全文获取类型
收费全文 | 38866篇 |
免费 | 18958篇 |
国内免费 | 43880篇 |
专业分类
测绘学 | 3142篇 |
大气科学 | 22142篇 |
地球物理 | 13500篇 |
地质学 | 37345篇 |
海洋学 | 16486篇 |
天文学 | 1102篇 |
综合类 | 4980篇 |
自然地理 | 3007篇 |
出版年
2024年 | 202篇 |
2023年 | 468篇 |
2022年 | 829篇 |
2021年 | 1136篇 |
2020年 | 2272篇 |
2019年 | 5615篇 |
2018年 | 5953篇 |
2017年 | 5598篇 |
2016年 | 5718篇 |
2015年 | 5046篇 |
2014年 | 4563篇 |
2013年 | 5084篇 |
2012年 | 4765篇 |
2011年 | 4557篇 |
2010年 | 4565篇 |
2009年 | 3900篇 |
2008年 | 3140篇 |
2007年 | 3153篇 |
2006年 | 2731篇 |
2005年 | 2638篇 |
2004年 | 2895篇 |
2003年 | 2616篇 |
2002年 | 2359篇 |
2001年 | 2161篇 |
2000年 | 1799篇 |
1999年 | 1883篇 |
1998年 | 1940篇 |
1997年 | 1979篇 |
1996年 | 1569篇 |
1995年 | 1487篇 |
1994年 | 1322篇 |
1993年 | 1286篇 |
1992年 | 1090篇 |
1991年 | 812篇 |
1990年 | 755篇 |
1989年 | 642篇 |
1988年 | 548篇 |
1987年 | 419篇 |
1986年 | 346篇 |
1985年 | 263篇 |
1984年 | 292篇 |
1983年 | 178篇 |
1982年 | 211篇 |
1981年 | 149篇 |
1980年 | 118篇 |
1979年 | 109篇 |
1978年 | 47篇 |
1977年 | 42篇 |
1971年 | 53篇 |
1970年 | 44篇 |
排序方式: 共有10000条查询结果,搜索用时 187 毫秒
81.
82.
Universal cokriging is used to obtain predictions when dealing with multivariate random functions. An important type of nonstationarity is defined in terms of multivariate random functions with increments which are stationary of orderk. The covariance between increments of different variables is modeled by means of the pseudo-cross-covariance function. Criteria are formulated to which the parameters of pseudo-cross-covariance functions must comply so as to ensure positive-definiteness. Cokriging equations and the induced cokriging equations are given. The study is illustrated by an example from soil science. 相似文献
83.
84.
85.
The purpose of this work was to reinvestigate the existing hydrogeological conceptual model of the basin of Madrid, Spain. A cumulative chemical isotopic diagram which enabled the distinction between different groups of water as well as calculation of the mode of their blending was used for this investigation. It was found that the groups of discharge were lighter in their isotopic composition than that of recharge. The previous explanation of this fact, backed by carbon-14 dating, was the long residence time due to flow lines going down to depths of more than 1000 m. This flow model assumes homogenous conditions to these depths. This assumption can not be supported by evidence from deep wells. Thus a modified model is suggested which maintains homogenous conditions only to about 300 m and a deep confined aquifer below containing paleowater. The higher degree of depletion of this water has been explained by a colder climate on top of an altitude effect. Another interesting observation was the correlation between the isotopic composition of the rains, the month of the rain event and the composition of the recharge group groundwater. It could be seen that the winter rains resemble the groundwater composition, which shows that practically all the spring and summer rains were evapotranspirated. 相似文献
86.
介绍一种自行设计和加工的直接雾化石墨炉进样装置的结构、工作原理和操作方法。该装置由微机控制与石墨炉加热程序同步工作,可以实现自动控制和自动测量。对雾化进样石墨炉分析的仪器参数和实验条件进行了研究和选择。结果表明,该装置与石墨炉结合具有自动化程度高、重现性好、样品利用率高和分析速度快等特点。 相似文献
87.
88.
89.
本文简述了国际天球参考架的发展历史和现在射电参考架的现状—基准源选择的标准和参考架的稳定性。描述了地面上光学观测在依巴谷参考架的维持和加密的一系列工作。介绍由天体测量卫星GAIA和SIM给出的天球参考架可能逵到的精度。详述了在今后十年中地面天体测量的作用以及正在开展有关天球参考架的研究课题 ,同时也列出了我国正在和即将开展天体测量的几个研究课题 相似文献
90.
Dolmatova L. S. Eliseykina M. G. Timchenko N. F. Kovaleva A. L. Shitkova O. A. 《中国海洋湖沼学报》2003,21(4):293-304
Pure fraction (92%–95%) of phagocytes (FP) and a mixture of amoebocytes (62%) and morula cells (38%)-FPMC- of the holothurianEupentacta fraudatrix' (Holothuroidea, Dendrochirota) were obtained by using ficoll-verographine step gradient. Basal production of reactive oxygen species (ROS) in FP quantified
by using reduction of nitroblue tetrazolium (NBT) was more than twice that in FPMC. Thermostable toxin ofYersinia pseudotuberculosis (TST) at different concentrations (0.2; 0.5; 2.5 μg/ml, but not 0.1 μg/ml) stimulated NBT reduction in FPMC after 24 h incubation.
In FP, TST at concentrations of 0.1 and 0.2 μg/ml inhibited and at concentrations of 0.5 and 2.5 μg/ml stimulated NBT reduction
after 24 h incubation. Maximal effect was observed in FP and FPMC at TST concentrations of 0.5 and 0.2 μg/ml, respectively.
Addition of catalase (0.7 μg/ml) to the cells treated with TST (2.5 μg/ml) was followed by a decrease in NBT reduction compared
to that under toxin treatment alone. TST stimulated superoxide dismutase activity in concentration-dependent manner (maximum
at 0.5 μg/ml concentration in FP) after 24 h treatment, and this stimulation was prevented by a commercial catalase. Plant
lectin concanavalin A stimulated NBT reduction more than 5-fold in FPMC compared to the control. With addition of TST, lectin
stimulated ROS to lesser extent than that with lectin alone. When catalase, TST, and lectin were added into the FPMC simultaneously,
ROS increase was similar to that under lectin treatment alone. On the whole, data obtained indicated that ROS generation in
holothurian coelomocytes especially occurs in both stimulated and not stimulated phagocytes, and that changes in ROS production
by these cells may be one of the mechanisms of antibacterial protection of holothurians.
This work was financially supported by the Russian Foundation for Fundamental Research Grant (No. 00-04-48949). 相似文献