全文获取类型
收费全文 | 2261篇 |
免费 | 49篇 |
国内免费 | 26篇 |
专业分类
测绘学 | 48篇 |
大气科学 | 107篇 |
地球物理 | 578篇 |
地质学 | 770篇 |
海洋学 | 238篇 |
天文学 | 372篇 |
综合类 | 10篇 |
自然地理 | 213篇 |
出版年
2021年 | 21篇 |
2020年 | 31篇 |
2019年 | 28篇 |
2018年 | 42篇 |
2017年 | 33篇 |
2016年 | 63篇 |
2015年 | 45篇 |
2014年 | 55篇 |
2013年 | 120篇 |
2012年 | 64篇 |
2011年 | 114篇 |
2010年 | 78篇 |
2009年 | 95篇 |
2008年 | 82篇 |
2007年 | 92篇 |
2006年 | 85篇 |
2005年 | 90篇 |
2004年 | 76篇 |
2003年 | 53篇 |
2002年 | 74篇 |
2001年 | 40篇 |
2000年 | 38篇 |
1999年 | 43篇 |
1998年 | 28篇 |
1997年 | 30篇 |
1996年 | 32篇 |
1995年 | 41篇 |
1994年 | 33篇 |
1993年 | 26篇 |
1992年 | 25篇 |
1991年 | 32篇 |
1990年 | 23篇 |
1989年 | 24篇 |
1988年 | 34篇 |
1987年 | 27篇 |
1986年 | 24篇 |
1985年 | 35篇 |
1984年 | 38篇 |
1983年 | 41篇 |
1982年 | 32篇 |
1981年 | 30篇 |
1980年 | 27篇 |
1979年 | 32篇 |
1978年 | 24篇 |
1977年 | 33篇 |
1976年 | 31篇 |
1975年 | 25篇 |
1973年 | 19篇 |
1972年 | 22篇 |
1971年 | 18篇 |
排序方式: 共有2336条查询结果,搜索用时 15 毫秒
161.
162.
Hydrogen speciation in synthetic quartz 总被引:1,自引:0,他引:1
Roger D. Aines Stephen H. Kirby George R. Rossman 《Physics and Chemistry of Minerals》1984,11(5):204-212
The dominant hydrogen impurity in synthetic quartz is molecular H2O. H-OH groups also occur, but there is no direct evidence for the hydrolysis of Si-O-Si bonds to yield Si-OH HO-Si groups. Molecular H2O concentrations in the synthetic quartz crystals studied range from less than 10 to 3,300 ppm (H/Si), and decrease smoothly by up to an order of magnitude with distance away from the seed. OH? concentrations range from 96 to 715 ppm, and rise smoothly with distance away from the seed by up to a factor of three. The observed OH? is probably all associated with cationic impurities, as in natural quartz. Molecular H2O is the dominant initial hydrogen impurity in weak quartz. The hydrolytic weakening of quartz may be caused by the transformation H2O + Si-O-Si → 2SiOH, but this may be a transitory change with the SiOH groups recombining to form H2O, and the average SiOH concentration remaining very low. Synthetic quartz is strengthened when the H2O is accumulated into fluid inclusions and cannot react with the quartz framework. 相似文献
163.
The valence and distribution of iron in vivianite, lazulite, babingtonite, rockbridgeite, acmite, aegirine-augite, hedenbergite, and ilvaite were studied with optical and Mössbauer spectroscopy. Optically activated intervalence charge transfer between Fe2+ and Fe3+ in neighboring sites through common edges or faces is observed in all these minerals irrespective of the polymerization of the iron-oxygen polyhedra ranging from finite clusters to infinite structural units. However, a distinct decrease occurs in the energy of the corresponding optical absorption band with increasing number of Fe2+ and Fe3+ ions involved in the charge transfer process. Thermally activated electron delocalization between Fe2+ and Fe3+ occurs only if Fe2+ and Fe3+ occupy crystallographically equivalent or geometrically very similar neighboring sites which share common edges to form extended structural units such as the ribbon in ilvaite. If the Fe-O polyhedra form finite clusters of two, three, or four polyhedra (e.g., in vivianite, lazulite, and babingtonite, respectively) no thermally-activated mixed-valence states of iron are observed. In aegirine, extended regions of the M1 chain are statistically occupied by Fe2+ and Fe3+ giving rise to thermally-activated electron delocalization in addition to the intervalence band in the optical absorption spectrum. The intensity of the optical intervalence absorption has been measured in a number of systems: ? values range from 60 to 210. 相似文献
164.
A two-dimensional numerical model is applied to a coastal ocean wherein alongshore elevation and density gradients, normally calculated by a three-dimensional model, are instead supplied by climatologically averaged data for the California Current System between 25 and 40°N. Surface wind stress is also obtained from climatological data. Both surface and bottom boundary layers are resolved in the model calculations; a second moment turbulence closure submodel supplies vertical diffusivities. Near steady state solutions are possible when surface buoyancy flux is imposed at the surface.Model results are as follows: Southward wind stress produces a broad equatorward current with an embedded coastal jet in accordance with previous studies. Positive wind stress curl reduces the jet current and produces a poleward undercurrent which then surfaces as the curl is increased. The jet currents are reduced and poleward flow increases as bottom steepness increases; to a lesser extent, inclusion of the beta effect has a similar effect. The existence of near bottom, poleward or equatorward flow is explained rather simply in terms of the bottom stress resulting from the alongshore balance of surface wind stress and vertically integrated pressure gradient, the latter involving the alongshore surface elevation and density gradient. A further finding is that the upwelling circulation associated with wind stress is confined to the top 200 to 300 m of the ocean along the California coast. 相似文献
165.
166.
Although several researchers have pointed out some advantages and disadvantages of various soil sampling designs in the presence of spatial autocorrelation, a more detailed study is presented herein which examines the geometrical relationship of three sampling designs, namely the square, the equilateral triangle, and the regular hexagon. Both advantages and disadvantages exist in the use of these designs with respect to estimation of the semivariogram and their effect on the mean square error or variance of error. This research could be used to design optimal sampling strategies; it is based on the theory of regionalized variables, in which the intrinsic hypothesis is satisfied. Among alternative designs, an equilateral triangle design gives the most reliable estimate of the semivariogram. It also gives the minimum maximum mean square error of point estimation of the concentration over the other two designs for the same number of measurements when the nugget effect is small relative to the variance. If the nugget effect is large (.90
2
or more), and the linear sampling density is >0.85r where r is the range, the hexagonal design is best. This study computes and compares the maximum mean square error for each of these designs. 相似文献
167.
George A. Griffiths 《Journal of Hydrology》1983,65(4):259-270
A theory of stable-channel design is deduced from the condition that, for any given steady flow, total sediment concentration maintains a balanced average in space and time within a stable channel. This condition for stability applies during changes in channel position but does not constrain them. A stability index is defined in terms of concentration of bedmaterial load: a function of steady, uniform or quasi-uniform water discharge, channel bedslope, water surface width and a typical diameter of cohesionless, granular bedmaterial. The index is constant only within a stable channel, and its mean magnitude may be determined from observations at cross-sections of more or less straight, wide reaches. Two distinct design methods, threshold and mobile-bed, are deduced from the theory. Design formula are presented for both methods as functions of the channel stability index, and are applicable to cross-sections or reaches, independent of planform, of both sand-silt and gravel-bed rivers. A relation describing variation along a channel in the ratio of bedload to suspended-sediment discharge, for a specified steady flow, is also inferred from the stability condition. 相似文献
168.
Effect of a kelp forest on coastal currents 总被引:1,自引:0,他引:1
Ocean currents supply a kelp ecosystem with nutrients, planktonic food, and larvae. We have found that these currents in a kelp forest (Macrocystis pyrifera) are slower than currents outside. At the Pt. Loma, San Diego, California, site that we studied, current velocities were about a third of those outside. A comparison of frequency spectra shows that semi-diurnal frequencies are preferentially passed by the kelp. This effect of a kelp forest on the currents that nurture it is similar to that of a terrestrial forest on local winds. 相似文献
169.
Mean growing season soil PCO2 data were obtained for 19 regions of the world in nine countries. Bivariate and multiple linear regression analysis with soil log(PCO2) as the dependent variable and TEMP, PRECIP, log(AET), and log(PET) as the four climatic independent variables demonstrated that AET was the best independent predictor of soil PCO2. An improved soil PCO2-AET model was developed by assuming (1) that as AET approaches zero, soil PCO2 approaches the atmospheric value and (2) that there is an upper limit to soil PCO2 at very high AET. This model has the form log(PCO2) = ?3·47 + 2·09 (1 ?e?0·0172 AET) where AET is in mm. It explains 67 per cent of the initial variation in the soil PCO2 data, predicts a soil log(PCO2) of ? 3·47 at AET = 0, and an upper limit of 3·5 per cent (log(PCO2) = ? 1·45) for mean growing season soil PCO2 at AET values of 2000 mm and above. The results of this study suggest that soil PCO2 levels in tropical areas are, on average, higher than those in temperate, alpine, and arctic regions. 相似文献
170.
Jeffrey P. Chanton Christopher S. Martens George W. Kipphut 《Geochimica et cosmochimica acta》1983,47(10):1791-1804
Sediment accumulation rate studies utilizing excess 210Pb and 137Cs were conducted as part of recent investigations of biogeochemical cycling at a single site in Cape Lookout Bight, a rapidly changing coastal basin on the Outer Banks of North Carolina (U.S.A.). Cores three meters in length reveal a depositional history for the bight interior characterized by a gradual transition in texture from coarse-grained to fine-grained material over the period 1946–1979. This transition is controlled by progressive enclosure of the bight by an active northerly migrating recurved spit. The textural gradation is periodically interrupted by layers of well-sorted sand associated with major storm events. Lead-210 data indicate that the upper meter of the sediment has accumulated at a rate of 3.35 to 4.71 g · cm?2 · yr?1 or approximately 8.4 to 11.8 cm · yr?1 (at ø = 0.84). Below 120 cm depth, dilution of clay and silt by low activity sand necessitates correction of the 210Pb profile in order to establish a geochronology. Grain size 210Pb distribution measurements at three depths reveal that the specific activity (dpm · g?1) of clay is 3.2 times that of silt and 24.7 times that of sand. Corrections of bulk sediment excess 210Pb activities based on these measurements lead to dates for textural changes which are consistent with charted changes in basin morphology and major storm events.Sixteen 137Cs measurements between 33–241 cm depth reveal a peak activity at 105–115 cm and indicate a minimum sedimentation rate of approximately 2.7 g · cm?2 · yr?1. 相似文献