全文获取类型
收费全文 | 971篇 |
免费 | 48篇 |
国内免费 | 14篇 |
专业分类
测绘学 | 33篇 |
大气科学 | 65篇 |
地球物理 | 311篇 |
地质学 | 339篇 |
海洋学 | 63篇 |
天文学 | 155篇 |
综合类 | 5篇 |
自然地理 | 62篇 |
出版年
2023年 | 5篇 |
2022年 | 23篇 |
2021年 | 14篇 |
2020年 | 27篇 |
2019年 | 22篇 |
2018年 | 47篇 |
2017年 | 48篇 |
2016年 | 51篇 |
2015年 | 52篇 |
2014年 | 65篇 |
2013年 | 64篇 |
2012年 | 63篇 |
2011年 | 62篇 |
2010年 | 61篇 |
2009年 | 81篇 |
2008年 | 57篇 |
2007年 | 43篇 |
2006年 | 30篇 |
2005年 | 39篇 |
2004年 | 26篇 |
2003年 | 28篇 |
2002年 | 26篇 |
2001年 | 13篇 |
2000年 | 7篇 |
1999年 | 13篇 |
1998年 | 10篇 |
1997年 | 7篇 |
1996年 | 6篇 |
1995年 | 2篇 |
1994年 | 3篇 |
1992年 | 1篇 |
1990年 | 3篇 |
1989年 | 1篇 |
1988年 | 2篇 |
1987年 | 2篇 |
1986年 | 2篇 |
1985年 | 1篇 |
1984年 | 4篇 |
1983年 | 3篇 |
1982年 | 4篇 |
1981年 | 2篇 |
1980年 | 2篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 1篇 |
1976年 | 1篇 |
1975年 | 1篇 |
1974年 | 1篇 |
1971年 | 1篇 |
1969年 | 3篇 |
排序方式: 共有1033条查询结果,搜索用时 0 毫秒
101.
Qualitative and quantitative aspects of the evolution of the planktonic populations in Lake Lugano 总被引:2,自引:0,他引:2
This study is a summary of all available information on the long-term and on the more recent development of the planktonic populations in Lago di Lugano with regard to its trophic evolution. The first effects of the lake eutrophication became manifest beginning in the 1950's and brought to important changes in the planktonic community as the appearance and the rapid increase of the filamentous algaeOscillatoria rubescens andStephanodiscus hantzschii, and the disappearance of the zooplanktonic Diaptomidae. Since 1980 the phosphorus concentration began to decrease in lake's epilimnion and the planktonic community showed a new composition, evidencing the presence of others predominant species:Oscillatoria redekei, Lyngbya limnetica, Stephanodiscus sp. (little form). Since 1989 algal standing crop reduced its value below 2 g m–2 (dry weight); furthermore a strong decrease of the Cyanophceae and the appearance of new predominant species (Tabellaria fenestrata, div. Ulotrichales) occurred. At the same time an increase of the herbivorous zooplankton (Daphnia hyalina) and the come-back of the Diaptomidae was observed. Only in the northern basin a decrease tendency of the primary production was observed, changing from 480 to about 300 g C m–2 yr–1 during the last 10 years. 相似文献
102.
Michele Bossi Gianantonio Guerrero Luciano Mantegazza Marco Scardia 《Astrophysics and Space Science》1982,85(1-2):25-29
Photoelectric observations of the Delta Scuti star KW 207 (HD 73576) are given. In spite of the low variations, the analysis of the light curves shows at least two sighificant periodicities, which seem to be related to non-radial pulsations.A third frequency, corresponding to the radial fundamental mode, is probably bame out by our data. 相似文献
103.
104.
Marco Franzini 《Contributions to Mineralogy and Petrology》1969,21(3):203-224
A discussion of the transition from the ideal hexagonal mica structure to the ideal ditrigonal one, leads to the conclusion that the single mica layer may have two different structures (labelled A and B). The recent literature data show that both the A and B structures have been detected in some triocahedral layer lattice silicates found in nature. An examination of the structural stability of the A and B structures suggests that the last one may not be realized by dioctahedral layer lattice silicates. The concept of two structurally different mica layers, which however have the same lattice constants, greatly improves the understanding of polymorphism and twin laws in layer lattice silicates.The structural features of the tetrahedral sheet, octahedral sheet and interlayer region are carefully examined. Thus we can reach the following conclusions: the tetrahedal sheet is not entirely free to reduce its lateral dimensions by the mechanism of tetrahedal rotation owing to the repulsion among Obas atoms; the octahedral sheet in layer lattice silicates, may increase or reduce its lateral dimensions as compared to the lateral dimensions it has in the hydroxide minerals; the interlayer region is characterized by a regular octahedral coordination of the Obas around the interlayer cation. On the ground of these conclusions, new structural models for some selected layer lattice silicates are proposed.Notations Obas
basal oxygen atoms of the (Al, Si)O4 tetrahedra
- Oap
apical oxygen atoms of the (Al, Si)O4 tetrahedra
-
b
tetr
b dimension which the tetrahedral sheet would assume if unconstrained
-
b
oct
b dimension which the octahedral sheet has in the hydroxide minerals
-
b
obs
observed value of b
- c
oct
*
thickness of the octahedral sheet
-
d
o
distance between an octahedral cation and an Oap atom
-
d
int
distance between an interlayer cation and an Obas atom
-
average tetrahedral rotation from ideal hexagonal symmetry 相似文献
105.
Monica Previati Marco Palma Giorgio Bavestrello Carla Falugi Carlo Cerrano 《Marine Ecology》2010,31(4):555-565
Despite the wide distribution of zoanthids, little is known about their pattern of reproduction. Here we investigate the reproductive biology of two Mediterranean species, the common Parazoanthus axinellae (Schmidt) and the rare Savalia savaglia (Bertoloni). For both species, samples were collected during an annual cycle, from January to December 2005, in the Western Mediterranean (Ligurian Sea, Italy). Both species are gonochoric. In P. axinellae the sex‐ratio (n colonies = 30) showed a slight predominance of male colonies (M/F = 1.35), whereas in the population of S. savaglia (n colonies = 15) a predominance of females was found (M/F = 0.3). In P. axinellae the first gametocytes were visible in March, whereas in S. savaglia they became visible in May. Both species reproduce at the end of autumn when seawater temperature begins to decrease. Parazoanthus axinellae (10 m depth) spawns eggs and sperms in November, whereas S. savaglia (67 m depth) spawns in December. In P. axinellae sexes were segregated on a rocky wall, with males occurring deeper, whereas male and female colonies of S. savaglia were irregularly dispersed in the population. The maximum number of oocytes differed between the species, being higher in P. axinellae than in S. savaglia. 相似文献
106.
107.
Carl ALWMARK Birger SCHMITZ Sanna HOLM Federica MARONE Marco STAMPANONI 《Meteoritics & planetary science》2011,46(8):1071-1081
Abstract– A method is described for imaging in 3‐D the interiors of meteoritic chromite grains and their inclusions using synchrotron radiation X‐ray tomographic microscopy. In ordinary chondrites, chromite is the only common mineral that survives long‐term weathering on Earth. Information about the silicate matrix of the original meteorite, however, can be derived from mineral inclusions preserved in the protecting chromite. The inclusions are crucial in the classification of fossil meteorites as well as sediment‐dispersed chromite grains from decomposed meteorites and larger impacts, as these are used for characterizing the past influx of material to Earth, but have previously been difficult to locate. The method is non‐destructive and time efficient for locating inclusions. The method allowed quantitative and morphological studies of both host chromite grains and inclusions in three dimensions. The study of 385 chromite grains from eight chondrites (H4–6, L4–6, LL4, LL6) reveals that inclusions are abundant and equally common in all samples. Almost two‐thirds of all chromite grains contain inclusions, regardless of group and type. The study also shows that the size of the inclusions and the host chromite grains, as well as the number of inclusions, within the host chromite grains vary with petrographic type. Thus, the petrographic type of the host of a suite of chromite grains can be determined based solely on inclusion content. The study also revealed that the amount of fractures in the host chromite can be correlated to previously assigned shock stages for the various chondrites. The study has thus shown that the features and inclusions of fossil chromite grains can give similar information about a former host meteorite as do studies of an unweathered whole meteorite, meaning that this technique is essential in the studies of ancient meteorite flux to Earth. 相似文献
108.
In this article, we draw attention to trends in land transformation in the West Bank since the Second Intifada, after which a surge of investment from Gulf countries entered Palestine, almost exclusively in the West Bank. The occupied Palestinian territories have attracted a great deal of attention from media and academics, yet the vast majority of scholarship focuses on the conflict and the variety of social, economic, and political repercussions of the on-going Israeli occupation. While the occupation has undeniably serious impacts on every aspect of life for Palestinians, the near-exclusive focus on the conflict means that significant new trends, such as urban mega-developments and emerging market-based urbanisation processes, have been largely overlooked. We outline three directions for future research: the assemblage of “neglected city-builders” shaping current urbanization processes in Palestine; the ideas, policies, and norms circulating in the West Bank; and the social, spatial, economic, and other impacts of these new urban developments. 相似文献
109.
Behavior of epidote at high pressure and high temperature: a powder diffraction study up to 10 GPa and 1,200 K 总被引:1,自引:0,他引:1
G. Diego Gatta Marco Merlini Yongjae Lee Stefano Poli 《Physics and Chemistry of Minerals》2011,38(6):419-428
The thermo-elastic behavior of a natural epidote [Ca1.925 Fe0.745Al2.265Ti0.004Si3.037O12(OH)] has been investigated up to 1,200 K (at 0.0001 GPa) and 10 GPa (at 298 K) by means of in situ synchrotron powder diffraction.
No phase transition has been observed within the temperature and pressure range investigated. P–V data fitted with a third-order Birch–Murnaghan equation of state (BM-EoS) give V
0 = 458.8(1)Å3, K
T0 = 111(3) GPa, and K′ = 7.6(7). The confidence ellipse from the variance–covariance matrix of K
T0 and K′ from the least-square procedure is strongly elongated with negative slope. The evolution of the “Eulerian finite strain”
vs “normalized stress” yields Fe(0) = 114(1) GPa as intercept values, and the slope of the regression line gives K′ = 7.0(4). The evolution of the lattice parameters with pressure is slightly anisotropic. The elastic parameters calculated
with a linearized BM-EoS are: a
0 = 8.8877(7) Å, K
T0(a) = 117(2) GPa, and K′(a) = 3.7(4) for the a-axis; b
0 = 5.6271(7) Å, K
T0(b) = 126(3) GPa, and K′(b) = 12(1) for the b-axis; and c
0 = 10.1527(7) Å, K
T0(c) = 90(1) GPa, and K’(c) = 8.1(4) for the c-axis [K
T0(a):K
T0(b):K
T0(c) = 1.30:1.40:1]. The β angle decreases with pressure, βP(°) = βP0 −0.0286(9)P +0.00134(9)P
2 (P in GPa). The evolution of axial and volume thermal expansion coefficient, α, with T was described by the polynomial function: α(T) = α0 + α1
T
−1/2. The refined parameters for epidote are: α0 = 5.1(2) × 10−5 K−1 and α1 = −5.1(6) × 10−4 K1/2 for the unit-cell volume, α0(a) = 1.21(7) × 10−5 K−1 and α1(a) = −1.2(2) × 10−4 K1/2 for the a-axis, α0(b) = 1.88(7) × 10−5 K−1 and α1(b) = −1.7(2) × 10−4 K1/2 for the b-axis, and α0(c) = 2.14(9) × 10−5 K−1 and α1(c) = −2.0(2) × 10−4 K1/2 for the c-axis. The thermo-elastic anisotropy can be described, at a first approximation, by α0(a): α0(b): α0(c) = 1 : 1.55 : 1.77. The β angle increases continuously with T, with βT(°) = βT0 + 2.5(1) × 10−4
T + 1.3(7) × 10−8
T
2. A comparison between the thermo-elastic parameters of epidote and clinozoisite is carried out. 相似文献
110.