全文获取类型
收费全文 | 49394篇 |
免费 | 640篇 |
国内免费 | 324篇 |
专业分类
测绘学 | 1334篇 |
大气科学 | 3830篇 |
地球物理 | 9285篇 |
地质学 | 16261篇 |
海洋学 | 4172篇 |
天文学 | 12164篇 |
综合类 | 108篇 |
自然地理 | 3204篇 |
出版年
2020年 | 339篇 |
2019年 | 337篇 |
2018年 | 828篇 |
2017年 | 810篇 |
2016年 | 1052篇 |
2015年 | 696篇 |
2014年 | 1170篇 |
2013年 | 2441篇 |
2012年 | 1110篇 |
2011年 | 1543篇 |
2010年 | 1436篇 |
2009年 | 1930篇 |
2008年 | 1774篇 |
2007年 | 1776篇 |
2006年 | 1678篇 |
2005年 | 1544篇 |
2004年 | 1508篇 |
2003年 | 1413篇 |
2002年 | 1357篇 |
2001年 | 1230篇 |
2000年 | 1155篇 |
1999年 | 1121篇 |
1998年 | 1056篇 |
1997年 | 1062篇 |
1996年 | 844篇 |
1995年 | 836篇 |
1994年 | 794篇 |
1993年 | 738篇 |
1992年 | 709篇 |
1991年 | 680篇 |
1990年 | 776篇 |
1989年 | 674篇 |
1988年 | 648篇 |
1987年 | 741篇 |
1986年 | 620篇 |
1985年 | 824篇 |
1984年 | 947篇 |
1983年 | 912篇 |
1982年 | 861篇 |
1981年 | 819篇 |
1980年 | 722篇 |
1979年 | 698篇 |
1978年 | 692篇 |
1977年 | 627篇 |
1976年 | 596篇 |
1975年 | 517篇 |
1974年 | 597篇 |
1973年 | 586篇 |
1972年 | 362篇 |
1971年 | 334篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
211.
The coupled mechanics of fluid-filled granular media controls the physics of many Earth systems, for example saturated soils, fault gouge, and landslide shear zones. It is well established that when the pore fluid pressure rises, the shear resistance of fluid-filled granular systems decreases, and, as a result, catastrophic events such as soil liquefaction, earthquakes, and accelerating landslides may be triggered. Alternatively, when the pore pressure drops, the shear resistance of these geosystems increases. Despite the great importance of the coupled mechanics of grain–fluid systems, the basic physics that controls this coupling is far from understood. Fundamental questions that must be addressed include: what are the processes that control pore fluid pressurization and depressurization in response to deformation of the granular skeleton? and how do variations of pore pressure affect the mechanical strength of the grains skeleton? To answer these questions, a formulation for the pore fluid pressure and flow has been developed from mass and momentum conservation, and is coupled with a granular dynamics algorithm that solves the grain dynamics, to form a fully coupled model. The pore fluid formulation reveals that the evolution of pore pressure obeys viscoelastic rheology in response to pore space variations. Under undrained conditions elastic-like behavior dominates and leads to a linear relationship between pore pressure and overall volumetric strain. Viscous-like behavior dominates under well-drained conditions and leads to a linear relationship between pore pressure and volumetric strain rate. Numerical simulations reveal the possibility of liquefaction under drained and initially over-compacted conditions, which were often believed to be resistant to liquefaction. Under such conditions liquefaction occurs during short compactive phases that punctuate the overall dilative trend. In addition, the previously recognized generation of elevated pore pressure under undrained compactive conditions is observed. Simulations also show that during liquefaction events stress chains are detached, the external load becomes completely supported by the pressurized pore fluid, and shear resistance vanishes. 相似文献
212.
R. L. Karale K. K. Narula V. Dayal K. M. Saini 《Journal of the Indian Society of Remote Sensing》1989,17(1):23-31
In order to check the premature siltation of the reservoirs and guard against the drop in the irrigation potential, the Government of India has launched the schemes of soil conservation and integrated watershed management in the catchments of RVPs and Flood Prone rivers. Owing to the large financial and manpower commitments needed to implement and execute soil conservation measures over vast catchment areas, a priority approach for treatment was identified. The methodology developed for prioritization of watersheds of a catchment area conceptualizes sedimentation of the reservoirs as a multiplicative function of erosivity value and the delivery ratio. This paper deals with the development of a computerized data base software module ‘WEIGHT’ for determination of erosivity values for the mapping units comprising assemblages of the varying combinations of climate, physiography and slope, land use and cover conditions, soil characteristics (texture, solumn thickness, permeability and pH) and the existing erosion and soil conservation measures. The WEIGHT software package is coded in FORTRON-4 for PDP 11/83 operating system. the data base comprises storage of the attributes of the different erosivity determinants of the mapping units with predetermined erosivity values sequentially on a disk and comparing the attributes of a new mapping unit to get the most probabilities erosivity value. The objective has been to eliminate the personal bias and bring about the objectivity in the process of assigning erosivity values to the different mapping units. The data base design, design logic and operational sequence of the data base are discussed in the paper. 相似文献
213.
Room temperature X-irradiation of some natural beryls produced several new absorption lines in the electron paramagnetic resonance (EPR) spectrum, a known series of optical absorption lines in the 500–700 nm range, and a shift of the absorption edge to lower energies. Several of the new EPR lines and part of the irradiation-induced shift of the absorption edge disappeared after a few days at room temperature, and were not examined in detail. However, three of the paramagnetic centres responsible for the new EPR lines were stable at room temperature and two of these have previously been identified as atomic hydrogen and the methyl radical, CH3. These species were stable to ~150 and ~450°C respectively. The third stable species, hitherto unreported, showed a single-line EPR spectrum of axial symmetry, with g∥=2.0051 and g⊥=2.0152. This spectrum was found to be intensity-correlated with the series of optical bands in the 500–700 nm range, after thermal bleaching at 175°C. The EPR and optical spectra are therefore assigned to the same species. It is argued that this species is the CO 3 ? molecular ion, located in the widest part of the structural channel and aligned with the plane of the molecule perpendicular to the c axis. The EPR spectrum is consistent with a 2 A′2 ground state of a CO 3 ? molecule with trigonal symmetry, and this requires that the optical transition has a 2 A′2 → 2 E′ character. Most of the features in the optical spectrum can be assigned to coupling of a totally symmetric mode of frequency ~1020 cm?1 onto a zero-phonon line at 14,490 cm?1 and a second weaker line at 16,020 cm?1. However, both of these two fundamental lines are structured, and the two components show strong temperature-dependent derivative-shaped magnetic circular dichroism (MCD). Furthermore, the overall sign of the MCD for the line at 16,020 cm?1 is opposite to that at 14,490 cm?1. The separation (~120 cm?1) of the two components of the 14,490 cm?1 line is much larger than that expected from spin-orbit interaction, and the origin of this splitting is not yet understood. 相似文献
214.
The galactic distribution and physical nature of X-ray transient sources is investigated. Two types of transients are considered. The observational data on 41 X-ray transient sources are given, and the average parameters of hard and soft X-ray transients are estimated. 相似文献
215.
216.
Summary. This paper is concerned with an examination of the possibility that there might exist a small scale of convective circulation beneath the oceanic lithosphere. Recent suggestions that this might be the case have been made in an effort to understand why the bathymetry of the sea-floor deviates from the prediction of boundary layer theory for ages in excess of about 100 Ma. The energy source which sustains the secondary motion is supposed to be found in the steep temperature gradient near the planetary surface which is itself presumably maintained by the large-scale convective circulation associated with plate creation and destruction. Here we investigate the extent to which the temperature dependence of viscosity may act so as to stabilize the upper boundary layer against disruption by such secondary instability. If the viscosity profile is monotonie and the asymptotic upper mantle viscosity is about 1022 poise, as suggested by post-glacial rebound data, then the existence of the second scale is extremely unlikely. On the other hand, if a sufficiently pronounced low viscosity zone does exist under old sea-floor then the development of such a second scale cannot be ruled out completely. Some recently obtained geophysical evidence is reviewed which suggests that this is unlikely to be the case. 相似文献
217.
Relationships among conductivity, temperature, total dissolved solids (TDS), and density were determined for hypersaline brine
(TDS, 83 g kg−1) from Mono Lake. A theoretical estimate of density based on ionic composition and calculated partial molar volumes compared
well with actual measurements (difference, 390 ppm). However, an empirically derived equation of state based on temperature
and conductivity provides the most accurate density estimates (residual errors, 190 ppm) and can be used with data collected
with CTD profilers. 相似文献
218.
219.
A. E. Sansom R. G. Izzard P. Ocvirk 《Monthly notices of the Royal Astronomical Society》2009,399(2):1012-1025
One of the complexities in modelling integrated spectra of stellar populations is the effect of interacting binary stars besides Type Ia supernovae (SNeIa). These include common envelope systems, cataclysmic variables, novae, and are usually ignored in models predicting the chemistry and spectral absorption line strengths in galaxies. In this paper, predictions of chemical yields from populations of single and binary stars are incorporated into a galactic chemical evolution model to explore the significance of the effects of these other binary yields. Effects on spectral line strengths from different progenitor channels of SNeIa are also explored. Small systematic effects are found when the yields from binaries, other than SNeIa, are included, for a given star formation history. These effects are, at present, within the observational uncertainties on the line strengths. More serious differences can arise in considering different types of SNIa models, their rates and contributions. 相似文献
220.