全文获取类型
收费全文 | 89237篇 |
免费 | 1978篇 |
国内免费 | 2112篇 |
专业分类
测绘学 | 3373篇 |
大气科学 | 6990篇 |
地球物理 | 17723篇 |
地质学 | 34746篇 |
海洋学 | 6658篇 |
天文学 | 16367篇 |
综合类 | 2492篇 |
自然地理 | 4978篇 |
出版年
2022年 | 460篇 |
2021年 | 762篇 |
2020年 | 770篇 |
2019年 | 803篇 |
2018年 | 6058篇 |
2017年 | 5283篇 |
2016年 | 4380篇 |
2015年 | 1504篇 |
2014年 | 1996篇 |
2013年 | 3493篇 |
2012年 | 3138篇 |
2011年 | 5367篇 |
2010年 | 4349篇 |
2009年 | 5313篇 |
2008年 | 4470篇 |
2007年 | 4906篇 |
2006年 | 2787篇 |
2005年 | 2287篇 |
2004年 | 2303篇 |
2003年 | 2197篇 |
2002年 | 1993篇 |
2001年 | 1582篇 |
2000年 | 1562篇 |
1999年 | 1305篇 |
1998年 | 1315篇 |
1997年 | 1253篇 |
1996年 | 1097篇 |
1995年 | 1063篇 |
1994年 | 935篇 |
1993年 | 843篇 |
1992年 | 789篇 |
1991年 | 783篇 |
1990年 | 831篇 |
1989年 | 757篇 |
1988年 | 692篇 |
1987年 | 850篇 |
1986年 | 697篇 |
1985年 | 858篇 |
1984年 | 1002篇 |
1983年 | 900篇 |
1982年 | 881篇 |
1981年 | 785篇 |
1980年 | 789篇 |
1979年 | 671篇 |
1978年 | 642篇 |
1977年 | 631篇 |
1976年 | 568篇 |
1975年 | 551篇 |
1974年 | 573篇 |
1973年 | 640篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
171.
Both pyruvic and α-ketobutyric acids are formed during heating experiments with modern Chione shells. These α-keto acids are also present in fossil specimens. These findings provide additional evidence that the dehydration reaction of free serine and threonine or the β-elimination reaction of peptide bound serine and threonine occurred in fossil materials. These experiments suggest that the formation of α-keto acids occurs early in protein diagenesis. A large fraction of the α-keto acids which were formed have been lost either by subsequent reactions or by diffusion out of the shell matrix. 相似文献
172.
173.
Summary. Rotational remanent magnetization, RRM, is the magnetization acquired when a sample is rotated during alternating field demagnetization. Although the existence of RRM has been well documented in different laboratories, until now no physical mechanism explaining its origin has been given. We propose that the RRM originates from thermal fluctuations biased by a precessional torque associated with the alternating field. Our theory is consistent with the observation that no directional preference exists in the experimental situation until the sample is rotated relative to the alternating field. Moreover, our theory predicts that the combined sample rotation and precession will produce a RRM that switches direction when the frequency of sample rotation increases from any value below the frequency of the alternating field to any value above that frequency as observed in experiments. Although no precise theory is given for the intensity of RRM, the model presented here can qualitatively explain previous intensity observations. 相似文献
174.
Dongqi Sun Liang Zhou Yu Li Haimeng Liu Xiaoyan Shen Zedong Wang Xixi Wang 《地理学报(英文版)》2017,27(8):943-966
The future development of new-type urbanization has drawn great attention from both the government and public alike. In this context, the present study had three related research aims. Firstly, it sought to predict the urbanization and population dynamics in China at both national and provincial levels for the period of 2015 to 2030. Secondly, on this basis, it sought to examine the spatial variation of urbanization given the predicted national urbanization rate of 70.12%. Thirdly, it sought to estimate and evaluate the national and provincial demands of investment in the development of new-type urbanization. The main conclusions from this study were as follows: (1) The population size and urbanization rate will reach 1.445 billion and 70.12%, respectively, from 2015 to 2030. (2) The demographic dividend will vanish when the population pressure reaches its maximum. During this period, there will be 70.16 million urban population born. The suburban population that becomes urbanized will be 316.7 million, and thus the net increase in urban population will reach 386 million. (3) Although the urbanization rate of every Chinese province will increase during 2015–2030, it will do so unequally, while differences in urbanization quality among provinces will also be substantial. In some provinces, moreover, the urbanization quality is not compatible with their eco-social development. (4) A total of 4,105,380 billion yuan is required to fund new-type urbanization and the investment demand for each province varies greatly; for example, Guangdong province requires the most funding, amounting to approximately 148 times that required by Tibet, the province in least need of funding. In the final part of this study, policy suggestions concerning the investment of the new-type urbanization are put forward and discussed. 相似文献
175.
176.
177.
Aggeliki Adamaki Eleftheria E. Papadimitriou George M. Tsaklidis Vassilios Karakostas 《Acta Geophysica》2011,59(4):748-769
Aftershock rates seem to follow a power law decay, but the assessment of the aftershock frequency immediately after an earthquake,
as well as during the evolution of a seismic excitation remains a demand for the imminent seismic hazard. The purpose of this
work is to study the temporal distribution of triggered earthquakes in short time scales following a strong event, and thus
a multiple seismic sequence was chosen for this purpose. Statistical models are applied to the 1981 Corinth Gulf sequence,
comprising three strong (M = 6.7, M = 6.5, and M = 6.3) events between 24 February and 4 March. The non-homogeneous Poisson process outperforms the simple Poisson process
in order to model the aftershock sequence, whereas the Weibull process is more appropriate to capture the features of the
short-term behavior, but not the most proper for describing the seismicity in long term. The aftershock data defines a smooth
curve of the declining rate and a long-tail theoretical model is more appropriate to fit the data than a rapidly declining
exponential function, as supported by the quantitative results derived from the survival function. An autoregressive model
is also applied to the seismic sequence, shedding more light on the stationarity of the time series. 相似文献
178.
LIU ZhiQiang LAI YuanMing ZHANG MingYi & ZHANG XueFu State Key Laboratory of Frozen Soil Engineering Cold Arid Regions Environmental Engineering Research Institute Chi- nese Academy of Sciences Lanzhou China School of Civil Engineering Lanzhou Jiao Tong University Lanzhou China Chongqing Jiao Tong University Chongqing China 《中国科学D辑(英文版)》2007,50(3):404-410
The stochastic finite element equations for random temperature are obtained using the first-order per-turbation technique taking into account the random thermal properties and boundary condition, based on heat transfer variational principle. The local average method for 2-D is used to discretize random fields. Then, the random temperature fields of embankment in cold regions are investigated on condi-tion that the thermal properties and boundary condition are taken as random fields, respectively, by using the program, which is written by the methods. The expected value of temperature field and the standard deviation of the temperature field of embankment in cold regions are obtained and analyzed. 相似文献
179.
Mass spectra in the range 2 ≦ M/Q ≦ 3 provided by a high resolution mode of the ISEE-3 Plasma Composition Experiment were evaluated for three selected periods during early 1980. The observed Ne/O ratios are compatible with estimated solar abundance ratios. In two of the three periods, the He/Ne-ratios agree with the Apollo foil results. Freezing-in temperatures for oxygen are similar to those obtained by other groups. Possible reasons for an unexpectedly high flux at M/Q = 2.4 are discussed. 相似文献
180.
Robert W. Johnson 《Astrophysics and Space Science》2011,332(1):73-79
To investigate the relation between observations of the 10.7 cm flux and the international sunspot number so that a physical
unit may be ascribed to historical records, both polynomial and power law models are developed giving the radio flux as a
function of sunspot number and vice versa. Bayesian data analysis is used to estimate the model parameters and to discriminate between the models. The effect on the
parameter uncertainty and on the relative evidence of normalizing the measure of fit is investigated. The power law giving
flux as a function of sunspot number is found to be the most plausible model and may be used to estimate the radio flux from
historical sunspot observations. 相似文献