全文获取类型
收费全文 | 41000篇 |
免费 | 749篇 |
国内免费 | 664篇 |
专业分类
测绘学 | 1050篇 |
大气科学 | 3398篇 |
地球物理 | 8376篇 |
地质学 | 13669篇 |
海洋学 | 3798篇 |
天文学 | 9314篇 |
综合类 | 157篇 |
自然地理 | 2651篇 |
出版年
2021年 | 264篇 |
2020年 | 334篇 |
2019年 | 331篇 |
2018年 | 734篇 |
2017年 | 698篇 |
2016年 | 1051篇 |
2015年 | 762篇 |
2014年 | 983篇 |
2013年 | 2106篇 |
2012年 | 1195篇 |
2011年 | 1662篇 |
2010年 | 1397篇 |
2009年 | 1999篇 |
2008年 | 1784篇 |
2007年 | 1656篇 |
2006年 | 1555篇 |
2005年 | 1422篇 |
2004年 | 1365篇 |
2003年 | 1315篇 |
2002年 | 1256篇 |
2001年 | 1129篇 |
2000年 | 1091篇 |
1999年 | 986篇 |
1998年 | 892篇 |
1997年 | 893篇 |
1996年 | 758篇 |
1995年 | 684篇 |
1994年 | 610篇 |
1993年 | 547篇 |
1992年 | 535篇 |
1991年 | 513篇 |
1990年 | 501篇 |
1989年 | 440篇 |
1988年 | 415篇 |
1987年 | 468篇 |
1986年 | 456篇 |
1985年 | 555篇 |
1984年 | 614篇 |
1983年 | 587篇 |
1982年 | 539篇 |
1981年 | 482篇 |
1980年 | 450篇 |
1979年 | 410篇 |
1978年 | 433篇 |
1977年 | 368篇 |
1976年 | 337篇 |
1975年 | 353篇 |
1974年 | 358篇 |
1973年 | 349篇 |
1972年 | 218篇 |
排序方式: 共有10000条查询结果,搜索用时 0 毫秒
81.
If utilized, the energy dissipative capability of seismically loaded shallow foundations due to inelastic behavior can result in more economic design, provided the consequences, such as excessive deformations are accounted for. In this article, a Beam‐on‐Nonlinear‐Winkler‐Foundation (BNWF) model is used to assess the performance of shearwall‐foundation systems with different attributes, when subjected to ground motions of varied hazard levels. The numerical study indicates that the force and drift demands to the shearwall reduce significantly, when nonlinear foundation behavior is realized, while permanent settlement is well below the permissible limit. These results support the concept of shallow foundation capacity mobilization in future design. Copyright © 2010 John Wiley & Sons, Ltd. 相似文献
82.
Summary The summer monsoon circulation shows various spatial and temporal oscillations and often a combination of systems produces
an integrated effect. In this study phases of the southwest (SW) monsoon have been identified in an objective manner with
the help of T-mode principal component analysis (PCA) of weekly rainfall anomalies. Mean composite charts have been prepared
utilising all available upper air data (1977–1986) for each category of the SW monsoon epochs identified by the PCA. These
sets of charts have been constructed for both the strong and weak phases associated with the first four significant principal
components (PCs).
A well defined east-west oriented trough system, extending from about 28° N Latitude/65° E Longitude to 20° N Latitude/90° E
Longitude, in the lower levels, has been the main feature associated with the strong phase of the monsoon corresponding to
PC I. The trough in the lower levels is more marked in the eastern half compared to the western half in both the sets of charts
associated with strong phases of the monsoon related to the PC II and PC III. With PC II, the position of the troughs in the
lower levels is further north of its location in PC III. The east-west trough system associated with the strong phase of PC
IV has a large southward tilt with height. The charts corresponding to the weak phases of these PCs have synoptic features,
such as the position of the trough close to the foothills of the Himalayas, and the shifting of middle and upper tropospheric
anticyclones to the south.
The study suggests an objective method of interpretation of principal components by utilising synoptic data. In addition,
synoptic models and data sets corresponding to different phases of the monsoon can also be prepared in an objective manner
by such PCA.
Received July 18, 1997 Revised April 30, 1999 相似文献
83.
FluBiDi is a two-dimensional model created to simulate real events that can take days and months, as well as short events (minutes or hours) and inclusive laboratory tests. To verify the robustness of FluBiDi, it was tested using a previous study with both designed and real digital elevation models. The results highlight good agreement between the models (i.e. Mike Flood, SOBEK, ISIS 2D, and others) tested and FluBiDi (around 90% for a specific instant and 95% for the complete time simulation). In the simulated hydrographs, the discharge peak value, time to peak, and water level results were accurate, reproducing them with an error of less than 5%. The velocity differences observed in a couple of tests in FluBiDi were associated with very short periods of time (seconds). However, FluBiDi is highly accurate for simulating floods under real topographical conditions with differences of around 2 cm when water depth is around 150 cm. The average water depth and velocities are precise, and the model describes with high accuracy the pattern and extent of floods. FluBiDi has the capability to be adjusted to different types of events and only requires limited input data. 相似文献
84.
85.
P. Rodríguez-Gil B. T. Gänsicke H.-J. Hagen S. Araujo-Betancor A. Aungwerojwit C. Allende Prieto D. Boyd J. Casares D. Engels O. Giannakis E. T. Harlaftis J. Kube H. Lehto I. G. Martínez-Pais R. Schwarz W. Skidmore A. Staude M. A. P. Torres 《Monthly notices of the Royal Astronomical Society》2007,377(4):1747-1762
86.
Elementary cluster analysis of induced seismicity in a South African gold mine has shown that there is a clear interaction amongst the clusters; and that the level of the interaction is a function of the distance. The clusering algorithm used is an adaptation of the single-link cluster analysis which considers both three-dimensional space and time. A high level of interaction between the clusters is demonstrated from the cross-correlation analysis of seismic activity rates and radiated energy. A distinct decrease in the value of correlation coefficients was detectable as distance increased. This was somewhat surprising, considering the simplicity of the technique used. Since no attempt is made to study the physical mechanisms of interaction, these results are very preliminary, but interesting from an observational point of view. 相似文献
87.
The study of trace metal speciation has benefited from a great deal of interest leading to the development and the diversification of sequential extraction schemes (SES), which triggered the need for harmonization by the standard, measurements and testing programme (SM & T). However, some uncertainties still persist in the application of the harmonized 3-step protocol, because of the difficulty in conceptualizing a technique that can be applied to environmental samples irrespective of their nature. The alternative method proposed in this study is based on the solubility of the sample components progressively dissolved during the course of an acidimetric titration by 1 mol L−1 HCl. The major HCl-reactive mineralogical components are identified using mass balance calculation of H+ consumed by their dissolution together with the amount of major elements released into solution. The speciation of minor and trace elements is investigated by comparing their titrations to those of the major elements. This approach is much simpler than SES because it uses simple non-selective H+ at room temperature instead of a complex experimental design of so-called specific reagents. The different mineral components of the solid are no longer operationally defined and the problem of selectivity is irrelevant to the titration approach. The method was applied to several sediment samples from the Scheldt estuary and the particulate phase was further examined by Scanning Electron Microprobe and X-ray Diffraction techniques. The nearly complete consumption of H+ in the suspension is balanced by the total dissolution of carbonates and Fe-oxyhydroxides. In contrast to the speciation inferred from the Tessier SES, the acidimetric titration has demonstrated that the carbonate phase does not significantly contain trace metals with the exceptions of 40% of the Mn and 30% of the Co. In contrast, the Fe-oxyhydroxides seem to play a major role and account for 70% of Pb and 20% of Cr, in addition to 60% of P and additional amounts of 20% Co and 40% Mn. 70% of the Cu also occur in the oxyhydroxide phase, more likely coprecipitated with gibbsite. 90% of the Cd and 85% of Zn can be attributed to the Acid Volatile Sulfide (AVS) phase as evidenced from S titration. The acidimetric titration method not only provides information on the speciation of trace metals but also allows the quantification of their reactivity and mobility, if one considers that the titration roughly mimics pH changes that may occur as a result of chemical disequilibrium in the environment. The results demonstrate the potential of the acidimetric titration as an alternative to SES protocols in geochemical and environmental regulation studies. This method is applicable to a wide variety of environmental materials with little or minor adjustments. 相似文献
88.
89.
Graça Cabeçadas M. José Brogueira M. Gelena Cavaco Célia Gonçalves 《Journal of Oceanography》2010,66(2):201-210
Although the circulation of intermediate water masses in the eastern North Atlantic remains poorly defined, the presence of
fresher intermediate waters, the Sub-Artic (SAIW) and the Antarctic Intermediate Water (AAIW), as well the saline intermediate
Mediterranean Water (MW), has been tracked using biogeochemical properties. Here we assess the hydrographic and chemical structures
of intermediate waters along the western Portuguese margin by examining the vertical distributions and property-property plots
of chemical tracers (oxygen and nutrients). AAIW was traced by low oxygen and high nutrients, while SAIW was recognized by
low nutrients. The Mediterranean Water (MW) undercurrent is shown to spread towards the eastern flank of Gorringe bank. Concurrently,
the fresher waters gained salt by direct incorporation of MW, while this water was enriched in nutrients on its way northward
and westward owing, to a great extent, to the entrainment of an AAIW branch. The distributions of nutrients and apparent oxygen
utilization are discussed in terms of regional ocean circulation. Our analysis suggests a circulation pattern of the various
intermediate waters along the western Portuguese margin: MW extends all over the area, but its presence is more pronounced
around cape St. Vincent; SAIW apparently moves southward, reaching the Gorringe bank region, and AAIW flows northward along
the coast and around the bank. 相似文献
90.
To study the crystal chemistry of bernalite, Fe(OH)3, and the nature of the octahedral Fe3+ environment, Mössbauer spectra were recorded from 80 to 350 K, optical spectra were recorded at room temperature and a sample was studied using transmission electron microscopy. The Mössbauer spectrum of bernalite consists of a single six-line magnetic spectrum at 80 K. A broadened six-line magnetic spectrum with significantly less intensity is observed at higher temperatures, and is attributed to a small fraction of bernalite occurring as small particles. The variation of hyperfine magnetic field data for bulk bernalite with temperature is well described by the Weiss molecular field model with parameters of H
0 = 55.7±0.3 T and T
N = 427±5K. The centre shift data were fitted to the Debye model with parameters 0=0.482±0.005 mm/s (relative to -Fe) and M=492±30 K. The quadrupole shift is near zero at 300 K, and does not vary significantly with temperature. Absorption spectra in the visible and near infrared range show three crystal field bands of Fe3+ at 11 300, 16000 and 23 200 cm-1, giving a crystal field splitting of 14 570 cm-1 and Racah parameters of B=629 cm-1 and C=3381 cm-1. Infrared reflection spectra show two distinct OH-stretching frequencies, which could correspond to two structurally different types of OH groups. A band was also observed at 2250 cm-1, suggesting the presence of molecular CO2 in the large cation site. Analytical transmission electron microscopy indicates that Si occurs within the bernalite structure as well as along domain boundaries. Electron diffraction and imaging show that bernalite is polysynthetically twinned along {100} planes with twin domains ranging from 3 to 20 nm in thickness. Results are discussed with respect to the nature of the octahedral Fe3+ site, and compared with values for other iron oxides and hydroxides. 相似文献