全文获取类型
收费全文 | 194篇 |
免费 | 7篇 |
专业分类
化学 | 141篇 |
晶体学 | 1篇 |
力学 | 13篇 |
数学 | 16篇 |
物理学 | 30篇 |
出版年
2023年 | 1篇 |
2022年 | 20篇 |
2021年 | 13篇 |
2020年 | 14篇 |
2019年 | 15篇 |
2018年 | 5篇 |
2017年 | 12篇 |
2016年 | 5篇 |
2015年 | 5篇 |
2014年 | 4篇 |
2013年 | 16篇 |
2012年 | 9篇 |
2011年 | 12篇 |
2010年 | 10篇 |
2009年 | 7篇 |
2008年 | 7篇 |
2007年 | 12篇 |
2006年 | 9篇 |
2005年 | 5篇 |
2004年 | 3篇 |
2003年 | 3篇 |
2002年 | 7篇 |
2001年 | 1篇 |
1999年 | 2篇 |
1998年 | 2篇 |
1989年 | 1篇 |
1988年 | 1篇 |
排序方式: 共有201条查询结果,搜索用时 0 毫秒
121.
We study the linear stability of a three-layer flow of immiscible liquids located in a periodic normal electric field. We consider certain porous media assumed to be uniform, homogeneous, and isotropic. We analytically and numerically simulate the system of linear evolution equations of such a medium. The linearized problem leads to a system of two Mathieu equations with complex coefficients of the damping terms. We study the effects of the streaming velocity, permeability of the porous medium, and the electrical properties of the flow of a thin layer (film) of liquid on the flow instability. We consider several special cases of such systems. As a special case, we consider a uniform electric field and solve the transition curve equations up to the second order in a small dimensionless parameter. We show that the dielectric constant ratio and also the electric field play a destabilizing role in the stability criteria, while the porosity has a dual effect on the wave motion. In the case of an alternating electric field and a periodic velocity, we use the method of multiple time scales to calculate approximate solutions and analyze the stability criteria in the nonresonance and resonance cases; we also obtain transition curves in these cases. We show that an increase in the velocity and the electric field promote oscillations and hence have a destabilizing effect. 相似文献
122.
Thanaa Abdel‐Moghny Elshafie Ahmed Gad Yasser Mostafa 《Journal of Dispersion Science and Technology》2013,34(1):133-141
Four samples from different crude oils were used for this study: light and heavy crude oils from Iran and two crude oils from Egypt, namely, Ras Gharb and Suez mix. The asphaltenes were separated from these crude oils and then the maltene (non‐asphaltenic fraction) was fractionated into waxes, aromatics, and resins. All fractions were characterized using FTIR and UV spectroscopic analyses in addition to gel permeation chromatograph (GPC). These fractions were tested for their emulsion stability. For chemometric analysis different parameters (variables) have been used to study the effect of different fractions (objects) on the emulsion stability. Such variables included the integrated areas under the stretching absorption peaks of CH in the range of 3000–2800 cm?1, C?O in the range of 1750–1650 cm?1, and the aromatic C?C in the range of 1650–1550 cm?1, as well as UV absorption value at 235 nm and average molecular weight (MW). Principal component analysis (PCA) and multiple linear regression (MLR) were conducted for examining the relationship between multiple variables and the stability of water‐in‐crude oil emulsions. The results of PCA explain the interrelationships between the observations and variables in multivariate data. The correlation coefficients between different parameters derived from PCA reveals that the UV absorption value and MW are strongly correlated with emulsion stability. It also reveals that the resins, asphaltenes, and maltene have better emulsion stability than waxes and lower molecular weight aromatics. The linear relationship between the parameters and the stability of water‐in‐crude oil emulsions using MLR was modeled according to the better statistical results. The obtained mathematical model can be used to predict the stability of water‐in‐crude oil emulsions from the chemical groups and functionalities in each crude oil fraction. 相似文献
123.
Chen G Fujimori K Lee H Nashed-Samuel Y Phillips J Rogers G Shen H Yee C 《Spectrochimica acta. Part A, Molecular and biomolecular spectroscopy》2011,78(5):1646-1650
To address the increasing concern that acetonitrile may be intentionally adulterated to meet the shortfall in global supplies resulting from a downturn in its manufacturing, three analytical techniques were examined in this study. Gas Chromatography with Thermal Conductivity Detection (GC-TCD), Near Infrared (NIR) spectroscopy and Fourier Transform Infrared (FT-IR) spectroscopy were assessed for their ability to detect and quantify potential adulterants including water, alternative organic solvents, and by-products associated with the production of acetonitrile. The results of the assessment of the three techniques for acetonitrile adulteration testing are discussed. 相似文献
124.
Manawwer Alam Naser M. Alandis Fahmina Zafar Eram Sharmin Yasser M. Al-Mohammadi 《高分子科学杂志,A辑:纯化学与应用化学》2013,50(10):698-708
AbstractVegetable oil based environmentally friendly polyurethane-TiO2 nanocomposite coatings have been synthesized by using sunflower oil derived diol, toluene diisocyanate and TiO2 nanoparticles. The chemical structure was confirmed by FTIR and NMR techniques while physico-chemical testing was carried out by standard laboratory methods. Physico-mechanical and anticorrosive tests of the coatings (in different corrosive media) have been investigated by standard methods. In addition to this the morphology and thermal stability behavior of the coatings have been carefully investigated by different techniques like XRD, TEM, TGA/DTG and DSC. The comparison of the performance of nanocomposites with the respective virgin polyurethane coatings reveals that the dispersion of nanoTiO2 enhanced the mechanical, corrosion and thermal stability behavior of the polymer. The synthesized nanocomposites can be used safely upto 250–275?°C. These sunflower oil derived polyurethane nanocomposites can be used in the world of protective coatings, as an alternative of petroleum derived corrosion protective coating materials. 相似文献
125.
Al Angari Yasser M. Ewais Hassan A. El-Shishtawy Reda M. 《Transition Metal Chemistry》2019,44(1):57-64
Transition Metal Chemistry - The redox decolorization of direct violet 31 [DV-31]? by Ag(I)/S2O82? under conditions similar to the Fenton process was studied kinetically in aqueous... 相似文献
126.
A binary mixture of hyoscine butylbromide and ketoprofen was determined by 4 different methods. The first involved determination of hyoscine butylbromide and ketoprofen using the ratio-spectra first-derivative spectrophotometric technique at 211 and 234 nm over the concentration ranges of 2-14 and 5-45 microg/mL with mean accuracies 99.84 +/-0.92 and 99.98+/- 0.64%, respectively. The second method utilized second-derivative spectrophotometry over the concentration ranges of 2-14 and 5-35 microg/mL with mean accuracies 99.32+/- 1.06 and 99.55+/-1.15%, respectively. The third method was based on the resolution of the 2 components by bivariate calibration depending on a simple and rapid mathematical algorithm and quantitative evaluation of the absorbances at 206 and 254 nm over concentration ranges of 2-16 and 5-35 microg/mL; mean accuracies of 100.21+/-1.30 and 100.19 +/-1.07% were obtained for hyoscine butylbromide and ketoprofen, respectively. The fourth method was reversed-phase liquid chromatography using 0.05 M ammonium dihydrogen phosphate-acetonitrile-methanol (20 + 30 + 6, v/v) as the mobile phase with ultraviolet detection at 220 nm over concentration ranges of 1-90 and 5-70 microg/mL; mean accuracies were 99.92+/-1.02 and 99.61+/- 0.98%, respectively. The suggested procedures were checked using laboratory-prepared mixtures and were successfully applied for the analysis of pharmaceutical preparations. The methods retained their accuracy and precision when the standard addition technique was applied. The results obtained by applying the proposed methods were statistically analyzed and compared with those obtained by the manufacturer's method. 相似文献
127.
Nadia Ragab Mohamed Manal Mohamed Talaat El‐Saidi Yasser Mahmoud Ali Mohamed Hilmy Elnagdi 《Journal of heterocyclic chemistry》2007,44(6):1333-1337
128.
Yasser M. Loksha Erik B. Pedersen Paolo La Colla Roberta Loddo 《Journal of heterocyclic chemistry》2007,44(6):1351-1356
129.
We reveal that the overall evanescent wave (EW) power captured by an unclad multimode fiber employed in a sensing configuration is determined by the tunneling modes, not the guided modes. While enormous in strength, most of this power is inaccessible using traditional EW power enhancers. However, we found that by roughening the fiber end face, this supposedly lost power can be recaptured and thus can boost the detectable power level significantly. Intensive mode mixing events across various mode categories are proposed to interpret the observed phenomenon. 相似文献
130.
Muneer Aziz Saleh Ahmad Termizi Ramli Yasser Alajerami Mohammed Damoom Abubakar Sadiq Aliyu 《Isotopes in environmental and health studies》2014,50(1):103-113
The radiation survey of the ambient environment was conducted using two gamma detectors, and the measurement results were used in the computation of the mean external radiation dose rate, mean-weighted dose rate and annual effective dose, which are 144 nGy h?1, 0.891 mSv y?1 and 178 μSv, respectively. A high-purity germanium detector was used to determine the activity concentrations of 232Th, 226Ra and 40K in soil samples. The results of the gamma spectrometry of the soil samples show radioactivity concentration ranges from 19±1 to 405±13 Bq kg?1 with a mean value of 137±5 Bq kg?1 for 232Th, from 21±2 to 268±9 Bq kg?1with a mean value of 78±3 Bq kg?1 for 226Ra and from 23±9 to 1268±58 Bq kg?1 with a mean value of 207±13 Bq kg?1 for 40K. Radium equivalent activity (Raeq) and external hazard index (Hex) were 290 Bq kg?1 and 0.784, respectively, which were safe for the population. The mean lifetime dose and lifetime cancer risk for each person living in the area with average lifetime (70 y) were 12.46 mSv and 7.25×10?4 Sv year, respectively. The results were compared with values given in United Nations Scientific Committee on the Effects of Atomic Radiation 2000. 相似文献