Modelling hydrothermal synthesis of ceramic composite NANOmaterials
R.R. Piticescu*, Roxana M. Piticescu*, A. Motoc* and D. Taloi**
*Institute for Non-ferrous and Rare Metals, 102 Biruintei Blvd., 73957
Pantelimon-Bucharest, Romania
** University POLITEHNICA Bucharest, 313 Splaiul Independentei,
Bucharest, Romania
Contact person : E-mail: [email protected]
http://www.imnr.ro
Hydrothermal procedures are interesting chemical routes
for synthesis of many important ceramic nanostructured materials.
Processes include hydrothermal reaction synthesis from solutions and
hydrothermal treatment of amorphous hydrous oxides. Nanostructured
materials can be obtained controlling nucleation by
solubilisation-reprecipitation mechanisms and particle growth.
Thermodynamic prediction of equilibrium compounds formed in the
process is difficult; a reliable estimation may be done using the
Pourbaix diagrams. Some important advantageous of these procedures
are: direct producing of crystalline materials at low synthesis
temperatures in one step, versatility, utilization of low cost
inorganic precursors and important reduction of pollutant effluents,
in accordance with the new environmental protection regulations.
Fundamental studies regarding the mechanisms and kinetics
of the synthesis processes were developed. Considering that in the
initial stage at low pH the nucleation takes place by the hydrolysis
reactions Mz+ +z H[2]O Ű M(OH)[z] + zH+ and knowing the initial molar
concentration of the metal in solution S[0] = [Mz+], putting the
equilibrium concentration of the precipitated species P[e] =
=[M(OH)[z]][e] and noting that according to reaction stoichiometry
[H+]z[e] = (1/z) P[e], one may calculate the equilibrium concentration
of precipitated complex from solution solving the polynomial
equation:
(1/z)z (P[e])z+1 + K[h,g] P[e] - K[h,g] S[0]=0
We approximated the nuclei radius from
r[0] ť (3 k[B] /4pr[0])1/3 (P[e])1/3 (-lnK[h,g])-1/3
[ln(1/1-(P[e]/S[0])]1/3
where k[B] is the Boltzmann constant.
Hydrothermal synthesis of nanopowders of yttria-doped
zirconia, zirconia-alumina nanocomposites, lead zirconate titanate or
barium titantate were successfully done starting form inorganic
soluble precursors.
Thin films of yttria doped zirconia ceramics or PZT could also be
deposited from solutions precursors and suspensions under hydrothermal
using our hydrothermal/electrochemical system composed from CORTEST 2L
Teflon autoclave/VOLTALAB galvanostat/potentiostat. Ni or Ti foils are
the working electrode, the counter-electrode being platinated Nb and
an external AgCl reference electrode with a saline junction was used.
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