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   121766篇
  免费   3002篇
  国内免费   3755篇
  2024年   70篇
  2023年   435篇
  2022年   965篇
  2021年   1614篇
  2020年   1080篇
  2019年   1356篇
  2018年   12811篇
  2017年   11432篇
  2016年   8601篇
  2015年   2468篇
  2014年   2501篇
  2013年   2682篇
  2012年   6919篇
  2011年   15138篇
  2010年   13436篇
  2009年   9558篇
  2008年   11293篇
  2007年   12643篇
  2006年   1580篇
  2005年   1607篇
  2004年   1951篇
  2003年   1923篇
  2002年   1616篇
  2001年   854篇
  2000年   651篇
  1999年   456篇
  1998年   286篇
  1997年   253篇
  1996年   233篇
  1995年   184篇
  1994年   235篇
  1993年   173篇
  1992年   165篇
  1991年   144篇
  1990年   113篇
  1989年   97篇
  1988年   102篇
  1987年   66篇
  1986年   50篇
  1985年   50篇
  1984年   42篇
  1983年   44篇
  1982年   25篇
  1975年   6篇
  1972年   247篇
  1971年   276篇
  1965年   13篇
  1962年   24篇
  1944年   12篇
  1940年   10篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
991.
Conflicts between humans and wildlife, especially wild boar (Sus scrofa), have caused serious problems across the world in recent years. It is necessary to effectively control wild boar agricultural damage that may be influenced by many factors. In this study, we collected data on agricultural damage caused by wild boars from November 2009 to October 2010 using field surveys and social interviews in Taohongling National Nature Reserve of Jiangxi Province, China. We constructed models using binary logistic regression analysis to predict damage risks and to identify the factors influencing damage risks. About 8.1 % of croplands were damaged by wild boars, and the damage to rice, cotton, and other crops were not distributed based on their respective availability as shown by the result of a chi-square goodness-of-fit test. Five factors (Japanese silvergrass, soil conditions, terrain, distances to settlements, and water sources) were explained in a model based on damage area (area-based model) with the prediction accuracy being 72.1 %. In addition to these five factors, one additional factor (i.e. distance to forest edge) was retained in a model based on damage frequency (frequency-based model) with the prediction accuracy being 83.1 %. Caution is needed when we apply these two models to predict boar damage to crops, and it is recommended that both models be used in combination to predict the damage probabilities more accurately.  相似文献   
992.
Bulk heterojunction solar cells containing molybdenum oxide hole extracting anode contacts have been fabricated with varying stoicheometry using radio frequency reactive sputtering from a Molybdenum metal target. A blend of the newly synthesised conjugated polymer poly[9‐(heptadecan‐9‐yl)‐9H‐carbazole‐2,7‐diyl‐alt‐(5,6‐bis(octyloxy)‐4,7‐di(thiophen‐2‐yl)benzo[c][1,2,5]thiadiazole)‐5,5‐diyl] (PCDTBT‐8) and fullerene [6,6]‐Phenyl‐C71‐butyric acid methyl ester (PC70BM) was used as the photoactive layer and device results show that anodes with greater than 98% Molybdenum (VI) oxide result in peak power conversion efficiencies of 3.7%.The presence of up to 28% of Mo (V) results in no significant reduction in efficiency, however the presence of metallic Mo (IV) and lower oxidation states lead to severe reductions in device performance due to a combination of a large hole extraction energy barrier of approximately 0.9eV and reduced device stability.  相似文献   
993.
994.
995.
High-purity fructooligosaccharides (FOS) were produced from sucrose by an innovative process incorporating immobilized Aspergillus japonicus and Pichia heimii cells. Intracellular FTase of A. japonicus converted sucrose into FOS and glucose, and P. heimii fermented glucose mainly into ethanol. The continuous production of FOS was carried out using a tanks-in-series bioreactor consisting of three stirred tanks. When a solution composed of 1 g L?1 yeast extract and 300 g L?1 sucrose was fed continuously to the bioreactor at a dilution rate of 0.1 h?1, FOS at a purity of up to 98.2 % could be achieved and the value-added byproduct ethanol at 79.6 g L?1 was also obtained. One gram of sucrose yielded 0.62 g FOS and 0.27 g ethanol. This immobilized dual-cell system was effective for continuous production of high-purity FOS and ethanol for as long as 10 days.  相似文献   
996.
A convenient and promising alternative to surface modification of carbon mesh anode was fulfilled by electrochemical oxidation in the electrolyte of nitric acid or ammonium nitrate at ambient temperature. It was confirmed that such an anode modification method was low cost and effective not only in improving the efficiency of power generation in microbial fuel cells (MFCs) for synthetic wastewater treatment, but also helping to reduce the period for MFCs start-up. The MFCs with anode modification in electrolyte of nitric acid performed the best, achieving a Coulombic efficiency enhancement of 71 %. As characterized, the electrochemical modification resulted in the decrease of the anode potential and internal resistance but the increase of current response and nitrogen-containing and oxygen-containing functional groups on the carbon surface, which might contribute to the enhancement on the performances of MFCs.  相似文献   
997.
In our previous study, we used a comparative proteomic approach based on 2DE to profile dynamic proteomes of cotton fibers and found 235 protein spots differentially expressed during the elongation process ranging from 5 to 25 days post‐anthesis. Of them, only 106 differentially expressed proteins (DEPs) were identified by MS due to database limitations at the time. In the present work, we successfully identified the remaining 129 DEPs from the same experimental system using high‐resolution MS with an updated database. Bioinformatic analysis revealed that proteins involved in carbohydrate and protein metabolism, transport, and redox homeostasis are the most abundant, and glycolysis was found to be the most significantly regulated process during fiber elongation. Our high‐confidence reference dataset, composed of 235 DEPs, provides a valuable resource for future studies on the molecular mechanism of cotton fiber elongation.  相似文献   
998.
通过对甘农3号、甘农5号和游客紫花苜蓿多元杂交后代选育的36个株系及其亲本的生长、产量、品质等相关指标的测定,采用灰色关联度理论,构造综合评价模型进行供试材料综合评价,筛选出速生12#、速生11#株系为最理想的优良株系,生长高度分别为105.44cm、105.42cm;生长速度分别为1.74cm/d、1.68cm/d;茎叶比分别为0.30、0.35;分枝数分别为23、17;鲜草产量分别为39.99 t/hm2、35.13 t/hm2;粗蛋白含量分别为19.95%、23.89%;相对饲用价值为153.15%、157.02%。多叶2#、速生5#、速生20#、速生21#等4个株系为较理想的优良株系。  相似文献   
999.
西瓜DUS测试标准品种SSR指纹图谱构建及应用   总被引:6,自引:0,他引:6  
本研究采用代表最大限度西瓜遗传多样性的SSR核心引物组合,分析了西瓜DUS测试指南中的24份标准品种遗传多样性与核酸指纹。以基于重测序获得的SNP标记构建的17份西瓜材料的系统发育树为参照,对24份标准品种进行了遗传多样性分析,在遗传相似系数0.80处将24份标准品种分为3大类群,分析表明:核酸指纹分类比传统形态学分类更为准确。采用二维(QR)编码构建了西瓜24份标准品种的SSR指纹图谱,并利用本技术以保护品种“京欣2号”与对照品种“京欣1号”为例,进行了DUS分子鉴定测试,共扩增32个SSR位点,“京欣1号”和“京欣2号”之间存在4个位点的差异,品种间遗传相似系数为0.89,比形态学鉴定的差异位点更多且更准。本研究建立的西瓜DUS标准品种SSR指纹图谱与分子检测技术,可以应用到西瓜品种DUS分子检测实践,同时也为西瓜品种纯度与真实性鉴定及遗传背景分析提供了技术方案。  相似文献   
1000.
Cellular redox state is regulated by numerous components. The thiol-disulfide compound, glutathione, is considered to be one of the most significant, owing to its antioxidant power and potential influence over protein structure and function. While signaling roles for glutathione in plants have been suggested for several years, hard proof is scarce. Recently, through an approach based on genetic manipulation of glutathione in an oxidative stress background, we reported evidence that glutathione status is important to allow intracellular oxidation to activate pathogenesis-related phytohormone signaling pathways. This effect does not seem to be caused by changes in glutathione antioxidant capacity, and appears to be distinct to regulation through known players in pathogenesis responses, such as NPR1. Our data therefore suggest that new glutathione-dependent components that link oxidative stress to response outputs await discovery.  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