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   157篇
  免费   24篇
  国内免费   35篇
测绘学   8篇
大气科学   14篇
地球物理   32篇
地质学   64篇
海洋学   53篇
天文学   3篇
综合类   13篇
自然地理   29篇
  2023年   6篇
  2022年   2篇
  2021年   5篇
  2020年   4篇
  2019年   8篇
  2018年   4篇
  2017年   5篇
  2016年   8篇
  2015年   7篇
  2014年   6篇
  2013年   8篇
  2012年   5篇
  2011年   9篇
  2010年   5篇
  2009年   13篇
  2008年   3篇
  2007年   10篇
  2006年   10篇
  2005年   5篇
  2004年   13篇
  2003年   8篇
  2002年   5篇
  2001年   5篇
  2000年   5篇
  1999年   7篇
  1998年   3篇
  1997年   7篇
  1996年   8篇
  1995年   4篇
  1994年   6篇
  1993年   5篇
  1992年   4篇
  1991年   3篇
  1990年   5篇
  1988年   3篇
  1987年   2篇
排序方式: 共有216条查询结果,搜索用时 15 毫秒
1.
北方劳德藻配子和复大孢子的产生   总被引:8,自引:0,他引:8  
于1986年10月在厦门近岸采集北方劳德藻,从培养中观察其有性生殖和光对生殖的影响。结果表明,该藻只有宽度为26-40μm的细胞能形成配子和复大孢子;在光照条件下每昼夜只发生1次性化;雌雄配子以不同发育方式形成,配子发生呈现一定的时间程序;强光照和16h光周期最适于性化。由此得知,引起性化既依赖适当的细胞大小,又依赖光诱导的外部因素;产生两种精子数目是与精原孢囊的体积大小紧密相关;性化与光强度之间  相似文献   
2.
本实验进行了不同雌雄比对海带雌配子体发育的影响。结果表明:雌雄比(鲜重)为1:1及2:1,实验组出现排卵时间最早,且发育率高,实验第20天,发育率超过70%,孢子体正常。5:1,10:1,20:1实验组随着雄性比例的减少,发育率逐渐降低,随着培养天数的增加,三个实验组均出现未受精卵及畸形孢子体,雌雄比越大,未受精卵数目越多。  相似文献   
3.
三疣梭子蟹雌雄隔离养殖技术研究   总被引:6,自引:0,他引:6  
在193亩池塘中开展三疣梭子蟹雌雄分养和雌雄混养对比试验。其中雌雄分养面积93亩,亩产商品蟹182公斤,平均壳宽12.6厘米,平均个体重230.1克,成活率40.2%。与雌雄混养相比,亩产量提高了53.6%,成活率提高了1.51倍。专家鉴定认为,梭子蟹雌雄分养,方式新颖,效果明显,技术水平属国内先进。  相似文献   
4.
The pattern of climate change in the Southern Hemisphere during the Younger Dryas (YD) chronozone provides essential constraint on mechanisms of abrupt climate change only if accurate, high-precision chronologies are obtained. A climate reversal reported previously at Kaipo bog, New Zealand, had been dated between 13,600 and 12,600 cal yr B.P. and appeared to asynchronously overlap the YD chron, but the chronology, based on conventionally radiocarbon-dated bulk sediment samples, left the precise timing questionable. We report a new high-resolution AMS 14C chronology for the Kaipo record that confirms the original chronology and provides further evidence for a mid-latitude Southern Ocean cooling event dated between 13,800 and 12,400 cal yr B.P. (2σ range), roughly equivalent to the Antarctic Cold Reversal.  相似文献   
5.
Deformation-induced garnet zoning   总被引:1,自引:0,他引:1  
Hyeong Soo Kim   《Gondwana Research》2006,10(3-4):379-388
Compositional zoning patterns in garnet porphyroblasts from kyanite-bearing samples of the Devonian Littleton Formation, north-central Massachusetts, reveal complex patterns of growth that are related to multiple deformation and metamorphic events. Garnet porphyroblasts exhibit asymmetrical and irregular zoning patterns in XMn, XCa and Fe/(Fe + Mg). Zoning reversals in Mn and Fe/(Fe + Mg) and patch distribution in Ca appear to occur around the boundaries of the textural zones. Also, the compositions of the garnet at the textural boundaries are variable for all traverses. These observations suggest that the garnet zoning was not only modified from diffusion processes, but was also influenced by pre-existing microfabrics through the effects of preferential dissolution and resorption in partial disequilibrium. Relationships between chemical and textural truncations indicate that the zoning patterns of garnet were strongly modified from preferential dissolution and precipitation during the development of successive foliations that occurred in zones of high strain/stress (cleavage seams) and zones of low strain/stress, respectively.  相似文献   
6.
A new genus and species of extinct rove beetle, Procileoporus burmiticus gen. and sp. n., is described from Upper Cretaceous Burmese amber (Cenomanian; ca. 99 Ma). Due to the condition of the fossil, parts of the body could not be examined, but sufficient characteristics were visible to allow the specimen to be placed within the subfamily Tachyporinae (tribe Tachyporini). Procileoporus is most similar to the extant genus Cileoporus Campbell in having a slender abdomen, but it differs in having much shorter abdominal setae, prominent striated microreticulation on the elytra, non-lobed tergite VIII, and in other details of the male genital segments. This is the first tachyporine rove beetle from Burmese amber and the oldest record of the subfamily in amber. The evolution of sexual dimorphism in the genital segments is briefly discussed.  相似文献   
7.
17β-雌二醇对长毛对虾生长的影响   总被引:4,自引:0,他引:4  
吴仲庆 《海洋科学》1990,14(2):53-56
  相似文献   
8.
Reliable,with high data rate,acoustic communication in time-varying,multipath shallow water environment is a hot research topic recently.Passive time reversal communication has shown promising results in improvement of the system performance.In multiuser environment,the system performance is significantly degraded due to the interference among different users.Passive time reversal can reduce such interference by minimizing the cross-correlated version of channel impulse response among users,which can be realized by the well-separated users in depth.But this method also has its shortcomings,even with the absence of relative motion,the minimization sometimes may be impossible because of the time-varying environment.Therefore in order to avoid the limitation of minimizing the cross-correlated channel function,an approach of passive time reversal based on space-time block coding (STBC) is presented in this paper.In addition,a single channel equalizer is used as a post processing technique to reduce the residual symbol interference.Experimental results at 13 kHz with 2 kHz bandwidth demonstrate that this method has better performance to decrease bit error rate and improve signal to noise ratio,compared with passive time reversal alone or passive time reversal combined with equalization.  相似文献   
9.
柱撑蛭石吸附去除废水中重金属离子的实验研究   总被引:2,自引:0,他引:2  
刘云  吴平霄  党志 《矿物岩石》2006,26(4):8-13
分别利用有机柱化剂十二烷基磺酸钠(SDS)和无机柱化剂聚羟基铝(HA)对蛭石进行柱撑制得十二烷基磺酸钠柱撑蛭石(SDS-V erm icu lite)和聚羟基铝柱撑蛭石(HA-V erm icu lite),并通过XRD、红外光谱、ZETA电位等表征手段对柱撑蛭石进行表征,同时针对柱撑蛭石对Cu2 ,Cd2 ,C r3 3种重金属离子的吸附进行研究,结果表明:吸附去除率受反应时间、重金属离子的初始浓度、pH值等因素的影响,经柱撑后的蛭石对重金属离子吸附的吸附性能比蛭石原矿要强。柱撑蛭石吸附3种重金属离子的动力学吸附过程可用E lov icb方程和双常数方程进行较好的拟合。  相似文献   
10.
The properties of rock resitivity were studied under pressure, particularly with “stress reversal”, a procedure in which the pressure applied was increased and decreased. It was observed that, 1) With pressure increasing, the main feature of resistivity change was increase-steady-decrease for high-saturation rock samples (saturation 70–100%). But the main feature for low-saturation samples was different. 2) In 10 out of 11 cases of “stress reversal” for high-saturation samples the resistivity droped (about 2%). Such drop could explain the anomalies in geoelectricity terms, which are commonly observed before earthquakes in China. 3) It was also observed shortly before rock failure that, a) the resistivity drops more dramatically (about 20%) during “stress reversal” period, which is much more than ordinary drops. b) these drops occurred not only during stress decrease but also during stress increase. c) Resistivity exhibits anisotropy: the resistivity along different directions may differ by 10%. These three features may indicate that the rock is nearing failure, while ordinary resistivity drops are only connected with “stress reversal” and may not mean the imminence of rock failure. 4) Resistivity increase was observed during the “stress reversal” period for low-saturation rock samples. The results mentioned above were explained with the effect of water flowing in and out of the cracks of rock. The temporary factors which yield a reduction of the maximum main stress, may enhence the possibility of earthquake occurrence.  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