全文获取类型
收费全文 | 26491篇 |
免费 | 244篇 |
国内免费 | 965篇 |
专业分类
测绘学 | 1598篇 |
大气科学 | 2221篇 |
地球物理 | 4953篇 |
地质学 | 12439篇 |
海洋学 | 1128篇 |
天文学 | 1899篇 |
综合类 | 2192篇 |
自然地理 | 1270篇 |
出版年
2023年 | 16篇 |
2022年 | 53篇 |
2021年 | 59篇 |
2020年 | 62篇 |
2019年 | 65篇 |
2018年 | 4837篇 |
2017年 | 4110篇 |
2016年 | 2753篇 |
2015年 | 358篇 |
2014年 | 270篇 |
2013年 | 275篇 |
2012年 | 1119篇 |
2011年 | 2817篇 |
2010年 | 2095篇 |
2009年 | 2386篇 |
2008年 | 1972篇 |
2007年 | 2404篇 |
2006年 | 119篇 |
2005年 | 238篇 |
2004年 | 436篇 |
2003年 | 434篇 |
2002年 | 270篇 |
2001年 | 65篇 |
2000年 | 77篇 |
1999年 | 33篇 |
1998年 | 37篇 |
1997年 | 18篇 |
1996年 | 10篇 |
1995年 | 10篇 |
1994年 | 18篇 |
1993年 | 13篇 |
1992年 | 6篇 |
1991年 | 24篇 |
1990年 | 21篇 |
1989年 | 14篇 |
1988年 | 10篇 |
1987年 | 17篇 |
1986年 | 15篇 |
1985年 | 15篇 |
1984年 | 12篇 |
1982年 | 6篇 |
1981年 | 25篇 |
1980年 | 24篇 |
1979年 | 8篇 |
1976年 | 9篇 |
1975年 | 6篇 |
1974年 | 11篇 |
1972年 | 8篇 |
1971年 | 5篇 |
1969年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 812 毫秒
271.
M. R. Ramesh Kumar Y. Sadhuram G. S. Michael L. V. Gangadhara Rao 《Boundary-Layer Meteorology》1990,52(1-2):177-191
The spatial variability of the structure of the lower troposphere over the northwestern Indian Ocean for the period 12th July to 2nd September, 1983 has been studied using upper air data collected during the first scientific cruise of ORV Sagar Kanya.An analysis of thermodynamic structure and kinematics of the marine boundary layer for different zonal and meridional sections revealed the following features: (a) Temperature and humidity inversions were generally absent over the study area except over a few locations in the western region; (b) Large-scale subsidence was found over the central equatorial Indian Ocean; (c) The convective activity over the western Indian Ocean was found to be moderately suppressed as compared to the eastern region; (d) The zonal and meridional components of winds along the equator and 10° N zonal section exhibited a mirror-image-like distribution. 相似文献
272.
Surface aeration experiments were conducted in two types of rectangular tanks of aspect ratios i.e., length to width ratio (L/W) of 1.5 and 2 and developed simulation equations to correlate the oxygen transfer coefficient, k and power number, P0 with a parameter governing theoretical power per unit volume X. The parameter X is defined as equal to F4/3R1/3, where F and R are impellers’ Froude and Reynolds numbers respectively). Results have shown that the P0 can not be simulated singularly with either Reynolds number, R or Froude number, F, which results in scale-effects; there appears to be a need to incorporate the effects of both F and R. It was found that P0 is uniquely related to X for rectangular aeration tanks of both aspect ratios, however, such relationships are different depending upon the aspect ratios. It has been demonstrated that energy can be saved substantially if the aeration tanks are run at relatively higher input powers. It is also demonstrated that smaller sized tanks are more energy conservative and economical when compared to big sized tanks, while aerating the same volume of water, and at the same time by maintaining a constant input power in all the tanks irrespective of their size. 相似文献
273.
J.K. Tomson Y.J. Bhaskar Rao T. Vijaya Kumar J. Mallikharjuna Rao 《Gondwana Research》2006,10(1-2):115
Major, trace element compositions and Sr–Nd isotopic characteristics of charnockitic gneisses from the Southern Granulite Terrain (SGT), South India are presented. The study region encompasses the central segment of the Cauvery Shear Zone system (CSZ) and regions within the Madurai Block (MB) immediately south of it (designated here as the CSZ/MB and MB domains). Differences in the compositions and source characteristics between charnockitic rocks of the CSZ vis-à-vis those of the CSZ/MB and MB regions are highlighted. Foremost, the charnockites and enderbites of the CSZ show highly fractionated REE patterns with positive Eu-anomalies, depleted HREE, Y and near chondritic εNd0 and initial-87Sr/86Sr at ca. 2.5 Ga, consistent with hydrous partial melting of amphibolitic crust with residual garnet and hornblende for the parental melts. By contrast, modeled at ca. 1.8 Ga and 0.8 Ga, the CSZ/MB and MB charnockitic rocks, which show a wider range of Ti and P, relatively lower degree of HREE depletion, commonly negative Eu-anomalies and undepleted Y, present clear evidence for involvement of Archaean crustal components in sources of their magmatic protoliths. There is also evidence for significant intracrustal melting processes within a thickened crust at elevated temperatures between 800 and 1000 °C. Implications to the controversial Archaean–Neoproterozoic terrane boundary problem of the SGT are discussed. 相似文献
274.
Subrata Kumar Majumder Kamal ChandnaDhiren Sankar De Gautam Kundu 《International Journal of Mineral Processing》2006
Experiments were carried out to investigate the rheological properties of coal–oil–water suspension containing solids of different sizes. Two different coal samples with mean particle sizes of 120 mesh, 175 mesh and 220 mesh were used. The coal concentration was varied from 5% to 25% by weight. Sodium silicate has been used as an additive to study the behavior of the variation of average viscosity of the suspension. A generalized correlation has been developed to predict the average viscosity of suspension in terms of particle diameter of the coal, concentration of coal, viscosity of the suspending medium and the concentration of water. Experimental investigations revealed that coal–oil–water suspensions show an increase in the viscosity with decrease in coal size but with the addition of an additive, the average viscosity tends to decrease initially up to a certain optimum dosages and thereafter it increases with further addition of additives. Two empirical correlations are proposed for average viscosity of the coal–oil–water suspension, μsL in terms of physical properties of the solid and viscosity of the suspending medium with and without additives. 相似文献
275.
This paper presents the results of investigation on uptake of radioactive species 90Sr and 137Cs present in the liquid effluents from nuclear processing plants. Chemical precipitation process is adopted to remove radioactivity from the effluents with low and intermediate level of activity. In this process, radioactive 90Sr and 137Cs are co-precipitated along with copper ferrocyanide, ferric hydroxide and either calcium phosphate or barium sulphate. These precipitates being fine in size require flocculation for enhanced settling rate in clarifier/thickener. The flocculation by some selected high molecular weight polyacrylamide based polymers has improved the uptake of radioactive metal ions. The adsorption of these radioactive species has been found to increase in the presence of these flocculants thereby improving the decontamination factor (DF). While flocculating the precipitates, there may be some complex formation with Sr2+/Cs+, flocculant and the substrate. This has enhanced the uptake of the radioactive metal ions from the liquid component. The plant trials have indicated the improvement of DF value due to flocculation by cationic flocculant. 相似文献
276.
277.
278.
K. V. Sanil Kumar P. V. Hareesh Kumar 《Journal of the Indian Society of Remote Sensing》2005,33(1):137-146
Eddies and planetary waves are identified as one of the important factors that control the dynamics of the Arabian Sea. During
10–14 January 1990, Ignat, Paulyuchenkov (USSR ship) conducted an experiment in the central Arabian Sea and of late TOPEX/POSEIDON
satellites collected data on sea surface height (SSH) anomalies of the Arabian Sea. These data sets give an opportunity to
understand the characteristic of eddies and planetary waves in this region during winter. The geostrophic flow revealed three
anticyclonic and two cyclonic eddies of diameters ranging from 75 to more than 150 km from surface to subsurface levels. Current
speeds around different eddies were maximum at surface and varied from 9 cm/s to 25 cm/s (at the middle point between the
center and periphery). The occurrence of eddies were further investigated with the TOPEX/POSEIDON altimetry for the years
1993–97. The analysis revealed multiple eddies of diameter 100 to 550 km occur every year with maximum number of eddies during
1997 and minimum during 1995. The calculated speed varied between 8–30 cm/s around various eddies.
Longitude-Time plots showed annual Rossby waves generating at the eastern Arabian Sea and propagating westwards with a phase
speed of ~ 10 cm/s along 16° N. Further, it was observed that these waves arrived in the study area by January. In addition,
another positive anomaly of SSH was found generating at the western Arabian Sea simultaneously and extended up to the study
region by April–June. Time series of SSH at selected locations along 16°N revealed many small-scale oscillations and their
spatial variability. These oscillations were delineated using the FFT analysis. Other than the Rossby wave, the major components
at the study region were 40–60 and 26–32 day oscillations. The implications of these long period waves associated with eddies
are discussed. 相似文献
279.
The reservoir surveys carried out using the conventional equipments like theodolite, plane table, sextant, range finders,
sounding rods, echo-sounders and slow moving boats etc. are time consuming and sometime it takes up to three years to complete
the survey of a major reservoir like Hirakud. During such longtime of survey, the siltation pattern and the bed levels also
get changed. So, the need to update the sediment measurement techniques and to introduce latest technology available in the
field was felt to overcome the difficulties faced in the conventional method especially in major reservoirs. With this objective,
the present study of the assessment of the sedimentation in Linganmakki reservoir is carried out using remote sensing and
GIS techniques. The revised capacities in the zone of reservoir levels (533.05 – 548.78 m) for the year 1989–1990 and 2001–2002
in the zone of 532.20 – 545.91 m of reservoir levels were determined, which is found 2837.84 M cum and 2207.95 M cum respectively.
Based on these results, the sedimentation rates in the zone (533.05 – 548.78 m) come out to be 8.96 ham/100 km2/year and for the zone (532.20 – 545.91 m) is 8.57 ha-m/100 km2/year. It is found that the results are comparable during and the uniform sediment rate of 8.57 ha-m/100 km2/ year can be considered. 相似文献
280.
本对斜交型扰动不稳定谱点的分布做了理论分析,得到了该谱点分布的半圆定理一该谱点分布在复一面上以原点为圆心以R0为半径的上半平面上,同时还对该不稳定增长率的上界作了估计。发现水平永度越小,模式顶越高则该估计值越大;垂直风切变的增大和纬度的增高对该增长率的增大有正贡献;当层结稳定度减小时,最大增长率随相对最大增长率得增大而减小。 相似文献