全文获取类型
| 收费全文 | 58379篇 |
| 免费 | 14711篇 |
| 国内免费 | 21437篇 |
专业分类
| 测绘学 | 10804篇 |
| 大气科学 | 8168篇 |
| 地球物理 | 10664篇 |
| 地质学 | 39191篇 |
| 海洋学 | 11171篇 |
| 天文学 | 785篇 |
| 综合类 | 5100篇 |
| 自然地理 | 8644篇 |
出版年
| 2024年 | 417篇 |
| 2023年 | 1322篇 |
| 2022年 | 3512篇 |
| 2021年 | 4190篇 |
| 2020年 | 3450篇 |
| 2019年 | 4198篇 |
| 2018年 | 3727篇 |
| 2017年 | 3560篇 |
| 2016年 | 3455篇 |
| 2015年 | 4162篇 |
| 2014年 | 4052篇 |
| 2013年 | 5006篇 |
| 2012年 | 5472篇 |
| 2011年 | 5541篇 |
| 2010年 | 5470篇 |
| 2009年 | 5035篇 |
| 2008年 | 5226篇 |
| 2007年 | 4920篇 |
| 2006年 | 4652篇 |
| 2005年 | 3959篇 |
| 2004年 | 3018篇 |
| 2003年 | 2121篇 |
| 2002年 | 2182篇 |
| 2001年 | 2000篇 |
| 2000年 | 1610篇 |
| 1999年 | 767篇 |
| 1998年 | 282篇 |
| 1997年 | 186篇 |
| 1996年 | 151篇 |
| 1995年 | 117篇 |
| 1994年 | 83篇 |
| 1993年 | 85篇 |
| 1992年 | 88篇 |
| 1991年 | 54篇 |
| 1990年 | 72篇 |
| 1989年 | 30篇 |
| 1988年 | 14篇 |
| 1987年 | 26篇 |
| 1986年 | 30篇 |
| 1985年 | 27篇 |
| 1984年 | 30篇 |
| 1983年 | 19篇 |
| 1982年 | 13篇 |
| 1981年 | 12篇 |
| 1979年 | 29篇 |
| 1978年 | 11篇 |
| 1965年 | 8篇 |
| 1957年 | 21篇 |
| 1954年 | 32篇 |
| 1933年 | 10篇 |
排序方式： 共有10000条查询结果，搜索用时 15 毫秒
81.
82.
Glacier change and glacier runoff variation in the Tuotuo River basin,the source region of Yangtze River in western China 总被引：3，自引：0，他引：3
Glaciers in the Tuotuo River basin, western China, have been monitored in recent decades by applying topographical maps and
high-resolution satellite images. Results indicate that most of glaciers in the Tuotuo River basin have retreated in the period
from 1968/1971 to 2001/2002, and their shrinkage area is 3.2% of the total area in the late 1960s. To assess the influence
of glacier runoff on river runoff, a modified degree–day model including potential clear-sky direct solar radiation has been
applied to the glaciated regions of the river basin over the period 1961–2004. It was found that glacier runoff has increased
in the last 44 years, especially in the 1990s when a two-thirds increase in river runoff was derived from the increase in
glacier runoff caused by loss of ice mass in the entire Tuotuo River basin. 相似文献
83.
84.
Wenjun Yong E. Dachs A. C. Withers E. J. Essene 《Contributions to Mineralogy and Petrology》2008,155(2):137-146
The low-temperature heat capacity (C
p) of Si-wadeite (K2Si4O9) synthesized with a piston cylinder device was measured over the range of 5–303 K using the heat capacity option of a physical
properties measurement system. The entropy of Si-wadeite at standard temperature and pressure calculated from the measured
heat capacity data is 253.8 ± 0.6 J mol−1 K−1, which is considerably larger than some of the previous estimated values. The calculated phase transition boundaries in the
system K2O–Al2O3–SiO2 are generally consistent with previous experimental results. Together with our calculated phase boundaries, seven multi-anvil
experiments at 1,400 K and 6.0–7.7 GPa suggest that no equilibrium stability field of kalsilite + coesite intervenes between
the stability field of sanidine and that of coesite + kyanite + Si-wadeite, in contrast to previous predictions. First-order
approximations were undertaken to calculate the phase diagram in the system K2Si4O9 at lower pressure and temperature. Large discrepancies were shown between the calculated diagram compared with previously
published versions, suggesting that further experimental or/and calorimetric work is needed to better constrain the low-pressure
phase relations of the K2Si4O9 polymorphs.
Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users. 相似文献
85.
86.
Understanding of isotopic variations in leaf water is important for reconstruction of paleoclimate and assessment of global biochemical processes. We report here a study of isotopic distributions within a single needle of two pine species, Pinus resinosa Ait and Pinus strobes L., with the objective of understanding how isotopic compositions of leaf water are controlled by environmental and physiological variables. A 2D model was developed to simulate along-leaf isotopic variations and bulk leaf water isotopic compositions. In addition to variables common to all leaf water isotopic models, this 2D model also takes into account the specific geometry and dimensions of pine needles and the isotopic transport in xylem and mesophyll. The model can successfully simulate oxygen isotopic variations along a single needle and averaged over a leaf (bulk leaf water). The simulations suggest that isotopic composition of the bulk leaf water does not always depend only upon the average transpiration rate, which in turn raises questions about using leaf water isotopic values to estimate transpiration rates. An unsuccessful attempt to simulate along-needle hydrogen isotopic variations suggests that certain unknown biological process(es) may not have been incorporated into our 2D model, and if so, it calls for a reevaluation of all other models for hydrogen isotopic simulations of leaf water since they too lack these processes.Existing leaf water isotopic models are reviewed in this work. In particular, we evaluate the most frequently used model, the stomatal boundary layer model (also referred to as the Craig-Gordon model). We point out that discrepancy between the boundary layer model and the measured bulk leaf water seems to depend upon relative humidity. Using our 2D model, we show that this humidity dependency is a result of an interplay between environmental and physiological conditions: if the transpiration rate of plant leaves decreases with increasing relative humidity, our 2D model can reproduce the pattern of isotopic discrepancy between boundary layer model predictions and observations, enabling us to understand better the reason behind this discrepancy. 相似文献
87.
运用3DEEMs及荧光偏振方法研究pH、离子强度及浓度效应对腐殖酸荧光光谱特性的影响 总被引：6，自引：0，他引：6
利用三维荧光激发.发射光谱(3DEEMs)及荧光偏振法研究了3例腐殖酸和1例湖泊溶解有机物(DOM)在不同腐殖酸浓度、离子强度和pH条件下的荧光光谱特性.实验结果表明,随着浓度增大,各腐殖酸样的荧光强度增大,其中垃圾渗滤液腐殖酸和Pahokee泥炭腐殖酸的荧光峰出现红移.随着浓度进一步增大,腐殖酸的荧光偏振值增大,由此推断在此浓度范围内腐殖酸由于分子间的排斥作用使其构型出现伸展.离子强度增大导致腐殖酸及阿哈湖DOM的荧光强度降低.随着氯化钾(KCI)浓度增大,Fluka腐殖酸的荧光峰A蓝移,荧光峰B的荧光偏振值出现减小,推测是由于静电中和作用使其荧光基团发生卷曲造成的.离子强度对其余腐殖酸样和阿哈湖DOM的其他荧光特性没有明显的影响.总的来说,pH值的变化对腐殖酸和湖泊DOM的荧光特征的影响是很明显的.随着pH值增大,腐殖酸和湖泊DOM的荧光强度增强.而pH值减小时,Fluka腐殖酸和Pahokee泥炭腐殖酸的荧光峰(A峰)蓝移,当pH值降到5时,Fhka腐殖酸的荧光峰突然红移到最大,随后又继续蓝移;垃圾渗滤液腐殖酸和阿哈湖DOM的荧光峰位无明显的漂移.所有样品的荧光偏振值都减小(但阿哈湖DOM的A峰变化不大),但在pHi≤5时又增大,这种现象表明随着pH值降低,腐殖酸和湖泊DOM中有机大分子发生卷曲,但在pH≤5时由于羧基的质子化又以平面伸展构型出现在溶液中. 相似文献
88.
89.
90.