Browsing by Author "Rezaie H.R."

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Effects of chelating agents on the sol-gel synthesis of nano-zirconia: Comparison of the Pechini and sugar-based methods
(2020-05-01) Kazemi F.; Arianpour F.; Taheri M.; Saberi A.; Rezaie H.R.
This study focused on the comparison of the Pechini and sugar-based combustion synthesis methods to produce nano-zirconia. Zirconium hydroxide was utilized as metal precursor and citric acid, sucrose, and fructose were used as chelating agents, followed by calcination at 500, 600, and 700°C in air, respectively. Characterization was conducted by thermal analysis, specific surface area measurement, Fourier transform infrared spectroscopy, X-ray diffraction, and scanning and transmission electron microscopy. When sucrose and citric acid were used as chelating agents during synthesis, mixtures of monoclinic and tetragonal phases were formed after calcination at 600 and 700°C. In the fructose samples, the tetragonal structure was the unique characterized phase. The tetragonal parameters in the fructose samples were determined using the diffraction data and the lattice parameter ratio was proven to increase with the temperature increase. Compared with the citrate and sucrose samples, the largest specific surface area (27 m2·g−1) and smallest particle size (39.1 nm) were obtained for the fructose sample after calcination at 700°C. The study revealed the formation of single-phase stabilized tetragonal zirconia using fructose as chelating agent after calcination at 500°C, and the presence and formation mechanism of stabilized tetragonal phase were also discussed on the basis of the X-ray and electron diffraction studies.
Kinetic study of carbothermal reduction of zirconia under vacuum condition
(2020-01-01) Kazemi F.; Arianpour F.; Rezaie H.R.
In this research, formation mechanism and kinetics of vacuum carbothermal synthesis of zirconium carbide using zirconium acetate and sucrose are discussed. The study of non-isothermal reduction was conducted by thermogravimetry analysis and heating the samples in argon and vacuum conditions up to 1773 K, and then the heat exchange values of reactions were calculated. Isothermal formation mechanism of carbide phase was investigated by heating the samples at 1473 K and 1673 K in argon and vacuum atmospheres followed by X-ray diffraction and quantitative phase analysis. Results showed that in non-isothermal state, the carbothermal reduction of zirconia is a heterogeneous reaction with multiple steps. For isothermal reaction, the kinetic parameters such as activation energy and pre-exponential factor were calculated as 70.56 kJ mol−1 and 11.22 × 10−2 S−1, respectively. It was presented that the activation energy value extracted from isothermal reaction is completely in accordance with the final step of non-isothermal results.
Thermodynamic study of zirconium carbide synthesis via a low-temperature pyrovacuum method
(2020-09-01) Arianpour F.; Kazemi F.; Rezaie H.R.
In this research, the thermodynamic aspect of the nano-sized zirconium carbide production is investigated via a facile, low-temperature and cost-effective carbothermal method under vacuum and argon atmospheres. The starting materials were zirconium acetate and sucrose as zirconium and carbon precursors, respectively. The gels were prepared based on 3, 4, 5, and 7 molar ratios of carbon to zirconium and heated at 1200 and 1400 °C under vacuum and argon atmospheres. The formation of zirconium carbides under different atmospheres were studied via thermogravimetric analysis and the results were compared. The phase composition and microstructural features were investigated using X-ray diffraction and scanning electron microscopy, respectively. According to the thermogravimetric results and performed thermodynamic calculations, it was revealed that the ZrC formation starts at 1200 °C under vacuum. It is also demonstrated that the formation of nano ZrC powder with crystallite sizes smaller than 30 nm, completely occurs after processing at 1400 °C in vacuum. The measured lattice parameter value of the optimized sample was equal to 4.7003 Å.