全文获取类型
收费全文 | 86243篇 |
免费 | 16408篇 |
国内免费 | 22216篇 |
专业分类
测绘学 | 7914篇 |
大气科学 | 17397篇 |
地球物理 | 20858篇 |
地质学 | 43728篇 |
海洋学 | 11215篇 |
天文学 | 3428篇 |
综合类 | 9299篇 |
自然地理 | 11028篇 |
出版年
2024年 | 381篇 |
2023年 | 1282篇 |
2022年 | 3840篇 |
2021年 | 4578篇 |
2020年 | 3740篇 |
2019年 | 4374篇 |
2018年 | 4816篇 |
2017年 | 4367篇 |
2016年 | 4985篇 |
2015年 | 4534篇 |
2014年 | 5242篇 |
2013年 | 5530篇 |
2012年 | 5411篇 |
2011年 | 5541篇 |
2010年 | 5504篇 |
2009年 | 5254篇 |
2008年 | 4835篇 |
2007年 | 4567篇 |
2006年 | 3882篇 |
2005年 | 3413篇 |
2004年 | 2685篇 |
2003年 | 2528篇 |
2002年 | 2554篇 |
2001年 | 2552篇 |
2000年 | 2769篇 |
1999年 | 3618篇 |
1998年 | 2999篇 |
1997年 | 2872篇 |
1996年 | 2667篇 |
1995年 | 2337篇 |
1994年 | 2131篇 |
1993年 | 1866篇 |
1992年 | 1502篇 |
1991年 | 1169篇 |
1990年 | 883篇 |
1989年 | 799篇 |
1988年 | 708篇 |
1987年 | 433篇 |
1986年 | 378篇 |
1985年 | 267篇 |
1984年 | 216篇 |
1983年 | 159篇 |
1982年 | 183篇 |
1981年 | 133篇 |
1980年 | 95篇 |
1979年 | 87篇 |
1978年 | 31篇 |
1977年 | 25篇 |
1976年 | 24篇 |
1958年 | 58篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
Mercury (Hg) was investigated in bone tissues of skua ( Catharacta maccormick) and penguin (Pygoscelis adeliae) collected in the maritime Antarctic using atomic fluorescence spectrometry (AFS) and synchrotron radiation X-ray fluorescence (SR-XRF) method. The total levels of mercury in bone tissues of penguin and skua are much lower than those in other organs (e. g. , kidney, liver). The toxic effects of mercury in bone tissues of seabirds in polar region are not known. We have used SR- XRF method to map the distribution of trace levels of mercury in bones. The levels of mercury are found to be enriched somewhere near the periosteal surface and/or endosteal surface. The distribution of mercury shows strongly correlation with that of some essential elements and probably poses negative effect on the bone metabolism inferring from the relationship of mercury with the other elements. These studies represent a first step toward understanding the toxic effects of mercury on bone of polar animals by suggesting the possible microscopic investigation. 相似文献
32.
A wide-field time-series CCD photometric survey of variable stars in the field of the open cluster NGC 2168 was carried out using the BATC Schmidt telescope. In total 13 new variable stars are discovered with three W UMa systems, one EA type and two EB type eclipsing binaries (one of them could be a W UMa system), and seven pulsating stars including three candidates of δScuti stars. 相似文献
33.
Zhi Xu Cheng Fang Wei-Qun Can Astronomy Department of Nanjing University Nanjing Purple Mountain Observatory Chinese Academy of Sciences Nanjing 《中国天文和天体物理学报》2005,5(5):519-530
Chromospheric lines, including Ha, Lyα, Lyβand CaⅡK, CaⅡ8542, are systemically and quantitatively investigated with respect to the non-thermal excitation and ionization due to particle beam bombardment for a series of solar semi-empirical atmospheric models. As a result we propose to use the contrast in the integrated intensity of hydrogen lines to estimate the total energy flux of the bombarding beam during the solar flare impulsive phase. Partial frequency redistribution is considered in the Lyαline calculation and a smaller intensity enhancement in the Ha line-centers is found than in the previous results of Fang et al. 相似文献
34.
太阳发电机理论主要解释太阳磁场的起源和演化,是太阳物理中最基本、最重要的问题之一.对它的研究不仅具有重要的科学价值,而且对空间天气的预测和空间技术的发展有一定的影响.随着日震学的发展,该理论也得到了很大改进。从相关观测入手,综述了发电机理论需解释的观测事实,以及为发电机理论提供约束的太阳内部动力学结构;介绍了基本理论和近来的主流模型,并指出尚待解决的问题和进一步的努力方向。 相似文献
35.
张希然 《云南地理环境研究》1990,2(1):72-81
粤东海岸东起闽粤交界的大埕湾,西迄深圳市的大鹏湾,岸线曲折,港湾较多,河溪人海泥沙在潮间带及紧邻5m 等深线以内形成大片海涂,土壤类型多样,以泥质和沙泥质土壤为主;除河口湾的土壤含盐量低外,其余较高;土壤有机质含量中等,钾素较丰富,磷素较低。因地处亚热带南缘,光、热和水资源条件较好,且多数海湾有深入内地的小海湾和潮沟,甚适合发展海水养殖与晒盐,韩江口的义丰河和海门龟头海大闸外等滩地尚有围垦价值。多处海湾潮滩沙土适合作海滨浴场,也有小片滩地红树林适合旅游观赏。 相似文献
36.
本区至今还是综合科学考察的空白区。为查明基本情况,1989年夏以乘汽车路线考察方式穿行该区而作了预查,搜集了当地地质、地理、生物等基本资料。经考察发现,区内野生动物资源丰富,上新世以来火山活动普遍,湖泊众多,矿产资源可能有良好前景;所在区又为自然地域和地质构造的重要界线,对青藏高原研究具有重大科学价值。 相似文献
37.
Parametric transduction offers valuable advantages for underwater acoustic communications. Perhaps the most significant benefit is the fact that high directivity is achieved by means of a physically small transmit transducer. This feature may, ultimately, be employed to permit long-range, low-frequency communication using a compact source. The high directivity is desirable to combat multipath propagation and to achieve data communications in water which is shallow by comparison with range. A real-time, high data-rate “model” differential phase shift keying (DPSK) communication system has been constructed and demonstrated. This system uses parametric transduction, with a 300-kHz primary frequency and a 50-kHz secondary frequency. Experimental results show that the system can be employed to combat multipath propagation in shallow water and can achieve high data-rate text and color image transmission at 10 and 20 kb s-1 for 2-DPSK and 4-DPSK, respectively, through a transmission bandwidth of 10 kHz. The “model” system was developed to confirm performance predictions for a future, operational long-range link employing a 50-kHz primary frequency and a 5-kHz secondary frequency 相似文献
38.
39.
Histological study on the ontogeny of the lymphoid organs, kidney, thymus and spleen of Japanese flounder, Paralichthys olivaceus, from hatching to 40 d was carried out. The pronephric kidney duct appeared early in hatching although the primordial haemopoietic
stem cells were observed within a week after hatching. The spleen was first seen after 8d of hatching. The thymus appeared
after 15d, situated near the pronephric kidney. Small lymphoid cells appeared during the later phase of the post-larval stage
in the sequence of thymus, kidney and spleen. During the 40d of observations, there were no distinct inner or outer zones
in thymus and no red or white pulp in spleen. These results suggest that the nonspecific defense immune system plays a very
important role in the early larval stage of Japanese flounder. 相似文献
40.
本文论述了珠穆朗玛峯地区土壤中稀土元素的含量,以及它们在土壤中的分布模式,并发现土壤中稀土元素的含量水平在很大程度上受土壤母质的制约。 相似文献