全文获取类型
收费全文 | 40460篇 |
免费 | 7866篇 |
国内免费 | 9184篇 |
专业分类
测绘学 | 2979篇 |
大气科学 | 7862篇 |
地球物理 | 11178篇 |
地质学 | 19288篇 |
海洋学 | 4799篇 |
天文学 | 2116篇 |
综合类 | 4202篇 |
自然地理 | 5086篇 |
出版年
2024年 | 137篇 |
2023年 | 523篇 |
2022年 | 1626篇 |
2021年 | 1892篇 |
2020年 | 1560篇 |
2019年 | 2000篇 |
2018年 | 2230篇 |
2017年 | 2121篇 |
2016年 | 2492篇 |
2015年 | 2242篇 |
2014年 | 2589篇 |
2013年 | 2594篇 |
2012年 | 2431篇 |
2011年 | 2495篇 |
2010年 | 2564篇 |
2009年 | 2436篇 |
2008年 | 2326篇 |
2007年 | 2117篇 |
2006年 | 1671篇 |
2005年 | 1601篇 |
2004年 | 1218篇 |
2003年 | 1225篇 |
2002年 | 1184篇 |
2001年 | 1223篇 |
2000年 | 1373篇 |
1999年 | 1744篇 |
1998年 | 1402篇 |
1997年 | 1303篇 |
1996年 | 1225篇 |
1995年 | 1054篇 |
1994年 | 932篇 |
1993年 | 853篇 |
1992年 | 676篇 |
1991年 | 510篇 |
1990年 | 376篇 |
1989年 | 348篇 |
1988年 | 304篇 |
1987年 | 177篇 |
1986年 | 163篇 |
1985年 | 99篇 |
1984年 | 106篇 |
1983年 | 81篇 |
1982年 | 72篇 |
1981年 | 64篇 |
1980年 | 36篇 |
1979年 | 33篇 |
1978年 | 12篇 |
1977年 | 13篇 |
1976年 | 7篇 |
1958年 | 31篇 |
排序方式: 共有10000条查询结果,搜索用时 343 毫秒
691.
Effects of dynamic recrystallization on lattice preferred orientation (LPO) in olivine were investigated through the combination of two SEM-based techniques, electron backscattered diffraction (EBSD) technique for crystallographic orientation measurement and backscattered electron imaging (BEI) for dislocation observation. Samples are experimentally deformed olivine aggregates in simple shear geometry. In the sample deformed at T=1473 K and high stresses (480 MPa), only incipient dynamic recrystallization is observed along grain-boundaries. Orientations of these small recrystallized grains are more random than that of relict grains, suggesting an important role of grain-boundary sliding at this stage of recrystallization. In the sample deformed at T=1573 K and low stress (160 MPa), dynamic recrystallization is nearly complete and the LPO is characterized by two [100] peaks. One peak is located at the orientation subparallel to the shear direction and is dominated by grains with high Schmid factor. The other occurs at high angles to the shear direction and is due to the contribution from grains with low Schmid factor. Grains with high Schmid factor tend to have higher dislocation densities than those with low Schmid factor. Based on these observations, we identify two mechanisms by which dynamic recrystallization affects LPO: (1) enhancement of grain-boundary sliding due to grain-size reduction, leading to the modification of LPO caused by the relaxation of constraint for deformation; (2) grain-boundary migration by which grains with lower dislocation densities grow at the expense of grains with higher dislocation densities. Based on the deformation mechanism maps and stress versus recrystallized grain-size relation, we suggest that the first mechanism always plays an important role whereas the second mechanism has an important effect only under limited conditions. 相似文献
692.
About 30 samples representing major lithologies of Sulu ultrahigh-pressure (UHP) metamorphic rocks were collected from surface exposures and exploration wells, and compressional (Vp) and shear wave (Vs) velocities and their directional dependence (anisotropy) were determined over a range of constant confining pressures up to 600 MPa and temperatures ranging from 20 to 600 °C. Samples range in composition from acidic to ultramafic. P- and S-wave velocities measured at 600 MPa vary from 5.08 to 8.64 km/s and 2.34 to 4.93 km/s, respectively. Densities are in the range from 2.60 to 3.68 g/cm3. To make a direct tie between seismic measurements (refraction and reflection) and subsurface lithologies, the experimental velocity data (corresponding to shallow depths) were used to calculate velocity profiles for the different lithologies and profiles of reflection coefficients at possible lithologic interfaces across the projected 5000-m Chinese Continental Scientific Drilling Program (CCSD) crustal segment. Comparison of calculated in situ velocities with respective intrinsic velocities suggests that the in situ velocities at shallow depths are lowered by an increased abundance of open microcracks. The strongly reflective zone beneath the Donghai drill site can be explained by the impedance contrasts between the different lithologies. Contacts between eclogite/peridotite and felsic rocks (gt-gneiss, granitic gneiss), in particular, may give rise to strong seismic reflections. In addition, shear-induced (lattice preferred orientation (LPO)-related) seismic anisotropy can increase reflectivity. For the explanation of the high velocity bodies (>6.4 km/s) around 1000 m and below 3200-m depth, large proportions of eclogite/peridotite (about 40 and 30 vol.%, respectively) are needed. 相似文献
693.
Shijie Qu Shuhua Hao Guangping Chen Baohui Li Guangzeng Bian 《Fragblast: International Journal for Blasting and Fragmentation》2002,6(1):85-103
Blast design is a critical factor dominating fragmentation and cost of actual bench blasts. However, due to the varying nature of rock properties and geology as well as free surface conditions, reliable theoretic formulae are still unavailable at present and in most cases blast design is carried out by personal experience. As an effort to find a more scientific and reliable tool for blast design, a computer-aided bench blast design and simulation system, the BLAST-CODE model, is developed for Shuichang surface mine, Mining Industry Company of the Capital Iron and Steel Corporation Beijing. The BLAST-CODE model consists of a database representing geological and topographical conditions of the mine and the modules Frag + and Disp + for blast design and prediction of resultant fragmentation and displacement of rock mass. The two modules are established in accordance with cratering theory qualitatively and modified quantitatively by regression of the data collected from 85 bench blasting practices conducted in 3 mines of the Shuichang surface mine. Blasting parameters are selected based upon quantitative and comprehensive evaluation on the effect of the factors such as rock properties, geology, free surface conditions and detonation characteristics of the explosive products in use. In order to ensure practicality and reliability of the system, the BLAST-CODE model allows automatic adjustment to the selected parameters such as burden B and spacing S as well as explosive charge amount Q of any blasthole under irregular topographic and/or varying blastability conditions of the rock mass to be blasted. Simulation of the BLAST-CODE model includes prediction of fragmentation and displacement that are demonstrated in terms of swell factor, characteristic rock size x c and size distribution coefficient n by Rossin-Ramler's equation, and 3-dimentional muck pile profile. The BLAST-CODE model also permits interactive parameter selection based on comparison of the predicted fragmentation and displacement as well as the cost for drilling, explosives, and accessories until the most effective option can be selected. 相似文献
694.
The Hongzuisi Section in Lanzhou area is used to infer the paleoclimate since the late glacial epoch. Through combined analysis of organic carbon isotope and CaCO3 content, paleoenvironment evolution of Hongzuisi Section since the late glacial epoch is discussed. There were changes in both drymoist and warm-cold. Especially the climate changes can be compared with synchronous sections of other areas, which provides useful information for our future forecast. 相似文献
695.
696.
697.
698.
699.
Introduction The stress status of the earths crust is closely related to global structure, the direction of plate movement, the drive mechanism of plate, earthquake cause, etc. Therefore the researches of the stress status of the earths crust and the process of dynamic evolution are paid a great attention to by many people around the world (Zoback, Zoback, 1980; Zoback, et al, 1989; Zoback, 1989; Spence, 1997; Fuchs, 1997; Plenefish, Bonjer, 1997; Muller, et al, 1997; XU, et al, 1989; WAN… 相似文献
700.