全文获取类型
收费全文 | 26031篇 |
免费 | 467篇 |
国内免费 | 215篇 |
专业分类
测绘学 | 527篇 |
大气科学 | 1986篇 |
地球物理 | 5219篇 |
地质学 | 9089篇 |
海洋学 | 2109篇 |
天文学 | 6031篇 |
综合类 | 47篇 |
自然地理 | 1705篇 |
出版年
2021年 | 161篇 |
2020年 | 197篇 |
2019年 | 210篇 |
2018年 | 500篇 |
2017年 | 465篇 |
2016年 | 569篇 |
2015年 | 438篇 |
2014年 | 622篇 |
2013年 | 1218篇 |
2012年 | 738篇 |
2011年 | 1027篇 |
2010年 | 882篇 |
2009年 | 1227篇 |
2008年 | 1060篇 |
2007年 | 1061篇 |
2006年 | 983篇 |
2005年 | 823篇 |
2004年 | 841篇 |
2003年 | 802篇 |
2002年 | 741篇 |
2001年 | 689篇 |
2000年 | 639篇 |
1999年 | 577篇 |
1998年 | 579篇 |
1997年 | 579篇 |
1996年 | 443篇 |
1995年 | 423篇 |
1994年 | 383篇 |
1993年 | 336篇 |
1992年 | 311篇 |
1991年 | 278篇 |
1990年 | 305篇 |
1989年 | 282篇 |
1988年 | 235篇 |
1987年 | 311篇 |
1986年 | 260篇 |
1985年 | 348篇 |
1984年 | 385篇 |
1983年 | 372篇 |
1982年 | 337篇 |
1981年 | 310篇 |
1980年 | 314篇 |
1979年 | 283篇 |
1978年 | 312篇 |
1977年 | 263篇 |
1976年 | 267篇 |
1975年 | 275篇 |
1974年 | 234篇 |
1973年 | 238篇 |
1972年 | 158篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
601.
Geochemistry and Intrusive History of the Ashland Pluton, Klamath Mountains, California and Oregon 总被引:3,自引:1,他引:2
GRIBBLE ROBERT F.; BARNES CALVIN G.; DONATO MARY M.; HOOVER JAMES D.; KISTLER RONALD W. 《Journal of Petrology》1990,31(4):883-923
The Ashland pluton is a calc-alkaline plutonic complex thatintruded the western Paleozoic and Triassic belt of the KlamathMountains in late Middle Jurassic time. The pluton comprisesa series of compositionally distinct magma pulses. The oldestrocks are hornblende gabbro and two-pyroxene quartz gabbro withinitial 87Sr/86Sr = 0{dot}7044, 18O = 8{dot}7%, and REE patternswith chondrite normalized La/Lu = 7. These units were followedby a suite of tonalitic rocks (LaN/LuN = 7) and then by a suiteof K2O- and P2O5 rocks of quartz monzodioritic affinity (LaN/LuN= 1321; LaN/SmN = 2{dot}43{dot}) The quartz monzodioriticrocks were then intruded by biotite granodiorite and granitewith lower REE abundances but more fractionated LREE(LaN/LuN= 1319; LaN/SmN = 4{dot}36 and they, in turn,were host to dikes and bosses of hornblende diorite. The latestintrusive activity consisted of aplitic and granitic dikes.Combined phase equilibria and mineral composition data, indicateemplacement conditions of approximately Ptotal = 2{dot}3kb,PH2O between 1{dot}5 and 2{dot}2 kb, and fO2 between the nickel-nickeloxide and hematite-magnetite buffers. Successive pulses of magma display increasing SiO2 togetherwith increasing 18O and decreasing initial 87Sr/86Sr. The isotopicdata are consistent with either (1) combined fractional crystallizationof andesitic magma and concurrent assimilation of crustal materialcharacterized by low Sr1 and high (18O or, more probably, (2)a series of partial melting events in which sources were successivelyless radiogenic but richer in 18O Each intrusive stage displaysevidence for some degree of crystal accumulation and/or fractionalcrystallization but neither process adequately accounts fortheir compositional differences. Consequently, each stage appearsto represent a distinct partial melting or assimilation event. The P2O5-rich nature of the quartz monzodiorite suite suggestsaccumulation of apatite. However, the suite contains abundantmafic microgranitoid enclaves and most apatite in the suiteis acicular. These observations suggest that magma mixing affectedthe compositional variation of the quartz monzodiorite suite.Mass balance calculations are consistent with a simple mixingprocess in which P2O5-rich alkalic basalt magma (representedby the mafic microgranitoid enclaves) was combined with a crystal-poorfelsic magma (represented by the tonalite suite), yielding aquartz monzodioritic magma that then underwent differentiationby crystal fractionation and accumulation. 相似文献
602.
Tests have been carried out on Kodak Spectroscopic Plates Type I-N, in order to find a treatment which could be easily applied to the infrared plates to be used with the Loiano 152 cm,f/8, Ritchey-Chrétien telescope. Different techniques have been tested, not only with the aim of achieving the higher speed gain, but also to obtain the better uniformity and reproducibility. Our results suggest that the advantages of preexposure techniques are underestimated in this respect. Furthermore, we find that blue and infrared preexposures (at 0.8 s) give different results, suggesting a different degree of dispersity in the latent image produced by pre-exposure in the two cases.Baths including both ammonia and Ag-compounds provide the higher speed gains, but the increment of the fog is often non-uniform and/or very high. The best results have been obtained with baths including AgNO3 and AgCl. The rejuvenation process found by Sanduleak and the results obtained by Jenkins and Farnell on the reversibility of the gains due to silver-nitrate baths have been confirmed. 相似文献
603.
The problem of single Compton scattering is considered and the resulting spectrum, angular distribution and polarization of scattered photons in a general case are obtained. The inverse Compton scattering (ICS) for arbitrary energies of electronsE and photons 0 is investigated in detail. In the case of isotropically-distributed initial photons and relativistic electrons, a strong rise of the scattered spectrum near the upper edge takes place, starting from the values of the characteristic parameterb4E
010 (in units of mc2=1). The energy-loss rate of relativistic electrons due to ICS is calculated. It is shown that the relativistic electrons of the energiesE100 MeV, when scattering on the X-rays with 0~10KeV, transmit the dominant part of their energy to the photons which fall after scattering into the energy range of the electrons (100 MeV).The radiation spectrum of ICS, as well as the energy-losses of relativistic electrons distributed by power-lawE
–, are calculated. The radiation spectrum reveals the power-law behaviour with the different indices in two limits: the dependence –(1)/2 at 01 gradually changes to –(+1) ln (0) law for 01. 相似文献
604.
During the last half of 1977 the UCSD/MIT Hard X-Ray and Low Energy Gamma-Ray Experiment of HEAO-1 observed two of the three gamma-ray bursts detected by at least three satellites. The first of these bursts (20 October, 1977) had a fluence of (3.1±0.5)×10–5 erg cm–2 integrated over the energy range 0.135–2.05 MeV and over its duration of 38.7 s, placing it among the largest bursts observed. The second (10 November, 1977) had a fluence of (2.1±0.8)×10–5 erg cm–2 integrated over the energy range 0.125–3 MeV and over its duration of 2.8 s. The light curves of both bursts exhibit time fluctuations down to the limiting time resolution of the detectors (0.1 s). The spectrum of the 20 October, 1977 burst can be fitted with a power law (index –1.93±0.16), which is harder than other reported gamma-ray burst spectral fits. This burst was detected up to 2.05 MeV, and approximately half of its energy was emitted at photon energies above 0.5 MeV. The spectrum of the 10 November, 1977 burst is softer (index –2.4±0.7) and is similar to the spectrum of the 27 April, 1972 burst.Paper presented at the Symposium on Cosmic Gamma-Ray Bursts held at Toulouse, France, 26–29 November, 1979. 相似文献
605.
F. Kneer G. Scharmer W. Mattig A. Wyller G. Artzner P. Lemaire J. C. Vial 《Solar physics》1981,69(2):289-300
Observations with the French (L.P.S.P.) experiment on board OSO-8 of a sunspot and nearby plage region are described. The behaviour of the emission cores of the Ca II H and K and Mg II h and k resonance lines is very similar and the correspondence in intensity between the four lines persists in all observed features. In contrast, the Lyman lines show little correlation with the other lines. Their emission regions appear broader in the spectroheliograms than the underlying sunspot structure and must not necessarily possess a counterpart in lower layers. From the central intensity of L above the umbra an electron density of 4.3 × 1010 cm-3 n
e
* 2.3 × 1011 cm-3 at 20 000 K is estimated.Mitteilungen aus dem Kiepenheuer-Institut Nr. 186.Stockholm Observatorium, S-13300 Saltsjöbaden, Sweden.Laboratoire de Physique Stellaire et Planétaire, CNRS, P.O. Box 10, F-91370 Verrières-le-Buisson, France. 相似文献
606.
A comparison is made between the flux-versus-time profile in the EUV band and the thick target electron flux profile as inferred from hard X-rays for a number of moderately large solar flares. This complements Kane and Donnelly's (1971) study of small flares. The hard X-ray data are from ESRO TD-1A and the EUV inferred from SFD observations.Use of a 2 minimising method shows that the best overall fit between the profile fine structures obtains for synchronism to 5 s which is within the timing accuracy. This suggests that neither conduction nor convection is fast enough as the primary mechanism of energy transport into the EUV flare and rather favours heating by the electrons themselves or by some MHD wave process much faster than acoustic waves.The electron power deposited, for a thick target model, is however far greater than the EUV luminosity for any reasonable assumptions about the area and depth over which EUV is emitted. This means that either most of the power deposited is conducted away to the optical flare or that only a fraction 1–10% of the X-ray emitting electrons are injected downwards. Recent work on H flare heating strongly favours the latter alternative - i.e. that electrons are mostly confined in the corona. 相似文献
607.
J. W. LAVELLE D. J. P. SWIFT P. E. GADD W. L. STUBBLEFIELD F. N. CASE† H. R. BRASHEAR† K. W. HAFF† 《Sedimentology》1978,25(6):823-842
Both spring-summer and fall-winter sand transport have been observed on the Long Island, New York, inner shelf at water depths of 20-22 m using a radio-isotope sand tracer system. The extent of dispersal of the tagged, fine sand was measured at 3 week intervals in two 70 day experiments. In the late spring and early summer, movement was primarily diffusive in nature, extending 100 m around the line of tracer injection, while late fall-winter patterns had strong advective features, including an ellipsoidal outline extending approximately 1500 m westward of the injection points after the passage of several storms with strong northeasterly winds. Near-bottom current observations made with Savonius rotor sensors identify the event responsible for the bulk of the transport over the 135 day observation period as a storm flow of 2 days duration. Tracer and current observations together suggest that westward winter storm flow along the Long Island shelf is the major mechanism of sand transport at these depths on a yearly time scale. A least-squares fit of several of the observed winter patterns with a plume model yields average sediment mass flux lower bounds of 3.2 × 10?3 gm/cm/sec and 1.7 × 10?1 gm/cm/sec for ‘typical’ and extreme winter storm activity. 相似文献
608.
609.
610.
James B. Pollack Donald W. Strecker Fred C. Witteborn Edwin F. Erickson Betty J. Baldwin 《Icarus》1978,34(1):28-45
We have measured the shape and absolute value of Venus' reflectivity spectrum in the 1.2-to 4.0-μm spectral region with a circular variable filter wheel spectrometer having a spectral resolution of 1.5%. The instrument package was mounted on the 91-cm telescope of NASA Ames Kuiper Airborne Observatory, and the measurements were obtained at an altitude of about 41,000 feet, when Venus had a phase angle of 86°. Comparing these spectra with synthetic spectra generated with a multiple-scattering computer code, we infer a number of properties of the Venus clouds. We obtain strong confirmatory evidence that the clouds are made of a water solution of sulfuric acid in their top unit optical depth and find that the clouds are made of this material down to an optical depth of at least 25. In addition, we determine that the acid concentration is 84 ± 2% H2SO4 by weight in the top unit optical depth, that the total optical depth of the clouds is 37.5 ± 12.5, and that the cross-sectional weighted mean particle radius lies between 0.5 and 1.4 μm in the top unit optical depth of the clouds. These results have been combined with a recent determination of the location of the clouds' bottom boundary [Marov et al., Cosmic Res.14, 637–642 (1976)] to infer additional properties about Venus' atmosphere. We find that the average volume mixing ratio of H2SO4 and H2O contained in the cloud material both equal approximately 2× 10?6. Employing vapor pressure arguments, we show that the acid concentration equals 84 ± 6% at the cloud bottom and that the water vapor mixing ratio beneath the clouds lies between 6 × 10?4 and 10?2. 相似文献