
农学 | 82篇 |
基础科学 | 413篇 |
135篇 | |
综合类 | 741篇 |
农作物 | 28篇 |
水产渔业 | 37篇 |
畜牧兽医 | 54篇 |
园艺 | 11篇 |
植物保护 | 17篇 |
出版年
2024年 | 13篇 |
2023年 | 28篇 |
2022年 | 63篇 |
2021年 | 53篇 |
2020年 | 63篇 |
2019年 | 66篇 |
2018年 | 27篇 |
2017年 | 53篇 |
2016年 | 58篇 |
2015年 | 61篇 |
2014年 | 83篇 |
2013年 | 88篇 |
2012年 | 115篇 |
2011年 | 112篇 |
2010年 | 76篇 |
2009年 | 85篇 |
2008年 | 66篇 |
2007年 | 77篇 |
2006年 | 81篇 |
2005年 | 85篇 |
2004年 | 61篇 |
2003年 | 42篇 |
2002年 | 33篇 |
2001年 | 30篇 |
2000年 | 30篇 |
1999年 | 22篇 |
1998年 | 18篇 |
1997年 | 9篇 |
1996年 | 14篇 |
1995年 | 23篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 7篇 |
1991年 | 8篇 |
1990年 | 2篇 |
1989年 | 3篇 |
1988年 | 3篇 |
1987年 | 1篇 |
排序方式: 共有1683条查询结果,搜索用时 15 毫秒
21.
电子商务的迅猛发展,不仅改变了企业的经营模式,也改变了用户的消费习惯和理念。饲料企业也开始寻求电子商务与传统交易方式相结合的经营模式,谋求企业的长远发展。本文基于"互联网+"背景下,深入分析了我国饲料企业电子商务的经营现状和存在的问题,探讨了电子商务在企业经营中的运行机制和不足之处,并提出了相关的优化对策,以期能帮助企业实现可持续发展。 相似文献
22.
23.
24.
海河二道闸和防潮闸优化调度研究 总被引:1,自引:0,他引:1
汛期为腾空海河河道并保证二道闸下咸水不致上溯,二道闸闸下水位应控制在1.0m左右。利用一维非恒定流模型对海河自西河闸至入海口河段进行数值模拟,研究了西河闸分流量为80、100、120m3/s3种情况下,海河防潮闸和二道闸优化调度结果。 相似文献
25.
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
26.
Aimed at soving the problem existing in the knowledge-acquisition directing-operation expert systems, a new method is proposed to acquire knowledge of expert-operation, excluding errors existed in examples and reinforce the accuracy of examples,the goal of knowledge acquisition can be achieved, the method presented in this paper solves the problem of self-enhancement of expert knowledge on line efficiently. 相似文献
27.
In the general directing-operation expert systems.the information of directingoperation is often shown after the subprocess has finished,The aim of the thesis is to enhance thefunction of directing-operation,and suggest that we add the self-tuning predictor in the expertsystems and use it to direct the present operation.We have shown the case oftemperature-predicting experiment made in the small wine distillatorv so that we can illustrate thatit is feasible to use the predictor to direct the operation. 相似文献
28.
A reservoir operational rule for irrigation in Japan 总被引:1,自引:0,他引:1
Yutaro Senga 《Irrigation and Drainage Systems》1991,5(2):129-140
Operation of a reservoir for irrigation is more complicated and difficult than that for domestic water supply in Japan, owing to the fact that a requirement for irrigation takes various value depending upon effective rainfall on farmland, soil condition, and so on. Therefore, the reservoirs should be operated to correspond with demand, inflow, and unknown droughts occurring in the future. However, the reservoirs have been operated under operators' experiences without formal operational rules, because any effective theories or methods have been neither developed nor used in Japan at present.On operation of the reservoirs, there are two targets — promotion of effective release and restriction of release in preparation for droughts — conflicting with each other. It is useful to set up the operation with Required Storage for Drought Curve (RSDC) Method to achieve reasonable operation harmonizing those two targets. RSDC is composed with Target Line (TL) and Restrictive Release Lines (RRLs). TL is a target storage level to guarantee that storage will never empty until the end of the irrigation period under a certain probability. If actual storage remains above TL, effective release will be promoted. RRLs are storage levels pointing out the basis of restricting release. If actual storage falls below TL, release will be restrained in accordance with RRLs in order to get over uncertain droughts occurring in the future.In this paper, the theory of RSDC is described and the effectiveness of Rule Curve (RC) based on RSDC Method is illustrated by simulation on a single reservoir for irrigation. 相似文献
29.
介绍了柴油机高压共轨电控喷射系统的组成与结构、电控单元ECU硬件电路的设计,嵌入式操作系统L inux应用在电控单元中。 相似文献
30.
In spite of several attempts at integrated operation planning, multiple reservoirs in Japan have been operated by trial and error without any formal rules. Subjects of integrated operation are not only showing optimal usage of daily storage levels as an operational policy but also providing a countermeasure for droughts.Objectives of this study are to make a formal operation rule of multiple reservoirs for irrigation using the theory of Required Storage for Drought Curve (RSDC) Method and to propose operational policy for multiple reservoirs as large water supply systems. The Iwaki river basin, on which there are four reservoirs for irrigation parallel with each other, is considered to be a model river basin with a large water system for this case study.From results of simulations using historical data, comparing an individual operation rule with an integrated operation rule on several indices, effectiveness of the latter rule is recognized. Under integrated operation, water losses are minimized and excessive water conservation can be avoided over the whole area to benefit while target river discharge is maintained at key control points because water usage from all reservoirs is well balanced in relation to water availability. 相似文献