全文获取类型
收费全文 | 88007篇 |
免费 | 1521篇 |
国内免费 | 764篇 |
专业分类
测绘学 | 2143篇 |
大气科学 | 6937篇 |
地球物理 | 18511篇 |
地质学 | 29673篇 |
海洋学 | 7359篇 |
天文学 | 19436篇 |
综合类 | 252篇 |
自然地理 | 5981篇 |
出版年
2021年 | 546篇 |
2020年 | 689篇 |
2019年 | 691篇 |
2018年 | 1444篇 |
2017年 | 1405篇 |
2016年 | 1942篇 |
2015年 | 1414篇 |
2014年 | 2009篇 |
2013年 | 4325篇 |
2012年 | 2177篇 |
2011年 | 3131篇 |
2010年 | 2744篇 |
2009年 | 3934篇 |
2008年 | 3528篇 |
2007年 | 3226篇 |
2006年 | 3206篇 |
2005年 | 2762篇 |
2004年 | 2857篇 |
2003年 | 2632篇 |
2002年 | 2491篇 |
2001年 | 2286篇 |
2000年 | 2209篇 |
1999年 | 1933篇 |
1998年 | 1948篇 |
1997年 | 1867篇 |
1996年 | 1627篇 |
1995年 | 1516篇 |
1994年 | 1375篇 |
1993年 | 1254篇 |
1992年 | 1192篇 |
1991年 | 1043篇 |
1990年 | 1236篇 |
1989年 | 1083篇 |
1988年 | 937篇 |
1987年 | 1174篇 |
1986年 | 1026篇 |
1985年 | 1294篇 |
1984年 | 1501篇 |
1983年 | 1437篇 |
1982年 | 1282篇 |
1981年 | 1235篇 |
1980年 | 1085篇 |
1979年 | 1061篇 |
1978年 | 1128篇 |
1977年 | 1004篇 |
1976年 | 981篇 |
1975年 | 936篇 |
1974年 | 902篇 |
1973年 | 918篇 |
1972年 | 580篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
981.
982.
Summary Boron-doped mullites were synthesized using aluminium nitrate-nonahydrate, tetraethoxysilane and boric acid in a sol–gel process
with subsequent annealing at 950 and 1300 °C for five hours. Two different bulk compositions with constant Al2O3 contents (60 and 70 mol%, respectively) and varying SiO2 plus B2O3 contents were investigated. X-ray powder diffraction analyses yielded a linear decrease of the lattice parameters with increasing
bulk B2O3 content, which was interpreted as to be due to boron incorporation. Related to the increasing boron content, corresponding
infrared spectra revealed a slight and continuous shift for most of the absorption bands. These data show that mullite is
able to incorporate large amounts of boron into its structure (up to about 20 mol% B2O3 depending on the bulk composition of the starting materials). Infrared analyses suggest that boron is incorporated into the
mullite structure in form of planar three-fold coordinated BO3 groups.
Author’s address: A. Beran, Institut für Mineralogie und Kristallographie, Universit?t Wien-Geozentrum, Althanstra?e 14, 1090 Wien, Austria 相似文献
983.
H. F. Rainsford 《Journal of Geodesy》1955,29(3):12-22
Summary This article examines the practical application of formulae for computing long lines on the ellipsoid. The main aim is to
eliminate the successive approximation generally required. For the inverse problem, this is achieved by the method ofE. M. Sodano, Army Map Service, U.S.A. An adaptation of a method produced byG. T. McCaw is used for the direct problem.
Results are given of five practical examples, including two which extend halfway round the world. Construction of further
special tables is recommended to simplify the computations required by a problem which has an ever increasing application.
Zusammenfassung Der Verfasser—ein bekannter Fachmann auf diesem Gebiet—untersucht kritisch die von verschiedenen Geod?ten angewandten oder vorgeschlagenen Verfahren zur Berechnung sehr langer geod?tischer Linien.
Resumen El autor, especialista bien conocido en la cuestión, examina y discute los differentes métodos utilizados o preconizados por diferentes geodestas para el cálculo de lineas geodésicas de gran longitud en la superficie terrestre.
Résumé L'auteur, spécialiste bien connu de la question, examine et discute les différentes méthodes utilisées ou préconisées par différents Géodésiens, pour le calcul des lignes géodésiques de grande longueur à la surface de la terre.
Riassunto L'Autore, specialista ben noto dell'argomento, esamina e discute i diversi metodi utilizzati o proposti dai differenti Geodeti per il calcolo delle geodetiche lunghe sulla superficie terrestre.相似文献
984.
985.
Hildegard?WestphalEmail author Iris?Surholt Christian?Kiesl Holger F. ?Thern Thomas?Kruspe 《Pure and Applied Geophysics》2005,162(3):549-570
Carbonate rocks are well known for their complex petrophysical behavior where, in contrast to siliciclastic rocks, different parameters, including porosity and permeability, usually are not directly related. This behavior is the result of thorough reorganization of porosity during diagenesis, and it turns prediction of reservoir quality of carbonate rocks into a challenge. The study presented here deals with the problem of utilizing NMR techniques in prediction of petrophysical properties in carbonates.We employ a visual porosity classification as a priori knowledge for better interpreting NMR data for prediction purposes. This allows for choice of suitable T2 cutoff values to differentiate movable from bound fluids adapted for the specific carbonate rock, thus resulting in better interpretation of NMR data. The approach of using a genetic pore type classification for adapting the conventional method for T2 cutoff determination, which originally was developed for siliciclastic rocks, is promising. Similarly, for permeability determination on the basis of NMR measurements, the classification of carbonate rocks based on porosity types also shows potential. The approach implemented here has the promise to provide a basis of standardized interpretation of NMR data from carbonate rocks.Acknowledgment We are grateful to Baker Hughes INTEQ, Celle, for permission to publish the results of this work. This study was part of I.S.s Masters Thesis at Hannover University that was carried out in collaboration with Baker Hughes INTEQ, Celle, Germany. We appreciate comments of an anonymous referee and PAGEOPH editor Brian J. Mitchell. 相似文献
986.
D.M. Worrall M. Birkinshaw M.J. Hardcastle C.R. Lawrence 《Monthly notices of the Royal Astronomical Society》2001,326(3):1127-1133
We report results of an 18-ks exposure with the ACIS instrument on Chandra of the powerful z =0.62 radio galaxy 3C 220.1. The X-ray emission separates into cluster gas of emission-weighted kT ∼5 keV , 0.7–12 keV luminosity (to a radius of 45 arcsec) of 5.6×1044 erg s−1 and unresolved emission (coincident with the radio core). While the extended X-ray emission is clearly thermal in nature, a straightforward cooling-flow model, even in conjunction with a point-source component, is a poor fit to the radial profile of the X-ray emission. This is despite the fact that the measured properties of the gas suggest a massive cooling flow of ∼130 M⊙ yr−1 , and the data show weak evidence for a temperature gradient. The central unresolved X-ray emission has a power-law spectral energy index α ∼0.7 and 0.7–12 keV luminosity of 1045 erg s−1 , and any intrinsic absorption is relatively small. The two-point spectrum of the core emission between radio and X-ray energies has α rx =0.75 . Since this is a flatter spectrum than seen in other sources where the X-ray emission is presumed to be radio-related, regions close to the active galactic nucleus (AGN) in this source may dominate the central X-ray output, as is believed to be the case for lobe-dominated quasars. Simple unification models would be challenged if this were found to be the case for a large fraction of high-power radio galaxies. 相似文献
987.
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
988.
Heidi Kreibich Veit Blauhut Jeroen C. J. H. Aerts Laurens M. Bouwer Henny A. J. Van Lanen Alfonso Mejia 《水文科学杂志》2020,65(3):491-494
ABSTRACTWe thank the authors, Brunella Bonaccorso and Karsten Arnbjerg-Nielsen for their constructive contributions to the discussion about the attribution of changes in drought and flood impacts. We appreciate that they support our opinion, but in particular their additional new ideas on how to better understand changes in impacts. It is great that they challenge us to think a step further on how to foster the collection of long time series of data and how to use these to model and project changes. Here, we elaborate on the possibility to collect time series of data on hazard, exposure, vulnerability and impacts and how these could be used to improve e.g. socio-hydrological models for the development of future risk scenarios. 相似文献
989.
Ruiz F 《Marine pollution bulletin》2001,42(6):482-490
The impact of river-transported metal pollution and industrial wastes on the metal distribution (Cr, Cu, Zn, Pb) in estuarine sediments was studied in the southwestern Spanish estuaries. Intertidal and subtidal surface sediments of the Tinto-Odiel Estuary are very highly polluted by heavy metals, with geoaccumulation indices up to 4 in the three sedimentary environments studied (channel, channel border and salt marsh). The single exception is the Punta Umbria channel, very protected from the point sources by salt marsh deposits and hydraulic processes. In the remaining two estuaries, pollution (Pb, Cu) was only significant near the harbour situated in the Piedras river mouth, whereas very low values were found in the Guadiana Estuary. In these last rivers, the enrichment factor increases from the channel to the salt marsh sediments. 相似文献
990.
Rigoberto G. Sanabria Castro Sandra M. C. Malta Abimael F. D. Loula Luiz Landau 《Computational Geosciences》2002,5(4):301-330
A finite element formulation is proposed to approximate a nonlinear system of partial differential equations, composed by an elliptic subsystem for the pressure–velocity and a transport equation (convection–diffusion) for the concentration, which models the incompressible miscible displacement of one fluid by another in a rigid porous media. The pressure is approximated by the classical Galerkin method and the velocity is calculated by a post-processing technique. Then, the concentration is obtained by a Galerkin/least-squares space–time (GLS/ST) finite element method. A numerical analysis is developed for the concentration approximation. Then, stability, convergence and numerical results are presented confirming the a priori error estimates. 相似文献