全文获取类型
收费全文 | 407篇 |
免费 | 25篇 |
国内免费 | 10篇 |
专业分类
化学 | 267篇 |
晶体学 | 3篇 |
力学 | 29篇 |
数学 | 50篇 |
物理学 | 93篇 |
出版年
2023年 | 5篇 |
2022年 | 10篇 |
2021年 | 14篇 |
2020年 | 21篇 |
2019年 | 18篇 |
2018年 | 23篇 |
2017年 | 22篇 |
2016年 | 38篇 |
2015年 | 25篇 |
2014年 | 26篇 |
2013年 | 53篇 |
2012年 | 47篇 |
2011年 | 33篇 |
2010年 | 16篇 |
2009年 | 13篇 |
2008年 | 14篇 |
2007年 | 13篇 |
2006年 | 10篇 |
2005年 | 9篇 |
2004年 | 1篇 |
2003年 | 4篇 |
2002年 | 3篇 |
2001年 | 2篇 |
2000年 | 3篇 |
1999年 | 2篇 |
1998年 | 3篇 |
1997年 | 1篇 |
1996年 | 2篇 |
1995年 | 1篇 |
1994年 | 2篇 |
1991年 | 1篇 |
1990年 | 2篇 |
1989年 | 2篇 |
1987年 | 2篇 |
1980年 | 1篇 |
排序方式: 共有442条查询结果,搜索用时 9 毫秒
1.
Polyoxometalates (POM) supported on zirconia, H3PW12O40/ZrO2, were prepared by incorporating polyphosphotungstate into a zirconia matrix via sol-gel technique that involving the hydrolysis of zirconium (IV) n-butoxide, Zr (n-OBu)4, as the ZrO2 source. This insoluble and readily separable catalyst was characterized by using XRD, FT-IR, SEM, and UV diffuse reflectance spectroscopy (UV-DRS), indicating that the polyphosphotungstate was chemically attached to the zirconia supports, and primary Keggin structure remained intact. The photocatalytic and sonocatalytic activity of the supported polyphosphotungstate was tested via degradation of different dyes in aqueous solutions. The POM-ZrO2 nanocomposite showed higher photocatalytic and sonocatalytic activity than pure polyoxometalate or pure ZrO2. 相似文献
2.
Efficient visible-light-induced photocatalytic degradation of MO on the Cr-nanocrystalline titania-S
Masood Hamadanian Ali Sadeghi SarabiAli Mihammadi Mehra Vahid Jabbari 《Applied Surface Science》2011,257(24):10639-10644
In order to improve visible light photocatalytic activities of the nanometer TiO2, a novel and efficient Cr,S-codoped TiO2 (Cr-TiO2-S) photocatalyst was prepared by precipitation-doping method. The crystalline structure, morphology, particle size, and chemical structure of Cr-TiO2-S were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and Fourier transform infrared (FT-IR) techniques, respectively. Results indicate that the doping of Cr and S, cause absorption edge shifts to the visible light region (λ > 420 nm) compare to the pure TiO2, reduces average size of the TiO2 crystallites, enhances desired lattice distortion of Ti, promotes separation of photo-induced electron and hole pair, and thus improves pollutant decomposition under visible light irradiation. The photocatalytic activities of Cr-TiO2-S nanoparticles were evaluated using the photodegradation of methyl orange (MO) as probe reaction under the irradiation of UV and visible light and it was observed that the Cr-TiO2-S photocatalyst shows higher visible photocatalytic activity than the pure TiO2. The optimal Cr-TiO2-S concentration to obtain the highest photocatalytic activity was 5 mol% for both of Cr and S. 相似文献
3.
Vahid Karimipour 《Letters in Mathematical Physics》1994,30(2):87-98
Quantum de Rham complexes on the quantum plane and the quantum group itself are constructed for the nonstandard deformation of Fun(SL(2)). It is shown that in contrast to the standardq-deformation of SL(2), the above complexes are unique for SL
h
(2). Also, as a byproduct, a new deformation of the two-dimensional Heisenberg algebra is obtained which can be used to construct models ofh-deformed quantum mechanics. 相似文献
4.
Behrouz Vahid 《Journal of fluorescence》2017,27(4):1339-1347
ZnO quantum dots (QDs) based molecularly imprinting polymer (MIP)-coated composite was described for specific detection of the dimethoate (DM) as a template. The MIP was synthesized by simple self-assembly of 3-aminopropyl triethoxysilane (APTES) monomers and tetraethyl ortho-silicate as cross linking agent in the presence of template molecules. The used imprinting course can improve the tendency of the prepared QDs toward the DM template molecules. The MIP-coated ZnO QDs showed a strong fluorescence emission which undergoes a quenching effect in the presence of DM. So, a selective probe could be designed based on these composites to recognize DM in water samples. Under optimized experimental conditions, a linear relationship between the emission intensity of MIP-coated ZnO QDs and concentration of DM, in the range of 0.02–3.2 mg L?1 with a detection limit of 0.006 mg L?1. Combination of high specificity of MIP element and distinct fluorescence features of ZnO QDs provides a sensitive and selective recognizing method for pesticide detection. The developed method was successfully applied for the determination of DM contamination in environmental water samples. 相似文献
5.
Nanoparticles of a three-dimensional coordination polymer, [Pb(L)(μ2-Br)(H2O)]n (1), (L? = 1H-1,2,4-triazole-3-carboxylate), have been synthesized by an ultrasonic method and characterized by scanning electron microscopy, X-ray powder diffraction, IR spectroscopy and elemental analyses. The thermal stability of compound 1 both its bulk and nano-size has been studied by thermal gravimetric (TG) and differential thermal (DTA) analyses and compared each other. Concentration of initial reagents effects and the role of power ultrasound irradiation on size and morphology of nano-structured compound 1, have been studied. Calcination of the compound 1 at 500 °C under air atmosphere yields Pb3O2Br2 nanoparticles. 相似文献
6.
We consider the tachyon-brane inflationary universe model in the context of a polytropic gas equation of state. In slow-roll approximation, we discuss general conditions of this model. For exponential potential, in high-energy limit the characteristics of the model are presented. By using the seven-year Wilkinson Microwave Anisotropy Probe (WMAP7) observational data, we constrain the cosmological parameters of the model. 相似文献
7.
In this study, alumina nanoparticles and film were synthesized by sol–gel method and thermally grown. The chemical composition, morphology, and structural properties of the grain nanostructures of the nanoaggregate aluminum atoms on the silicon surface are characterized by X-ray diffraction, energy dispersive spectroscopy, and scanning electron microscopy techniques which show that the atomic ratio of Al and O elements is about 60:45 and the mass ratio is about 70:31. This point reveals that there is no intermediate layer between Al2O3 and the silicon substrate. In conclusion, this nanoparticle shows good electron conductivity at the surface of a carbon paste electrode that shows its ability in the preparation of a nanosensor in morphine analysis. The linear response range and detection limit were found to be 0.1–550 and 0.03 μmol L?1, respectively. The proposed sensor was successfully applied for the determination of morphine in real samples such as human urine and drug. 相似文献
8.
This paper presents the design principles of a novel optimized microring-based uni-traveling carrier photodiode (MR-UTC-PD), for wavelength division multiplexing (WDM) optical systems. The fundamental parameters for output characteristics such as quantum efficiency and 3-dB bandwidth of MR-UTC-PD are discussed. We analyze the effects of basic parameters as effective ring radius, definite coupling condition and the suitable cross section on the MR-UTC-PD main characteristics for high performance of the device. It is shown that the maximum quantum efficiency and most efficient performance of the device can be obtained at critical coupling condition. We show that the maximum 3-dB bandwidth and an efficient approach for high bandwidth-efficiency product can be achieved at overcoupling condition. In this regard, some design curves are presented for the optimized MR-UTC-PD. 相似文献
9.
Effect of operational parameters on degradation of Malachite Green by ultrasonic irradiation 总被引:1,自引:0,他引:1
The aim of the present study was to apply ultrasonic technique to remove Malachite Oxalate Green (MG) from aqueous solution. An ultrasonic bath with frequency of 35 kHz was used to investigate the effect of different operational parameters such as MG concentration, power density, temperature, mechanical agitation and addition of EtOH, 2-PrOH and iso-BuOH. Decolorization of MG follows a first order kinetics and hydroxyl radicals have an important role in degradation of MG. The apparent reaction rate constant (k(ap)) was influenced by variation of operational parameters. The activation energy was 30.95 kJ/mol in temperature range of 21-34 degrees C, suggesting a diffusion-controlled reaction. Alcohols act as hydroxyl radicals scavengers having undesirable contribution. UV-vis spectral change of MG showed hypsochromic shift occurred with increasing sonication time, proposing N-demethylation process of MG. 相似文献
10.
A novel robust proportional-integral (PI) adaptive observer design for chaos synchronization 下载免费PDF全文
In this paper, chaos synchronization in the presence of parameter uncertainty, observer gain perturbation and exogenous input disturbance is considered. A nonlinear non-fragile proportional-integral (PI) adaptive observer is designed for the synchronization of chaotic systems; its stability conditions based on the Lyapunov technique are derived. The observer proportional and integral gains, by converting the conditions into linear matrix inequality (LMI), are optimally selected from solutions that satisfy the observer stability conditions such that the effect of disturbance on the synchronization error becomes minimized. To show the effectiveness of the proposed method, simulation results for the synchronization of a Lorenz chaotic system with unknown parameters in the presence of an exogenous input disturbance and abrupt gain perturbation are reported. 相似文献