全文获取类型
收费全文 | 20309篇 |
免费 | 3695篇 |
国内免费 | 5947篇 |
专业分类
测绘学 | 569篇 |
大气科学 | 5962篇 |
地球物理 | 4362篇 |
地质学 | 6313篇 |
海洋学 | 4967篇 |
天文学 | 5361篇 |
综合类 | 999篇 |
自然地理 | 1418篇 |
出版年
2024年 | 98篇 |
2023年 | 248篇 |
2022年 | 545篇 |
2021年 | 682篇 |
2020年 | 750篇 |
2019年 | 991篇 |
2018年 | 721篇 |
2017年 | 706篇 |
2016年 | 701篇 |
2015年 | 916篇 |
2014年 | 1180篇 |
2013年 | 1265篇 |
2012年 | 1354篇 |
2011年 | 1437篇 |
2010年 | 1358篇 |
2009年 | 1800篇 |
2008年 | 1673篇 |
2007年 | 1753篇 |
2006年 | 1563篇 |
2005年 | 1409篇 |
2004年 | 1255篇 |
2003年 | 1066篇 |
2002年 | 907篇 |
2001年 | 809篇 |
2000年 | 765篇 |
1999年 | 681篇 |
1998年 | 613篇 |
1997年 | 412篇 |
1996年 | 409篇 |
1995年 | 382篇 |
1994年 | 329篇 |
1993年 | 269篇 |
1992年 | 188篇 |
1991年 | 180篇 |
1990年 | 92篇 |
1989年 | 82篇 |
1988年 | 76篇 |
1987年 | 39篇 |
1986年 | 32篇 |
1985年 | 45篇 |
1984年 | 36篇 |
1983年 | 28篇 |
1982年 | 20篇 |
1981年 | 18篇 |
1980年 | 18篇 |
1979年 | 4篇 |
1978年 | 9篇 |
1977年 | 21篇 |
1954年 | 11篇 |
1875年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
Kaizad F. Patel Ivan J. Fernandez Sarah J. Nelson Stephen A. Norton Cheryl J. Spencer 《水文研究》2021,35(5):e14147
The Bear Brook Watershed in Maine (BBWM) is a long-term research site established to study the response of forest ecosystem function to environmental disturbances of chronic acidic deposition and ecosystem nitrogen enrichment. Starting in 1989, the West Bear (treated) watershed received bimonthly applications of ammonium sulfate [(NH4)2SO4] fertilizer from above the canopy, whereas East Bear (reference) received ambient deposition. The treatments were stopped in 2016, marking the beginning of the recovery phase. Research at the site has focused on soils, streams, and vegetation. Here, we describe data collected over three decades at the BBWM—input and stream output nutrient fluxes, quantitative soil pits and soil chemistry, and soil temperature and moisture. 相似文献
2.
利用香港卫星定位参考站网GNSS观测数据,提取强热带风暴"塔拉斯"与热带风暴"洛克"影响期间各测站天顶方向对流层延迟,反演香港区域大气可降水量;根据香港区域49个天文台气象站提供的实测降雨量数据,分析大气可降水量与实际降雨量的相关性,以及两次台风对香港区域水汽时空分布的不同影响。结果表明,大气可降水量在台风影响前期均上升,在大量降雨后回落,但在连续台风的间歇期间,仍高于台风来临前的水平;水汽累积是大量降雨的前提条件,当水汽累积量相近时,水汽累积时长与累积降雨量呈正相关;台风期间大气可降水量值超过65 mm的区域面积与台风等级相关,台风路径对局部水汽分布有一定的影响。 相似文献
3.
Analytical study of ground motion caused by seismic wave propagation across faulted rock masses 下载免费PDF全文
Studying seismic wave propagation across rock masses and the induced ground motion is an important topic, which receives considerable attention in design and construction of underground cavern/tunnel constructions and mining activities. The current study investigates wave propagation across a rock mass with one fault and the induced ground motion using a recursive approach. The rocks beside the fault are assumed as viscoelastic media with seismic quality factors, Qp and Qs. Two kinds of interactions between stress waves and a discontinuity and between stress waves and a free surface are analyzed, respectively. As the result of the wave superposition, the mathematical expressions for induced ground vibration are deduced. The proposed approach is then compared with the existing analysis for special cases. Finally, parametric studies are carried out, which includes the influences of fault stiffness, incident angle, and frequency of incident waves on the peak particle velocities of the ground motions. 相似文献
4.
《Astroparticle Physics》2002,16(4):183-386
Frequency distributions of local muon densities in high-energy extensive air showers (EAS) are presented as signature of the primary cosmic ray energy spectrum in the knee region. Together with the gross shower variables like shower core position, angle of incidence, and the shower sizes, the KASCADE experiment is able to measure local muon densities for two different muon energy thresholds. The spectra have been reconstructed for various core distances, as well as for particular subsamples, classified on the basis of the shower size ratio Nμ/Ne. The measured density spectra of the total sample exhibit clear kinks reflecting the knee of the primary energy spectrum. While relatively sharp changes of the slopes are observed in the spectrum of EAS with small values of the shower size ratio, no such feature is detected at EAS of large Nμ/Ne ratio in the energy range of 1–10 PeV. Comparing the spectra for various thresholds and core distances with detailed Monte Carlo simulations the validity of EAS simulations is discussed. 相似文献
5.
6.
7.
8.
9.
In the atmospheric Čerenkov technique γ-rays are detected against the abundant background produced by hadronic showers. In
order to improve the signal to noise ratio of theexperiment, it is necessary to reject a significant fraction of hadronic
showers. Traditional background rejection methods based on image shape parameters have been extensively used for the data
from imaging telescopes. However, non-imaging Čerenkov telescopes have to develop very different means of statistically identifying
and removing cosmic ray events. Some of the parameters, which could be potentially important for non-imaging arrays, are the
temporal and spectral differences, the lateral distributions and density fluctuations of Čerenkov photons generated by γ-ray
and hadron primaries. Here we study the differences in fluctuations of Čerenkov photon density in the light pool at the observation
level from showers initiated by photons and those initiated by protons or heavier nuclei. The database of simulated events
for the PACT array has been used to evaluate the efficiency of the new technique. Various types of density fluctuations like
the short range and medium range fluctuations as well as flatness parameter are studied. The estimated quality factors reflect
the efficiencies with which the hadrons can be rejected from the data. Since some of these parameters are independent, the
cuts may be applied in tandem and we demonstrate that the proton rejection efficiency of ∼90% can be achieved. Use of density
fluctuations is particularly suited for wavefront sampling observations and it seems to be a good technique to improve the
signal to noise ratio.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
10.
The kinetics of the reactions of C2H radical with ethane (k1), propane (k2), and n-butane (k3) are studied over the temperature range of T = 96-296 K with a pulsed Laval nozzle apparatus that utilizes a pulsed laser photolysis-chemiluminescence technique. The C2H decay profiles in the presence of both the alkane reactant and O2 are monitored by the CH(A2Δ) chemiluminescence tracer method. The results, together with available literature data, yield the following Arrhenius expressions: k1(T) = (0.51 ± 0.06) × 10−10 exp[(−76 ± 30)K/T] cm3 molecule−1 s−1 (T = 96-800 K), k2(T) = (0.98 ± 0.32) × 10−10exp[(−71 ± 60)K/T] cm3 molecule−1 s−1 (T = 96-361 K), and k3(T) = (1.23 ± 0.26) × 10−10 cm3 molecule−1 s−1 (T = 96-297 K). At T = 296 K, k1 is measured as a function of total pressure and has little or no pressure dependence. The results from this work support a direct hydrogen abstraction mechanism for the title reactions. Implications to the atmospheric chemistry of Titan are discussed. 相似文献