全文获取类型
收费全文 | 2378篇 |
免费 | 67篇 |
国内免费 | 128篇 |
专业分类
系统科学 | 110篇 |
丛书文集 | 65篇 |
教育与普及 | 38篇 |
理论与方法论 | 11篇 |
现状及发展 | 19篇 |
研究方法 | 17篇 |
综合类 | 2313篇 |
出版年
2024年 | 6篇 |
2023年 | 26篇 |
2022年 | 41篇 |
2021年 | 55篇 |
2020年 | 32篇 |
2019年 | 12篇 |
2018年 | 23篇 |
2017年 | 32篇 |
2016年 | 35篇 |
2015年 | 46篇 |
2014年 | 89篇 |
2013年 | 63篇 |
2012年 | 67篇 |
2011年 | 72篇 |
2010年 | 90篇 |
2009年 | 85篇 |
2008年 | 98篇 |
2007年 | 104篇 |
2006年 | 73篇 |
2005年 | 83篇 |
2004年 | 63篇 |
2003年 | 103篇 |
2002年 | 150篇 |
2001年 | 132篇 |
2000年 | 92篇 |
1999年 | 150篇 |
1998年 | 76篇 |
1997年 | 103篇 |
1996年 | 97篇 |
1995年 | 77篇 |
1994年 | 70篇 |
1993年 | 59篇 |
1992年 | 57篇 |
1991年 | 52篇 |
1990年 | 50篇 |
1989年 | 37篇 |
1988年 | 39篇 |
1987年 | 21篇 |
1986年 | 8篇 |
1985年 | 1篇 |
1984年 | 2篇 |
1982年 | 2篇 |
排序方式: 共有2573条查询结果,搜索用时 109 毫秒
81.
B–Y modified silicide coatings were prepared on Nb–Si based alloy by pack cementation at 1300 ℃ for 10 h. The effect of Y_2O_3 content in the pack mixtures on microstructure and oxidation resistance of the coatings was investigated. The results show that the four coatings have similar structures, which possess a(Nb,X)Si_2 outer layer and a(Nb,X)_5Si_3 transitional layer. Y_2O_3 content in the pack mixtures has an obvious effect on the Si content in the coating. The mass gains of the coatings prepared with 0.5, 1, 2 and 3 wt% Y_2O_3 in pack mixtures are 2.33, 1.96, 2.05 and 2.86 mg/cm~2 after oxidation at 1250 ℃ for 100 h, respectively. The coating prepared with 1 wt% Y_2O_3 exhibits the best oxidation resistance due to the formation of a dense glass-like borosilicate scale. 相似文献
82.
Telomere dysfunction may result in chromosomal abnormalities, DNA damage responses, and even cancer. Early studies in lower organisms have helped to establish the crucial role of telomerase and telomeric proteins in maintaining telomere length and protecting telomere ends. In Oxytricha nova, telomere G-overhangs are protected by the TEBP-alpha/beta heterodimer. Human telomeres contain duplex telomeric repeats with 3' single-stranded G-overhangs, and may fold into a t-loop structure that helps to shield them from being recognized as DNA breaks. Additionally, the TEBP-alpha homologue, POT1, which binds telomeric single-stranded DNA (ssDNA), associates with multiple telomeric proteins (for example, TPP1, TIN2, TRF1, TRF2 and RAP1) to form the six-protein telosome/shelterin and other subcomplexes. These telomeric protein complexes in turn interact with diverse pathways to form the telomere interactome for telomere maintenance. However, the mechanisms by which the POT1-containing telosome communicates with telomerase to regulate telomeres remain to be elucidated. Here we demonstrate that TPP1 is a putative mammalian homologue of TEBP-beta and contains a predicted amino-terminal oligonucleotide/oligosaccharide binding (OB) fold. TPP1-POT1 association enhanced POT1 affinity for telomeric ssDNA. In addition, the TPP1 OB fold, as well as POT1-TPP1 binding, seemed critical for POT1-mediated telomere-length control and telomere-end protection in human cells. Disruption of POT1-TPP1 interaction by dominant negative TPP1 expression or RNA interference (RNAi) resulted in telomere-length alteration and DNA damage responses. Furthermore, we offer evidence that TPP1 associates with the telomerase in a TPP1-OB-fold-dependent manner, providing a physical link between telomerase and the telosome/shelterin complex. Our findings highlight the critical role of TPP1 in telomere maintenance, and support a yin-yang model in which TPP1 and POT1 function as a unit to protect human telomeres, by both positively and negatively regulating telomerase access to telomere DNA. 相似文献
83.
报告了在复合型纳米硫正极材料、纳米储锂合金负极材料和用原位合成工艺掺入纳米二氧化硅的凝
胶型聚合物电解质的研制方面所取得的进展;所研制的复合型纳米硫正极材料与凝胶电解质及锂金属负极配合
制成扣式实验电池进行测试,容量已达到700mAh桙g,发现该材料放电电压是现有锂钴氧材料放电电压的一半,
双电池串联可以与现有锂钴氧材料电池互换;采用微乳液新工艺合成的Cu-Sn纳米合金材料,以石墨与金属锡
复合的材料,以及以金属氧化物作为原料,采用乳液法制备碳微球镶嵌金属锡的球形复合材料等高容量负极材
料取得了较大的进展 相似文献
84.
85.
全光纤安防系统模式识别混合编程的实现 总被引:1,自引:0,他引:1
介绍了宽域全光纤监控预警系统及光纤扰动信号模式识别算法.利用小波包分解对信号进行分解与重构,并提取各频段内信号分量能量谱作为特征向量;应用综合考虑观测数据和确定性先验知识的BVC-RBF神经网络实现信号分类,提高了网络的泛化能力.利用Matlab引擎、动态链接库(DLL)、组件对象模型(COM)3种混合编程方法将Mat... 相似文献
86.
提出了一种基于软迭代的叠加训练序列信道估计方法.该方法将周期性训练序列与用户信息序列叠加后一起发送,通过软迭代方法,在接收端利用信道译码器输出的软信息来计算信息序列的一阶统计量,并将其反馈给信道估计器,用于消除观测量中来自未知信息的干扰,因此提高了信道的利用率和信道参数的估计精度.仿真结果表明,该方法有效地改善了信道估计性能,降低了接收机的误码率,提高了功率效率. 相似文献
87.
Yong Wei QiuGang Zong ZuYin Pu WeiXing Wan JianJun Liu SuiYan Fu QuanQi Shi 《科学通报(英文版)》2011,56(11):1117-1122
It is believed that a southward interplanetary magnetic field (IMF) is mainly responsible for the energy input from solar
wind into the magnetosphere. This paper presents an unusual case of strong anti-sunward plasma flow (up to 2 km/s) in the
polar cap ionosphere and large cross-polar cap potential (CPCP) during a period of horizontal IMF (|B
Z
| < 2 nT) observed by both ACE (at the L1 point) and Geotail (on the dusk flank of the magnetosheath). The CPCP is even higher
than that under preceding B
Z
≈ −23 nT. Furthermore, GOES8 observed that the magnetosheath field turns northward as the anti-sunward plasma flow and CPCP
start to increase, which implies that the magnetosheath field interacting with the Earth’s magnetopause has significantly
rotated and differs from the IMF observed by ACE and Geotail. In accordance with previous theoretical work, we suggest that
the magnetic field line draping produces a southward magnetosheath field and enhances anti-sunward plasma flow and the CPCP. 相似文献
88.
Angular dependent NEXAFS study of the molecular orientation of PTCDA multilayers on Au (111) surface
CAO Liang ZHANG WenHua HAN YuYan CHEN TieXin ZHENG ZhiYuan WAN Li XU FaQiang IBRAHIM Kurash QIAN HaiJie WANG JiaOu 《科学通报(英文版)》2011,56(33):3575-3577
The molecular orientation of perylene-3,4,9,10-tetracarboxylic acid dianhydride (PTCDA) multilayers adsorbed on Au (111) surface has been investigated using angular dependent O K-edge near edge X-ray absorption fine structure (NEXAFS) spectroscopy. The significant angular dependence of important resonant structures (π* and σ*) reveals that PTCDA molecules adopt an ordered geometry on the substrate surface. The average tilt angle of the PTCDA molecular planes is 27°±10° from the Au (111) surface. 相似文献
89.
研究了带转移机制且股票价格服从几何Levy过程的均值-方差投资组合选择模型。 用一个连续时间平稳马氏链表示市场所处的状态,文中主要参数,比如资产收益、Levy测度等均依赖于所处的市场状态。 分析了最优投资组合策略的存在性,用动态规划方法得到了最优投资组合策略、最优目标函数和有效前沿。 相似文献
90.