全文获取类型
收费全文 | 84915篇 |
免费 | 4207篇 |
国内免费 | 5976篇 |
专业分类
林业 | 10867篇 |
农学 | 7044篇 |
基础科学 | 4203篇 |
10538篇 | |
综合类 | 41653篇 |
农作物 | 3879篇 |
水产渔业 | 2583篇 |
畜牧兽医 | 7677篇 |
园艺 | 2449篇 |
植物保护 | 4205篇 |
出版年
2024年 | 451篇 |
2023年 | 1146篇 |
2022年 | 1718篇 |
2021年 | 1888篇 |
2020年 | 1928篇 |
2019年 | 2470篇 |
2018年 | 1316篇 |
2017年 | 2422篇 |
2016年 | 3083篇 |
2015年 | 2764篇 |
2014年 | 3906篇 |
2013年 | 4045篇 |
2012年 | 6047篇 |
2011年 | 6423篇 |
2010年 | 5319篇 |
2009年 | 5288篇 |
2008年 | 4923篇 |
2007年 | 5905篇 |
2006年 | 5207篇 |
2005年 | 4343篇 |
2004年 | 3363篇 |
2003年 | 2894篇 |
2002年 | 2252篇 |
2001年 | 1960篇 |
2000年 | 1827篇 |
1999年 | 1453篇 |
1998年 | 1278篇 |
1997年 | 1170篇 |
1996年 | 1158篇 |
1995年 | 1113篇 |
1994年 | 1051篇 |
1993年 | 921篇 |
1992年 | 859篇 |
1991年 | 725篇 |
1990年 | 664篇 |
1989年 | 571篇 |
1988年 | 444篇 |
1987年 | 314篇 |
1986年 | 159篇 |
1985年 | 67篇 |
1984年 | 38篇 |
1983年 | 28篇 |
1982年 | 31篇 |
1981年 | 27篇 |
1980年 | 28篇 |
1979年 | 25篇 |
1978年 | 23篇 |
1977年 | 14篇 |
1976年 | 16篇 |
1955年 | 13篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
41.
42.
Self-bonding is the main factor of the performance expression of binderless boards, and therefore its clarification is considered
to be an important issue. For this purpose, a series of chemical analyses were conducted on kenaf core binderless boards and
their chemical changes during the hot-pressing process are discussed in this article. First of all, binderless boards were
prepared from kenaf core powder at different pressing temperatures (without steam-explosion process) and were used for chemical
analyses after they were reduced into powders and extracted with methanol. To investigate their chemical changes, lignin,
holocellulose, and neutral sugar contents were determined, Fourier transform infrared (FTIR) spectra were recorded, and the
nitrobenzene oxidation procedure was applied. As a result, it was found that parts of lignin and hemicelullose were decomposed
during the hot-pressing process; however, the contribution of the resulting fractions to selfbonding was not observed. In
addition, progress of condensation reactions in lignin and the formation of chemical bonds by low molecular weight conjugated
carbonyl compounds in methanol extractives were observed. Thermal softening of lignin is also suggested to play an important
role in the expression of board performance. 相似文献
43.
目的 试图给出确定森林群落树种丰富度的方法,解决物种丰富度测度问题,完善森林生物多样性研究的方法。 方法 采用Monod模型表达种-面积曲线,通过数学手段确定最小森林群落面积和最大树种数。 结果 用本研究提出的方法对不同气候带森林物种丰富度进行测算,结果表明:热带森林群落具有更高的最大树种数和更大的群落最小面积,物种丰富度随气候带由热带到寒温带逐渐减少。 结论 用最大树种数能够确切表达不同气候带森林群落树种丰富度,克服了现有研究中直接把不同大小调查面积上的树种数量视为树种丰富度的弊端。 相似文献
44.
Eduardo R. S. Roldan 《Reproduction in domestic animals》2019,54(Z4):14-21
Sperm competition is a powerful selective force that has influenced many reproductive traits in males and females although additional evolutionary explanations may help to understand the diversity of mammalian reproduction. Sperm morphology varies considerably in mammals with extreme examples in several rodent lineages in which a wide range of sizes and complex head morphologies have been identified. Mammalian spermatozoa also differ in function, with swimming velocity and trajectory showing much divergence. Underlying processes mediating function have received little attention so far, but differences in timing and proportion of sperm undergoing capacitation or acrosomal exocytosis may be related to variation in signalling processes. Furthermore, energy required for sperm functions (such as motion, signalling and overall maintenance of cell integrity) can be produced and consumed, following different patterns among species and this could be the result of several selective forces. A more thorough understanding of the diversity in structure and function of sperm cells, and underlying selective forces, may help us develop better methods to assess them taking into account particulars and generalities of sperm form and performance. Such tests could then become more reliable in estimations of the impact of cryopreservation or effect of changes in the environment and their relevance for fertility. 相似文献
45.
李旎 《农业图书情报学刊》2012,24(10)
南图阅读节是国内首个以省馆名义举办的阅读节。在活动主体,活动主题,活动内容等方面都有许多创新。文章从6个方面总结了南图阅读节的经验。 相似文献
46.
八个竹种海涂引种试验报告 总被引:1,自引:0,他引:1
本文从土壤含盐量着手对八个竹种在引种成活率、新竹抽发数及其质量、角竹笋产量等方面进行了较系统的研究。结果表明,土壤盐分影响竹子的生长发育,此影响一般随含盐量的增加而增加;不同竹种对土壤盐分的适应能力不同;角竹笋能获得高产,单产达20800kg/ha。参照产地母竹和引种地新竹的生长情况,我们建议:海涂地区发展竹子时可选用角竹、桂竹和红竹。 相似文献
47.
考虑动力稳定装置的轮轨非线性接触、轮轨间的间隙传动、夹钳油缸和水平油缸的等效刚度建立了动力稳定装置动态特性键合图模型.使用分段函数表达式实现了轮轨间的间隙传动.针对不同夹持力对于能量的传递效率与作业效果进行了深入分析与探究,确定了所需最小的水平油缸的推力. 相似文献
48.
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
49.
通过对浙江省丽水市责任山产权登记现状的调查,分析各县、市、区存在登记差异的原因,并提出了解决问题的建议。 相似文献
50.