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   23043篇
  免费   754篇
  国内免费   1595篇
系统科学   1301篇
丛书文集   655篇
教育与普及   452篇
理论与方法论   56篇
现状及发展   167篇
研究方法   59篇
综合类   22696篇
自然研究   6篇
  2024年   103篇
  2023年   207篇
  2022年   356篇
  2021年   448篇
  2020年   263篇
  2019年   149篇
  2018年   250篇
  2017年   295篇
  2016年   327篇
  2015年   605篇
  2014年   888篇
  2013年   904篇
  2012年   1018篇
  2011年   1184篇
  2010年   1215篇
  2009年   1242篇
  2008年   1435篇
  2007年   1400篇
  2006年   1183篇
  2005年   1144篇
  2004年   889篇
  2003年   867篇
  2002年   1026篇
  2001年   984篇
  2000年   784篇
  1999年   908篇
  1998年   723篇
  1997年   743篇
  1996年   666篇
  1995年   544篇
  1994年   490篇
  1993年   394篇
  1992年   366篇
  1991年   353篇
  1990年   271篇
  1989年   272篇
  1988年   207篇
  1987年   148篇
  1986年   72篇
  1985年   19篇
  1984年   10篇
  1982年   3篇
  1981年   4篇
  1980年   3篇
  1976年   3篇
  1973年   3篇
  1972年   3篇
  1971年   5篇
  1969年   5篇
  1957年   2篇
排序方式: 共有10000条查询结果,搜索用时 46 毫秒
191.
《福建植物志》补遗   总被引:1,自引:0,他引:1  
本文介绍了顺昌县生物多样性科学考察队于1995—1996年间在顺昌调查采集中发现有的种类在《福建植物志》尚未收载。本文将其归入21科28属,并列出中名、学名、原始文献、异名、主要生境及分布,以供参考。  相似文献   
192.
The relaxation of the highly vibrationally excited CO (v = 1–8) by CO2 is studied by timeresolved Fourier transform infrared emission spectroscopy (TR FTIR). 193 nm laser photolysis of the mixture of CHBr3 with O2 generates the highly vibrationally excited CO(v) molecules. TR FTIR records the intense infrared emission of CO(v→v-1). The vibrational populations of each level of CO(v) have been determined by the method of spectral simulation. Based on the evolution of the time resolved populations and the differential method, 8 energy transfer rate constants of CO(v = 1–8) to CO2 molecules areobtained: (5.7±0.1), (5.9±0.1), (5.2±0.2), (3.4±0.2), (2.4±0.3), (2.2±0.4), (2.0±0.4) and (1.8±0.6) (1014 cm3 · molecule−1 · s−1), respectively. A two-channel energy transfer model can explain the feature of the quenching of CO(v) by CO2. For the lower vibrational states of CO, the vibrational energy transfers preferentially to the u3 mode of CO2 For the higher levels, the major quenching channel changes to the vibrational energy exchange between CO(v→v-1) and the u1 mode of CO2.  相似文献   
193.
Two national stalagmite standard (GBW04412, GBW04413) and one international coral standard (RKM_4) have been determined by using thermal ionization mass spectrometry (TIMS) method. The values of ( 234 U/ 238 U) act , ( 230 Th/ 234 U) act and age are all consistent in error range with the standard values obtained by using α spectrometry.  相似文献   
194.
The187Os/188Os ratios of spinel lherzolite xenoliths from Panshishan determined with N-TIMS are lower than that of the primitive mantle, which shows depleted mantle characteristics. Their positive correlation with Al2O3/MgO suggests that the geochemical behavior of trace elements Re and Os is similar to that of Al2O3 and MgO respectively during the magmatic evolution and Re-Os isotopic system is largely immune to mantle metasomatism. A model age of 2.8–3.4 Ga obtained by187Os/188Os-Al2O3/ MgO correlation might represent the homogeneous age of the mantle lithosphere beneath the area.  相似文献   
195.
Chronology of environmental events over East Asia during the past 130 ka   总被引:3,自引:0,他引:3  
In the transitional region between the Loess Plateau and deserts, 4 sections located respectively at Yuling, Hengshan, Dingbian and Huining of China are studied. The high-resolution paleoclimatic records obtained show that environmental events on varying time scales have happened over East Asia. These events are dated with the time scales established using the recently-developed approach of Automatic Orbital Tuning. This leads to the establishment of the chronology of the environmental events.  相似文献   
196.
The effects of calmoddin (CaM) antagonist W7-agarose, anti-CaM serum and exogenous purified CaM on pollen germination and tube growth ofForsythia suspensu were studied. The pollen germination and tube growth were inhibited or completely stopped by CaM antagonist W7-agarose. The addition of exogenous purified CaM stimulated pollen germination and tube growth, whereas the same amount of bovine serum albumin (BSA) had no effect. The inhibitory effects caused by W7-agarose and anti-CaM serum could be reversed completely by the addition of exogenous purified CaM. The tube growth of germinated pollen was also inhibited or completely stopped by W7-agarose. The results suggest that endogenous extracellular CaM initiates pollen germination and tube growth, whereas exogenous CaM enhances the above processes.  相似文献   
197.
Having reviewed the major classification systems proposed by various scholars across the world, it is found thatindica andjaponica underO. sativa L. are two major directions thoroughly differentiated from the Asian cultivated rice, forming the framework of the classification structure. A system withindica andjaponica as the only two subspecies is therefore reiterated. There are various ways to determine the indica-japonica identity of hybrid rice, but the “combined morphological trait index” (CMT index) method is more efficient and easier to handle, although the isozyme analysis, molecular marker analysis and grain quality assay methods are also feasible.  相似文献   
198.
以沥青砂浆材料为例,通过薄板的蠕变断裂及单轴拉伸蠕变的试验结果和数据分析,推求出沥青砂浆在不同温度下的断裂特性参数Γ和σmI1。  相似文献   
199.
研究用稳态平板法测不良导体导热率时存在的系统和方法误差,提出了误差修正方法。  相似文献   
200.
The nuclear capsid protein gene (vp39) ofBombyx mori nuclear polyhedrosis virus (BmNPV) was amplified successfully by PCR technique and inserted into pGEM 3zf(+). The 5′ and 3′ terminal area of the amplified vp39 gene were sequenced with silver-staining dideoxy method. Bmvp39 gene was sub-cloned into the expression vector pRSET-A, and transformed intoE. coli BL21. This gene was highly expressed by IPTG induction. SDS-PAGE analysis showed that the expressed protein is about 38 kd, and the expressed amount reached maxium in 4 h with IPTG induction. Supported by the National Natural science Foundation of China and the Doctoral Foundation of Edn carto Committee Liu Deli: born in 1954, Doctoral Candidate from Huazhong Normal University To whom correspondence should be addressed: (027-7882712-2938)  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