全文获取类型
收费全文 | 5474篇 |
免费 | 558篇 |
国内免费 | 160篇 |
专业分类
测绘学 | 239篇 |
大气科学 | 613篇 |
地球物理 | 1985篇 |
地质学 | 2222篇 |
海洋学 | 308篇 |
天文学 | 383篇 |
综合类 | 189篇 |
自然地理 | 253篇 |
出版年
2023年 | 2篇 |
2022年 | 10篇 |
2021年 | 25篇 |
2020年 | 11篇 |
2019年 | 21篇 |
2018年 | 451篇 |
2017年 | 392篇 |
2016年 | 268篇 |
2015年 | 164篇 |
2014年 | 127篇 |
2013年 | 135篇 |
2012年 | 672篇 |
2011年 | 433篇 |
2010年 | 130篇 |
2009年 | 153篇 |
2008年 | 137篇 |
2007年 | 128篇 |
2006年 | 131篇 |
2005年 | 837篇 |
2004年 | 878篇 |
2003年 | 655篇 |
2002年 | 180篇 |
2001年 | 71篇 |
2000年 | 47篇 |
1999年 | 15篇 |
1998年 | 6篇 |
1997年 | 17篇 |
1996年 | 14篇 |
1991年 | 12篇 |
1990年 | 10篇 |
1989年 | 6篇 |
1987年 | 4篇 |
1983年 | 2篇 |
1981年 | 2篇 |
1980年 | 3篇 |
1976年 | 3篇 |
1975年 | 4篇 |
1973年 | 2篇 |
1969年 | 2篇 |
1968年 | 2篇 |
1965年 | 3篇 |
1963年 | 2篇 |
1961年 | 2篇 |
1959年 | 2篇 |
1956年 | 1篇 |
1955年 | 2篇 |
1954年 | 2篇 |
1951年 | 2篇 |
1948年 | 2篇 |
1925年 | 1篇 |
排序方式: 共有6192条查询结果,搜索用时 5 毫秒
991.
Mathesia darderi, a slender cylindrical monopleurid genus, formerly documented from the late Aptian-Albian of Spain, France, Tunisia, Algeria, Egypt and Turkey, has been discovered in the upper Barremian and the lower Aptian of Bulgaria and Spain. Notwithstanding some morphological changes, Barremian-lower Aptian forms and those of the upper Aptian-Albian possess the same myocardinal organisation and the same microstructural attributes. The inner shell margin of the right valve displays scalloped, festooned, tubular and vermiform microstructures. The most prominent evolutionary trait of M. darderi is the increase in body size through time. A statistical analysis of size distributions show that populations of the late Barremian-early Aptian, and the late Aptian, and those of the early to middle Albian, are significantly different; a pattern which has a biostratigraphic potential. Ecological changes through time are expressed by a displacement of communities from the central/distal part, to the proximal part of carbonate platforms. M. darderi is present locally in the upper Barremian-lower Aptian, and has its major spreading over the European and Arabo-African margins of the Mediterranean Tethys during the Clansayesian-lower to middle Albian. The disappearance of the species at the Middle-Upper Albian boundary, correlates with a critical, spatial reduction of carbonate platforms. 相似文献
992.
On February 22, 2011, an earthquake of magnitude 6.3 occurred very near to the city of Christchurch, New Zealand. The consequence
came as a shock to many seismologists and earthquake engineers as New Zealand is known as the homeland of modern earthquake-resistant
design techniques. After the earthquake, the focus of discussion has been on the collapse of buildings, while few queried
the adequacy of design requirements. Importantly, similar “inadequacy” seems to repeat all around the world more frequently
than expected. Hence, the question statement in the title concerns not only Christchurch, but anywhere in the world. 相似文献
993.
The Islands of Réunion and Mayotte are French territories in the Indian Ocean and are prone to numerous and recurrent hazards
as well as to increasing vulnerabilities. This paper examines disaster risk reduction on non-sovereign islands through the
case studies of Réunion and Mayotte. The approach used is embedded in standard geographical approaches to risk analysis, focusing
on place-based vulnerability, in order to understand disaster risk and disaster risk reduction on both islands. In addition
to establishing baseline information and visiting both islands for further data, 21 semi-structured individual interviews
were completed to understand the people’s perceptions of the hazards and of their vulnerabilities. Disaster risk reduction
strategies, and attitudes from governing bodies implementing the strategies, tend to assume similar vulnerability contexts
as mainland France, despite obvious differences on the islands. Many policies are implemented in a top–down manner by those
from mainland France who have little interest in, and who do not fully take into consideration, local realities. This paper
contributes new case studies to the literature, especially in comparing two non-Anglophone non-sovereign islands, and reinforces
recommendations factoring in local contexts when addressing ‘vulnerability of a place’ to disasters. 相似文献
994.
Composition and origin of organic matter in surface sediments of Lake Sarbsko: A highly eutrophic and shallow coastal lake (northern Poland) 总被引:1,自引:0,他引:1
Micha? Woszczyk Achim Bechtel Reinhard Gratzer Maciej J. Kotarba Miko?aj Kokociński Jens Fiebig Roman Cie?liński 《Organic Geochemistry》2011,42(9):1025-1038
We present an organic geochemical study of surface sediments of Lake Sarbsko, a shallow coastal lake on the middle Polish Baltic coast. The aim was to provide evidence concerning the origin of the organic matter (OM) and its compositional diversity in surface deposits of this very productive, highly dynamic water body. The content and composition of the OM in the bottom sediments were investigated at 11 sampling stations throughout the lake basin. OM sources were assigned on the basis of bulk indicators [total organic carbon (TOC), total nitrogen (TN), δ13CTOC and δ15N and extractable OM yield], biomarker composition of extractable OM and compound-specific C isotope signatures. The source characterization of autochthonous compounds was verified via phytoplankton analysis. The distribution of gaseous hydrocarbons in the sediments, as well as temporal changes in lake water pH, the concentration of DIC (dissolved inorganic carbon) and δ13CDIC were used to trace OM decomposition.The sedimentary OM is composed mainly of well preserved phytoplankton compounds and shows minor spatial variability in composition. However, the presence of CH4 and CO2 in the bottom deposits provides evidence for microbial degradation of sedimentary OM. The transformation of organic compounds in surface, bottom and pore waters via oxidative processes influences carbonate equilibrium in the lake and seasonally favours precipitation or dissolution of CaCO3.The data enhance our understanding of the relationships between the composition of sedimentary OM and environmental conditions within coastal ecosystems and shed light on the reliability of OM proxies for environmental reconstruction of coastal lakes. 相似文献
995.
Mobilisation of people living with HIV/AIDS in the 1980s was tied to social identity and the public rendering of AIDS as a
gay disease. This activism has continued in large cities alongside new streams of mobilization addressing the needs of groups
such as women and Aboriginal peoples living with HIV. We draw on interviews conducted with six people living with HIV/AIDS
in a small Ontario city where the population does not allow for services tailored to specific groups. We investigate experiences
of social engagement amongst people living with, and affected by, HIV/AIDS and examine why efforts to establish a community-based
participatory research project was unsuccessful. Drawing from indicators of community capacity developed by Jackson et al.
(Health Promot Int 18(4): 339–350, 2003) we identify the role of stigma both amongst different groups living with HIV/AIDS
and the wider public as undermining a common sense of community and, potentially, our efforts to establish a community-based
participatory research process. 相似文献
996.
G��nther J. Redhammer Anatoliy Senyshyn Martin Meven Georg Roth Sebastian Prinz Astrid Pachler Gerold Tippelt Clemens Pietzonka Werner Treutmann Markus Hoelzel Bj?rn Pedersen Georg Amthauer 《Physics and Chemistry of Minerals》2011,38(2):139-157
The compound NaFeGe2O6 was grown synthetically as polycrystalline powder and as large single crystals suitable for X-ray and neutron-diffraction experiments to clarify the low temperature evolution of secondary structural parameters and to determine the low temperature magnetic spins structure. NaFeGe2O6 is isotypic to the clinopyroxene-type compound aegirine and adopts the typical HT-C2/c clinopyroxene structure down to 2.5?K. The Na-bearing M2 polyhedra were identified to show the largest volume expansion between 2.5?K and room temperature, while the GeO4 tetrahedra behave as stiff units. Magnetic susceptibility measurements show a broad maximum around 33?K, which marks the onset of low-dimensional magnetic ordering. Below 12?K NaFeGe2O6 transforms to an incommensurately modulated magnetic spin state, with k?=?[0.323, 1.0, 0.080] and a helical order of spins within the M1-chains of FeO6 octahedra. This is determined by neutron-diffraction experiments on a single crystal. Comparison of NaFeGe2O6 with NaFeSi2O6 is given and it is shown that the magnetic ordering in the latter compound, aegirine, also is complex and is best described by two different spin states, a commensurate one with C2??/c?? symmetry and an incommensurate one, best being described by a spin density wave, oriented within the (1 0 1) plane. 相似文献
997.
998.
Nb-Ta-Ti-bearing oxide minerals (Nb-Ta-bearing rutile, columbite-group minerals) represent the most common Nb-Ta host in topaz-albite granites and related rocks from the Krásno-Horní Slavkov ore district. Tungsten-bearing columbite-(Fe), W-bearing ixiolite, wodginite and tapiolite-(Fe) are extremely rare in these rocks. Rutile contains significant levels of Ta (up to 37?wt.% Ta2O5) and Nb (up to 24?wt.% Nb2O5), with Ta/(Ta?+?Nb) ratio ranging from 0.04 to 0.61. Columbite-group minerals are represented mostly by columbite-(Fe) and rarely by columbite-(Mn), with Mn/(Mn?+?Fe) ratio ranging from 0.23 to 0.94. The exceptionally rare Fe-rich, W-bearing ixiolite occurs only as inclusions in Nb-Ta-bearing rutile from quartz-free alkali-feldspar syenites (Vysoky Kámen stock). Wodginite was found only in the topaz-albite microgranite of gneissic breccia matrix that occurs in the upper most part of the Hub topaz-albite granite stock. In wodginite, the Mn/(Mn?+?Fe) ratio is 0.42?C0.51, whereas the coexisting tapiolite-(Fe) has a distinctly lower Mn/(Mn?+?Fe) ratio close to 0.06. 相似文献
999.
Contrasting tourmaline types from peraluminous granites: a case study from Moslava?ka Gora (Croatia)
Two texturally and chemically distinct types of tourmaline are found inside peraluminous granites of the Moslava?ka Gora, Croatia: nodular tourmaline in the two-mica granite and disseminated tourmaline in the cross-cutting leucogranite dykes. Both tourmaline types belong to alkali tourmaline group, nodular tourmaline being dravite to schorl and disseminated tourmaline corresponding to schorl. Comparison of characteristic parameters of nodular (Nt) and disseminated tourmaline (Dt) shows significant differences in #Fe (0.40?C0.65 for Nt vs. 0.74?C0.85 in Dt) along with variations in the calculated X-site vacancy (0.22?C0.37 pfu in Nt and 0.33?C0.44 in Dt) and ??/(??+Na) ratio (0.23?C0.40 in Nt and 0.34?C0.45 in Dt). Disseminated tourmaline from the MG leucogranites is regarded as an early crystallized magmatic phase, while the interstitial tourmaline from the cores of tourmaline nodules originated from more complex mineralogical and chemical interactions inside the two-mica granite melt. Major element gain (Mg) and loss (Fe, Ca, Na, K) for the ??idealized nodule?? (34 vol. % core + 66 vol. % halo) when compared to the host granite shows that the nodule??s volume is not a completely independent and closed system. Based on the observed characteristics, nodule??s halo can be considered as a ??transitional zone?? between the tourmaline-bearing core and the host granite, texturally and mineralogically related to the host two-mica granite, chemically being an integral part of the nodule??s volume at the same time. 相似文献
1000.
Thomas?Oommen Debasmita?MisraEmail author Anupma?Prakash Sukumar?Bandopadhyay Sathy?Naidu John?J.?Kelley 《Natural Resources Research》2011,20(1):11-24
Multiple Regressive Pattern Recognition Technique (MRPRT) is an adapted approach for improved geologic resource estimation.
We developed and tested this approach for the Platinum (Pt) bearing region near Goodnews Bay, Alaska, which presents an example
of a complex depositional environment. We applied geospatial and pattern recognition methods to assess the spatial distribution
of offshore Pt in the Goodnews Bay area from point data collected by various agencies. We used the coefficient of correlation
(r) and the Nash–Sutcliffe efficiency (E) to quantitatively assess the degree of accuracy of the estimated Pt distribution. We split the study area, based on trend
analysis, into two regions: inside the Bay and outside the Bay. We could not obtain appreciable estimates from the geospatial
and pattern recognition methods. Using MRPRT, we were able to improve r from 0.57 to 0.93 and the E from 28.31 to 92.91 inside the Bay. We achieved improvement in r from 0.55 to 0.61 and E from 28.46 to 34.52 outside the Bay. The reasons for a non-significant improvement outside the Bay have been discussed. The
results indicate that the proposed MRPRT has wide application potential in georesource estimation where input data is often
scarce. 相似文献