全文获取类型
收费全文 | 1449篇 |
免费 | 257篇 |
国内免费 | 448篇 |
专业分类
测绘学 | 81篇 |
大气科学 | 322篇 |
地球物理 | 341篇 |
地质学 | 768篇 |
海洋学 | 208篇 |
天文学 | 126篇 |
综合类 | 116篇 |
自然地理 | 192篇 |
出版年
2024年 | 5篇 |
2023年 | 20篇 |
2022年 | 64篇 |
2021年 | 56篇 |
2020年 | 42篇 |
2019年 | 42篇 |
2018年 | 70篇 |
2017年 | 61篇 |
2016年 | 67篇 |
2015年 | 60篇 |
2014年 | 89篇 |
2013年 | 67篇 |
2012年 | 79篇 |
2011年 | 90篇 |
2010年 | 87篇 |
2009年 | 86篇 |
2008年 | 79篇 |
2007年 | 86篇 |
2006年 | 69篇 |
2005年 | 66篇 |
2004年 | 42篇 |
2003年 | 57篇 |
2002年 | 71篇 |
2001年 | 73篇 |
2000年 | 76篇 |
1999年 | 71篇 |
1998年 | 72篇 |
1997年 | 78篇 |
1996年 | 39篇 |
1995年 | 49篇 |
1994年 | 37篇 |
1993年 | 47篇 |
1992年 | 23篇 |
1991年 | 26篇 |
1990年 | 18篇 |
1989年 | 17篇 |
1988年 | 15篇 |
1987年 | 11篇 |
1986年 | 8篇 |
1985年 | 6篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 3篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1975年 | 1篇 |
1973年 | 2篇 |
1971年 | 1篇 |
1958年 | 2篇 |
排序方式: 共有2154条查询结果,搜索用时 62 毫秒
981.
Zhuming Yang Kuishou Ding Jeffrey de Fourestier Qian Mao He Li 《Mineralogy and Petrology》2013,107(2):163-169
The Fe-rich Li-bearing magnesionigerite-6N6S occurs in the Xianghualing tin-polymetallic ore field, Linwu County, Hunan Province, Peoples Republic of China. It was found near the outer contact zone of the Laizhiling granite body and in the Middle-Upper Devonian carbonate rocks of Qiziqiao Formation. The mineral formed during the skarn stage. Its empirical formula is Sn1.81Li0.67(Fe1.43Zn1.19 Mn0.41)Σ3.03(Al14.89Mg1.46 Ti0.11Si0.01)Σ16.47O30(OH)2. The structure for magnesionigerite-6N6S was solved and refined in space group R-3?m, with a?=?5.7144(8), c?=?55.446(11) Å, V?=?1568.0(4) Å3, to R1?=?0.0528. Based on the structural refinement of single crystal diffraction data the formula of magnesionigerite-6N6S is Sn1.80Li0.97(Fe1.89Zn0.91) Σ2.80 (Al14.60Mg1.63 Ti0.20)Σ16.43O30(OH)2 with Z?=?3. Fe-rich Li-bearing magnesionigerite-6N6S contains 0.74 wt.% Li2O. The idealized charge-balanced composition of magnesionigerite-6N6S may be expressed by bivalent and trivalent cations: (Mg2+)4(Al3+)18O30(OH)2. The simplified general formula for the 6N6S polysomes in the nigerite and högbomite groups can be given as A x B18-x O30(OH)2, x?=?~4, where A?=?Mg2+, Fe2+, Zn2+; B?=?Al3+, Sn4+, Ti4+, Li+, □. 相似文献
982.
983.
Humification degree of peat and its implications for Holocene climate change in Hani peatland, Northeast China 总被引:2,自引:0,他引:2
The humification degree of peat is a significant climatic proxy for paleoclimate change.Using the alkali-extraction method,a time series of absorbance values of the Hani peatland,Northeast China,was determined,which is used as an indicator for the humification degree of peat.Combined with14C dating data of peat cellulose,and compared withδ18O andδ13C time series of the cellulose in the Hani peatland,the evidence for the existence of14 ka paleoclimate was provided.Higher humification degrees hint a warmer-wetter climate,and vice versa.It also reconstructs the four stages of Holocene climate evolution in this region:11.5–9.8 cal ka B.P.,warm and wet period;9.8–9.0 cal ka B.P.,cold and dry period;9.0–4.8 cal ka B.P.,warm and wet period;and 4.8–0 cal ka B.P.,warm-wet and dry-cold alternation period.Meanwhile,it is revealed that the abrupt climate shifts signals such as the "8.2 ka" event and the "4.2 ka" event.Results showed that the Hani peat humification degree is of sensitive response to paleoclimate change.Therefore,it is a feasible method to analyze the relationship between paleoclimate change and peat humification degree. 相似文献
984.
Guangxuan Han Liqiong Yang Junbao Yu Guangmei Wang Peili Mao Yongjun Gao 《Estuaries and Coasts》2013,36(2):401-413
Using the Eddy Covariance (EC) technique, we analyzed temporal variation in net ecosystem CO2 exchange (NEE) and determined the effects of environmental factors on the balance between ecosystem photosynthesis and respiration in a reed (Phragmites australis) wetland in the Yellow River Delta, China. Our results indicated that diurnal and seasonal patterns of NEE and its components (ecosystem respiration (R eco), gross primary production (GPP)) varied markedly among months for the growing season (May to October). The cumulative CO2 emission was 1,657 g CO2 m?2, while 2,612 g CO2 m?2 was approximately accumulated as GPP, which resulted in the reed wetland being a net sink of 956 g CO2 m?2. The ratio of R eco to GPP in reed wetland was 0.68, which was close to other temperate wetlands. Soil temperature and soil moisture exerted the primary controls on R eco during the growing season. Daytime NEE values during the growing season were strongly correlated with photosynthetically active radiation. Aboveground biomass showed significant linear relationships with 24-h average NEE, daytime GPP, and R eco, respectively. Thus, we conclude that the coastal wetland acted as a carbon sink during the growing season despite the variations in environmental conditions, and long-term flux measurements over these ecosystems are undoubtedly necessary. 相似文献
985.
Multivariate analysis of heavy metals in surface sediments from lower reaches of the Xiangjiang River, southern China 总被引:4,自引:0,他引:4
Longjiang Mao Duowen Mo Yuanyuan Guo Qiang Fu Jinghong Yang Yaofeng Jia 《Environmental Earth Sciences》2013,69(3):765-771
The concentrations of heavy metals (Cr, Mn, Co, Ni, Cu, Zn, Cd, and Pb) in 16 samples collected from the lower reach (Changsha–Xiangtan–Zhuzhou section) of the Xiangjiang River in southern China were determined by high-resolution inductively coupled plasma mass spectroscopy (HR-ICPMS). Multivariate analysis, such as principal component analysis and cluster analysis, coupled with correlation coefficient analysis, was used to analyze the analytical data and to identify possible pollution sources of heavy metals. The results showed that the eight studied heavy metals accumulated in the sediments from the lower Xiangjiang River, especially Mn, Cu, Zn, Pb and Cd, which were 2.0–2.6, 1.7–2.6, 3.5–3.8, 3.2–3.6 and 189.5–152.8 times the soil trace element background for Hunan Province and UCC background values, respectively. Principal component analysis and cluster analysis, coupled with correlation coefficient analysis, revealed that the sediments from lower Xiangjiang River were mainly influenced by two sources: Cr, Co, Ni, Cu, Zn, Cd and Pb mainly originated from industrial sources, whereas Mn was derived from both industrial and natural sources, but mainly from natural sources due to weathering and erosion. 相似文献
986.
987.
Dingrong Xiong Haojing Zhang Xiong Zhang Yonggang Zheng Wenguang Liu Lisheng Mao Bangrong Huang Ting Kang Yongjuan Cha Fei Guo 《Astrophysics and Space Science》2013,343(1):345-360
Using γ-ray data detected by Fermi Large Area Telescope (LAT) and multi-wave band data for 40 TeV active galactic nuclei (AGNs), we have studied the correlations between flux densities (F R, F IR, F O, F X and F γ ) in the radio, infrared, optical, X-ray and γ-ray wave bands. Our results are the following: (1) For TeV HSP BL Lacertae objects (THBLs), there are strong correlations between F γ and F R and between F γ and F IR in all states (average/high/low); (2) The TeV radio galaxies (TRGs) deviate from the area occupied by THBLs; (3) The TeV flat-spectrum radio quasars (TFSRQs) have much stronger γ-ray emission than THBLs; (4) For THBLs, there are weak correlations between F γ and F X in all states as well as between F γ and F O in both average and high states, and a strong correlation between F γ and F O in the low state; (5) For THBLs, there are strong correlations between F O and F R in both low and average states as well as between F O and F IR in all states and between F IR and F R in all states, but no strong correlations among other bands are found. From these results, we suggest that for THBLs, the synchrotron self-Compton radiation (SSC) is the main mechanism of high energy γ-ray emission and the inverse Compton scattering of circum-nuclear dust is likely to be a important complementary mechanism. Compared with THBLs, TRGs and TFSRQs may have a different origin of high energy γ-ray. 相似文献
988.
WANG Chunguang HUANG Jiaqi XIANG Peng WANG Yanguo XU Zhenzu GUO Donghui LIN Mao 《海洋学报(英文版)》2014,33(6):95-102
Fifty-seven stations(48 grid stations and nine stratified stations) were sampled across the study region(67.000°–88.394°N, 152.500°–178.643°W) during the 4th Chinese National Arctic Research Expedition(CHINARE 4) from July to August 2010 by the icebreaker R/V Xuelong. A total of 24 species of Hydromedusae were identified from 130 zooplankton samples, of which seven species belonged to Automedusa, eight species to Anthomedusae, four species to Leptomedudae, and three species to Siphonophora. Catablema multicirratum Kishinouye, 1910, Bougainvillia bitentaculata Uchida, 1925, and Euphysa japonica(Maas, 1909) were recorded for the first time in the Arctic sea. In the present paper, 18 species of Hydromedusae were described and illustrated, of which three species were described for the first time in the Arctic sea, and 15 species were described for the first time in China. 相似文献
989.
To simulate the soil moisture variation in cropland, a two-parameter exponential recession model was derived to depict the recession process of soil moisture in the root zone. The model is based on the assumption that the recession rate of soil water is propor- tional to the potential evapotranspiration rate and the difference of soil water content and steady soil water content. Two parameters in this model are soil texture-dependent recession constant and steady soil water content. The model was calibrated and validated with measured soil water data at two experiment sites in North China with different soil textures and cropping systems. Coefficients of de- termination between measured and model simulated soil water content were all greater than 0.7, indicating that both models gave satis- factory simulation results. Results showed that values of two parameters mentioned above are both larger for finer soil than those for coarser soil. At the same potential evapotranspiration rate and soil water content, the recession rate of finer soil is usually lower than that of coarser soil. The proposed model can be used in irrigation management to predict approximate date for irrigation, as well as be em- bedded into watershed hydrological models to estimate the antecedent precipitation index. 相似文献
990.
Jing Yang Qing Bao Duoying Ji Daoyi Gong Rui Mao Ziyin Zhang Seong-Joong Kim 《大气科学进展》2014,31(5):1147-1156
Two parallel sets of numerical experiments (an ozone-hole simulation and a non-ozone-hole simulation) were performed to investigate the effect of ozone depletion on surface temperature change using the second spectral version of the Flexible Global Ocean-Atmosphere-Land System model (FGOALS-s2), focusing on the eastern Antarctica (EA) continent in austral summer. First, we evaluated the ability of the model to simulate the EA surface cooling, and found the model can successfully reproduce the cooling trend of the EA surface, as well as the circulation change circling the South Pole in the past 30 years. Second, we compared the two experiments and discovered that the ozone depletion causes the cooling trend and strengthens the circumpolar westerly flow. We further investigated the causes of the EA surface cooling associated with the ozone hole and found two major contributors. The first is the ozone-hole direct radiation effect (DRE) upon the surface that happens because the decrease of the downward longwave (LW) radiation overcomes the increase of the downward shortwave (SW) radiation under clear sky. The second is the cloud radiation effect (CRE) induced by ozone depletion, which happens because the decreased downward SW radiation overcomes the increased downward LW radiation in the case of increased cloud. Although the CRE is theoretically opposite to the DRE, their final net effect makes comparable contributions to the EA surface cooling. Compared with the surface radiation budget, the surface heat flux budgets have a much smaller contribution. We additionally note that the CRE is basically ascribed to the circulation change. 相似文献