全文获取类型
收费全文 | 2358篇 |
免费 | 116篇 |
国内免费 | 124篇 |
专业分类
系统科学 | 121篇 |
丛书文集 | 59篇 |
教育与普及 | 113篇 |
理论与方法论 | 5篇 |
现状及发展 | 25篇 |
研究方法 | 13篇 |
综合类 | 2262篇 |
出版年
2024年 | 19篇 |
2023年 | 45篇 |
2022年 | 66篇 |
2021年 | 77篇 |
2020年 | 39篇 |
2019年 | 28篇 |
2018年 | 36篇 |
2017年 | 27篇 |
2016年 | 31篇 |
2015年 | 95篇 |
2014年 | 83篇 |
2013年 | 84篇 |
2012年 | 93篇 |
2011年 | 122篇 |
2010年 | 150篇 |
2009年 | 111篇 |
2008年 | 129篇 |
2007年 | 132篇 |
2006年 | 89篇 |
2005年 | 110篇 |
2004年 | 77篇 |
2003年 | 69篇 |
2002年 | 79篇 |
2001年 | 51篇 |
2000年 | 73篇 |
1999年 | 81篇 |
1998年 | 82篇 |
1997年 | 89篇 |
1996年 | 87篇 |
1995年 | 50篇 |
1994年 | 56篇 |
1993年 | 49篇 |
1992年 | 47篇 |
1991年 | 37篇 |
1990年 | 23篇 |
1989年 | 27篇 |
1988年 | 24篇 |
1987年 | 15篇 |
1986年 | 2篇 |
1985年 | 3篇 |
1984年 | 1篇 |
1983年 | 1篇 |
1982年 | 2篇 |
1981年 | 1篇 |
1972年 | 1篇 |
1970年 | 2篇 |
1969年 | 2篇 |
1965年 | 1篇 |
排序方式: 共有2598条查询结果,搜索用时 46 毫秒
181.
Concentration level and distribution of polycyclic aromatic hydrocarbons in soil and grass around Mt. Qomolangma, China 总被引:2,自引:0,他引:2
WANG XiaoPing YAO TanDong CONG ZhiYuan YAN XinLiang KANG ShiChang ZHANG Yong 《科学通报(英文版)》2007,52(10):1405-1413
High mountains may serve as a condenser for persistent organic pollutants. In the present study, soil and grass samples from Mt. Qomolangma region, China were collected from 4600 to 5620 m a.s.I and were analyzed for polycyclic aromatic hydrocarbons (PAHs) to determine if they are concentrated at colder, more elevated sites and to evaluate their possible resources and fractionation. The total PAHs concentration in soil samples was 〈 600 ng g^-1, the critical value to differentiate PAHs levels in remote and urban regions. This implied the PAHs levels at Mt. Qomolangma are relatively low and what one might expect in such a remote region of the world. These low values may represent a soil background for mid-latitude Northern Hemisphere soils away from the direct influence of an anthropogenic source. As for the distribution pattern, the low molecular weight PAHs were prone to accumulate at higher altitude, while the high molecular weight PAHs inversely related or unrelated with elevation. Based on high concentration of phenanthrene at elevated sample site and the ratios of individual PAHs, we deduced that home-heating combustion and vehicle emission may result in the constitution trait of PAHs in this mountain region. Monsoon traveling over India, Pakistan and other countries with dense population may carry contaminant to Mt. Qomolangma region. 相似文献
182.
电泳显示双层微胶囊的制备 总被引:1,自引:0,他引:1
以尿素-甲醛原位聚合法与明胶-阿拉伯胶复凝聚法相结合制备了电泳显示双层微胶囊.考察了系统调节剂、搅拌速度、pH值和加醛方式对单层微胶囊粒径分布及表面形貌的影响,以及双层微胶囊与单层脲醛树脂微胶囊在干燥过程中的破损率.实验结果表明,以明胶为系统调节剂,pH值为3.0—3.5,采用滴加方式加入甲醛制备的单层脲醛树脂微胶囊,囊壁透明、光滑、球形度好;单层脲醛树脂微胶囊包覆明胶后制备的双层微胶囊壁材强度和韧性有显著提高. 相似文献
183.
184.
185.
186.
Xie Aihong Qin Dahe Ren Jiawen Qin Xiang Xiao Cunde Hou Shugui Kang Shichang Yang Xingguo Jiang Youyan 《自然科学进展(英文版)》2007,17(7):828-837
Mount Everest,the highest point on the Earth is often referred to as the earth's third pole as such the place is relatively inaccessible and little is known about its meteorology.In April 2005,an automatic weather station was installed at the mountain's North Col(6523 m a.s.l.).According to the observational 10-minute mean and daily records,the meteorological characteristics were analyzed. All the meteorological elements displayed obvious diurnal variations during May 1 to July 22,2005.The monthly variation of daily meteo- rological elements on Mount Everest coincided with that on Dingri,the closest routine meteorological station,with the high correlation co- efficients of 0.928,0.877,0.682,0.755,0.826 and 0.676(n=83,p<0.001)for mean temperature,minimum temperature,max- imum temperature,relative humidity,pressure and wind speed,respectively.Furthermore,the vertical mean gradient of temperature was above 0.6℃/100 m,especially for the daily maximum temperature.Most weather events on Mount Everest prominently appeared on the same day as those on Dingri,especially those from daily mean pressure,temperature and relative humidity with the cross-correlation coeffi- cients of 0.673,0.485 and 0.487(n=83,p<0.001),respectively.Some other weather events on Mount Everest lagged one-day be- hind those on Dingri.Furthermore,forecasting of the weather events on Mount Everest from pressure on Dingri was more reliable than those from the other meteorological elements.The conclusions are much important for research on meteorology and climate changes in the region. 相似文献
187.
Scanning for sisnatures of geosraphically restricted selection based on population genomics analysis 总被引:1,自引:0,他引:1
DENG LiBin TANG XiaoLi KANG Jian WANG QingYun ZENG ChangQing 《科学通报(英文版)》2007,52(19):2649-2656
Natural selection, as the driving force of human evolution, has direct impact on population differentiation. However, it is still unclear to what extent the genetic differentiation has been caused by natural selection. To explore this question, we performed a genome-wide scan with single nucleotide polymorphism (SNP) data from the International HapMap Project. Single locus FsTanalysis was applied to assess the frequency difference among populations in autosomes. Based on the empirical distribution of FsT, we identified 12669 SNPs correlating to population differentiation and 1853 candidate genes subjected to geographic restricted natural selection. Further interpretation of gene ontogeny revealed 121 categories of biological process with the enrichments of candidate genes. Our results suggest that natural selection may play an important role in human population differentiation. In addition, our analysis provides new clues as well as research methods for our understanding of population differentiation and natural selection. 相似文献
188.
YUAN YiZhong KANG HaiFeng SUN ZhenRong WANG ZuGeng 《科学通报(英文版)》2007,52(14):1897-1901
The thin film of a heptamethine cyanine chromophore HC was prepared by spin-coating technique. Its surface morphology and linear optical property were characterized by atomic force microscopy (AFM) and UV-visible absorption spectroscopy. The results show that HC molecules are arranged in a well-ordered H-aggregate type. The third-order nonlinear optical properties of the spin-coating film were also measured by degenerate four-wave mixing (DFWM) measurement. Enhanced third-order nonlinear susceptibility can be attributed to molecular aggregation effects, and the corresponding mechanism was dealt with by collective electronic oscillator (CEO) approach. 相似文献
189.
LI ZhiFeng LUE LingLing KANG JingWan LU XiaoQuan 《科学通报(英文版)》2007,52(15):2035-2041
The insertion reaction mechanism of CBr2 with CH3CH2O has been studied by using the B3LYP/6-311G(d) and CCSD(T)/6-311G(d) at single point. The geometries of reactions,transition state and products were completely optimized. All the transition state is verified by the vibrational analysis and the internal re-action coordinate (IRC) calculations. The results show that reaction (1) is the dominant reaction path,which proceeds via two steps: i) two reactants form an intermediate (IM1),which is an exothermal re-action of 8.62 kJ·mol?1 without energy barrier; ii) P1 is obtained via the TS1 and the H-shift,in which the energy barrier is 44.53 kJ·mol?1. The statistical thermodynamics and Eyring transition state theory with Wigner correction are used to study the thermodynamic and kinetic characters of this reaction in temperature range from 100 to 2200 K. The results show that the appropriate reaction temperature ranges from 200 to 1900 K at 1.0 atm,in which the reaction has a bigger spontaneity capability,equi-librium constant (K) and higher rate constant (k). 相似文献
190.