全文获取类型
收费全文 | 14659篇 |
免费 | 757篇 |
国内免费 | 1805篇 |
专业分类
林业 | 1748篇 |
农学 | 2277篇 |
基础科学 | 1380篇 |
2695篇 | |
综合类 | 4056篇 |
农作物 | 810篇 |
水产渔业 | 559篇 |
畜牧兽医 | 2165篇 |
园艺 | 437篇 |
植物保护 | 1094篇 |
出版年
2024年 | 42篇 |
2023年 | 145篇 |
2022年 | 410篇 |
2021年 | 584篇 |
2020年 | 514篇 |
2019年 | 504篇 |
2018年 | 359篇 |
2017年 | 471篇 |
2016年 | 544篇 |
2015年 | 707篇 |
2014年 | 652篇 |
2013年 | 733篇 |
2012年 | 830篇 |
2011年 | 1003篇 |
2010年 | 875篇 |
2009年 | 867篇 |
2008年 | 801篇 |
2007年 | 891篇 |
2006年 | 774篇 |
2005年 | 738篇 |
2004年 | 366篇 |
2003年 | 318篇 |
2002年 | 258篇 |
2001年 | 279篇 |
2000年 | 354篇 |
1999年 | 417篇 |
1998年 | 359篇 |
1997年 | 356篇 |
1996年 | 304篇 |
1995年 | 310篇 |
1994年 | 270篇 |
1993年 | 223篇 |
1992年 | 183篇 |
1991年 | 173篇 |
1990年 | 139篇 |
1989年 | 123篇 |
1988年 | 131篇 |
1987年 | 60篇 |
1986年 | 37篇 |
1985年 | 21篇 |
1984年 | 21篇 |
1983年 | 21篇 |
1982年 | 22篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 7篇 |
1965年 | 1篇 |
1964年 | 3篇 |
1956年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
国产人造板宽带砂光机功率谱密度和试验模态分析 总被引:3,自引:0,他引:3
以国产B229型四四砂架人造板双面宽带砂光机为研究对象,从易产生横纹缺陷入手,对其砂架部件、整机等分别采用试验模态分析(EMA)及功率谱密度法(PSD)来开展振动动态特性研究。在模态试验中,采取测量点固定、改变激振点的跑点测量方法,得到固定测量点对各激振点频响函数,并应用SISO频响函数识别法,通过频响函数曲线拟合来完成模态参数识别,获到典型的整机、砂架部件的频响函数图和接触辊的二阶振型图。通过PSD分析,获得砂光机的PSD图谱和激励频率等。 相似文献
992.
灵香草浸膏系列产品的研究 总被引:5,自引:0,他引:5
以报春花科珍珠菜属植物灵香草为原料,采用不同的工艺,制取各类浸膏的研究结果。首先采用各种剂对灵香草进行浸提试验,并对得到的乙醇浸膏进行脱色浸膏、净油的研制;对得到的各类产品的物性数据进行研究测试,结果表明乙醇浸膏、脱色浸膏和净油具有许多优良特性,对它们的进一步研究开发有重大意义。 相似文献
993.
994.
以安楚高速公路某一立交匝道测设数据为例 ,给出坐标计算公式及编程思路 ,节省了坐标计算的时间及提高了计算数据的精度 ,为立交匝道放样提供了方便 相似文献
995.
Plant diversity of secondary forests in response to anthropogenic disturbance levels in montane regions of northeastern China 总被引:4,自引:0,他引:4
The effects of anthropogenic disturbances on forest structure and plant diversity of secondary forest ecosystems were evaluated
based on the classification by site factors in the montane regions of northeastern China. Forty-five sample plots containing
720 sub-plots of overstory species (8 m × 8 m), 1,440 quadrats of understory species (2 m × 2 m), and 1,440 quadrats of herbaceous
layer species (1 m × 1 m) were clustered into five groups (G1, G2, G3, G4 and G5) by site variables with Hierarchical Cluster
Analysis. Meanwhile, the disturbance levels corresponding to the five groups were determined according to the factors influencing
human disturbances (D0-G2, D1-G1, D2-G3, D3-G5, and D4-G4). Species diversities of overstory, understory, herbaceous layer
and overall species were evaluated using species number, Margalef index, Pielou index, Shannon–Wiener index, and Simpson index;
and β-diversities (Whittaker index, Cody index). Basal area of stands exhibited a decreasing trend along the disturbance level
due to a gradual increase in the extraction of timber by the human disturbances. The indices of species diversity suggested
that overstory and understory species were distributed evenly among the groups or disturbance levels. There were no absolutely
dominant tree species in the secondary forest ecosystems. The differences in site factors and the current disturbance intensities
were not intense enough to lead to loose changes in overstory and understory species. The species diversity indices exhibited
the maximum values at D3 (G5) for herbaceous layer species; this may suggest that the current disturbance intensities were
intense enough to lead the changes of herbaceous layer species. Three rare and endangered species (Juglans mandshurica, Fraxinus mandshurica and Acanthopanax senticosus) were found within the secondary forests. These rare and endangered species appeared in each clustered group or disturbance
level, which suggested that the current disturbance intensity in the study area was not strong enough to influence the distribution
of rare and endangered species. The current disturbances in the secondary forests may not lead to a decrease in stability
and complexity of the overstory and understory species, but the higher disturbance level may be intense enough to change the
habitat fitness for the herbaceous species.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized authors. 相似文献
996.
Based on paper of “Theoretical derivation of risk-ratios for assessing wind damage in coastal forest“,wind damage in the pine coasteal forest,which was thinned at four levels in December of 1997,was investigated for four successive growing seasons.Besides wind damage,the wind profiles outside and inside the coastal forest stand and the distributions of optical stratification porosity (OSP) were also observed.Based on these data,risk-ratios of wind damage for both individual trees and stands were estimated according to the methods developed in “Theoretical derivation of risk-ratios for assessing wind damage in a coastal forest“.The results showed that risk-ratios of wind damage,which were calculated from the meen height and diameter only and from the combination of wind and stand sructure profiles,accurately predicted wind damage in the plantation.Relationships between different thinning ratios and incidence of wind damage showed that stand stability decreased soon after the thinning.This was due to the immediate effects of thinning on increasing the canopy roughness and wind load,and on decreasing the sheltering effects from surrounding trees.However,thinning strategies could improve the stability by long-term effects on growthand development of trees against extreme wind.Only canopy damage was recorded during the experimental period,no stem damage was found,even though the maximum 10-min wind speed outside the coastal forest attained 30.2m s^-1.The results obtained in this study indicate that thinning is the most effective silvicultural strategy available for managing coastal forest despite the increased probability of wind damage soon after thinning. 相似文献
997.
The effect of tool angles on the shapes of chips generated by parallel-to-grain and end-grain milling was explored for China
fir and maple under fixed spindle and feed speeds and cutting depth. The milling path was up-milling by straight router-bits
with a diameter of 12 mm. The chip shapes could be distinguished as five types: spiral, splinter, flow, thin, and granules
or powder. The flow and thin chips were generated most often (on a weight percentage basis) for all tool angles investigated
for parallel-to-grain and end-grain milling of China fir and maple. More granule chips were produced with parallel-to-grain
milling than with end-grain milling for both woods. The measured chip thickness (t′) was thicker than the calculated thickness (t
max). Thicker and longer maple chips were produced by end-grain milling than by parallel-to-grain milling. The tool geometries
of 40°/15° (sharpness of the angle–rake angle), 50°/15°, and 60°/15° for China fir and 40°/25°, 50°/5°, and 60°/5° for maple
produced relatively more flow chips with parallel-to-grain milling. Furthermore, the tool geometries of 40°/5°, 50°/15° and
60°/25° produced more flow chips (weight percentage) by end-grain milling of China fir and maple.
Received: May 23, 2001 / Accepted: June 28, 2002
Acknowledgment This study was supported by a grant from the National Council of Science, Taiwan (NSC89-2313-B-415-011). 相似文献
998.
农田防护林主要间距离的确定——基于林带结构与风速降低的关系 总被引:3,自引:0,他引:3
Relative windspeed reduction was measured behind nine relatively narrow, homogenous tree windbreaks with porosities between
0.13–0.33, and behind 28 combinations of model stubble barriers representing 25 different optical porosities (0.00–0.80).
The optimum porosities observed were 0.25 and 0.13 for tree windbreaks and stubble barriers respectively. Based on the relationship
between windbreak structure (optical porosity) and wind reduction, the chief indices for determining spacing interval, i.e.,
the windbreak structure index (δ) and the parameter of microclimate, represented by the problem wind (L
rp
), were determined. Additionally, investigations on shelterbelt trees were carried out, and stem-analysis techniques were
used, to develop a method for determining the mature height of tree windbreaks (H
0). Optimal spacing intervals between windbreaks could be predicted from the indices of a given windbreak structure, percentage
of reduction of windspeed desired and tree growth model. A hypothetical example for determining the spacing interval of principal
poplar windbreaks is given at the end of this paper. The results can be applied not only to tree windbreak design but also
to other plant materials and artificial barriers for wind protection.
Foundation item: This study was supported by Innovation Research Project of Chinese Academy of Sciences
Biography: ZHU Jiao-jun (1965-), male, Ph. Doctor, PhD advisor. Professor of Institute of Applied Ecology, the Chinese Academy of Sciences,
China, Scholar researcher of Faculty of Agriculture, Niigata University, Japan.
Responsible editor: Song Funan 相似文献
999.
1000.