全文获取类型
收费全文 | 33772篇 |
免费 | 609篇 |
国内免费 | 371篇 |
专业分类
测绘学 | 725篇 |
大气科学 | 2214篇 |
地球物理 | 6173篇 |
地质学 | 12774篇 |
海洋学 | 3361篇 |
天文学 | 7706篇 |
综合类 | 79篇 |
自然地理 | 1720篇 |
出版年
2022年 | 307篇 |
2021年 | 503篇 |
2020年 | 563篇 |
2019年 | 622篇 |
2018年 | 1164篇 |
2017年 | 1117篇 |
2016年 | 1245篇 |
2015年 | 657篇 |
2014年 | 1145篇 |
2013年 | 1933篇 |
2012年 | 1293篇 |
2011年 | 1653篇 |
2010年 | 1458篇 |
2009年 | 1817篇 |
2008年 | 1512篇 |
2007年 | 1568篇 |
2006年 | 1467篇 |
2005年 | 905篇 |
2004年 | 871篇 |
2003年 | 821篇 |
2002年 | 843篇 |
2001年 | 714篇 |
2000年 | 663篇 |
1999年 | 553篇 |
1998年 | 563篇 |
1997年 | 509篇 |
1996年 | 464篇 |
1995年 | 410篇 |
1994年 | 434篇 |
1993年 | 352篇 |
1992年 | 344篇 |
1991年 | 352篇 |
1990年 | 372篇 |
1989年 | 269篇 |
1988年 | 267篇 |
1987年 | 333篇 |
1986年 | 253篇 |
1985年 | 373篇 |
1984年 | 353篇 |
1983年 | 315篇 |
1982年 | 325篇 |
1981年 | 270篇 |
1980年 | 303篇 |
1979年 | 241篇 |
1978年 | 274篇 |
1977年 | 212篇 |
1976年 | 213篇 |
1975年 | 206篇 |
1974年 | 208篇 |
1973年 | 202篇 |
排序方式: 共有10000条查询结果,搜索用时 93 毫秒
151.
152.
Seung Ryeol Lee J. Wright Horton Richard J. Walker 《Meteoritics & planetary science》2006,41(6):819-833
Abstract— The osmium isotope ratios and platinum‐group element (PGE) concentrations of impact‐melt rocks in the Chesapeake Bay impact structure were determined. The impact‐melt rocks come from the cored part of a lower‐crater section of suevitic crystalline‐clast breccia in an 823 m scientific test hole over the central uplift at Cape Charles, Virginia. The 187Os/188Os ratios of impact‐melt rocks range from 0.151 to 0.518. The rhenium and platinum‐group element (PGE) concentrations of these rocks are 30–270x higher than concentrations in basement gneiss, and together with the osmium isotopes indicate a substantial meteoritic component in some impact‐melt rocks. Because the PGE abundances in the impact‐melt rocks are dominated by the target materials, interelemental ratios of the impact‐melt rocks are highly variable and nonchondritic. The chemical nature of the projectile for the Chesapeake Bay impact structure cannot be constrained at this time. Model mixing calculations between chondritic and crustal components suggest that most impact‐melt rocks include a bulk meteoritic component of 0.01–0.1% by mass. Several impact‐melt rocks with lowest initial 187Os/188Os ratios and the highest osmium concentrations could have been produced by additions of 0.1%–0.2% of a meteoritic component. In these samples, as much as 70% of the total Os may be of meteoritic origin. At the calculated proportions of a meteoritic component (0.01–0.1% by mass), no mixtures of the investigated target rocks and sediments can reproduce the observed PGE abundances of the impact‐melt rocks, suggesting that other PGE enrichment processes operated along with the meteoritic contamination. Possible explanations are 1) participation of unsampled target materials with high PGE abundances in the impact‐melt rocks, and 2) variable fractionations of PGE during syn‐ to post‐impact events. 相似文献
153.
154.
Abstract— Primary minerals in calcium‐aluminum‐rich inclusions (CAIs), Al‐rich and ferromagnesian chondrules in each chondrite group have δ18O values that typically range from ?50 to +5%0. Neglecting effects due to minor mass fractionations, the oxygen isotopic data for each chondrite group and for micrometeorites define lines on the three‐isotope plot with slopes of 1.01 ± 0.06 and intercepts of ?2 ± 1. This suggests that the same kind of nebular process produced the 16O variations among chondrules and CAIs in all groups. Chemical and isotopic properties of some CAIs and chondrules strongly suggest that they formed from solar nebula condensates. This is incompatible with the existing two‐component model for oxygen isotopes in which chondrules and CAIs were derived from heated and melted 16O‐rich presolar dust that exchanged oxygen with 16O‐poor nebular gas. Some FUN CAIs (inclusions with isotope anomalies due to fractionation and unknown nuclear effects) have chemical and isotopic compositions indicating they are evaporative residues of presolar material, which is incompatible with 16O fractionation during mass‐independent gas phase reactions in the solar nebula. There is only one plausible reason why solar nebula condensates and evaporative residues of presolar materials are both enriched in 16O. Condensation must have occurred in a nebular region where the oxygen was largely derived from evaporated 16O‐rich dust. A simple model suggests that dust was enriched (or gas was depleted) relative to cosmic proportions by factors of ~10 to >50 prior to condensation for most CAIs and factors of 1–5 for chondrule precursor material. We infer that dust‐gas fractionation prior to evaporation and condensation was more important in establishing the oxygen isotopic composition of CAIs and chondrules than any subsequent exchange with nebular gases. Dust‐gas fractionation may have occurred near the inner edge of the disk where nebular gases accreted into the protosun and Shu and colleagues suggest that CAIs formed. 相似文献
155.
It was found recently that fast electrons travel through the plasma of the solar corona in the form of beam-plasma structure
(BPS), which consists of electrons and Langmuir waves. In this paper the influence of scattering BPS Langmuir waves off plasma
ions (l+i=l+i) on BPS velocity is studied. We show that the maximum BPS velocity equals 0.35c, which is close to the velocity of Type III bursts sources. 相似文献
156.
Andrei V. Ivanov Nataliya N. Kononkova S. Vincent Yang Michael E. Zolensky 《Meteoritics & planetary science》2003,38(5):725-737
Abstract— Clasts of alkaline (the second find in meteorites) and subalkaline rocks were found in the Kaidun meteorite. One of them (#d4A) is a large crystal of albite with inclusions of fluorapatite, arfvedsonite, aenigmatite, and wilkinsonite. The two latter minerals were previously unknown in meteorites. Another clast (#d[3–5]D) has a melt crystallization texture of mainly feldspar (oligoclase) composition and contains relict grains of both high‐Ca and low‐Ca pyroxene and fluorapatite. The mineralogical characteristics of these clasts suggest a genetic relationship and an origin from the same parent body. The textural and mineralogical characteristics of the clasts indicate origin by extensive igneous differentiation. Such processes most likely took place in a rather large differentiated body. The material of clast #d(3–5)D is similar in some mineralogical respects to basaltic shergottites. 相似文献
157.
158.
159.
160.
A high-velocity ionized outflow and XUV photosphere in the narrow emission line quasar PG1211+143 总被引:1,自引:0,他引:1
K. A. Pounds J. N. Reeves A. R. King K. L. Page P. T. O'Brien M. J. L. Turner 《Monthly notices of the Royal Astronomical Society》2003,345(3):705-713
We report on the analysis of a ∼60-ks XMM–Newton observation of the bright, narrow emission line quasar PG1211+143. Absorption lines are seen in both European Photon Imaging Camera and Reflection Grating Spectrometer spectra corresponding to H- and He-like ions of Fe, S, Mg, Ne, O, N and C. The observed line energies indicate an ionized outflow velocity of ∼24 000 km s−1 . The highest energy lines require a column density of N H ∼ 5 × 1023 cm−2 , at an ionization parameter of log ξ∼ 3.4 . If the origin of this high-velocity outflow lies in matter being driven from the inner disc, then the flow is likely to be optically thick within a radius of ∼130 Schwarzschild radii, providing a natural explanation for the big blue bump (and strong soft X-ray) emission in PG1211+143. 相似文献