全文获取类型
收费全文 | 9446篇 |
免费 | 1668篇 |
国内免费 | 2216篇 |
专业分类
测绘学 | 475篇 |
大气科学 | 1914篇 |
地球物理 | 2557篇 |
地质学 | 4774篇 |
海洋学 | 947篇 |
天文学 | 526篇 |
综合类 | 928篇 |
自然地理 | 1209篇 |
出版年
2024年 | 53篇 |
2023年 | 163篇 |
2022年 | 419篇 |
2021年 | 475篇 |
2020年 | 421篇 |
2019年 | 510篇 |
2018年 | 572篇 |
2017年 | 484篇 |
2016年 | 571篇 |
2015年 | 462篇 |
2014年 | 605篇 |
2013年 | 520篇 |
2012年 | 486篇 |
2011年 | 570篇 |
2010年 | 543篇 |
2009年 | 502篇 |
2008年 | 457篇 |
2007年 | 477篇 |
2006年 | 355篇 |
2005年 | 290篇 |
2004年 | 289篇 |
2003年 | 281篇 |
2002年 | 292篇 |
2001年 | 261篇 |
2000年 | 302篇 |
1999年 | 389篇 |
1998年 | 335篇 |
1997年 | 336篇 |
1996年 | 278篇 |
1995年 | 241篇 |
1994年 | 288篇 |
1993年 | 221篇 |
1992年 | 170篇 |
1991年 | 130篇 |
1990年 | 104篇 |
1989年 | 78篇 |
1988年 | 98篇 |
1987年 | 51篇 |
1986年 | 45篇 |
1985年 | 41篇 |
1984年 | 36篇 |
1983年 | 33篇 |
1982年 | 35篇 |
1981年 | 27篇 |
1980年 | 7篇 |
1979年 | 10篇 |
1978年 | 5篇 |
1975年 | 2篇 |
1958年 | 7篇 |
1954年 | 2篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
81.
W. C. Zhao J. P. Cheng Z. Y. Yu Q. L. Tang F. Cheng Y. W. Yin W. H. Wang 《International Journal of Environmental Science and Technology》2013,10(3):487-494
People living in the urban area and the surrounding suburban area have disparities in exposure and health risks due to different levels of ambient air pollutants. The main objective of this study is to investigate the concentrations, seasonal variations, and related health risks of ambient air pollutants (PM10, NO2, and SO2) in urban and suburban areas of Ningbo, China. The results showed that the average PM10, NO2, and SO2 concentrations in the urban area were 85.2, 49.3, and 37.4 μg/m3, which were 1.13, 1.25, and 1.41 times the values of the suburban area during the period of March 2009 to February 2010. For the potential health risk analysis, the residents have been divided into four age categories namely, infants, children (1 year), children (8–10 years), and adults. The analysis took into account age-specific breathing rates, body weights for different age categories. The results showed that the potential health risks to respiratory disease for all age categories living in urban area were higher than those in suburban area. 相似文献
82.
吉林西部地区高砷地下水砷的阈值分析及风险评价 总被引:1,自引:0,他引:1
高砷地下水导致的砷中毒是吉林西部地区近年来新发现的地方病。为评价饮用水中砷对人体健康产生的潜在危害,在野外调查、采样分析的基础上,开展了水砷安全阈值分析 ,确定了砷的总质量浓度0.05 mg/L为研究区的水砷安全阈值,建立了地下水健康风险评价模型,进行了地下水砷健康风险评价。结果表明:研究区地下水砷引起的个人年均风险度最高达2.11×10-3 a-1,其中高砷的第四系承压水中97.06%的采样点水砷对人体的个人年均致癌风险度,大于国际辐射防护委员会(ICRP)推荐的最大可接受值5.0×10-5 a-1;作为备用水源的第三系大安组、泰康组承压水69.57%采样点砷的致癌风险度也超过最大可接受风险水平。通过对风险度评价结果与砷中毒病情等资料的对比分析,风险评价结果与砷中毒病情基本一致,验证了评价结果的合理性。 相似文献
83.
Hongyan Geng Min Sun Chao Yuan Wenjiao Xiao Weisheng Xian Guochun Zhao Lifei Zhang Kenny Wong Fuyuan Wu 《Chemical Geology》2009,266(3-4):373-398
Voluminous granitic intrusions are distributed in the West Junggar, NW China, and they can be classified as the dioritic rocks, charnockite and alkali-feldspar granite groups. The dioritic rocks (SiO2 = 50.4–63.8 wt.%) are calc-alkaline and Mg enriched (average MgO = 4.54 wt.%, Mg# = 0.39–0.64), with high Sr/Y ratios (average = 21.2), weak negative Eu (average Eu/Eu = 0.80) and pronounced negative Nb–Ta anomalies. Their Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7035–0.7042, εNd(t) = 4.5–7.9, εHf(t) = 14.1–14.5) show a depleted mantle-like signature. These features are compatible with adakites derived from partial melting of subducted oceanic crust that interacted with mantle materials. The charnockites (SiO2 = 60.0–65.3 wt.%) show transitional geochemical characteristics from calc-alkaline to alkaline, with weak negative Eu (average Eu/Eu = 0.75) but pronounced negative Nb–Ta anomalies. Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7037–0.7039, εNd(t) = 5.2–8.0, εHf(t) = 13.9–14.7) also indicate a depleted source, suggesting melts from a hot, juvenile lower crust. Alkali-feldspar granites (SiO2 = 70.0–78.4 wt.%) are alkali and Fe-enriched, and have distinct negative Eu and Nb–Ta anomalies (average Eu/Eu = 0.26), low Sr/Y ratios (average = 2.11), and depleted Sr–Nd and zircon Hf isotopic compositions ((87Sr/86Sr)i = 0.7024–0.7045, εNd(t) = 5.1–8.9, εHf(t) = 13.7–14.2). These characteristics are also comparable with those of rocks derived from juvenile lower crust. Despite of the differences in petrology, geochemistry and possibly different origins, zircon ages indicate that these three groups of rocks were coevally emplaced at ~ 305 Ma.A ridge subduction model can account for the geochemical characteristics of these granitoids and coeval mafic rocks. As the “slab window” opened, upwelling asthenosphere provided enhanced heat flux and triggered voluminous magmatisms: partial melting of the subducting slab formed the dioritic rocks; partial melting of the hot juvenile lower crust produced charnockite and alkali-feldspar granite, and partial melting in the mantle wedge generated mafic rocks in the region. These results suggest that subduction was ongoing in the Late Carboniferous and, thus support that the accretion and collision in the Central Asian Orogenic Belt took place in North Xinjiang after 305 Ma, and possibly in the Permian. 相似文献
84.
Integrated Ocean Drilling Program (IODP) Expeditions 304/305 recovered a total of 1.4 km sequence of lower crustal gabbroic and minor ultramafic rocks from the Atlantis Massif oceanic core complex on the western flank of the Mid Atlantic Ridge (MAR) at 30°N. We conducted an integrated paleomagnetic and rock magnetic study on this sequence to help address the interplay between magmatism and detachment faulting. Detailed thermal and alternating-field demagnetization results demonstrate that stable components of magnetization of mainly reversed polarity with unblocking temperatures below the Curie temperature of magnetite are retained in gabbroic rocks at IODP Site U1309. Several samples also contain multicomponent remanences of both normal and reversed polarities that were acquired over sharply defined blocking temperature intervals, providing evidence for localized reheating of some intervals during both normal and reversed polarity periods. Results from a series of rock magnetic measurements corroborate the demagnetization behavior and show that titanomagnetites are the main magnetic carrier rocks recovered at Site U1309D. The overall magnetic inclination of Hole U1309D is -35°, implying significant (up to ~ 50° counterclockwise, viewed to the north) rotation of the footwall around a horizontal axis parallel to the rift axis (010°) may have occurred. The tectonic rotations inferred by the paleomagnetic data suggest that the original fault orientation dipped relative steeply toward the spreading axis and subsequently rotated to a shallower angle. Coupled with the newly published U–Pb zircon ages for Hole U1309D rocks [Grimes, C.B., John, B.E., Wooden, J.L., 2008. Protracted construction of gabbroic crust at a slow-spreading ridge: Constraints from 206Pb/238U zircon ages from Atlantis Massif and IODP Hole 1309D, (30°N, MAR). Geochem. Geophys. Geosyst. 9, Q08012. doi:1029/2008GC002063], the new paleomagnetic data provide temporal and thermal constraints on the accretion history of the Atlantis Massif. 相似文献
85.
Sanzhong Li Timothy M. Kusky Xiaochun Liu Guowei Zhang Guochun Zhao Lu Wang Yuejun Wang 《Gondwana Research》2009,16(2):294
The western Dabie orogen (also known as the Hong'an block) forms the western part of the Dabie–Sulu HP–UHP belt, central China. Rocks of this orogen have been subjected to pervasive ductile deformation, and include numerous quartz schists and felsic mylonites cropping out in ductile shear zones. Quartz textures in these mylonites contain important clues for understanding the movement sense of late-collisional extrusion and exhumation of high-pressure–ultrahigh-pressure (HP–UHP) rocks from the lower crustal level to the upper crustal level during Middle Triassic and Early Jurassic. The orientation and distribution of quartz crystallographic axes were used to confirm the regional shear sense across the orogen. The asymmetry of c-axis patterns consistently indicates top-to-the-southeast thrusting across the orogen in early structural stages. Later stages of deformation show different senses of movement in northern and southern parts of the orogen, with top-to-the-northwest sinistral shearing recorded in rocks north of the Xinxian HP–UHP eclogite-facies belt, and top-to-the-southeast dextral shearing south of the same unit.Based on our study on quartz c-axis fabrics and marco- to micro-scale structures, simultaneous southeastward shearing within a large part of the orogen and normal faulting north of the Xinxian HP–UHP unit is explained by upward extrusion in early stages of deformation. The extrusion process has been attributed to syn- and late-collisional processes, accounting for some earlier deformation in the western Dabie orogen such as metamorphic sequences around the core of the Xinxian HP–UHP eclogite-facies unit. Much higher pressure of deformation is also indicated in the aligned glaucophane and omphacite from blueschist and eclogite in the field. An orogen-parallel eastward extrusion of the Xinxian HP–UHP eclogite-facies unit, however, occurred diachronously in later stages of deformation. Therefore, a tectonic model combining an early upward extrusion with a later eastward extrusion is presented. Two different stages and types of extrusion for exhumation of HP–UHP rocks are suitable to all of east central China. Geochronological data shows that the first, upward extrusion occurred during Middle Triassic, the second, eastward extrusion occurred during Late Triassic to Early Jurassic. These two extrusions are correlative with two stages of rapid exhumation of the Dabie HP–UHP rocks, respectively. These two-stage late-collisional (Middle Triassic to Early Jurassic) extrusion events bridge the gap between syn-collisional (Early to Middle Triassic) vertical extrusion and post-collisional (Cretaceous) eastward-directed lateral escape and provide vital clues to understanding the more detailed processes of exhumation of HP–UHP rocks. 相似文献
86.
Geocell reinforcement has been increasingly applied to embankment engineerings. Deformation calculation is one of the major concerns during the design process. Using Winkler foundation model and with consideration of the interface resistance effect, a deformation control differential equation for the geocell reinforced mattress under the vertical symmetric loads is presented in this paper. The corresponding power-series semi analytic solutions for the displacements and the internal forces of geocell reinforcement are also presented. Furthermore, the influence factors such as loads, length and flexural rigidity of geocell reinforcement, coefficient of subgrade reaction and the interface resistance on the stress-deformation characteristic of geocell reinforcement are discussed. Finally, two examples are employed to verify the presented method. This study suggests that the effect of the interface resistance on the deformation of geocell reinforcement should be considered in engineering design. 相似文献
87.
Junlai Liu Anjian Wang Haoran Xia Yunfeng Zhai Lan Gao Qunye Xiu Zhaochong Zhang Zhidan Zhao Dianhua Cao 《Mineralium Deposita》2010,45(6):567-582
There are two types of lead–zinc ore bodies, i.e., sandstone-hosted ores (SHO) and limestone-hosted ores (LHO), in the Jinding
giant sulfide deposit, Yunnan, SW China. Structural analysis suggests that thrust faults and dome structures are the major
structural elements controlling lead–zinc mineralization. The two types of ore bodies are preserved in two thrust sheets in
a three-layered structural profile in the framework of the Jinding dome structure. The SHO forms the cap of the dome and LHO
bodies are concentrated beneath the SHO cap in the central part of the dome. Quartz, feldspar and calcite, and sphalerite,
pyrite, and galena are the dominant mineral components in the sandstone-hosted lead–zinc ores. Quartz and feldspar occur as
detrital clasts and are cemented by diagenetic calcite and epigenetic sulfides. The sulfide paragenetic sequence during SHO
mineralization is from early pyrite to galena and late sphalerite. Galena occurs mostly in two types of cracks, i.e., crescent-style
grain boundary cracks along quartz–pyrite, or rarely along pyrite–pyrite boundaries, and intragranular radial cracks in early
pyrite grains surrounding quartz clasts. The radial cracks are more or less perpendicular to the quartz–pyrite grain boundaries
and do not show any overall (whole rock) orientation pattern. Their distribution, morphological characteristics, and geometrical
relationships with quartz and pyrite grains suggest the predominant role of grain-scale cracking. Thermal expansion cracking
is one of the most important mechanisms for the generation of open spaces during galena mineralization. Cracking due to heating
or cooling by infiltrating fluids resulted from upwelling fluid phases through fluid passes connecting the SHO and LHO bodies,
provided significant spaces for crystallization of galena. The differences in coefficients of thermal expansion between pyrite
and quartz led to a difference in volume changes between quartz grains and pyrite grains surrounding them and contributed
to cracking of the pyrite grains when temperature changed. Combined thermal expansion and elastic mismatch due to heating
and subsequent cooling resulted in the radial and crescent cracking in the pyrite grains and along the quartz–pyrite grain
boundaries. 相似文献
88.
To restore China’s degraded ecological environment, the government has launched an environmental restoration project named
the “Grain for Green Project” (GGP) in 1999. From 1999 to 2010, the government will spend 40 billion dollars to convert 147
million ha of croplands and 173 million ha of wastelands into forestlands and grasslands in 25 provinces, municipalities and
autonomous regions. A primary goal is to replace cropping and livestock grazing in fragile areas with trees and grass. Given
the tremendous scale and great number of participants in the project, the attitudes of the affected farmers and the future
development in the area where GGP is implemented have a direct influence on the success of the project. To gain a clear idea
about the farmers’ attitudes towards the project and put forward the development models for the forestlands converted from
croplands, two case sites in the mountain-gorge region in Nujiang River are selected as the study areas, and the methods of
field survey and semi-structured interview are adopted to make interviews with more than 100 households in 2002 and 2003 in
order to quantify the farmers’ opinions about the GGP and how it has affected their livelihood, socio-cultural and industrial
structures, etc. The results are as follows: 1) the project has a certain influence on the farmers with better economic basis
and exerts greater influence on the farmers living in the low-elevation regions than on those living in the regions with middle-high
elevation; 2) the production models of the local farmers has changed from cultivation and animal husbandry to forestry and
sidelines due to the project and the income structure has changed from animal husbandry as main income source to state subsidy
and sideline as main income sources; 3) the reduction in the grain income and decrease in the quantity of livestock because
of the project have led to the diminution in the total income of the farmers; 4) the project has resulted in changes in the
lifestyles and architecture styles of the local farmers, and the traditional “huotang” culture has gone away after the implementation
of the project; 5) energy utilization has changed from firewood to methane and electricity in the wake of the implementation
of the project. The above-mentioned study results have indicated that the GGP has truly exerted influence on the livelihood
and production of the local farmers. Therefore, it is very necessary to make a research into the development models in the
forestlands converted from croplands to resolve the problems of the farmers’ livelihood and production. The study results
will provide some references for the sustainable development of the mountain-gorge regions. 相似文献
89.
90.
不同成因类型煤中烷基环己烷和烷基苯的特征探讨 总被引:2,自引:0,他引:2
通过对藻煤、烛煤、腐殖煤共生煤层的分析,将不同成因类型煤中的烷基环己烷和烷基苯进行了比较。结果表明,不同成因类型煤中所含的烷基环己烷和烷基苯明显不同。各种成因类型煤中烷基环己烷和烷基苯所提供的母质信息与煤岩结果吻合。氧化环境不利于烷基环己烷和烷基苯的形成。烷基环己烷和烷基苯有时可以作为沉积环境和成熟度的标志。 相似文献