全文获取类型
收费全文 | 17036篇 |
免费 | 3874篇 |
国内免费 | 5105篇 |
专业分类
测绘学 | 1897篇 |
大气科学 | 3198篇 |
地球物理 | 4353篇 |
地质学 | 9440篇 |
海洋学 | 2946篇 |
天文学 | 542篇 |
综合类 | 1576篇 |
自然地理 | 2063篇 |
出版年
2024年 | 91篇 |
2023年 | 320篇 |
2022年 | 872篇 |
2021年 | 1017篇 |
2020年 | 846篇 |
2019年 | 959篇 |
2018年 | 1001篇 |
2017年 | 860篇 |
2016年 | 971篇 |
2015年 | 931篇 |
2014年 | 1055篇 |
2013年 | 1169篇 |
2012年 | 1182篇 |
2011年 | 1210篇 |
2010年 | 1216篇 |
2009年 | 1183篇 |
2008年 | 1158篇 |
2007年 | 1109篇 |
2006年 | 947篇 |
2005年 | 881篇 |
2004年 | 633篇 |
2003年 | 511篇 |
2002年 | 583篇 |
2001年 | 593篇 |
2000年 | 541篇 |
1999年 | 602篇 |
1998年 | 471篇 |
1997年 | 446篇 |
1996年 | 434篇 |
1995年 | 332篇 |
1994年 | 323篇 |
1993年 | 286篇 |
1992年 | 263篇 |
1991年 | 202篇 |
1990年 | 166篇 |
1989年 | 137篇 |
1988年 | 121篇 |
1987年 | 60篇 |
1986年 | 69篇 |
1985年 | 41篇 |
1984年 | 43篇 |
1983年 | 22篇 |
1982年 | 41篇 |
1981年 | 18篇 |
1980年 | 20篇 |
1979年 | 16篇 |
1978年 | 10篇 |
1977年 | 7篇 |
1958年 | 13篇 |
1954年 | 12篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
11.
概略地叙述了西宁地区的泥石流形成条件和形成机理,并将西宁地区泥石流划分为稀性泥石流类及泥石质泥石流种类。在此基础上提出防治泥石流灾害的意见。 相似文献
12.
13.
利用Logisitic曲线模型和扩展支撑模型对居民可支配收入预测和单项消费预测,以河北省居民的收入及其消费支出为实例,研究了城镇居民和农民的可支配收入分配和单项边际消费倾向,合理建立了预测模型,并理论计算了1999~2003年的通信消费方面的边际消费倾向、预测了2004~2008年通信消费情况。最后,根据预测结果对通信网络建设提出了一些有价值的建议。 相似文献
14.
Hong-Guang Wang Guo-Jun Qiao Ren-Xin XuDepartment of Astronomy Peking University Beijing 《中国天文和天体物理学报》2003,3(5):443-452
The inner vacuum gap model has become the foundation stone of most theories on pulsar radio emission. The fundamental picture of this model is the sparking, which was conjectured to be induced by magnetic absorption of background gamma photons. However, a question is, can the sparking be triggered in the millisecond pulsars (MSPs) with magnetic fields (B) only about 108 G? We investigate this problem by including the pair production above the inner gap. Under the assumption that the magnetic field is dipolar, our results show the background gamma-ray emission can not be the key factor that triggers the sparking, at least not in MSPs with B - 108 G, if the temperature in the polar cap region is only so high as is observed (< 4 × 106 K). Some other mechanisms are required. 相似文献
15.
H. L. Xu S. Svanberg R. D. Cowan P.-H. Lefèbvre P. Quinet E. Biémont 《Monthly notices of the Royal Astronomical Society》2003,346(2):433-440
Radiative lifetime measurements were performed with time-resolved laser-induced fluorescence techniques for 24 levels of Nd ii in the energy range 20 500–32 500 cm−1 . For 17 levels, no previous experimental data exist. These results have allowed the testing of new theoretical calculations with the relativistic Hartree–Fock method taking configuration interactions and core-polarization effects into account, and a satisfying agreement has been found for this complex ion. A new set of calculated oscillator strengths, accurate within a few per cent for the strongest transitions, is presented for 107 lines of astrophysical interest appearing in the wavelength range 358.0–1100.0 nm. These results will be useful to evaluate abundance values of neodymium in chemically peculiar stars in relation with cosmochronology. 相似文献
16.
The effect of ocean wave breaking as a non-Bragg mechanism on backscattering cross-section and modulation transfer functions (MTF) of radar was investigated based on Bragg resonance theory and parametric method. The result showed that the additional effect of wave breaking on backscattering cross-section is not more than 20% except for the small incident angle of VV polarized electromagnetic (e.m.) wave but is significant for HH polarized e.m. wave. Breaking waves lead to increase in the modulus of tilt modulation MTF and the larger the wind speed, the faster the increase. For large incident angle, the modulus of tilt modulation MTF with wave breaking decreases quickly with incident angle for HH polarization and approach to that without wave breaking for VV polarization. The hydrodynamic MTF increases 30%-60% when considering wave breaking and the increase is larger for HH polarization than for VV polarization. 相似文献
17.
Jianhua FAN Bingfei SHI 《东北亚地学研究》2006,9(2):217-219
Kunming Basin locates middle of Yunnan altiplano and has a particularity in geography,topographic and geological environment.With the urban dilation quickly,add the reason of the unreasonable city layout,conflicts between environment and urban resources consumption become shrill increasingly.It is human being activities that lead to vulnerability and depravation of geological environment in local.Take a few examples on geological environment to expatiate relationship between urban construction and geological environment carrying capacity,and find a way how to make a better plan for urban sustainable development to achieve new balance between man and nature in local. 相似文献
18.
1 INTRODUCTIONIn the previous paper (on et al. 1999, hereafter Paper I), we investigated the wavelength-dependence of four colltribution functions (CFs) derived from dmerent formal solutions andreferring to different emergellt quantities in the unpolarized case. Because one cannot generallyassign a single formation region to the whole line band in a real stellar atmosphere, e.g., the solaratmosphere, instead, the line formation region can be defined as the layers deviating farthestfrom t… 相似文献
19.
对20个氯代苯酚类化合物进行了CNDO/2量子化学计算,讨论了化合物电子结构与其对水生物翻车鱼(Bluegill)、花鳉鱼(Guppy)、和虾(Shrimp)半致死量之间的关系,分别获得了表示其构效关系的三个线性方程,它们的显著性均远高于α=0.01水平。结果表明:氯代苯酚化合物苯环上碳原子的兀电荷之和(∑Qπ_R)越大苯环上碳原子的净电荷之和(∑Q_R)越大,或LUMO轨道能(E_(LUMO))越低,均使化合物对Bluegill和Guppy水生物的毒性增大;而化合物苯环上羟基邻位的碳原子的兀电荷布居(P_3)越大,同时HOMO轨道能E_(HOMO)越高,则化合物对Shrimp毒性也越大。据此,可预测氯代苯酚类化合物对上述三种水生物的毒性。 相似文献
20.
An improved approach is given for deriving the physical parameters of a molecular cloud by its NH3 rotation-inversion hyperfine spectra. The optical depth τ0(1, 1) of NH3 (1,1) is obtained by considering the blending effect of the magnetic hyperfine spectral lines, the (1-1) excitation temperature Tex (1, 1) is calculated by a two level model, the effect of different collision rate and the thermalization of the (1, 1) inversion lines are discussed, the rotation temperature TR ( 2, 1) between the NH3 levels (2, 2) and (1, 1) and the column density N( 1, 1) of the NH3 (1, 1) inversion level are derived, the results of the total column density from different assumptions for the abundance ratio of ortho-NH3 and para-NH3 are also discussed.
This approach is used for the molecular cloud NGC 2023 by using the relevant observed data and its optical depth and other physical parameters are obtained. 相似文献