首页 | 本学科首页   官方微博 | 高级检索  
文章检索
  按 检索   检索词:      
出版年份:   被引次数:   他引次数: 提示:输入*表示无穷大
  收费全文   2032篇
  免费   95篇
  国内免费   370篇
林业   265篇
农学   261篇
基础科学   170篇
  518篇
综合类   705篇
农作物   72篇
水产渔业   176篇
畜牧兽医   150篇
园艺   39篇
植物保护   141篇
  2024年   24篇
  2023年   62篇
  2022年   73篇
  2021年   82篇
  2020年   93篇
  2019年   98篇
  2018年   87篇
  2017年   104篇
  2016年   121篇
  2015年   116篇
  2014年   130篇
  2013年   195篇
  2012年   148篇
  2011年   142篇
  2010年   104篇
  2009年   85篇
  2008年   83篇
  2007年   95篇
  2006年   101篇
  2005年   84篇
  2004年   63篇
  2003年   39篇
  2002年   48篇
  2001年   38篇
  2000年   46篇
  1999年   30篇
  1998年   34篇
  1997年   28篇
  1996年   27篇
  1995年   18篇
  1994年   14篇
  1993年   15篇
  1992年   13篇
  1991年   12篇
  1990年   13篇
  1989年   14篇
  1988年   6篇
  1987年   2篇
  1986年   2篇
  1985年   1篇
  1984年   1篇
  1983年   3篇
  1982年   1篇
  1979年   1篇
  1978年   1篇
排序方式: 共有2497条查询结果,搜索用时 12 毫秒
51.
运用自动热脱附仪和气相色谱/质谱联用技术,通过对木制品中释放的多种挥发性有机物(VOCs)进行测试,得到较优的测试处理条件.研究发现,该方法的重现性和稳定性较好;较低的采样流速,可有效的降低苯在Tenax TA吸附管中的穿透行为.  相似文献   
52.
导热油为热介质的转子式刨花干燥机的热量计算   总被引:1,自引:1,他引:1  
阐述了利用导热油为热介质的转子式刨花干燥机优良的干燥性能,并对转子式刨花干燥机单位耗热量进行了计算,并根据耗热量计算出了转子的散热面积。为了便于导热油的传热和防止结焦现象,根据雷诺准则确定了导热油在管束内的流速。  相似文献   
53.
不同竹龄毛竹增强相的动态热机械研究   总被引:1,自引:0,他引:1  
利用动态热机械分析仪检测不同竹龄的毛竹,实验证明:在常温条件下,毛竹竹青的刚性随毛竹竹龄的增加而增加,当毛竹到了一定竹龄后,其竹青的刚性渐渐趋于稳定。一般5~6年生的成熟毛竹的竹青部分的常温存储模量位于1010Pa(10GPa)数量级以上,一般能满足做为风力发电叶片复合材料中增强相材料的使用。因此,最佳的风力发电叶片复合材料增强相材料是5~6年生的成熟毛竹。  相似文献   
54.
铅笔板高温热压改性技术研究   总被引:1,自引:0,他引:1  
为克服高温烤板处理的缺点 ,进行了椴木铅笔板热压干燥基本规律及温度、时间与含水率的关系研究和热压干燥的铅笔板与烤板的吸湿性、脆性对比试验。研究结果表明 :高温热压可使木材进行热分解 ,热压干燥后的铅笔板的吸湿性和脆性能达到烤板的要求 ,并在质量稳定性方面优于烤板 ,更重要的是它解决了烤板对环境的污染问题  相似文献   
55.
Forest soils are pivotal to understanding global carbon (C) cycling and evaluating policies for mitigating global change. However, they are very difficult to monitor because of the heterogeneity of soil characteristics, the difficulty of representative sampling, and the slow time scale of response to environmental change. Here we demonstrate that use of gamma-ray spectroscopy facilitates in situ non-destructive analysis of C and other elements in forest soils. In this approach the element-specific gamma-rays are induced by fast and thermal neutrons interacting with the nuclei of the elements present in the soil. Background gamma-rays emanating from naturally occurring radionuclides in the forest are recorded as well. We applied this approach in a mature northern hardwood forest on glacial till soils at the Bartlett Experimental Forest in New Hampshire, USA. The inelastic neutron scattering (INS) system yielded strong signals in gamma-ray counts/h, from C and other elements present in the soil matrix that included silicon, oxygen, hydrogen, iron, aluminum, manganese and potassium. The INS sensitivity for carbon was 20.656 counts h−1 kg−1 C m−2 based on current net C gamma-ray counts and the data for the O horizon and mineral soil to a depth of 30 cm obtained from a nearby quantitative soil pit (7.35 kg C m−2). We estimate the minimum detectable change to be ∼0.34 kg C m−2, which is ∼5% of the current soil C content, and the minimum detectable limit to be ∼0.23 kg C m−2. Eight % reproducibility from 11 measurements was limited, in part, by the large variability in the system counting geometry due to the uneven forest microtopography. The INS approach has the potential to revolutionize belowground monitoring of C and other elements, because the possibility of detecting a 5% change in forest soils has not been possible with destructive sampling methods.  相似文献   
56.
IntroductionCharcoal, produced by the paFtial pyrolysis of woodin an inert atmosphere, is a complex material of whichthe physical and chemical characteristics may varywidely. The properties may vary with the type of woodused, the part of the tree from which the wood is cut,as well as the pyrolysis regime tO which the wood issubjeCt6d. The aim of the present work is tO develop arange of t6sts for the charaCt6rization of compositionand reactivity of charcoals for use in gunpoWder.Compositiona…  相似文献   
57.
Abstract

Wood is being used heavily in single-family residential dwellings. Therefore, it is important to categorize their response when exposed to elevated temperatures for a sustained period of time. An important aspect of structural fire design is to assess postfire residual strength of existing structures. This study addresses this issue by developing models to predict strength degradation of wood after exposure to elevated temperature. The objectives were to (a) study the effect of exposure time on bending strength [Modulus of Rupture (MOR)] of wood at elevated temperatures, (b) interpret any relationships between different temperature and time of exposure using kinetics and a statistical approach, and (c) compare the two approaches. Two hundred thirty-two samples in total were tested in flexure as a function of exposure time and several temperatures. MOR of wood decreased as a function of temperature and exposure time. Rate of degradation was higher at higher temperatures. These results were fit to a simple kinetics model, based on the assumption of degradation kinetics following an Arrhenius activation energy model with apparent activation energy of 37.4 kJ/mol. A regression-based statistical model was also developed. The kinetics-based model fit the data better with one less parameter and predictions consistently matched the observed values, making the model preferred over the regression approach.  相似文献   
58.
Convenience products of fish that are heat processed in-pack are often subjected to a compromise between quality and durability within the framework of required safety. The purpose of this study was to provide knowledge for optimizing pasteurization of cod muscle by quantifying changes in water holding capacity (WHC), cook loss, color, and texture within a wide range of processing times and temperatures. A heat load equivalent to 2 min at 70°C may be applied to the cod while maintaining the WHC above 66%, the cook loss below 5.6%, and keeping hardness and whiteness low, provided that the processing temperature is kept in the range 68 ± 4°C.  相似文献   
59.
杉木采伐对集水区土壤热状况的影响   总被引:2,自引:0,他引:2  
康文星  闫文德 《林业科学》2003,39(5):156-160
土壤作为森林生态系统的子体系 ,是林木扎根并赖以生存的基础 ,也是土壤微生物一切生命活动的场所。因此 ,有关土壤养分、水分及其土壤物理结构特征等方面的研究报告颇多 (王政权等 ,2 0 0 0 ;张合平等 ,1 997)。但是有关森林土壤的热状况 ,以及林木采伐后对土壤热状况影响的研究报导很少 (康文星等 ,1 989;刘煊景等 ,1 993)。本文在获得采伐前连续 3年和采伐后连续 3年的观测数据的基础上 ,就杉木人工林生态系统土壤的热状况各因子之间 ,土壤各组成成分对热状况的单一或组合效应 ,以及皆伐这种人为强烈干扰对土壤热状况的影响进行了剖析。…  相似文献   
60.
Low temperature is one of the major limiting environmental factors which constitutes the growth, development,productivity and distribution of plants. Over the past several years, the proteins and genes associated with freezing resistance of plants have been widely studied. The recent progress of domestic and foreign research on plant antifreeze proteins and the identification and characterization of plant antifreeze protein genes, especially on expression regulatory mechanism of plant antifreeze proteins are reviewed in this paper. Finally, some unsolved problems and the trend of research in physiological functions and gene expression regulatory mechanism of plant antifreeze proteins are discussed.  相似文献   
设为首页 | 免责声明 | 关于勤云 | 加入收藏

Copyright©北京勤云科技发展有限公司  京ICP备09084417号