全文获取类型
收费全文 | 388217篇 |
免费 | 81137篇 |
国内免费 | 102358篇 |
专业分类
测绘学 | 64850篇 |
大气科学 | 74734篇 |
地球物理 | 68280篇 |
地质学 | 199243篇 |
海洋学 | 66987篇 |
天文学 | 7018篇 |
综合类 | 40355篇 |
自然地理 | 50245篇 |
出版年
2024年 | 4504篇 |
2023年 | 15821篇 |
2022年 | 19215篇 |
2021年 | 20335篇 |
2020年 | 15841篇 |
2019年 | 22792篇 |
2018年 | 21606篇 |
2017年 | 14483篇 |
2016年 | 15941篇 |
2015年 | 19607篇 |
2014年 | 26566篇 |
2013年 | 24322篇 |
2012年 | 25981篇 |
2011年 | 26569篇 |
2010年 | 25541篇 |
2009年 | 24928篇 |
2008年 | 24500篇 |
2007年 | 22725篇 |
2006年 | 21826篇 |
2005年 | 18878篇 |
2004年 | 17730篇 |
2003年 | 16298篇 |
2002年 | 14519篇 |
2001年 | 13539篇 |
2000年 | 11161篇 |
1999年 | 8977篇 |
1998年 | 6487篇 |
1997年 | 5681篇 |
1996年 | 5216篇 |
1995年 | 4903篇 |
1994年 | 4580篇 |
1993年 | 4377篇 |
1992年 | 4367篇 |
1991年 | 3927篇 |
1990年 | 3694篇 |
1989年 | 3863篇 |
1988年 | 3932篇 |
1987年 | 3822篇 |
1986年 | 3246篇 |
1985年 | 3306篇 |
1984年 | 2969篇 |
1983年 | 2781篇 |
1982年 | 2301篇 |
1981年 | 1725篇 |
1980年 | 1283篇 |
1979年 | 806篇 |
1978年 | 399篇 |
1959年 | 321篇 |
1958年 | 345篇 |
1957年 | 326篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
791.
From elements formed in interstellar furnaces to humans peering back at the stars, the evolution of life has been a long, intricate and perhaps inevitable process. Life as we know it requires a planet orbiting a star at just the right distance so that water can exist in liquid form. It needs a rich supply of chemicals and energy sources. On Earth, the combination of chemistry and energy generated molecules that evolved ways of replicating themselves and of passing information from one generation to the next. Thus, the thread of life began. This chart traces the thread, maintained by DNA molecules for much of its history, as it weaves its way through the primitive oceans, gaining strength and diversity along the way. Organisms eventually moved onto the land, where advanced forms, including humans, ultimately arose. Finally, assisted by a technology of its own making, life has reached back out into space to understand its own origins, to expand into new realms, and to seek other living threads in the cosmos. 相似文献
792.
Theoretical rotational excitation rates were computed for C3H2 in collisions with He atoms at temperatures from 30 to 120 K. The intermolecular forces were obtained from accurate self-consistent field and perturbation theory calculations, and collision dynamics were treated within the infinite-order sudden approximation. The accuracy of the latter was examined by comparing with the more exact coupled states approximation. 相似文献
793.
Chandna RC Sharma D 《Population geography : a journal of the Association of Population Geographers of India》1987,9(1-2):45-55
Using a sample of 885 females (1/2 of all married women of reproductive age), this study examines the role of education as a determinant of fertility among couples in Kullu town of Himachol Pradesh, India. Of the 885 respondents, only 149 were illiterate. The average family size was 2.88. Findings reveal that 1) the average family size was largest for illiterate respondents (3.57) and for illiterate males (3.76); 2) average family size declined consistently after the middle level of education to 1.29 for post graduate respondents and 2.33 for post graduate males; 3) a negative correlation exists between fertility and education of both husband and wife, with the wife's education having a stronger negative correlation with fertility; and 4) couples with an educational level of matriculation and above have a distinctly smaller family size than those less educated. 相似文献
794.
795.
利用降水平均递增率求山地最大降水高度 总被引:6,自引:0,他引:6
本文引入降水平均递增率的概念,改进了文献中的计算方法,可以一次求出山地降水量与海拔的经验关系式中全部参数的最佳值,其中包括山地最大降水高度。并用实例说明了本法的步骤及特点。 相似文献
796.
This article deals with the interaction of zinc with δ-MnO2,γ-MnOOH and manganite existing in natural water systems. The mechanism of the reaction has been studied in detail. From the fact that the "ratio of ion exchange (%)-pH" graph is an "S shaped" curve, it is possible to deduce that the chemical reaction is of the nature of cation exchange. And since the pH range of ion exchange = 4, it is possible to further deduce that the reaction can be explained by the mechanism of monovalence cation exchange.The main result of this article is the discovery of a new type of isotherm which has not been mentioned in previous literature here and abroad. This isotherm cannot be represented by any presently available adsorption isotherm equations in marine chemistry. The characteristics of this new type of isotherm are as follows: the isotherm has two "knees" and three "plateaus", the heights of these three "plateaus" are in the ratio 1:2:3. In order to explain theoretically our new isotherm, this article suggest 相似文献
797.
798.
造山带中富集型上地幔的成因——以萨尔托海蛇绿岩块为例 总被引:1,自引:0,他引:1
发育于造山带中的蛇绿岩,其剖面下部的地幔橄榄岩部分是造山带地区富集型上地幔的直接标本。其地球化学特点是:主要元素Al_2O_3、TiO_2、CaO、Na_2O、K_2O强烈亏损,而REE,痕量元素和87Sr/86Sr则强烈富集;同时,143Nd/144Nd<0.511836,亦表明它们属于一个富集型的源区。 形成富集型上地幔的主要机制是地幔交代作用,富含不相容元素的低熔岩浆和富Ca-LREE流体与已亏损的地幔橄榄岩发生脉状交代和渗透交代反应,从而造成上地幔中不相容元素的富集。造山带富集型上地幔形成的构造环境是:洋壳从扩张脊向两侧运移并最终拼入造山带这段时间内。富集型上地幔不但存在于大陆区,而且亦存在于造山带地区,它可能是一种全球性的地球内部的化学作用。 相似文献
799.
作者用透射电子显微镜研究了沉积钙质白云石中的带状面缺陷。在衍衬象中面缺陷呈现出特有的条纹衬度,并具有一定的择优取向,它们沿着{018}面排列。面缺陷也可分叉,分枝部分平等于{2-/10}面。根据白云石面缺陷的取向,本文提出了鞍状白云石宏观弯曲面和微观面缺陷的取向关系。 相似文献
800.
本文系统描述了中国南部二叠、三叠纪“混生生物群”。根据笔者的统计,目前巳经发现有“二叠纪型”分子36属53种与三叠纪菊石、双壳类、牙形剌共生。 笔者认为中国南部可以以牙形剌Anchignathodus parvus 的出现作为三叠系的底界,双壳类Pteria ussurica variabilis,菊石 Hypophiceras的出现可以作为三叠系开始的辅助标志。文中还将这一界线与世界上其它地区的二叠、三叠系界线进行了对比。 相似文献