全文获取类型
收费全文 | 5587篇 |
免费 | 201篇 |
国内免费 | 70篇 |
专业分类
测绘学 | 134篇 |
大气科学 | 378篇 |
地球物理 | 1287篇 |
地质学 | 2061篇 |
海洋学 | 472篇 |
天文学 | 972篇 |
综合类 | 22篇 |
自然地理 | 532篇 |
出版年
2021年 | 60篇 |
2020年 | 90篇 |
2019年 | 109篇 |
2018年 | 129篇 |
2017年 | 131篇 |
2016年 | 179篇 |
2015年 | 143篇 |
2014年 | 173篇 |
2013年 | 289篇 |
2012年 | 177篇 |
2011年 | 280篇 |
2010年 | 227篇 |
2009年 | 291篇 |
2008年 | 252篇 |
2007年 | 259篇 |
2006年 | 238篇 |
2005年 | 227篇 |
2004年 | 208篇 |
2003年 | 150篇 |
2002年 | 167篇 |
2001年 | 107篇 |
2000年 | 104篇 |
1999年 | 102篇 |
1998年 | 81篇 |
1997年 | 85篇 |
1996年 | 86篇 |
1995年 | 83篇 |
1994年 | 61篇 |
1993年 | 56篇 |
1992年 | 52篇 |
1991年 | 71篇 |
1990年 | 45篇 |
1989年 | 49篇 |
1988年 | 65篇 |
1987年 | 64篇 |
1986年 | 47篇 |
1985年 | 65篇 |
1984年 | 64篇 |
1983年 | 59篇 |
1982年 | 58篇 |
1981年 | 61篇 |
1980年 | 53篇 |
1979年 | 48篇 |
1978年 | 39篇 |
1977年 | 52篇 |
1976年 | 48篇 |
1975年 | 38篇 |
1974年 | 39篇 |
1973年 | 39篇 |
1971年 | 31篇 |
排序方式: 共有5858条查询结果,搜索用时 15 毫秒
31.
This study presents a finite element (FE) micromechanical modelling approach for the simulation of linear and damage‐coupled viscoelastic behaviour of asphalt mixture. Asphalt mixture is a composite material of graded aggregates bound with mastic (asphalt and fine aggregates). The microstructural model of asphalt mixture incorporates an equivalent lattice network structure whereby intergranular load transfer is simulated through an effective asphalt mastic zone. The finite element model integrates the ABAQUS user material subroutine with continuum elements for the effective asphalt mastic and rigid body elements for each aggregate. A unified approach is proposed using Schapery non‐linear viscoelastic model for the rate‐independent and rate‐dependent damage behaviour. A finite element incremental algorithm with a recursive relationship for three‐dimensional (3D) linear and damage‐coupled viscoelastic behaviour is developed. This algorithm is used in a 3D user‐defined material model for the asphalt mastic to predict global linear and damage‐coupled viscoelastic behaviour of asphalt mixture. For linear viscoelastic study, the creep stiffnesses of mastic and asphalt mixture at different temperatures are measured in laboratory. A regression‐fitting method is employed to calibrate generalized Maxwell models with Prony series and generate master stiffness curves for mastic and asphalt mixture. A computational model is developed with image analysis of sectioned surface of a test specimen. The viscoelastic prediction of mixture creep stiffness with the calibrated mastic material parameters is compared with mixture master stiffness curve over a reduced time period. In regard to damage‐coupled viscoelastic behaviour, cyclic loading responses of linear and rate‐independent damage‐coupled viscoelastic materials are compared. Effects of particular microstructure parameters on the rate‐independent damage‐coupled viscoelastic behaviour are also investigated with finite element simulations of asphalt numerical samples. Further study describes loading rate effects on the asphalt viscoelastic properties and rate‐dependent damage behaviour. Copyright © 2006 John Wiley & Sons, Ltd. 相似文献
32.
Continuous wavelet analyses of hourly time series of air temperature, stream discharge, and precipitation are used to compare the seasonal and inter‐annual variability in hydrological regimes of the two principal streams feeding Bow Lake, Banff National Park, Alberta: the glacial stream draining the Wapta Icefields, and the snowmelt‐fed Bow River. The goal is to understand how water sources and flow routing differ between the two catchments. Wavelet spectra and cross‐wavelet spectra were determined for air temperature and discharge from the two streams for summers (June–September) 1997–2000, and for rainfall and discharge for the summers of 1999 and 2000. The diurnal signal of the glacial runoff was orders of magnitude higher in 1998 than in other years, indicating that significant ice exposure and the development of channelized glacial drainage occurred as a result of the 1997–98 El Niño conditions. Early retreat of the snowpack in 1997 and 1998 led to a significant summer‐long input of melt runoff from a small area of ice cover in the Bow River catchment; but such inputs were not apparent in 1999 and 2000, when snow cover was more extensive. Rainfall had a stronger influence on runoff and followed quicker flow paths in the Bow River catchment than in the glacial catchment. Snowpack thickness and catchment size were the primary controls on the phase relationship between temperature and discharge at diurnal time scales. Wavelet analysis is a fast and effective means to characterize runoff, temperature, and precipitation regimes and their interrelationships and inter‐annual variability. The technique is effective at identifying inter‐annual and seasonal changes in the relative contributions of different water sources to runoff, and changes in the time required for routing of diurnal meltwater pulses through a catchment. However, it is less effective at identifying changes/differences in the type of the flow routing (e.g. overland flow versus through flow) between or within catchments. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
33.
Monazite electron microprobe U–Th–Pb and garnet Sm–Nd isotopic data from metapelitic assemblages in the Willyama Supergroup in the southern Curnamona Province, south‐central Australia, indicate that the terrain underwent regional greenschist to amphibolite‐grade metamorphism during the c. 500 Ma Delamerian Orogeny. The Delamerian‐aged mineral assemblages include prograde garnet–staurolite and kyanite‐bearing associations that overprint andalusite‐ and sillimanite‐bearing assemblages that are interpreted to have developed during the c. 1600 Ma Olarian Orogeny. Importantly, the development of secondary kyanite‐bearing assemblages in the southern Curnamona Province has been used previously to suggest that the Olarian Orogeny followed an anticlockwise P–T evolution. If such assemblages are the product of c. 500 Ma metamorphism, then the anticlockwise P–T path is an apparent path, due to the overprint of a distinct metamorphic cycle c. 1100 Ma later. Making such distinctions is therefore extremely important when using the textural and metamorphic evolution of polycyclic terrains to model the thermal behaviour of the crust during orogeny. This study highlights the utility of in situ geochronology, linking age data to petrologically important phases and assemblages. 相似文献
34.
35.
36.
Frank Stefan Tautz Volker Heine Martin T. Dove Xiaojie Chen 《Physics and Chemistry of Minerals》1991,18(5):326-336
A molecular dynamics simulation of quartz at different temperatures both in the a and in the phase has been conducted. The - phase transition could be observed. A phonon analysis of the -phase confirms and rounds out in a quantitative way the origin of the incommensurate (ic) modulated phase. In particular it traces the optic soft mode at becoming (to a good approximation) a so-called rigid unit mode (RUM) at q0, and elucidates its coupling to the transverse acoustic mode which precipitates the incommensurate transition. This success underpins and illuminates the concept of RUMs and their role in structural phase transitions. 相似文献
37.
Food, energy and mineral resources appeared to be available in limitless quantities. The accelerated growth in global population during recent centuries brought about a new situation. We reached the resource limits of Spaceship Earth. This, along with the dawning awareness of the extent of environmental damage to date, have made it necessary to turn to new attitudes as to the use of renewable and non-renewable resources.IRM (Integrated Resource Management) is proposed as a way to rationally balance the use and conservation of natural resources. A databank based on the Geographic Information System (GIS) concept, bringing together all relevant resources of a nation, is the centerpiece of IRM and will make it possible to establish strategies to optimize the long term use of resources, including all relevant environmental factors. 相似文献
38.
Martin Beech 《Astrophysics and Space Science》1986,122(1):57-61
We perform a Monte Carlo analysis of the eclipse probabilities of short-period binary systems in an attempt to estimate the distribution of ellipsoidal variable system with Main-Sequence components. Our results suggest that possibly as many as one in four of the non-eclipsing, spectroscopic binaries withP(d)10, and Main-Sequence components withM
v
10 could be ellipsoidal variable systems. This result, while an upper limit, would seem to hold irrespective of primary spectral type (B5 to G5). 相似文献
39.
Martin Beech 《Astrophysics and Space Science》1985,114(2):345-350
In the standard cosmological model symmetry breaking in grand unified theories will occur at times 10–39 s after the initial singularity when the Universe has cooled to a temperature 1016 GeV. We investigate here whether it is possible for a uniform, large-scale, magnetic field present in the early universe to delay significantly the time at which symmetry breaking occurs. Given the present magnitude of the intergalactic B-field (10–11–10–9 G) it is found that no significant effects are introduced. 相似文献
40.
A set of 41 focal mechanisms (1989–2006) from P-wave first polarities is computed from relocated seismic events in the Giudicarie–Lessini region (Southern Alps). Estimated hypocentral depths vary from 3.1 to 20.8 km, for duration magnitudes (MD) in the range 2.7–5.1. Stress and strain inversions are performed for two seismotectonic zones, namely G (Giudicarie) and L (Lessini). This subdivision is supported by geological evidence, seismicity distribution, and focal mechanism types. The available number of data (16 in G, 22 in L) does not make possible any further subdivisions. Seismotectonic zones G and L are undergoing different kinematic regimes: thrust with strike-slip component in G, and strike-slip in L. Principal stress and strain axes in each sub-region show similar orientations. The direction of maximum horizontal compressive stress is roughly perpendicular to the thrust fronts along the Giudicarie Belt in zone G, and compatible with right-lateral strike-slip reactivation of the faults belonging to the Schio-Vicenza system in zone L. On the whole, kinematic regimes and horizontal stress orientations show a good fit with other stress data from focal mechanisms and breakouts and with geodetic strain rate axes. 相似文献