全文获取类型
收费全文 | 2375篇 |
免费 | 409篇 |
国内免费 | 598篇 |
专业分类
测绘学 | 103篇 |
大气科学 | 479篇 |
地球物理 | 575篇 |
地质学 | 1267篇 |
海洋学 | 262篇 |
天文学 | 160篇 |
综合类 | 237篇 |
自然地理 | 299篇 |
出版年
2024年 | 6篇 |
2023年 | 61篇 |
2022年 | 140篇 |
2021年 | 155篇 |
2020年 | 96篇 |
2019年 | 122篇 |
2018年 | 169篇 |
2017年 | 137篇 |
2016年 | 155篇 |
2015年 | 121篇 |
2014年 | 153篇 |
2013年 | 138篇 |
2012年 | 118篇 |
2011年 | 133篇 |
2010年 | 149篇 |
2009年 | 116篇 |
2008年 | 112篇 |
2007年 | 85篇 |
2006年 | 72篇 |
2005年 | 76篇 |
2004年 | 40篇 |
2003年 | 63篇 |
2002年 | 48篇 |
2001年 | 44篇 |
2000年 | 73篇 |
1999年 | 111篇 |
1998年 | 92篇 |
1997年 | 91篇 |
1996年 | 89篇 |
1995年 | 95篇 |
1994年 | 66篇 |
1993年 | 61篇 |
1992年 | 42篇 |
1991年 | 28篇 |
1990年 | 22篇 |
1989年 | 24篇 |
1988年 | 21篇 |
1987年 | 11篇 |
1986年 | 12篇 |
1985年 | 9篇 |
1984年 | 5篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 2篇 |
1980年 | 1篇 |
1979年 | 1篇 |
1976年 | 1篇 |
1958年 | 2篇 |
排序方式: 共有3382条查询结果,搜索用时 62 毫秒
41.
This study explores the potential existence of local stocks of Synechogobius ommaturus using otolith chemical signatures. The concentrations of 10 elements (K, Ca, Na, Mg, Sr, Pb, Ba, Mn, Co, Zn) in the otoliths from eight stocks (Dandong, Dalian, Tianjin, Dongying, Weihai, Qingdao, Ganyu and Xiamen) of S. ommaturus were measured by Inductively Coupled Plasma Mass Spectrometry and regressed against otolith weight. No significant correlation between otolith weight and concentrations of Na, Mg, Ca, Sr, Co, Ba, Zn and Pb were observed. Concentrations of K and Mn were correlated with otolith weight. Consequently, the residuals of the regressions were used instead of the original concentrations in subsequent analyses. Analysis of variance (ANOVA) showed no significant difference between males and females in otolith chemical signatures. Differences were found among the locations for K, Sr, Mn, Co, Ba, Pb, but not for Ca, Na, Mg and Zn. Residuals of the regressions of elemental concentrations against otolith weight that fitted the normal distribution and homogeneity were studied by discriminant analysis. The success of the discriminant function to predict the location of S. ommaturus capture ranged from 100% for Weihai and Xiamen stocks, to only 44.4% and 62.5% for Qingdao and Dandong stocks, respectively. The average prediction success was 80.3%. 相似文献
42.
Yuexin Ma Feixue Sun Congyao Zhang Pengyun Bao Shuqing Cao Meiyan Zhang 《中国海洋大学学报(英文版)》2014,13(6):1061-1066
A marine bacterium, Pseudoalteromonas sp. BC228 was supplemented to feed in a feeding experiment aiming to determine its ability of enhancing the digestive enzyme activity and immune response of juvenile Apostichopus japonicus. Sea cucumber individuals were fed with the diets containing 0(control), 105, 107 and 109 CFU g-1 diet of BC228 for 45 days. Results showed that intestinal trypsin and lipase activities were significantly enhanced by 107 and 109 CFU g-1 diet of BC228 in comparison with control(P 0.01). The phagocytic activity in the coelomocytes of sea cucumber fed the diet supplemented with 107 CFU g-1 diet of BC228 was significantly higher than that of those fed control diet(P 0.05). In addition, 105 and 107 CFU g-1 diet of BC228 significantly enhanced lysozyme and phenoloxidase activities in the coelomic fluid of sea cucumber, respectively, in comparison with other diets(P 0.01). Sea cucumbers, 10 each diet, were challenged with Vibrio splendidus NB13 after 45 days of feeding. It was found that the cumulative incidence and mortality of sea cucumber fed with BC228 containing diets were lower than those of animals fed control diet. Our findings evidenced that BC228 supplemented in diets improved the digestive enzyme activity of juvenile sea cucumber, stimulated its immune response and enhanced its resistance to the infection of V. splendidus. 相似文献
43.
介绍了一种高精度、高性价比的GPS定位定向仪的设计及实现(〈1mil,2σ)方法,系统利用了实际应用中的有利约束条件,对基于LAMBDA方法的载波相位整周模糊度解算算法进行了改进,经过系统工程设计,在实际应用中实现了单频、高精度、快速定向数据的输出。可以满足绝大多数环境下的使用需求,大大降低了系统成本。 相似文献
44.
The trends of distribution, translocation and seasonal change of heavy metal Pb were studied based on the surface and bottom water sampling in Jiaozhou Bay in 1979, and compared with those in 1990's. The results showed that the source of Pb in the bay was from wastewater and sewage in the east of Jiaozhou Bay from ocean vessels. Pb concentration was higher in spring and lower in summer and autumn, and remained stable through sedimentation in the bottom layer. The overall water quality was good in 1970's. Compared with the environmental monitoring data of 1995-1999, Pb pollution had become serious. Therefore, more efforts should be made to protect the bay from Pb pollution. 相似文献
45.
There are two different dust storms in Northeast China, normal dust and alkali dust, from Kerqin sand land and Songnen saline land. As well as visible damages of the dust storms, there are some invisible influences. The authors provide evidences that the bacteria carried by dusts from remote area should not be ignored, but the damages of the alkali dusts are still not clear. 相似文献
46.
Xiaozheng Chen Wei Wang Qinghua Shang Yue Lou Xinchun Liu Changqun Cao Yue Wang 《Precambrian Research》2009,170(3-4):223-230
Research on the origin of eukaryotes often focuses on the exceptional preservation found in silicified Precambrian fossils. Nuclei like subcellular structures in well-preserved fossil becomes confusion and arguments, which focus on whether the partial degradation of prokaryotes produces artifacts that resemble a ‘nucleus’, or fossilized nuclei of eukaryotes. In order to understand the mechanisms of silicification and identify the fossilized subcellular and microstructures in rocks, a series of laboratory controlled experiments were performed for simulating the silicification process. The effects of different silica solutions in eukaryote fossilization were studied in our experiments by exposing onion skin cells (epidermis) to silica solutions. Onion skin provides a good experimental model because of its well characterized cellular structures which are easily observed. The designed experiments revealed that the possibility of onion cell preserved as a “fossil” with nuclear structures, the first week fossilization, or mineralization as rapid as 1 week is important. And the experiment also revealed interactions between silica and the onion skin cell wall surface functional groups were weak. The preservation of nuclei in the onion skin model was due to precipitation in highly supersaturated silica solutions rather than simply the high silica concentration. When the silica gel precipitates slowly at low supersaturation states, the nuclei were not well preserved, but the rapid precipitation at high supersaturated silica conditions preserved nuclear structures. A better understanding of the processes involved in onion skin fossilization will further contribute to issues concerning the silicification of other eukaryotic materials. 相似文献
47.
Brine shrimp (Artemia parthenogenetica) which had ingested three water-insoluble antibacterial drugs i.e. sulfadiazine(SD), oxytetracycline (OTC) and erythromycin
estolate (ERY-Es) were fed toTilapia and Mysis IIIof Penaeus orientalis K. The drug contents in the predators were then determined. After administration of drugs toTilapia and Mysis III, through the bio-encapsulation of the brine shrimp, efficacious therapeutical concentration of OTC and ERY-Es
(but not SD) in the predators could be reached and maintained for more than 8 hours. 相似文献
48.
Yun Xu Xin-Wu Cao Shanghai Astronomical Observatory Chinese Academy of Sciences Shanghai Graduate School of Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2006,6(6):649-654
The geometry of broad-line regions (BLRs) in active galactic nuclei (AGNs) is still controversial. We use a sample of BL Lac objects, of which the black hole masses Mbh are estimated from their host galaxy absolute magnitude at R-band, MR, by using the empirical relation between MR and black hole mass Mbh. The sizes of the broad-line regions for Mgn are derived from the widths of Mgn lines and the black hole masses. Compared with the empirical relation between BLR size RBLR and MgII line luminosity LMgII, it is found the BLR sizes in the BL Lac objects derived in this paper are 2-3 orders of magnitude higher. If the BLR geometry of these sources is disklike, then the viewing angle between the axis and the line of sight is in the range of ~ 2°-15°, which is consistent with the unification scheme. 相似文献
49.
Ji-Cheng Zhang Chen Cao Nan Song Fei-Ge Wang Xiao-Tong Zhang 《Chinese Astronomy and Astrophysics》2011,35(4):409
In order to make more accurate estimates of the physical parameters of extrasolar planets, the observation and research on the phenomena of transits of the 7 known stars with the planetary systems TrES-1, TrES-3, XO-2, WASP-1, WASP-2, WASP-3 and HAT-P-7 are carried out with the 1-m reflecting telescope at Shandong University Weihai Astronomical Observatory/Weihai Municipal Astronomical Observatory. The fundamental conditions of the observation and data processing are introduced, and the transit light curves as well as the parameters of some planets derived from them are given. At the same time, when the results are summarized and analyzed, the prospects for the more in-depth and detailed researches which will be further carried out are also described. 相似文献
50.
The 1.6 m clear aperture solar telescope in Big Bear is operational and with its adaptive optics (AO) system it provides diffraction limited solar imaging and polarimetry in the near-infrared (NIR). While the AO system is being upgraded to provide diffraction limited imaging at bluer wavelengths, the instrumentation and observations are concentrated in the NIR. The New Solar Telescope (NST) operates in campaigns, making it the ideal ground-based telescope to provide complementary/supplementary data to SDO and Hinode. The NST makes photometric observations in Hα (656.3 nm) and TiO (705.6 nm) among other lines. As well, the NST collects vector magnetograms in the 1565 nm lines and is beginning such observations in 1083.0 nm. Here we discuss the relevant NST instruments, including AO, and present some results that are germane to NASA solar missions. 相似文献