I don't have the data, but as alpha is a compact phase whereas gamma is porous, I guess that the thermal conductivity of alpha is higher. But the difference should not be that high.

Alain

Dear Alain

Thank you, in general I agree with you the porosity is one of the main issues (but there is no data about that, if you can you share it with me). 

Another issue how about the effect of the crystal structure?

Regards

M.Shahien

About porosity (in fact, vacancies), look at the thread below.

Alpha has a higher density than gamma (3.97 against 3.65 g/cm3), and this is enough to give a difference of conductivity. I don't se how to compare the effects of crystal structure alone, as one is defective and the other not.

Alain

https://www.researchgate.net/post/What_are_the_differences_between_alpha_and_gamma_alumina

The Effect of Structure on the Thermal Conductivity of Plasma Sprayed Alumina

1983 MRS Meeting.

H.C. Fiedlera1

a1 Metallurgy Laboratory, General Electric Company, Corporate Research and Development, Schenectady, New York 12301

ABSTRACT

Plasma sprayed deposits of alumina are normally of the metastable gamma phase, which transforms to alpha upon heating to an elevated temperature. The poor thermal conductivity of gamma phase is improved by transforming to alpha, but the improvement is restricted by the fragmented nature of the structure, a consequence of alpha being a denser phase than gamma.

Alpha phase has been reported to form when deposits are made on preheated substrates. It was found that reducing the distance between gun and deposition surface, thereby raising the temperature of the latter, resulted in transforming gamma to alpha, and as the temperature continued to rise, alpha formed directly from the liquid. The thermal conductivity of alpha formed directly from the liquid is 0.27 Watt/cm K at 323 K, which approaches the thermal conductivity given for 99.5% pure, 98% dense, polycrystalline alpha alumina.

Dear Alain

Thank you so much.

Regards

M.Shahien

Dear Aseel

Thank you and I will try to get that paper, because it fit the main target of my question.

Regards

M.Shahien

density can be related to thermal conductivity,

are these two phases have the same density

???

The Alpha has a higher density (about 3.97 g/cm3) than gamma (about 3.65 g/cm3).

So the thermal conductivity of alpha is higher.

Dear Mohammed Shahien ,

Dear Dr. Shahien:

The previous approach is thrue but also is not sufficient to an unequivocal  assignement of polymorphic phases, same chemical formula but distinct crystalline structure. There are another possibilities that can be leads to phase mismatch identification. Some ideas below commented can be added to concept of packing density to a reliable indexation of phases.

See, for solids, the thermal conductivity (K)  exhibits well defined but not well behaviored mechanism of thermal conduction. A priori, two mechanisms of thermal conductivity are operational depending in part of gradient temperature and nature of material.

The first mechanism is based on electrons with strong correlation to the electrical conductivity of solid (for a non metal solid: possibility  of a long range conductivity and or short range conducivity). The second mechanism of thermal conductivity is Lattice Vibration Waves or phonon ( in fact a quantized lattice vibration). Here, the temperature undergo an increasing with increases of the amplitude of the lattice vibration.

Typically one of these mechanisms above dominate the thermal conductivity. Furthermore, sometimes, the dominant mechanism of thermal conductivity is can be a surprise, as an example in metal Cu. In a solid ionic, a mechanism based on the phonon can be expected. In ionic structures, the phonon tend to propagate with a "kind" of further efficience in a more densily close packed structure. But, in a broad sense, at oxide materials, a serie of factors have influence under the magnitude of thermal conductivity as follow: chemical bond nature (ionic to covalent balance), heterogeneity of sub-lattice cationic, presence of a heteroatom at lattice (non detected previously), extensive presence of another materials (commonly the thermal conductivity at alumina oxide is reported at material with 94% of purity) and band structure (extra level at gap can affect the thermal conductivity). It is important have in mind that in a general way, the assessment of parameter thermal conductivity has as a fundamental characterisitic of measurement the stationary condiction, in which the parameter temperature is a constant during the measurement of thermal conductivity. Then, alfa and beta varieties can have the same thermal conductivity if proper tempertures be choosed. Also, alfa and beta alumina can be show opposite level of magnitude of therma conductivity, at a same temperture if a proper dopant/additive be present in solid solution.

See, above in specific ceramic refractory based in alumina was not approached in specific. But, two aspects should be highligthed, the first is that temperature at which the thermal conductivity is measured is important, as well as the amount of additives or contaminant in solid solution in the crystalline lattice.

Kind regards.

Marcos Nobre

Dear Dr Marcos

Thank you and I agree with you that several aspects must be considered like the purity, measurement temp, additives, also if the Al2O3 in included in some composite the other phases may affect,----

Best Regards

M.Shahien

Hi

different phases according to their calcination temperature i.e. gamma alumina around 500 °C however, alpha alumina is more than 1200 °C

Alpha has a higher density than gamma (3.97 against 3.65 g/cm3), and this is enough to give a difference of conductivity, which is higher in alpha phase

hope this is helping

Have a very good day