Synthesis and oxidation behavior of MgAlON prepared from different starting materials

Article abstract of DOI:10.1111/j.1551-2916.2009.03408.x

The formation and oxidation of magnesium-aluminum oxynitride (MgAlON) were investigated with emphasis on the effect of impurities through using synthetic raw materials and natural raw materials. It was observed that the formation of MgAlON was not much in

Full text of DOI:10.1111/j.1551-2916.2009.03408.x

J. Am. Ceram. Soc., 93 [2] 322–325 (2010)
DOI: 10.1111/j.1551-2916.2009.03408.x
r 2009 The American Ceramic Society
ournal
J
Synthesis and Oxidation Behavior of MgAlON Prepared from Different
Starting Materials
Guotian Ye,w Junli Shang, Dongrui Zhang, Min Liang, and Yaosheng Chen
High-Temperature Ceramic Institute, Zhengzhou University, Zhengzhou, China
The formation and oxidation of magnesium–aluminum oxynit-
ride (MgAlON) were investigated with emphasis on the effect of
impurities through using synthetic raw materials and natural
raw materials. It was observed that the formation of MgAlON
was not much influenced by the starting materials, but MgAlON
prepared from the natural starting materials exhibited acceler-
ated oxidation. The different oxidation behaviors of MgAlON
prepared from the different raw materials were discussed from
the viewpoint of impurities in the materials. The formation tem-
perature and oxidation temperature of MgAlON produced in
this work were compared with those of MgAlON prepared using
other processes.
MgAlON follows8 MgAlON1O2-MgAl2O4SS1Al2O31N2. It
was also observed13 that variation in the composition of the
MgAlON solid solution did not exert a noticeable influence on
the onset oxidation temperature and the temperature at which
a-Al2O3 began to appear.
In the above reports, monolithic MgAlON was synthesized at
temperatures between 16501 and 17001C. The earlier investiga-
tions on the oxidation behavior of MgAlON were based on
MgAlON synthesized with relatively pure starting materials,
without considering the effect of impurities. However, the use of
natural raw materials with a certain amount of impurities for the
production of synthetic raw materials for refractories is a com-
mon practice. In view of the above concern, in this work, natural
starting materials (magnesite and bauxite) with relatively higher
amounts of impurities (Fe2O3, SiO2, and TiO2) and synthetic
starting materials with relatively lower amounts of impurities
were used to prepare MgAlON, with an emphasis on the effect
of impurities. Then the synthesized MgAlON was oxidized in air
at temperatures between 11001 and 15001C to study the influ-
ence of impurities on the oxidation behavior of MgAlON.
I. Introduction
AGNESIUM–ALUMINUM OXYNITRIDE (MgAlON) possesses
M
high mechanical strength at high temperatures, high re-
sistance to slag and liquid metal,1 and good stability under room
temperature.2–4 Therefore, MgAlON can be used as high-per-
formance refractories.1,5–7 Different approaches have been used
for the synthesis of MgAlON. Dai et al.8,9 prepared MgAlON
using spark plasma sintering (SPS) at 17001C in an N2 atmo-
sphere with MgO, Al2O3, and AlN as the starting materials.
Monolithic MgAlON was produced after firing at 16501C by
carbothermal reduction and nitridation (CRN)10 of MgO,
Al2O3, and C and by a solid–gas reaction11 of Al, Al2O3, and
MgAl2O4 embedded in C and Al2O3 powders under air. Ban-
dyopadhyay et al.12 obtained monolithic MgAlON by a solid-
state reaction of MgO, AlN, and Al2O3 at 16751C in an N2
atmosphere.
II. Experimental Procedure
The samples with a higher purity (HP) were prepared with MgO,
Al2O3, and aluminum powder (Al>98 wt%, o0.074 mm) in a
molar proportion of 1.0:1.86:1.0. MgO was obtained by calci-
nation of commercial 4MgCO3 Á Mg(OH)2 Á 4H2O (ꢀ 99% pu-
rity) at 5001C and Al2O3 by calcination of commercial Al(OH)3
(ꢀ 99% purity) at 6001C. The samples with a lower purity (LP)
were composed of magnesite, bauxite, and aluminum powder in
the same MgO:Al2O3:Al molar ratio as shown above. The chem-
ical compositions of magnesite powder (o0.074 mm) and baux-
ite powder (o0.074 mm) are shown in Table I.
As mentioned above, MgAlON can be used in refractory
materials. However, MgAlON is inevitably oxidized in an oxi-
dation atmosphere at high temperatures, which would change
the phase compositions and properties of the materials contain-
ing MgAlON and consequently influence the high-temperature
performance of the material in industrial vessels. Therefore,
there has been increased interest in the oxidation behaviors of
MgAlON in recent years. It was found that oxidation of
MgAlON powder started at about 7501C,8,11 even if the oxida-
tion rate of MgAlON was very slow below 10001C.13 Although
the oxidation rate increased dramatically above 10001C from the
mass gain results of the MgAlON oxidation test, X-ray powder
diffractometry (XRD) results showed that only monolithic
MgAlON was observed at different oxidation temperatures
up to 11001C.13 Only at 12001C or above, was another phase
of a-Al2O3 detected in the oxidized product besides the cubic
spinel crystalline phase.13,14 The overall oxidation behavior of
The raw materials were meticulously weighed and mixed for
24 h in a rotary mill with zirconia balls (5 mm in diameter) using
ethanol as dispersion with a ball to powder weight ratio of 3:1.
The slurries were dried at 801C, sieved, and uniaxially pressed
under a pressure of 40 MPa to obtain cylindrical samples 20 mm
in diameter and 10 mm in height. The samples were heated with
carbon embedding to 14501 and 15501C and held for 5 h, re-
spectively, in a MoSi2 furnace under a nitrogen (99.99% purity)
atmosphere (0.02 MPa). Carbon embedding was used to keep
the oxygen partial pressure thermodynamically low enough for
the synthesis of MgAlON.14
XRD (X’Pert Pro, Philips, Eindhoven, the Netherlands) was
conducted to identify the phase compositions of the fired sam-
˚
ples using monochromatized CuKa radiation (1.54056 A). For
MgO–Al2O3 spinel, the strongest diffraction peak corresponds
to the (311) plane.15 The lattice constant of MgAlON was cal-
culated based on XRD data of the (311) plane as MgAlON has
the same cubic crystallographic structure as MgAl2O4.16 Mag-
nesium aluminate spinel (AR-78, Almatis, Ludwigshafen, Ger-
many) was used as a reference to compare with MgAlON in the
shifts of XRD peak positions. The morphology of the fired
samples was examined via scanning electronic microscopy
S. Seetharaman—contributing editor
Manuscript No. 26317. Received June 23, 2009; approved August 24, 2009.
Supported by National Natural Science Foundation of China, under Grant No.
50872125.
wAuthor to whom correspondence should be addressed. e-mail: gtye@zzu.edu.cn
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Rapid Communications of the American Ceramic Society
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Table I. Chemical Compositions of the Natural Raw
Materials (wt%)
Al2O3 Fe2O3 SiO2 CaO MgO
TiO2 Na2O LOI
Magnesite 0.64 0.79 0.84 1.25 89.28 0.04 0.09 7.17
Bauxite 86.82 2.57 6.05 0.18 0.13 3.72 0.53
_
(JSM-5610LV, JEOL, Tokyo, Japan). The oxidation tests of
the synthesized MgAlON powder were carried out under
flowing dry air by thermogravimetry (TG; STA449C, Netzsch,
Selb, Germany) at a heating rate of 101C/min. The oxidation
behavior of the synthesized MgAlON was also studied with
XRD by identifying the phases of the samples heated in air for
3 h at different temperatures between 11001 and 15001C.
B
III. Results and Discussion
A
It can be seen from Fig. 1 that a significant amount of MgAlON
was formed from both the synthetic raw materials (HP) and the
natural raw materials (LP) fired at 14501C. When the temper-
ature was increased to 15501C, monolithic MgAlON was ob-
tained in the HP sample, and the major phase of MgAlON in the
LP sample. As shown in Fig. 2, MgAlON formation is evi-
30
40
50
60
70
2 Theta (°)
Fig. 2. X-ray powder diffraction patterns of stoichiometric magnesium
aluminate spinel (A) and higher purity MgAlON obtained after firing at
15501C (B), showing shifts of the peak positions of MgAlON from
MgAl2O4 (%, MgAlON; ~, MgAl2O4).
(a)
denced by shifts in the peak positions, as compared with mag-
nesium aluminate spinel, which is in agreement with the results
reported by Bandyopadhyay et al.12
It is shown in Fig. 1(b) that a small amount of FeO was also
present in the LP sample fired at 15501C, which can be attrib-
uted to reduction of Fe2O3 in bauxite, which was used to pre-
pare the LP samples. As can be seen from Table I, 6.05% SiO2
and 3.72% TiO2 are present in bauxite. However, no phases
containing SiO2 and TiO2 were detected in the bauxite-contain-
ing LP samples after firing at both 14501 and 15501C (Fig. 1(b)).
Possibly, parts of SiO2 and TiO2 were evaporated as SiO(g) and
TiO(g) during firing because of the reduction reactions of SiO2
and TiO2 by carbon/carbon monoxide, and possibly parts of
SiO2 and TiO2, including iron oxide, were dissolved in MgA-
lON. It was found in the oxidation test that SiO2 and TiO2
were present in MgAlON from the LP sample fired at 15501C.
The above results indicate that impurities do not have any in-
fluence on the temperature at which MgAlON was formed, ex-
cept for FeO present in MgAlON from the LP starting mixture.
As reported by others, the firing temperature for monolithic
MgAlON formation was 16501C using CRN,10 16501C by a
solid–gas reaction,11 16751C through a solid-state reaction,12
and 17001C via SPS.8 It can be seen from Fig. 1(a) that the
temperature for forming monolithic MgAlON was 15501C,
which is at least 1001C lower compared with that via a solid–
gas reaction,11 CRN,10 and solid-state reaction,12 possibly be-
cause the in situ formed MgAl2O4 and AlN in the heating
process have a higher reactivity than MgAl2O4 and AlN in the
starting materials.11,12
1550
°
°
C
C
1450
10
20
30
40
50
60
70
80
2 Theta (°)
(b)
1550
°
C
Figures 3(a) and (b) show the morphology of the HP and LP
samples after firing at 15501C with carbon embedding. It can be
seen that the particles are agglomerated in the LP sample
(Fig. 3(b)), because the impurities may enhance sintering of
the powder sample. In contrast, the particles are loosely dis-
persed in the HP sample (Fig. 3(a)) as the impurities in the sam-
ple are relatively much less and consequently sintering would
occur to a lower degree as compared with the LP sample.
Figure 4 shows the weight change of the 15501C-synthesized
HP and LP samples heated in air. It can be seen from Fig. 4 that
the LP sample showed an increase in weight at temperatures
between 4001 and 6501C. The XRD results (Fig. 5) confirmed
that the weight gain in this temperature range was due to the
oxidation of FeO to Fe2O3. The TG results (Fig. 4) indicate that
oxidation of MgAlON prepared from natural raw materials
1450
°
C
10
20
30
40
50
60
70
80
2 Theta (°)
Fig. 1. X-ray powder diffraction patterns of the higher purity sample
(a) and the lower purity sample (b) after nitridation at different temper-
atures with carbon embedding (%, MgAlON; ꢁ, a-Al2O3; m, FeO).

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Vol. 93, No. 2
(a)
B
A
10
20
30
40
50
60
70
80
(b)
2 Theta (°)
Fig. 5. X-ray powder diffraction patterns of the 15501C-synthesized
lower purity (LP) sample (A) and the 15501C-synthesized LP sample
oxidized at 6501C for 3 h (B) (%, MgAlON; m, FeO; }, Fe2O3).
from MgAl2O4. In comparison, MgAlON prepared from the
latter raw materials was entirely oxidized to MgAl2O4 and
Al2O3 at 15001C (Fig. 6(a)). It is interesting to note that appre-
ciable amounts of Fe2O3, TiO2, and SiO2 were present in the
MgAlON sample from natural raw materials after oxidation in
air between 11001 and 13001C (Fig. 6(b)); in contrast, lower
amounts of FeO and no TiO2 and SiO2 were observed in the
as-synthesized MgAlON sample (Fig. 1(b)) from the same
starting materials, indicating that Fe2O3/FeO, TiO2, and SiO2
were present as solutes in the as-synthesized MgAlON solid so-
lution. The presence of Fe21, Ti41, and Si41 in MgAlON would
deform the lattice structure of MgAlON as the radii of Fe21
Ti41, and Si41 are different from those of Mg21 and Al31
,
.
Fig. 3. Scanning electron micrographs of the higher purity sample (a)
and the lower purity sample (b) after nitridation at 15501C with carbon
embedding.
Indeed, the calculated lattice constant (0.80239 nm) of the as-
synthesized MgAlON from natural raw materials is larger than
that (0.80217 nm) from synthetic raw materials. During oxida-
tion in air, MgAlON from natural raw materials with a de-
formed lattice structure possibly has a higher tendency to be
oxidized. Furthermore, the precipitation of Fe2O3 and TiO2
from MgAlON would activate the MgAlON lattice and conse-
quently make MgAlON more vulnerable to oxidation. In sum-
mary, compared with MgAlON from synthetic raw materials,
the lower oxidation temperature of MgAlON prepared from the
natural raw materials was possibly due to the impurities present
in the as-synthesized MgAlON as the natural raw materials
contain appreciable amounts of Fe2O3 and TiO2.
(LP) started at B7501C, while oxidation of MgAlON prepared
from synthetic raw materials (HP) started at B8501C.
It can be seen from the XRD results that alumina began to
precipitate at 12001 and 14001C from MgAlON prepared from
natural raw materials (Fig. 6(b)) and synthetic raw materials
(Fig. 6(a)), respectively. MgAlON prepared from the former raw
materials was completely oxidized to MgAl2O4 and Al2O3 at
13001C (Fig. 6(b)), as evidenced by the peak shifts of MgAlON
7
It can be seen that the complete oxidation temperature of
MgAlON prepared by different approaches with high-purity
raw materials is about 13001C.13,14 In contrast, MgAlON
prepared with high-purity raw materials in this work was
completely oxidized at 15001C. It is not clear why MgAlON
prepared in this work showed higher oxidation resistance than
that in the above-mentioned investigations.
6
HP
5
4
3
2
LP
IV. Conclusions
1
0
MgAlON was obtained from two mixtures with different
amounts of impurities after firing at 15501C under a nitrogen
atmosphere and the only difference in the product phases
was FeO, which existed in the sample from the natural raw
materials. The oxidation of MgAlON prepared from natural
raw materials started at B7501C, while oxidation of MgAlON
prepared from synthetic raw materials started at B8501C. Al-
pha alumina was detected in the oxidized products of MgAlON
prepared from natural raw materials and synthetic raw materials
at 12001 and 14001C, respectively. MgAlON prepared from the
–1
0
200
400
600
800
1000 1200 1400
Temperature (˚C)
Fig. 4. Oxidation temperature dependence of the weight change of the
15501C-synthesized higher purity (HP) sample and lower purity (LP)
sample heated in air.

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Rapid Communications of the American Ceramic Society
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former raw materials and the latter raw materials was com-
pletely oxidized at 13001 and 15001C, respectively. The results
indicate that, in comparison with MgAlON from the synthetic
raw materials, MgAlON from the natural raw materials tends to
be oxidized at the lower temperatures possibly due to the pres-
ence of impurities in the as-synthesized MgAlON. It is proposed
that the deformed lattice structure of MgAlON due to the pres-
ence of impurity and/or precipitation of impurities from MgA-
lON would accelerate oxidation of MgAlON prepared using the
natural raw materials.
(a)
F
E
D
C
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Fig. 6. X-ray powder diffraction patterns of the products of the
15501C-synthesized higher purity sample (a) and lower purity sample
(b) oxidized under the different conditions: as-synthesized MgAlON
sample (A), oxidized for 3 h at 11001C (B), 12001C (C), 13001C (D),
14001C (E), and 15001C (F) (~, MgAl2O4; ꢁ, a-Al2O3; &, TiO2;
m, FeO; }, Fe2O3; |, SiO2).
(2000).
&