全文获取类型
收费全文 | 6558篇 |
免费 | 1615篇 |
国内免费 | 787篇 |
专业分类
化学 | 429篇 |
晶体学 | 40篇 |
力学 | 130篇 |
综合类 | 51篇 |
数学 | 414篇 |
物理学 | 2026篇 |
无线电 | 5870篇 |
出版年
2024年 | 11篇 |
2023年 | 78篇 |
2022年 | 124篇 |
2021年 | 173篇 |
2020年 | 209篇 |
2019年 | 161篇 |
2018年 | 182篇 |
2017年 | 271篇 |
2016年 | 344篇 |
2015年 | 386篇 |
2014年 | 582篇 |
2013年 | 580篇 |
2012年 | 575篇 |
2011年 | 628篇 |
2010年 | 486篇 |
2009年 | 432篇 |
2008年 | 534篇 |
2007年 | 540篇 |
2006年 | 500篇 |
2005年 | 399篇 |
2004年 | 289篇 |
2003年 | 274篇 |
2002年 | 211篇 |
2001年 | 165篇 |
2000年 | 157篇 |
1999年 | 115篇 |
1998年 | 106篇 |
1997年 | 72篇 |
1996年 | 75篇 |
1995年 | 63篇 |
1994年 | 40篇 |
1993年 | 48篇 |
1992年 | 25篇 |
1991年 | 32篇 |
1990年 | 12篇 |
1989年 | 14篇 |
1988年 | 17篇 |
1987年 | 10篇 |
1986年 | 5篇 |
1985年 | 5篇 |
1984年 | 11篇 |
1983年 | 2篇 |
1982年 | 4篇 |
1981年 | 4篇 |
1979年 | 1篇 |
1978年 | 1篇 |
1977年 | 2篇 |
1976年 | 2篇 |
1975年 | 1篇 |
1973年 | 1篇 |
排序方式: 共有8960条查询结果,搜索用时 125 毫秒
121.
122.
《Surface and interface analysis : SIA》2003,35(5):445-449
Surface states of polydimethylsiloxane (PDMS) treated by plasma were investigated by x‐ray photoelectron spectroscopy and surface voltage decay. X‐ray photoelectron spectroscopy confirmed the formation of a silica‐like (SiOx, x = 3–4) oxidative surface layer. This layer increased in thickness with increasing exposure duration of plasma. Plasma exposure lowers the surface resistivity from 1.78 × 1014 to 1.09 × 1013 Ω □?1 with increasing plasma treatment time. By measuring the decay time constant of surface voltage, the calculated surface resistivity was compared with the value measured directly by a voltage–current method; good agreement between the two methods was obtained. It was observed that plasma treatment led to a decrease in the thermal activation energy of the surface conduction from 31.0 kJ mol?1 for an untreated specimen to 21.8 kJ mol?1 for a plasma‐treated specimen for 1 h. Our results allow the examination of effects of plasma on the electrical properties of PDMS. Copyright © 2003 John Wiley & Sons, Ltd. 相似文献
123.
《Arabian Journal of Chemistry》2022,15(7):103933
In this study, a fingerprint-activity relationship modeling between chemical fingerprints and antirheumatic activity was established, and multivariate statistical analysis was used to evaluate the quality of Taxilli Herba (TH) from different hosts. Characteristic fingerprints of 20 batches of TH samples were generated by high-performance liquid chromatography coupled with triple quadrupole-time of flight tandem mass spectrometry (HPLC-Triple TOF-MS/MS), and the similarity analysis was calculated based on thirteen common characteristic peaks by hierarchical clustering analysis (HCA). Subsequently, nine efficacy markers were discovered by combining fingerprints and antirheumatic activity through grey correlation analysis (GCA) and bivariate correlation analysis (BCA). Meanwhile, the content of 5 constituents in 9 markers was determined by high-performance liquid chromatography coupled with triple quadrupole-linear ion trap tandem mass spectrometry (HPLC-QTRAP-MS/MS). The comprehensive quality of TH was assessed using multivariate statistical analysis, including principal components analysis (PCA) and technique for order preference by similarity to ideal solution (TOPSIS). The results showed that a high dose of TH extract could markedly ameliorate arthritis damage compared to other doses, with flavonoids playing an important role in the antirheumatic activity. The comprehensive quality of samples from Morus alba L. (SS) was superior to those from Liquidambar formosana Hance (FXS). The present study will demonstrate the markers associated with efficacy, and provide an applicable strategy for more comprehensive quality control and evaluation of TH. 相似文献
124.
Electron ionization (EI) mass spectra of 46 compounds from several different compound classes were measured. Their molecular ion abundances were compared as obtained with 70‐eV EI, with low eV EI (such as 14 eV), and with EI mass spectra of vibrationally cold molecules in supersonic molecular beams (Cold EI). We further compared these mass spectra in their National Institute of Standards and Technology (NIST) library identification probabilities. We found that
- Low eV EI is not a soft ionization method, and it has little or no influence on the molecular ion relative abundances for large molecules and those with weak or no molecular ions.
- Low eV EI for compounds with abundant or dominant molecular ions in their 70 eV mass spectra results in the reduction of low mass fragment ions abundances thereby reducing their NIST library identification probabilities thus rarely justifies its use in real‐world applications.
- Cold EI significantly enhances the relative abundance of the molecular ions particularly for large compounds; yet, it retains the low mass fragment ions; hence, Cold EI mass spectra can be effectively identified by the NIST library.
- Different standard EI ion sources provide different 70 eV EI mass spectra. Among the Agilent technologies ion sources, the “Extractor” exhibits relatively abundant molecular ions compared with the “Inert” ion source, while the “High efficiency source” (HES) provides mass spectra with depleted molecular ions compared with the “Inert” ion source or NIST library mass spectra.
125.
一种基于门限的光突发交换受限偏射路由算法 总被引:6,自引:3,他引:6
提出了一种基于突发丢失门限的条件偏射路由算法(LDFD),该算法采用丢弃少量偏射的高优先级突发来保证偏射路由上较低优先级非偏射突发的服务质量(QoS)。当偏射的高优先级突发到达核心节点而该节点没有空闲数据信道时,就启用定义的偏射条件检测函数来判断是丢弃该突发还是允许其抢占非偏射的低优先级突发的资源,从而减少偏射突发与偏射路由上原有突发的竞争。仿真表明,该算法可以很好地控制偏射突发对偏射路由上正常流量的影响,并有效地提高整个网络的突发丢失性能。 相似文献
126.
研究了薄基区HBT合金温度对残余电压Voffset和欧姆接触电阻Rcontact的影响,给出了薄基区HBT的最佳合金温度区域.用肖特基钳位理论解释了合金温度过高导致Voffset偏大的现象.从晶体管基本物理机制推导出Voffset与集电极、发射极面积比Ac/Ae的关系,并用此解释了U形发射极HBT具有较小Ac/Ae的原因,进一步证明了U型发射极结构的优越性. 相似文献
127.
128.
提出了体硅LDMOS漂移区杂质浓度分布的一种二维理论模型,根据该模型,如果要使带有场极板的LDMOS得到最佳的性能,那么LDMOS漂移区的杂质浓度必须呈分段线性分布.用半导体专业软件Tsuprem-4和Medici模拟证明了该模型十分有效,根据该模型优化得到的新型LDMOS的击穿电压和导通电阻分别比常规LDMOS增加58.8%和降低87.4%. 相似文献
129.
通过在共平面波导上周期性地分布微机械电容,外加电压驱动改变电容值,可实现级联式MEMS移相器.本文讨论了优化相移特性对共平面波导特性阻抗及下拉电压的要求,通过工艺参数优化制备了高阻硅基上的Ka波段级联式MEMS移相器,测试结果表明制备器件具有较低的驱动电压,8V时即产生明显的相移量,在36GHz处15V驱动电压时相移量为118°,25V时为286°.对微结构弹性膜的机械振动寿命测试表明,13级级联的MEMS移相器所有弹性膜同步振动的寿命为3×106次.为器件的实用化提供了重要保障. 相似文献
130.