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   7200篇
  免费   1390篇
  国内免费   1674篇
测绘学   350篇
大气科学   1564篇
地球物理   1980篇
地质学   3448篇
海洋学   836篇
天文学   359篇
综合类   767篇
自然地理   960篇
  2024年   31篇
  2023年   122篇
  2022年   336篇
  2021年   438篇
  2020年   352篇
  2019年   390篇
  2018年   450篇
  2017年   412篇
  2016年   436篇
  2015年   353篇
  2014年   445篇
  2013年   409篇
  2012年   395篇
  2011年   428篇
  2010年   406篇
  2009年   450篇
  2008年   383篇
  2007年   350篇
  2006年   258篇
  2005年   255篇
  2004年   209篇
  2003年   196篇
  2002年   194篇
  2001年   205篇
  2000年   240篇
  1999年   324篇
  1998年   254篇
  1997年   252篇
  1996年   221篇
  1995年   202篇
  1994年   152篇
  1993年   167篇
  1992年   117篇
  1991年   87篇
  1990年   46篇
  1989年   72篇
  1988年   58篇
  1987年   30篇
  1986年   22篇
  1985年   18篇
  1984年   18篇
  1983年   9篇
  1982年   19篇
  1981年   9篇
  1980年   9篇
  1979年   4篇
  1978年   5篇
  1975年   2篇
  1974年   2篇
  1958年   13篇
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
41.
大黄鱼病原弧菌拮抗放线菌的筛选与人工诱变   总被引:4,自引:0,他引:4  
从厦门集美海滩底泥分离得到38株放线菌,用琼脂块法测定它们对大黄鱼病原弧菌-溶藻弧菌和副溶血弧菌的拮抗效果,选择两株拮抗效果较好的放线菌进行诱变,测定诱变后各菌株的抗菌效果,并选择抗菌效果较好的菌株进行第二次诱变,如此反复诱变3次,共得到97株放线菌,结果表明;38株放线菌中的22株对两株病原菌有一定的拮抗作用;用紫外线照射可以获得少量对两株病原菌拮抗作用加强的菌株。  相似文献   
42.
Researchonthespectralanalysisandtestmethodofperiodsignalsinmonthlymeansealevel¥MaJirui;TianSuzhen;ZhengWenzhenandChaiXinmin(R...  相似文献   
43.
The basis, process and results of the demonstration of the main dimensions of a 75000t floating production and storage vessel are discussed in this paper. A simple but reliable orthogonal design method is applied in the main dimension optimization. The ideas of gradual approximation and feedback from various aspects are put into effect. During the demonstration, in order to make the model tally with the actual situation, the draft design is closely related to the computational analysis, so that the demonstration model can be verified at any time; the handling of the overall system is closely related to the research of each item, which is beneficial not only to the mastery of various regularities, but also to the balance of decisions. Finally, according to the computational results and the regularities obtained from analysis, the main dimensions are determined.  相似文献   
44.
动荷载作用下海底粉土的孔压响应及其动强度   总被引:5,自引:0,他引:5  
本文选用近海分布较广的粉土为研究对象 ,利用室内动三轴试验结果 ,找出动荷载作用下粉土的动应力应变关系 ,分析模拟波浪荷载作用下粉土中的孔压响应、临界循环次数 ,确定波浪作用下粉土的应力状态、破坏临界循环次数 ,判断不同深度处的粉土发生液化的可能性及发生液化所需要的时间 ;研究粉土在动荷载作用下的强度降低 ,为海上工程设计和施工提供科学依据。  相似文献   
45.
Study on form characteristics of nitrogen in marine sediments is the primary method to research its biogeochemical cycling and nitrogen form characteristics in core sediments can reflect the process and results of early diagenesis in a certain degree. In this paper, Sequential extraction process in natural grain size was used for studying the existent forms of nitrogen in five core sediments of the southern Bohai Sea for the first time. Nitrogen was divided into two parts -transferable and fixed based on whether it could be extracted by the reagent. Distributions and early diagenesis of transferable nitrogen forms in the southern Bohai Sea were researched integratedly. Results indicate that IEF - N and OSF-N are predominant forms in transferable part in the studied core sediments. Contents of different nitrogen forms vary differently with depth, and have different diagenesis process. Decomposition constant of organic nitrogen (ON) and OC are about 15.51 × 10~(-3)a~(-1)and 4.79× 10~(-3)a~(-1) respective  相似文献   
46.
Temperature data at different layers of the past 45 years were studied and we found adiploe mode in the thermocline layer (DMT): anomalously cold sea temperature off the coast of Sumatra and warm sea temperature in the western Indian Ocean. First, we analyzed the temperature and the temperature anomaly (TA) along the equatorial Indian Ocean in different layers. This shows that stronger cold and warm TA signals appeared at subsurface than at the surface in the tropical Indian O-cean. This result shows that there may be a strong dipole mode pattern in the subsurface tropical Indian Ocean. Secondly we used Empirical Orthogonal Functions (EOF) to analyze the TA at thermocline layer. The first EOF pattern was a dipole mode pattern. Finally we analyzed the correlations between DMT and surface tropical dipole mode (SDM), DMT and Nino 3 SSTA, etc. and these correlations are strong.  相似文献   
47.
麻痹性贝毒(Paralytic Shellfish Poisoning,PSP)毒素由石房蛤毒素(saxitoxin,STX)及其衍生物组成,目前己发现20余种,在赤潮研究、分子生物学和神经生物学基础研究、医药、军事防化等方面都有应用潜力[其结构、类型和应用见本集刊王云峰等(2003)“麻痹性贝毒毒素的应用研究进展”一文]。由于PSP毒素的稀有来源和国际社会对STX交易的禁止,限制了国内PSP毒素应用及研究的全面深入展开,因此本文作者对PSP毒素进行了制备,并采用不同方法对制备的PSP毒素进行了研究。 PSP毒素能够选择性地可逆抑制可兴奋膜的电压依赖钠离子通道的开放,从而阻止神经冲动的发生和传导,使神经、肌肉丧失兴奋性(Frace et al.,1986;Penzotti et al.,1998)。本文利用神经束膜下记录和全细胞膜片钳技术,报道了从塔玛亚历山大藻(Alexandrium tamarense)中提取的PSP粗毒素对小鼠运动神经末梢膜电流和NG108-15细胞钠离子通道的作用研究结果,并与STX标准毒素的作用结果进行了比较。 NG108-15细胞是由小鼠神经母细胞瘤和大鼠胶质细胞瘤融合的杂交细胞,经分化剂分化后,显示诸+L1196如兴奋性、合成和释放乙酰胆碱、与培养肌细胞形成突触联系等多种神经细胞的基本特性(Hamprcht,1977)和Na+、K+、Ca2+等多种离子通道,已作为神经细胞模型被广泛应用于分析药物对离子通道作用的研究(Enomoto et al.,1992; Docherty et al.,1992;Shi et al.,1993;Hu et al.,1997a;Hu et al.,1997b);发育出具有Na+内流支持的锋电位(Hamprecht,1977),该电位是分析作用于膜钠离子通道药物的好材料。  相似文献   
48.
A combination of CTD casts, discrete bottle sampling and in situ voltammetric microelectrode profiling was used to examine changing redox conditions in the water column at a single station south of the Bay Bridge in the upper Chesapeake Bay in late July/early August, 2002–2005. Short-term (2–4 h) fluctuations in the oxic/suboxic/anoxic interface were documented using in situ voltammetric solid-state electrodes. Profiles of dissolved oxygen and sulfide revealed tidally-driven vertical fluctuations of several meters in the depth and thickness of the suboxic zone. Bottom water concentrations of sulfide, Mn2+ and Fe2+ also varied over the tidal cycle by approximately an order of magnitude. These data indicate that redox species concentrations at this site varied more due to physical processes than biogeochemical processes. Based on analysis of ADCP data, tidal currents at this station were strongly polarized, with the principal axis of tidal currents aligned with the mainstem channel. Together with the chemical data, the ADCP analysis suggests tidal flushing of anoxic bottom waters with suboxic water from north of the site. The present study is thus unique because while most previous studies have focused on processes across relatively stable redox interfaces, our data clearly demonstrate the influence of rapidly changing physical mixing processes on water column redox chemistry.Also noted during the study were interannual differences in maximum bottom water concentrations of sulfide, Mn2+ and Fe2+. In 2003, for example, heavy spring rains resulted in severe hypoxia/anoxia in June and early July. While reported storm-induced mixing in late July/early August 2003 partially alleviated the low-oxygen conditions, bottom water concentrations of sulfide, Mn2+ and Fe2+ were still much higher than in the previous year. The latter implies that the response time of the microbial community inhabiting the suboxic/anoxic bottom waters to changing redox conditions is slow compared to the time scale of episodic mixing events. Bottom water concentrations of the redox-sensitive chemical species should thus be useful as a tracer to infer prior hypoxic/anoxic conditions not apparent from ambient oxygen levels at the time of sampling.  相似文献   
49.
本文介绍一种声遥测海底平面层系声阻抗分布的模拟实验结果。实验在油漆厂冷却水池中进行,采用取样平均迭代的反演方法,对三介质层系的遥测结果与实测值较好地接近。  相似文献   
50.
贻贝(Mytilus edulis L.)是一种冷水性双壳类软体动物,它在我国沿岸分布的南限是胶州湾,但仅见于港内码头及船底上,且数量不多。 1958年青岛市水产局、原青岛市海水养殖场和中国科学院海洋研究所等单位曾一起自大连和烟台移贻贝苗至青岛试养,次年养殖架上也曾有少量幼苗附着,但未能大量繁殖起来。1971年后胶南县海水养殖试验场及青岛市第二海水养殖场等先后进行数十亩乃至数百亩移苗养殖,附苗量虽有所增长,但直至1974年仍缺乏生产意义。这种情况引起我们的思考:胶州湾贻贝苗源能否大量发展?它的限制因素是什么? 1972年至1973年,我们与烟台地区海水养殖试验场等单位曾对烟台沿岸贻贝苗源进行调查研究,确认当时烟台苗种产量不高的主要原因是附着基不足,其次是亲贝不足,据此试验成功了“废旧草绠采苗法”。之后,随着采苗器材的增加及养殖面积的不断扩大,烟台芝罘湾已发展成为一个较好的苗场,通常可供万亩以上生产用苗。这个经验对我们开发胶州湾的苗源很有启发。 1975年胶南县在胶州湾大力推广“废旧草绠采苗法”,收割海带后保留废旧海带架两万台采贻贝苗, 1976年进一步扩大贻贝养殖面积,继续留绠采苗。我们在青岛市水产局及胶南县水产局等单位的协助下,于1975年9月及1976年8-9月,对胶州湾内及湾口(青岛前海)留绠采苗的情况进行了调查,这些调查资料即为本文的主要依据。  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