全文获取类型
收费全文 | 4981篇 |
免费 | 195篇 |
国内免费 | 206篇 |
专业分类
系统科学 | 220篇 |
丛书文集 | 187篇 |
教育与普及 | 149篇 |
理论与方法论 | 9篇 |
现状及发展 | 25篇 |
研究方法 | 11篇 |
综合类 | 4781篇 |
出版年
2024年 | 21篇 |
2023年 | 69篇 |
2022年 | 75篇 |
2021年 | 105篇 |
2020年 | 59篇 |
2019年 | 16篇 |
2018年 | 33篇 |
2017年 | 49篇 |
2016年 | 40篇 |
2015年 | 113篇 |
2014年 | 169篇 |
2013年 | 122篇 |
2012年 | 149篇 |
2011年 | 205篇 |
2010年 | 155篇 |
2009年 | 213篇 |
2008年 | 216篇 |
2007年 | 229篇 |
2006年 | 166篇 |
2005年 | 197篇 |
2004年 | 127篇 |
2003年 | 124篇 |
2002年 | 130篇 |
2001年 | 150篇 |
2000年 | 168篇 |
1999年 | 288篇 |
1998年 | 244篇 |
1997年 | 293篇 |
1996年 | 235篇 |
1995年 | 217篇 |
1994年 | 180篇 |
1993年 | 184篇 |
1992年 | 132篇 |
1991年 | 114篇 |
1990年 | 115篇 |
1989年 | 96篇 |
1988年 | 89篇 |
1987年 | 56篇 |
1986年 | 28篇 |
1985年 | 10篇 |
1984年 | 1篇 |
排序方式: 共有5382条查询结果,搜索用时 15 毫秒
901.
Li Yuhua Chen Hui Guo Kunyuan Xie Yongmei Wang Feng Xie Kuangcheng Huang Jiangsheng Ren Daming 《科学通报(英文版)》2001,46(13):1107-1111
A live attenuated AraA- autotrophic mutant ofSalmonella typhimurium (SL3261) was used as carrier for eukaryotic expression vectors EGFPN1, pCMVmIL-12, pCMVhIL-12, pCMVmGM-CSF and pCMVhGM-CSF
and was administered orally to BALB/c and C57BL/6 mice. After 6 weeks, these mice were challenged with 4T1 and Lewis tumor
cells respectively. GFP expression and gene integration could be detected in mice’s livers, spleens, intestines, kidneys and
tumors. The serum level of cytokines increased significantly in treated mice, so did the ratio of CD
8
+
/CD
4
+
, which resulted in the tumor regression and prolongation of the survival time of those mice. These researches laid an experimental
foundation for the tumor gene therapy using live attenuated salmonella. 相似文献
902.
Lianzhi Li Aixin Song Yi Xie Zhongxian Huang Ellen de Waal Kolczak Urszula Gerard W. Canters 《科学通报(英文版)》2001,46(19):1608-1611
The key subunit Ⅱ of cytochrome c oxidase (CcO) contains a soluble binuclear copper center (CuA) domain. The CuA domain of Paracoccus versutus was cloned, expressed, purified and characterized. The gene encoding the CuA domain in pET11d vector was expressed in E. coli BL21 (DE3). The results showed that the CuA domain was expressed mostly in inclusion bodies and the CuA domain protein synthesized in E. coli cells represents approximately 10 percent of the total cellular proteins. Dissolved in urea, dialyzed and recombined with Cu+/Cu2+ and purified by the Q-sepharose fast flow anion-exchange column and Sephadex G-75 gel filtration column, the soluble purple-colored protein, which shows a single band in electrophoresis, was obtained. The UV-visible absorption spectrum of CuA domain showed that there are intense band at 478 nm and a shoulder peak at 530 nm, and two weak bands at 360 and 806 nm respectively, which can be assigned to the charge transfer and the interactions of obitals of Cu—S and Cu——Cu in the mixed-valence binuclear metal center (Cu2S2R2). The far-UV CD spectrum indicated that this domain is predominantly in β-sheet structure. The fluorescence spectra showed that its maximal excitation wavelength and maximal emission wavelength are at 280 and 345 nm, respectively. 相似文献
903.
The occurrence of the dominant C23 n-alkane derived from bryophyte was demonstrated to be related to the cold climate, based on the GC and GC/MS analyses on a 40-cm long peat core, in which this compound exponentially varies in abundance with the depth corresponding to the well-known second Little Ice Age. Both the variation of the chemical components of modern bryophyte resulted from climatic change and the ecological data about the biological growth influenced by climate shed light on the point that the variation of this compound in the peat was related to the cold climate. This was also evidenced by the previously-reported lacustrine data. In addition, the variations of the ratios of ketones/esters and C24 n-alkene/n-alkane (C24︰1/C24︰0) display the trend identical with that of organisms, implicating the replacement between bryophyte and monocotyledon occurring in the peat core. 相似文献
904.
Elastic wave velocity in rocks from Dabieshan and its constraints for lithospheric composition and crust-mantle recycling 总被引:1,自引:0,他引:1
ZHAO Zhidan ZHAO Zhidan Nicolas I.CHRISTENSEN ZHOU Wenge XIE Hongsen ZHANG Zeming 《自然科学进展(英文版)》2001,11(2):115-122
P-and S-wave velocities in eclogites and granulites from the Dabie ultrahigh pressures (UHP) meta morphic belt, China, were measured at room temperature under the hydrostatic pressures up to 1.0 GPa. The ultrahigh pressure eclogites had the highest densities (3.3 ~ 3.6 g.cm-3) , high velocities and the lowest anisotropy (1.4%c ~ 2.6 % ) . The lowest densities (2.8 ~ 3.1 g. cm-3 ) and the highest Poisson' s ratios (0.28 ~ 0.29) were found in gran ulites, whereas the strongest anisotropies (6. 1 % ~8.4% ) were found in the high-pressure (HP) eclogites. Compari son of the velocities in rocks with that observed in the deep seismic sounding profile crossing Dabieshan suggests that e clogites might exist in the lower crust of Dabieshan, but the quantity might be small. The upper mantle has very similar velocities as the UHP eclogites and serpentinizated/water-bearing dunite. The formation of eclogite represents the crust mantle recycling processes. Crustal material is delaminated and sinks into the mantle by way of eclogite, whereas only a small part of the eclogite could return to the crust. 相似文献
905.
906.
907.
谢秉川 《山东师范大学学报(自然科学版)》2001,16(2):142-145
研究了由超导体及正常导体构成的无规网络,在d维修下的临越行为,采用(i)有效介质伫似(EMA),(ii )多重分形近似两种方法,来确定渗流系统的临界指数M(β)和N(β)。 相似文献
908.
从N-S方程出发,考虑稀薄气体效应,推导出长微圆管道中压力驱动的可压缩气体流动的简化控制方程组;采用迭代方法求解此方程组,得到了流动的速度分布、密度分布、质量流量;并分析了影响流量的因素。 相似文献
909.
通过构造有界的平面三次系统,证实了(1)其有限奇点的5-4(5个奇点指标为+1,另4个奇点指标为-1),3-2,2-1,+1四种分布均可实现;(2)仅有一个指标为+1的有限奇点的有界三次系统至少有11种类型;(3)赤道附近轨线拓扑结构相同的有界三次系统它们有限奇点的分布可以有不同类型。 相似文献
910.
在我国现行的低电压配电网络中一般采用TT、TN配电方式,即三相四线制配电,为了使分布式发电机组与四线制低压电网相连接,需要选择一个合适的变流器拓扑结构。同时为了提高电网中的电能质量,分布式发电机组需要添加有源电力滤波器(APF)后再接入四线制低压电网中。本篇论文主要进行两种四线制拓扑结构——非分裂链路式四桥臂变流器和分裂链路式三桥臂变流器拓扑结构的性能进行对比分析。主要在控制的难易程度、电磁兼容性(EMC)、中点控制、直流侧母线电压利用率和电路复杂性等方面进行对比分析。本篇论文总结出分裂链路式三桥臂拓扑结构变流器主要性能要优于非分裂链路式四桥臂变流器,主要体现于在相同电路复杂性和直流母线利用率的参照标准下,分裂链路式三桥臂拓扑结构的变流器在电流控制方面更加简单,同时有较小的电磁兼容性问题。 相似文献