全文获取类型
收费全文 | 18807篇 |
免费 | 1189篇 |
国内免费 | 1916篇 |
专业分类
林业 | 1607篇 |
农学 | 2023篇 |
基础科学 | 1448篇 |
2595篇 | |
综合类 | 7369篇 |
农作物 | 1105篇 |
水产渔业 | 892篇 |
畜牧兽医 | 2858篇 |
园艺 | 951篇 |
植物保护 | 1064篇 |
出版年
2024年 | 96篇 |
2023年 | 297篇 |
2022年 | 809篇 |
2021年 | 898篇 |
2020年 | 815篇 |
2019年 | 840篇 |
2018年 | 645篇 |
2017年 | 882篇 |
2016年 | 649篇 |
2015年 | 979篇 |
2014年 | 951篇 |
2013年 | 1230篇 |
2012年 | 1603篇 |
2011年 | 1520篇 |
2010年 | 1494篇 |
2009年 | 1365篇 |
2008年 | 1206篇 |
2007年 | 1065篇 |
2006年 | 884篇 |
2005年 | 674篇 |
2004年 | 385篇 |
2003年 | 341篇 |
2002年 | 400篇 |
2001年 | 345篇 |
2000年 | 284篇 |
1999年 | 273篇 |
1998年 | 153篇 |
1997年 | 140篇 |
1996年 | 118篇 |
1995年 | 113篇 |
1994年 | 75篇 |
1993年 | 66篇 |
1992年 | 57篇 |
1991年 | 56篇 |
1990年 | 45篇 |
1989年 | 39篇 |
1988年 | 31篇 |
1987年 | 21篇 |
1986年 | 16篇 |
1985年 | 8篇 |
1984年 | 8篇 |
1983年 | 4篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1980年 | 4篇 |
1979年 | 3篇 |
1968年 | 2篇 |
1966年 | 2篇 |
1956年 | 2篇 |
1955年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
171.
172.
Investment on construction projects has become an important factor in improving the development of economy in China.The non-price bottom bidding is a representation that the investment on projects is becoming standardized.During the years of trying out the non-price bottom bidding,the colluding in bidding or bidding below the cost has happened several times,which have damaged the benefits of owners.To solve theses problems,this paper firstly studies the reason of these problems-the failure in the normal relationship between principal and agent.Then,some measures have been presented as follows: building systematic and scientific system of qualification prejudication,judging behaviors of colluding in bidding and bidding below the cost with reason and providing a surety mechanism of the project.The authors hope that these measures are beneficial to non-price bottom bidding. 相似文献
173.
Jointing of titanium alloy and stainless steel is very often in the fields like aviation, chemical plants etc. The combination of materials has the advantages of both titanium alloy and stainless steel and will reduce the consumption of titanium resource. The phase transformation diffusion bonding of TA17 titanium alloy to 0CriSNigTi stainless steel are carried out on Gleeble - 1500D. The effcet of maximum thermal cycle temperature, the tensile strength, the fracture surface and microstruture of the joints are investigated. The results show that TiFe and TiFe2 intermetallics and some solid solutions are formed along the interface, when maximum thermal cycle temperature increase, the area proportion of intermetallics in the fracture surface increase, which are detrimental to the mechanical properties of the transition joints. 相似文献
174.
为对菲牛蛭抗凝血活性物质的提取进行优化,通过采取不同方法对菲牛蛭活体(6种)、干体(6种)及唾液腺分泌物(3种)进行提取得出最适提取方法。结果表明对于菲牛蛭活体和干体均是采取80%预冷丙酮提取得到的抗凝血物质含量高,最高分别达至(61.20±1.13)AT-U/g和(89.19±1.13)AT-U/g;对于菲牛蛭唾液腺分泌物则是采取10%的三氯乙酸提取得到的抗凝血物质含量高,最高达(24.41±0.98)AT-U/g。研究结果得出了适合菲牛蛭活体、干体和唾液腺分泌物中提取抗凝血活性物质的最适方法,有利于推动水蛭素的大规模生产和广泛的临床应用。 相似文献
175.
176.
The existence of ettringite in sulphoaluminate cement hardened paste is mainly in the form of pole-like crystal,which causes unfavorable effect on the cement.In this paper the strength and microstructure of the sulphoaluminate cement containing ultra-fine CaCO_3 weze discussed.Experimet results show that the strength of sulphoaluminate cement was obviously improved by adding 3% of ultra-fine CaCO_3.After curing for 28d,the compressive strength of cement paste and mortar is 100.6MPa and 94.1MPa respectively,and the bending strength of the mortar reached 12.5MPa.SEM study on hardened paste of sulphoaluminate cement containing ultra-fine CaCO_3 showed that its structure was dense and homogeneous,there was hardly any massive crystall detected. 相似文献
177.
A dynamic mathematical model of the steam-cooled cyclone separator in a 440 t/h CFB boiler has been established.Based on the mathematical model,simulation model of the cyclone separator is created with the advanced continuous simulation language(ACSL).The validity of this model has been verified by the simulation experiments and analysis.This model can provide a theoretical guidance for operation and optimal design of the cyclone separator. 相似文献
178.
In this paper, the finite element method is used to analyze the behavior of the prestressed V-shape Slab Roof structure, especially the effect of temperature change on the stress of slab. The calculated results indicate that when the temperature changes, the tensile stress at the lower extreme fibers of the flange plate near rib exceeds the tensile strength of concrete subjected to the combined action of triaxial stresses and results in the longitudinal cracks of the flange plate. The deflection analysis is based on the mechanical model of the prestressed V shape slab with the longitudinal cracks. The calculated results are in general agreement with the field observations. 相似文献
179.
Intrusion detection system is an essential component of network security protection mechanisms. Most intrusion detection system can not adapt the variation of network environment. Aiming at this problem, an adaptive strategy that composed of condition space and strategy space is proposed. The condition space describes the network environment and the strategy space describes the strategy. There is an exclusive strategy corresponds to a certain environment state in condition space. On the base of the adaptive strategy, an adaptive intrusion detection system based on agent and data mining is designed. The simulation experiments indicate that the adaptive strategy is effective. 相似文献
180.
The following details are discussed in the present paper, after high titanium slag and H3PO4 is put in the closed system,the .reaction takes place under controlled temperature conditions, and the gas produced is collected. The two phases of gas liquid attain kinetic equilibrium, ihe gas above the liquid is used to make GC analysis with the aid of chromatographic column of 5A molecular sieve, TCD and N2 30ml/min. Satisfactory quantitative results can be achieved with simple instruments and uncomplicated operation technique. 相似文献