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    Applications of the zero-order reaction rate model and transition state theory on the intra-particle sintering of an alumina powder by using surface area measurements
    (Elsevier Science Sa, 2007) Sarikaya, Y.; Ada, K.; Önal, M.
    The cylindrical compacts with the diameter of 14mm were prepared under 32MPa of an alumina powder having agglomeration degree of 80% and maximum surface area of 81 m(2) g(-1) after calcination at 900 degrees C for 2 h. Each compact was fired isothermally at a different temperature between 950 and 1150 degrees C for 2 h. The rate constant for each temperature was obtained from the application of the zero-order reaction rate model on the surface area measurements after sintering. Arrhenius equation for the sintering was obtained in the form: k = (8.09 x 10(6) m(2) mol(-1) s(-1)) exp (-187 643 J mol(-1)/RT). Transition state theory was applied to the sintering and thermodynamic parameters of the activation were calculated. The relation of these parameters in SI units can be summarized in the forms: Delta H-# = 178 643 - 8.314T, Delta S-# = -73.6 - 8.314 ln T, and ln K-# = -Delta G(#)/RT = -(Delta H-# - T Delta S-#)/RT= -21 487/T - ln T - 7.853, where Delta G(#), Delta H-#, Delta S-# and K-# are the Gibbs free energy, enthalpy, entropy, and equilibrium constant of activation. (c) 2006 Elsevier B.V. All rights reserved.
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    Investigation of the intra-particle sintering kinetics of a mainly agglomerated alumina powder by using surface area reduction
    (Elsevier, 2006) Ada, K.; Önal, M.; Sarıkaya, Y.
    An alumina precursor was prepared by the aluminium sulphate (0.20 M) and excess urea reaction in boiling aqueous solution. The precursor was calcined at 900 degrees C for 2 h and then delta-Al2O3 Powder having volumetric agglomeration degree of 80% was obtained. Cylindrical compacts having diameter of 14 mm were prepared under 32 MPa by axial pressing using oleic acid as binder. Each compact was fired isothermally at various temperatures between 950 and 1400 degrees C. The firing time was changed from zero to 2 h. The fired compacts were examined by scanning electron microscopy (SEM) and nitrogen adsorption techniques. The specific surface areas (S/m(2) g(-1)) of the samples were calculated using the Brunauer, Emmett, and Teller (BET) procedure. The rate constant (k) and mechanism-characteristic parameter (n) were obtained for different temperatures between 950 degrees C and 1150 degrees C from the application of the neck-growth sintering rate (NGRM) model on the surface area reduction data. An Arrhenius equation and the parameter n for the sintering were found in the forms of k = (7.648 x 106 h(-1)) exp (-186,234 J mol(-1)/RT) and n = 4.0 x 10(-7) T-3 - 1.7 x 10(-3) T-2 + 2.3 T- 1030.8 respectively. The parameter n changes in the interval 0.61 < n < 1.34 with rising temperature having maximum at about 1025 degrees C. Based on the SEM images and NGRM data, the intra-particle sintering was discussed. (c) 2006 Elsevier B.V. All rights reserved.
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    The investigation of the pore structure of fine alumina powders prepared for ceramic production by the technique of precipitation from solution
    (2000) Ada, K.; Sarıkaya, Y.; Alemdaroğlu, T.; Önal, M.
    In this study, alumina precursors were prepared in three different ways and were converted into three different alumina powders (Al2O3) by calcination. The precipitation methods of the alumina precursors were as follows: i) Addition in excess of a 25% NH3 solution dropwise for two hours, to a boiling 0.08 M A12(SO4)3 solution which was already buffered by NH3 / NH4Cl. ii) Boiling for two hours a 0.006 M Al2(SO4)S solution which contains C2H4(COOH)2, NH4Cl and (NHi)2CO. iti) Boiling for two hours a 0.2 M Al2(SO4)S solution which contains excess (NHj)2CO. In order to find out the effect of the followed procedure on the pore structure, the following determinations were realized. The powder shapes and sizes were determined by electron microscopy. The specific surface areas and the adsorption heats of these powders were calculated from the adsorption data. The mesopore size distribution curves were plotted by using the desoiption data. The specific micropore, mesopore and micropore-mesopore volumes were determined by extrapolating these curves. Finally, the dependence of the pore structure on the procedure of preparation was discussed in detail.