全文获取类型
收费全文 | 42489篇 |
免费 | 764篇 |
国内免费 | 327篇 |
专业分类
测绘学 | 983篇 |
大气科学 | 3087篇 |
地球物理 | 8290篇 |
地质学 | 14558篇 |
海洋学 | 3958篇 |
天文学 | 9959篇 |
综合类 | 75篇 |
自然地理 | 2670篇 |
出版年
2021年 | 420篇 |
2020年 | 422篇 |
2019年 | 469篇 |
2018年 | 972篇 |
2017年 | 869篇 |
2016年 | 1131篇 |
2015年 | 634篇 |
2014年 | 1045篇 |
2013年 | 2131篇 |
2012年 | 1166篇 |
2011年 | 1681篇 |
2010年 | 1521篇 |
2009年 | 2057篇 |
2008年 | 1707篇 |
2007年 | 1787篇 |
2006年 | 1645篇 |
2005年 | 1365篇 |
2004年 | 1308篇 |
2003年 | 1294篇 |
2002年 | 1220篇 |
2001年 | 1078篇 |
2000年 | 1009篇 |
1999年 | 828篇 |
1998年 | 827篇 |
1997年 | 844篇 |
1996年 | 684篇 |
1995年 | 669篇 |
1994年 | 629篇 |
1993年 | 581篇 |
1992年 | 540篇 |
1991年 | 507篇 |
1990年 | 514篇 |
1989年 | 504篇 |
1988年 | 483篇 |
1987年 | 566篇 |
1986年 | 503篇 |
1985年 | 623篇 |
1984年 | 669篇 |
1983年 | 613篇 |
1982年 | 552篇 |
1981年 | 587篇 |
1980年 | 492篇 |
1979年 | 472篇 |
1978年 | 444篇 |
1977年 | 437篇 |
1976年 | 384篇 |
1975年 | 381篇 |
1974年 | 380篇 |
1973年 | 388篇 |
1971年 | 226篇 |
排序方式: 共有10000条查询结果,搜索用时 33 毫秒
991.
The peralkaline Kaffo albite—riebeckite granite is an albitised, low-temperature intrusion in Liruei Complex, one of the oldest of the ring-complexes in the Younger Granite province of Nigeria. Analyses of borehole samples from different parts of the intrusion show it to be compositionally heterogeneous, especially in respect of Si, Al, Na, K and F distribution and this, in part, can be correlated with the variable degree of albitisation. Isotopically the granite is a normal plutonic type with δ18 O values of , and albitisation does not seem to have been accompanied by exchange of isotopes between albitising fluid and the granite. Co-existing riebeckitic-arfvedsonite and aegirine pairs from borehole samples show extreme enrichment in Na and Fe; the amphibole also shows considerable substitution of Fe by Ti, Zn and Mn, and of OH by F. Isotopically the amphibole and pyroxene are different from others, having low, variable δ18 O values (+5.3–+6.4 and , respectively), and the fractionation value Δ Px — Am is always large, negative and constant (—). The low δ18 O values are considered to be due to special features of the crystal chemistry of the alkali amphiboles and pyroxenes, and the spread of each set of values may be due to sub-solidus isotope exchange between the minerals and albitising fluid. 相似文献
992.
D. Whitehead 《Chemical Geology》1976,18(2):149-153
An atomic-absorption spectrophotometric method is described for the determination of Ca and Mg in carbonate rocks after an acetic-acid decomposition. KCl is used as a releasing agent instead of the more commonly used LaCl3. Results obtained using both releasing agents compare favourably in accuracy and precision, KCl having the advantage that it is much less expensive. 相似文献
993.
D. C. Schwank 《Astrophysics and Space Science》1976,43(2):459-468
Weight functions for the determination of the periods of linear adiabatic non-radial oscillations have been calculated in the same manner as Epstein's classic treatment of purely radial oscillations. Quadrupole (l=2) oscillations for thef and lower orderp andg-modes were considered. One group of static models were polytropes in the range 1.0n4.0 with
; thus included were configurations that were convectively stable, unstable and neutrally stable throughout. Another group consisted ofn=3.0 polytropes with convective shells or convective cores; 1 was set at different values in each region in order to produce stability (
) or instability (
). The weight function provides a pictorial means for assessing the relative importance of each region of a given static model with respect to generating a given non-radial mode. 相似文献
994.
995.
996.
Three silicified limestone horizons of D1 age from the Visean of the Isle of Man contain calcitic concretions with peripheral silica crusts, occasionally surrounded by a further calcitic layer. Components of the original sediment include carbonate skeletons, carbonaceous grains, sponge spicules and muscovite. Diagenetic products include calcite, dolomite, pyrite, sphalerite, clays, feldspar and quartz. The concretions are composed of neomorphic calcite. The time of recrystallization and the identity of the neomorphic precurosor are both unknown. Displacive, fibrous calcite is chemically similar to neomorphic calcite and both are of early diagenetic age. Granular and rhombic ferroan calcites are of late diagenetic age and were precipitated from pore-waters with Sr/Ca, Mg/Ca and Fe/Ca ratios unlike those of seawater. The difference between early silicification which produced silica crusts and later diffuse silicification of the host sediment is related to a change in sediment transmissivity between the two silicification periods. A four-fold scheme of concretionary growth is proposed. The supply of silica is from sponge spicules and that of carbonate from seawater via porewater. The distribution of organic matter, either as sporadic large carcasses or as small carcasses concentrated in particular horizons, is believed to be vital for carbonate precipitation and controls the distribution of concretions. Awareness of the multiplicity of diagenetic changes is essential in interpretation of early porewater systems and in the origin of products which are often metastable and destined to subsequent changes. No single model is an explanation for all types of concretionary growth. 相似文献
997.
998.
T. P. Armstrong S. M. Krimigis D. Hovestadt B. Klecker G. Gloeckler 《Solar physics》1976,49(2):395-407
Comprehensive measurements of the temporal variations of the Z 6 charge composition in the 4 July 1974 solar event made with experiments aboard IMP-7 and 8 show that large variations in the charge composition occur in three-hour-averaged intensities. Hourly averaged fluxes show variations as large as factors of 3 to 4 from one hour to the next and 3 hour averages show nearly a factor of 10 peak to minimum over the event. The precision of the measurements are limited by counting statistics of the Fe-group channel. Iron to oxygen variations are established by both rate channel and pulse height analysis techniques. Comparison of measurements from IMP-7 and 8, separated by about 70 R
E shows that, while significant differences in composition and intensity exist for brief periods, the gross compositional variations are reproduced well at both spacecraft. These observations provide particularly stringent conditions for theories of the acceleration, release, and propagation of solar energetic particles. 相似文献
999.
D. E. Blackwell P. A. Ibbetson C. A. Murray N. M. Pratt 《Astrophysics and Space Science》1976,43(2):469-473
A quantitative definition of limiting magnitude for stellar photographs is proposed, together with a method of measurement employing a Racine prism. Limiting magnitudes inU, B andV for the Isaac Newton telescope at Herstmonceux derived from a total of 72 prime focus photographs are presented and the results correlated with the size of the photographed image. The limiting magnitude for an extrapolated value of the seeing of 1 isB=22m.0, in good agreement with a value obtained by scaling the limiting magnitudes of the 200 inch telescope. 相似文献
1000.
Lawrence D. Kavanagh 《Icarus》1975,25(1):166-170
A number of charged-particle radiation belt models for Uranus and Neptune are postulated, and the synchrotron emission spectrum for each is calculated over the frequency range 18.75 to 2400 MHz. Although no radio observations of these planets exist at frequencies below 1400MHz, available observations are used in conjunction with the synchrotron calculations to establish a rough upper limit to the size and strength of the planets' magnetic field strength and radiation belt intensity. The magnitude of synchrotron radio emission expected in many of the models is within the detection range of several existing ground-based telescopes, at decimetric and metric wavelengths. 相似文献