الفهرس 
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Abstract
The thesis aims to prepare and study the structural analysis and the semiconductor behavior of Ag-added BaTi03-CuO sensor samples. Also, the present study was performed with the object of investigating the influence of silver additions and sintering temperature on the sensing properties of BaTi03-CuO sensors toward the carbon dioxide gas. This was done by studying some physico-chemical properties using, X-ray diffraction (XRD), thermogravimetric and differential thermal analyses (TGA-DTA), infrared absorption spectra (IR), scanning electron microscopy (SEM), electrical conductivity measurements and the sensitivity measurements toward different concentrations of carbon dioxide gas at different temperatures. The procedure of preparation, investigation and results obtained can be summarized as follow: I- Preparation: 1.1- Preparation of pure BaTiQ3 The semiconductor BaTi03 powder used in this investigation was prepared by the solid state reaction method using BaC03 and Ti02 (1:1 molar ratio). The mixed powder was presintered in a platinum crucible at 1073 K for two hours and then heated for 8 hours at 1473 K in air. 1.2- Sensor samples preparation The BaTi03-CuO based sensor was prepared by mixing the powders of CuO and the obtained BaTi03 in equimolar mixture (1:1 molar ratio). The mixture was mechanically mixed in an agate mortar for 24 h. BaTi03-CuO samples with silver additions were prepared by adding silver nitrate (AgN03) to the mixture of BaTi03 and CuO in different ratios to obtain Ag content after sintering processes with the concentrations of 1, 1.5 and 2 mol %. The materials were compressed to form pellets. The pellets were divided into two groups; the first one was sintered in air at 773 K for 5 h in an electric furnace while the second group was sintered at 973 K under the same experimental conditions. II- Samples characterization: 11.1. X-ray diffraction (XRD) X-ray diffraction investigation of the prepared BaTi03 indicated that all diffraction peaks of the prepared BaTi03 agree well with those of the ICSD card for the tetragonal phase BaTi03. Thus, the prepared BaTi03 by the solid state reaction between BaC03 and Ti02 has only single phase with tetragonal structure. The X-ray diffraction pattern of the CuO, starting material, showed that the CuO powder used in the sensor samples preparation is a pure single phase with monoclinic structure. The X-ray diffraction studied of the prepared BaTi03-CuO based sensor samples sintered at 773 and 973 K showed that no new diffraction peaks appeared except for those from BaTi03 and CuO. Also, no changes in the diffraction angles nor in the relative intensities of each diffraction peaks from BaTi03 and CuO can be recognized. This means that no solid reaction took place between BaTi03 and CuO during the sintering and the BaTi03-CuO mixed oxide is only a mixture of two phases. BaTi03-CuO sensor samples with silver additions of 1.5 and 2 mol % gave weak peaks at 20 = 38.6 (d = 2.35) which coincide with the 100 % peak of silver (d = 2.359, ICSD card). The crystallite sizes estimated from XRD measurements were found to increase with increasing the sintering temperature for all components in the different sensor samples. 11.2. Thermal analysis The thermogravimetric analysis (TGA) of the silver nitrate, starting material, showed that it begins to decompose at nearly 713 K and the complete decomposition took place at 773 K. So the silver nitrate added to the BaTi03-CuO mixed oxide convert to silver particles after sintering at 773 and 973 K for five hours. The TGA of BaTi03-CuO mixed oxide shows the thermal stability of this mixed oxide up to 1073 K. II.3. Infrared absorption studies The IR absorption spectrum of the prepared BaTiC>3 showed that, it has two main absorption bands, the two bands appeared at 550 cm’ and around 400 cm”1 are assigned as Vi and v2 respectively. The higher frequency band Vi can be attributed to the Ti-Oi stretching vibration and the lower frequency band to Ti-On bending vibration. The obtained data are in agreement with those reported by different authors. With respect to CuO starting material, the spectrum showed four absorption bands below 1000 cm’ . The observed bands are assigned as vi, V2, v3 and v4 at 580, 500, 420 and 320 cm”1 respectively. These bands can be attributed to Cu-0 stretching modes. The IR patterns of BaTiOa-CuO and Ag-added BaTi03-CuO samples sintered at 773 and 973 K look like a combination of those of BaTi03 and CuO, all samples show four bands assigned to that of BaTi03 and CuO. This was expected since no chemical reaction took place between them as explained from X-ray investigation results. There is a slight shift in bands positions of BaTi03-CuO samples toward the lower frequency with increasing Ag content. The observed shift may be due to the effect of Ag on bands polarity of metal-oxygen vibration of both BaTi03 and CuO. II.4. Scanning electron microscopy The morphology and particle size of the prepared BaTi03 sample showed a modestly uniform grained microstructure and nearly flat grain edges with some pores at grain boundaries, the average grain size of BaTi03 is 2.3 urn. The microstructure of CuO component showed irregular morphology shape with some pores at grain boundary and little fine pores entrapped within the grains. The average grains size of the CuO is 1.6 um which was less than that of the BaTi03 The SEM micrographs of BaTi03-CuO sensor sample and those of Ag-added BaTi03-CuO with different concentration of Ag and sintered at 773 K were characterized. With respect to the BaTi03-CuO sample the presence of larger BaTi03 particles and smaller CuO particles is clearly observed with some pores at grain boundaries. It was appeared that the mixed oxide of BaTi03-CuO is only a mixture of two phases and the clods of BaTi03 grains just contact mechanically to those of CuO. The microstructure of Ag-added sensor samples showed that the particle morphology varied with the composition. Surface smoothness and particle sphericity improved with increasing Ag content. SEM micrographs of the BaTi03-CuO and Ag-added BaTi03-CuO sensor samples sintered at 973 K were taken. As Ag content increase a similar variation in the morphology of sensor samples with that found in the samples sintered at 773 K. The micrographs showed that with increase the sintering temperature from 773 to 973 K, the average grain size of the sensor samples relatively increase. Also, the particle morphology is more uniform and spherical. Ill- Electrical measurements: 1II.1. Electrical conductivity stmdies The a. c. electrical conductivity as a function of temperature was measured for BaTi03, CuO, BaTi03-CuO and Ag-added BaTi03-CuO sensor samples sintered at 773 and 973 K; the measurements were carried out in the temperature range from 303 up to 783 K. CuO showed a semiconductor behavior in the measuring temperature region, while the other samples showed a typical semiconductor temperature dependence of the electrical conductivity at high temperature region (563-783 K). With increasing the sliver content in the samples and the sintering temperature, the electrical conductivity was found to increase. The activation energy of the BaTi03 and CuO were found to be 1.17 and 0.6 eV respectively. The height of the potential barrier between BaTi03 and CuO of the Ag-added BaTi03-CuO samples slightly decreases with increasing Ag content. Meanwhile there is a slightly change in its values with sintering temperature. This may be due to the ionization of the Ag-particles which leads to the increase in charge carrier concentration and as a result a decrease in the resistance of the sample. III.2. Carbon dioxide sensitivity studies The sensitivity to 2% C02 gas of BaTi03, CuO and BaTi03-CuO sensor sample sintered at 773 and 973 K as a function of temperature was measured. The experimental data indicate that the capacitance of BaTi03 and CuO nearly have the same values in the two atmospheres (air and air plus 2% C02), which means that the sensitivity of BaTi03 and CuO components to C02 gas is very weak. While, the BaTi03-CuO sensor samples revealed a good sensitivity to C02 gas, also it was noted that the sensitivity of the BaTi03-CuO sensor samples begins to increase around 600 K and attains the maximum value at 713 K, which was taking as the working temperature for BaTi03-CuO samples in this investigation. The dependence of sensitivity to 2% C02 on temperature for Ag-added BaTi03-CuO sensor samples sintered at 773 and 973 K showed that, the sensitivity of Ag-added BaTi03-CuO sensor samples sintered at 773 K sharply increases with increasing the operating temperature and attained a maximum at 730 K. The sensor samples sintered at 973 K showed higher sensitivity values than those sintered at 773 K, but also attained the same maximum sensitivity at 730 K. The optimum operating temperature of BaTi03-CuO without additives was 713 K. The addition of Ag slightly increases the optimum operating temperature, but the sensitivity of Ag-added BaTi03-CuO samples to 2% C02 at 730 K is higher than that of BaTi03-CuO without additives by large value. The sensitivity increases with increasing Ag content in the BaTi03-CuO sensor sample but it seems that 1 mol % Ag addition is enough for increasing the sensitivity for practical use . The variation of the sensitivity with carbon dioxide concentration for the prepared sensor samples sintered at 773 and 973 K were measured. The sensitivity of BaTi03-CuO sensor sintered at 773 and 973 K increases with increasing the C02 concentration up to 3 %, then after there is a slight increase in the sensitivity .The sensitivity of the sensor samples with Ag additions sintered at 773 and 973 K showed large increase in sensitivity with C02 concentration up to 10 %. The concentration of the C02 in the normal atmosphere, work places and automotive exhaust was estimated to be 350, 5000 and 104-105 ppm respectively, so this sensor can be used to monitor the carbon dioxide concentration in different purposes. Although the sensor samples without silver additions have nearly the same linear range as that of the sensors with silver additions, but the values of the sensitivity of BaTi03-CuO sensors are smaller. The response and recovery time measurements for the sensor samples showed that, the complete response and recovery time after exposure to 2% C02 and re-exposure to air respectively take place within several seconds and tend to become constant around 100 second. In the final it seems that, the sensor sample consist of BaTi03-CuO with 1 mol % Ag which sintered at 973 K has enough sensitivity to be used in the fabrication of the sensitive part for CO2 gas sensor which used for detection of carbon dioxide gas concentration up to 10”% (100000 ppm) at the working temperature of 730 K and applied frequency of 50 KHz.