Mechanochemical activation-assisted low-temperature synthesis of CaZrO 3

Article abstract of DOI:10.1111/j.1551-2916.2006.01335.x

Calcium zirconate (CaZrO₃, CZ) was prepared using a solid-state reaction with mechanochemical activation through vibro-milling, aiming at completing the reaction CaO+ZrO₂ = CaZrO₃ at relatively low calcination temperatures. Changes in the crystallite size and homogeneity of the mixed components CaO and ZrO₂ in the starting mixtures were observed with different milling times. The influence of milling on the incipient temperature of CZ formation and completion of CZ formation was investigated. It is concluded that milling of the reactants for 20 h lowered the incipient temperature of CZ formation from 800° to 600°C, and the temperature of complete CZ formation from above 1100° to 800°C.

Full text of DOI:10.1111/j.1551-2916.2006.01335.x

J. Am. Ceram. Soc., 90 [1] 287–290 (2007)
DOI: 10.1111/j.1551-2916.2006.01335.x
r 2006 The American Ceramic Society
ournal
J
Mechanochemical Activation-Assisted Low-Temperature Synthesis of
CaZrO3
Guotian Yew and Tom Troczynski
Department of Materials Engineering, University of British Columbia, Vancouver, BC, Canada V6T 1N5
Calcium zirconate (CaZrO3, CZ) was prepared using a solid-
state reaction with mechanochemical activation through
vibro-milling, aiming at completing the reaction CaO1ZrO2
5 CaZrO3 at relatively low calcination temperatures. Changes
in the crystallite size and homogeneity of the mixed components
CaO and ZrO2 in the starting mixtures were observed with dif-
ferent milling times. The influence of milling on the incipient
temperature of CZ formation and completion of CZ formation
was investigated. It is concluded that milling of the reactants for
20 h lowered the incipient temperature of CZ formation from
8001 to 6001C, and the temperature of complete CZ formation
from above 11001 to 8001C.
investigate the possibility of synthesis CZ by a solid-state reac-
tion with the aid of the mechanochemical activation process
through vibro-milling, with the principal objective of decreasing
the process temperature necessary to achieve phase-pure CZ.
II. Experimental Procedures
The starting materials included calcium oxide (CaO; Fisher
Scientific, Ottawa, Canada) and zirconyl chloride octahydrate
(ZrOCl2 ꢂ 8H2O; purity 498%, Fisher Scientific, Ottawa, Ca-
nada). ZrOCl2 ꢂ 8H2O was pre-calcined at 5001C to obtain small
crystal size ZrO2 with high reactivity. The mixtures of the react-
ants (pre-calcined ZrO2 plus CaO), in a molar ratio correspond-
ing to ZrO21CaO, were dry mixed for 15 min using a mortar
and pestle. The mechanochemical activation was carried out in
5-g batches for 5, 10, and 20 h, respectively, with a vibratory
Spex 8000 mixer/mill (Metuchen, NJ) in a WC/Co cylindrical
vial (40 mm in diameter and 40 mm in length), containing one
stainless-steel ball (12.7 mm in diameter). The grinding was sus-
pended for 0.5 h after every 0.5 h of milling, to maintain the vial
temperature below 501C. The vial was air tight during grinding
and opened only after it was cooled to room temperature. The
mixtures before and after milling were heat treated in air at
temperatures ranging from 6001 to 11001C for 5 h. The phases
and crystallinity of the precursors and the calcined samples were
determined using an X-ray diffractometer (XRD; Rigaku Mul-
tiFlex, Tokyo, Japan). The decomposition behavior and phase
change of the precursors with temperature were analyzed using
thermogravimetric (TGA) and differential thermal analysis
(DTA; TG-96, Setaram, Caluir, France); the measurement was
performed at 101C/min heat-up under a flow of helium. The
particle size and morphology of the mixture powders after mill-
ing and calcination were examined by scanning electron micro-
scopy (SEM S3000N; Hitachi, Tokyo, Japan). The apparent
crystallite sizes of CaO and monoclinic ZrO2 (m-ZrO2) in the
mixtures milled for different times were calculated from X-ray
diffraction data through the Scherrer equation using the strong-
I. Introduction
ALCIUM zirconate (CaZrO3, CZ) has phase stability in con-
C
trast to zirconia,1 a high melting temperature (23451C),2
and a relatively low thermal expansion coefficient (6.5–
8.5 ꢀ 10ꢁ6 Cꢁ1 2
,
as compared with 10.0 ꢀ 10ꢁ6 Cꢁ1 of cubic
ZrO2,3 12.0ꢀ 10ꢁ6 Cꢁ1 of tetragonal ZrO2, and 13.5ꢀ 10ꢁ6 Cꢁ1
of MgO).4 Additionally, CZ has a relatively high strength (B300
MPa),5 high chemical stability,6 and thus excellent corrosion re-
sistance against alkali, earth alkali oxides, and basic slags.7 As a
result, CZ is a good refractory ceramic for applications such as
anti-clogging nozzles for continuous steel casting and hot zone
bricks of cement rotary kilns.7–10 CZ-based ceramics can also be
used as a sensor to monitor oxygen, humidity, and hydrogen
because of its good ionic conductivity at elevated tempera-
tures.6,11 CZ for refractory applications is normally produced
by a solid-state reaction of calcium oxide or carbonate and
zirconia, at temperatures typically above 13001C, to obtain
phase-pure CZ.12 Hydrothermal synthesis,13 self-propagating
combustion synthesis, and precipitation technique11 were also
used to prepare small amounts of CZ. However, these methods
cannot be used to produce CZ for large-scale refractory appli-
cations due to their high cost.
The mechanochemical activation process has proved to be an
effective technique to enhance a solid-state reaction at relatively
low temperatures.14 In such a process, the ‘‘mechanical’’ effects
of milling, such as reduction of particle size and mixture hom-
ogenization,15,16 are accompanied by chemical effects, such as
partial decomposition of salts or hydroxides,17,18 resulting in
very active reactants. It is expected that mechanochemical acti-
vation could promote CZ formation from solid-state precursors.
To our knowledge, this processing technique has not been re-
ported for preparation of CZ. Hence, this work was initiated to
ꢀ
est diffraction peaks of CaO (200) plane and m-ZrO2 ð111Þ
plane, respectively. XRD machine calibration in peak broaden-
ing was performed using the strongest (101) plane diffraction
line of well-crystallized quartz (4800 nm, powder diffraction file
No. 046-1045) for crystallite size calculation.
III. Results and Discussion
Figure 1 shows the effect of milling activation on the crystallin-
ity and phase composition of the starting mixtures of m-ZrO2,
(powder diffraction file No. 013-0307), tetragonal zirconia (pow-
der diffraction file No. 050-1089), and lime (CaO; powder dif-
fraction file No. 037-1497). After milling for 5 h, the peaks
corresponding to m-ZrO2, tetragonal zirconia, and lime signifi-
cantly decreased and broadened, indicating reduced crystal
sizes. As shown in Table I, CaO crystallite size (calculated
from Scherrer equation) was reduced from 43 to 12 nm and
J. Wang—contributing editor
Manuscript No. 21965. Received June 30, 2006; approved August 11, 2006.
This work was financially supported by the Natural Science and Engineering Council of
Canada.
wAuthor to whom correspondence should be addressed. e-mail: guotian@interchange.
ubc.ca
287

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Vol. 90, No. 1
20 h
10 h
5 h
0 h
30
40
50
20
2Theta (؇)
Fig. 1. X-ray diffractometer pattern of the starting mixtures with and
without milling. &, CaO; J, monoclinic ZrO2; & , tetragonal ZrO2.
Table I. Crystallite Size, Total Weight Loss, and Exothermic
Peak Temperature of the Mixtures Milled for Different Times
Milling time (h)
0
5
10
20
Crystallite size (nm)
CaO
m-ZrO2
43
10
12
6
12
6
11
5
Total weight loss (%)
12.9 12.3 10.0 7.2
Exothermic peak temperature (1C) 895 821 801 772
CaO, Calcium oxide; m-ZrO2, monoclinic zirconia.
m-ZrO2 from 10 to 6 nm after milling for 5 h. With extended
milling up to 20 h, the crystallite sizes of CaO and m-ZrO2 were
11 and 5 nm, respectively, indicating that prolonging the milling
time from 5 to 20 h did not change much the crystal sizes of CaO
and m-ZrO2.
Figure 2 illustrates the influence of high-energy milling on the
decomposition of the starting materials and CZ formation. The
total weight loss of the starting mixture up to 12001C was
12.9%, while the total weight losses of the mixtures after mill-
ing for 5, 10, and 20 h are 12.3%, 10.0%, and 7.2%, respectively
(Fig. 2(b)). The decrease in the total weight loss of the precursor
with the milling time could be ascribed to the decomposition of
partially hydrated CaO and residual ZrOCl2 during milling (as
confirmed through decreasing endothermic peak at B4301C in
Fig. 3. Scanning electronic microscopy micrographs of mixtures after
milling for (a) 0 h and (b) 20 h.
Fig. 2(a)) and loss of water during transfer of the sample from
the mill to TGA. Figure 2(a) also shows the impact of mechan-
ical activation on the temperature corresponding to spinel for-
mation (as confirmed later through XRD). The exothermic peak
at 8951C for the starting mixture in Fig. 2(a), indicative of CZ
formation, shifted to 8211, 8011, and 7721C after 5, 10, and
20 h of milling, respectively. As shown in Fig. 1, the intensity of
monoclinic and tetragonal zirconia, as well as lime peaks,
was decreased and broadened with grinding times, indicating
that the starting components’ crystal size decreased (and also
20 h
10 h
5 h
0 h
20 h
10 h
5wt%
5 h
0 h
0
300
600
900
1200
0
300
600
900
1200
Temperature (˚C)
Temperature (˚C)
Fig. 2. (a) Differential thermal analysis and (b) thermogravimetric patterns of the precursors milled for different times.

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Communications of the American Ceramic Society
289
(a)
(b)
11100ЊC
1000ЊC
1000ЊC
900ЊC
800ЊC
900ЊC
800ЊC
700ЊC
600ЊC
700ЊC
600ЊC
20
30
40
50
20
30
40
50
2 Theta (Њ)
2 Theta (Њ)
(c)
(d)
900ЊC
900ЊC
800ЊC
700ЊC
600ЊC
800ЊC
700ЊC
600ЊC
20
30
40
50
20
30
40
50
2 Theta (Њ)
2 Theta (Њ)
Fig. 4. X-ray diffractometer patterns of mixtures fired at different temperatures for 5 h, after milling for (a) 0 h, (b) 5 h, (c) 10 h, and (d) 20 h. &, CaO;
J, monoclinic ZrO2; & , tetragonal ZrO2; , cubic ZrO2; n, CaZrO3.
ꢃ
possibly they were structurally disordered) and as a result, the
reactivity of the mixtures increased. Accordingly, the mechanical
activation of the precursor favored the solid-state reaction of
calcium and zirconia and the temperature of CZ formation was
lowered considerably.
particle size of the starting materials was substantially decreased
and the number of close particle contacts increased during mill-
ing, resulting in homogenization of the starting components.
Figures 4(a)–(d) shows the dependence of CZ formation on
the milling time and calcination temperatures. The phases of the
unground mixture after firing at 6001C were the same as those in
the starting mixture, i.e., including m-ZrO2, tetragonal zirconia,
and CaO (Fig. 4(a)). CZ began to form in the unground mixture
at 8001C. m-ZrO2 peaks decreased with firing temperature and
disappeared at 10001C. At the same time, the peaks character-
istic of cubic zirconia and CZ increased with the firing tempera-
ture up to 11001C. Phase-pure CZ was not obtained after firing
at 11001C as cubic zirconia and CaO were still present in the
fired samples (Fig. 4(a)).
Milling for 5 and 10 h did not show a noticeable effect on the
phase composition of the mixtures after firing at 6001C
(Fig. 4(b) and (c)). However, CZ peaks (CaZrO3, powder dif-
fraction File No. 076-2401) appeared in the mixture after 20 h of
milling and firing at 6001C (Fig. 4(d)). In comparison, CZ for-
mation started at 8001C (Fig. 4(a)) for the unground mixture
and 7001C for the mixtures milled for 5 (Fig. 4(b)) and 10 h
(Fig. 4(c)). These results indicate that milling of the reactants for
Figure 3 shows the particle size and morphology change of
the mixture during milling. The backscattering electron images
demonstrated large zirconia (white gray) particles of about 10
mm, fine zirconia particles of 3–5 mm, and CaO (dark gray) par-
ticles in the starting mixture (Fig. 3(a)). After milling for 5 h, the
zirconia and CaO particles were not individually visible in back-
scattering electron images and the 1–10 micrometers large ag-
glomerates were composed of homogenously mixed zirconia and
CaO particles (figure not shown). The agglomerate sizes in the
mixture after milling for 5 h were in the same range as the par-
ticle sizes of the starting mixture shown in Fig. 3(a). Mixtures
milled for 20 h (Fig. 3(b)) showed a decrease in agglomerate size
when compared with the mixtures without milling (Fig. 3(a))
and those milled for 5 h. With prolonged milling (20 h), the
larger agglomerates above 5 mm disappeared. Individual sub-
micron particles below 0.6 mm were within the agglomerates
after milling for 20 h (the figure not shown), confirming that the

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Vol. 90, No. 1
Fig. 5. Scanning electronic microscopy micrographs of calcium zirconate mixtures after calcination for 5 h at 10001C (a) without milling and (b) with 5
h of milling.
up to 20 h lowered the incipient temperature of CZ formation
from 8001 to 6001C.
6001C, and the temperature of complete CZ formation from
above 11001 to 8001C. CZ for refractory industrial applications
can be produced by milling a CaO and ZrO2 mixture for 5 h,
followed by firing for 5 h at 8001C.
It is interesting to note that the lime peaks were still present in
the unground mixture fired at 11001C (Fig. 4(a)), but disap-
peared at 9001C in the mixture milled for 5 h (Fig. 4(a)), and at
7001C in the mixtures milled for 10 and 20 h (Figs. 4(c),(d)). This
is expected as extending the mill time decreased the crystallite
size (Fig. 1 and Table I) and homogenized the starting materials
to a sub-micron level, thus increasing the reaction surface and
decreasing the diffusion distance between the reactants. These
effects are well documented and quantified in numerous studies
of solid-state reactions.15–18
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&