الفهرس 
AbstractOnly 14 pages are availabe for public view
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Abstract
Three different samples of CeO2/ZnO nanocomposites have been synthesized by two different techniques, co-precipitation and sol–gel with and without dispersing agent. The microstructure and crystal structure were determined by SEM, TEM and XRD-patterns. The frequency and temperature dependent of the electrical characteristics and the dielectric behavior of the prepared materials have been studied in details. The changes in ac-conductivity with frequencies at various temperatures obey universal Jonscher power law. The dependent of dc-conductivity on the temperature was found to follow Arrhenius plot. The frequency- dependent behavior of the extracted parameters from nonlinear fitting of ac-conductivity shows that the ac-conduction mechanism could be attributed to the correlated barrier hopping model (CBH) at high frequency, whereas at low frequency (42–100 Hz) the quantum mechanical tunneling model (QMT) is the appropriate conduction mechanism. The dielectric constant ɛ′ has high values with a strong size dependent for CeO2/ZnO nano-composites which appear low dependent on temperature for S2 and S3 at a temperature range of 25–250°C which was attributed to the reduction in grain size for these samples. However, the dielectric constant ɛ′ for S1 has a strong dependence on temperature which may be a result of the reduction in surface polarization with increasing crystallite size. A key conclusion from the recent studies is that the method of preparation has a great effect on the crystal structure, grain size and electrical properties of the prepared nano-composites. The properties of dielectric materials with high dielectric constants make them new candidates for voltage-tunable devices such as tunable filter and phase shifter, etc.
Following the third technique, Mn, Mg and Ni doped CeO2/ZnO nanocomposites were prepared with two different molar ratio of each dopant (2 and 5 molar %). Furthermore, XRD pattern of these multi-metal oxides revealed that all samples have both cubic-CeO2 and hexagonal-ZnO with lower peak intensity than those of the pure oxides owing to the alteration in scattering factors between Zn2+, Ce4+ and the other ions referring to decrease in the crystal size. SEM data confirmed that both 5Mn and 5Ni have porous form while 5Ni sample’s surface has more homogeneous pores than 5Mn. In addition, TEM images gave well-matched data with XRD results indicating that the particle size of each sample is less by 10nm than that of the pristine oxides (” ” ” ” " ~ " ” ” ” ”39 and 38 nm). UV-VIS analysis showed that there are two peaks (264 and 336 nm) were recognized due to the localized-charge transfer transitions of Ce-O including a number of surface Ce4+ ions with different coordination numbers differing between 4 and 8. Because of the great bathochromic shift, 5Mn has the lowest band-gap value. The electrical properties were examined for all samples at the same frequency and temperature range. The results clarified that as the amount of Mn ions increased the space charge polarization enhanced. Since, Mn ions, as a transition metal, have various oxidation states. The increase of ɛˋ may be attributed to the hop of electrons between the same elements and ac-conductivity of 5Mn and 2Mn showed a semiconductor- metal phase transition. The conduction mechanism was changed from electronic to ionic mechanism for 5Mn. While for 2Ni and 5Ni samples, the dielectric values are comparatively higher than those of S3 referring to increasing the polarization hence increasing the conductivity. The activation energy indicated that the conduction mechanism of 5Ni is electronic and of 2Ni converted from electronic to ionic mechanism with mainly CBH and QMT models. Moreover, conduction mechanism of both 2Mg and 5Mg samples electronic mechanism following QMT, NSPT and CBH.
Nanocomposite modified glassy carbon electrodes of 5Mn and 5Ni were prepared separately and utilized as an electrochemical sensor to determine Pb2+, Hg2+ and Cd2+ ions with concentration range 15-30 ppm. The capability of the prepared electrode to determine these ions was estimated by the gathering the ions for 5 min on the electrode surface at potential of -1.3 V versus Ag/AgCl, KCl (3.0 M) and its electrochemical stripping in the electrolyte by using anodic stripping voltammetry technique. The desired analyte was electro-deposited on the surface of the working electrode during a reduction step followed by the oxidation from the electrode surface during the stripping step. The anodic peaks of Pb2+, Cd2+ and Hg2+ were −0.40, −0.71 and 0.22V, respectively. The calibration curve of each electrode with each ion was measured according to (I_p=aC+b). LOQ and LOQ were calculated for every analyte with each electrode and the results revealed that 5Ni/GC electrode has the lowest values because of the homogeneous and lower pore size that increasing the surface area exposed to the electrochemical reaction. Also, the effect of the scan rate was tested for Pb2+ for each electrode. The diffusion coefficient of 5NiGC was 9.072×10−6 cm2/s while it was 9.196×10−6 cm2/s for 5Mn/GC.
The synthetic wastewater polluted by heavy metal treated by commercial lime and hydrothermal prepared Na-A zeolite. The effect of time was studied and the best time for complete precipitation was 30 min. Then, the effect of initial concentration was investigated. The removal capacity of the precipitation as hydroxide was nearly 98.5% of lead ions and then decreased to 70.9% for 600 ppm. The same behavior was observed for cadmium ions where the efficiency decreased from 80.44 to 62.1% of 500 and 600 ppm respectively.
The removal efficiency of Pb2+ and Cd2+ by Na-A zeolite increased with increase the metal ion concentrations till 300 and 200 ppm respectively, and then dropped with raising the concentration. The ion exchange process is easy by exchanging ions in a small amount in the surface interiors of the zeolite, thus allowing the continuation of the ion exchange process inside the zeolite until it reaches a certain limit of exchange. The surface ion exchange process will increase, and this prevents the process of entering of ions into zeolite interiors and accordingly the exchange process will decrease despite of the presence of a large amount of ions required to be removed from the solutions.