全文获取类型
收费全文 | 74266篇 |
免费 | 26221篇 |
国内免费 | 51850篇 |
专业分类
测绘学 | 11485篇 |
大气科学 | 24764篇 |
地球物理 | 18890篇 |
地质学 | 57179篇 |
海洋学 | 22119篇 |
天文学 | 1233篇 |
综合类 | 7474篇 |
自然地理 | 9193篇 |
出版年
2024年 | 534篇 |
2023年 | 1336篇 |
2022年 | 3394篇 |
2021年 | 4276篇 |
2020年 | 4340篇 |
2019年 | 7494篇 |
2018年 | 7115篇 |
2017年 | 6750篇 |
2016年 | 6766篇 |
2015年 | 7054篇 |
2014年 | 6377篇 |
2013年 | 7724篇 |
2012年 | 7992篇 |
2011年 | 7953篇 |
2010年 | 7911篇 |
2009年 | 6993篇 |
2008年 | 6506篇 |
2007年 | 6359篇 |
2006年 | 5805篇 |
2005年 | 5058篇 |
2004年 | 4625篇 |
2003年 | 3761篇 |
2002年 | 3569篇 |
2001年 | 3188篇 |
2000年 | 2664篇 |
1999年 | 1974篇 |
1998年 | 1779篇 |
1997年 | 1825篇 |
1996年 | 1399篇 |
1995年 | 1330篇 |
1994年 | 1189篇 |
1993年 | 1156篇 |
1992年 | 999篇 |
1991年 | 734篇 |
1990年 | 718篇 |
1989年 | 585篇 |
1988年 | 494篇 |
1987年 | 391篇 |
1986年 | 322篇 |
1985年 | 253篇 |
1984年 | 285篇 |
1983年 | 164篇 |
1982年 | 210篇 |
1981年 | 150篇 |
1980年 | 109篇 |
1979年 | 130篇 |
1978年 | 49篇 |
1977年 | 45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.
5个新发现的X选BL Lac天体 总被引:1,自引:0,他引:1
利用ROSATVLA方法筛选,从ROSATX射线源中选出了一批新的BLLac天体和类星体的候选体.1996年12月8日至17日,利用北京天文台2.16m望远镜和新从美国引进的OMR摄谱仪,对这批新候选体进行了光谱认证.经SUN工作站处理,又发现了5个新的X选BLLac天体.此外,还对去年作者在OMR引进之前发现的BLLac天体进行了检验,结果发现,去年发现的7个BLLac天体中,2322+343的CaIIH&K“Breakstrength”为26.9%,稍大于判据25%,因此,2322+343到底是BLLac天体还是射电星系还有待于进一步测光及偏振观测的验证. 相似文献
82.
ImODUcrroNTheVertchrateepidennisisinintirnatecontaCtwiththeenvironmentandre~environmentaladaptions.TheepidermisofterreStrialanbolsiskeratinedwhilethatofamPhibiousandaquaticfonnsisrelataltomucus-produCti0n.Theepidennisadsasafi-nalbatherbetweentheorganismanditsenvironmentandoneofitSrnainfunCtionsistoproteCttheorganismagainstextemalpathogens.lnadditiontoIneCanicalproedonofthebody,theepidertnisofamPhibiansandfishescansededinsandotherantibacterialpeptidesthathelpinthebonedefenSeoftheanhal(Su… 相似文献
83.
1 INTRODUCTIONCompact Symmetric Objects (CSOs) are powerful and compact sources (overall size <1 bpc) with lobe emission on both sides of the central engine. The small size of these sourcesis almost certainly to be attributed to the youth of the sources themselves (ages < 104 yr) andnot due to a dense coallning medium (Readhead 1996). The unification scenario assumes thatCSOs evolve into compact steep spectrum (CSS) sotirces and then into Fanaroff-Riley type 11objects (Fanti 1995). … 相似文献
84.
太阳线性无力场评注和快速傅氏分析法的应用 总被引:1,自引:0,他引:1
首先对30年来用无力场计算太阳活动区磁场方法作了一些评注,特别研究了线性无力场解中的几个误区,提出了准线性无力场方法并探讨了快速傅氏变换在线性无力场计算中的应用,以2000年9月15日NOAA 9165活动区磁场为例,应用3种不同的方法计算出磁力线走向,然后同紫金山天文台赣榆观测站观测的精细Hα图形对比。 相似文献
85.
Yanjun ZHANG Fenglong ZHANG Full QI Fuquan NI 《东北亚地学研究》2006,9(2):199-203
In view of the situation of excavation of open coal mine for the underground water disaster,we should carry out simulation studies for the numerical value of the water lowering project and improve the accuracy and the level of the water lowering project.On the basis of the hydrological geological conditions of certain open mine digging,a more reasonable seepage numerical model was built according to MODFLOW.It was simulated in advance that the process of the confined water level descending with the time,and combining with the actual observations to test the correctness of the model.The calculation showed that the results coincided well with the results of actual measurement.Based on this,different water lowering numerical simulations were built for the open coal mine digging.It could be simulated and forecast that the changes of the groundwater level in drainage process within and outside the mine pit,and it was quantitatively assessed that the possible water lowering result of the opencast water drainage process,which provide an important basis for the actual water lowering project and the possible project disposal. 相似文献
86.
87.
Axial surveys were performed in the two river tributaries of the Cochin estuary, SW India during November 1988. Surficial sediments were subjected to sequential chemical extractions to delineate five metal fractions, namely, exchangeable, carbonate bound, easily reducible, organic/sulfide bound, and residual. The results indicated selective accumulation of Mn and Ni in carbonate bound and organic/sulfide forms, along with marginal amounts of Co in the exchangeable fraction. Large portions of Fe and Cr occurred in the residual fraction, whereas composite fractionation of Zn species was noticed. The exchangeable fractions of Fe and Cr as well as of easily reducible cobalt were below detection limits. The levels of Cr and Zn indicate anthropogenic inputs in this estuary, whereas Co and Ni show regional contamination exceeding natural levels. The analytical speciation procedure helps to deduce the sedimental diagenetic processes in the estuarine environment. 相似文献
88.
89.
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
90.
南水北调西线工程(简称西线调水)是从长江上游引水至黄河上游的跨流域调水工程。其重要目标是开发西北,繁荣发展中西部经济和改良生态环境。 本世纪末;下世纪初,我国经济建设的重点将转向大西北,实现区域资源开拓的重点转移。显然这对振兴大西北,实现区域持续发展是一项重大的战略决策。 相似文献