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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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    The effect of Al3+ concentration on the properties of alumina powders obtained by reaction between aluminium sulphate and urea in boiling aqueous solution
    (Elsevier Sci Ltd, 2002) Sarikaya, Y.; Ada, K.; Alemdaroğlu, T.; Bozdoğan, I.
    Homogeneous precipitation of alumina precursors was carried out in boiling aqueous solutions that contained excess urea and 0.010 0.200 M Al3+. The obtained precursor precipitates were dried for 2 h at 100 C to yield precursor powders and the precursor powders were calcined for 2 h at 1000 degreesC to yield alumina powders. Precursor powders and alumina powders were investigated by X-ray diffraction (XRD), electron microscopy (EM), particle size distribution (PSD) analysis and nitrogen adsorption-desorption techniques for characterisation. Inspection of the XRD data showed that the precursor powders were amorphous in nature and the final product was delta-Al2O3, Investigation of the EM and PSD data of the powders showed that they contained unequally sized spheres and agglomerates. It was determined that, as the Al3+ concentration in the boiling aqueous solution increased, the number of 0.3-1.0 mum sized independent particles decreased and the sizes of agglomerates grew between 1.00 100 mum while their volume percentages increased from 50 to 80%. It was also observed that the specific surface areas of the alumina powders decreased from 98 m(2) g(-1) to 54 m(2) g(-1) and their specific micropore-mesopore volumes decreased from 0.31 cm(3) g(-1) to 0.14 cm(3) g(-1) as the Al3+ concentration in the boiling aqueous solution increased. It was observed that it was possible to prepare alumina powders that have different particle size distributions and different porosities by changing the Al3+ concentration while keeping the [urea]/[Al3+] ratio constant at a value of 5.4 in the boiling aqueous solution. (C) 2002 Elsevier Science Ltd. All rights reserved.
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    An indirect thermodynamic model developed for initial stage sintering of an alumina compacts by using porosity measurements
    (Springer, 2012) Sarikaya, Y.; Onal, M.; Ada, K.
    The specific micro- and mesopore volumes (V) of alumina compacts fired between 900 and 1250 A degrees C for 2 h were determined from nitrogen adsorption/desorption data. The V value was taken as a sintering equilibrium parameter. An arbitrary sintering equilibrium constant (K (a)) was estimated for each firing temperature by assuming K (a) = (V (i) - V)/V, where V (i) is the largest value at 900 A degrees C before sintering. Also, an arbitrary Gibbs energy (Delta G (a) A degrees) of sintering was calculated for each temperature using the K (a) value. The graph of ln K (a) versus 1/T and Delta G (a) A degrees versus T were plotted, and the real enthalpy (Delta HA degrees) and the real entropy (Delta SA degrees) of sintering were calculated from the slopes of the obtained straight lines, respectively. On the contrary, real Delta GA degrees and K values were calculated using the real Delta HA degrees and Delta SA degrees values in the Delta GA degrees = -RT lnK = 165814 - 124.7T relation in SI units.