全文获取类型
收费全文 | 4580篇 |
免费 | 202篇 |
国内免费 | 518篇 |
专业分类
化学 | 3272篇 |
晶体学 | 57篇 |
力学 | 97篇 |
综合类 | 6篇 |
数学 | 69篇 |
物理学 | 1799篇 |
出版年
2024年 | 11篇 |
2023年 | 127篇 |
2022年 | 109篇 |
2021年 | 73篇 |
2020年 | 127篇 |
2019年 | 146篇 |
2018年 | 103篇 |
2017年 | 120篇 |
2016年 | 151篇 |
2015年 | 178篇 |
2014年 | 196篇 |
2013年 | 232篇 |
2012年 | 202篇 |
2011年 | 373篇 |
2010年 | 325篇 |
2009年 | 342篇 |
2008年 | 316篇 |
2007年 | 339篇 |
2006年 | 242篇 |
2005年 | 195篇 |
2004年 | 195篇 |
2003年 | 165篇 |
2002年 | 126篇 |
2001年 | 93篇 |
2000年 | 101篇 |
1999年 | 82篇 |
1998年 | 94篇 |
1997年 | 64篇 |
1996年 | 64篇 |
1995年 | 52篇 |
1994年 | 53篇 |
1993年 | 32篇 |
1992年 | 41篇 |
1991年 | 44篇 |
1990年 | 16篇 |
1989年 | 16篇 |
1988年 | 15篇 |
1986年 | 17篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 7篇 |
1982年 | 10篇 |
1981年 | 18篇 |
1980年 | 12篇 |
1979年 | 7篇 |
1978年 | 11篇 |
1977年 | 6篇 |
1976年 | 9篇 |
1975年 | 5篇 |
1974年 | 6篇 |
排序方式: 共有5300条查询结果,搜索用时 0 毫秒
31.
R E Sepponen 《Magnetic resonance imaging》1985,3(2):163-167
A phase encoding method for imaging of chemical shift or magnetic field distributions is described. The method utilizes the spin-echo principle and the time period between signal collection and excitation is constant but the time period between excitation and the 180 degrees pulse is varied by constant steps. The method is relatively easy to apply with the Fourier or projection reconstruction methods. 相似文献
32.
33.
Sven Erlander 《Mathematical Programming》1981,21(1):137-151
This paper treats entropy constrained linear programs from modelling as well as computational aspects. The optimal solutions to linear programs with one additional entropy constraint are expressed in terms of Lagrange-multipliers. Conditions for uniqueness are given. Sensitivity and duality are studied. The Newton—Kantorovich method is used to obtain a locally convergent iterative procedure. Related problems based on maximum entropy or minimum information are discussed. 相似文献
34.
通过对19个酚类化合物1H NMR谱的研究和分析发现,酚羟基形成分子内氢键后,使对位芳氢化学位移值向高场变化的幅度大于邻位,其差值约为0.1ppm-0.3ppm. 相似文献
35.
36.
采用非接触的色散荧光发射光谱和时间分辨光谱方法,在大气压条件下,对脉冲电晕放电脱除NO的化学反应动力学过程进行了实验研究。在该研究中,首先利用色散荧光发射光谱方法,得到纯NO气体脉冲电晕放电中各活性粒子的色散荧光谱,并对其进行归属,确定了各活性粒子的灵敏指纹跃迁谱线;在此基础上,通过测量NO气体放电过程中所产生的N+,O,N2及NO分子的灵敏指纹跃迁谱线的时间行为特性,研究脉冲电晕放电脱除NO的化学反应机理。实验结果表明:在脉冲电晕放电过程中,NO分子首先与高能电子发生非弹性电离碰撞,变成NO+离子,随后NO+离子发生解离反应,形成N+离子和O原子;N+离子在向阴极运动过程中与电子碰撞结合成激发态N原子,继而和其他激发态N原子结合成为激发态N2;而O原子则应与其它O原子结合成为O2。由此建立了大气压条件下纯NO气体脉冲电晕放电脱除NO的化学反应动力学模型。 相似文献
37.
S. Sharmila B. Senthilkumar V.D. Nithya Kumaran Vediappan Chang Woo Lee R. Kalai Selvan 《Journal of Physics and Chemistry of Solids》2013
Submicron-sized polyhedral Li4Ti5−xSnxO12 (x=0.0, 0.05, and 0.1) materials were successfully prepared by a single-step molten salt method. The structural, morphological, transport and electrochemical properties of the Li4Ti5−xSnxO12 were studied. X-ray diffraction patterns showed the formation of a cubic structure with a lattice constant of 8.31 Å, and the addition of dopants follows Vegard's law. Furthermore, FT-IR spectra revealed symmetric stretching vibrations of octahedral groups of MO6 lattice in Li4Ti5O12. The formation of polyhedral submicron Li4Ti5−xSnxO12 particles was inferred from FE-SEM images, and a particle size reduction was observed for Sn-doped Li4Ti5O12. The chemical composition of Ti, O and Sn was verified by EDAX. The DC electrical conductivity was found to increase with increasing temperature, and a maximum conductivity of 8.96×10−6 S cm−1 was observed at 200 °C for Li4Ti5O12. The galvanostatic charge–discharge behavior indicates that the Sn-doped Li4Ti5O12 could be used as an anode for Li-ion batteries due to its enhanced electrochemical properties. 相似文献
38.
Size controlled cubic Fe3O4 nanoparticles in the size range 90–10 nm were synthesized by varying the ferric ion concentration using the oxidation method. A bimodal size distribution was found without ferric ion concentration and the monodispersity increased with higher concentration. The saturation magnetization decreased from 90 to 62 emu/g when the particle size is reduced to 10 nm. The Fe3O4 nanoparticles with average particle sizes 10 and 90 nm were surface modified with prussian blue. The attachment of prussian blue with Fe3O4 was found to depend on the concentration of HCl and the particle size. The saturation magnetization of prussian blue modified Fe3O4 varied from 10 to 80 emu/g depending on the particle size. The increased tendency for the attachment of prussian blue with smaller particle size was explained based on the surface charge. The prussian blue modified magnetite nanoparticles could be used as a radiotoxin remover in detoxification applications. 相似文献
39.
Alain Adenier Eva Cabet-Deliry Sophie Griveau Florence Mercier Christine Vautrin-Ul 《Surface science》2006,600(21):4801-4812
This study investigates the spontaneous grafting of different para-substituted phenyl groups on carbon and metallic surfaces from diazonium salts solutions. Glassy carbon, nickel, zinc and iron plates were allowed to react with an acetonitrile solution of aryldiazonium tetrafluoroborate salt by simple dipping. The surfaces were characterized before and after their immersion by XPS and AFM to evidence the formation of a coating on the different materials. The results are indicative of the presence of substituted phenyl groups on all the investigated surfaces. This study also aims at correlating grafting efficiency with metal reactivities and diazonium salt electronic properties by means of AFM and FT-IRRAS. For this purpose, zinc and nickel were chosen due to their opposite reducing properties and two diazonium salts were selected with electron-donor or -withdrawing para-substituents. The results tend to indicate that redox properties of both partners (diazonium + metal) are of prime importance for grafting to occur. 相似文献
40.
Non-stoichiometric ternary chalcogenides (Zn,Fe)S were prepared in the film form by pyrolytic spray deposition technique, using air/nitrogen as the carrier gas. The precursor solution comprised of ZnCl2, FeCl2 and thiourea. The depositions were carried out under optimum conditions of experimental parameters viz. carrier gas (air/nitrogen) flow rate, concentration of precursor constituents, nozzle substrate distance and temperature of quartz substrate. The deposited thin films were later sintered in argon at 1073 K for 120 min.The structural, compositional and optical properties of the sintered thin films were studied. X-ray diffraction studies of the thin films indicated the presence of (Zn,Fe)S solid solution with prominent cubic sphalerite phase while surface morphology as determined by scanning electron microscopy (SEM) revealed a granular structure.The chemical composition of the resulting thin films as analyzed by energy dispersive X-ray analysis (EDAX) reflected the composition of the precursor solutions from which the depositions were carried out with Fe at% values ranging from 0.4 up to 33.SEM micrographs of thin films reveal that the grain sizes of the thin films prepared using air as carrier gas and N2 as carrier gas are in the vicinity of 300 and 150 nm, respectively.The diffuse transmittance measurements for thin films, as a function of wavelength reveal the dependence of direct optical band gap on Fe content and type of phase. 相似文献