全文获取类型
收费全文 | 9372篇 |
免费 | 1558篇 |
国内免费 | 898篇 |
专业分类
化学 | 5245篇 |
晶体学 | 41篇 |
力学 | 446篇 |
综合类 | 46篇 |
数学 | 920篇 |
物理学 | 2877篇 |
无线电 | 2253篇 |
出版年
2024年 | 41篇 |
2023年 | 250篇 |
2022年 | 281篇 |
2021年 | 380篇 |
2020年 | 419篇 |
2019年 | 337篇 |
2018年 | 295篇 |
2017年 | 297篇 |
2016年 | 401篇 |
2015年 | 466篇 |
2014年 | 517篇 |
2013年 | 675篇 |
2012年 | 813篇 |
2011年 | 858篇 |
2010年 | 581篇 |
2009年 | 541篇 |
2008年 | 574篇 |
2007年 | 532篇 |
2006年 | 505篇 |
2005年 | 414篇 |
2004年 | 318篇 |
2003年 | 215篇 |
2002年 | 231篇 |
2001年 | 163篇 |
2000年 | 172篇 |
1999年 | 194篇 |
1998年 | 169篇 |
1997年 | 187篇 |
1996年 | 170篇 |
1995年 | 157篇 |
1994年 | 130篇 |
1993年 | 124篇 |
1992年 | 93篇 |
1991年 | 77篇 |
1990年 | 68篇 |
1989年 | 39篇 |
1988年 | 37篇 |
1987年 | 34篇 |
1986年 | 15篇 |
1985年 | 26篇 |
1984年 | 11篇 |
1983年 | 7篇 |
1982年 | 4篇 |
1981年 | 6篇 |
1980年 | 1篇 |
1979年 | 2篇 |
1916年 | 1篇 |
排序方式: 共有10000条查询结果,搜索用时 15 毫秒
821.
针对复杂海陆环境中的无线信号传播预测问题,研究了适用于抛物方程的信号时延与到达角估计方法。将自由空间中抛物方程轴向波前信号视为本地副本信号,然后利用信号的自相关特性,将接收信号与副本信号进行互相关运算,最后通过相关函数的峰值检索,得到脉冲信号在复杂环境中传播的附加时延。采用数值算例,验证了该方法的正确性和有效性。此外,采用多重信号分类算法,由抛物方程构建接收阵列的协方差矩阵,并对其进行特征值分解,然后利用信号子空间和噪声子空间的正交性,实现复杂环境中的信号到达角估计。仿真结果表明,相比于传统的平面波谱方法,该方法具有更高的多径分辨率。基于上述方法,并结合数字地图,在典型的海陆环境中进行了仿真实验,分析了蒸发波导对脉冲信号传播时延和到达角的影响。 相似文献
822.
Ferroelectric behaviour of 30nm BaTiO3 ceramics prepared by high pressure assisted sintering 下载免费PDF全文
Dense nanocrystalline BaTiO3 ceramics with a homogeneous grain size of 30 nm was obtained by pressure assisted sintering. The ferroelectric behaviour of the ceramics was characterized by the dielectric peak at around 120 ℃, the P-E hysteresis loop and some ferroelectric domains. These experimental results indicate that the critical grain size for the disappearance of ferroelectricity in nanocrystalline BaTiO3 ceramics fabricated by pressure assisted sintering is below 30 nm. The ferroelectric property decreasing with decreasing grain size can be explained by the lowered tetragonality and the 'dilution' effect of grain boundaries. 相似文献
823.
824.
Liao J Sun X Cui X Yu K Zhu J Deng J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(11):2611-2615
An important synthon, tert-butanethiosulfinate (2), has been effectively resolved by forming molecular complexes with (R)-2,2'-dihydroxy-1,1'-binaphthyl (BINOL, 3) in high enantioselectivity (>99 % ee). The present procedure represents the first example of the resolution of thiosulfinate. The mechanism of chiral discrimination is discussed in terms of molecular recognition based on IR and Xray analyses of the diastereomeric complexes during the resolution. In the less-soluble complex, (R)-3 and (R)-2 self-assembled as a linear supramolecule; however, in the more-soluble complex, (R)-3 and (S)-2 formed a simple bimolecular complex by one stronger hydrogen bond. Hydrogen bonding is the major driving force for effective resolution. 相似文献
825.
826.
邓群 《宁波大学学报(理工版)》1997,(2)
将矩阵论中重要的Wielandt-Hoffman定理推广到实四元数体上,并给出它的几个应用。 相似文献
827.
Li4Ti5O12 (LTO)/carbon nanotubes (CNTs) composite material is synthesized based on a solid-state method by sand-milling, spray-drying and calcining at 850 ℃ under N2 flow. The LTO/CNTs samples with 1 wt% and 3 wt% weight ratio of CNTs addition and the pristine LTO sample are prepared. The rate performance and the thermal stability of these samples are investigated based on LiMn2O4 (LMO)/LTO full-cell. The results show that theweight ratio of CNTs addition has distinct effect on LTO performances. The composite materials of LTO composited CNTs have better performance at high-rate due to the intercalation enhancement by conductive network of CNTs. At second, the overcharging temperature response of the cell's surface with 1 wt% CNTs addition is the lowest. The particle size distribution is measured and the most uniform particles are obtained with 1 wt% CNTs addition. This trend could explain that the mediumquantity of CNTs is optimal to improve the heat and mass transfer and prevent the problems of crystallite growing interference and aggregation during the calcination process. 相似文献
828.
Deng S Mattner J Zang Z Bai L Teyton L Bendelac A Savage PB 《Organic & biomolecular chemistry》2011,9(22):7659-7662
Natural killer T (NKT) cells recognize glycolipids produced by Sphingomonas bacteria, and these glycolipids contain C6-oxidized sugars, either glucuronic acid or galacturonic acid, linked to ceramides. Glycolipids with gluco stereochemistry are the most prevalent. Multiple studies have demonstrated that galactosylceramides are more potent stimulators of NKT cells than their glucose isomers. To determine if this stereoselectivity is retained in the context of the C6-oxidized sugars found in bacterial glycolipids, we prepared two sets of gluco and galacto-glycolipids oxidized at their C6 positions and compared their NKT stimulatory properties. In the context of carboxylic acid groups at C6, gluco stereochemistry gave the more potent responses. We also prepared bacterial glycolipids containing more complex ceramide groups to determine if these chains impact NKT cell responses. 相似文献
829.
本文通过对新疆农业生产函数的分析。讨论了生产要素转移情况。得出:1 新疆的土地资源已经过度使用;2 劳动力资源潜力很大,其转移方向正从流向第三产业转向第二产业;3 新疆农业资金缺乏,部分农业资金被非农业侵占。 相似文献
830.