Abstract
As a part of the study of sintering behaviors of oxides, the thermal decomposition processes of La2(SO4)39H2O and La2(CO3)3 were investigated by using T. G. A. and X-ray diffraction. From the results obtained, the detailed mechanisms of their decomposition were inferred.
(1) The thermo-gravimetric curves of the specimens were obtained in air, N2, and CO2 gas. The thermal decomposition curves showed three stages for La2(SO4)39H2O and two stages for La2(CO3)3. The chemical reaction of each stage was determined as follows.
La2(SO4)3·9H2O:
La2(SO4)3·9H2O→La2(SO4)3+9H2O (the first stage)
La2(SO4)3→La2O2(SO4)+2SO2+O2 (the second stage)
La2O2(SO4)→La2O3+SO3 (the third stage)
La2(CO3)3:
La(CO3)3→La2O3CO2+2CO2 (the first stage)
La2O3CO2→La2O3+CO2 (the second stage)
(2) These reactions were interpreted to be the first order reaction in all atmospheres. The rate constants for three decomposition stages in air of La2(SO4)39H2O were represented by
k1=105.4exp(-13000/RT) (the first stage)
k2=1012exp(-65000/RT) (the second stage)
k3=1017exp(-134000/RT) (the third stage)
and those for the two decomposition stages in air of La2(CO3)3 were
k1=1013exp(-50000/RT) (the first stage)
k2=1012exp(-67000/RT) (the second stage)
(3) The lattice constants of La2O3 obtained were a=3.89Å and c=6.08Å. Lanthanum sulfate and carbonate decomposed to La2O3 at about 1500°C and 1000°C, respectively.
