全文获取类型
收费全文 | 54213篇 |
免费 | 578篇 |
国内免费 | 438篇 |
专业分类
测绘学 | 1437篇 |
大气科学 | 4460篇 |
地球物理 | 10114篇 |
地质学 | 22207篇 |
海洋学 | 3943篇 |
天文学 | 10387篇 |
综合类 | 218篇 |
自然地理 | 2463篇 |
出版年
2020年 | 180篇 |
2019年 | 197篇 |
2018年 | 3550篇 |
2017年 | 3384篇 |
2016年 | 2310篇 |
2015年 | 565篇 |
2014年 | 704篇 |
2013年 | 1406篇 |
2012年 | 1877篇 |
2011年 | 3780篇 |
2010年 | 3540篇 |
2009年 | 3894篇 |
2008年 | 3117篇 |
2007年 | 3695篇 |
2006年 | 1087篇 |
2005年 | 1378篇 |
2004年 | 1230篇 |
2003年 | 1328篇 |
2002年 | 1112篇 |
2001年 | 775篇 |
2000年 | 817篇 |
1999年 | 660篇 |
1998年 | 627篇 |
1997年 | 666篇 |
1996年 | 575篇 |
1995年 | 540篇 |
1994年 | 482篇 |
1993年 | 426篇 |
1992年 | 418篇 |
1991年 | 416篇 |
1990年 | 421篇 |
1989年 | 398篇 |
1988年 | 378篇 |
1987年 | 464篇 |
1986年 | 433篇 |
1985年 | 464篇 |
1984年 | 557篇 |
1983年 | 560篇 |
1982年 | 501篇 |
1981年 | 503篇 |
1980年 | 463篇 |
1979年 | 433篇 |
1978年 | 447篇 |
1977年 | 394篇 |
1976年 | 358篇 |
1975年 | 355篇 |
1974年 | 405篇 |
1973年 | 389篇 |
1972年 | 245篇 |
1971年 | 224篇 |
排序方式: 共有10000条查询结果,搜索用时 203 毫秒
231.
The transport of thermal radiation has been considered within a finite slab which absorb and scatter anisotropically. The problem involves the space-dependent single-scattering albedow(x). Two approximations are taken forw(x). In the first it is represented in exponential form asw(x)=w
0 exp(–x/s), wherew
0 ands are given constants andx is the optical variable. The second approximation assumes the formw(x) =
r=0
R
d
r
*
p
r
(x/a), whered
r
*
are known expansion coefficients anda is the half optical thickness of the slab. Analytic expressions for the forward, backward radiation intensities and fluxes are given in each approximation. The solution of the linear transport equation is performed on the basis of integral Fourier transforms. 相似文献
232.
We report and discuss solar systemN
R
abundances for nuclidesA>70, obtained as differences between measured solar system abundances and calculatedS-process contributions. The abundance peak atA163 in the rare Earth element region reveals properties which are similar to those of theR-process peaks corresponding to magic neutron numbersN=82 andN=126. We observe that systematic differences in theN
R
abundances of even-A and odd-A nuclides are restricted to specific mass regions. We discuss possible interpretations and conclude that these differences are most probably related to the properties of nuclear species during – to the stability valley.... A genesis of the elements such as is sketched out would not be confined to our little Solar System, but would probably follow the same general sequence of events in every center of energy now visible as a Star. Sir William Crookes (1886)Paper dedicated to Professor Hannes Alfvén on the occasion of his 80th birthday, 30 May 1988. 相似文献
233.
Irvine WM Friberg P Hjalmarson A Ishikawa S Kaifu N Kawaguchi K Madden SC Matthews HE Ohishi M Saito S Suzuki H Thaddeus P Turner BE Yamamoto S Ziurys LM 《The Astrophysical journal》1988,334(2):L107-L111
We report the astronomical identification of the cyanomethyl radical, CH2CN, the heaviest nonlinear molecular radical to be identified in interstellar clouds. The complex fine and hyperfine structures of the lowest rotational transitions at about 20.12 and 40.24 GHz are resolved in TMC-1, where the abundance appears to be about 5 x 10(-9) relative to that of H2. This is significantly greater than the observed abundance of CH3CN (methyl cyanide) in TMC-1. In Sgr B2 the hyperfine structure is blended in the higher frequency transitions at 40, 80, and 100 GHz, although the spin-rotation doubling is clearly evident. Preliminary searches in other sources indicate that the distribution of CH2CN is similar to that for such carbon chain species as HC3N or C4H. 相似文献
234.
Rowland RH 《Soviet geography》1988,29(9):809-829
"This article examines union republic migration trends in the USSR between 1979 and 1987 and prospects for indigenous out-migration from rural areas in Central Asia. The study is based on migration data derived by the residual technique and migration data from the 1985 microcensus. Results indicate that a south-to-north and probably Russian-dominated migration trend emerged in the 1980s, one which marks an almost complete reversal from earlier periods, especially 1959-70. Although Central Asia continues to have low levels of indigenous out-migration, labor surpluses and relatively waning capital investment in Central Asia may change this situation." 相似文献
235.
A. O. Allakhverdiyev O. H. Guseinov F. K. Kasumov I. M. Yusifov 《Astrophysics and Space Science》1986,121(1):21-44
A new way of choosing of the calibrators for constructing (D)-dependence of supernova remnants (SNR) is suggested. A knowledge of the distance of an SNR is not sufficient to consider it as a calibrator. It is shown that neither more accurate determination of distance to calibrators, nor an increase in their number, nor an increase in the precision of the remnant angular size and radio flux determination can lead to a considerable improvement of simple (D)-dependence which is suggested in the given work.For remnants with small surface brightnesses there is no (D)-dependence at all. The most accurate remnant distances currently known, are given: these are liable to change only after a large amount of additional observational data accurately compiled. 相似文献
236.
H. J. Staude 《Astrophysics and Space Science》1986,128(1):179-197
The topic is reviewed with emphasis on observations in the optical and near infrared spectral range.Paper presented at a Workshop on The Role of Dust in Dense Regions of Interstellar Matter, held at Georgenthal, G.D.R., in March 1986. 相似文献
237.
We have investigated the evolutionary behaviour of intermediate mass (2, 3, 4, 5, and 7M
) Population I stars, assuming two different rates of rotation at the threshold of stability.In the first part of the study, stars are assumed to start with a critical rotation (fast rotation model) and to progress to the point of rotational instability. The stars evolve by losing mass and become rotationally unstable before they reach the zero-age Main Sequence. It is argued that multiple star systems might be formed through the evolution of rapidly rotating stars. An expression for the rotational mass loss rate is derived as a function of the physical parameters of stars.In the second part of the study, stars are assumed to rotate at a rate below the critical value (slow rotation model). The evolution of slowly rotating stars is followed as far as zero-age Main Sequence on the theoretical Hertzsprung-Russell diagram and compared with that of normal stars. The evolutionary paths are found to be more or less similar to those of normal stars; but their positions on the Main Sequence are characterized by effective temperatures and luminosities lower than those of normal stars. The zero-age Main-Sequence times of these stars are longer than those of normal stars. The rotational rates obtained for the zero-age Main Sequence are in good agreement with observed values. 相似文献
238.
Recent observational and theoretical studies of the structure of umbral chromosphres have led us to propose new models of the region. We combine space and ground based determinations of the densitytemperature structure of the transition region with existing Caii H, K, and IR triplet spectral data to establish relatively high density, gradient models of the sunspot chromosphere.Avrett in 1981 presented theSunspot Sunspot Model which can be characterized as a physically extended, low density, plateau model. However, in a similar study, the authors (Beebeet al., 1982) pointed out that the relatively high transition region pressures derived from space observations indeed are required to reproduce high resolution Caii spectral features. Studies of the umbral chromosphere as a resonant cavity for slow-mode magneto-acoustic waves producing umbral oscillations (Zhugzhdaet al., 1984) also lead to atmospheres of relatively small physical thickness, thus higher densities.Paper presented at the IAU Third Asian-Pacific Regional Meeting, held in Kyoto, Japan, between 30 September–6 October, 1984. 相似文献
239.
The data of Paper I (Doddet al., 1986), representing a catalogue of parameters measured for bright galaxies in field 444 of the Southern Sky Survey, are examined in a search for non-random effects. A global trend of bimodal alignment of galaxies in this field is found. This effect is present even for galaxies of large apparent ellipticity (e0.5) and is interpreted as revealing a real physical phenomenon on the large scale in this field. Analysis of nearest neighbouring pairs of galaxies reveals evidence also for small-scale interactions. The difference in apparent ellipticities between close pairs is larger than would be expected if the galaxies were randomly distributed. Nearest neighbouring galaxies also show a strong preference to be aligned with their major axes parallel. The results are discussed with regard to theories for galaxy formation. 相似文献
240.
The production of X-rays and gamma-rays in bursts is believed to be due to the rapid burning of matter accreted onto a neutron star surface from its companion, most probably a giant star. The accreted matter consists mainly of hydrogen and helium and a very small amount of heavy elements. Due to the infall of matter, the temperature at the bottom layers is raised to a value of the order of 108 K. The neutron star surface density is>107 g cm–3. As hydrogen burning is a slow process under any temperature and density conditions, we consider the helium-burning reactions as the source of gamma-rays in the neutron star surface. Under high-density conditions the ordinary laboratory reaction rates should become modified. At high-density conditions, the strong screening effect due to the polarising cloud of electrons around the ions become important and enhances the reaction rates considerably. The helium-burning reactions are calculated under such conditions. The abundances of helium-burning products such as12C, 116O, and20Ne, etc., are computed. Under high-density and temperature conditions carbon is found to be more abundant than oxygen. Neon is completely absent in almost all the relevant physical conditions in which a strong screening effect is operative. It is suggested that explosive burning of accreted helium of 10–13
M
will account for the observed energy of gamm-ray burst. 相似文献