全文获取类型
收费全文 | 1667篇 |
免费 | 295篇 |
国内免费 | 362篇 |
专业分类
测绘学 | 88篇 |
大气科学 | 376篇 |
地球物理 | 374篇 |
地质学 | 830篇 |
海洋学 | 145篇 |
天文学 | 153篇 |
综合类 | 168篇 |
自然地理 | 190篇 |
出版年
2023年 | 35篇 |
2022年 | 73篇 |
2021年 | 79篇 |
2020年 | 67篇 |
2019年 | 57篇 |
2018年 | 105篇 |
2017年 | 102篇 |
2016年 | 104篇 |
2015年 | 90篇 |
2014年 | 117篇 |
2013年 | 92篇 |
2012年 | 78篇 |
2011年 | 78篇 |
2010年 | 96篇 |
2009年 | 79篇 |
2008年 | 61篇 |
2007年 | 79篇 |
2006年 | 53篇 |
2005年 | 43篇 |
2004年 | 55篇 |
2003年 | 42篇 |
2002年 | 41篇 |
2001年 | 44篇 |
2000年 | 50篇 |
1999年 | 82篇 |
1998年 | 56篇 |
1997年 | 60篇 |
1996年 | 63篇 |
1995年 | 52篇 |
1994年 | 52篇 |
1993年 | 38篇 |
1992年 | 30篇 |
1991年 | 29篇 |
1990年 | 24篇 |
1989年 | 29篇 |
1988年 | 16篇 |
1987年 | 9篇 |
1986年 | 14篇 |
1985年 | 13篇 |
1984年 | 5篇 |
1983年 | 7篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1980年 | 3篇 |
1978年 | 2篇 |
1976年 | 3篇 |
1974年 | 1篇 |
1967年 | 1篇 |
1965年 | 1篇 |
1964年 | 2篇 |
排序方式: 共有2324条查询结果,搜索用时 15 毫秒
21.
Paeonia delavayi is a wild tree peony species endemic to high-altitude regions in southwestern China. Recent agricultural land expansion, however, not only causes its dwindling population size, but also poses a severe threat to its long-term persistence. Since our knowledge of the reproductive biology of P. delavayi is very limited, and some management misconceptions have further exacerbated the already worrisome situation, the aim of the present study is to provide some scientific evidence regarding habitat preference and breeding system of P. delavayi, and to help correct some management misconceptions. Specifically, morphological traits of P. delavayi populations growing in two different habitats (i.e., the open area versus forest understory), including basal stem diameter, plant height and flower numbers per plant, were measured and compared with each other. A range of floral manipulation experiments was further conducted to study the extent of self-compatibility and pollen limitation. The results showed that P. delavayi preferred the open habitat over forest understory as it generally grew bigger with thicker stems, and produced a higher number of flowers in the former habitat. Therefore, the open habitat should receive immediate priority for conservation action instead of being converted to agricultural land; also, P. delavayi is pollen limited, as evidenced by the fact that experimental flowers receiving supplemental non-self pollen generally displayed elevated seed production than flowers subjected to other treatments (e.g., non-supplemental self pollen, non-supplemental non-self pollen and supplemental self pollen), which is consistent with reported observations that alpine plants are more likely to be pollen-limited than lowland plants. We suggest that human intervention might be necessary to guarantee the long-term persistence of P. delavayi as harsh alpine environment, intense competition for pollinators and different anthropogenic perturbations co-limit its reproductive success. 相似文献
22.
23.
地形在金字塔沙山形成发育过程中起着重要的作用。金字塔沙山通常形成于山前地带,应属于地形屏障影响下形成的一种沙丘类型。首先,上升气流的发育是山前风阻区气流的主要特征,地形屏障是上升气流发生发展的主要原因。沙山的坡脚、坡中及坡顶分别是上升气流的启动区、发育区及衰退区。上升气流是沙山增高增大发育的主要机制。其次,金字塔沙山多发育于局地环流发育较强的地带,局地环流与区域风况配置是形成复杂沙丘类型的重要因素。本区局地环流偏南风不仅持续时间长,且受到鸣沙山微地形的影响,下坡气流较强。实地观测结果破解了常规气候台观测数据难以揭示上升气流及局地环流对金字塔沙丘形成发育的影响。第三,下附地形在沙山形成发育过程中决定着沙丘发育的“临界尺度”,即丘体达到“临界尺度”的时候,坡面上升气流及风速放大作用逐渐显现,促使丘体增高增大发育,随着沙丘形态与上升气流的互馈作用进一步增强,金字塔沙丘逐渐形成演化为高大沙山。实地观测进一步证实了金字塔沙山是纵向(横向)沙丘形变的一种形式。并提出了金字塔沙山在地形条件下形成演化的一种新模式。 相似文献
24.
25.
26.
The flow patterns in the near wake of a cylinder (either circular or square in shape, D=25 mm) placed in the proximity of a fully developed turbulent boundary layer (thickness δ=0.4D) are investigated experimentally using particle image velocimetry (PIV). The effects of changing the gap height (S) between the cylinder bottom and the wall surface, over the gap ratio range S/D=0.1–1.0, have been investigated. The results show that both the ensemble-averaged and instantaneous flow fields are strongly dependent on S/D. The flow patterns for the two types of cylinders share many similarities with respect to the change in S/D, such as the reduced recirculation length and increased velocity fluctuation in the near wake with increasing S/D, as well as the trend of suppression of vortex shedding at small S/D and onset of vortex shedding at large S/D. However, developments of the shear layers, in terms of wake width, flow curvature, etc., are considerably different for these two types of cylinders. In general, the wake development and momentum exchange for the square cylinder are slower those for the circular cylinder at the same gap ratio. Correspondingly, it is shown that the periodic vortex shedding is delayed and weakened in the case of square cylinder, as compared to that of the circular cylinder at the same S/D. 相似文献
27.
Characteristics of mineral element content of alpine vegetation
in permafrost region on Qinghai-Tibetan Plateau 总被引:1,自引:0,他引:1
The content characteristics of 16 elements (Al, Ca, Cr, Cu, Fe, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, Sr, and Zn) in 23 plant species
collected from the Qinghai-Tibetan Plateau permafrost region were investigated using ICP-OES. Results show that the average
contents of Ca, K, Mg, Fe and P were higher than 1,000 mg/kg, those of Al, Na, Zn and Cr ranged between 10–1,000 mg/kg and
those of Cu, Li, Pb and Mo were less than 10 mg/kg. The levels of Al, Ca, K, Mg and Na were within the scope of the reported
terrestrial plant element content, those of Sr, Fe and Cr were higher than the average of the terrestrial plants and the maximum
content of Mn was higher than the upper limit of the reported Mn content. The main character of the element content was of the
Ca>K type, however, in terms of Cyperaceae species the element content character was K>Ca type. The contents of Ca, Li, Mg
and Sr in Gramineae and Cyperaceae species were higher than those in other species and the contents of Ca, K, Mg, Fe, P, Al and
Na in all collected plants were higher than those of other elements. Zn had weak variability with the lowest coefficient (i.e.,
7.81%), while other elements had strong variability. The ratio of maximum content to minimum content indicated Ca and K had
less change than other elements in the Qinghai-Tibetan Plateau permafrost region. Element content of alpine vegetation in the
Qinghai-Tibetan Plateau permafrost region mainly shows a positive correlation, among which the correlation coefficient between
Al and Pb, Al and Fe, Mo and Cr, Pb and Fe, Sr and Li were higher than 0.9, and negative correlation had no statistical significance.
The correlation between Al and Fe, Mg, Mn in the Qinghai-Tibetan Plateau permafrost region were consistent with that reported
in Kunlun Mountains. 相似文献
28.
报道一种合成2,2-二甲基-4-戊烯酸的新方法。由异丁醛与烯丙醇缩合得到2,2-二甲基-4-戊烯醛,后者经氧化银选择氧化生成2,2-二甲基-4-戊烯酸。此法氧化产率(96%)高于分子氧氧化法(61%)和歧化法(41%)。反应中析出的金属银能定量回收,循环使用。 相似文献
29.
30.
Since soil moisture and vegetation index are direct and important indicators for surface drought status, a new drought monitoring method (MPDI1) is developed in NIR-Red reflectance space. It is a combination of two satellite-derived variables—a soil moisture component using the Perpendicular Drought Index (PDI), and a vegetation component using the Perpendicular Vegetation Index (PVI). Enhanced Thematic Mapper Plus (ETM+) image and in-situ ground observation are introduced to validate the accuracy of the proposed method. Results indicate that MPDI1 is highly consistent to the in-situ ground observation with the coefficient of determination (R2?=?0.49) between MPDI1 and 5–20 cm mean soil moisture, which is slightly higher than the coefficient of determination (R2?=?0.42) between MPDI1 and 10 cm soil moisture. Compared with drought indices such as PDI and the Modified Perpendicular Drought Index (MPDI), MPDI1 provides quite similar trends for bare soil or lower vegetated surface, but it demonstrates a better performance in measuring densely vegetated surface. This paper concludes that MPDI1 provides correct and sufficient information on surface drought status in soil-plant continuum, which appears to have robust available and great potential for surface drought estimation in China and other countries. 相似文献