全文获取类型
收费全文 | 285篇 |
免费 | 27篇 |
国内免费 | 4篇 |
专业分类
化学 | 228篇 |
力学 | 7篇 |
数学 | 23篇 |
物理学 | 58篇 |
出版年
2023年 | 1篇 |
2022年 | 4篇 |
2021年 | 3篇 |
2020年 | 13篇 |
2019年 | 5篇 |
2018年 | 24篇 |
2017年 | 19篇 |
2016年 | 20篇 |
2015年 | 16篇 |
2014年 | 16篇 |
2013年 | 22篇 |
2012年 | 26篇 |
2011年 | 35篇 |
2010年 | 26篇 |
2009年 | 10篇 |
2008年 | 15篇 |
2007年 | 14篇 |
2006年 | 11篇 |
2005年 | 11篇 |
2004年 | 7篇 |
2003年 | 5篇 |
2002年 | 4篇 |
2001年 | 3篇 |
2000年 | 2篇 |
1999年 | 1篇 |
1993年 | 1篇 |
1982年 | 1篇 |
1979年 | 1篇 |
排序方式: 共有316条查询结果,搜索用时 15 毫秒
111.
Amir A. Mehrdad Sharif Ali M. Astaraki Parviz Aberoomand Azar Saeed Abedini Khorrami Shahram Moradi 《Arabian Journal of Chemistry》2012,5(1):41-44
The effect of NaCl and Na2SO4 concentrations in aqueous phase on the phase inversion temperature (PIT) in nanoemulsions of water/Brij30/n-hexadecane system has been studied separately, and then compared. The variation of conductivity with temperature was measured by Cyber Scan PC510 conductivity meter for emulsions with 20 wt% hexadecane and 8 wt% Brij30 concentration and different concentrations of NaCl and Na2SO4 in aqueous phase. The results showed that with increasing concentrations of NaCl and Na2SO4 in aqueous phase, the PIT of nanoemulsions decreases. The effect of the elevation of concentration on the decrease of PIT was more for Na2SO4 in aqueous phase than NaCl with equal concentrations. 相似文献
112.
A new diamine containing sulfone,sulfide and amide groups was synthesized via a three-step reaction process. The nucleophilic substitution reaction of 4-aminothiophenol with 4-nitrobenzoyl chloride in the presence of propylene oxide (PO) afforded N-(4-mercaptophenyl)-4-nitrobenzamide(MPNB).The catalytic reduction of the nitro group in MPNB to amino group was accomplished by using Pd/C and hydrazine monohydrate to produce 4-amino-N-(4-mercapto phenyl)benzamide(AMPB).Reaction of two moles of AMPB with bis(... 相似文献
113.
Nahid Mashkouri Najafi Mohammad Eidizadeh Shahram Seidi Ensieh Ghasemi Reza Alizadeh 《Microchemical Journal》2009,93(2):159-163
Electrodeposition is known to be proper for separation and preconcentration of extremely low concentrations of analytes from the bulk sample which is instrumentally very simple. In the present research, a combination of electrodeposition with arc atomic emission spectrometry (ED-AAES) method has been developed in order to improve the analytical performance of this spectrometry technique. The results show that sensitivity and detection limits by using ED-AAES were improved 1000–2000 folds over those of normal arc atomic emission spectrometry in determination of the selected elements. The detection limits for measurement of Ni, Cr and Pb were 2.56, 3.05 and 2.11 µg L− 1 for monodeposition and 3.31, 3.72 and 3.25 µg L− 1 for simultaneously deposition, respectively. The precision of determination was in the range of 2–4% RSD. Typical calibration graphs for these elements were linear up to 100 µg L− 1, depending on the element and matrix.Application of this technique was also tested on determination of the studied elements in an electroplating plant's waste water. The accuracy of technique was verified by comparing the results of the waste water analysis with those of electrothermal atomic absorption spectroscopy as a reference standard method.The obtained results show that the combined technique (ED-AAES) has been progressed substantially toward the ultimate goal of direct interference-free determination of trace analysis in complex samples by AAES. 相似文献
114.
Hadi Taghavifar Shahram Khalilarya Samad Jafarmadar Hamid Taghavifar 《Theoretical and Computational Fluid Dynamics》2016,30(4):349-362
A multidimensional computational fluid dynamic code was developed and integrated with probability density function combustion model to give the detailed account of multiphase fluid flow. The vapor phase within injector domain is treated with Reynolds-averaged Navier–Stokes technique. A new parameter is proposed which is an index of plane-cut spray propagation and takes into account two parameters of spray penetration length and cone angle at the same time. It was found that spray propagation factor (SPI) tends to increase at lower r/d ratios, although the spray penetration tends to decrease. The results of SPI obtained by empirical correlation of Hay and Jones were compared with the simulation computation as a function of respective r/d ratio. Based on the results of this study, the spray distribution on plane area has proportional correlation with heat release amount, NO x emission mass fraction, and soot concentration reduction. Higher cavitation is attributed to the sharp edge of nozzle entrance, yielding better liquid jet disintegration and smaller spray droplet that reduces soot mass fraction of late combustion process. In order to have better insight of cavitation phenomenon, turbulence magnitude in nozzle and combustion chamber was acquired and depicted along with spray velocity. 相似文献
115.
In this work, porous poly (2-methoxyaniline) film prepared in the presence of sodium dodecyl sulfate (P2MA-SDS) is used as a potent support for dispersion of bimetallic palladium-copper nanoparticles (Pd-Cu NPs) towards the electrooxidation of formic acid (HCOOH) in sulfuric acid solution. The Pd-Cu NPs are prepared through galvanic replacement reaction between PdII ions and Cu particles. The SEM, EDS, and electrochemical methods were used to characterize the prepared Pd-Cu NPs. The obtained results show that the Pd-Cu exhibited significantly high current density of HCOOH oxidation compared to other catalysts and utilization of Cu NPs enhances the electrocatalytic activity towards electrooxidation. The enhanced performance is related to the synergetic effect between Pd-Cu NPs and P2MA-SDS film. The effects of galvanic replacement time, potential sweep rates, and concentration of HCOOH on the catalytic activity of the electrocatalyst as well as long-term stability are investigated by the various electrochemical techniques. 相似文献
116.
We present a Chaplygin gas Friedmann-Robertson-Walker quantum cosmological model in the presence of the cosmological constant.
We apply the Schutz’s variational formalism to recover the notion of time, and this gives rise to Wheeler-DeWitt equation
for the scale factor. We study the early and late time universes and show that the presence of the Chaplygin gas leads to
an effective positive cosmological constant for the late times. This suggests the possibility of changing the sign of the
effective cosmological constant during the transition from the early times to the late times. For the case of an effective
negative cosmological constant for both epoches, we solve the resulting Wheeler-DeWitt equation using the Spectral Method
and find the eigenvalues and eigenfunctions for positive, zero, and negative constant spatial curvatures. Then, we use the
eigenfunctions in order to construct wave packets for each case and obtain the time-dependent expectation value of the scale
factors, which are found to oscillate between finite maximum and minimum values. Since the expectation value of the scale
factors never tend to the singular point, we have an initial indication that this model may not have singularities at the
quantum level. 相似文献
117.
Hossein Salavati Shahram Tangestaninejad Majid Moghadam Valiollah Mirkhani Iraj Mohammadpoor-Baltork 《Ultrasonics sonochemistry》2010,17(1):145-152
A Keggin-type heteropolyanion compound (HPO) was doped within the montmorillonite (MMT) structure by impregnation method. The synthesized catalyst was characterized by FT-IR, XRD, UV–vis, CV, SEM and elemental analysis. Based on chemical adsorption between HPO, and hydroxyl surface groups, HPOs nanoparticles were successfully located on the MMT. Moreover, the obtained nanocomposite was found as an efficient catalyst for oxidation of hydrocarbons under reflux and ultrasonic irradiation conditions. 相似文献
118.
Khatamnejad Hassan Khalilarya Shahram Jafarmadar Samad Mirsalim Mostafa Gharehghani Ayat Daei Niaki Seyed Omid 《Journal of Thermal Analysis and Calorimetry》2019,138(6):3977-3986
Journal of Thermal Analysis and Calorimetry - Usage of natural gas in an internal combustion engine that has different combustion technologies is considered a possible solution to reduce engine... 相似文献
119.
Electrical properties of C/Ni films are studied using four mosaic targets made of pure graphite and stripes of nickel with the surface areas of 1.78,3.21,3.92 and 4.64%.The conductivity data in the temperature range of400–500 K shows the extended state conduction.The conductivity data in the temperature range of 150–300 K shows the multi-phonon hopping conduction.The Berthelot-type conduction dominates in the temperature range of 50–150 K.The conductivity of the films in the temperature range about 50 K is described in terms of variable-range hopping conduction.In low temperatures,the localized density of state around Fermi level(F)for the film deposition with 3.92% nickel has a maximum value of about 56.2×10~(17)cm~(-3)eV~(-1) with the minimum average hopping distance of about 3.43 × 10~(-6) cm. 相似文献
120.
Gap States of ZnO Thin Films by New Methods:Optical Spectroscopy,Optical Conductivity and Optical Dispersion Energy 下载免费PDF全文
Vali Dalouji Shahram Solaymani Laya Dejam Seyed Mohammad Elahi Sahar Rezaee Dariush Mehrparvar 《中国物理快报》2018,(2)
The optical reflectance and transmittance spectra in the wavelength range of 300–2500 nm are used to compute the absorption coefficient of zinc oxide films annealed at different post-annealing temperatures 400, 500 and 600°C.The values of the cross point between the curves of the real and imaginary parts of the optical conductivity ɑ_1 and ɑ_1 with energy axis of films exhibit values that correspond to optical gaps and are about 3.25–3.3 eV. The maxima of peaks in plots dR/dλ and dT/dλ versus wavelength of films exhibit optical gaps at about 3.12–3.25 eV.The values of the fundamental indirect band gap obtained from the Tauc model are at about 3.14–3.2 eV. It can be seen that films annealed at 600°C have the minimum indirect optical band gap at about 3.15 eV. The films annealed at 600°C have Urbach's energy minimum of 1.38 eV and hence have minimum disorder. The dispersion energy d of films annealed at 500°C has the minimum value of 43 eV. 相似文献