全文获取类型
收费全文 | 136篇 |
免费 | 7篇 |
国内免费 | 15篇 |
专业分类
林业 | 19篇 |
农学 | 15篇 |
基础科学 | 22篇 |
24篇 | |
综合类 | 35篇 |
农作物 | 5篇 |
水产渔业 | 5篇 |
畜牧兽医 | 29篇 |
园艺 | 1篇 |
植物保护 | 3篇 |
出版年
2022年 | 2篇 |
2021年 | 2篇 |
2020年 | 3篇 |
2019年 | 8篇 |
2018年 | 2篇 |
2017年 | 6篇 |
2016年 | 10篇 |
2015年 | 11篇 |
2014年 | 14篇 |
2013年 | 11篇 |
2012年 | 15篇 |
2011年 | 5篇 |
2010年 | 7篇 |
2009年 | 4篇 |
2008年 | 1篇 |
2007年 | 6篇 |
2006年 | 8篇 |
2005年 | 4篇 |
2004年 | 3篇 |
2003年 | 3篇 |
2002年 | 2篇 |
2001年 | 4篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 4篇 |
1997年 | 2篇 |
1995年 | 1篇 |
1993年 | 2篇 |
1992年 | 3篇 |
1991年 | 2篇 |
1990年 | 4篇 |
1989年 | 1篇 |
1988年 | 1篇 |
1987年 | 1篇 |
1985年 | 1篇 |
排序方式: 共有158条查询结果,搜索用时 31 毫秒
101.
科尔沁沙地人工杨树(Populus simonii)林生态防风效应研究 总被引:5,自引:0,他引:5
通过4~7月对人工杨树林迎风和背风区6倍树高处0.25,0.5,1,2m4个高度风速廓线的观测,定量评价了林地对不同风向及同风向不同风速的生态防护效应,探讨了林地叶面积指数的季节变化对林地生态防风效应的影响。主要结论是:(1)各月可蚀风(风速≥4m/s)的出现频率及风速特征明显不同,4月和5月可蚀风的出现频率及平均风速显著高于6月和7月。(2)各月不同风向的出现频率及风速特征明显不同,4月以SSW、NNW、NW、S、WNW、N和WSW风为主,5月以NW、NNW、W、WSW、WNW和SW风为主,6月以S、SSW、ESE和NNE风为主,7月以SSW、S和SW风为主。(3)杨树林对N、NNE、NE、ENE和NNW风的生态防护效能最好,对NW、E、SW、WSW和W风的防护效能次之,对SE、SSE、S和ESE风的防护效能最差。(4)观测期内林地叶面积指数与0.25,0.5m两个高度的平均风速减弱系数呈显著正相关,其关系可分别用对数和指数函数来描述,而与1,2m两个高度的平均风速减弱系数呈显著非完全正相关,其关系可用三次曲线来描述。 相似文献
102.
Until 1990 the Bitterfeld region was heavily polluted by mining,an obsolete chemical industry and the uncontrolled deposition ofchemical wastes. The groundwater is extensivly contaminated inthe area of a disused chemical factory which once produced andapplied halogenated compounds on a large scale. At the researcharea in Bitterfeld, highly mobile chlorinated aliphatics andaromatics have been detected in both groundwater and sediments,especially in a lignite seam. The modern analytical method ofsolid-phase micro extraction (SPME) was successfully used toanalyse the pollution level of groundwater and sediments. Thelignite seam was found to have accumulated large quantities ofhalogenated hydrocarbons functioning as a giant adsorptionfilter, and hence recording past groundwater contamination.The vertical and lateral pollution profiles of sediments and thedistribution coefficients between lignite and groundwater atequilibrium conditions were determined for the key components.The consequences of the present situation for the progressivegroundwater quality of the quaternary aquifer are assessed in aforthcoming article. 相似文献
103.
木本植物群落具有遮阴和辐射消减的功能,可以缓解城市夏季的热岛效应。为定量研究城市公园木本植物群落的辐射消减效应,选择江门市城区公园24个木本植物景观单元,采用半球面影像技术获取林冠影像,分析模拟城市公园植被冠层结构和林下光照条件。结果表明:1)江门城区公园木本植物景观单元的叶面积指数(ILAI)处于0.88~2.22之间,低于自然环境植被IN,LAI水平;2)冠层对光照的辐射消减率随ILAI的增加而增大,ILAI与直射光消减率和散射光消减率均呈极显著正相关;3)城区公园树木景观单元林冠下直射光的变异程度强于林下散射光,林冠对直射光的影响大于散射光;4)城市公园林冠对直射光和散射光的消减作用极显著,月际变化呈单峰分布,以6~8月消减量最大。 相似文献
104.
北京地区高产小麦品种的冠层形成、光截获和产量 总被引:18,自引:2,他引:18
北京地区的小麦在平均亩产突破300公斤之后,要继续高产稳产,极需进一步地改善品种的产量潜力。本研究将叶面积指数(LAI)、光截获(LI)、光衰减系数(?)、叶面积持续时间(LAD)、干物质积累(DMA)、作物生长速度(CGR)等性状与冠层形成联系起来,研究它们与产量的关系。几年来,通过7个高产品种的14个冠层参数对籽粒产量的相关、通径 相似文献
105.
本文比较分析了三种x射线源(~(55)Fe、~(238)Pu和x光管)对木材进行微密度测定的精度,并对不同树种木材的质量衰减系数进行了测定和研究。 相似文献
106.
金属改性13x分子筛在木材阻燃中的抑烟减毒作用 总被引:2,自引:0,他引:2
抑烟、减毒是减少火灾中人员伤亡的重要途径。采用锥形量热法、热分析法和扫描电镜研究了铁和铜改性13x分子筛和聚磷酸铵复合阻燃木材的燃烧、烟气释放和成炭特性。结果表明:铁、铜改性分子筛与聚磷酸铵复合处理木材的总热释放量(THR)与空白样相比分别降低了36.8%、39.8%,总烟释放量(TSP)降低了69.3%、72.8%,CO平均产率(YCO)降低了40.2%、44.5%,均具有优异的阻燃、抑烟和减毒效果;热分析和电镜实验表明,APP的催化脱水作用有利于炭层形成,铁、铜改性分子筛与聚磷酸铵的协同作用使炭层结构紧密。APP对木材具有高效阻燃作用,但产生大量有毒气体,铁、铜改性分子筛与聚磷酸铵复合阻燃剂在高效阻燃的同时具有少烟低毒的特性。 相似文献
107.
108.
《Wood material science & engineering》2013,8(6):437-444
ABSTRACTThe estimation of the pixel-wise distribution of the moisture content (MC) in wood using X-ray computed tomography (CT) requires two scans of the same wood specimen at different MCs, one of which is known. Image-processing algorithms are needed to compensate for the anisotropic distortion that wood undergoes as it dries. An alternative technique based on dual-energy CT (DECT) to determine MC in wood has been suggested by several authors. The purpose of the present study was to evaluate the hypothesis that DECT can be used for the determination of MC in real time. A method based on the use of the quotient between the linear attenuation coefficients (μ) at different acceleration voltages (the so-called quotient method) was used. A statistical model was created to estimate the MC in solid sapwood of Scots pine, Norway spruce and brittle willow. The results show a regression model with R2?>?0.97 that can predict the MC in these species with a RMSE of prediction of 0.07, 0.04 and 0.11 (MC in decimal format) respectively and at MC levels ranging from the green to the totally dry condition. Individual measurements of MC show an uncertainty of up to ±0.4. It is concluded that under the conditions prevailing in this study, and in studies referred to in this paper, it is not possible to measure MC with DECT. 相似文献
109.
110.
To discuss the optical properties of water body in Xiaojiang river backwater area for the early impounding stage of the Three Gorges Reservoir (TGR), the spatiotemporal variations of visible light attenuation coefficients (Kd) and Secchi Disc transparency (SD) are investigated during the period from January, 2008 to December, 2008. The correlations between Kd and SD, total particulate matters (TPM), dissolved organic carbon (DOC) and chlorophyll-a (Chla) are also analyzed according to the monitoring data. In the study area, there are no significant spatial variations in the mean Kd at five sampling stations, but the seasonal variations are obvious with the highest value of Kd in the winter and the lowest value in the summer. The range of Kd is 0.23~4.82 m-1, and the mean value is (1.03±0.07) m-1. Statistics analysis suggests that there are remarkable correlations between Kd and SD, TPM, TIM as revealed by the coefficient of determination R2=0.779 1, R2 =0.728 5 and R2=0.763 7, respectively. The relationship between Kd and DOC is less significant as shown by the low value of R2=0.128 9. However, the correlation between Kd and Chla is weak which can be seen from the value of R2=0.006. The analysis suggests that total inorganic particulate matters largely influenced the optical properties of water body in Xiaojiang river backwater area. 相似文献