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B. Prochowska-Klisch, A. Malecki / Thermochimica Acta 335 (1999) 99±104
While the kinetics of Co O thermal decomposition
3
3. Results and discussion
4
is quite well described, the detailed mechanism of
microscopic processes corresponding to diffusion and
chemical reaction remains unknown. Doping of
The preliminary measurements showed that decom-
position of cobalt spinel doped by nickel ions carried
out in constant temperature is diffusing inhibited, just
like it was found for Co O decomposition. In result
2
Co O by Ni ions should in¯uence the electron
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4
transport process [3] that could help to explain the
microscopic nature of Co O thermal decomposition.
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4
only in the ®rst, relatively short period the rate of
reaction was high enough for recording. After this
time diffusion processes took the control over the
decomposition and the reaction rate has signi®cantly
decreased. Finally the reaction rate became so low that
recording was almost impossible though the conver-
sion degree has reached only 0.4±0.6, depending on
the temperature. The way to omit this problem was to
measure the kinetics of the process at constant oxygen
pressure and in conditions of rising temperature.
At ®rst the mixed-control model of reaction was
tested for the kinetic analysis of the results obtained.
The attempt to introduce temperature as a function of
time into the mixed-control model of Co O thermal
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4
The goal of the present work is to ®nd the effect of
nickel ions presence on the kinetics of the studied
reaction.
2
. Experimental
2
The powder samples of Co O doped with Ni
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3
ions used in experiments were prepared by thermal
decomposition of cobalt(II) and nickel(II) nitrate(V)
mixtures (analytically pure). The nitrate mixtures
of appropriate composition were decomposed at
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7
50 K in oxygen ¯ow and next annealed for 1 h at
00 K [4].
The X-ray diffraction analysis of the samples pre-
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4
decomposition has shown that analytical form of
equation connecting conversion degree ꢀ, with the
reaction time is impossible to derive. The main cause
of this is that producing of the nuclei of new phase
(CoO) and the rate of its growth are the complex
functions of temperature. The consequence of this is
that in discussed mixed-control model of reaction, the
ratio of chemical reaction and diffusion in the rate of
the whole process changes not only as a function of ꢀ,
but also as a function of temperature and what is more,
this function is unknown. In consequence it is impos-
sible to obtain the simple equation that could be the
analogue of Eq. (1) for non-isothermal conditions.
pared showed the presence of just one phase corre-
sponding to Co O spinel phase, with slightly shifted
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4
most intensive lines in the spectrum, that might be the
effect of nickel ions built in the spinel structure. The
grain size of the powder samples was lower than
2
� 3
3
0 mm and the speci®c area was 7.7±9.2 m cm
Gravimetric analysis and atomic absorption spec-
.
troscopy method were used to determine the chemical
composition of the samples obtained. In experiments
the samples of the following composition were used:
Co2.90Ni0.10O , Co2.85Ni0.15O , Co2.80Ni0.20O .
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4
4
The kinetics of decomposition of the prepared
samples as well as Co O was measured in the volu-
Fig. 1 shows how the rate of Co2.85Ni0.15O decom-
4
position changes during the course of reaction.
For the kinetic description of the obtained ꢀ(t)
dependencies it was necessary to create the simpli®ed
model of reaction proceeding in non-isothermal con-
ditions.
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4
metric kinetic apparatus. The measurements were
carried out at the constant oxygen pressure and at
�
1
heating rate ꢁ 3 K min . The experimental oxygen
pressures were 2.7, 4.0, 6.7, 10.0, 13.3, 20.0 kPa.
The sample mass was 20±140 mg, depending on
experimental oxygen pressure. In the preliminary
measurements it was stated that mechanism of exam-
ined reaction is not in¯uenced by the mass of sample
in the mass range used in experiments.
It was assumed that the rate of Co3 NixO decom-
� x
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position is proportional to Áp p � p and to nuclea-
r
tion rate J:
V kꢀpr � p Á J Á gꢀꢀ; (3)
where g(ꢀ) is some function of decomposition degree
which has to satisfy condition:
The conversion degree ꢀ, was determined as a ratio
of oxygen volume evolved from the sample till
moment t, to oxygen volume evolved after the com-
plete decomposition of sample.
ꢀli!m1gꢀꢀ 0:
(4)