الفهرس 
Chapter one: Theoretical introductionOnly 14 pages are availabe for public view
 |  | from | 122 |  |  |
| | | from | 122 | | |
Abstract
The aim of the present study was to inspect the microstructure, electrical and mechanical properties of ZVM varistors in addition to the grain growth kinetics have been investigated.
Solid State Reaction method was used for preparing series of solid samples for the required research as follows:
The preparation of samples was performed according to the following chemical formula: (99.5-x)mol.%-ZnO-0.5mol.%V2O5-xMn3O4, x takes on the values of (0 → 1) mol.% .where then the samples are burnt at 120 °C for 12 hours and then thermally treated (sintering) at 900 °C for 5 hours. Several techniques have also been used, such as X-ray diffraction (XRD), scanning electron microscope (SEM), spectrophotometric analysis of the dispersion of the energy of the X-ray (EDX), density in a manner Archimedes (density Archimedes) and micro-hardness to study the structure, morphology and elemental analysis of the samples along with it was also the study of the mechanical properties and electrical properties of some samples prepared.
Also, other varistor samples, which has the chemical formula: 99 mol. % ZnO-0.5 mol.% V2O5- 0.5 mol. % Mn3O4 were also prepared. Then they burned at a temperature of 120 °C for 12 hours and these samples were divided into two groups: the first group is thermally treated at 900 °C for different times (0.5, 1, 2, 3, 4 hours). And the other is thermally treated for 3 hours at different temperatures (825, 850, 875, 900 and 950 °C). The outcome results from the above measurements are abstracted as follows:
1 - X-ray diffraction patterns confirmed the presence of Wurtzite zinc oxide as major phase and the presence of secondary minor phases, such as (Mn-rich) and Zn3 (VO4) 2 , ZnV2O4 and VO2.
2 - Electron microscope images revealed the presence of zinc oxide major phase, other secondary phases, such as (Mn-rich) and Zn3 (VO4) 2 , ZnV2O4 and VO2, in consistence with the above XRD results.
3 - (EDX) results revealed the presence of secondary phases on the boundaries of the phase-grained zinc oxide and manganese ions, as well as vanadium ions exist within the zinc oxide grain and on the boundary.
4 -the dependence of the density and hardness of the samples on the doping content of manganese oxide, x mol. % Mn3O4, it is noted that both of them has decreased with an increase in the Mn3O4 content. This is attributed to the increased molar volume of the
ABSTRACT
III
samples and weak linkage between the grains of zinc oxide, as well as existence of some porosity during sintering process.
Also, the results of heat treatment (sintering) dependence of ceramic density showed a decrease in density with increasing temperature of sintering in the range from 825 to 950°C due to volatility of some particles of vanadium, which has a low melting point (690 °C), but an increase in density with increasing time of sintering from 0.5 to 4 hours at the same temperature (900 °C) is obtained. This is due to the promotion of V-species liquid phase e.g., Zn3 (VO4)2, for densification of ZnO grains and porosity release during sintering at different time.
5 - The results of the ultrasounds measurements of the samples, showed that the average grain size has a significant impact on: speed of the longitudinal ultrasonic waves, longitudinal modulus of elasticity and coefficient of attenuation of longitudinal waves scattered in the ceramics studied, where it was observed that the decrease in average grain size increases both the speed of longitudinal waves and elastic modulus and reduces the attenuation coefficient of the waves and vice versa.
6 - The characteristics (J-E) curves show the presence of two zones on each curve: ohmic region before breakdown voltages and other non-ohmic region after the breakdown field. All the electrical parameters, mentioned below, are calculated from these characteristic curves.
7 - Electrical conductivity, voltage height barrier, donor concentration of charge carriers, density of surface states and the barrier width, all depend on the mol. % of Mn3O4 content.
8 - The breakdown field, nonlinear coefficient and leakage current density are dependent on the varistor composition. Anyhow, the highest coefficient of nonlinearity obtained at 0.5 mol. % of Mn3O4 and sintered at 900 °C for 5 h time is α = 16.29, so it has been elected for extending the study of this composition with different temperatures at same sintering time as well as, with different times at the same sintering temperature where this may play an important role in the varistor properties.
9 - The sintering processes (temperatures and times) have shown to affect on the electrical properties of the varistor (breakdown field, nonlinear coefficient, leakage current density, voltage barrier, donor concentration, density of surface levels and barrier width). These effects are discussed.
10 -The grain growth kinetics of studied varistor whose chemical formula: 99 mol. %
ABSTRACT
IV
ZnO- 0.5 mol. % V2O5- 0.5 mol. % Mn3O4 was performed by using the well-known phenomenological equation of grain growth kinetics. The results showed that the grain growth exponent (n) is equal to 4 and the activation energy (Q) of 143.72 kJ/mol. Comparing these results with previously published ones, it is found that the obtained value of n = 4 is greater than those reported for purified zinc oxide (n = 3 and Q = 224 kJ/mole) [84] and even bigger than that of binary system ZnO- 0.5 mol % V2O5 (n = 1.6) [85]. These results illustrate that the addition of manganese to the varistor matrix leads to reduce the growth rate of zinc oxide grains.