Grain growth in CaTiO3-perovskite plus FeO-wustite aggregates

Grain growth kinetics in CaTiO3-perovs-kite + FeO-wustite aggregates were studied at the conditions of T = 1223-1623 K, P = 0.1 MPa and P = 200 MPa. Starting samples were fabricated by hot-pressing mechanically mixed powders of CaTiO3 + FeO with FeO = 0%, 1%, 3%, 6%, 10%, 20% and 100% by weight in a gas-medium apparatus at 1323 K and 300 MPH for 5 h. The increase of grain size (G) of CaTiO3 with time (I) follows a growth law: G(n) - G(n)(o) = kappa . t(kappa = kappa(0) exp(- (Q/RT)). Two grain growth regimes are observed at T < 1523 K and T greater than or equal to 1523 K. For T < 1523 K, the best fits of the data to the growth law yield growth exponents of n = 2.2 +/- 0.2, 3.0 +/- 0.3 and 3.5 +/- 0.3 for samples with FeO = 0%, 3% and 10% respectively. Under these conditions the rate constants, K, obey an Arrhenius relation with Q = 206 +/- 35 kJ/mol and 385 +/- 65 kJ/ mol for samples with FeO = 3% and 10%. Grain growth of CaTiO3 becomes sluggish when FeO content exceeds 64%. For T greater than or equal to 1523 K, the best fits of the data to the growth law yield n = 2.5 +/- 0.2 for both samples with FeO - 3% and 10%. The activation energies (Q) were determined as 71 +/- 30 kJ/mol and 229 +/- 45 kJ/ mol for samples with FeO = 3% and 10%, respectively. The TEM observations show a remarkable difference in the distribution and geometry of FeO below and above 1523 K: nanometer-sized particles of FeO were observed alone CaTiO3 grain boundaries in samples annealed at T < 1523 K. No FeO particles were detected along CaTiO3 grain boundaries in samples annealed at T greater than or equal to 1523 K, but large clusters of FeO particles are observed locally indicating a fast separation of FeO from CaTiO3. Thus we conclude that the slow growth rate CaTiO3 at T < 1523 K is due to the pinning by FeO particles at grain boundary, and that the change of grain growth kinetics in CaTiO3 at T greater than or equal to 1523 K may relate to the separation of FeO from CaTiO3, which we interpret as due to the phase transformation of CaTiO3 at around 1523 K.