全文获取类型
收费全文 | 2146篇 |
免费 | 68篇 |
国内免费 | 12篇 |
专业分类
测绘学 | 50篇 |
大气科学 | 216篇 |
地球物理 | 418篇 |
地质学 | 665篇 |
海洋学 | 170篇 |
天文学 | 494篇 |
综合类 | 1篇 |
自然地理 | 212篇 |
出版年
2020年 | 16篇 |
2019年 | 29篇 |
2018年 | 44篇 |
2017年 | 27篇 |
2016年 | 40篇 |
2015年 | 36篇 |
2014年 | 55篇 |
2013年 | 127篇 |
2012年 | 70篇 |
2011年 | 91篇 |
2010年 | 63篇 |
2009年 | 85篇 |
2008年 | 79篇 |
2007年 | 73篇 |
2006年 | 69篇 |
2005年 | 73篇 |
2004年 | 88篇 |
2003年 | 72篇 |
2002年 | 60篇 |
2001年 | 49篇 |
2000年 | 49篇 |
1999年 | 40篇 |
1998年 | 36篇 |
1997年 | 36篇 |
1996年 | 29篇 |
1995年 | 31篇 |
1994年 | 38篇 |
1993年 | 30篇 |
1992年 | 30篇 |
1991年 | 20篇 |
1990年 | 22篇 |
1989年 | 23篇 |
1988年 | 26篇 |
1987年 | 27篇 |
1986年 | 29篇 |
1985年 | 45篇 |
1984年 | 38篇 |
1983年 | 37篇 |
1982年 | 32篇 |
1981年 | 32篇 |
1980年 | 32篇 |
1979年 | 38篇 |
1978年 | 36篇 |
1977年 | 36篇 |
1976年 | 21篇 |
1975年 | 18篇 |
1974年 | 17篇 |
1973年 | 22篇 |
1972年 | 26篇 |
1971年 | 19篇 |
排序方式: 共有2226条查询结果,搜索用时 15 毫秒
171.
I. Choe R. Ingalls J. M. Brown Y. Sato-Sorensen 《Physics and Chemistry of Minerals》1992,19(4):236-239
We have measured in situ Mössbauer transmission spectra of iron silicate spinel (γ-Fe2SiO4) in a diamond anvil cell at room temperature and pressures up to 16 GPa. The observed spectra show a doublet characteristic of the paramagnetic state. The isomer shift and quadrupole splitting at atmospheric pressure are 1.10 and 2.63 mm/s, respectively, which are smaller than those of fayalite (α-Fe2SiO4). Both the isomer shift and quadrupole splitting decrease linearly with pressure with slope of ?0.003(1) and ?0.020(1) mm/sec · GPa, respectively. This simple linear trend suggests that no electronic or polymorphic transitions occur under 16 GPa except for those due to the small and continuous changes of volume and local symmetry under pressure. On the basis of a crystalline field calculation, the negative pressure derivative of the quadrupole splitting is associated with a trend towards an ideal cubic symmetry of the oxygen sublattice. 相似文献
172.
Neal D. Durant Vernon B. Myers Lawrence A. Eccles 《Ground Water Monitoring & Remediation》1993,13(1):151-158
The U.S. Environmental Protection Agency (EPA) recently proposed to amend federal regulations to require vadose zone monitoring at certain hazardous waste facilities. To support this proposal, EPA evaluated previous policy on vadose zone monitoring and examined advances in vadose zone monitoring technology. Changes in EPA vadose zone monitoring policy were driven by demonstrated advances in the available monitoring technology and improvements in understanding of vadose zone processes/When used under the appropriate conditions, currently available direct and indirect monitoring methods can effectively detect contamination that may leak from hazardous waste facilities into the vadose zone. Direct techniques examined include soil-core monitoring and soil-pore liquid monitoring. Indirect techniques examined include soil-gas monitoring, neutron moderation, complex resistivity, ground-penetrating radar, and electrical resistivity. Properly designed vadose zone monitoring networks can act as a complement to saturated zone monitoring networks at numerous hazardous waste facilities. At certain facilities, particularly those in arid climates where the saturated zone is relatively deep, effective vadose zone monitoring may allow a reduction in the scope of saturated zone monitoring programs. 相似文献
173.
J. Rosenbloom P. Mock P. Lawson J. Brown H.J. Turin 《Ground Water Monitoring & Remediation》1993,13(3):159-169
Cleanup standards for volatile organic compounds in thick vadose zones can be based on indirect risk (transport to ground water) when contamination is below depths of significant direct risk. At one Arizona Superfund site, a one-dimensional vadose zone transport model (VLE-ACH) was used to estimate the continued transport of VOCs from the vadose zone to ground water. VLEACH is a relatively simple and readily available model that proved useful for estimating indirect risk from VOCs in the vadose zone at this site. The estimates of total soil concentrations used as initial conditions for VLF.ACH incorporated a variety of data from the site. Soil gas concentrations were found to be more useful than soil matrix data for estimating total soil concentrations at this arid-zone site. A simple mixing cell model was used with the VLEACH-derived mass loading estimates from the vadose zone over time to estimate the resulting changes in ground water concentrations. For this site, the results of the linked VLEACH/mixing cell simulations indicate it is likely that the federal MCI. for TCE will be exceeded in underlying ground water if remedial action on I he vadose zone is not pursued. 相似文献
174.
175.
The production of granitic melts during ultrametamorphism 总被引:8,自引:0,他引:8
Data concerning the occurrence and mineralogy of granitic rocks and migmatites indicate that granitic liquids are not generally water saturated and do not form on minimum melting curves. Experiments on the partial fusion of crustal materials containing water only in hydrated phases demonstrate that water undersaturated melts with compositional affinities to the granite family are produced at temperatures and pressures in accord with observations on the metamorphio state of migmatite terrains. 相似文献
176.
Summary The crystal structure of Ca5(PO4)2SiO4 (silico-carnotite) has been determined from 3358 x-ray diffraction data collected by a counter method and has been refined toR
w
=0.038,R=0.045, in space group Pnma. The unit cell parameters area=6.737 (1) Å,b=15.508 (2) Å andc=10.132 (1) Å at 24°C;Z=4. The observed density is 3.06 and the calculated density is 3.03 g · cm–3. The crystal contains about 2.5% V2O5 as an impurity. The bond lengths within the tetrahedral anions suggest that substitution or disorder of PO4
3–, SiO4
4– and possibly VO4
3– occurs among the anion sites. The structure has some relationship to that of Ca5(PO4)3OH, the predominant inorganic phase in the human body, but suggests that the Ca5(PO4)3OH type structure may not be stable without some of the OH positions being filled. Ca5(PO4)2SiO4 is more closely related to K3Na(SO4)2 (glaserite) if it is considered that there are systematic cation vacancies in Ca5(PO4)2SiO4.This type of structure is consistent with the view that cation vacancies in the glaserite-type structure account for solid solutions between Ca2SiO4 and Ca3(PO4)2 and between Ca3(PO4)2 and CaNaPO4.
With 9 Figures 相似文献
Die Kristallstruktur vonCa 5(PO 4)2 SiO 4 (Silicocarnotit)
Zusammenfassung Die Kristallstruktur von Ca5(PO4)2SiO4 (Silicocarnotit) wurde aus 3358 Röntgendiffraktometer-Daten bestimmt und in Raumgruppe Pnma aufR w =0,038,R=0,045 verfeinert. Die Gitterkonstanten (bei 24° C) sind:a=6,737 (1) Å,b=15,508 (2) Å undc=10,132 (1) Å,Z=4; Dobs.=3,06 g · cm–3, Dexp.=3,03 g · cm–3. Der Kristall enthält etwa 2,5% V2O5 als Verunreinigung. Die Bindungslängen in den tetraedrischen Anionen legen nahe, daß unter den Anionenplätzen gegenseitige Vertretung oder Unordnung von PO4 3–, SiO4 4– und möglicherweise VO4 3– auftritt. Die Struktur zeigt einige Verwandtschaft zu der von Ca5(PO4)3OH, der wichtigsten anorganischen Substanz im menschlichen Körper, weist aber darauf hin, daß eine Struktur vom Ca5(PO4)3OH-Typ ohne Besetzung eines Teiles der OH-Position nicht stabil ist. Ca5(PO4)2SiO4 zeigt engere Beziehungen zu K3Na(SO4)2 (Glaserit), wenn man berücksichtigt, daß in Ca5(PO4)3SiO4 systematische Kationen-Leerstellen sind. Dieser Strukturtyp ist mit derAuffassung in Übereinstimmung, daß Kationenleerstellen für die festen Lösungen zwischen Ca2SiO4 und Ca3(PO4)2 und zwischen Ca3(PO4)2 und CaNaPO4 verantwortlich sind.
With 9 Figures 相似文献
177.
Summary If for a point on the earth's surface, the apparent (moveout) seismic velocities have been determined for a number of profiles in different directions, a possibility is offered to calculate true strike and dip of crustal boundaries. In the present paper the theory is developed, partly for a sloping Mohorovii discontinuity only, partly for sloping Conrad and Mohorovii discontinuities with parallel strikes. The theory for the sloping Mohorovii discontinuity is applied to field measurements at a place about 30 km west of Uppsala performed in June 1969. With a crustalP-wave velocity of 6.3 km/sec, a good fit to the observations is obtained for aPn-velocity of 8.55 km/sec and a downdip of Moho of 7.2° in the direction S 3.8° E. These numerical values are not the most important output of this paper. On the other hand, the testing of the method appears far more important, and also that improvements can be suggested in the experimental procedure in future applications of this method. 相似文献
178.
Edwin H. Brown 《Contributions to Mineralogy and Petrology》1971,31(4):275-299
Phase relations of biotite and stilpnomelane and associated silicate minerals have been studied in rocks of the greenschist facies, chiefly from Otago, New Zealand and western Vermont, but also from Scotland, Minnesota-Michigan iron range, and northwest Washington. That stilpnomelane in the greenschicht facies crystallizes initially with nearly all iron in the ferrous state is indicated by chemical analyses, high p-T experiments, and phase relationships. Alteration of stilpnomelane after metamorphism not only oxidizes iron but leaches potassium; corrections for both effects must be made in using analyses of brown stilpnomelane in studies of phase relations. Two discontinuous reactions which produce biotite at the biotite isograd have been identified:
- muscovite+stilpnomelane+actinolite→ biotite+chlorite+epidote
- chlorite+microcline→ biotite+muscovite. Biotite produced by the first of these reactions has a limited range of variation in Fe/Mg. As grade advances within the biotite zone more magnesian and ferruginous biotites become stable in consequence of the two continuous reactions:
- muscovite+actinolite+chlorite→ biotite (Mg-rich)+epidote
- muscovite+stilpnomelane→ biotite (Fe-rich)+chlorite.
179.
The theory of heat and mass transfer in turbulent pipe flow is applied to a semi-infinite moist-walled cylindrical pipe to determine the longitudinal distributions of both temperature and moisture content as functions of external conditions, pipe radius and wall temperature, and flow velocity. Since many cave and mine passages approximate this model, the results are directly applicable to cave microclimate studies and mine ventilation problems. The results are found to agree well with previously published microclimate observations. The theory is also applicable to water flow in free-flow karst aquifers; specifically to the study of temperature variations and of solution kinetics under turbulent flow conditions. 相似文献
180.
The time-dependent failure of rock subjected to repeated loading can be explained in terms of the residual strength locus: catastrophic failure occurs when the accumulation of deformation is such that the residual strength locus is intercepted. In a preliminary experimental study of the fatigue characteristics of rock masses, highly idealized models of jointed rock were subjected to a series of cyclic uniaxial compression tests. The results of these experiments show that intact and failed models of jointed rock are extremely susceptible to cyclic fatigue failure, and that in these cases the residual strength locus interpretation of time-dependent failure is correct. 相似文献