全文获取类型
收费全文 | 14430篇 |
免费 | 1361篇 |
国内免费 | 833篇 |
专业分类
化学 | 1099篇 |
晶体学 | 69篇 |
力学 | 948篇 |
综合类 | 150篇 |
数学 | 2060篇 |
物理学 | 2744篇 |
综合类 | 9554篇 |
出版年
2024年 | 73篇 |
2023年 | 198篇 |
2022年 | 356篇 |
2021年 | 414篇 |
2020年 | 421篇 |
2019年 | 371篇 |
2018年 | 358篇 |
2017年 | 465篇 |
2016年 | 456篇 |
2015年 | 532篇 |
2014年 | 875篇 |
2013年 | 922篇 |
2012年 | 893篇 |
2011年 | 975篇 |
2010年 | 680篇 |
2009年 | 786篇 |
2008年 | 817篇 |
2007年 | 887篇 |
2006年 | 742篇 |
2005年 | 685篇 |
2004年 | 607篇 |
2003年 | 604篇 |
2002年 | 502篇 |
2001年 | 438篇 |
2000年 | 351篇 |
1999年 | 336篇 |
1998年 | 259篇 |
1997年 | 267篇 |
1996年 | 237篇 |
1995年 | 183篇 |
1994年 | 156篇 |
1993年 | 121篇 |
1992年 | 134篇 |
1991年 | 124篇 |
1990年 | 104篇 |
1989年 | 72篇 |
1988年 | 75篇 |
1987年 | 51篇 |
1986年 | 26篇 |
1985年 | 18篇 |
1984年 | 4篇 |
1983年 | 8篇 |
1982年 | 9篇 |
1981年 | 3篇 |
1980年 | 3篇 |
1979年 | 8篇 |
1978年 | 3篇 |
1976年 | 3篇 |
1957年 | 3篇 |
1955年 | 3篇 |
排序方式: 共有10000条查询结果,搜索用时 31 毫秒
51.
52.
53.
采用随机域外奇点法对轴拉杆件进行了分析.考虑杨氏模量的不确定性,得到了不同相关类型、不同相关长度下的解析解,并分析了相关类型、相关长度以及随机场中点离散法对位移方差的影响. 相似文献
54.
55.
本文采用解析的方法计算了应变Si1-xGex层中p型杂质电离度与Ge组分x、温度T以及掺杂浓度N的关系.发现常温时,在同一Ge组分下,随着掺杂浓度的升高,杂质的电离度的先变小,而后又迅速上升到1.在同一掺杂浓度下,轻掺杂时,杂质的电离度随Ge组分的增加先变大,而后几乎不变;重掺杂时,杂质电离能变为0后,杂质电离度为1.低温下,轻掺杂时,载流子低温冻析效应较为明显,杂质的电离度普遍较小,当掺杂浓度大于Mott转换点时,载流子冻析效应不再明显,电离率迅速上升到1.在同一Ge组分下,随着掺杂浓度的升高,杂质的电离度先变小,后变大,而后又迅速上升到1.在同一掺杂浓度下,轻掺杂时,杂质的电离度随Ge组分的增加变大;重掺杂时,杂质电离能变为0后,杂质电离度为1. 相似文献
56.
57.
Purple Soil distributes extensively and mainly in China. Because of abundant easily weatherable parent rocks/materials and unstable soil structure, and also influenced by parent materials, usage systems, and slope gradients, erosion and sediment production of purple soils are very severe with main fashions of water erosion and gravitational erosion. Basing on observed data in small watersheds, rainfall erosivity,vegetation coverage, previous soil water content, flow and relating factors such as climate, topograph of small watershed,land usage, and soil kinds are all the influence factors of erosion and erodibility of purple soil as well as sediment production and transport in small watershed of purple hilly areas.The effective technological countermeasures of ecosystem restoration, agricultural tillage for water conservation and erosion prevention, agriculture project, and soil changing for fertility and anti-erosion were provided. 相似文献
58.
Considering a plane hyperbolic system with time-periodic coefficients, we construct a version of the direct Lyapunov method with the condition on the derivative of the Lyapunov functional along the trajectories of the system which is weakened by use of periodicity of the coefficients. We exhibit an illustrative example. 相似文献
59.
60.
Sarah Josefsson Rikard Westbom Lennart Mathiasson Erland Bjrklund 《Analytica chimica acta》2006,560(1-2):94-102
Pressurized liquid extractions were performed on eight sediments in order to investigate if a modified US EPA method (100 °C, 100 bar, n-heptane/acetone (1:1), 2 × 5 min) provided exhaustive extractions of polychlorinated biphenyls (PCBs) from sediment, and to study if the extractability of PCBs from the different sediments was affected by characteristics of the sediment. The recovery from the eight native sediments, contaminated in nature, was between 96.4% and 98.9%, as an average of the recoveries from 10 PCB congeners. Hundred percent recovery was defined as the sum of two consecutive extractions (2 × 5 min each) at the stated conditions. The recoveries of the individual congeners were above 94%, except for one congener in one sediment, which had a recovery of 92%. When the recoveries and different characteristics of the sediments were compared, no correlation appeared between recoveries and sediment PCB concentration, total organic carbon (TOC), soot carbon (SC) or amorphous carbon (AC). The fact that carbon did not influence the extractions was somewhat surprising, since previous experiments have indicated a connection. Instead, statistically significant (p < 0.05) correlations were observed for water content and carbon/nitrogen (C/N) ratio. The decrease in recoveries with decreased water content was attributed to less access of the solvent to the analytes due to less matrix swelling. The lowered recoveries with increased C/N ratio can indicate that a difference in structure of the organic matter exists, which influences the binding strength between the analytes and the matrix. The difference in structure can possibly be explained by different origin of the organic matter or by aging effects. Overall the method was found to be exhaustive and the excellent recoveries show that sediment characteristics do not influence the extractions markedly. 相似文献