全文获取类型
收费全文 | 985篇 |
免费 | 34篇 |
国内免费 | 58篇 |
专业分类
系统科学 | 47篇 |
丛书文集 | 44篇 |
教育与普及 | 28篇 |
理论与方法论 | 3篇 |
现状及发展 | 6篇 |
综合类 | 949篇 |
出版年
2024年 | 7篇 |
2023年 | 18篇 |
2022年 | 23篇 |
2021年 | 21篇 |
2020年 | 19篇 |
2019年 | 5篇 |
2018年 | 16篇 |
2017年 | 15篇 |
2016年 | 10篇 |
2015年 | 22篇 |
2014年 | 31篇 |
2013年 | 41篇 |
2012年 | 28篇 |
2011年 | 39篇 |
2010年 | 41篇 |
2009年 | 50篇 |
2008年 | 37篇 |
2007年 | 51篇 |
2006年 | 44篇 |
2005年 | 45篇 |
2004年 | 26篇 |
2003年 | 32篇 |
2002年 | 44篇 |
2001年 | 50篇 |
2000年 | 46篇 |
1999年 | 55篇 |
1998年 | 36篇 |
1997年 | 37篇 |
1996年 | 34篇 |
1995年 | 33篇 |
1994年 | 31篇 |
1993年 | 22篇 |
1992年 | 17篇 |
1991年 | 11篇 |
1990年 | 10篇 |
1989年 | 11篇 |
1988年 | 12篇 |
1987年 | 5篇 |
1985年 | 2篇 |
排序方式: 共有1077条查询结果,搜索用时 31 毫秒
991.
毛蓼挥发油主要化学成分的研究 总被引:1,自引:0,他引:1
采用GC-MS-COMP联用方法,对毛蓼(Polygonum barbatumL.)全草通过超临界二氧化碳萃取(SFE-CO2)并加CHCl3作改性剂的挥发油成分进行发分离和鉴定。从中分出36个色谱峰,鉴定了33个化合物,其中,β-谷甾醇15.747%,桉叶油醇3.703%,维生素E类物质占3.603%。 相似文献
992.
Study on the Application of Standing Collar Pattern 总被引:1,自引:0,他引:1
The relations between the lining collar and the face collar in pattern design of standing collar by experiments and theoretical calculation is elucidated. The collar's shape and the fabric thickness, although with the same construction, influence the lining collar and the face collar, especially the back center of the lining and the two collar angles. With a proportional relation, the thicker the fabric is, the more the total variation is. It is supposed to have some practical value to the collar pattern design, as well as to the fashion CAD. 相似文献
993.
本文介绍了一种马铃薯淀粉生产设备--DFB-65-50-230淀粉泵的设计方法,为其它技术性能同类泵的研制提供了设计实例。 相似文献
994.
995.
Van der Pol方程是三极管电路的数学模型,利用著名的Poincare-Bendixon环域定理,给出了Van der Pol振子在正阻尼(μ>0)情形下稳定闭轨存在性的一种证明。 相似文献
996.
997.
利用李雅普诺夫方法给出了广义双线性系统的一个镇定条件,利用变结构控制方法给出了广义双线性系统的变结构控制器的设计方法,保证了闭环系统的渐近稳定. 相似文献
998.
999.
研究了氩气中圆柱形空心阴极放电的自脉冲现象.测试了空心阴极放电不同阶段的伏安特性及自脉冲频率和电流脉冲上升沿随放电平均电流、气压和电路电容的变化;讨论了自脉冲的形成机理. 结果表明,自脉冲现象出现在空心阴极放电的负阻特性阶段,并具有稳定的频率.自脉冲频率随平均电流的增加而线性增大,气压和孔径的乘积pD值对频率的影响与其大小有关,但电极间的并联电容不影响脉冲频率;电流脉冲上升沿几乎与平均电流、气压或并联电容无关. 实验结果表明,自脉冲现象是空心阴极本身的一种特征放电过程. 相似文献
1000.
SHANG Yi XIAO Jin MA LuLin WANG HaiYan QI ZengJun CHEN PeiDu LIU DaJun WANG XiuE 《科学通报(英文版)》2009,54(18):3249-3257
DON, as a virulence factor, plays an important role in the infection of Fusarium graminearum in wheat. The infection ability of F. graminearum depends on its capacity of producing DON. The production of DON by F. graminearum is significantly decreased in the wheat varieties with scab resistance. In this study, GeneChip analysis indicated that an EST encoding an ATP-binding cassette (ABC) transporter was up-regulated by 45 times in a wheat landrace Wangshuibai, which is resistant to DON accumulation. A pair of EST-derived primers were designed based on the EST sequence, and a clone was then isolated from a wheat genomic DNA TAC library. The TAC clone was sequenced using chromosome walking and gene prediction was conducted using Softberry. A cDNA clone of this gene was subsequently isolated from Wangshuibai induced by DON using gene-specific primers designed according to the untranslated sequence of the gene. The genome size of the gene is 7377 bp, consisting of 19 exons with coding sequences of 4308 bp. It encodes a protein with 1435 amino acid residues and the calculated molecular weight is about 161 kD. BLAST analysis indicated that the gene may belong to pleiotropic drug resistance (PDR) sub-family, and hence designated as TaPDR1 (Triticum aestivum pleiotropic drug resistance). TaPDR1 was located on chromosome 5A of wheat using nullisomic-tetrasomic lines of Chinese Spring. TaPDR1 was up-regulated by induction of both DON and F. graminearum. Expression patterns of TaPDR1 were different in wild-type Wangshuibai and the fast-neutron induced Wangshuibai mutant lacking FHB1, a major QTL of FHB resistance and DON resistance in chromosome arm 3BS. These results suggested that TaPDR1 might be a candidate gene responsible for DON accumulation resistance. The expression profile showed that TaPDR1 expression was neither induced by hormones typically involved in biotic stress, such as JA and SA, nor by abiotic stresses, such as heat, cold, wounding and NaCI. However, TaPDR1 expression was regulated by Al^3+ and [Ca^2+], indicating that [Ca^2+]1 might mediate the signal of TaPDR1 expression. 相似文献