全文获取类型
收费全文 | 4825篇 |
免费 | 1293篇 |
国内免费 | 1876篇 |
专业分类
测绘学 | 871篇 |
大气科学 | 692篇 |
地球物理 | 1078篇 |
地质学 | 3402篇 |
海洋学 | 784篇 |
天文学 | 80篇 |
综合类 | 441篇 |
自然地理 | 646篇 |
出版年
2024年 | 70篇 |
2023年 | 220篇 |
2022年 | 437篇 |
2021年 | 498篇 |
2020年 | 418篇 |
2019年 | 484篇 |
2018年 | 438篇 |
2017年 | 436篇 |
2016年 | 437篇 |
2015年 | 413篇 |
2014年 | 418篇 |
2013年 | 423篇 |
2012年 | 479篇 |
2011年 | 407篇 |
2010年 | 333篇 |
2009年 | 325篇 |
2008年 | 251篇 |
2007年 | 218篇 |
2006年 | 196篇 |
2005年 | 140篇 |
2004年 | 117篇 |
2003年 | 100篇 |
2002年 | 92篇 |
2001年 | 104篇 |
2000年 | 78篇 |
1999年 | 52篇 |
1998年 | 41篇 |
1997年 | 42篇 |
1996年 | 42篇 |
1995年 | 33篇 |
1994年 | 35篇 |
1993年 | 25篇 |
1992年 | 24篇 |
1991年 | 22篇 |
1990年 | 11篇 |
1989年 | 8篇 |
1988年 | 10篇 |
1987年 | 10篇 |
1986年 | 13篇 |
1985年 | 11篇 |
1984年 | 14篇 |
1983年 | 4篇 |
1982年 | 8篇 |
1981年 | 11篇 |
1980年 | 10篇 |
1977年 | 5篇 |
1976年 | 6篇 |
1974年 | 5篇 |
1973年 | 4篇 |
1954年 | 4篇 |
排序方式: 共有7994条查询结果,搜索用时 0 毫秒
71.
72.
73.
为了全面评价和考核湘江长沙段各洲滩近几年来采取的不同灭螺措施对洲滩钉螺(Oncomelania hu-pensis)螺情的影响,采用以系统抽样结合环境抽查的查螺方法,2003~2008年对湘江长沙段20个有螺洲滩进行了跟踪调查.结果表明,2008年与2003年相比,活螺洲滩数、有螺框出现率、钉螺平均密度和活螺率分别下降了40.00%,99.10%,99.74%,51.09%.经统计学处理,各洲滩有螺框出现率(Z=91.560 2,P<0.01)、活螺率(Z=65.133 2,P<0.01)不相同,但年度下降趋势明显.在20个有螺洲滩中,有感染性钉螺分布的洲滩5个,占有螺洲滩总数的25.00%;有感染性钉螺分布的洲滩逐年减少,感染性钉螺的平均密度和感染率逐年降低,2007~2008年所有洲滩无感染性钉螺分布.湘江长沙段有螺洲滩采取的各种防治措施中,药物与环改灭螺相结合的效果远好于单纯的药物灭螺.各种环改灭螺措施,又以抬洲降滩、翻耕垦种的灭螺效果更持久,更彻底. 相似文献
74.
75.
六盘山植物区系基本特征的初步分析 总被引:11,自引:2,他引:9
六盘山位于陕甘宁交接地区,属于半湿润气候向半干旱气候的过渡带,是黄土高原生物多样性最丰富的地区之一,有维管植物836种,隶属于93科,359属。本区属于泛北极植物区、中国-日本森林植物亚区、华北植物地区的黄土高原亚地区。全部种子植物可分为13个分布区类型和12个变型,以北温带分布类型为主,与其他植物区系联系广泛,但特有属较贫乏。六盘山与小陇山、太白山和中条山的属相似性系数较高,均达60%以上,表明几座山体处于相似的生物气候背景之下,地理隔离性不明显。与贺兰山的属相似性系数低于60%,因为贺兰山位于干旱的荒漠与半荒漠区,可能有地理隔离作用。 相似文献
76.
The discovery of Mercury's magnetosphere by Mariner 10 was surprising since the conventional view of regenerative planetary dynamos had been that the spin requirement would likely have been in excess of the observed spin rate of Mercury. Also internal fluid motions were not expected to be sufficiently large. This paper explores the alternative model of the formation of Mercury's magnetosphere via electromagnetic induction forced by the solar wind. It is shown, however, that the constraints are so severe as to limit severely the applicability of such a model. Although induction is easily observed on the Moon, the modification of the magnetic boundary condition associated with a plasma magnetosphere on Mercury rules out its formation via induction except for interplanetary driving fields which are decreasing in amplitude. That model is explored but retains the difficulty that induced magnetospheres tend to be of small radial and temporal extent compared to that inferred by Ness et al. for Mercury. 相似文献
77.
为探究不同悬沙浓度下潮滩粒度特征的变化,对黄河三角洲潮滩进行了物理模拟实验。使用采自研究区的潮滩表层沉积物,设定与实际潮滩尺寸按比例缩小的室内水槽以及波浪潮汐动力模拟,对不同悬沙浓度条件(0~14.9 kg/m3)下的模拟“潮滩”进行粒度分析。结果表明,随着悬沙浓度的减少,潮滩粒度特征发生了明显变化。随着含沙量的减少,整个潮滩的细颗粒沉积物分布范围减小,总体上沉积物呈现粗化特征,但颗粒较大的极粗粉砂的分布同样减少;在不同含沙量条件下,分选系数、偏度和峰度均值总体呈现出潮下带上部高、潮间带和潮下带下部低的特征,反映了模拟的潮间带上部和潮下带前缘的分选较潮滩中部更好的特征。含沙量的变化同样影响了模拟潮滩的微地貌形态,随着含沙量的降低,潮滩微地貌逐渐侵蚀破碎。潮滩物理模型实验有助于快速再现潮滩冲淤形态和变化特征,使特定条件下潮滩的变化具备可预见性,从而为人类有针对性地进行生产活动提供参考。 相似文献
78.
It is generally believed that the complexity and variability of the light curves of gamma-ray bursts (GRBs) are caused by the internal shocks, which would occur when a rapid shell catches up a slower one and collides with it. The electrons in the shock layer are heated by the shocks and radiate via the mechanisms of synchrotron radiation and inverse Compton scattering. Based on relativistic kinematics, a relation between the photon number of the emission from the rapidly moving shock layer and the number of the photons received by an observer is derived. Then, employing the angular spreading of the internal shock emission, the curve equation and profile of a single pulse are obtained, and the shape is typically in the shape of a fast rise and exponential decline. Furthermore, by using the model of the successive collisions of multiple shells under the condition of reasonable parameters, the observed light curves are fitted with a rather good effect. Therefore, by this means, more different types of light curves of GRBs can be explained. 相似文献
79.
In this work, we collected a sample of 69 TeV blazars from TeVCat, obtained their multiwavelength observations, and fitted their spectral energy distributions by using a second degree polynomial function. The structure parameters of synchrotron bumps for 68 blazars and those of inverseCompton bumps for 56 blazars were derived. Then, we conducted statistical analysis on the parameters(curvature, peak frequency, peak luminosity, bolometric luminosity and X/γ-ray spectral indexes).From our analysis and discussions, we can conclude the following:(1) There is a clear positive correlation between the synchrotron peak frequency, log ν_p~s, and the inverse-Compton peak frequency, log ν_p~(IC),and between the synchrotron peak luminosity, log ν_p~sL_(νp)~s, and the inverse-Compton peak luminosity,log ν_p~(IC)L_(νp)~(IC).(2) The correlation between the peak frequency and the curvature of synchrotron bump is clearly different from that of the inverse-Compton bump, which further indicates that there are different emission mechanisms between them.(3) There is a correlation between log νIC pand γ-ray spectral index, α_γ, for the TeV blazars: log ν_p~(IC)=-(4.59 ± 0.30)α_γ+(32.67 ± 0.59), which is consistent with previous work.(4) An "L-shape" relation is found between log ν_p~s and α_X for both TeV blazars and Fermi blazars. A significant correlation between log νs pand X-ray photon index(α_X) is found for the TeV blazars with high synchrotron peak frequency: log ν_p~s=-(3.20 ± 0.34)α_X+(24.33 ± 0.79),while the correlation is positive for low synchrotron peaked TeV sources.(5) In the αX-αγdiagram,there is also an "L-shape." The anti-correlation is consistent with results available in the literature, and we also find a positive correlation between them.(6) Inverse-Compton dominant sources have luminous bolometric luminosities. 相似文献
80.
本文基于Vening Meinesz区域均衡模型,通过试验不同参数计算Vening Meinesz均衡补偿深度,将其与CRUST1.0模型给出的莫霍面深度进行拟合,得到适应于天山及邻区的平均补偿深度、"地区性指标"以及区域补偿半径.结合地球重力场模型EIGEN-6C4与地形数据,利用球冠体积分方法进行地形效应、沉积层效应计算和均衡校正,得到了研究区的Vening Meinesz均衡重力异常.结果显示天山及邻区的均衡重力异常幅值在-110~120 mGal之间,表明了天山及周边盆地岩石圈所处于的均衡状态,同时揭示了研究区的壳幔密度分布特征.天山、塔里木盆地、准噶尔盆地等块体的地壳垂向形变可能部分地由均衡调整引起,且均衡调整趋势与地面形变测量结果相契合.通过对均衡重力异常成因的解释,从地壳均衡角度分析了该地区复杂的构造背景及其新生代以来的演化历程. 相似文献