全文获取类型
收费全文 | 2759篇 |
免费 | 72篇 |
国内免费 | 286篇 |
专业分类
化学 | 1380篇 |
晶体学 | 13篇 |
力学 | 778篇 |
综合类 | 11篇 |
数学 | 224篇 |
物理学 | 711篇 |
出版年
2024年 | 6篇 |
2023年 | 118篇 |
2022年 | 82篇 |
2021年 | 73篇 |
2020年 | 80篇 |
2019年 | 62篇 |
2018年 | 60篇 |
2017年 | 95篇 |
2016年 | 77篇 |
2015年 | 79篇 |
2014年 | 105篇 |
2013年 | 134篇 |
2012年 | 108篇 |
2011年 | 219篇 |
2010年 | 152篇 |
2009年 | 229篇 |
2008年 | 192篇 |
2007年 | 221篇 |
2006年 | 143篇 |
2005年 | 112篇 |
2004年 | 77篇 |
2003年 | 102篇 |
2002年 | 78篇 |
2001年 | 73篇 |
2000年 | 68篇 |
1999年 | 51篇 |
1998年 | 83篇 |
1997年 | 40篇 |
1996年 | 33篇 |
1995年 | 33篇 |
1994年 | 24篇 |
1993年 | 20篇 |
1992年 | 18篇 |
1991年 | 7篇 |
1990年 | 10篇 |
1989年 | 1篇 |
1988年 | 17篇 |
1987年 | 13篇 |
1986年 | 3篇 |
1985年 | 2篇 |
1984年 | 3篇 |
1983年 | 1篇 |
1982年 | 3篇 |
1981年 | 3篇 |
1979年 | 3篇 |
1978年 | 3篇 |
1975年 | 1篇 |
排序方式: 共有3117条查询结果,搜索用时 13 毫秒
131.
Song YQ Hürlimann MD Flaum C 《Journal of magnetic resonance (San Diego, Calif. : 1997)》2003,161(2):222-233
This paper describes a method to determine molecular displacements as a function of time in just two scans: one reference scan using the Carr-Purcell-Meiboom-Gill (CPMG) sequence, a second scan using a modified CPMG sequence (KCPMG). Measurements on free diffusion in bulk fluids, and on restricted diffusion in porous rock samples are reported. This technique can also be used for rapid measurement of flow and chemical exchange. 相似文献
132.
N. Sankara Subramanian R. Vivek Sabaapathy P. Vickraman G. Vimal Kumar R. Sriram B. Santhi 《Ionics》2007,13(5):323-328
P-type porous silicon (PS) structure has been prepared by anodic electrochemical etching process under optimized conditions.
Photoluminescence studies of the PS structure show emission at longer wavelengths (red) for the excitation at 365 nm. Scanning
electron microscope investigations of the PS surface confirm the formation of uniform porous structure, and the pore diameter
have been estimated as 25 μm. Pd:SnO2/PS/p-Si heterojunction with top gold ohmic contact developed by conventional methods has been used as the sensor device.
Sensing properties of the device towards liquefied petroleum gas (LPG) and NO2 gas have been investigated in an indigenously developed sensor test rig. The response and recovery characteristics of the
sensor device at different operating temperatures show short response time for LPG. From the studies, maximum sensitivity
and optimum operating temperature of the device towards LPG and NO2 gas sensing has been estimated as 69% at 180 °C and 52% at 220 °C, respectively. The developed sensor device shows a short
response time of 25 and 57 s for sensing LPG and NO2 gases, respectively.
Paper presented at the Third International Conference on Ionic Devices (ICID 2006), Chennai, Tamilnadu, India, Dec. 7–9, 2006. 相似文献
133.
This paper introduces a relative structural complexity measure for the characterization of disordered surfaces. Numerical solutions of 2d+1 KPZ equation and scanning force microscopy (SFM) patterns of porous silicon samples are analyzed using this methodology. The results and phenomenological interpretation indicate that the proposed measure is efficient for quantitatively characterize the structural complexity of disordered surfaces (and interfaces) observed and/or simulated in nano, micro and ordinary scales. 相似文献
134.
Graft polymerization of acrylic acid (AA) onto porous polyethersulfone (PES) membrane surfaces was developed using corona discharge in atmospheric ambience as an activation process followed by polymerization of AA in aqueous solution. The effects of the corona parameters and graft polymerization conditions on grafting yield (GY) of AA were investigated. The grafting of AA on the PES membranes was confirmed by ATR-FTIR and X-ray photoelectron spectroscopy (XPS) analysis. Porosimetry measurements indicate the average pore diameters and porosities of the modified membranes decrease with the increase of the GY. The hydrophilicity and surface wetting properties of the original and modified membranes were evaluated by observing the dynamic changes of water contact angles. It is found that the grafting of AA occurs not only on the membrane surfaces, but also on the pore walls of the cells inside the membrane. The permeability experiments of protein solution reveal that the grafting of PAA endows the modified membranes with enhanced fluxes and anti-fouling properties. The optimized GY of AA is in the range of 150-200 μg/cm2. In addition, the tensile experiments show the corona discharge treatment with the power lower than 150 W yields little damage to the mechanical strength of the membranes. 相似文献
135.
R. Cristescu A. Doraiswamy G. Socol E. Axente R.J. Narayan A. Moldovan I.N. Mihailescu D.B. Chrisey 《Applied Surface Science》2007,253(19):7702-7706
We report the successful deposition of the porous polymer poly(d,l-lactide) by matrix assisted pulsed laser evaporation (MAPLE) using a KrF* excimer laser (248 nm, τ = 7 ns) operated at 2 Hz repetition rate. The chemical structure of the starting materials was preserved in the resulting thin films. Fluence played a key role in optimizing our depositions of the polymer. We demonstrated MAPLE was able to improve current approaches to grow high quality thin films of poly(d,l-lactide), including a porosity control highly required in targeted drug delivery. 相似文献
136.
This paper reports physical properties of porous silicon and oxidized porous silicon, manufactured by anodisation from heavily p-type doped silicon wafers as a function of experimental parameters. The growth rate and refractive index of the layers were studied at different applied current densities and glycerol concentrations in electrolyte. When the current density varied from 5 to 100 mA/cm2, the refractive index was between 1.2 and 2.4 which corresponded to a porosity range from 42 to 85%. After oxidation, the porosity decreased and was between 2 and 45% for a refractive index range from 1.22 to 1.46. The thermal processing also induced an increase in thickness which was dependent on the initial porosity. This increase in thickness was more important for the lowest porosities. Lastly, the roughness of the porous layer/silicon substrate interface was studied at different applied current densities and glycerol concentrations in solution. Roughness decreased when the current density or glycerol concentration increased. Moreover, roughness was also reduced by thermal oxidation. 相似文献
137.
Based on a modified-Darcy–Brinkman–Maxwell model, stability analysis of a horizontal layer of Maxwell fluid in a porous medium heated from below is performed. By solving the eigenvalue problems, the critical Rayleigh number, wave number and frequency for overstability are determined. It is found that the critical Rayleigh number for overstability decreases as the relaxation time increases and the elasticity of a Maxwell fluid has a destabilizing effect on the fluid layer in porous media. On the other hand, the critical Rayleigh number for overstability increases by increasing the porous parameter which acts to stabilize the system. In limiting cases, some previous results for viscoelastic fluids in nonporous media are recovered from our results. 相似文献
138.
Pallab Banerji 《Applied Surface Science》2007,253(11):5129-5132
Atomic scale properties of thin porous silicon (PSi) layers, characterized by the formation of positronium, are investigated using positron annihilation lifetime spectroscopy in the temperature range 20-300 K under 10−7 Torr vacuum. The longest orthopositronium as well as the shortest parapositronium components are found to have quite low intensities in the thin layer at room temperature. It is also found that at temperatures ≤240 K, these two components do not show up in the spectrum. The reason for this absence of the longest lifetime component is suggested. 相似文献
139.
In this paper, time-varying photoconductivity (PC) and the photoluminescence (PL) of different complexes were studied. Due to thick polymer layer hindering light penetrating into porous silicon (PS) layer, intrinsic PS luminescence in polymer/PS system disappeared. The physical origin of PL may be related to the recombination mechanisms involving surface defect states such as silicon oxide, siloxene. Due to carrier transfer controlled by different energy barrier, different devices prepared from different doped Si wafer showed opposite current-voltage characteristic. 相似文献
140.
This paper reports the formation and properties of nano-composite pyroelectric thin films. They consist of pyroelectric triglycine sulfate (TGS) single-crystal nano-rods grown inside a highly dense array of alumina pores (about 65 nm diameter and density of 1011 cm−2). The nucleation and growth of the TGS single crystals are obtained by precipitation from a supersaturated aqueous solution. Nucleation is preferred only at the bottom of the pores due to a tight control of temperature, composition and pore diameter. Growth of single crystals with preferred crystallographic orientation is obtained with the aid of an applied electric field. Various crystallographic orientations (1 0 0) (−1 1 0) (−1 2 0) are obtained separately as a single preferred orientation by changing the amplitude of the electric field during crystal growth. The films exhibit ferroelectric behavior. 相似文献