全文获取类型
收费全文 | 64901篇 |
免费 | 23467篇 |
国内免费 | 47517篇 |
专业分类
测绘学 | 5289篇 |
大气科学 | 25690篇 |
地球物理 | 19502篇 |
地质学 | 50398篇 |
海洋学 | 18574篇 |
天文学 | 2282篇 |
综合类 | 7820篇 |
自然地理 | 6330篇 |
出版年
2024年 | 282篇 |
2023年 | 855篇 |
2022年 | 2093篇 |
2021年 | 2616篇 |
2020年 | 3342篇 |
2019年 | 6677篇 |
2018年 | 7225篇 |
2017年 | 6745篇 |
2016年 | 7037篇 |
2015年 | 6268篇 |
2014年 | 6061篇 |
2013年 | 6649篇 |
2012年 | 6375篇 |
2011年 | 6230篇 |
2010年 | 6106篇 |
2009年 | 5402篇 |
2008年 | 4540篇 |
2007年 | 4479篇 |
2006年 | 3917篇 |
2005年 | 3596篇 |
2004年 | 3568篇 |
2003年 | 3277篇 |
2002年 | 2965篇 |
2001年 | 2737篇 |
2000年 | 2542篇 |
1999年 | 2954篇 |
1998年 | 2703篇 |
1997年 | 2704篇 |
1996年 | 2242篇 |
1995年 | 2070篇 |
1994年 | 1896篇 |
1993年 | 1711篇 |
1992年 | 1424篇 |
1991年 | 1082篇 |
1990年 | 950篇 |
1989年 | 805篇 |
1988年 | 703篇 |
1987年 | 507篇 |
1986年 | 422篇 |
1985年 | 343篇 |
1984年 | 325篇 |
1983年 | 197篇 |
1982年 | 248篇 |
1981年 | 175篇 |
1980年 | 128篇 |
1979年 | 131篇 |
1978年 | 55篇 |
1977年 | 44篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
21.
高速滑坡问题是当前工程地质和环境地质学界极为关注的问题。本文对诸如滑体高速滑动所对应的时空界限、滑体滑动过程所产生的热效应以及空气浮托力等高速滑坡产生机制问题进行了初步探讨。 相似文献
22.
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
23.
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
24.
Xue-Fei Chen Zhan-Wen HanNational Astronomical Observatories Yunnan Observatory Chinese Academy of Sciences. Kunming Graduate School of the Chinese Academy of Sciences Beijing 《中国天文和天体物理学报》2005,5(1):65-76
We have studied the influence of different choices of core-envelope transition point on the final merger of contact binaries with two main-sequence components. A binary of 1.00 + 0.90M⊙ with an initial orbital period of 0.35d is examined. The mass fraction of the primary mixed with the matter of the secondary, qmix, determined by the chosen core-envelope transition point, ranges from 0.04 to 1.00 in our analysis. If as qmix< 0.8, none of the 相似文献
25.
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
26.
1 IntroductionBacteriaandtheiractivitiesplayanimportantroleintheelementalbiogeochemicalcyclesandenergytransformingintheocean (Zhenetal.1 997) .DortchandPackard(1 989) proposedthatfoodwebsintheeutrophicwatersaredominatedbythebiomassofprimaryproducerswhilefoodwebsintheoligotrophicwatersaredominatedbythebiomassofmicrobes.Heterotrophicbacteriahadbeenshowntoplayanimportantroleinthedecompositionoflarge ,rapidlysinkingorganicparticleswithinandbelowtheeuphot iczone ,andfurthertoaffecttheelementaldyn… 相似文献
27.
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
28.
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
29.
30.