全文获取类型
收费全文 | 39218篇 |
免费 | 634篇 |
国内免费 | 241篇 |
专业分类
测绘学 | 758篇 |
大气科学 | 2595篇 |
地球物理 | 7719篇 |
地质学 | 14171篇 |
海洋学 | 3580篇 |
天文学 | 8810篇 |
综合类 | 87篇 |
自然地理 | 2373篇 |
出版年
2022年 | 257篇 |
2021年 | 446篇 |
2020年 | 503篇 |
2019年 | 582篇 |
2018年 | 1016篇 |
2017年 | 1019篇 |
2016年 | 1089篇 |
2015年 | 587篇 |
2014年 | 1042篇 |
2013年 | 1885篇 |
2012年 | 1211篇 |
2011年 | 1644篇 |
2010年 | 1453篇 |
2009年 | 1805篇 |
2008年 | 1650篇 |
2007年 | 1709篇 |
2006年 | 1578篇 |
2005年 | 1086篇 |
2004年 | 1099篇 |
2003年 | 1131篇 |
2002年 | 1016篇 |
2001年 | 876篇 |
2000年 | 803篇 |
1999年 | 728篇 |
1998年 | 718篇 |
1997年 | 728篇 |
1996年 | 593篇 |
1995年 | 575篇 |
1994年 | 502篇 |
1993年 | 455篇 |
1992年 | 409篇 |
1991年 | 427篇 |
1990年 | 440篇 |
1989年 | 394篇 |
1988年 | 370篇 |
1987年 | 402篇 |
1986年 | 415篇 |
1985年 | 511篇 |
1984年 | 547篇 |
1983年 | 548篇 |
1982年 | 497篇 |
1981年 | 456篇 |
1980年 | 430篇 |
1979年 | 408篇 |
1978年 | 375篇 |
1977年 | 385篇 |
1976年 | 346篇 |
1975年 | 356篇 |
1974年 | 340篇 |
1973年 | 373篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
991.
A 132 kVA electrical transmission line has been proposed to connect the towns of Eket and Ikot Abasi in the eastern Niger delta in Nigeria. A geotechnical and hydrological study was performed to geotechnically characterize the route alignment and to provide data for design of the foundations for the transmission towers. In this paper, we introduce the study location and its geological, topographical and physiological characteristics, present the hydrological and geotechnical investigations and their results, and make recomendations for the design of the transmission tower foundations. Hydrological inspection of the power line alignment revealed that the route is traversed by numerous tributaries of the Qua-Iboe and Imo Rivers, the two major rivers that control the drainage of the study area. In-situ geotechnical tests consisted of cone penetration tests (CPTs), vane shear tests, test pit sampling (0–2.0 m depth) and lithologic boreholes (20 and 50 m deep). Laboratory tests included grain size analysis, pH determination and Atterberg Limits. Based on the CPT log pattern, no of stratum, and refusal depths, the soils along the transmission alignment were classified into twelve structural domains having common geotechnical properties. Because of the elevated water levels, and the nature of the soils, deep pile foundations were recommended for the towers. Software from the Louisiana Transportation Research Center, was employed to estimate skin friction, end bearing, and ultimate bearing capacities for assumed 225 mm square shaped reinforced concrete pile in each of the different zones of the alignment. 相似文献
992.
Mixed-layer corrensite-chlorites and their formation mechanism in the glauconitic sandstone-clayey rocks (Riphean, Anabar Uplift) 总被引:1,自引:0,他引:1
V. A. Drits T. A. Ivanovskaya B. A. Sakharov B. B. Zviagina N. V. Gor’kova E. V. Pokrovskaya A. T. Savichev 《Lithology and Mineral Resources》2011,46(6):566-593
The paper presents the first detailed mineralogical, structural, and crystal-chemical characteristics of the mixed-layer corrensite-chlorites
from the glauconitic sandy-clayey rocks that make up the bottom (0.10 m) of a basal member (1.50 m) of the lower subformation
of the Yusmastakh Formation (Riphean, Anabar Uplift, North Siberia). Like the overlying mudstones (1.40 m) in the basal member,
these rocks are generally transformed up to the deep catagenesis level and included in a thick dolomite sequence. In mudstones
represented by the dioctahedral micas, the corrensite-type minerals are observed as traces. 相似文献
993.
K.E. Eiermann 《Astronomische Nachrichten》2012,333(2):182-185
The Pioneer anomaly is one of the most important problems in modern physics. The observed blueshift of the Doppler signals coming back from the space probes Pioneer 10 and 11 is interpreted as being due to an anomalous acceleration ap = (8.74 ± 1.33) × 10–8 cm s–2 towards the Sun. In this paper the blueshift is explained by the frequency shifts of the receivers. These frequency shifts result from an increase in elementary particle masses in time, the rate of increase being tied up with the present‐day Hubble parameter H0. The result is that the seeming acceleration ap is the product of H0 and the velocity of light. Taking new physics into consideration, this paper presents a new explanation of the Pioneer anomaly based on the assumption that the Universe is eternal and infinite without expansion or contraction (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献
994.
The paper is focused on the two tectonic-geodynamic factors that made the most appreciable contribution to the transformation of the lithospheric and hydrocarbon potential distribution at the Barents Sea continental margin: Jurassic-Cretaceous basaltic magmatism and the Cenozoic tectonic deformations. The manifestations of Jurassic-Cretaceous basaltic magmatism in the sedimentary cover of the Barents Sea continental margin have been recorded using geological and geophysical techniques. Anomalous seismic units related to basaltic sills hosted in terrigenous sequences are traced in plan view as a tongue from Franz Josef Land Archipelago far to the south along the East Barents Trough System close to its depocentral zone with the transformed thinned Earth’s crust. The Barents Sea igneous province has been contoured. The results of seismic stratigraphy analysis and timing of basaltic rock occurrences indicate with a high probability that the local structures of the hydrocarbon (HC) fields and the Stockman-Lunin Saddle proper were formed and grew almost synchronously with intrusive magmatic activity. The second, no less significant multitectonic stress factor is largely related to the Cenozoic stage of evolution, when the development of oceanic basins was inseparably linked with the Barents Sea margin. The petrophysical properties of rocks from the insular and continental peripheries of the Barents Sea shelf are substantially distinct as evidence for intensification of tectonic processes in the northwestern margin segment. These distinctions are directly reflected in HC potential distribution. 相似文献
995.
E. Curti K. Fujiwara J. Tits A. Kitamura W. Müller 《Geochimica et cosmochimica acta》2010,74(12):3553-5422
High-purity synthetic barite powder was added to pure water or aqueous solutions of soluble salts (BaCl2, Na2SO4, NaCl and NaHCO3) at 23 ± 2 °C and atmospheric pressure. After a short pre-equilibration time (4 h) the suspensions were spiked either with 133Ba or 226Ra and reacted under constant agitation during 120-406 days. The pH values ranged from 4 to 8 and solid to liquid (S/L) ratios varied from 0.01 to 5 g/l. The uptake of the radiotracers by barite was monitored through repeated sampling of the aqueous solutions and radiometric analysis. For both 133Ba and 226Ra, our data consistently showed a continuous, slow decrease of radioactivity in the aqueous phase.Mass balance calculations indicated that the removal of 133Ba activity from aqueous solution cannot be explained by surface adsorption only, as it largely exceeded the 100% monolayer coverage limit. This result was a strong argument in favor of recrystallization (driven by a dissolution-precipitation mechanism) as the main uptake mechanism. Because complete isotopic equilibration between aqueous solution and barite was approached or even reached in some experiments, we concluded that during the reaction all or substantial fractions of the initial solid had been replaced by newly formed barite.The 133Ba data could be successfully fitted assuming constant recrystallization rates and homogeneous distribution of the tracer into the newly formed barite. An alternative model based on partial equilibrium of 133Ba with the mineral surface (without internal isotopic equilibration of the solid) could not reproduce the measured activity data, unless multistage recrystallization kinetics was assumed. Calculated recrystallization rates in the salt solutions ranged from 2.8 × 10−11 to 1.9 × 10−10 mol m−2 s−1 (2.4-16 μmol m−2 d−1), with no specific trend related to solution composition. For the suspensions prepared in pure water, significantly higher rates (∼5.7 × 10−10 mol m−2 s−1 or ∼49 μmol m−2 d−1) were determined.Radium uptake by barite was determined by monitoring the decrease of 226Ra activity in the aqueous solution with alpha spectrometry, after filtration of the suspensions and sintering. The evaluation of the Ra uptake experiments, in conjunction with the recrystallization data, consistently indicated formation of non-ideal solid solutions, with moderately high Margules parameters (WAB = 3720-6200 J/mol, a0 = 1.5-2.5). These parameters are significantly larger than an estimated value from the literature (WAB = 1240 J/mol, a0 = 0.5).In conclusion, our results confirm that radium forms solid solutions with barite at fast kinetic rates and in complete thermodynamic equilibrium with the aqueous solutions. Moreover, this study provides quantitative thermodynamic data that can be used for the calculation of radium concentration limits in environmentally relevant systems, such as radioactive waste repositories and uranium mill tailings. 相似文献
996.
Izvestiya, Atmospheric and Oceanic Physics - In this paper, seasonal and interannual upwelling variability in the Sea of Okhotsk off the eastern coast of Sakhalin Island based on wind data obtained... 相似文献
997.
The Portage Lake Lava Series of the Keweenaw Peninsula, northernMichigan, is composed of over 5000 m of tholeiitic lava flows.Chemical and petrologic study of thin undifferentiated flowsfrom this sequence shows that the bulk of the flows are olivinetholeiites; highly ironenriched tholeiites comprise up to 10per cent of the volume. In addition, a few small rhyolitic intrusives,exceedingly rich in alkalis, are exposed in the region. Duringburial the amygdular tops of each flow acted as channelwaysfor migrating fluids during a regional metamorphic event thatproduced a progressive sequence of secondary phases within theflow tops. In the upper part of the stratigraphic section, laumontite,analcime, albite, and chlorite dominate assemblages characteristicof the zeolite facies. Stratigraphically deeper. flow tops arecharacterized by metamorphic assemblages of the prehnite-pumpellyitefacies. Within the higher rank facies, rocks along fractureshave been transformed to calcium-rich monomineralic domains(metadomains) of epidote or pumpellyite. These metadomains gradeoutward through incompletely reconstituted rocks containingalbitized feldspars, and finally, in flow centers, to basaltdisplaying few signs of mineralogical readjustment. Extremechemical disparity exists within the altered flow tops, particularlywith respect to calcium and sodium content. Calculations revealthat bulk compositions of metamorphically adjusted flow topsare similar to unaltered basalts. Thus, the chemical variationdisplayed by the metamorphic rock types resulted from localizedmetamorphic differentiation. The Keweenawan sequence displays progressive dehydration fromthe top to the base of the stratigraphic section. At the topchlorite (H2O = 12 per cent) and pumpellyite (H2O = 6 per cent)metadomains formed by dehydration. The fact that most of theserocks were hydrated strongly suggests that fluid pressures wereless than total pressures during the metamorphic event. In rocksundergoing hydration in the uper part of the stratigraphic section,therefore, water pressures decreased away from channelways.As a result, the most hydrous rock types formed in these regions.Conversely, in rock undergoing dehydration water pressures increasedaway from channelwasys, so that the most dehydrated rocks formedin such regions. Water content of an individual rock is dependenton both position in the stratigraphic pile and position withrespect to fluid channelways. 相似文献
998.
Chepurov A. I. Sonin V. M. Zhimulev E. I. Chepurov A. A. Tomilenko A. A. 《Doklady Earth Sciences》2011,441(2):1738-1741
This work presents data on the experimental study of CaCO3 stability (3.0–5.5 GPa; 1300–1400°C) under reducing conditions modeling the presence of metallic iron. It is established
that CaCO3 is stable at the above P-T parameters under reducing conditions (in the presence of metallic Ti). CaCO3 decomposed only when it chemically interacted with iron, forming Ca-ferrites and releasing solid carbon in the form of graphite
in the closed system (in sealed Pt-ampoules). 相似文献
999.
J.D. Martín-Martín J. Tritlla E. Cardellach D. Gmez-Gras 《Journal of Geochemical Exploration》2006,89(1-3):267
Fluid inclusions and clay mineralogy of the Permo-Triassic rocks from the Espina and Espadà Ranges (SE Iberian Chain, Spain) have been investigated to establish their relationship with hydrothermal fluid circulation during the Alpine Orogeny. Primary fluid inclusions in quartz-filled tension gashes in Permo-Triassic sandstones reveal maximum temperatures around 230 °C and very constant salinities of 8.5% wt. eq. NaCl. Secondary fluid inclusions found in quartz from the Santonian Ba–Cu–Hg deposits show similar compositional and thermodynamic characteristics, denoting an Alpine recrystallization. Clay mineral composition of Permo-Triassic mudrocks is characterized by pyrophyillite, indicating low-grade metamorphic conditions. Field observations and experimental data suggest that the crystallization of quartz in tension gashes, the formation of secondary fluid inclusions and the development of the metamorphism are contemporaneous and related to fluid circulation during the Alpine compression. Fluid flow took place along the Hercynian fault system that was reactivated during the Mesozoic rift stage and inverted during the Alpine deformation. 相似文献
1000.
A basic model for the formation of non-equilibrium rotational energy distributions is described for reactive, homo-polar diatomic molecules and ions in the interstellar medium. Kinetic models were constructed to calculate the rotational populations of $\mathrm{C}_{2}^{+}$ under the conditions it would experience in the diffuse interstellar medium. As the non-polar ion reacts with molecular hydrogen, but not atomic hydrogen, the thermalization of a hot nascent rotational population will be arrested by chemical reaction when the H2 density begins to be significant. Populations that deviate strongly from the local thermodynamic equilibrium are predicted for $\mathrm{C}_{2}^{+}$ in environments where it may be detectable. Consequences of this are discussed and a new optical spectrum is calculated. 相似文献