全文获取类型
收费全文 | 48252篇 |
免费 | 2127篇 |
国内免费 | 276篇 |
专业分类
测绘学 | 1036篇 |
大气科学 | 3086篇 |
地球物理 | 11897篇 |
地质学 | 17554篇 |
海洋学 | 4071篇 |
天文学 | 10033篇 |
综合类 | 123篇 |
自然地理 | 2855篇 |
出版年
2022年 | 307篇 |
2021年 | 595篇 |
2020年 | 693篇 |
2019年 | 863篇 |
2018年 | 1641篇 |
2017年 | 1620篇 |
2016年 | 1767篇 |
2015年 | 1160篇 |
2014年 | 1644篇 |
2013年 | 2623篇 |
2012年 | 1762篇 |
2011年 | 2205篇 |
2010年 | 1988篇 |
2009年 | 2284篇 |
2008年 | 2093篇 |
2007年 | 2017篇 |
2006年 | 1854篇 |
2005年 | 1337篇 |
2004年 | 1317篇 |
2003年 | 1360篇 |
2002年 | 1206篇 |
2001年 | 1039篇 |
2000年 | 980篇 |
1999年 | 799篇 |
1998年 | 813篇 |
1997年 | 803篇 |
1996年 | 649篇 |
1995年 | 649篇 |
1994年 | 591篇 |
1993年 | 510篇 |
1992年 | 466篇 |
1991年 | 475篇 |
1990年 | 494篇 |
1989年 | 439篇 |
1988年 | 417篇 |
1987年 | 440篇 |
1986年 | 463篇 |
1985年 | 556篇 |
1984年 | 586篇 |
1983年 | 587篇 |
1982年 | 547篇 |
1981年 | 499篇 |
1980年 | 472篇 |
1979年 | 450篇 |
1978年 | 423篇 |
1977年 | 424篇 |
1976年 | 371篇 |
1975年 | 394篇 |
1974年 | 369篇 |
1973年 | 410篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
31.
A numerical scheme is developed in order to simulate fluid flow in three dimensional (3‐D) microstructures. The governing equations for steady incompressible flow are solved using the semi‐implicit method for pressure‐linked equations (SIMPLE) finite difference scheme within a non‐staggered grid system that represents the 3‐D microstructure. This system allows solving the governing equations using only one computational cell. The numerical scheme is verified through simulating fluid flow in idealized 3‐D microstructures with known closed form solutions for permeability. The numerical factors affecting the solution in terms of convergence and accuracy are also discussed. These factors include the resolution of the analysed microstructure and the truncation criterion. Fluid flow in 2‐D X‐ray computed tomography (CT) images of real porous media microstructure is also simulated using this numerical model. These real microstructures include field cores of asphalt mixes, laboratory linear kneading compactor (LKC) specimens, and laboratory Superpave gyratory compactor (SGC) specimens. The numerical results for the permeability of the real microstructures are compared with the results from closed form solutions. Copyright © 2004 John Wiley & Sons, Ltd. 相似文献
32.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ~2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ~0.04 at 1.25 μm and ~0.05 at 2.2 μm. 相似文献
33.
34.
35.
36.
D. Montes M. C. Gálvez M. J. Fernández-Figueroa I. Crespo-Chacón 《Astrophysics and Space Science》2006,304(1-4):367-369
High resolution echelle spectroscopic observations taken with the FEROS spectrograph at the 2.2 m telescope ESO confirm the binary nature of the flare M3.5V star LU Vel (GJ 375, RE J0958-462) previously reported by Christian and Mathioudakis (2002). Emission of similar intensity from both components is detected in the Balmer, Na i D1&D2, He i D3, Ca ii H&K, and Ca ii IRT lines. We have determined precise radial velocities by cross correlation with radial velocity standard stars, which have allowed us to obtain for the first time the orbital solution of the system. The binary consists of two near-equal M3.5V components with an orbital period shorter than 2 days. We have analyzed the behaviour of the chromospheric activity indicators (variability and possible flares). In addition, we have determined its rotational velocity and kinematics. 相似文献
37.
E.A. Evstigneeva R.R. de Carvalho A.L. Ribeiro H.V. Capelato 《Astrophysics and Space Science》2003,284(2):487-490
We present the preliminary results of a study of how small stellar systems merge to form larger ones. As we display the families
of galaxies in the μe - Re plane (effective surface brightness versus effective radius) we realize that different morphological types occupy different
loci, evidencing the different physical mechanisms operating in each family. As proposed by Capaccioli et al. (1992) this
diagram is the logical equivalent of the HR diagram for stars. Here we take some initial steps in understanding of how we
can establish the evolutionary tracks, solely due to dynamical processes, in the μe - Re plane, ultimately making a dwarf elliptical to turn into a normal elliptical galaxy.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
38.
Virgil E. Barnes 《Meteoritics & planetary science》1990,25(3):149-159
Abstract— Fifty-odd years of tektite research are reviewed, proceeding from the discovery of the first North American tektites in 1936. This included the early recognition that tektites were terrestrial objects rather than meteorites and that the glassy particles in tektites were fused quartz (lechatelierite). Later, during National Science Foundation-supported research, it was found that some tektites appeared to have formed as puddles of melt, that the content and character of bubbles in lechatelierite can be used as a relative temperature scale, that rayed bubbles in tektites formed from hydrous minerals, that bubbles in tektites formed chiefly from water which was absorbed into the walls of the bubbles leaving vacuums, and that “fingers” in the surficial part of some tektites may have formed by differential volatilization. Some unpublished observations and adventures are briefly reported. 相似文献
39.
Observations of the brightness, color, and polarization of the fuor V1057 Cyg over more than 30 years are presented and briefly discussed. Variability of the linear polarization was found. 相似文献
40.
A new procedure to separate the instrumental and atmospheric components of stray light is presented. It is based on the dependence of the aureole's atmospheric component on the air mass and is applied to measurements taken with the Vacuum Newton Telescope (VNT) at the Observatorio del Teide (Tenerife). The resulting instrumental part is independent of the air mass.The variation of both components with wavelength is also studied. The instrumental component shows no dependence on wavelength, in contrast to the atmospheric one which is greater in the blue than in the red.It is concluded that observations with air masses larger than two will probably be strongly affected by stray light. 相似文献