全文获取类型
收费全文 | 50609篇 |
免费 | 850篇 |
国内免费 | 614篇 |
专业分类
测绘学 | 1282篇 |
大气科学 | 3326篇 |
地球物理 | 9457篇 |
地质学 | 18547篇 |
海洋学 | 4906篇 |
天文学 | 12039篇 |
综合类 | 115篇 |
自然地理 | 2401篇 |
出版年
2022年 | 361篇 |
2021年 | 604篇 |
2020年 | 682篇 |
2019年 | 790篇 |
2018年 | 1551篇 |
2017年 | 1486篇 |
2016年 | 1716篇 |
2015年 | 884篇 |
2014年 | 1591篇 |
2013年 | 2778篇 |
2012年 | 1780篇 |
2011年 | 2256篇 |
2010年 | 2054篇 |
2009年 | 2572篇 |
2008年 | 2251篇 |
2007年 | 2313篇 |
2006年 | 2169篇 |
2005年 | 1443篇 |
2004年 | 1444篇 |
2003年 | 1331篇 |
2002年 | 1299篇 |
2001年 | 1147篇 |
2000年 | 1122篇 |
1999年 | 942篇 |
1998年 | 964篇 |
1997年 | 909篇 |
1996年 | 742篇 |
1995年 | 704篇 |
1994年 | 737篇 |
1993年 | 572篇 |
1992年 | 577篇 |
1991年 | 507篇 |
1990年 | 601篇 |
1989年 | 468篇 |
1988年 | 466篇 |
1987年 | 527篇 |
1986年 | 425篇 |
1985年 | 589篇 |
1984年 | 593篇 |
1983年 | 560篇 |
1982年 | 566篇 |
1981年 | 440篇 |
1980年 | 491篇 |
1979年 | 396篇 |
1978年 | 409篇 |
1977年 | 370篇 |
1976年 | 335篇 |
1975年 | 353篇 |
1974年 | 336篇 |
1973年 | 325篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
901.
Turbulent fluctuations have been investigated in the internal boundary layer (IBL) which forms after a dry-to-wet surface transition. The IBL is defined as that part of the atmospheric surface layer where the influence of the downstream surface is noticeable. The results of the application of three different quadrant analysis techniques are presented. The three techniques, in increasing order of the amount of information supplied, provide:
- the diurnal variation of quadrant contribution (C i), number fraction (T i) and conditional average (% MathType!MTEF!2!1!+-% feaafeart1ev1aaatCvAUfeBSjuyZL2yd9gzLbvyNv2CaerbuLwBLn% hiov2DGi1BTfMBaeXafv3ySLgzGmvETj2BSbqefm0B1jxALjhiov2D% aebbfv3ySLgzGueE0jxyaibaiiYdd9qrFfea0dXdf9vqai-hEir8Ve% ea0de9qq-hbrpepeea0db9q8as0-LqLs-Jirpepeea0-as0Fb9pgea% 0lrP0xe9Fve9Fve9qapdbaqaaeGacaGaaiaabeqaamaabaabcaGcba% GaeyykJeUabm4DayaafaGabm4CayaafaGaeyOkJe-aaSbaaSqaaiaa% dMgaaeqaaaaa!4215!\[\langle w's'\rangle _i \], with s = T or q) of vertical sensible and latent heat fluxes,
- the quadrant contribution and number of samples of different sizes depending on the relative magnitude of each sample, and
- the distribution of the nondimensional probability density function.
902.
A numerical model based upon fundamental principles, a standard (k, ) turbulence closure and a finite element integration technique, is applied to separated flows over hills. Predictions are compared to experimental data from a wind tunnel. Although few-equation turbulence closures have been shown to have obvious deficiencies with respect to comparable flows, the model predicts remarkably accurately, without coefficient adjustments of any kind. Even the turbulent intensity is predicted quite realistically. 相似文献
903.
N. Ya. Sotnikova 《Astrophysics》1996,39(2):141-152
Computer programs developed to study large-scale, transient gas structures in galaxies are described and test results are given. Gas-dynamic quantities are determined on the basis of a three-dimensional algorithm using so-called "smoothed particle hydrodynamics" (SPH). Preliminary calculations were made to simulate the formation of a gas ring around a spheroidal galaxy when it absorbs a low-mass, gas-rich companion, as well due to gas accretion during the flyby of a spiral galaxy of comparable mass. The evolution of tidal gas tails of disk galaxies is investigated.Translated from Astrofizika, Vol. 39, No. 2, pp. 265–284, April–June, 1996. 相似文献
904.
An H filament eruption on November 5, 1992 was fully observed in H with the Hida Flare Monitoring Telescope, while Yohkoh's Soft X-ray Telescope observed the pre- and post-eruption evolution of the coronal magnetic fields. From the H data, including the red and blue wings, we have reconstructed the rise of the filament, including trajectory, velocity, and acceleration. In combination with the Yohkoh data this reconstruction suggests that the filament had several interactions with other coronal magnetic fields during the eruption. The Yohkoh data also shows pre-eruption changes in the coronal fields and several post-eruption bright coronal structures. The pre-eruption changes are interpreted as a partial opening of the corona, indicating that it is not necessary to have a complete opening of the corona in order for a filament to erupt and we discuss the several possible contributions from emerging flux. The post-event bright coronal structures are compared with theory and with a cleaner filament eruption event on July 31, 1992. These comparisons suggest that, although there are many similarities, it is hard to completely reconcile the observations with the existing theory. 相似文献
905.
We have obtained images in solar coronal emission lines under high sky-background conditions by making precise differential measurements between the coronal emission line and the near-by continuum, which is primarily due to scattered light from the solar disk. Chopping between the two wavelengths was performed at 100 kHz to avoid artifacts from fast-flying dust particles and other aerosols, and also from seeing effects. The differential signal was detected with a novel CCD camera that demodulates signals up to 100 kHz. These preliminary observations show coronal emission at the 0.2% level of the scattered-light background and pave the way to efficient and precise imaging of coronal emission features under less than ideal coronal-sky conditions.Operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. 相似文献
906.
Summary Direct measurements of the thermal plasma parameters in the topside ionosphere reveal variations of the plasmasphere boundary in the dusk sector. The ACTIVE satellite's near-polar orbits at altitudes of 500 – 1800 km around winter solstice 1989 were used to study the bulge region of the plasmasphere during intervals with different levels of geomagnetic agitation. The narrow, sharply defined trough in electron concentration corresponding to the plasmapause under quiet conditions situated at L = 6 – 7 moved to lower L-values with increasing geomagnetic activity. This narrow trough can be found in all main ion constituents. During periods of moderate geomagnetic activity, following the onset of a weak magnetic storm, a portion of the plasmaspheric bulge region was separated from the main plasmaspheric body. This can be seen in the outer ionosphere as an inner narrow trough at lower L-value. Troughs in light ions need no longer coincide with this in electron concentration. He+ is the most sensitive constituent reflecting the dusk sector plasmaspheric situation at this altitude.Dedicated to the Memory of Professor Karel P 相似文献
907.
M. N. Taran K. Langer A. N. Platonov V. Indutny 《Physics and Chemistry of Minerals》1994,21(6):360-372
The effect of raising temperature on spin-allowed dd-transitions of octahedral Cr3+ was studied for various point symmetries of the Cr3+-bearing structural sites, i.e. 3 m and 3 with inversion center in spinel and garnets, respectively, or 32, 3, 2 and 1, lacking the inversion centre, in beryl, corundum, diopside and topaz, respectively. For this purpose, crystals of Cr3+-bearing spinel, pyrope, andradite, grossular, uvarovite, emerald, ruby, diopside and topaz were analyzed by microprobe, oriented, and measured in polarized radiation (except for the cubic minerals) in the spectral range 30 000 to 11 100 cm-1 and at temperatures between 77 and 797 K. The evaluation of the intensities, half widths, and energy positions of bands due to Cr3+-transitions derived from
4
A
2g
4
T
2g
(F) and
4
T
1g
(F) as well as of Dq- and B-values derived, had the following results:In all cases, red shift of the above bands and, hence, independent on the site symmetry of Cr3+, decreases in the Dq-values were obtained. The dependcies of Dq on T are nearly linear above room temperature and amount between -1.6% in topaz and -5.1% in pyrope in the temperature range studied. From this, values for the local thermal expansion of the Cr3+-centered octahedra,
loc, were derived on the basis of the R
M-0
-5
-proportionality of 10Dq. Such values are consistently higher than those obtained from X-ray refinements, a method averaging rm-o for all the respective octahedral positions. 相似文献
908.
The present paper deals with the specification of bed erosion flux that accounts for the effects of sediment-induced stratification in the water column. Owing to difficulties in measuring the bed shear stress b and the erosive shear strength s, we suggest a series of methods that combine laboratory and numerical experiments. A simplified turbulent transport model that includes these effects helps to quantify b and s. Focusing on soft stratified beds, the present study considers erosion rate formulas of the form =f exp {[Tb-Ts]} where is a model constant (=1 for Gularte's (1978) formula and =1/2 for Parchure's (1984) formula). First, the bed erosive strength profile s(Z) is adjusted by forcing the turbulent transport model with measured erosion rates. Second, three procedures are suggested to determine the erosion rate formula coefficients f and : a global procedure and two different layer-by-layer procedures. Each procedure is applied to an erosion experiment conducted in a rotating annular flume by Villaret and Paulic (1986). The use of the layer-by-layer procedure based on a least squares fitting technique provides a closer fit than the global procedure. The present study points out the complementarity of experimental and numerical approaches and also suggests possible improvements in laboratory test procedures. 相似文献
909.
Correlation by Rb-Sr geochronology of garnet growth histories from different structural levels within the Tauern Window,Eastern Alps 总被引:1,自引:1,他引:0
John N. Christensen Jane Selverstone John L. Rosenfeld Donald J. DePaolo 《Contributions to Mineralogy and Petrology》1994,118(1):1-12
In order to evaluate rates of tectonometamorphic processes, growth rates of garnets from metamorphic rocks of the Tauern Window, Eastern Alps were measured using Rb-Sr isotopes. The garnet growth rates were determined from Rb-Sr isotopic zonation of single garnet crystals and the Rb-Sr isotopic compositions of their associated rock matrices. Garnets were analyzed from the Upper Schieferhülle (USH) and Lower Schieferhülle, (LSH) within the Tauern Window. Two garnets from the USH grew at rates of 0.67
–0.13
+0.19
mm/million years and 0.88
–0.19
+0.34
mm/million years, respectively, indicating an average growth duration of 5.4±1.7 million years. The duration of growth coupled with the amount of rotation recorded by inclusion trails in the USH garnets yields an average shear-strain rate during garnet growth of 2.7
–0.7
+1.2
×10-14 s-1. Garnet growth in the sample from the USH occurred between 35.4±0.6 and 30±0.8 Ma. The garnet from the LSH grew at a rate of 0.23±0.015 mm/million years between 62±1.5 Ma and 30.2±1.5 Ma. Contemporaneous cessation of garnet growth in both units at 30 Ma is in accord with previous dating of the thermal peak of metamorphism in the Tauern Window. Correlation with previously published pressure-temperature paths for garnets from the USH and LSH yields approximate rates of burial, exhumation and heating during garnet growth. Assuming that theseP — T paths are applicable to the garnets in this study, the contemporaneous exhumation rates recorded by garnet in the USH and LSH were approximately 4
–2
+3
mm/year and 2±1 mm/year, respectively. 相似文献
910.
M. T. McCulloch T. K. Kyser J. D. Woodhead L. Kinsley 《Contributions to Mineralogy and Petrology》1994,115(3):303-312
A comprehensive Sr–Nd–Pb–O isotopic study is reported for rhyolites from the Maroa Volcanic Centre in the Taupo Volcanic Zone (TVZ) of New Zealand. The Sr–Nd isotopic compositions of the rhyolites (87Sr/86Sr=0.705236 to 0.705660 and Nd = 2.0 to 0.2) are intermediate between those of primitive basalts (87Sr/86Sr=0.70387 and Nd = 5.3) and the Torlesse basement (87Sr/86Sr=0.709 and Nd = -4.5). The relatively low mantle-like oxygen isotopic compositions of 18 O = 7 ± 0.5 are consistent with the Nd-Sr isotopic constraints in that they can be accounted for by 15% to 25% crustal contamination of a basaltic parent by relatively 18 O-rich Torlesse metasediment. High precision Pb isotopic analyses of plagioclase separates from the Maroa rhyolites show that they have essentially the same compositions as the Torlesse metasedimentary terrane which is itself distinctive from the Western or Waipapa metasediments. Due to the high concentration of Pb in the Torlesse metasediments (>20 ppm) compared to the basalts (<2 ppm), the Pb isotopic composition of the volcanics may be controlled by relatively small amounts (>10%) of crustal contamination. All these results are shown to be consistent with derivation of the rhyolites by 15% to 25% contamination of relatively primitive basaltic magmas with Torlesse metasedimentary crust, followed by extensive, essentially closed system fractionation of the basalt to a magma of rhyolite composition. It is argued that the processes of assimilation and fractionation are separated in both space and time. The voluminous high silica rhyolites, which make up >97% of the exposed volcanism in the continental margin back-are basin environment of the TVZ, therefore appear to be a product of predominantly new additions to the crust with assimilation-recycling of pre-existing crust being of secondary importance. 相似文献