Solid-state transformations in crystalline material

[tom0112358]

Hi,

I am an inorganic chemist and I am looking for some guidance on where to find a mathematical/physical description of phenomena which I have been observing in a solid-state transformation. I am working with a crystalline oxide (MoO3) which I expose to elevated temperatures (750C) in a strongly reducing atmosphere (H2/CH4) to effect the transformation:
MoO3 --> MoO2 --> Mo2C

The materials have successively increasing density (cf. 4g/cm3 - 6g/cm3 - 9g/cm3) and for reasons I won't go into, the overall morphology and (aggregate) particle sizes are retained through the reaction albeit in highly fragmented forms. This results in a large increase in surface area with each transformation. The final surface area appears to be unrelated to the initial surface area of the starting material which makes me wonder what basic principles lie behind the fracturing of the initial crystal form and formation of the final contracted particles.

Please let me know if the problem is not clear enough and I will try to explain more fully. Thanks for reading this far!

cheers
Tom

 

[Mapes]

Hi Tom, welcome to PF. You might be interested in Porter and Easterling's Phase Transformations in Metals and Alloys. I know these are ceramics that you're working with, but the concepts of nucleation and growth of new phases apply to them too. P&E provide a very nice description of the factors at play during solid state transformations: the volumetric free energy decrease that drives the reaction, and the surface energy penalty of the new phase and its interface with the surrounding material. The interplay of these factors--along with subtleties such as orientation-dependent surface energy--explain a great number of experimental observations such as crystal faceting and dendritic formation. Besides the strong theoretical base, there are plenty of good photographs, some of which may match what you've seen in your samples and may give a deeper understanding of the fundamentals.

 

[tom0112358]

Thanks Mapes,

Luckily it appears that we have that book in our library... I will check it out.

cheers
Tom