1070-4272/05/7802-0200 + 2005 Pleiades Publishing, Inc.
Russian Journal of Applied Chemistry, Vol. 78, No. 2, 2005, pp. 200!203. Translated from Zhurnal Prikladnoi Khimii, Vol. 78, No. 2, 2005,
Original Russian Text Copyright + 2005 by Kamaev, Archugov, Mikhailov.
OF SYSTEMS AND PROCESSES
Study and Thermodynamic Analysis of the ZrO
D. N. Kamaev, S. A. Archugov, and G. G. Mikhailov
South-Ural State University, Chelyabinsk, Russia
Received November 2, 2004; in final form, September 2004
Abstract-The system ZrO
was additionally studied by means of differential thermal analysis.
The temperatures of the solid-state decomposition of zircon and the coordinates of the eutectic point were
refined. A thermodynamic calculation of the system in terms of the theory of subregular ionic solutions was
carried out on the basis of the experimental data obtained.
The positive effect of zirconium on steel properties
 is caused by its vigorous reaction with oxygen,
nitrogen, sulfur, and silicon to give stable chemical
compounds . In particular, zirconium additives
make smaller the grain size, raise strength, improve
weldability, and enhance oxidation and corrosion re-
sistance. A thermodynamic analysis of how zirconium
added to a steel interacts with oxygen dissolved in
it is necessary for solving problems associated with
steel alloying with zirconium in metallurgy. If silicon
is present in a steel, information on the interaction
of zirconium and silicon with oxygen can be obtained
when precise data on the Fe3Zr3Si3O system are
available. In an attempt to carry out a thermodynamic
analysis, we found that published data on the phase
diagram of the ZrO
system are ambiguous.
It has been shown  that heating of zircon
) to high temperatures leads to its dissociation
into oxide components. There is an opinion  that
zircon melts incongruently at 1775 + 10oC, whereas
its eutectic temperature is appreciably lower: 1675oC
(94 mol % SiO
). According to , the melting point
of zircon is 1720 + 20oC. The authors of  believe
decomposes even in the solid phase at
According to experimental data on the high-tem-
perature behavior of zircon , it decomposes into
component oxides at 1676oC, and its eutectic point
is 1687oC. These data are consistent with the results
of , but differ from the data of . However,
the eutectic point was not determined in , and only
the decomposition of zircon in the solid phase and
the approximate eutectic composition (about 97 mol %
) were reported.
However, assessment  of the results obtained in
a study of the zircon synthesis from ZrO
 shows that zircon is thermodynamically stable ap-
proximately up to 1727oC, and the free energy of its
dissociation into component oxides is zero at 1771oC.
This value differs by only 4oC from the temperature
reported in .
To eliminate these contradictions and to obtain a
thermodynamic description of the system ZrO
we determined the temperature of zircon decomposi-
tion, its eutectic point, and eutectic composition.
A modified installation for precision high-tempera-
ture thermal analysis  was used in the experiments.
The addition of the third crucible with a reference
substance with well-known properties  into the cell
maked it possible to obtain data accurate to no less
than +10oC in the temperature range 170032100oC.
Earlier, experimental data of high precision have been
obtained on a similar installation [11, 12].
Samples for the experiments were prepared from
analytically pure silicon and zirconium oxides. To re-
move moisture and adsorbed admixtures, the start-
ing oxides were calcined in air at 1200oC for 3 h and
immediately after that were placed in weighing bottles
stored in a desiccator in dry air. The accuracy rating
of the balance used to prepare weighed samples guar-
antied that the composition deviated from a calculated
value by no more than 0.01 wt %.
Samples with the SiO
content varied in the range
803100 mol % with a step of 2% and a sample con-
taining 50 mol % SiO
were prepared. After being