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   26860篇
  免费   635篇
  国内免费   1578篇
测绘学   1556篇
大气科学   2479篇
地球物理   5082篇
地质学   12955篇
海洋学   1314篇
天文学   1787篇
综合类   2360篇
自然地理   1540篇
  2024年   11篇
  2023年   82篇
  2022年   164篇
  2021年   177篇
  2020年   109篇
  2019年   141篇
  2018年   4888篇
  2017年   4145篇
  2016年   2723篇
  2015年   374篇
  2014年   247篇
  2013年   186篇
  2012年   1109篇
  2011年   2860篇
  2010年   2152篇
  2009年   2439篇
  2008年   1996篇
  2007年   2448篇
  2006年   131篇
  2005年   273篇
  2004年   449篇
  2003年   474篇
  2002年   303篇
  2001年   100篇
  2000年   130篇
  1999年   146篇
  1998年   127篇
  1997年   96篇
  1996年   97篇
  1995年   103篇
  1994年   73篇
  1993年   65篇
  1992年   44篇
  1991年   29篇
  1990年   24篇
  1989年   25篇
  1988年   21篇
  1987年   14篇
  1986年   15篇
  1985年   10篇
  1984年   5篇
  1983年   9篇
  1982年   6篇
  1981年   23篇
  1980年   20篇
  1979年   1篇
  1976年   7篇
  1958年   2篇
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
Non-specific immune response of bullfrog Rana catesbeiana to pathogenic Aeromonas hydrophila was studied to 60 individuals in two groups. Each bullfrog in bacterium-injected group was injected intraperitoneally (i.p.) with 0.2 ml bacterial suspension at a density of 5.2 × 10^6 CFU/ml, while each one in control group injected i.p. with 0.2 ml sterile saline solution (0.85%, w/v). Three bullfrogs in both groups were sampled at 0, 1, 3, 7, 11, 15 and 20 days post-injection (dpi) for the evaluation of non-specific immune parameters. It was observed that intraperitoneal injection of A. hydrophila significantly increased the number of leucocytes and that of NBT-positive cells in peripheral blood. Significant increases in serum bactericidal activity and serum acid phosphatase activity were also observed in the bacterium-injected frogs when compared with those in the control group. However, a significant reduction was detected in vitro in phagocytosis activity of peripheral blood phagocytes. No significant difference in changes in the number of peripheral erythrocytes, serum superoxide dismutase (SOD) activity, and lysozyme activity was detected between the two groups. It is suggested that bullfrogs may produce a series of non-specific immune reactions in response to the A. hydrophila infection.  相似文献   
192.
Excessive alcohol consumption leads to liver disease. Extensive evidence suggests that C-phycocyanin (C-PC), a chromophore phycocyanobilin derived from Spirulina platensis, exerts protective effects against chemical-induced organ damage. In this study, we investigated whether C-PC could protect against ethanol-induced acute liver injury. Serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), triglyceride (TG), total cholesterol (CHOL), low-density lipoprotein (LDL), liver homogenate malondialdehyde (MDA), superoxide dismutase (SOD) content were measured, and pathological examination of liver sections were examined. C-PC showed obvious inhibitory effects on serum ALT, AST, TG, CHOL, LDL and MDA, and SOD content significantly increased in the liver. The structure of hepatic lobules was clear, liver sinus returned to normal, and liver cell cords were arranged in neat rows. Cloudiness, swelling, inflammatory cell infiltration and spotty necrosis of liver cells were significantly reduced. Therefore, C-PC can significantly protect against ethanol-induced acute liver injury.  相似文献   
193.
During the last two decades, the first generation of beam combiners at the Very Large Telescope Interferometer has proved the importance of optical interferometry for high-angular resolution astrophysical studies in the near- and mid-infrared. With the advent of 4-beam combiners at the VLTI, the u ? v coverage per pointing increases significantly, providing an opportunity to use reconstructed images as powerful scientific tools. Therefore, interferometric imaging is already a key feature of the new generation of VLTI instruments, as well as for other interferometric facilities like CHARA and JWST. It is thus imperative to account for the current image reconstruction capabilities and their expected evolutions in the coming years. Here, we present a general overview of the current situation of optical interferometric image reconstruction with a focus on new wavelength-dependent information, highlighting its main advantages and limitations. As an Appendix we include several cookbooks describing the usage and installation of several state-of-the art image reconstruction packages. To illustrate the current capabilities of the software available to the community, we recovered chromatic images, from simulated MATISSE data, using the MCMC software SQUEEZE. With these images, we aim at showing the importance of selecting good regularization functions and their impact on the reconstruction.  相似文献   
194.
We made model fitting to the mid-to-far infrared spectral energy distributions (SEDs) for different categories of galaxies in the main extragalactic field of the Spitzer First Look Survey with the aid of spectroscopic information from the Sloan Digital Sky Survey. We find that the mid-to-far infrared SEDs of HII galaxies, mixture type galaxies and LINERs can be well fitted by the one-parameter (α) dust model of Dale et al. plus the 13 Gyr dust-free elliptical galaxy model. The statistics of α values indicates that all these galaxies tend to be quiescent, although the HII galaxies are relatively more active than the LINERs. The mid- infrared SEDs of absorption galaxies are well fitted simply by the 13 Gyr dust-free elliptical galaxy template, and the near-to-mid infrared SEDs of QSOs can be represented by AGN NGC 5506.  相似文献   
195.
The effects of natural fish oil,DHA oil and soybean lecithin in microparticulate diets on stress tolerance of larval gilthead seabream(Sparus aurata)were investigated after 15 days feeding trials.The tolerance of larval gilthead seabream to various stress factors such as exposure to air(lack of dissolved oxygen),changes in water temperature(low)and salinity(high) were determined.This study showed that microparticulate diet with natural fish oil and soybean lecithin was the most effective for in-creasing the tolerance of larval gilthead seabream to various stresses,and that microparticulate diet with natural fish oil and palmitic acid(16:0)was more effective than microparticulate diet with DHA oil and soybean lecithin.  相似文献   
196.
利用第29次南极科考期间在普里兹湾海区释放的8个Argos漂流浮标所获的总计662 d的数据,对普里兹湾及其邻近海域的表层海流特征进行了分析,并和以前的研究结果进行对比,结果表明:普里兹湾西部和北部表层海流基本呈现向西的流动,说明此地区的表层海流以沿南极大陆的逆时针沿岸流为主。普里兹湾东部海流呈现先向南再向西或先向南再向北的流动,普里兹湾西北部海流呈现向南的流动。8个浮标平均流速在0.02—0.20 m·s~(-1)之间,最大流速为1.57 m·s~(-1),流速1.0 m·s~(-1)的大流速区主要集中在普里兹湾、克洛斯角外侧、达恩利角外侧海域,流速1.5 m·s~(-1)的大流速区只出现在普里兹湾、克洛斯角外侧两处海域。  相似文献   
197.
A sediment sample was collected from a deep-sea hydrothermal vent field located at a depth of 2 951 m on the Southwest Indian Ridge. Phylogenetic analyses were performed on the prokaryotic community using polymerase chain reaction(PCR) amplification of the 16 S rRNA and nifH genes. Within the Archaea, the dominant clones were from marine benthic group E(MBGE) and marine group I(MGI) belonging to the phyla Euryarchaeota and Thaumarchaeota, respectively. More than half of the bacterial clones belonged to the Proteobacteria, and most fell within the Gammaproteobacteria. No epsilonproteobacterial sequence was observed. Additional phyla were detected including the Actinobacteria, Bacteroidetes, Planctomycetes, Acidobacteria, Nitrospirae, Chloroflexi, Chlorobi, Chlamydiae, Verrucomicrobia, and candidate divisions OD1, OP11, WS3 and TM6, confirming their existence in hydrothermal vent environments. The detection of nifH gene suggests that biological nitrogen fixation may occur in the hydrothermal vent field of the Southwest Indian Ridge. Phylogenetic analysis indicated that only Clusters I and III NifH were present. This is consistent with the phylogenetic analysis of the microbial 16 S rRNA genes, indicating that Bacteria play the main role in nitrogen fixation in this hydrothermal vent environment.  相似文献   
198.
The present paper reports 9 species of pontoniine shrimps collected from Hainan IS-land ,South China Sea ,by the Joint Chinese-German Marine Biology Expeditions,in which Peri-climenaeus arabicus (Calman,1939) and Periclimenaeus hecate (Nobili,1904) are recorded for the first time from Hainan Island.  相似文献   
199.
Soil sediment samples of 10 layers with a spacing of 10 cm each were collected in different floodplain zones adjacent to Huolin River in the Xianghai Nature Reserve, and contents of total N, total P and organic matters were analyzed. The results showed that contents of total N, total P and organic matters were generally decreasing with the increase of distance from sample locations to the river channel, and contents of the three items were generally higher in the upper soil layer than that in the lower soil layer. The content variations displayed how flooding functions influenced nutrient matter content variations in floodplain soils since the flood inundation frequencies of the sample locations varied. The correlation analysis displayed that there were remarkable relativities between total N, total P and organic matters within definite spatial distance from the Huolin River channel.  相似文献   
200.
Boundary faults associated with thick sedimentary basins are more often curved in cross section rather than planar. We develop a space domain-based automatic gravity inversion technique to quantify such listric fault sources from a set of observed gravity anomalies. The density contrast within the hanging wall of fault morphology is presumed to be known according to a prescribed exponential law. Furthermore, the fault plane is described by a polynomial function of arbitrary but specific degree, whose coefficients become the unknown parameters to be estimated from a set of observed gravity anomalies in addition to the thickness of the fault structure. Using a set of characteristic anomalies, the present inversion identifies approximate parameters pertaining to the origin of fault plane and depth to decollement horizon. Based on the errors between the observed and model gravity anomalies of the structure, the algorithm constructs and solves a system of normal equations to estimate the improvements in depth and coefficients of the polynomial in an iterative approach until one of the specified convergence criteria is fulfilled. The efficacy of the algorithm is shown with the analysis of gravity anomalies attributable to a synthetic model of a listric fault source in the presence of pseudorandom noise. Application of the proposed inversion technique on the observed gravity anomalies of the Ahri-Cherla master fault of the Godavari subbasin in India using the derived exponential density contrast model has yielded an interpretation that is consistent with the available/reported information.  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