全文获取类型
收费全文 | 8491篇 |
免费 | 1780篇 |
国内免费 | 2299篇 |
专业分类
测绘学 | 717篇 |
大气科学 | 1694篇 |
地球物理 | 2316篇 |
地质学 | 4425篇 |
海洋学 | 1092篇 |
天文学 | 394篇 |
综合类 | 839篇 |
自然地理 | 1093篇 |
出版年
2024年 | 47篇 |
2023年 | 139篇 |
2022年 | 455篇 |
2021年 | 505篇 |
2020年 | 390篇 |
2019年 | 513篇 |
2018年 | 520篇 |
2017年 | 505篇 |
2016年 | 562篇 |
2015年 | 497篇 |
2014年 | 549篇 |
2013年 | 559篇 |
2012年 | 548篇 |
2011年 | 514篇 |
2010年 | 491篇 |
2009年 | 488篇 |
2008年 | 486篇 |
2007年 | 432篇 |
2006年 | 313篇 |
2005年 | 267篇 |
2004年 | 261篇 |
2003年 | 294篇 |
2002年 | 334篇 |
2001年 | 317篇 |
2000年 | 303篇 |
1999年 | 361篇 |
1998年 | 256篇 |
1997年 | 248篇 |
1996年 | 245篇 |
1995年 | 209篇 |
1994年 | 157篇 |
1993年 | 163篇 |
1992年 | 131篇 |
1991年 | 90篇 |
1990年 | 87篇 |
1989年 | 69篇 |
1988年 | 68篇 |
1987年 | 36篇 |
1986年 | 36篇 |
1985年 | 26篇 |
1984年 | 22篇 |
1983年 | 12篇 |
1982年 | 13篇 |
1981年 | 11篇 |
1980年 | 9篇 |
1979年 | 5篇 |
1962年 | 4篇 |
1958年 | 9篇 |
1957年 | 2篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 504 毫秒
891.
F. X. Hu G. X. Wu G. X. Song Q. R. Yuan S. Okamura 《Astrophysics and Space Science》2006,302(1-4):43-59
The progress of the studies on the orientation of galaxies in the Local Supercluster (LSC) is reviewed and a summary of recent
results is given. Following a brief introduction of the LSC, we describe the results of early studies based on two-dimensional
analysis, which were mostly not conclusive. We describe next the three-dimensional analysis, which is used widely today. Difficulties
and systematic effects are explained and the importance of selection effects is described. Then, results based on the new
method and modern databases are given, which are summarized as follows. When the LSC is seen as a whole, galaxy planes tend
to align perpendicular to the LSC plane with lenticulars showing the most pronounced tendency. Projections onto the LSC plane
of the spin vectors of Virgo cluster member galaxies, and to some extent, those of the total LSC galaxies, tend to point to
the Virgo cluster center. This tendency is more pronounced for lenticulars than for spirals. It is suggested that ‘field’
galaxies, i.e., those which do not belong to groups with more than three members, may be better objects than other galaxies
to probe the information at the early epoch of the LSC formation through the analysis of galaxy orientations. Field lenticulars
show a pronounced anisotropic distribution of spin vectors in the sense that they lay their spin vectors parallel to the LSC
plane while field spirals show an isotropic spin-vector distribution. 相似文献
892.
The feasibility of using fluorescence excitation-emission matrix (EEM) along with parallel factor analysis (PARAFAC) and nonnegative least squares (NNLS) method for the differentiation of phytoplankton taxonomic groups was investigated. Forty-one phytoplankton species belonging to 28 genera of five divisions were studied. First, the PARAFAC model was applied to EEMs, and 15 fluorescence components were generated. Second, 15 fluorescence components were found to have a strong discriminating capability based on Bayesian discriminant analysis (BDA). Third, all spectra of the fluorescence component compositions for the 41 phytoplankton species were spectrographically sorted into 61 reference spectra using hierarchical cluster analysis (HCA), and then, the reference spectra were used to establish a database. Finally, the phytoplankton taxonomic groups was differentiated by the reference spectra database using the NNLS method. The five phytoplankton groups were differentiated with the correct discrimination ratios (CDRs) of 100% for single-species samples at the division level. The CDRs for the mixtures were above 91% for the dominant phytoplankton species and above 73% for the subdominant phytoplankton species. Sixteen of the 85 field samples collected from the Changjiang River estuary were analyzed by both HPLC-CHEMTAX and the fluorometric technique developed. The results of both methods reveal that Bacillariophyta was the dominant algal group in these 16 samples and that the subdominant algal groups comprised Dinophyta, Chlorophyta and Cryptophyta. The differentiation results by the fluorometric technique were in good agreement with those from HPLC-CHEMTAX. The results indicate that the fluorometric technique could differentiate algal taxonomic groups accurately at the division level. 相似文献
893.
We evaluated the effect of ration level(RL) on the growth and energy budget of lenok Brachymystax lenok. Juvenile lenok(initial mean body weight 3.06±0.13 g) were fed for 21 d at five different ration levels: starvation,2%,3%,4% bwd(body weight per day,based on initial mean values),and apparent satiation. Feed consumption,apparent digestibility,and growth were directly measured. Specific growth rates in terms of wet weight,dry weight,protein,and energy increased logarithmically with an increase in ration levels. The relationship between specific growth rate in terms of wet weight(SGRw,%/d) and RL(%) was characterized by a decelerating curve: SGRw=-1.417+3.166ln(RL+1). The apparent digestibility coefficients of energy exhibited a decreasing pattern with increasing ration level,and there was a significant difference among different RLs. Body composition was significantly affected by ration size. The relationship between feed efficiency rate in terms of energy(FERe) and RL was: FERe=-14.167+23.793RL–3.367(RL)2,and the maximum FERe was observed at a 3.53% ration. The maintenance requirement for energy of juvenile lenok was 105.39 k J BW(kg)-0.80/d,the utilization efficiency of DE for growth was 0.496. The energy budget equation at satiation was: 100IE=29.03FE+5.78(ZE+UE)+39.56 HE+25.63 RE,where IE is feed energy,FE is fecal energy,ZE+UE is excretory energy,HE is heat production,and RE is recovered energy. Our results suggest that the most suitable feeding rate for juvenile lenok aquaculture for wet weight growth is 2.89% bwd,whereas for energy growth,the suggested rate is 3.53% bwd at this growth stage. 相似文献
894.
2012年夏季中国第5次北极科学考察期间,对北冰洋楚科奇海及其北部边缘海浮游细菌丰度和生产力进行了测定,并将其与环境因子进行了相关性分析。结果显示,楚科奇海浮游细菌丰度的变化范围为0.56×108~6.41×108 cells/dm3,平均为2.25×108 cells/dm3;细菌生产力介于0.042~1.92mg/(m3·d)(以碳计)之间,平均为0.54mg/(m3·d)(以碳计),与已有研究结果基本相当。陆架区细菌丰度和生产力要明显高于北部边缘区,但前者的单位细菌生产力则较低。与环境因子的相关性分析显示,细菌丰度与温度和叶绿素a浓度存在显著正相关(p0.01),表明北极变暖导致的海水升温及浮游植物生物量的增加均会促进细菌的生长,从而进一步提高细菌在海洋生态系统和碳循环中的作用。但陆架区的细菌生产力与环境参数均没有显著相关性,表明其影响因素较为复杂;生产力在北部边缘区则仅与叶绿素a存在显著正相关(p0.01),表明浮游植物生长过程产生的溶解有机碳(DOC)是细菌生长最为主要的碳源,碳源的单一可能制约细菌的生产从而导致该海域无冰状态下细菌丰度的增加不如预期,但融冰过程带来的大量DOC将促进细菌活性的增加。 相似文献
895.
896.
虚拟漫游技术已成功应用到社会各个领域,传统漫游技术如VRML(Virtual Reality Modeling Language)、Quick Time、PTGui(Panorama Tools)等存在集成难度大、建模时间长、投入成本高等问题。本文基于全景相机以及Pano2vr、Dynamic-Photo HDR、Photoshop等软硬件环境,研发搭建了一整套旅游景区全景影像采集与展示平台。该平台具有良好的沉浸性、多视角浏览、实现方法简单、搭建快速等优点,为旅游景点、酒店、历史文化名胜等场景的360度实景展示提供了有效地工程化解决途径。 相似文献
897.
898.
We sampled the sediments of the East China Sea during 2005 and 2006, and analysed the contents of the biogenic mat-ters: biogenic silica, organic carbon, and organic nitrogen. From the surface distribu... 相似文献
899.
吉尔吉斯斯坦北天山构造带的矿床学数据缺乏,制约了天山造山带境内外成矿对比。布丘克金矿床位于吉尔吉斯斯坦北天山构造带中部。金矿体为石英复脉,呈带状发育于NWW向韧性剪切带中。矿体倾向SSW,倾角60°~70°,赋矿围岩主要为侵入于早古生代变质碎屑杂岩中的正长斑岩。布丘克金矿床成矿期石英流体包裹体观察、石英H-O同位素、硫化物S同位素测试结果显示,布丘克金矿床石英脉中包裹体大小集中在2~10μm之间,类型以H2O-CO2型、富CO2型、水溶液型包裹体为主,成分以富CO2、含CH4为特征。成矿流体具有中温(200~320℃)、低盐度(3%~7%NaCleqv)特征;石英δDV-SMOW值介于-108.1‰~-90.2‰之间,δ18O流体值介于4.86‰~9.26‰之间;黄铁矿δ34S分布在0‰左右(-0.9‰~1.6‰)。综合本文数据、矿床地质特征、区域地质资料,本文认为布丘... 相似文献
900.