全文获取类型
收费全文 | 24894篇 |
免费 | 717篇 |
国内免费 | 239篇 |
专业分类
系统科学 | 1453篇 |
丛书文集 | 384篇 |
教育与普及 | 77篇 |
理论与方法论 | 332篇 |
现状及发展 | 7921篇 |
研究方法 | 877篇 |
综合类 | 14277篇 |
自然研究 | 529篇 |
出版年
2018年 | 764篇 |
2017年 | 771篇 |
2016年 | 527篇 |
2015年 | 159篇 |
2013年 | 297篇 |
2012年 | 793篇 |
2011年 | 2004篇 |
2010年 | 1193篇 |
2009年 | 808篇 |
2008年 | 1074篇 |
2007年 | 1408篇 |
2006年 | 674篇 |
2005年 | 650篇 |
2004年 | 557篇 |
2003年 | 510篇 |
2002年 | 428篇 |
2001年 | 531篇 |
2000年 | 596篇 |
1999年 | 434篇 |
1992年 | 313篇 |
1991年 | 286篇 |
1990年 | 268篇 |
1989年 | 257篇 |
1988年 | 234篇 |
1987年 | 238篇 |
1986年 | 216篇 |
1985年 | 271篇 |
1984年 | 254篇 |
1983年 | 179篇 |
1982年 | 191篇 |
1981年 | 171篇 |
1980年 | 213篇 |
1979年 | 481篇 |
1978年 | 366篇 |
1977年 | 405篇 |
1976年 | 288篇 |
1975年 | 345篇 |
1974年 | 487篇 |
1973年 | 411篇 |
1972年 | 405篇 |
1971年 | 448篇 |
1970年 | 519篇 |
1969年 | 462篇 |
1968年 | 452篇 |
1967年 | 474篇 |
1966年 | 410篇 |
1965年 | 315篇 |
1958年 | 236篇 |
1957年 | 187篇 |
1955年 | 160篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
191.
DONGWei LIUJianhui PANYuerong 《科学通报(英文版)》2003,48(5):485-491
The Yuanmou Basin of Yunnan Province is a hotspot for the research of early homind evolution and its environmental background.During the implementation of the Program“Research of the Origin and Evolution of Early Man and Its Environmental Baqckground” from 1998 to 2001,a huge quantity of hominoid and mammal fossils were collected.The present report systematically describes an Euprox robustus sp.nov.identified during the recent laboratory work on the collected material.Euprox is a group of earliest cervids with true antlers.The new species is the third one of the genus discovered in China.Judged by its morphologic characteristics,the new species feed on juicy and tender leaves of dicotyledon.It implies that the vegetation of its epoch in the Yuanmou Basin is a kind of southern subtropical evergreen forest and the climate is humid and temperate with evident seasonality.The latter is mostly influenced by the monsoon and secondly by the latitude. 相似文献
192.
Inorganic chemical composition and source signature of PM2.5 in Beijing during ACE-Asia period 总被引:7,自引:0,他引:7
ZHANGRenjian XUYongfu HANZhiwei 《科学通报(英文版)》2003,48(10):1002-1005
Aerosol samples for PM2.5 were collected in Beijing for 38 consecutive days from March to April 2001 using an IMPROVE Sampler. Concentrations of 20 elements in PM2.5 were determined using a PIXE method. Results show that the average mineral dust concentration of PM2.5 was 14.6 Ilg/m3 during the observation period. On the sand-dust event days of March 21 and April 10, dust PM2.5 mass concentrations were 62.4 and 54.1 μg/m^3, respectively.These demonstrate that fine particle pollution by dust event in Beijing was very severe. The enrichment factors of S and Cu reached minimums on the dusty days and were high on the non-dusty days. It is considered that enrichment factors of elements in PM2.5, which are associated with human activities, can probably provide an effective method to distinguish local sources from external sources of dust. Factor analysis on the chemical composition in PM2.5 shows that sources of crustal matters, anthropogenic emission, and oil combustion contributed to PM2.5 levels in air in the springtime of 2001 in Beijing. 相似文献
193.
Progress in the study of tectophysicochemistry 总被引:1,自引:0,他引:1
LǖGuxian 《科学通报(英文版)》2003,48(11):1069-1079
Tectophysicochemistry is a research area that deals with the interrelations between physical and chemical changes of crustal materials caused by tectonism. A given tectonic force may be resolved into two parts: homogeneous stress and differential stress. Homogeneous stress refers to isotopic pressure, which is superimposed on the original pressure and affects the equilibrium of various chemical reactions. It is also a factor that influences petrogenesis,metallogenesis and metamorphism. Differential stress is commonly produced when external forces act on a given solid body. It may cause deformation of crustal materials and generate various kinds of structural features. Tectophysicochemistry pays special attention to pressure, temperature and other physicochemical conditions produced or induced by tectonism and studies the influences of additional tectonic parameters on various chemical equilibria, so it gradually develops into an independent area of science. 相似文献
194.
吕明玉 《渝西学院学报(自然科学版)》2003,2(4):65-67
本针对大学预科班学生的特点,从大学预科化学课程的基本目的和要求出发,讨论大学预科化学课程教学中应注意的问题. 相似文献
195.
首次采用第四统计力学群子理论对心血管中药中生命动力元素的分布规律进行了系统的研究 ,结果表明心血管中药的生命动力元素按原子序数分布呈现出如下四种分布规律,即不同中药有轻度元素分布、较轻度元素分布、较重度元素分布和重度元素分布 ,并且每一种分布根据R2 /R1的值又可分为两个分支。本文还首次阐述了群子统计参数R1、R2 、R2 /R1及R1·R2 在中药生命动力元素分布中的物理内涵。这些研究为进一步定量地研究中药的性味和药效之间的关系提供了必要的依据。 相似文献
196.
To ensure reliable land vehicle positioning, the Global Positioning System (GPS) is one of the best techniques commonly used in present-day positioning systems. However, GPS signals are not available if line of sight with the satellites is lost. In this respect, the addition of the Dead Reckoning (DR) method to complement the GPS unit would enhance the reliability of land vehicle positioning system. For implementing the DR method, the magnetic sensor and accelerometer are used for obtaining heading and velocity or distance information. Calibration of two sensors' dynamic model and DR algorithm are also introduced in this paper. The test result utilizing the GPS and DR methods is analyzed in this paper. It utilize spositioning information from GPS receiver when GPSsignal is available, otherwise DR is started to compensate GPS outage with the GPS output as the original point. The results showed that the error in east and north produced by the DR algorithm for a short time interval can be limitedwithin an acceptable range. 相似文献
197.
This paper introduces the in-car navigation system prototype development and test results under the context of Singapore. In the system, integrated GPS/DR is used for continuous vehicle positioning. A digital road network map database is built for the whole Singapore. The database is designed to support map matching, route determination, and route guidance functions. The prototype system demonstrates GPS positioning of the vehicle in real-time, tracking the vehicle onto a digital map with fix updates every one or two seconds.With the map matched vehicle location on the map, voice and/or text guidance is provide to the driver. The vehicle position can also be transferred to control centre through communication data link for the purpose of fleet management. 相似文献
198.
The Lattice-Boltzmann method is an effective tool for solving fluid mechanics problems, but there isn’t still a good scheme
to determinate some parameters in Boltzmann equations. In this paper, a technique using evolutionary algorithm to automatically
model Boltzmann equations is introduced. Numerical simulation shows that the designed scheme is fast and efficient.
Biography: Chen Ju-hua ( 1963-), male, Associate professor, research direction: Lattice-Boltzmann method, evolutionary algorithm. 相似文献
199.
WANG Yi CHU Wuyang SU Yanjing GAO Kewei & QIAO Lijie Department of Materials Physics University of Science & Technology Beijing Beijing China 《科学通报(英文版)》2003,48(12):1203-1206
It is known that some types of ceramics, like Al2O3 and ZrO2, undergo stress corrosion cracking (SCC) or static fatigue fracture under static loading, ever in a rela-tively mild environment, such as moist atmosphere or water at room temperature, which leads to a time-dependent fracture[1—4]. For a transmission electron microscopy specimen of YBa2Cu3O7-x ceramics superconductor, SCC induced by residual stress can occur in an atmosphere with low moisture content[5]. For a lead zirconate ti… 相似文献
200.
Do multiple cycles of aeolian deposit-pedogenesis exist in the reticulate red clay sections in southern China? 总被引:4,自引:0,他引:4
HU Xuefeng CHENG Tianfan & WU Hexin Department of Environmental Science Engineering Shanghai Uni-versity Shanghai China Correspondence should be addressed to Hu Xuefeng 《科学通报(英文版)》2003,48(12):1251-1258
Since the Quaternary scientists successfully dis-closed the paleoclimatic significance of the loess-paleo- sol sequence in the Loess Plateau, Northwest China, the loess-paleosol sequence has become well-known terres-trial materials revealing the multiple cyclic changes of Quaternary climates. As a carrier of paleoenvironmental information, it could compare beauty with the deep ocean deposit[8]. In recent years, however, the researchers began to put their sights to southern China. They try t… 相似文献