全文获取类型
收费全文 | 118562篇 |
免费 | 30917篇 |
国内免费 | 30511篇 |
专业分类
化学 | 76798篇 |
晶体学 | 2802篇 |
力学 | 8048篇 |
综合类 | 1740篇 |
数学 | 16958篇 |
物理学 | 73644篇 |
出版年
2024年 | 526篇 |
2023年 | 1096篇 |
2022年 | 2313篇 |
2021年 | 2164篇 |
2020年 | 2394篇 |
2019年 | 2884篇 |
2018年 | 2765篇 |
2017年 | 4009篇 |
2016年 | 3610篇 |
2015年 | 4593篇 |
2014年 | 4855篇 |
2013年 | 7197篇 |
2012年 | 7865篇 |
2011年 | 9737篇 |
2010年 | 13088篇 |
2009年 | 13390篇 |
2008年 | 7829篇 |
2007年 | 6716篇 |
2006年 | 6337篇 |
2005年 | 6069篇 |
2004年 | 6132篇 |
2003年 | 4612篇 |
2002年 | 4588篇 |
2001年 | 4763篇 |
2000年 | 4232篇 |
1999年 | 3529篇 |
1998年 | 2604篇 |
1997年 | 2296篇 |
1996年 | 2589篇 |
1995年 | 3039篇 |
1994年 | 3032篇 |
1993年 | 3134篇 |
1992年 | 2644篇 |
1991年 | 2286篇 |
1990年 | 1933篇 |
1989年 | 2017篇 |
1988年 | 2011篇 |
1987年 | 1262篇 |
1986年 | 1305篇 |
1985年 | 910篇 |
1984年 | 1053篇 |
1982年 | 937篇 |
1981年 | 772篇 |
1980年 | 813篇 |
1979年 | 581篇 |
1978年 | 563篇 |
1977年 | 647篇 |
1976年 | 1050篇 |
1972年 | 538篇 |
1971年 | 451篇 |
排序方式: 共有10000条查询结果,搜索用时 406 毫秒
991.
关于Ramsey数下界的部分结果 总被引:3,自引:1,他引:2
刘富贵 《数学的实践与认识》2002,32(1):97-99
本文得到 Ramsey数下界的一个计算公式 :R( l,s+ t-2 )≥ R( l,s) + R( l,t) -1 ,(式中 l、s、t≥ 3) .用此公式算得的 Ramsey数的下界比用其它公式算得的下界好 . 相似文献
992.
Sonya V. Roberson Albert J. Fahey Amit Sehgal Alamgir Karim 《Applied Surface Science》2002,200(1-4):150-164
We present a simple method for chemical modification of chlorosilane self-assembled monolayers (SAMs) on Si surfaces by exposure to a gradient of UV-ozone radiation to create stable substrates with a range of contact angles (θH2O≈5–95°) and surface energies on a single substrate. These gradient energy substrates are developed to potentially generate libraries for combinatorial studies of thin film phenomenology, where a systematic variation of interfacial surface energy represents one of the significant parameters along one axis. The graded oxidation process presents a systematic variation of surface chemical composition. We have utilized contact angle measurements and time-of-flight secondary ion mass spectrometry (ToF-SIMS) to investigate this variation for a series of ions, among which are SiCH3+, SiOH+ and COOH−. We show that the macroscopic measurements of surface free energy/contact angle correlate with the detailed analysis of surface chemistry (as assessed by ToF-SIMS) on these test substrates. 相似文献
993.
994.
Summary Measurements have been made of the sea-level size spectrum of cosmic-ray showers at a number of zenith angles and the relationship
between shower size and atmospheric depth has been derived for a number of integral cosmic-ray fluxes. These measurements
extend the available data to ∼ 1400 g·cm−2 and emphasize the discrepancy between measurements made on Mt. Chacaltaya and those made at other observatories.
Riassunto Sono state fatte misurazioni dello spettro delle dimensioni a livello del mare di sciami di raggi cosmici ad un certo numero di angoli zenitali ed è stata dedotta la relazione tra la grandezza dello sciame e la profondità atmosferica per un certo numero di flussi integrali di raggi cosmici. Queste misurazioni estendono i dati disponibili a∼1400 g·cm−2 ed evidenziano la discrepanza tra le misurazioni fatte sul Monte Chalcaltaya e quelle di altri osservatori.
Резюме Проводятся измерения спектра размеров ливней космических лучей на уровне моря при различных зенитных углах и соотношения между размером ливня и толщиной атмосферы для ряда интегральных потоков космических лучей. Эти измерения уточняют имеющиеся данные до толщии∼1400 г·см−2. Отмечается расхождение между измерениями, проведенными на горе Чакалтайа, и измерениями в других обсерваториях.相似文献
995.
996.
载能离子穿过固体界面引起界面原子迁移使界面原子混合和物质成分变化,从而导致界面发生材料相变。简要介绍了载能离子辐照引起金属/绝缘体界面混合效应及相变现象的主要实验研究进展、低能离子和高能离子辐照引起金属/绝缘体界面现象差异,并对离子辐照引起界面混合及相变的机制进行了初步探讨。When penetrating an interface between two kind of solids, energetic ions can induce atomic diffusion at both sides of the interface and then result in intermixing, atom re-distribution or composition change, as well as phase transformation. Main progress on the study of intermixing and phase change at metal/insulator interface induced by energetic ion irradiations, the difference of phenomena occurred at metal/insulator interfaces induced by high-and low-energy ions were briefly reviewed. Furthermore, the possible mechanisms related to intermixing and phase change at metal/insulator interface produced by energetic ion irradiations were also discussed in short words. 相似文献
997.
采用从头算CCSD(T)/6-311 G(2d,2p)//B3LYP/6-311G(d,p)方法,研究了自由基-分子反应F CH2CHCH3的各种不同的反应通道.该反应主要是通过复合物形成机制进行,即F分别加到碳碳双键的两端形成自由基复合物1和2.这两种亚稳态自由基会解离成三种产物:H C3H5F、CH3 C2H3F和HF C3H5.理论计算结果表明,生成CH3 C2H3F是反应的主要通道,而生成H C3H5F和HF C3H5对产物也有一定的贡献.这一结果和实验符合得很好. 相似文献
998.
Qingfeng Zhang Shanjia Xu 《International Journal of Infrared and Millimeter Waves》2006,27(11):1539-1552
A novel uniplanar 2-D composite right/left-handed transmission line (CRLH-TL) structure is proposed based on a general transmission
line theory and the way to increase the relative operating bandwidth in the left-handed (LH) region and lower the LH operating
frequency is illustrated. In addition, a new method to extract parameters and calculate the Bloch impedance of the structure
is presented. Numerical results of the dispersion as well as the extracted parameters are given, which are calculated based
on full-wave simulation. The present 2-D uniplanar CRLH-TL structure is applicable to 2-D left-handed materials in lower and
wider LH frequency range.
Supported by the National Natural Science Foundation of China (No. 60371010) (No. 60471037) (No. 63531020) 相似文献
999.
在四阳极直流放电装置上,测量并分析了辉光放电的电流-电压和发光特征随气压的变化关系。结果表明,采用稳流放电模式比稳压放电具有更宽的稳定放电气压和电流范围,能在从1~800Pa的较宽气压范围内实现氦气辉光放电,放电电流可达到500mA左右。随着电极表面亮斑的变化,对于同一气压,在低电流区,放电电压几乎成指数增长;随电流增大,电压的增长速度变缓;对于高的气压,碰撞频率的增大使得电压随电流升高的速率变小。分析表明,放电处于异常辉光区。从放电管的CCD图像可以看出,对于同一放电电流,随气压的升高,等离子体往阴极收缩。 相似文献
1000.
The ab initio calculations were performed at the RHF/4-31G level with the reaction pathways of the iso-merization and dehydrogenation of methylnitrene by the intrinsic reaction coordinate method. The results show that the transformation from methylnitrene to methylenimine would be very easy. This accountes for the experimental fact that one couldn' t find the methylnitrene, but only obtained the methylenimine in the pyrolysis of methyl azide. The mode-selective study reveals the reaction coordinates (IRC) of isomerization and dehydrogenation of methylnitrene are associated with the molecular deformation mode of 1191 cm-1 and the methyl group unsymmetrical stretch mode, respectively. The coupling between normal coordinates is favourable to select the reaction channel of isomerization. 相似文献