全文获取类型
收费全文 | 19167篇 |
免费 | 3468篇 |
国内免费 | 5593篇 |
专业分类
测绘学 | 2281篇 |
大气科学 | 3976篇 |
地球物理 | 4547篇 |
地质学 | 7925篇 |
海洋学 | 3821篇 |
天文学 | 1618篇 |
综合类 | 1605篇 |
自然地理 | 2455篇 |
出版年
2024年 | 76篇 |
2023年 | 245篇 |
2022年 | 656篇 |
2021年 | 735篇 |
2020年 | 971篇 |
2019年 | 1134篇 |
2018年 | 840篇 |
2017年 | 961篇 |
2016年 | 993篇 |
2015年 | 1183篇 |
2014年 | 1208篇 |
2013年 | 1361篇 |
2012年 | 1272篇 |
2011年 | 1290篇 |
2010年 | 1054篇 |
2009年 | 1206篇 |
2008年 | 1252篇 |
2007年 | 1473篇 |
2006年 | 1424篇 |
2005年 | 1240篇 |
2004年 | 1027篇 |
2003年 | 876篇 |
2002年 | 829篇 |
2001年 | 610篇 |
2000年 | 732篇 |
1999年 | 687篇 |
1998年 | 583篇 |
1997年 | 411篇 |
1996年 | 364篇 |
1995年 | 282篇 |
1994年 | 244篇 |
1993年 | 225篇 |
1992年 | 194篇 |
1991年 | 137篇 |
1990年 | 83篇 |
1989年 | 100篇 |
1988年 | 56篇 |
1987年 | 45篇 |
1986年 | 44篇 |
1985年 | 28篇 |
1984年 | 22篇 |
1983年 | 16篇 |
1982年 | 14篇 |
1981年 | 12篇 |
1980年 | 5篇 |
1979年 | 3篇 |
1978年 | 7篇 |
1977年 | 6篇 |
1973年 | 2篇 |
1954年 | 7篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
51.
The energy budget ofBellamya earuginosa in a shallow algal lake, Houhu Lake (Wuhan, China) was investigated by the measurement of flesh production (32.8 kJ/(m2·a)), egestion (337.7 kJ/(m2·a)), metabolism (246.7 kJ/(m2·a)), and estimation of excretion (21.4kJ/(m2·a)). The net growth efficiency of the species is about 10.9%, which accords with the generally reported value for gastropods.
In addition, the relationships between starvation respiration (R, mgO2/(Ind·d)), body weight (Wd, mg in dry wt) and temperature (T, °C) were also determined. The regression equationR=0.044Wd
0.537
e
0.061T
was obtained by the least square method, The measured SDA of the species is 26.51% of its gross metabolism.
Project No. 3960019 and 39430101 supported by NSFC and also a granted for the East Lake Ecological Experimental Station, CAS,
Wuhan. 相似文献
52.
V. Castellani S. Degl'Innocenti P. G. Prada Moroni 《Monthly notices of the Royal Astronomical Society》2001,320(1):66-72
We compare theoretical stellar models for main sequence (MS) stars with the Hipparcos data base for the Hyades cluster to give a warning against the uncritical use of available theoretical scenarios and to show how formal MS fittings can be fortuitous if not fictitious. Moreover, we find that none of the current theoretical scenarios appears able to account for an observed mismatch between theoretical predictions and observations of the coolest Hyades MS stars. Finally, we show that current theoretical models probably give too faint He burning luminosities unlike the case of less massive He burning models, with degenerate progenitors, which have been suggested to suffer the opposite discrepancy. 相似文献
53.
54.
D. Prandle 《Estuarine, Coastal and Shelf Science》1985,20(5):615-635
An examination is made of the circulation in narrow estuaries subject to a predominant tidal forcing. Velocity structures are derived separately for residual flow components associated with (a) river flow, (b) wind stress, (c) a well-mixed longitudinal density gradient and (d) a fully stratified saline wedge. Dimensionless parameters are introduced to indicate the magnitude of each component and these parameters are evaluated for 9 major estuaries, thereby revealing their sensitivity to each component.For a channel of constant breadth and depth, formulae are deduced for the length of saline intrusion, L. Comparisons with observed data show that such formulae may be used with confidence to predict changes in L arising from variations in river flow, tidal range or channel depths.The level of stratification is shown to be related to a product of two parameters, one associated with velocity structure and a second involving the square of the ‘flow ratio’ (i.e. residual velocity/amplitude of the tidal velocity). This relationship provides a simple classification system for estuarine stratification which can be used to indicate the sensitivity of any particular estuary to changing conditions. 相似文献
55.
56.
Randall D. Campbell 《Experimental Astronomy》2002,14(1):57-60
Keck's Long Wavelength Spectrometer (LWS), is the facility instrument used for imaging and spectroscopy in the wavelength
range of 3–28 μm at the Keck Observatory. LWS uses an 128 × 128 Si:As blocked impurity band (BIB) array manufactured by the
Boeing Corporation. This paper discusses the method used for optimizing the detector's operating parameters at a temperature
of 8.5 K and bias voltage of 1.2 V. A process for characterizing detective quantum efficiency of BIB detectors is also presented.
This revised version was published online in July 2006 with corrections to the Cover Date. 相似文献
57.
WANG Yachun 《东北亚地学研究》2002,(1)
The paper deals with the methods of formation pressure evaluation for a single well by using the very common ac-cepted parameters, such as drilling exponent , and flowline temperature , etc. which is part of compiling the end well report. 相似文献
58.
59.
研究油藏内部油水渗流方向时,主要考虑渗透率影响,引入两个水平方向渗透率比、水平与垂向渗透率比参数来分析。前者反映样品所在储层平面上两个水平方向渗流能力的差异,后者反映样品所在储层水平与垂向渗流能力的差异,并经小柱样测试和全岩心测试两种方法确定两个参数。通过对塔中4号油藏CⅠ油组9口井2306个样品两种渗透率比值分布直方图分析,研究了CⅠ油组渗透率的非均质性及其渗流方向:CI油组储层在水平方向的渗流能力有很大差异,具有明显的方向性。储层垂向渗流能力明显小于水平方向的渗流能力。 相似文献
60.
1 IntroductionDiscontinuousDeformationAnalysis(DDA)developedbyShiin 1 980’s[1 ,2 ] maybeusedtocalculatethedeformationanddisplacementsinamulti blocksystemandtoanalyzetherelationbe tweenforcesanddisplacementsinthesystem .Ifthesurveydisplacementofeverypointordirectionisknown,alldisplacementsanddeformationsofblocks,eventheglide ,theclosureandthestretchoftheboundary planeofblocks ,canbecalculated .TheresultisoptimallyfittedaccordingtotheLeastSquarePrinciple.DDAismainlyusedinrockblockssystems… 相似文献