全文获取类型
收费全文 | 1302篇 |
免费 | 79篇 |
国内免费 | 13篇 |
专业分类
测绘学 | 62篇 |
大气科学 | 182篇 |
地球物理 | 293篇 |
地质学 | 524篇 |
海洋学 | 81篇 |
天文学 | 165篇 |
综合类 | 2篇 |
自然地理 | 85篇 |
出版年
2023年 | 6篇 |
2022年 | 9篇 |
2021年 | 24篇 |
2020年 | 30篇 |
2019年 | 28篇 |
2018年 | 58篇 |
2017年 | 39篇 |
2016年 | 63篇 |
2015年 | 42篇 |
2014年 | 54篇 |
2013年 | 78篇 |
2012年 | 65篇 |
2011年 | 76篇 |
2010年 | 66篇 |
2009年 | 94篇 |
2008年 | 85篇 |
2007年 | 58篇 |
2006年 | 61篇 |
2005年 | 50篇 |
2004年 | 48篇 |
2003年 | 46篇 |
2002年 | 21篇 |
2001年 | 25篇 |
2000年 | 26篇 |
1999年 | 19篇 |
1998年 | 21篇 |
1997年 | 15篇 |
1996年 | 20篇 |
1995年 | 14篇 |
1994年 | 13篇 |
1993年 | 12篇 |
1992年 | 9篇 |
1991年 | 9篇 |
1990年 | 6篇 |
1989年 | 4篇 |
1987年 | 8篇 |
1986年 | 8篇 |
1984年 | 4篇 |
1983年 | 6篇 |
1982年 | 3篇 |
1981年 | 7篇 |
1979年 | 5篇 |
1978年 | 3篇 |
1977年 | 8篇 |
1976年 | 4篇 |
1975年 | 4篇 |
1974年 | 4篇 |
1969年 | 6篇 |
1968年 | 5篇 |
1967年 | 4篇 |
排序方式: 共有1394条查询结果,搜索用时 31 毫秒
81.
Emission rates of biogenic volatile organic compounds emitted by the forests were estimated for five geographical regions as well as for all Switzerland. Monoterpene and isoprene emissions rates were calculated for each main tree species separately using the relevant parameters such as temperature, light intensity and leaf biomass density. Biogenic emissions from the forests were found to be about 23% of the total annual VOC emissions (anthropogenic and biogenic) in Switzerland. The highest emissions are in July and lowest in January. Calculations showed that the coniferous trees are the main sources of the biogenic emissions. The major contribution comes from the Norway spruce (picea abies) forests due to their abundance and high leaf biomass density. Although broad-leaved forests cover 27% of all the forests in Switzerland, their contribution to the biogenic emissions is only 3%. Monoterpenes are the main species emitted, whereas only 3% is released as isoprene. The highest emission rates of biogenic VOC are estimated to be in the region of the Alps which has the largest forest coverage in Switzerland and the major part of these forests consists of Norway spruce. The total annual biogenic VOC emission rate of 87 ktonnes y–1 coming from the forests is significantly higher than those from other studies where calculations were carried out by classifying the forests as deciduous and coniferous. The difference is attributed to the high leaf biomass densities of Norway spruce and fir (abies alba) trees which have a strong effect on the results when speciation of trees is taken into account. Besides the annual rate, emission rates were calculated for a specific period during July 4–6, 1991 when a photochemical smog episode was investigated in the Swiss field experiment POLLUMET. Emission rates estimated for that period agree well with those calculated for July using the average temperatures over the last 10 years. 相似文献
82.
Stefan Keller 《Surveys in Geophysics》1995,16(5-6):661-670
Laboratory simulation of powder snow avalanches serves for the validation of numerical models and as a tool in its own right for consulting and research purposes. Two-phase gravity currents are simulated in water with quartz powder or glass spheres as the particle phase. Particle-phase velocity profiles and particle-phase volume fraction profiles are measured with an ultrasonic device. The experimental setup and the current state of the measurement technique are described and preliminary results are presented. 相似文献
83.
The lunar crust at the Apollo 16 landing site contains substantial amounts of a “primitive component” in which the ferromagnesian group of elements is concentrated. The composition of this component can be retrieved via an analysis of mixing relationships displayed by lunar breccias. It is found to be a komatiite which is compositionally similar to terrestrial komatiites both in major and minor elements. The komatiite component of the lunar crust is believed to have formed by extensive degrees of melting of the lunar interior at depths greater than were involved in the formation of the lunar magma ocean which was parental to the crust. After formation of the anorthositic crust, it was invaded by extensive flows and intrusions of komatiite magma from these deeper source regions. The komatiites became intimately mixed with the anorthosite by intensive meteoroid impacts about 4.5 b.y. ago, thereby accounting for the observed mixing relationships displayed by the crust. The compositional similarity between lunar and terrestrial komatiites strongly implies a corresponding similarity between the compositions of their source regions in the lunar interior and the Earth's upper mantle. The composition of the lunar interior can be modelled more specifically by combining the komatiite composition with its liquidus olivine composition (as determined experimentally) in proportions chosen so as to produce a cosmochemically acceptable range of Mg/Si ratios for the bulk Moon. Except for higher FeO and lower Na2O, the range of compositions thereby obtained for the bulk moon is very similar to the composition of the Earth's upper mantle.The effects of meteoritic contamination on the abundances of cobalt and nickel in lunar highland breccias were subtracted on the assumption that the contaminating projectiles were chondritic. The cobalt and nickel residuals thereby obtained were found to correlate strongly with the (Mg + Fe) content of the breccias, demonstrating that the Co and Ni are associated with the ferromagnesian component of the breccias and are genuinely indigenous to the Moon. The lunar highland Co and Ni residuals also display striking Ni/Co versus Ni correlations which follow a similar trend to those displayed by terrestrial basalts, picrites and komatiites. The lunar trends provide further decisive evidence of the indigenous nature of the Co and Ni residuals and suggest the operation of extensive fractionation controlled by olivine-liquid equilibria in producing the primitive component of the lunar breccias. Indigenous nickel abundances at the Apollo 14, 15 and 17 sites are much lower than at the Apollo 16 site, although rocks from all sites follow the same Ni/Co versus Ni trends. It is suggested that the primitive component at the Apollo 14, 15 and 17 sites was generally of basaltic composition, in contrast to the komatiitic nature of the Apollo 16 primitive component. 相似文献
84.
85.
Zusammenfassung Die chemische Analyse des neuen Minerals Johillerit mit der Elektronenmikrosonde ergab: Na2O 5,4, MgO 18,3, ZnO 5,4, CuO 15,8 und As2O5 55,8, Summe 100.7%. Aus diesem Ergebnis wurde die idealisierte Formel Na(Mg, Zn)3 Cu(AsO4)3 abgeleitet. Johillerit ist monoklin mit der RaumgruppeC2/c. Die Gitterkonstanten sind:a=11,870 (3),b=12,755 (3),c=6,770 (2) , =113,42 (2)°,Z=4. Die stärksten Linien des Pulverdiagramms sind: 4,06 (5) (22
), 3,50 (4) (310), 3,25 (8) (11
), 2,75 (10) (330, 240), 2,64 (5) (311, 13
, 40
), 1,952 (4) (13
, 35
), 1,682 (4) (20
, 460), 1,660 (5) (40
, 71
, 550, 64
), 1,522 (4) (442, 153, 13
). Es bestehen enge strukturelle Beziehungen zwischen Johillerit und O'Danielit, Na(Zn, Mg)3H2(AsO4)3, sowie einigen synthetischen. Verbindungen.Johillerit ist violett durchscheinend. Die Spaltbarkeit nach {010} ist ausgezeichnet und nach {100} und {001} gut.H (Mohs)3.D=4,15 undD
X
=4,21 g·cm–3. Das Mineral ist optisch zweiachsig positiv, 2V80 (5)°. Die Werte der Lichtbrechung sindn
=1,715 (4),n
=1,743 (4) undn
=1,783 (4). Die Auslöschung istn
b und auf (010)n
c16°. Johillerit ist stark pleochroitisch mit den AchsenfarbenX=violett-rot,Y = blauviolett undZ = grünblau. Das neue Mineral kommt in radialstrahligen Massen gemeinsam mit kupferhaltigem Adamin und Konichalcit in zersetzem Kupfererz von Tsumeb, Namibia, vor. Die Benennung erfolgte nach Prof. Dr.J.-E. Hiller (1911–1972).
Mit 1 Abbildung 相似文献
Johillerite, Na(Mg, Zn) 3 Cu(AsO 4 ) 3 , a new mineral from Tsumeb, Namibia
Summary Electron microprobe analysis of the new mineral johillerite gave Na2O 5.4, MgO 18.3, ZnO 5.4, CuO 15.8, and As2O5 55.8, total 100.7%. From this result, the ideal formula is given as Na(Mg, Zn)3 Cu(AsO4)3. Johillerite crystallizes monoclinic,C2/c. The unit cell dimensions are:a=11.870(3),b=12.755 (3),c=6.770 (2) , =113.42 (2)°,Z=4. The strongest lines on the X-ray powder diffraction pattern are: 4,06 (5) (22 ), 3,50 (4) (310), 3,25 (8) (11 ), 2,75 (10) (330, 240), 2,64 (5) (311, 13 , 40 ), 1,952 (4) (13 , 35 ), 1,682 (4) (20 , 460), 1,660 (5) (40 , 71 , 550, 64 ), 1,522 (4) (442, 153, 13 ). There is a close relationship between johillerite, o'danielite, Na(Zn, Mg)3H2(AsO4)3, and some synthetic compounds. Johillerite is violet in colour, transparent. Cleavage is {010} perfect, {100} and {001} good.H (Mohs)3.D=4.15 andD X =4.21 g·cm–3. The mineral is optically biaxial positive, 2V80 (5)°. The refractive indices are:n =1.715 (4),n =1.743 (4),n =1.783 (4). The extinction isn b and on (010)n c16°. Strongly pleochroic with axial coloursX=violet-red,Y=bluish violet andZ=greenish blue. The new mineral was found in radiated masses together with cuprian adamite and conichalcite in an oxidized copper ore from Tsumeb, Namibia. It is named in honour of Prof. Dr.J.-E. Hiller (1911–1972).
Mit 1 Abbildung 相似文献
86.
The effects of grain size on the analysis of fabrics by X-ray diffraction using both the fabric camera and the texture goniometer are discussed. Simple calculations show the interdependance between the mean grain diameter and the number of crystals which are measured by each instrument. The effects of both grain size and modal composition of the specimen are illustrated by comparative analyses. Both the fabric camera and texture goniometer are well suited to the analysis of specimens in which the mean grain diameter is between approximately 10 and 100 microns. The texture goniometer is more suitable when the grain size is less than this, while the fabric camera is better suited when the grain size is coarser, especially when the specimen is not monomineralic. 相似文献
87.
Summary The spatial and temporal variability of winter precipitation and its links to the large-scale atmospheric circulation patterns in Romania are examined. The data set is composed of observed rainfall at 30 meteorological stations during the 1961–1996 period. The large-scale field is represented by the observed geopotential height at 500 hPa (Z500) over the same period, covering the latitudinal belt between 20° N–90° N (resolution 2.5°×2.5°).The Standard Normal Homogeneity Test (SNHT) is applied to detect inhomogeneities in the data, and the Mann-Kendall and Pettitt non-parametric tests are used in order to identify trends and change points in the winter precipitation time series. The empirical orthogonal functions (EOF) technique is used for data reduction in order to highlight the basic patterns of rainfall variability in Romania. The covariance map between precipitation EOF time series (PCs) and the Z500 field, as well as the correlation coefficients between the PCs and circulation indices are calculated in order to identify the influence of large-scale circulation patterns on winter precipitation in Romania.A significant decreasing trend is identified in winter precipitation with a downward shift in winter 1969/1970, most significant from a statistical point of view in the extra-Carpathian region. This change seems to be real since the SNHT test does not reveal any inhomogeneity during the period tested. Significant relationships are found between winter precipitation variability in Romania and the large-scale circulation pattern, such as the North Atlantic Oscillation and the blocking phenomenon in the Atlantic-European sector. The positive phase of the NAO and the reduction in blocking activity could be one of the causes of the decrease in winter precipitation in Romania. 相似文献
88.
The mechanisms which control the formation and maintenance of pool–riffles are fundamental aspects of channel form and process. Most of the previous investigations on pool–riffle sequences have focused on alluvial rivers, and relatively few exist on the maintenance of these bedforms in boulder-bed channels. Here, we use a high-resolution two-dimensional flow model to investigate the interactions among large roughness elements, channel hydraulics, and the maintenance of a forced pool–riffle sequence in a boulder-bed stream. Model output indicates that at low discharge, a peak zone of shear stress and velocity occurs over the riffle. At or near bankfull discharge, the peak in velocity and shear stress is found at the pool head because of strong flow convergence created by large roughness elements. The strength of flow convergence is enhanced during model simulations of bankfull flow, resulting in a narrow, high velocity core that is translated through the pool head and pool center. The jet is strengthened by a backwater effect upstream of the constriction and the development of an eddy zone on the lee side of the boulder. The extent of flow convergence and divergence is quantified by identifying the effective width, defined here as the width which conveys 90% of the highest modeled velocities. At low flow, the ratio of effective width between the pool and riffle is roughly 1:1, indicating little flow convergence or divergence. At bankfull discharge, the ratio of effective width is approximately 1:3 between the pool and downstream riffle, illustrating the strong flow convergence at the pool head. The effective width tends to equalize again with a ratio of 1:1 between the pool and riffle during a modeled discharge of a five-year flood, as the large roughness elements above the pool become drowned out. Results suggest that forced pool–riffle sequences in boulder-bed streams are maintained by flows at or near bankfull discharge because of stage-dependent variability in depth-averaged velocity and tractive force. 相似文献
89.
Very soft organic harbour mud is increasingly used as a filling and construction material in harbour construction and reorganization.
The undrained shear strength of such soft sediments is the critical geotechnical soil parameter with regard to any specific
construction design. Field and laboratory vane shear testing is a standard method to quickly determine this important parameter.
So far, the effect of rod friction on vane shear tests in very soft organic soils is unclear. In this study we present results
from laboratory experiments on harbour mud from a construction site in northern Germany. Relations among vane and rod geometry,
penetration depth, water content, rod friction and undrained shear strength are derived. Based on these relations the influence
of rod friction on vane shear test results is investigated. The results indicate that field and laboratory vane shear test
measurements may be significantly influenced by rod friction. Methods are proposed to correct for the rod influence, which
is shown to increase with rising water contents. 相似文献
90.