全文获取类型
收费全文 | 5288篇 |
免费 | 1152篇 |
国内免费 | 2403篇 |
专业分类
测绘学 | 66篇 |
大气科学 | 319篇 |
地球物理 | 1071篇 |
地质学 | 5810篇 |
海洋学 | 963篇 |
天文学 | 48篇 |
综合类 | 266篇 |
自然地理 | 300篇 |
出版年
2024年 | 48篇 |
2023年 | 145篇 |
2022年 | 218篇 |
2021年 | 228篇 |
2020年 | 282篇 |
2019年 | 309篇 |
2018年 | 298篇 |
2017年 | 274篇 |
2016年 | 313篇 |
2015年 | 335篇 |
2014年 | 402篇 |
2013年 | 424篇 |
2012年 | 414篇 |
2011年 | 426篇 |
2010年 | 359篇 |
2009年 | 422篇 |
2008年 | 411篇 |
2007年 | 413篇 |
2006年 | 432篇 |
2005年 | 346篇 |
2004年 | 316篇 |
2003年 | 290篇 |
2002年 | 236篇 |
2001年 | 217篇 |
2000年 | 210篇 |
1999年 | 161篇 |
1998年 | 184篇 |
1997年 | 131篇 |
1996年 | 126篇 |
1995年 | 105篇 |
1994年 | 93篇 |
1993年 | 68篇 |
1992年 | 46篇 |
1991年 | 35篇 |
1990年 | 36篇 |
1989年 | 19篇 |
1988年 | 18篇 |
1987年 | 10篇 |
1986年 | 10篇 |
1985年 | 9篇 |
1984年 | 11篇 |
1983年 | 5篇 |
1982年 | 2篇 |
1977年 | 2篇 |
1976年 | 1篇 |
1974年 | 1篇 |
1954年 | 2篇 |
排序方式: 共有8843条查询结果,搜索用时 15 毫秒
81.
A. V. Krivov L. L. Sokolov V. V. Dikarev 《Celestial Mechanics and Dynamical Astronomy》1995,63(3-4):313-339
This paper deals with dynamics of impact ejecta from Phobos and Deimos initially on near-circular equatorial orbits around Mars. For particles emitted in a wide size regime of 1 micron and greater, and taking into account the typical particle lifetimes to be less than 100 years, the motion is governed by two perturbing forces: solar radiation pressure and influence of Mars' oblateness. The equations of motion of particles in Lagrangian non-singular elements are deduced and solved, both analytically and numerically, for different-sized ejecta. We state that the coupled effect of both forces above is essential so that on no account can the oblateness of Mars are be neglected. The dynamics of grains prove to be quite different for the ejecta of Phobos and Deimos. For Deimos, the qualitative results are relatively simple and imply oscillations of eccentricity and long-term variations of orbital inclination, with amplitudes and periods both depending on grain size. For Phobos, the dynamics are shown to be much more complicated, and we discuss it in detail. We have found an intriguous peculiar behavior of debris near 300 µm in size. Another finding is that almost all the Phobos ejecta with radii less than 30 µm (against the values of 5 to 20 µm adopted earlier by many authors) should be rapidly lost by collisions with martian surface. The results of the paper may be the base for constructing an improved model of dust belts that presumably exist around Mars. 相似文献
82.
R. J. Flower 《Journal of Paleolimnology》1991,5(2):175-188
In wind-stressed and unstratified Lough Neagh low aspect ratio sediment traps, which allow for internal resuspension, collected sediment at a rate close to the natural sediment accumulation rate inferred from dated sediment cores. High aspect ratio traps grossly overestimated the natural rate of sediment accumulation as did, but to a lesser extent, burial rate measurements on an artificial stratigraphic marker. The former type of trap deployed over long exposure periods is therefore recommended as providing the best estimate of net downward particle flux in lakes such as Lough Neagh. However, it is emphasized that all sediment trap types, when operated in shallow turbulent lakes, will to a greater or lesser extent be contaminated by secondary or redeposited material and so will not provide a direct measure of primary sedimentation. Use of tube traps in lakes such as Lough Neagh should not, however, be discounted since they can provide a record of the quality of sedimenting material through time. 相似文献
83.
Nicole Borderies Pierre-yves Longaretti 《Celestial Mechanics and Dynamical Astronomy》1990,49(1):69-98
The purpose of this paper is to present a model for the radiation pressure acceleration of a spherical satellite, due to the radiation reflected by a planet with a uniform albedo. A particular choice of variables allows one to reduce the surface integrals over the lit portion of the planet visible to the satellite to one-dimensional integrals. Exact analytical expressions are found for the integrals corresponding to the case where the spacecraft does not "see" the terminator. The other integrals can be computed either numerically, or analytically in an approximate form. The results are compared with those of Lochry (1966). The model is applied to Magellan, a spacecraft orbiting Venus. 相似文献
84.
Rock fracturing by explosive energy: review of state-of-the-art 总被引:1,自引:0,他引:1
M. R. Saharan H. S. Mitri J. L. Jethwa 《Fragblast: International Journal for Blasting and Fragmentation》2006,10(1):61-81
A study of the dynamic rock fracture initiation and propagation due to explosive energy is presented through a detailed state-of-the-art review. Explosive energy dissipation in crushing and fracturing is examined and the various means to enhance the explosive energy utilization for dynamic rock fracturing are reviewed. The study highlights the need for a better understanding of the dynamic fracturing process particularly in the presence of in situ stresses in the rock mass. 相似文献
85.
The stochastic nature of the cyclic swelling behavior of mudrock and its dependence on a large number of interdependent parameters was modeled using Time Delay Neural Networks (TDNNs). This method has facilitated predicting cyclic swelling pressure with an acceptable level of accuracy where developing a general mathematical model is almost impossible. A number of total pressure cells between shotcrete and concrete walls of the powerhouse cavern at Masjed–Soleiman Hydroelectric Powerhouse Project, South of Iran, where mudrock outcrops, confirmed a cyclic swelling pressure on the lining since 1999. In several locations, small cracks are generated which has raised doubts about long term stability of the powerhouse structure. This necessitated a study for predicting future swelling pressure. Considering the complexity of the interdependent parameters in this problem, TDNNs proved to be a powerful tool. The results of this modeling are presented in this paper. 相似文献
86.
To estimate static and seismic active earth pressure (Pad) on a rigid retaining wall, numerical analyses using different step sizes have been carried out in this paper, based on the
modified Culmann line method by considering Coulomb’s planar rupture surface. Equivalent pseudo-static seismic forces are
considered in the analysis. A new concept of modified unit weight by considering ground surcharge is introduced under static
and seismic conditions. By numerical analysis, area of soil (A) has been estimated to obtain the ratio of A/A0 where A0 is θh2, θ is the angle between retaining structure and ground surface and h is the vertical height of the wall. This ratio remains constant for a particular type of soil and has been used to estimate
the maximum active earth pressure using force diagram. Results are provided in tabular form for easy calculation of the coefficient
of static and seismic active earth pressure. Present results by considering the new technique, compares well with the results
obtained by earlier researchers. 相似文献
87.
S. Tadisetty K. Matsui H. Shimada R. N. Gupta 《Rock Mechanics and Rock Engineering》2006,39(4):383-393
Summary. Discontinuous manual observations and irregular caving characteristics of roof rocks often lead to improper decisions resulting
in accidents and production loss. Hence, systematic monitoring of the hanging roof behind the chock shields is necessary for
safe and productive mining operations. A real-time application was successfully implemented in an Indian mine for forecasting
of hanging roof behaviour to enhance safety and productivity. This paper reports the functioning of real-time TWAP (time weighted
average pressure) analysis in the forecasting of hanging roof behaviour in real time. 相似文献
88.
Koltjärnen and Nylandssjön are two closely situated lakes (<2 km apart) in northern Sweden. During the past century, distinct varved sediments have formed in these lakes. Nylandssjön has two varved, deep basins. Since lake and catchment characteristics superficially appear very similar for the two lakes and they are exposed to the same climate, one would expect the sediment varves to be similar. This investigation compares the varves in the two deep basins of Nylandssjön, and in the two lakes. The comparison of basins of Nylandssjön shows that varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1970–2003). The grey-scale curves are only clearly similar in about 50% of the varves. In the between-lake comparison varve thickness, water content and annual accumulation rates of organic matter and nitrogen are correlated for the period (1950–1996). However, the annual accumulation rates of dry mass, minerogenic matter and biogenic silica differ between the lakes, as well as within-varve successions in grey-scale. A general explanation to the differences is that the prerequisites for varve formation are not totally similar because of differences in catchment size, catchment- to-lake material fluxes, lake productivity and land-use influence. This study illustrates the complex relationships that exist between a lake, its catchment, in-lake productivity and formation of sediment varves. As a consequence, we must not apply a too simplistic view of the potential of varves as past climatic indicators, especially if the lakes are affected by land-use. 相似文献
89.
由于煤层气的解吸热效应,煤层气的运移过程是一个非等温过程。因此,温度场对煤层气渗流有着重要的影响。前人在研究煤层气渗流规律的研究中并没有同时考虑滑脱效应和温度场因素的影响,而在实际的深部开采中综合考虑滑脱效应和温度场的影响对研究深部煤层气运移规律有着重要的意义。因此本文建立了考虑滑脱效应及温度场的煤层气渗流数学模型,利用有限元数值方法研究了考虑滑脱效应和温度场耦合的煤层气运移规律;研究了考虑滑脱效应和温度场对压力场分布的影响;对考虑滑脱效应及温度场因素的煤层气产量进行了预测。得出了随着温度的升高煤储层的压力在降低,温度的升高对煤层气的产量有着负面的影响这一重要结论。 相似文献
90.
E. M. Chamorro Pérez I. Daniel J.-C. Chervin P. Dumas J. D. Bass T. Inoue 《Physics and Chemistry of Minerals》2006,33(7):502-510
High-pressure synchrotron infrared (IR) absorption spectra were collected between 650 and 4,000 cm−1 at ambient temperature for hydrous Mg-ringwoodite (γ-Mg2SiO4) up to 30 GPa. The main feature in the OH− stretching region is an extremely broad band centred at 3,150 cm−1. The hydrogen bond is strong for most protons and the most probable site for protonation is the tetrahedral edge. With increasing pressure, this band shifts downward while decreasing its integrated intensity until disappearance at a pressure of 25 GPa. Only one band at 2,450 cm−1 and an absorption plateau persist with a maximum wavenumber of 3,800 cm−1. This behaviour is reversible upon pressure release. We interpret this as a second-order phase transition occurring in hydrated Mg-ringwoodite at high pressure (beyond ∼ 25 GPa). This result is compatible with the observation by Kleppe et al. (Phys Chem Miner 29:473–476, 2002a) who suggested the presence of Si–O–Si linkages and/or partial increase in the coordination of Si. Beyond the phase transition, the protons are delocalized and their environment on the ringwoodite structure is probably quite different from that at low pressure. Data obtained in situ at high pressures and temperatures are needed to better understand the effect of protonation on the structure and to better constrain this phase transition. 相似文献