全文获取类型
收费全文 | 8716篇 |
免费 | 891篇 |
国内免费 | 529篇 |
专业分类
测绘学 | 458篇 |
大气科学 | 992篇 |
地球物理 | 2845篇 |
地质学 | 3517篇 |
海洋学 | 685篇 |
天文学 | 661篇 |
综合类 | 350篇 |
自然地理 | 628篇 |
出版年
2023年 | 43篇 |
2022年 | 110篇 |
2021年 | 137篇 |
2020年 | 121篇 |
2019年 | 126篇 |
2018年 | 592篇 |
2017年 | 513篇 |
2016年 | 390篇 |
2015年 | 286篇 |
2014年 | 290篇 |
2013年 | 306篇 |
2012年 | 794篇 |
2011年 | 620篇 |
2010年 | 284篇 |
2009年 | 332篇 |
2008年 | 262篇 |
2007年 | 239篇 |
2006年 | 249篇 |
2005年 | 935篇 |
2004年 | 983篇 |
2003年 | 770篇 |
2002年 | 321篇 |
2001年 | 199篇 |
2000年 | 138篇 |
1999年 | 95篇 |
1998年 | 93篇 |
1997年 | 92篇 |
1996年 | 65篇 |
1995年 | 64篇 |
1994年 | 48篇 |
1993年 | 46篇 |
1992年 | 53篇 |
1991年 | 42篇 |
1990年 | 32篇 |
1989年 | 35篇 |
1988年 | 22篇 |
1987年 | 28篇 |
1985年 | 21篇 |
1984年 | 30篇 |
1983年 | 21篇 |
1982年 | 20篇 |
1981年 | 31篇 |
1980年 | 27篇 |
1979年 | 22篇 |
1978年 | 27篇 |
1977年 | 17篇 |
1976年 | 18篇 |
1975年 | 28篇 |
1974年 | 13篇 |
1973年 | 15篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
The economic development in China and the rising of the living standard need to speed up the urbanisa-tion.The development of small towns is an important way to Chinese urbanization.Land use plays a very important role in the development of small towns.However there are many problems in the development of small towns,esp.in land use.The paper first discusses the land problems in the development of small towns.Such as much cultivated land lies idle,under-utilization and waste of land,increasing illegal use of land,unstable contractural relationship for land use.The relationship between the development of small towns and land use is also studied.Then the guidelines for the land system innovations of small towns are put forward.Namely the sustainability of social and economic development,the bal-ance between land reservation and land utilization,the provision of service to village ,agriculture and farmer,the manage-ment of land resource and land assets,the parsimonious andlegal use of land.The basic framework of land system innova-tions of small towns is put forward finally.It include the land replacement policy for small towns and the permanent tenan-cy of farmland,the overall plan for land uses and other plans in harmony,the establishment of a flexible system of land supplies,using land with payments ,the transfer of agricultural land and refining the land law related to the construction of small towns. 相似文献
2.
Mallory S. E. Roberts Crystal L. Brogan Bryan M. Gaensler Jason W. T. Hessels C.-Y. Ng Roger W. Romani 《Astrophysics and Space Science》2005,297(1-4):93-100
A remarkable number of pulsar wind nebulae (PWN) are coincident with EGRET γ-ray sources. X-ray and radio imaging studies of unidentified EGRET sources have resulted in the discovery of at least six new pulsar wind nebulae (PWN). Stationary PWN (SPWN) appear to be
associated with steady EGRET sources with hard spectra, typical for γ-ray pulsars. Their toroidal morphologies can help determine the geometry of the
pulsar which is useful for constraining models of pulsed γ-ray emission. Rapidly moving PWN (RPWN) with more cometary morphologies
seem to be associated with variable EGRET sources in regions where the ambient medium is dense compared to what is typical for the ISM. 相似文献
3.
4.
5.
The HCN emission features near 3 μm recently detected by Geballe et al. (2003, Astrophys. J. 583, L39) are analyzed with a model for fluorescence of sunlight in the ν3 band of HCN. The emission spectrum is consistent with current knowledge of the atmospheric temperature profile and the HCN distribution inferred from millimeter-wave observations. The spectrum is insensitive to the abundance of HCN in the thermosphere and the thousand-fold enhancement relative to photochemical models suggested by Geballe et al. (2003, Astrophys. J. 583, L39) is not required to explain the observations. We find that the spectrum can be matched with temperatures from 130 to 200 K, with slightly better fits at high temperature, contrary to the temperature determination of 130±10 K of Geballe et al. (2003, Astrophys. J. 583, L39). The HCN emission spectrum is sensitive to the collisional de-excitation probability, P10, for the ν3 state and we determine a value of 10−5 with an accuracy of about a factor of two. Analysis of absorption lines in the C2H2ν3 band near 3 μm, detected in the same spectrum, indicate a C2H2 mole fraction near 0.01 μbar of 10−5 for P10=10−4. The derived mole fraction, however, is dependent upon the value adopted for P10 and lower values are required if P10 at Titan temperatures is less than its room temperature value. 相似文献
6.
7.
8.
Long-range sidescan sonar can be used to map sediment distributions over wide expanses of deep ocean floor. Seven acoustic facies that arise from differing sediment or rock types have been mapped over the low-relief Saharan continental rise and Madeira abyssal plain. These have been calibrated with sampling, profiling and camera studies and the facies can be traced confidently on a regional scale using the sidescan data. The mapping of the sediment distribution shows that a complex interplay of turbidity current and debris flow processes can occur at a continental rise/abysaal plain transition over 1000 km from the nearest continental slope. 相似文献
9.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
10.
In this paper the second order characteristic (discontinuous bifurcation) condition is derived for the granular flow (fully plastic) equations. This second order bifurcation equation is shown to be formally identical to the first order localization requirement during steady elastoplastic deformation provided the elastic compliance tensor is substituted for the product of the plastic multiplier with the flow Hessian. For isotropic yield and flow functions the invariant form of the characteristic condition is given in detail, as well as an alternative expression in adapted co‐ordinates. The characteristic condition can be regarded as defining a hardening function which is maximized to identify the critical angles. When the method is applied to 3D Coulomb flow, Mohr's 3D fracture plane conditions are obtained uniquely. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献