全文获取类型
收费全文 | 11584篇 |
免费 | 2524篇 |
国内免费 | 3417篇 |
专业分类
测绘学 | 1372篇 |
大气科学 | 2294篇 |
地球物理 | 2768篇 |
地质学 | 6428篇 |
海洋学 | 1640篇 |
天文学 | 345篇 |
综合类 | 1214篇 |
自然地理 | 1464篇 |
出版年
2024年 | 60篇 |
2023年 | 254篇 |
2022年 | 736篇 |
2021年 | 815篇 |
2020年 | 611篇 |
2019年 | 842篇 |
2018年 | 813篇 |
2017年 | 758篇 |
2016年 | 818篇 |
2015年 | 780篇 |
2014年 | 802篇 |
2013年 | 861篇 |
2012年 | 895篇 |
2011年 | 818篇 |
2010年 | 834篇 |
2009年 | 741篇 |
2008年 | 656篇 |
2007年 | 630篇 |
2006年 | 479篇 |
2005年 | 416篇 |
2004年 | 326篇 |
2003年 | 277篇 |
2002年 | 328篇 |
2001年 | 365篇 |
2000年 | 327篇 |
1999年 | 347篇 |
1998年 | 284篇 |
1997年 | 250篇 |
1996年 | 206篇 |
1995年 | 211篇 |
1994年 | 183篇 |
1993年 | 160篇 |
1992年 | 147篇 |
1991年 | 103篇 |
1990年 | 80篇 |
1989年 | 65篇 |
1988年 | 60篇 |
1987年 | 39篇 |
1986年 | 21篇 |
1985年 | 27篇 |
1984年 | 18篇 |
1983年 | 20篇 |
1982年 | 10篇 |
1981年 | 12篇 |
1980年 | 7篇 |
1979年 | 6篇 |
1978年 | 6篇 |
1977年 | 3篇 |
1958年 | 11篇 |
1957年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 250 毫秒
991.
992.
������������ģ�ļ��� 总被引:1,自引:0,他引:1
??????????????????(Runge??Kutta)????????PREM??????????????????λ??????????????????????????????200 s??????????????????ε??????????????????????????????????仯???????????????о??????λ???????е?仯?????????????:??????????????????????????????仯?????, 20???????????????μ????????????Dziewonski??Anderson??????, ?????????????0.25%????? 相似文献
993.
Three methods for calculation of geocenter motion are intruduced. 相似文献
994.
卫国 《地球科学与环境学报》1990,(3)
对每个奇素数p,我们提出了关于p的两个命题:(I)不存在正整数m和n,使代数曲线f(y)=y~3-(2m+1)y-2n的三个零点同时为p方有理整数;(I)不存在正整数m,使不定方程x~2+3y~2=4(2m+1)具有三组正整数解x_i,y_i(i=1,2,3),满足x_1=x_2+x_3,x_1~2+x_2~2+x_3~2=6(2m+1)。本文旨在证明(I)及(I)都与关于素数p的Fermat猜想等价。 相似文献
995.
DNA barcode assessment of Ceramiales (Rhodophyta) in the intertidal zone of the northwestern Yellow Sea 总被引:1,自引:0,他引:1
A total of 142 specimens of Ceramiales(Rhodophyta) were collected each month from October 2011 to November 2012 in the intertidal zone of the northwestern Yellow Sea. These specimens covered 21 species,14 genera,and four families. Cluster analyses show that the specimens had a high diversity for the three DNA markers,namely,partial large subunit r RNA gene(LSU),universal plastid amplicon(UPA),and partial mitochondrial cytochrome c oxidase subunit I gene(COI). No intraspecific divergence was found in our collection for these markers,except for a 1–3 bp divergence in the COI of Ceramium kondoi,Symphyocladia latiuscula,and Neosiphonia japonica. Because short DNA markers were used,the phylogenetic relationships of higher taxonomic levels were hard to evaluate with poor branch support. More than half species of our collection failed to find their matched sequences owing to shortage information of DNA barcodes for macroalgae in Gen Bank or BOLD(Barcode of Life Data) Systems. Three specimens were presumed as H eterosiphonia crispella by cluster analyses on DNA barcodes assisted by morphological identification,which was the first record in the investigated area,implying that it might be a cryptic or invasive species in the coastal area of northwestern Yellow Sea. In the neighbor-joining trees of all three DNA markers,H eterosiphonia japonica converged with D asya spp. and was distant from the other Heterosiphonia spp.,implying that H. japonica had affinities to the genus Dasya. The LSU and UPA markers amplified and sequenced easier than the COI marker across the Ceramiales species,but the COI had a higher ability to discriminate between species. 相似文献
996.
D. maculates is a kind of specialized Schizothoracinae fish has been locally listed as a protected animal in Xinjiang Province,China. Ili River located in north of Tianshan Mountain and Tarim River located in north of Qinghai-Tibetan Plateau were two main distribution areas of this fish. To investigate the genetic diversity and genetic structure of D. maculates,four populations from Tarim River system and two populations from Ili River system were collected in this study. A 570-bp sequence of the control region was obtained for 105 specimens. Twenty-four haplotypes were detected from six populations,only Kunes River population and Kashi River population shared haplotypes with each other. For all the populations examined,the haplotype diversity(h) was 0.904 8±0.012 6,nucleotide diversity(π) was 0.027 9±0.013 9,and the average number of pairwise nucleotide differences(k) was 15.878 3±7.139 1. The analysis of molecular variance(AMOVA) showed that 86.31% of the total genetic variation was apportioned among populations,and the variation within sampled populations was 13.69%. Genetic differences among sampled populations were highly significant. F st statistical test indicated that all populations were significantly divergent from each other(P 0.01). The largest F st value was between Yurungkash River population and Muzat River population,while the smallest F st value was between Kunes River population and Kashi River population. NJ phylogenetic tree of D-loop haplotypes revealed two main clades. The neutrality test and mismatch distribution analysis suggested that the fish had went through a recent population expansion. The uplift of Tianshan Mountain and movement of Qinghai-Tibetan Plateau might contribute to the wide genetic divergence of D. maculates in northwest China. 相似文献
997.
998.
999.
高密度采集福建前湖湾海岸剖面133个泥沙样品做粒度分析,其频率曲线显示多种粒度分布特征,预示多种沉积环境的变化。采集研究剖面周边海滩、河口浅滩、河口、滨海沼泽和海岸沙丘等已知环境的样品作为目标样本。将目标样本沉积参数平均粒径(Mz)、标准偏差(σ)、峰态(Kg)设为判别参数,利用Excel计算海岸剖面样品与目标样本参数的距离,筛选最小距离样品,归为已知目标样本的同类。据此,分析了海岸剖面蕴含的6个沉积环境变化阶段。利用剖面底部淤泥测年14C=(328 15±170)a BP,和剖面上部泥炭测年14C=(24 130±100)a BP,了解这一变化发生的年代。自(32 815±170)a BP以来,前湖湾经历了河口浅滩-河口、沙丘-河口-滨海沼泽-海滩-沙丘等海岸环境演变过程。 相似文献
1000.
泛素结合酶(UbcE2)是蛋白泛素化过程中所必需的酶,在泛素转移和底物的特异性识别方面发挥着重要的作用。本实验利用RACE技术克隆获得了全长为993bp的松江鲈泛素结合酶E2-D2基因cDNA序列(命名为TfUbcE2-D2),其开放阅读框为444bp,编码147个氨基酸。通过SMART预测得知,TfUbcE2-D2含有一个UBCc结构域。同源比对结果表明该基因与其他物种的同源性为92.31%。实时荧光定量PCR显示,TfUbcE2-D2广泛表达于松江鲈各组织,在肾脏中的相对表达量最高,其次为鳃。鳗弧菌(Vibrio anguillarum)刺激后,TfUbcE2-D2在血液、脾脏、肝脏和鳃中表达均上调,其中,脾脏和鳃中的表达量上调约40倍。由以上实验结果推测,TfUbcE2-D2可能参与松江鲈的先天免疫防御。 相似文献