全文获取类型
收费全文 | 1697篇 |
免费 | 84篇 |
国内免费 | 48篇 |
专业分类
测绘学 | 59篇 |
大气科学 | 131篇 |
地球物理 | 360篇 |
地质学 | 601篇 |
海洋学 | 117篇 |
天文学 | 411篇 |
综合类 | 10篇 |
自然地理 | 140篇 |
出版年
2024年 | 5篇 |
2023年 | 13篇 |
2022年 | 10篇 |
2021年 | 36篇 |
2020年 | 42篇 |
2019年 | 39篇 |
2018年 | 81篇 |
2017年 | 68篇 |
2016年 | 78篇 |
2015年 | 74篇 |
2014年 | 79篇 |
2013年 | 113篇 |
2012年 | 67篇 |
2011年 | 98篇 |
2010年 | 81篇 |
2009年 | 90篇 |
2008年 | 88篇 |
2007年 | 91篇 |
2006年 | 76篇 |
2005年 | 59篇 |
2004年 | 64篇 |
2003年 | 49篇 |
2002年 | 48篇 |
2001年 | 36篇 |
2000年 | 44篇 |
1999年 | 34篇 |
1998年 | 21篇 |
1997年 | 23篇 |
1996年 | 19篇 |
1995年 | 11篇 |
1994年 | 13篇 |
1993年 | 11篇 |
1992年 | 15篇 |
1991年 | 8篇 |
1990年 | 9篇 |
1989年 | 6篇 |
1988年 | 6篇 |
1987年 | 7篇 |
1986年 | 10篇 |
1985年 | 12篇 |
1984年 | 7篇 |
1983年 | 9篇 |
1982年 | 5篇 |
1981年 | 7篇 |
1980年 | 7篇 |
1979年 | 8篇 |
1978年 | 7篇 |
1977年 | 5篇 |
1976年 | 5篇 |
1973年 | 4篇 |
排序方式: 共有1829条查询结果,搜索用时 156 毫秒
981.
NMR studies of chemical structural variation of insoluble organic matter from different carbonaceous chondrite groups 总被引:1,自引:0,他引:1
Solid-state 1H and 13C Nuclear Magnetic Resonance (NMR) spectroscopic experiments have been performed on isolated meteoritic Insoluble Organic Matter (IOM) spanning four different carbonaceous chondrite meteorite groups; a CR2 (EET92042), a CI1 (Orgueil), a CM2 (Murchison), and the unique C2 meteorite, Tagish Lake. These solid state NMR experiments reveal considerable variation in bulk organic composition across the different meteorite group’s IOM. The fraction of aromatic carbon increases as CR2 < CI1 < CM2 < Tagish Lake. The increases in aromatic carbon are offset by reductions in aliphatic (sp3) carbon moieties, e.g., “CHx,” and “CHx(O,N).” Oxidized sp2 bonded carbon, e.g., carboxyls and ketones grouped as “CO,” are largely conservative across these meteorite groups. Single pulse (SP) 13C magic angle spinning (MAS) NMR experiments reveal the presence of nanodiamonds with an apparent concentration ranking in the IOM of CR2 < CI1 < CM2 < Tagish Lake. A pair of independent NMR experiments reveals that, on average, the aromatic moieties in the IOM of all four meteoritic IOM fractions are highly substituted. Fast spinning SP 1H MAS NMR spectral data combined with other NMR experimental data reveal that the average hydrogen content of sp3 bonded carbon functional groups is low, requiring a high degree of aliphatic chain branching in each IOM fraction. The variation in chemistry across the meteorite groups is consistent with alteration by low temperature chemical oxidation. It is concluded that such chemistry principally affected the aliphatic moieties whereas the aromatic moieties and nanodiamonds may have been largely unaffected. 相似文献
982.
S. Taylor C.M.O.’D. Alexander P. Ma G.F. Herzog 《Geochimica et cosmochimica acta》2005,69(10):2647-2662
Atmospheric heating alters the compositions and textures of micrometeorites. To understand the changes and to test a proposed relationship between a micrometeorite’s petrographic texture and its degree of heating, we made elemental and multiple isotope analyses of stony cosmic spherules (sCS) collected from the South Pole Water well. Specifically, we analyzed the elemental compositions of 94 sCS and the isotopic ratios of Fe, K and O, on 43, 12 and 8 of these sCS, respectively.Our results show that sCS classified as strongly heated generally have lower concentrations of volatile and moderately volatile elements. Of the 43 spherules analyzed for Fe isotopes, only 5 have δ57Fe >5‰. In contrast, enrichment of 41K is pervasive (δ41K >0 in all 12 spherules analyzed) and large (up to 183‰). The determination of K isotope abundances in sCS may therefore be useful in deciphering thermal histories. Three of the eight sCS analyzed for O isotopes are mass fractionated with δ18O >30‰. We attribute two of these three δ18O enrichments to evaporative losses of oxygen in the atmosphere and the third to the presence in the parent material of an exotic phase, perhaps a sulfate or a carbonate. The K isotope and O isotope data are broadly consistent with the proposed textural classification.Because most spherules were not heated enough to fractionate Al, Mg, or Si, we compared the measured Mg/Al and Si/Al ratios directly to those of conventional meteorites and their matrices. ∼30% of the sCS have compositions outside the range defined by the bulk and the matrix compositions of known meteorite groups but consistent with those of pyroxene- and olivine-rich materials and may be samples of chondrules. The other 70% have Mg/Al and Si/Al ratios similar to those of CI, CM, CO, and CV chondrites. Natural variability of the Mg/Al and Si/Al ratios precludes the assignment of an individual sCS to a particular class of C-chondrite. 相似文献
983.
Longwave Measurements for the Coast of British Columbia and Improvements to the Tsunami Warning Capability 总被引:1,自引:0,他引:1
A few years ago the Canadian Hydrographic Service initiated a major upgrade toall tide gauges and tsunami stations on the coast of British Columbia (B.C.). Thisprogram was undertaken to address shortcomings of the earlier digital systems andwas driven by concerns about emergency response continuity in the year 2000. By1999, thirteen tide gauge stations had been installed and were operational. Three ofthese stations (Tofino, Winter Harbour, and Langara) were selected for use as tsunamiwarning stations. Several years of continuous, high quality data have now been collectedat these stations and used for analysis of long waves in the tsunami frequency band.Careful examination of these data revealed two weak tsunamis recorded by severalB.C. stations: a distant tsunami of June 23, 2001 generated by the Peru Earthquake(Mw = 8.4), and a local tsunami of October 12, 2001 induced by the Queen Charlotte Earthquake (Mw = 6.3$). Spectral characteristics of these two tsunamis are compared with the spectral characteristics of long waves generated by a strong storm (October, 2000) and of ordinary background oscillations. The topographic admittance functions (frequency responses) constructed for all stations showed that most of them (in particular, Winter Harbour, Tofino, Bamfield, Port Hardy, and Victoria) have strong resonance at periods from 2.5 to 20 min, indicating that these locations are vulnerable to relatively high-frequency tsunamis. The Winter Harbour station also has two strong resonant peaks with periods of 30 and 47 min and with amplification factors of about 7. The estimated source functions show very clear differences between long waves associated with the seismic source (typical periods 10–30 min) and those generated by a storm, which typically have shorter periods and strong energy pumping from high-frequencies due to non-linear interaction of wind waves. 相似文献
984.
A two-dimensional fuzzy random model of soil pore structure 总被引:1,自引:0,他引:1
A new conceptual model for soil poresolid structure is formalized. Soil poresolid structure is proposed to comprise spatially
abutting elements each with a value which is its membership to the fuzzy set “pore, ” termed porosity. These values have a
range between zero (all solid) and unity (all pore). Images are used to represent structures in which the elements are pixels
and the value of each is a porosity. Twodimensional random fields are generated by allocating each pixel a porosity by independently
sampling a statistical distribution. These random fields are reorganized into other poresolid structural types by selecting
parent points which have a specified local region of influence. Pixels of larger or smaller porosity are aggregated about
the parent points and within the region of interest by controlled swapping of pixels in the image. This creates local regions
of homogeneity within the random field. This is similar to the process known as simulated annealing. The resulting structures
are characterized using one and twodimensional variograms and functions describing their connectivity. A variety of examples
of structures created by the model is presented and compared. Extension to three dimensions presents no theoretical difficulties
and is currently under development. 相似文献
985.
986.
987.
The rate constants of the cyclisation reaction of 2,3-dimethylbiphenyl to give 1-methylfluorene under non-catalytic conditions have been obtained at five temperatures between 460 and 500°C. The activation energy, activation entropy and frequency factor for the reaction have been determined from the experimental data. These kinetic parameters were then used to calculate the temperature ranges where the reaction occurs under natural conditions in sedimentary sequences which have experienced different heating rates. The coincidence of the measured and calculated values has been taken as evidence that the cyclisation reaction occurs in sediments under non-catalytic conditions. 相似文献
988.
Alexander Zaporozec Arvi Valmari Hans-Georg Bohle G. Kunkel Rolf H. K?se et al. 《GeoJournal》1989,18(4):456-458
The International Geographical Union Reports
IGU Commission on Geographic Information SystemsGoegraphic information systems in higher education — A workshop — Columbus, Ohio, June 10–11, 1989 相似文献989.
990.
Alexander P. Anufriev Ivan Cupal Reviewer G. Siráň 《Studia Geophysica et Geodaetica》1987,31(1):37-42
Summary The behaviour of the poloidal and toroidal magnetic field at the core-mantle boundary is analysed in more detail, assuming that the conductive layer in the lowest mantle is thin. We can conclude that, in the case of the Z-model of the nearly symmetric hydromagnetic dynamo, the poloidal field may be considered potential everywhere in the mantle and that the azimuthal field depends on the geostrophic azimuthal velocity in the same manner as derived in[1] and[3].
aau ¶rt;-amu n¶rt;nuu m n¶rt; amuu aauum n¶rt;u nu¶rt;a u mu¶rt;a n. am ¶rt;, m Z-¶rt;u nmu umuu¶rt;aum ¶rt;ua aum nu¶rt;a n umam nmua ¶rt; amuu a n¶rt;u . ¶rt;m¶rt;am na [1] u [3] auum auma aum n m auma mu.相似文献