全文获取类型
收费全文 | 1137篇 |
免费 | 38篇 |
专业分类
化学 | 662篇 |
晶体学 | 40篇 |
力学 | 46篇 |
数学 | 86篇 |
物理学 | 341篇 |
出版年
2024年 | 6篇 |
2023年 | 12篇 |
2022年 | 21篇 |
2021年 | 29篇 |
2020年 | 35篇 |
2019年 | 31篇 |
2018年 | 29篇 |
2017年 | 38篇 |
2016年 | 43篇 |
2015年 | 32篇 |
2014年 | 55篇 |
2013年 | 114篇 |
2012年 | 84篇 |
2011年 | 71篇 |
2010年 | 62篇 |
2009年 | 52篇 |
2008年 | 43篇 |
2007年 | 52篇 |
2006年 | 36篇 |
2005年 | 35篇 |
2004年 | 28篇 |
2003年 | 15篇 |
2002年 | 19篇 |
1999年 | 5篇 |
1998年 | 8篇 |
1997年 | 7篇 |
1995年 | 7篇 |
1994年 | 8篇 |
1993年 | 13篇 |
1992年 | 10篇 |
1991年 | 6篇 |
1990年 | 6篇 |
1987年 | 8篇 |
1986年 | 4篇 |
1985年 | 9篇 |
1984年 | 10篇 |
1983年 | 5篇 |
1982年 | 7篇 |
1981年 | 6篇 |
1980年 | 7篇 |
1979年 | 12篇 |
1978年 | 9篇 |
1976年 | 6篇 |
1975年 | 4篇 |
1974年 | 5篇 |
1973年 | 6篇 |
1972年 | 5篇 |
1966年 | 6篇 |
1963年 | 10篇 |
1962年 | 4篇 |
排序方式: 共有1175条查询结果,搜索用时 312 毫秒
101.
Ashutosh Tiwari Rajendra Kumar Mani Prabaharan Ravi R. Pandey Premlata Kumari Anurag Chaturvedi A. K. Mishra 《先进技术聚合物》2010,21(9):615-620
A nanofibrous polyaniline (PANI) thin film was fabricated using plasma‐induced polymerization method and explored its application in the fabrication of NO2 gas sensor. The effects of substrate position, pressure, and the number of plasma pulses on the PANI film growth rate were monitored and an optimum condition for the PANI thin film preparation was established. The resulting PANI film was characterized with UV–visible spectrophotometer, FTIR, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The PANI thin film possessed nanofibers with a diameter ranging from 15 to 20 nm. The NO2 gas sensing behavior was studied by measuring the change in electrical conductivity of PANI film with respect to NO2 gas concentration and exposure time. The optimized sensor exhibited a sensitivity factor of 206 with a response time of 23 sec. The NO2 gas sensor using nanofibrous PANI thin film as sensing probe showed a linear current response to the NO2 gas concentration in the range of 10–100 ppm. Copyright © 2009 John Wiley & Sons, Ltd. 相似文献
102.
Reenamoni Saikia Chaliha K. Annapurna Anal Tarafder V.S. Tiwari P.K. Gupta Basudeb Karmakar 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2010,75(1):243-250
Precursor glass of composition 25K2O–25Nb2O5–50SiO2 (mol%) doped with Er2O3 (0.5 wt% in excess) was isothermally crystallized at 800 °C for 0–100 h to obtain transparent KNbO3 nanostructured glass–ceramics. XRD, FESEM, TEM, FTIRRS, dielectric constant, refractive index, absorption and fluorescence measurements were carried out to analyze the morphology, dielectric, structure and optical properties of the glass–ceramics. The crystallite size of KNbO3 estimated from XRD and TEM is found to vary in the range 7–23 nm. A steep rise in the dielectric constant of glass–ceramics with heat-treatment time reveals the formation of ferroelectric nanocrystalline KNbO3 phase. The measured visible photoluminescence spectra have exhibited green emission transitions of 2H11/2, 4S3/2 → 4I15/2 upon excitation at 377 nm (4I15/2 → 4G11/2) absorption band of Er3+ ions. The near infrared (NIR) emission transition 4I13/2 → 4I15/2 is detected around 1550 nm on excitation at 980 nm (4I15/2 → 4I11/2) of absorption bands of Er3+ ions. It is observed that photoluminescent intensity at 526 nm (2H11/2 → 4I15/2), 550 nm (4S3/2 → 4I15/2) and 1550 nm (4I13/2 → 4I15/2) initially decrease and then gradually increase with increase in heat-treatment time. The measured lifetime (τf) of the 4I13/2 → 4I15/2 transition also possesses a similar trend. The measured absorption and fluorescence spectra reveal that the Er3+ ions gradually enter into the KNbO3 nanocrystals. 相似文献
103.
Enhancement in nighttime total electron content (TEC) near the crest of equatorial ionization anomaly at Bhopal (Geog. 23.2°N,
77.4°E, and MLAT 14.2°N) has been studied for the solar minimum period March 2005- November 2006. TEC data recorded by GPS
Ionospheric Scintillation and TEC Monitor (GISTM) GSV4004A receiver is used for the study and results are presented in the
paper. More than 10% increase in TEC with respect to background content is considered for analysis. Out of total 138 enhancements,
65 observed during pre midnight hours and 73 during post midnight hours. It is observed that nighttime enhancement in TEC
at Bhopal occurs in all seasons; it is more frequent during summer, less during equinox and least during winter months. The
enhancement in nighttime TEC can be observed both in geomagnetic disturbed time and in quiet time. We found that mean peak
amplitude for pre-midnight TEC enhancement are more in equinox and less in winter, while in post-midnight TEC enhancement
it is highest in summer and less in winter. Post-midnight enhancements have smaller peak amplitude as compared to pre-midnight.
Also the most probable values for pre- and post-midnights are 4.4 TECU and 2.17 TECU respectively. The percentage occurrence
of nighttime TEC enhancement does not show any dependence on solar activity whereas the peak amplitude depends on solar activity.
The percentage occurrence of nighttime TEC enhancement decreases as the magnetic activity increases whereas there is no such
dependency found with peak amplitude. It is also observed that majority of nighttime TEC enhancements are occurred without
scintillation. The localised nighttime TEC enhancements near the crest of equatorial ionization anomaly region have been observed
most of the nights during the period of study. 相似文献
104.
ZnS:Cu nanophosphors were prepared by wet chemical methods and characterized by X-ray diffraction (XRD). The typical morphologies of the nanophosphors were investigated by scanning electron microscopy (SEM). The thermoluminescence (TL) properties of inorganically and organically passivated ZnS:Cu nanophosphors were investigated after γ-irradiation using a 60Co source at room temperature. The TL glow curve of capped ZnS:Cu showed variation in TL peak and intensity as the capping agent was changed. Amongst the synthesized samples the TL glow curve of SiO2 capped ZnS:Cu showed the highest TL intensity. It has been found that TL response of SiO2 capped ZnS:Cu is linear in the range 10-550 Gy. A discussion of the obtained results is also presented. 相似文献
105.
In this study, in situ control growth of bismuth nanoparticles (Bi0 NPs) was demonstrated in bismuth-based glass dielectrics under an electron beam (EB) irradiation at room temperature. The
effects of EB irradiation were investigated in situ using transmission electron microscopy (TEM), selected-area electron diffraction
and high-resolution transmission electron microscopy. The EB irradiation for 2–8 min enhanced the construction of bismuth
nanoparticles with a rhombohedral structure and diameter of 4–9 nm. The average particle size was found to increase with the
irradiation time. Bismuth metal has a melting point of 271 °C and this low melting temperature makes easy the progress of
energy induced structural changes during in situ TEM observations. This is a very useful technique in nano-patterning for
integrated optics and other applications. 相似文献
106.
M. Tiwari D. V. Seletskiy V. M. Kenkre 《The European Physical Journal B - Condensed Matter and Complex Systems》2011,80(2):147-153
In the presence of 1/f
β noise, we investigate the logical
stochastic resonance (LSR) in an asymmetric bistable model driven by various
cycling combinations of two logic inputs. The probability of correct logic
outputs is calculated according to true table of logic relationships. Two
major results are presented. Firstly, it is shown that the LSR effect can be
obtained by changing noise strength. Over entire range of noise variance,
white noise can be considered to be better than 1/f noise or 1/f
2 noise
to obtain clean logic operation. At a smaller noise level, 1/f noise can
realize higher output probability than white noise or 1/f
2 noise. In
the sense, 1/f noise can be considered to be better than white noise or
1/f
2. On the other hand, the correct probability can evolves
nonmonotonically as noise exponent β increases, and a kind of SR-like
effect can be obtained as a result of β. At certain intermediate
noise variance, the output probability is able to attain its minimum at
β = 1. It is also shown that actually some finite β sometime can
be better than β = 0 at small range of noise variance. The study might
provide some potential complement to LSR effect in the presence of
1/f
β noise. 相似文献
107.
R. Jangir Tapas GanguliPragya Tiwari S. PorwalHimanshu Srivastava S.K. RaiB.Q. Khattak S.M. Oak 《Applied Surface Science》2011,257(22):9323-9328
β-Ga2O3 nanostructures including nanowires, nanoribbons and nanosheets were synthesized via thermal annealing of gold coated GaAs substrates in N2 ambient. GaAs substrates with different dopants were taken as the starting material to study the effect of doping on the growth and photoluminescence properties of β-Ga2O3 nanostructures. The nanostructures were investigated by Grazing Incident X-ray Diffraction, Scanning Electron Microscopy, Transmission Electron Microscopy, Energy Dispersive X-ray Spectroscopy, room temperature photoluminescence and optical absorbance. The selected area electron diffraction and High resolution-TEM observations suggest that both nanowires and nanobelts are single crystalline. Different growth directions were observed for nanowires and nanoribbons, indicating the different growth patterns of these nanostructures. The PL spectra of β-Ga2O3 nanostructures exhibit a strong UV-blue emission band centered at 410 nm, 415 nm and 450 nm for differently doped GaAs substrates respectively. A weak red luminescence peak at 710 nm was also observed in all the samples. The optical absorbance spectrum showed intense absorption features in the UV spectral region. The growth and luminescence mechanism in β-Ga2O3 nanostructures are also discussed. 相似文献
108.
Dielectric properties of polymer blend of polyvinylidenefluoride (PVDF) and polysulfone (PSF) of different wt. % have been studied to understand the molecular motion and their relaxation behavior in the frequency range of 100 Hz to 10 kHz at different temperatures between 30 and 190 °C. The dielectric constant of the blend decreased with frequency and increased with the increasing temperature and PSF content in the blend. The magnitude of dielectric loss also increased with increase in temperature and PSF content. The observed characteristic has been consistently explained in terms of dipolar motions and the plasticization effect brought about by blending of PSF with PVDF. At constant frequency and temperature, the blend follows a linear relationship between logarithm of their dielectric constant and different ratios of blend. The appearance of a peak for each concentration in dielectric loss suggests the presence of relaxing dipoles in the blend. In addition of PSF with PVDF, the peak shifts toward higher frequency side suggesting the speed up the relaxation process. AC dielectric data is also combined with thermally stimulated depolarization current (TSDC) data which is generally studied for low-frequency dielectric properties of polymers blends so as to produce the results in a wide frequency range. The glass transition temperature (Tg) of the blend was studied by differential scanning calorimetric technique (DSC), the Tg was compared and correlated with TSDC peak. The blend samples were characterized by X-ray diffraction (XRD) and scanning electron microscopy (SEM) to study the formation of blend and micro structural properties of the materials. The shifting of peak toward lower diffraction angle side confirms the reduction in particle size with increasing amorphous content in the blend. 相似文献
109.
110.
The present study investigates the utility of composite beads of nano-particles of iron oxide and chitosan for removing Ni (II) ions from aqueous solution by batch and column adsorption techniques. In the batch mode experiment, the influence of pH, concentration, adsorbent dose, temperature, column mode, bed height, flow rate and initial concentration were studied on the adsorption profiles of nickel ions. The maximum uptake of Ni (II) ions was obtained at pH 4.0 in 30 min at room temperature. 相似文献