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   14050篇
  免费   623篇
  国内免费   1527篇
林业   1580篇
农学   1435篇
基础科学   587篇
  1553篇
综合类   6275篇
农作物   813篇
水产渔业   628篇
畜牧兽医   1575篇
园艺   987篇
植物保护   767篇
  2024年   57篇
  2023年   154篇
  2022年   320篇
  2021年   390篇
  2020年   447篇
  2019年   571篇
  2018年   460篇
  2017年   589篇
  2016年   754篇
  2015年   597篇
  2014年   666篇
  2013年   747篇
  2012年   934篇
  2011年   951篇
  2010年   826篇
  2009年   763篇
  2008年   726篇
  2007年   742篇
  2006年   638篇
  2005年   481篇
  2004年   373篇
  2003年   351篇
  2002年   283篇
  2001年   254篇
  2000年   242篇
  1999年   228篇
  1998年   205篇
  1997年   293篇
  1996年   257篇
  1995年   235篇
  1994年   267篇
  1993年   261篇
  1992年   291篇
  1991年   295篇
  1990年   247篇
  1989年   104篇
  1988年   57篇
  1987年   38篇
  1986年   23篇
  1985年   10篇
  1984年   11篇
  1982年   9篇
  1981年   7篇
  1980年   6篇
  1979年   8篇
  1978年   7篇
  1977年   7篇
  1976年   4篇
  1956年   5篇
  1955年   5篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
991.
由Cercospora sequoiae var.juniperi引起的杜松赤枯病在黑龙江省分布比较普遍。两季发病严重,能导致被害株死亡。6月中旬至7月上旬喷药。在双效灵、甲基托布津可湿性粉剂和多菌灵三种农药中,以50%多菌灵,用量为70ml/a时,防治效果最好。  相似文献   
992.
郭垂根  王清文 《林业研究》2007,18(3):203-207
MA-SEBS as compatibilizer and impact modifier was incorporated into Polypropylene/Wood Fiber (PP/WF) to enhance interface adhesion and impact strength of the composite. The effect of MA-SEBS content on the impact fracture behavior of PP/WF composites was studied. The impact properties of composites with 8% MA-SEBS reached the maximum value. And further increasing of MA-SEBS content to 10% did not improve the fracture toughness, but improved the stiffness of composites by DMA analysis. This was attributed to the improved PP/WF adhesion. As the MA-SEBS content is more than 8%, the molecule interaction of PP and WF was expected to much stronger than lower MA-SEBS. Scanning electron microscopy (SEM) was performed to analyze the impact fracture surface and showed a stronger affinity for the wood surfaces.  相似文献   
993.
A series of novel 4-O-alkoxytrityl chlorides (1) with different chain lengths was synthesized as a novel reagent for obtaining 6-O-alkylated cellulose with high regioselectivity via trityl groups in one reaction step without the use of any protective groups. These chlorides were reacted with methyl -d-glucoside, which was used as a model compound, to examine the reactivities toward the primary hydroxyl groups of cellulose to afford a series of 6-O-alkylated methyl -d-glucosides in high yields. The product compounds were found to have interesting solubilities and thermal properties. Thus, newly prepared trityl chloride derivatives were found to be useful regioselective derivatization reagents on the primary hydroxyl group in carbohydrates, especially in cellulose.  相似文献   
994.
二次调节静液储能拖拉机传动系统   总被引:1,自引:0,他引:1  
二次调节静液储能拖拉机传动系统是由柴油机,液压蓄能器,液压泵/马达(二次元件),差动变速器和驱动轴等组成,该系统充分利用了液压蓄能功率密度大,二次元件具有较高的控制质量,可实现四象限工作等特点;实现了柴油机和拖拉机行驶载荷的分离,工作过程中当外负载扭矩发生变化时,将引起二次元件的转速变化,从而引起二次元件排量的变化,通过调节液压马达的变量机构使其与工作负载所需转速相适应。  相似文献   
995.
Domestication of Irvingia gabonensis, a fruit tree grown in agroforestry systems in West and Central Africa, offers considerable scope for enhancing the nutritional and economic security of subsistence farmers in the region. Assessments of phenotypic variation in ten fruit, nut and kernel traits were made on twenty-four ripe fruits from 100 Irvingia gabonensis trees in Ugwuaji village in southeast Nigeria, a center of genetic diversity for this species. There were important differences between the young planted trees of this study and the older unplanted trees of a similar study in Cameroon. Significant and continuous tree-to-tree variation was found in fruit mass(69.0–419.8 g), flesh mass (59.5–388.8 g), nut mass (9.5–40.6 g), shell mass (4.9–30.9 g) and kernel mass (0.41–7.58 g); fruit length (49.2–89.3 mm) and width (46.2–100.5 mm) and flesh depth (12.9–31.4 mm), as well as considerable variation in flesh colour, skin colour, fruit taste and fibrosity. Some fruits were considerably bigger than those found in Cameroon. These quantitative results will help in the development of cultivars within participatory approaches to agroforestry tree domestication, and so promote poverty alleviation and sustainable agriculture. This revised version was published online in June 2006 with corrections to the Cover Date.  相似文献   
996.
杨木/无机硅化物复合材处理工艺初探   总被引:9,自引:0,他引:9  
刘磊  朱玮  赵砺 《木材工业》2001,15(3):8-11
用硅酸单体溶液(SAMS)处理及用硅酸钠-三氧化二硼-乙酸复合处理改性文县杨(Populus wenxianesis),制备杨木/无机硅化物复合材(WIC)。结果表明,在所选择的工艺条件下,SAMS处理的杨木WIC具有很好的尺寸稳定性和抗流失性,复合处理的杨木WIC的阻燃性能和防腐性能有了明显提高。  相似文献   
997.
应用热扩散式树干茎流计(TDP)对辽宁省章古台樟子松人工固沙林生长季期间树干液流速率进行了连续5年的测定,结合同步观测的降水、林冠截留量、林内蒸发散量和土壤水分含量,利用水量平衡的方法,推算了现阶段樟子松应有的合理密度。结果表明:4月上中旬沙地樟子松液流启动,10月中下旬液流基本结束,生长季(4-10月)各月平均液流速率分别为2.58、4.78、5.19、5.01、4.84、4.30、1.93 cm·h-1,以6月份的最高,其次依次为7、8、5、9月份,4、10两月平均液流速率较低。通过推算,径阶14、16、18、20 cm的樟子松纯林的合理密度分别应为688、531、422、344株·hm-2,现有同径阶的林分普遍存在密度过大的问题。研究结果对于现有林分的科学经营具有重要的指导意义。  相似文献   
998.
Our objective was to investigate the differences in the spectral trajectory of the LANDSAT Thematic Mapper with stand volume development between Japanese larch (Larix kaempferi) and Japanese oak (Quercus mongolica var. crispula) in the experimental forest of Kyushu University, Hokkaido, Japan. We used nonlinear regression and a categorical variable to investigate the relationship between spectral reflectance factor and stand volume for each forest type. This research showed that the reflectance factor of Japanese larch and Japanese oak reached saturation and that both forest types had different spectral trajectories with stand development. In addition, we found that middle of development stage (stand volume was between 100 and 200 m3/ha) of each forest type had similarity in reflectance factor.  相似文献   
999.
Sugi (Cryptomeria japonica D. Don) wood was treated with supercritical water (374°C, 22.1MPa), and fractionated into the water-soluble portion, the methanol-soluble portion, and the methanol-insoluble residue. The methanol-soluble portion mainly consisted of the lignin-derived products. To characterize the compounds in the methanol-soluble portion, gel permeation chromatographic (GPC) and gas chromatographic-mass spectrometric (GC-MS) analyses were performed. The GPC analysis indicated that the methanol-soluble portion contained lignin-derived monomeric and dimeric products. GC-MS analysis detected 31 products which were expected to be monomeric compounds, and 18 of these were identified to be guaiacol, methylguaiacol, ethylguaiacol, vinylguaiacol, eugenol, propylguaiacol, vanillin, cis-isoeugenol, homovanillin, trans-isoeugenol, acetoguaiacone, propioguaiacone, guaiacylacetone, 2-methoxy-4-(1-hydroxypropyl)phenol, homovanillic acid, 2-methoxy-4-(prop-1-en-3-one)phenol, coniferyl aldehyde, and ferulic acid. In addition, 22 dimeric products were detected, and 4 of these were believed to be compounds with biphenyl type (5-5), diphenylethane type (-1), stilbene type (-1), and phenylcoumaran type (-5) structures. These results clearly indicated that the methanol-soluble portion included various monomeric and dimeric compounds produced as a result of the cleavage of ether linkages and propyl chains of lignin.  相似文献   
1000.
蒋明亮 《林业科学》2001,37(4):107-110
百菌清(CTL)、氨浴季铵铜(ACQ)、氨溶柠檬酸铜(ACC)处理马尾松(Pinus massoniana)、毛白杨(Populus tomentosa)后对木材尺寸稳定性及MOE/MOR的影响本文进行了研究。结果表明,除剂量11.48kg/m^3的百菌清处理的毛白杨外,以上防腐剂处理试材后吸水至饱和时的尺寸稳定性与对照相比有不同程度的增加,幅度为3.9%-28.05;各处理对抗弯弹性模量(MOE)影响幅度不大,其中对于马尾松,MOE差异不明显,对于毛白杨,各处理对MOE稍有增加(百菌清乳剂处理除外),幅度6.3%-8.1之间。各制剂处理马尾松、毛白杨后对抗弯强度(MOR)影响不明显。  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