全文获取类型
收费全文 | 13773篇 |
免费 | 2871篇 |
国内免费 | 4391篇 |
专业分类
测绘学 | 2453篇 |
大气科学 | 1993篇 |
地球物理 | 2203篇 |
地质学 | 7782篇 |
海洋学 | 3120篇 |
天文学 | 485篇 |
综合类 | 1076篇 |
自然地理 | 1923篇 |
出版年
2024年 | 105篇 |
2023年 | 295篇 |
2022年 | 761篇 |
2021年 | 954篇 |
2020年 | 800篇 |
2019年 | 900篇 |
2018年 | 793篇 |
2017年 | 740篇 |
2016年 | 785篇 |
2015年 | 868篇 |
2014年 | 885篇 |
2013年 | 1126篇 |
2012年 | 1196篇 |
2011年 | 1178篇 |
2010年 | 1183篇 |
2009年 | 1112篇 |
2008年 | 1181篇 |
2007年 | 1062篇 |
2006年 | 1156篇 |
2005年 | 827篇 |
2004年 | 656篇 |
2003年 | 435篇 |
2002年 | 502篇 |
2001年 | 479篇 |
2000年 | 350篇 |
1999年 | 165篇 |
1998年 | 61篇 |
1997年 | 43篇 |
1996年 | 29篇 |
1995年 | 29篇 |
1994年 | 21篇 |
1993年 | 23篇 |
1992年 | 28篇 |
1991年 | 14篇 |
1990年 | 10篇 |
1989年 | 9篇 |
1988年 | 10篇 |
1987年 | 21篇 |
1986年 | 14篇 |
1985年 | 17篇 |
1984年 | 25篇 |
1983年 | 19篇 |
1982年 | 17篇 |
1981年 | 10篇 |
1980年 | 11篇 |
1979年 | 19篇 |
1971年 | 8篇 |
1970年 | 10篇 |
1969年 | 8篇 |
1954年 | 9篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
1.
试论新世纪地理信息科学的发展 总被引:1,自引:0,他引:1
本文重点分析了当前国内外地理信息科学的形成与发展趋势和地理信息科学不断发展的历程 ,提出近期发展的前沿 ,以及当前地理信息产业化面临的一些重要问题。最后提出对地理信息科学发展的意见。 相似文献
2.
TANGYa XIEJiasui SUNHui 《山地科学学报》2004,1(1):38-45
Dry valleys are a striking geographic landscape in Hengduan Mountains Region and are characterized by low rainfall, desert type of vegetation and fragile environment. Past efforts and resources have been concentrated mainly on rehabilitation of degraded ecosystem and fragile environment, particularly reforestation, while socio-economic development has been largely overlooked. Despite successes in pocket areas, the overall trend of unsustainability and environmental deterioration are continuing. It is important to understand that uplift of the Tibetan Plateau is the root cause of development of dry valleys, and development and formation of dry valleys is a natural process. Human intervention has played a secondary role in development of dry valleys and degradation of dry valleys though human intervention in many cases has speeded up environmental degradation of the dry valleys. It is important to understand that dry valleys are climatic enclaves and an integrated approach that combines rehabilitation of degraded ecosystems and socio-economic development should be adopted if the overall goal of sustainable development of dry valleys is to be achieved. Promotion of niche-based cash crops, rural energy including hydropower, solar energy, biogas and fuelwood plantation is recommended as the priority activities. 相似文献
3.
广西海洋资源丰富 ,品种繁多 ,为了更好开发 ,建议从法制、宣传教育、普及海洋资源有关知识 ,提高人们整体素质做起 ,进一步抓住机遇 ,合理开发 ,才能取得良好效益 ,达到预定目标 相似文献
4.
双酶法制备低黏度辛烯基琥珀酸淀粉钠的工艺条件 总被引:4,自引:0,他引:4
对双酶法制备用作微胶囊壁材的低黏度辛烯基琥珀酸淀粉钠的工艺路线与工艺条件进行了研究。正交实验结果表明 ,在酯化淀粉乳质量浓度 2 0 %、每 g淀粉α 淀粉酶的用量为 6SKBU的条件下 ,影响产品性能的各因素中 ,糖化酶的用量对产品的DE值和流度的影响最大 ,各因素的重要性依次是 :糖化酶的用量、糖化酶作用时间、α 淀粉酶保温作用时间。最佳工艺条件是 :α 淀粉酶保温作用时间 4min ,每 g淀粉糖化酶用量 0 .2GAU ,糖化酶作用时间 2 .5h ,此时产品的DE值为 30 ,流度为 80左右 相似文献
5.
在GPS数据处理中 ,存在着误差影响、影响波的干扰、周跳和数据量大等问题。误差影响和影响波的干扰实质是在接收卫星信号时受到其它因素的影响 ;周跳是由于卫星信号的失锁而造成信号的不连续 ;数据量大是因为GPS观测需要采样间隔小又连续观测所致。由于小波理论具有时频分析、波形分解、特征提取和快速小波变换等特性 ,应用小波变换和波形分解可以解决误差影响和影响波的干扰的问题 ;应用特征提取可以解决周跳检测问题 ;应用快速小波变换可进行数据压缩 相似文献
6.
网络时代的到来,对传统企业的组织结构、运作模式、管理方法均提出了严峻的挑战。传统的企业组织结构是适应大工业生产而形成的等级组织结构形式,它有很大的局限性;电子商务企业的组织结构是适应网络社会而形成扁平化、双向多对多、以信息为基础的新型组织结构形式。电子商务企业以其组织结构扁平化和中空化,决策分散化,运作虚拟化,灵活高效为特点,它将成为传统企业在电子商务发展阶段变革的方向和目标。 相似文献
7.
The Siberian Solar Radio Telescope: the current state of the instrument,observations, and data 总被引:2,自引:0,他引:2
Grechnev V.V. Lesovoi S.V. Smolkov G. Ya. Krissinel B.B. Zandanov V.G. Altyntsev A.T. Kardapolova N.N. Sergeev R.Y. Uralov A.M. Maksimov V.P. Lubyshev B.I. 《Solar physics》2003,216(1-2):239-272
The Siberian Solar Radio Telescope (SSRT) is one of the world's largest solar radio heliographs. It commenced operation in
1983, and since then has undergone several upgrades. The operating frequency of the SSRT is 5.7 GHz. Since 1992 the instrument
has had the capability to make one-dimensional scans with a high time resolution of 56 ms and an angular resolution of 15 arc sec.
Making one of these scans now takes 14 ms. In 1996 the capability was added to make full, two-dimensional images of the solar
disk. The SSRT is now capable of obtaining images with an angular resolution of 21 arc sec every 2 min. In this paper we describe
the main features and operation of the instrument, particularly emphasizing issues pertaining to the imaging process and factors
limiting data quality. Some of the data processing and analysis techniques are discussed. We present examples of full-disk
solar images of the quiet Sun, recorded near solar activity minimum, and images of specific structures: plages, coronal bright
points, filaments and prominences, and coronal holes. We also present some observations of dynamic phenomena, such as eruptive
prominences and solar flares, which illustrate the high-time-resolution observations that can be done with this instrument.
We compare SSRT observations at 5.7 GHz, including computed `light curves', both morphologically and quantatively, with observations
made in other spectral domains, such as 17 GHz radio images, Hα filtergrams and magnetograms, extreme-ultraviolet and X-ray
observations, and dynamic radio spectra. 相似文献
8.
在目前常用的周跳探测与修复方法基础上 ,提出了首先将观测资料按照观测历元不连续分成若干小弧段 ,然后利用差分法进行周跳探测 ,根据差分后周跳放大的特性判断周跳和野值 ,并确定其位置利用宽带组合和电离层组合的方法解算周跳大小。通过实例验证了其有效性。 相似文献
9.
Abstract— Hydrogen isotope ratios of organic compounds in carbonaceous chondrites provide critical information about their origins and evolutionary history. However, because many of these compounds are obtained by aqueous extraction, the degree of hydrogen‐deuterium (H/D) exchange that occurs during the process needs to be quantitatively evaluated. This study uses compound‐specific hydrogen isotopic analysis to quantify the H/D exchange during aqueous extraction. Three common meteoritic dicarboxylic acids (succinic, glutaric, and 2‐methyl glutaric acids) were refluxed under conditions simulating the extraction process. Changes in δD values of the dicarboxylic acids were measured following the reflux experiments. A pseudo‐first order rate law was used to model the H/D exchange rates which were then used to calculate the isotope exchange resulting from aqueous extraction. The degree of H/D exchange varies as a result of differences in molecular structure, the alkalinity of the extraction solution and presence/absence of meteorite powder. However, our model indicates that succinic, glutaric, and 2‐methyl glutaric acids with a δD of 1800%***o would experience isotope changes of 38o, 10o, and 6o, respectively during the extraction process. Therefore, the overall change in δD values of the dicarboxylic acids during the aqueous extraction process is negligible. We also demonstrate that H/D exchange occurs on the chiral α‐carbon in 2‐methyl glutaric acid. The results suggest that the racemic mixture of 2‐methyl glutaric acid in the Tagish Lake meteorite could result from post‐synthesis aqueous alteration. The approach employed in this study can also be used to quantify H/D exchange for other important meteoritic compounds such as amino acids. 相似文献
10.
织金矿区含煤地层为上二叠统龙潭组,含主要可采煤6层.煤系地层以含玄武岩屑、海绿石为特征的5类碎屑岩夹碳酸盐岩,水平、沙纹、波状层理发育,煤系中稳定的灰岩、硅铁质灰岩标志层较多,有利于煤层的划分与对比,并产丰富的动植物化石及碎块.属海、陆交替环境下的标型海陆交互相沉积. 相似文献