全文获取类型
收费全文 | 55741篇 |
免费 | 5457篇 |
国内免费 | 6695篇 |
专业分类
测绘学 | 1783篇 |
大气科学 | 7688篇 |
地球物理 | 12414篇 |
地质学 | 24561篇 |
海洋学 | 5873篇 |
天文学 | 8175篇 |
综合类 | 3105篇 |
自然地理 | 4294篇 |
出版年
2023年 | 561篇 |
2022年 | 1447篇 |
2021年 | 1767篇 |
2020年 | 1612篇 |
2019年 | 1749篇 |
2018年 | 2595篇 |
2017年 | 2372篇 |
2016年 | 2732篇 |
2015年 | 1798篇 |
2014年 | 2660篇 |
2013年 | 3161篇 |
2012年 | 2502篇 |
2011年 | 2823篇 |
2010年 | 2779篇 |
2009年 | 3001篇 |
2008年 | 2640篇 |
2007年 | 2655篇 |
2006年 | 2311篇 |
2005年 | 1630篇 |
2004年 | 1473篇 |
2003年 | 1449篇 |
2002年 | 1424篇 |
2001年 | 1305篇 |
2000年 | 1485篇 |
1999年 | 1846篇 |
1998年 | 1596篇 |
1997年 | 1567篇 |
1996年 | 1395篇 |
1995年 | 1226篇 |
1994年 | 1202篇 |
1993年 | 988篇 |
1992年 | 839篇 |
1991年 | 724篇 |
1990年 | 644篇 |
1989年 | 523篇 |
1988年 | 492篇 |
1987年 | 439篇 |
1986年 | 354篇 |
1985年 | 428篇 |
1984年 | 345篇 |
1983年 | 315篇 |
1982年 | 345篇 |
1981年 | 259篇 |
1980年 | 273篇 |
1979年 | 224篇 |
1978年 | 221篇 |
1977年 | 169篇 |
1976年 | 170篇 |
1975年 | 174篇 |
1974年 | 168篇 |
排序方式: 共有10000条查询结果,搜索用时 62 毫秒
21.
对青藏东北缘现今块体划分、运动及变形的初步研究 总被引:19,自引:9,他引:10
利用2维非连续变形分析方法(DDA),以位移代替围压作为边界约束力,研究青藏东北缘现今块体划分及其运动变形。根据该地区地质构造及地震活动,以GPS点测量位移作为模拟结果约束点,得出了较合理的块体划分模型和随时间演化的主应变分布图,并把应变高值区与近几年来发生的5级以上地震作对比,得出了研究区内地震危险性可能较大的区域。另外,对模拟的甘青块体与阿拉善块体的边缘带断裂左旋运动做了大概计算。 相似文献
22.
IwrRODUcrlONStainlessstalhasbeenusedwidelyasbuildingrnateria1,especiallyasdecoratingrnate-rialinbuildings.Ordinarystainlesssteehasonlyonemonot0noussilverywhitCcolourwhichsomeimesdoesnotrnatchwithitSsurmundingenvironment.lnordert0improveitSdereratingperfonnaneeandexpanditsuse,muchresearchhasbeenconductedsincethe4O'st0trytoformabright,uniform,highcorrosi0nandwearresistantco1ourfilmonastainlesssteesurface.Duringtheeariystage,anirnmersionedhodwasused,i.e.inimrs-ingthestainlesssteelinahots0luti… 相似文献
23.
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
24.
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
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.
The explicit forms of the metric as well as the equations of motion in the first-order post-Newtonian approximation are worked out under several gauge conditions. It is noted that the so-called EIH (Einstein, Infeld, and Hoffman) equation of motion for an assembly ofN finite mass points mutually interacting via gravitation is identically obtained under three different gauge conditions, namely the harmonic gauge, Chandrasekhar gauge and a composite Chandrasekhar gauge used by Misneret al. (1970), even though the solutions for the metric are found to be all different. In one case the metric has a component apparently diverging, but finally generates regular affine connections so that the equations of motions become free from any singularity. By use of the Chandrasekhar gauge and his formulation, the second-order contribution to the acceleration of planets in the limit of test particle motion around the Sun has been calculated, the inclusion of which in the EIH set of the equations of motion would extend the relative accuracy of computing the total acceleration of any planet to better than one part in 1017. 相似文献
27.
In astronomical photometry, the sensitivity of observations is limited by the dark counts of the photomultiplier tube. In the present work, the effect of dark count noise in photon counting systems is investigated by theory and experimental measurements. Dark counts are considered to be originating from two sources, namely: dc fluctuations and random pulses.Experimental measurements were carried out to determine noise effects in different operating regions of noise dominance. The results provide strong evidence that: in normal operating mode, where the effect of random pulses is dominant, dark counts do not follow Poisson statistics. The observed noise shows strong (1/f) power spectrum, where the observed noise power is found to increase with time of observation.The results are important in photon counting systems operating under dark count limited mode. The conclusions drawn can be useful in obtaining more accurate error estimates and in assessing astronomical photometric observations and data reduction techniques. 相似文献
28.
29.
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
30.
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