全文获取类型
收费全文 | 12923篇 |
免费 | 4137篇 |
国内免费 | 2383篇 |
专业分类
测绘学 | 1074篇 |
大气科学 | 2500篇 |
地球物理 | 3299篇 |
地质学 | 7318篇 |
海洋学 | 1803篇 |
天文学 | 518篇 |
综合类 | 1335篇 |
自然地理 | 1596篇 |
出版年
2025年 | 17篇 |
2024年 | 361篇 |
2023年 | 454篇 |
2022年 | 797篇 |
2021年 | 899篇 |
2020年 | 895篇 |
2019年 | 790篇 |
2018年 | 859篇 |
2017年 | 782篇 |
2016年 | 817篇 |
2015年 | 697篇 |
2014年 | 853篇 |
2013年 | 820篇 |
2012年 | 796篇 |
2011年 | 828篇 |
2010年 | 803篇 |
2009年 | 699篇 |
2008年 | 656篇 |
2007年 | 583篇 |
2006年 | 486篇 |
2005年 | 449篇 |
2004年 | 327篇 |
2003年 | 321篇 |
2002年 | 291篇 |
2001年 | 320篇 |
2000年 | 343篇 |
1999年 | 510篇 |
1998年 | 407篇 |
1997年 | 372篇 |
1996年 | 379篇 |
1995年 | 296篇 |
1994年 | 286篇 |
1993年 | 262篇 |
1992年 | 220篇 |
1991年 | 149篇 |
1990年 | 128篇 |
1989年 | 104篇 |
1988年 | 84篇 |
1987年 | 69篇 |
1986年 | 50篇 |
1985年 | 41篇 |
1984年 | 28篇 |
1983年 | 27篇 |
1982年 | 21篇 |
1981年 | 10篇 |
1980年 | 10篇 |
1979年 | 11篇 |
1978年 | 6篇 |
1977年 | 4篇 |
1958年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
21.
基于离散 Possion积分提出了一种灵活直接的向下延拓新方法 ,证明了方法的存在性 ,利用模拟数据进行的数值实验取得了很好的效果。 相似文献
22.
本文分析了影响我国近海的温带气旋气候特征及其影响因素。分析得出:3-7月是温带气旋的盛季,4月份是全年中温带气旋最多的月份。赤道东太平洋海温异常增暖的年份,温带气旋增多。 相似文献
23.
24.
25.
Dynamic responses of two buildings connected by viscoelastic dampers under bidirectional earthquake excitations 总被引:1,自引:1,他引:0
In this study,dynamic responses of two buildings connected by viscoelastic dampers under bidirectional excitations are extensively investigated.The two buildings are a 10-story building and a 16-story building,with the shorter building on the left.Viscoelastic dampers are installed at all fl oors of the shorter building.Equations of motion are formulated using a fractional derivative model to represent the viscoelastic dampers.Three cases are considered with mass eccentricities at 0,10% and-10% with respect to the dimensions of the buildings.The responses of the buildings are numerically predicted at different damper properties.The simulation results indicated that the maximum horizontal responses of the buildings without eccentricities are signifi cantly mitigated.However,torsional effects are adversely increased.For asymmetric buildings,the effectiveness of the connecting dampers is affected by building eccentricities.As a result,mass eccentricities must be taken into account in damper selection.When compared with vibrations induced by unidirectional excitations,bidirectional excitations can increase the responses of coupled asymmetric buildings.In addition,installing dampers only at the top fl oor of the shorter building may cause a sudden change in lateral stiffness of the taller building.Consequently,the story shear envelopes of the taller building are changed. 相似文献
26.
Based on the observed and NCEP reanalysis data from 1985 to 2006,the climate background and synoptic situation of fog at Great Wall Station were analyzed. It is shown that the seasonal variation of fog is controlled by the change of general circulation and local pressure field. Three favorable typical synoptic situations for fog development are found, the Front-of-A-Depression type, the Saddle-Shaped-Field type and the Passing-Weak-Cyclone type. The first one is the most important situation. Advection cooling fog is dominant at Great Wall Station, but there are other kinds of fog as well. As a result, some helpful principles for local fog forecasting are given. 相似文献
27.
The distribution and abundance of euphausiid larvae and salps was studied from samples collected in 2002 and 2006 from Prydz Bay, Antarctica.Larvae of Thysanoessa macrura and Euphausia superba were mainly distributed in the north of the continental shelf. T. macrura was more abundant and had a relatively wider distribution. In 2006, with ice having retreated and higher seawater temperatures and chlorophyll a levels, E. superba and T. macrura occurred in higher abundances and at more mature developmental stages. Euphausia crystallorophias was mainly distributed in the neritic region. In 2002, with severe ice conditions in the neritic region, abundance of E. crystallorophias was consisted of metanauplius (MN) and calyptopis I (CI). Salps, mostly Salpa thompsoni, had a low abundance in Prydz Bay. In 2002, S. thompsoni was only m)-3. In 2006, S. thompsoni was found at three stations located near the contimental factors, such as the timing of ice melt, polynya formation and food concentration appear to have a marked effect on the distribution and abundance of euphausiid larvae and salps. 相似文献
28.
Yang Li YueHeng Yang ShuJuan Jiao FuYuan Wu JinHui Yang LieWen Xie Chao Huang 《中国科学:地球科学(英文版)》2015,58(12):2134-2144
The hafnium isotopic analysis using laser ablation has been widely conducted on Hf-rich minerals(zircon/baddeleyite/ calzirtite/eudialyte), however, little work has been reported on Hf-poor(100 ppm) minerals. This work presents a detailed procedure of in situ hafnium isotopic analysis from rutile using laser ablation multiple collector inductively coupled plasma mass spectrometry(LA-MC-ICP-MS). The rutile U-Pb dating reference material JDX shows homogeneous hafnium isotopic ratios, with 176Hf/177Hf=0.281795±0.000015(2SD, n=33) and 176Lu/177Hf=0.000018±0.000004(2SD, n=17) that suggest the possibility of using JDX as a new reference material hafnium isotopic measurement. We also measure hafnium isotopic compositions of other rutile U-Pb dating reference material(R10, Sugluk-4 and PCA-S207) and the 176Hf/177 Hf values are similar to previously reported results, which confirms that we can acquire accurate and precise hafnium isotopic compositions using our developed analytical protocol. We analyzed hafnium isotopic compositions and U-Pb ages of rutile in high-temperature and ultrahigh-temperature granulites from various terrains of the Khondalite Belt from the North China Craton, combined with zircon results in the same area, suggesting that the metamorphic evolution history of the granulite is much more complicated than previously thought. 相似文献
29.
Hongfeng Yang Dun Wang Rumeng Guo Mengyu Xie Yang Zang Yue Wang Qiang Yao Chuang Cheng Yanru An Yingying Zhang 《地震研究进展(英文)》2022,2(1):100113
The MS 6.9 Menyuan earthquake in Qinghai Province, west China is the largest earthquake by far in 2022. The earthquake occurs in a tectonically active region, with a background b-value of 0.87 within 100 ?km of the epicenter that we derived from the unified catalog produced by China Earthquake Networks Center since late 2008. Field surveys have revealed surface ruptures extending 22 ?km along strike, with a maximum ground displacement of 2.1 ?m. We construct a finite fault model with constraints from InSAR observations, which showed multiple fault segments during the Menyuan earthquake. The major slip asperity is confined within 10 ?km at depth, with the maximum slip of 3.5 ?m. Near real-time back-projection results of coseismic radiation indicate a northwest propagating rupture that lasted for ~10 ?s. Intensity estimates from the back-projection results show up to a Mercalli scale of IX near the ruptured area, consistent with instrumental measurements and the observations from the field surveys. Aftershock locations (up to January 21, 2022) exhibit two segments, extending to ~20 ?km in depth. The largest one reaches MS 5.3, locating near the eastern end of the aftershock zone. Although the location and the approximate magnitude of the mainshock had been indicated by previous studies based on paleoearthquake records and seismic gap, as well as estimated stressing rate on faults, significant surface-breaching rupture leads to severe damage of the high-speed railway system, which poses a challenge in accurately assessing earthquake hazards and risks, and thus demands further investigations of the rupture behaviors for crustal earthquakes. 相似文献
30.
Chinese central government made a commitment to achieve a 40–45% reduction in carbon dioxide (CO2) per unit of GDP by 2020 compared with 2005. This targeted reduction was allocated averagely among all the provinces rather than individually according to different situations of each province. Though some research has been done regarding this rough allocation, two shortcomings in previous studies exist: Firstly, CO2 marginal abatement cost (MAC) has been ignored as one of the CO2 emission reduction allocation indexes. Secondly, either subjective or objective method has been used rather than comprehensively of both subjective and objective method to calculate the weight of each index in the previous studies. In order to fill the gaps, this paper builds a two-stage Shapley information entropy model to allocate CO2 emission reduction quota among the Chinese provinces based on the equity and efficiency principles. Afterward, three CO2 emission reduction quota allocation scenarios have been proposed. The results show that the CO2 MAC is an indispensable index in CO2 emission reduction quota allocation, because its value of CO2 Shapley information entropy is the highest among five indexes. CO2 emission reduction quota of lower-MAC provinces should be allocated larger, while the quota of higher-MAC provinces should be allocated smaller. Therefore, two suggested policies have been proposed: First, differential CO2 emission reduction quota allocation should be proposed. Second, synergetic development should be promoted. 相似文献