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   10434篇
  免费   797篇
  国内免费   944篇
  12175篇
  2024年   25篇
  2023年   158篇
  2022年   327篇
  2021年   573篇
  2020年   353篇
  2019年   505篇
  2018年   453篇
  2017年   279篇
  2016年   485篇
  2015年   693篇
  2014年   809篇
  2013年   914篇
  2012年   1009篇
  2011年   877篇
  2010年   502篇
  2009年   485篇
  2008年   508篇
  2007年   454篇
  2006年   411篇
  2005年   309篇
  2004年   281篇
  2003年   244篇
  2002年   175篇
  2001年   158篇
  2000年   133篇
  1999年   121篇
  1998年   121篇
  1997年   103篇
  1996年   88篇
  1995年   84篇
  1994年   72篇
  1993年   53篇
  1992年   83篇
  1991年   59篇
  1990年   38篇
  1989年   43篇
  1988年   23篇
  1987年   25篇
  1986年   25篇
  1985年   25篇
  1984年   11篇
  1983年   9篇
  1982年   10篇
  1981年   6篇
  1980年   5篇
  1979年   9篇
  1976年   4篇
  1973年   5篇
  1971年   4篇
  1968年   6篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
41.
The power conversion efficiencies (PCEs) of all-polymer solar cells (all-PSCs) have already exceeded 17%. However, the limited absorption range of an all-polymer system results in significantly reduced short-circuit current density (Jsc), which eventually influences the PCE improvement. To broaden the light absorption of polymer acceptors, herein, benzotriazole is introduced in the core unit of small molecule acceptors and thus two narrow-bandgap polymer acceptors named PTz-BO and PTz-C11 featuring the same molecular backbone and different side-chain length are synthesized. Compared with PTz-C11, the PTz-BO based-all PSCs deliver a slightly reduced Jsc, a large open-circuit voltage (Voc) and a low voltage loss below 0.50 V. Moreover, ternary all-PSCs are constructed by introducing PTz-C11 as a guest component. Benefiting from the reduced recombination, improved exciton generation and dissociation, and balanced charge transport, a high efficiency of 16.58% is obtained for the ternary all-PSCs, with a high Jsc over 25 mA cm−2 without sacrificing the Voc. Such result represents the highest efficiency reported for benzotriazole-based all-PSCs in the literature thus far. This work demonstrates the great potential of benzotriazole for the synthesis of efficient narrow-bandgap polymer acceptors.  相似文献   
42.
The unscientific application of synthetic pesticides has brought various negative effects on the environment, hindering the sustainable development of agriculture. Nanoparticles can be applied as carriers to improve pesticide delivery, showing great potential in the development of pesticide formulation in recent years. Herein, a star polymer (SPc) was constructed as an efficient pesticide nanocarrier/adjuvant that could spontaneously assemble with thiocyclam or monosultap into a complex, through hydrophobic association and hydrogen bonding, respectively, with the pesticide-loading contents of 42.54% and 19.3%. This complexation reduced the particle sizes of thiocyclam from 543.54 to 52.74 nm for pure thiocyclam, and 3 814.16 to 1 185.89 nm for commercial preparation (cp) of thiocyclam. Interestingly, the introduction of SPc decreased the contact angles of both pure and cp thiocyclam on plant leaves, and increased the plant uptake of cp thiocyclam to 2.4–1.9 times of that without SPc. Meanwhile, the SPc could promote the bioactivity of pure/cp thiocyclam against green peach aphids through leaf dipping method and root application. For leaf dipping method, the 50% lethal concentration decreased from 0.532 to 0.221 g/L after the complexation of pure thiocyclam with SPc, and that decreased from 0.390 to 0.251 g/L for cp thiocyclam. SPc seems a promising adjuvant for nanometerization of both pure and cp insecticides, which is beneficial for improving the delivery efficiency and utilization rate of pesticides.  相似文献   
43.
渗透胁迫对高粱根K~+吸收的影响   总被引:4,自引:0,他引:4  
以CaSO40.5mmolL-1溶液培养高粱得到低钾植株,在渗透胁迫下当PEG-1000使外界渗透势下降时,刺激了高粱根高K+亲和系统的净K+积累,其表观Km值和Tmax值分别为18μmolL-1和49.6μmolg-1DWh-1。此高亲和系统不受可渗物质乙二醇和外界pH变化的影响,为蛋白质合成抑制剂环己酰亚胺抑制。  相似文献   
44.
悬钩子属种质的评价   总被引:30,自引:0,他引:30  
在进行了7省悬钩子资源调查的基础上,在南京建立了悬钩子属田间基因库。3年来对田间基因库内保存并开花结果的30个种进行了开花结果性状的记栽和评价。内容包括果实特征,糖、酸、维生素类、氨基酸、矿质元素含量分析,染色体计数等。分析了种间和种内多样性物存在及在良种选育中利用的可能性。  相似文献   
45.
尹长民  赵敬钊 《蛛形学报》1996,5(2):117-119
报道我国狼蛛属1新种:二监狼蛛Lycosa erjianensis sp.nov.。  相似文献   
46.
神经节苷脂GM_3对小鼠腹腔常驻巨噬细胞(R-M)和Ge-132体内激活的巨噬细胞(Ge-132-M)的磷脂代谢转换有显著的影响,当这两种M在体外用GM_3处理时,表现出[ ̄(32)P]Pi和[ ̄3H]肌醇参入PI降低,参入PIP、PIP_2增加;但在[ ̄(32)P]Pi和[ ̄3H]胆碱参入PC上,R-M与Ge-132-M不同,即GM_3促进同位素前体参入R-M的PC,抑制它们参入Ge-132-M的PC.以上结果表明GM3可能提高了PI或PIP的磷酸激酶的活性,致使[ ̄(32)P]PIP和[ ̄(32)P]PIP_2增多,[ ̄(32)P]PI减少.激活的M(Ge-132-M)本身PC代谢转换率较R-M高,当Ge-132-M再受GM_3刺激,PC代谢转换率降低,这提示GM_3对激活的M的PC代谢转换有调节作用.  相似文献   
47.
Summary Xanthate floatation compounds, added to cultures of Thiobacillus ferrooxidans leaching chalcopyrite, inhibited growth in the concentration range 1–10 mM and caused a drop in formation of soluble copper and iron. Maximum (80–90%) growth inhibition was at 10 mM for isobutyl-, amyl- and ethyl xanthate while isopropyl xanthate was innocuous but caused a four-fold increase in lag phase. Copper production was depressed by 30% for isopropyl-, 53% for isobutyl-, ethyl- and 77% for amyl xanthate at 10 mM additions.  相似文献   
48.
原生质体电融合酵母多倍体生理特性的研究   总被引:5,自引:0,他引:5  
对原生质体电融合技术获得的11株酵母多倍体融合株,进行了一系列生理特性的分析比较.结果发现,在电融合过程中,融合株的细胞体积及DNA含量并不随着核倍性呈线性递增.而是有其特殊的变化规律。当糖化酵母sta1、sta2和sta3在细胞核中各自纯合时,明显表现出表达的剂量效应。其中sta1、sta2纯合的融合株.在YEPS培养中,GA分泌可被淀粉大量诱导,表现出一定的二次生长特性。从分析结果推测,在亲株5301-14D及HU-TY—1A中可能有对GA分泌不利的因素。  相似文献   
49.
Applying a double-labelling immunofluorescence technique, six types of substance P-containing nerve fibres were distinguished in the human adrenal gland according to the immunohistochemical colocalization of (I) calcitonin gene-related peptide (CGRP), (II) cholecystokinin, (III) nitric oxide synthase, (IV) dynorphin, (V) somatostatin, and (VI) vasoactive intestinal polypeptide. Fibre populations I to IV in their mediator content resembled the respective subpopulations of primary sensory neurons in human thoracic dorsal root ganglia, while populations V and VI revealed no correspondence with dorsal root neurochemical coding. Nerve fibres with the combination substance P/nitric oxide synthase occurred only in the adrenal cortex, whereas all other fibre types were present in both cortex and medulla. As revealed by immuno-electron microscopy, substance P-immunolabelled axon varicosities (a) exhibited synaptic contacts with medullary chromaffin cells or with neuronal dendrites, (b) were directly apposed to cortical steroid cells and (c) were separated from fenestrated capillaries only by the interstitial space. These findings provide immunochemical support for an assumed sensory innervation of the human adrenal gland, and additionally suggest participation of substance P in efferent autonomic pathways. Furthermore, the results are indicative for a differentiated involvement of substance P in the direct and indirect regulation of neuroneuronal and neuroendocrine interactions.  相似文献   
50.
蓟属两种植物的染色体研究   总被引:2,自引:0,他引:2  
鄢本厚  尹祖棠   《广西植物》1995,15(2):172-175
本文对蓟属(Cirsium)的两个形态相似的近缘种大刺儿菜和小刺儿菜进行了染色体研究,其中后者为首次报道。观察结果表明:两个种的染色体数目均为2n=2x=34:它们的核型是:大刺儿菜.2n=2x=34=20m+12sm+2st:小刺儿菜.2n=2x=34=22m+10km(2SAT)+2st。通过核型比较,认为它们是两个独立的种.而且后者比前者进化。  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