Mechanical activation of the decomposition and sintering of kyanite

Article abstract of DOI:10.1111/j.1151-2916.2002.tb00475.x

The influence of attrition milling on the thermal decomposition of kyanite (Al₂O₃·SiO₂) to mullite (3Al₂O₃·2SiO₂) and SiO₂, and its subsequent sintering, was studied. A commercial kyanite was attrition-milled for times up to 12 h. Dilatometry confirmed that as-received unmilled kyanite decomposes between 1300° and 1435°C. The decomposition reaction is slow initially and accelerates during the later stages until about one-half of the decomposition occurs in the last 35°C. For the attrition-milled kyanite, the onset decomposition temperature decreases, the transformation temperature interval is reduced, and both the decomposition reaction and subsequent sintering are accelerated. A dense microstructure of fine equiaxed mullite grains in the 1 μm size range, evenly dispersed in a glassy matrix, is obtained by sintering the attrition-milled kyanites. These results are explained in terms of the energy accumulated during attrition milling, a reduction of the milled kyanite particle size, and the presence of a liquid phase during sintering.

Full text of DOI:10.1111/j.1151-2916.2002.tb00475.x

J. Am. Ceram. Soc., 85 [10] 2425–31 (2002)
Mechanical Activation of the Decomposition and Sintering of Kyanite
Joaqu´ın Aguilar-Santilla´n, Ricardo Cuenca-Alvarez, and Heberto Balmori-Ram´ırez*
journal
Department of Metallurgical Engineering, ESIQIE—Instituto Polite´cnico Nacional, A.P. 75-872,
Me´xico City, 07300 Mexico
Richard C. Bradt**
Department of Metallurgical and Materials Engineering, The University of Alabama,
Tuscaloosa, Alabama 35487-0202
The influence of attrition milling on the thermal decomposition
of kyanite (Al2O3⅐SiO2) to mullite (3Al2O3⅐2SiO2) and SiO2,
and its subsequent sintering, was studied. A commercial
kyanite was attrition-milled for times up to 12 h. Dilatometry
confirmed that as-received unmilled kyanite decomposes be-
tween 1300° and 1435°C. The decomposition reaction is slow
initially and accelerates during the later stages until about
one-half of the decomposition occurs in the last 35°C. For the
attrition-milled kyanite, the onset decomposition temperature
decreases, the transformation temperature interval is reduced,
and both the decomposition reaction and subsequent sintering
are accelerated. A dense microstructure of fine equiaxed
mullite grains in the 1 ␮m size range, evenly dispersed in a
glassy matrix, is obtained by sintering the attrition-milled
kyanites. These results are explained in terms of the energy
accumulated during attrition milling, a reduction of the milled
kyanite particle size, and the presence of a liquid phase during
sintering.
andalusite to yield mullite and silica. That investigation revealed
that attrition milling accelerates the decomposition rate of anda-
lusite and reduces the decomposition temperature by nearly 200°C.
The silica is expelled as crystalline quartz. Together with kyanite
and sillimanite, andalusite forms a group of minerals, which share
the same chemical formula (Al2O3⅐SiO2), but differ in their
crystalline structures (Table I).8 None of the sillimanite com-
pounds are present in the Al2O3–SiO2 equilibrium phase diagram,9
although all three occur in nature and decompose on heating to
yield a mixture of mullite (3Al2O3⅐2SiO2) and SiO2. The mullite
content produced by the sillimanite minerals during decomposition
is large (ϳ80%). For this reason they are used as an important
source of mullite in commercial applications.10
The decomposition characteristics are quite different for the
three sillimanite minerals. Sillimanite decomposes at the highest
temperature, while kyanite does so at the lowest temperature.
During their decomposition, the three minerals display a volume
expansion (⌬V/V) that is very large for kyanite (ϳ20%), relatively
small for andalusite (ϳ4%), and intermediate for sillimanite
(ϳ8%). Finally, while the SiO2 expelled by kyanite during the
decomposition is in the form of cristobalite, it is usually amor-
phous silica for the other two group members.11 Both Wilson12
and Jain et al.13 have documented that reducing the particle size
accelerates the decomposition of kyanite.
I. Introduction
IGH-ENERGY, high-intensity mechanical milling has been ap-
H
plied to numerous materials for a variety of reasons, including
the alloying of metallic and ceramic powders,1 the synthesis of
compounds,2,3 activation of chemical reactions, and phase trans-
formations.4 Mechanically alloyed powders are usually character-
ized by the development of supersaturated solid solutions, meta-
stable crystalline or amorphous phases, and nanocrystalline grains
and particles. During mechanical synthesis a chemical reaction is
often completed in situ or is activated by the input mechanical
energy. For example, TiN powder can be produced by milling
metallic titanium powder in a nitrogen-containing atmosphere5 and
the chemical reactions between BaCO3 and TiO2 can be acceler-
ated by milling.6 Attrition milling has been documented to activate
thermal processes.4
Schneider and Majdic14 have suggested that the mullite pro-
duced by the decomposition initiates at preferential nucleation
sites such as grain surfaces, grain boundaries, and defects and then
proceeds into the grains. Jain et al.13 and Ildefonse and co-
workers15 confirmed these hypotheses for kyanite and andalusite,
respectively. The mullite crystals grow in a direction perpendicular
to the elongated direction of the host grains, maintaining a close
relation with the c-axes of the minerals.16 However, the morphol-
ogies of the resulting mullite crystals which are grown from the
different minerals vary. Tomba and co-workers found that elon-
gated mullite crystals are produced during the thermal decompo-
sition of sillimanite.17 In kyanite, elongated, fibrous mullite grains
are developed at low temperatures, but their morphology changes
to a tabular one with rounded edges at elevated temperatures.18
During the thermal decomposition of andalusite single crystals,
mullite single crystals develop in the interior of the andalusite
parent grains, leaving an interconnected capillary network filled
with a silica-rich glass.15
Recently, the thermal decomposition of attrition-milled anda-
lusite (Al2O3⅐SiO2) was reported.7 It was observed that the
mechanical energy input activates the thermal decomposition of
M. A. Stett—contributing editor
Sintering of the sillimanite group of minerals has never been
investigated in detail. It is believed that the thermal expansion that
accompanies the mullitization is detrimental to densification.
However, the large thermal expansion displayed by the decompo-
sition of kyanite has been used to counteract the sintering shrink-
age of some refractories.19 Dilatometric studies of large crystals of
sillimanite,17 kyanite,18 and andalusite7 suggest that these species
may possess rather poor sintering characteristics. However,
pressed compacts of attrition-milled andalusite do attain very high
Manuscript No. 188283. Received September 5, 2000; approved December 13,
2001.
Presented at the 101st Annual Meeting of the American Ceramic Society,
Indianapolis, IN, May 1999 (Refractory Ceramics Division, Paper No. R-015-99).
Financial support for R. Cuenca-Alvarez and J. Aguilar-Santilla´n was provided by
CONACYT under Grant No. 26176a and by CGPI-IPN under Grant No. 981014. H.
Balmori-Ram´ırez received financial support from COFAA-IPN and is a member of
SNI.
*Member, American Ceramic Society.
**Fellow, American Ceramic Society.
2425

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Journal of the American Ceramic Society—Aguilar-Santilla´n et al.
Vol. 85, No. 10
Table I. Characteristics of the Sillimanite-Group Minerals
Kyanite
Andalusite
Sillimanite
Mullite
Chemical formula
Al2O3⅐SiO2
5.5–7
Al2O3⅐SiO2
6.5–7.5
Al2O3⅐SiO2
6.5–7.5
3Al2O3⅐2SiO2
6–7
Hardness
Specific gravity (g/cm3)
Final decomposition temperature (°C)
% expansion
3.53–3.65
1410
3.13–3.16
1500
3.23–3.27
1625
3.15
Ͼ20
Triclinic
ϳ4
ϳ8
Crystalline structure
Orthorhombic
Orthorhombic
Orthorhombic
densities after sintering at 1500°C.7 The resulting microstructures
consist of fine interconnected mullite (ϳ1 ␮m cross section)
platelets.
Studies of the formation of mullite in mixtures of the sillimanite
minerals combined with alumina are rare. Ildefonse et al.15
obtained a mullite with ϳ5% retained glass by the reactive
sintering of micronized andalusite (0.4–10 ␮m) and alumina at
1600°C. There were larger amounts of residual glass when larger
andalusite particles were used. Mullite and mullite–corundum
aggregates have been produced from mixtures of beach sand
sillimanite and corundum fired at 1550°C with additions of 4 wt%
TiO220 or 2 wt% Fe2O3.21 These additives improve the sinterabil-
ity of the aggregates through liquid-phase sintering, but their
effects on the sillimanite decomposition are unknown. No publi-
cations of the mullitization of kyanite–alumina mixtures have been
reported.
The present investigation addresses the influence of attrition
milling on the thermal decomposition of a commercial kyanite.
These experiments are part of a larger investigation to understand
the thermal changes and mullitization processes of the attrition-
milled sillimanite group of minerals.
II. Experimental Procedure
(1) Attrition Milling of Kyanite
A commercial kyanite with the chemical composition (wt%)
58.30% Al2O3, 38.6% SiO2, 0.40% Fe2O3, 1.78% TiO2, 0.82%
Na2O, and 0.10% K2O was used as the starting material. It is
slightly richer in silica than stoichiometric kyanite (62.92% Al2O3
and 37.08% SiO2), and also contains minor impurities.
Batches of kyanite (100 g) were attrition-milled (Union Pro-
cess) in a 304 stainless steel container (3.5 L) employing 3 kg of
stainless steel milling media (3 mm diameter) at a rotational speed
of 400 rpm. The kyanite was milled for 1, 6, and 12 h in distilled
water, recovered by filtration, and then leached in a HCl solution
to remove any iron contamination from the mill and media. The
milled kyanite was then washed in distilled water and recovered by
filtration, dried at 70°C, and sieved through a 325-mesh sieve. The
residual iron content of the leached, attrition-milled kyanite
powders was measured by atomic absorption spectroscopy. These
attrition-milled powders were then uniaxially die-pressed into
disks of 10 mm diameter ϫ 5 mm thickness at 200 MPa and then
cold isostatically pressed at 400 MPa. Sintering was done between
1200° and 1600°C for 1 h in air. A heating rate of 5°C/min was
used.
(2) Characterization
The attrition-milled powders were characterized by X-ray dif-
fraction (XRD, CuK␣ radiation, Model D-5000, Siemens), scan-
ning electron microscopy (SEM, Model 6300, JEOL), and specific
surface area. Chemical composition was analyzed by atomic
absorption spectroscopy.
Dilatometry (Model TMA92, Setaram) was conducted on small
samples (5 mm height) cut from the pressed disks. Heating was to
1600°C at 5°C/min. The samples were placed on a high-alumina
plate and linear dimensional changes were monitored with a
high-alumina probe. Platinum foils separated the sample surfaces
from the alumina fixtures. Phase changes after heating were
followed by XRD. The microstructures of the samples sintered at
1600°C were observed in a scanning electron microscope (SEM,
Fig. 1. Micrographs of the kyanite attrition-milled for different times: (a)
as-received, unground kyanite; (b) kyanite attrition-milled 1 h; and (c)
kyanite attrition-milled 12 h.

October 2002
Mechanical Activation of the Decomposition and Sintering of Kyanite
2427
Fig. 2. XRD patterns of the kyanite at different attrition-milling times. k ϭ kyanite.
Jeol 6100). Samples for SEM were prepared by cutting the sintered
pellets in the transverse direction, grinding and polishing following
standard techniques, and thermally etching at 1400°C for 30 min.
Surfaces of the SEM specimens were coated with a thin layer of an
Au–Pd alloy. The density and porosity of the sintered samples was
measured by the Archimedes technique.
by a very large peak at 26.6° 2␪, the (200) planes, while the
relative intensities of the remaining reflections are smaller as
reported in the JCPDS Card No. 11-46.22 This texture can be
explained by the packing of the raw kyanite particles, which lie on
either of the prism faces that are parallel to the crystallographic
c-directions exposing a large quantity of the (200) planes. The
attrition-milled particles are more rounded, and therefore, this
texture disappears. The XRD line broadening and higher back-
ground levels observed in the patterns of the milled powders are
caused by the combined milling effects such as the reduced
particle size, creation of defects, and lattice strains.
III. Results
(1) Characterization of the Attrition-Milled Kyanite
The iron oxide content of the acid-leached kyanite was 0.40%
and 0.43% for the powders ground for 1 and 12 h, respectively.
These are similar to the initial Fe2O3 content of 0.40% and confirm
that the acid leaching treatment removed any iron contamination
from the milling process.
(2) Decomposition of the Attrition-Milled Kyanite
The linear thermal dimensional changes of the as-received
kyanite and of the attrition-milled kyanite are reported in Fig. 3.
The curve for the kyanite attrition-milled for 6 h is not presented
for the sake of clarity. It lies between those of the kyanites
attrition-milled for 1 and 12 h. The as-received kyanite exhibits a
total linear expansion of 23% that occurs between 1300° and
1435°C. This expansion initiates slowly and then accelerates
about 1400°C. Nearly half of the expansion occurs over a
narrow temperature interval of only 35°C, from 1400° to
1435°C. These temperatures coincide well with those reported
by Sainz et al.,18 but are slightly higher than those of Wilson12
and Schneider and Majdic.23 Above this temperature, shrinkage
occurs, but it is insufficient to compensate for the large lower
temperature expansion.
Figure 1 illustrates the kyanite particles before and after milling.
The as-received kyanite had prismatic crystals with nearly equi-
axed cross sections (ϳ50 ␮m ϫ 100 ␮m) and lengths larger than
300 ␮m. For the 1 h milling time, many of the broken crystals still
retained some evidence of those geometric forms as cleavage
dominates the milling process. However, there were also numerous
submicrometer-size particles. Distinctive geometric forms were
completely eliminated after 12 h of milling, yielding typically
equiaxed submicrometer particles with rough surfaces. Particles
were agglomerated as a consequence of the acid leaching and
drying treatments that followed the attrition milling. The specific
surface area of the powders increased from 0.25 m2/g in the
as-received powder to 12 m2/g after 1 h, 27 m2/g after 6 h, and 43
m2/g after 12 h of attrition milling, respectively. This reflects the
particle size reduction observed in the micrographs.
The dilatometry curves of the kyanite after attrition milling for
1 and 12 h are also presented in Fig. 3. Unlike the as-received
kyanite, these two samples do not exhibit the large expansion.
Rather, after milling, they contract significantly before they
expand. As the milling time increases, the contraction initiates at
The X-ray diffraction patterns for the kyanite after the milling
are shown in Fig. 2. The as-received kyanite pattern is dominated

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Journal of the American Ceramic Society—Aguilar-Santilla´n et al.
Vol. 85, No. 10
Fig. 3. Dilatometry curves of as-received kyanite, kyanite attrition-milled
1 h, and kyanite attrition-milled 12 h.
lower temperatures and a larger amount of shrinkage occurs before
the decomposition expansion initiates. The expansions that follow
are only 6.5%, 3.2% and 2.1% in the samples milled for 1, 6, and
12 h, respectively, much less than the 23% for the raw mineral.
The expansion occurs from 1290° to 1365°C for the sample milled
for 1 h and from 1280° to 1320°C for the sample milled for 12 h.
Shrinkage reoccurs or continues after the decomposition is com-
pleted. The dilatometer curve of the sample milled for 12 h
suggests that the sintering process is near completion at tempera-
tures above about 1500°C.
The nature of the expansions observed in the dilatometer
curves was determined from the XRD of samples heated at
selected temperatures. The results for the as-received, unground
kyanite are presented in Fig. 4(a). The XRD pattern of the
as-received kyanite fired 1 h at 1300°C is similar to the pattern
of the as-received kyanite shown in Fig. 2. Any mullite that
might have formed at this temperature is in such a small amount
that it is undetectable by X-ray diffraction. For the sample
heat-treated at 1400°C, there is a mixture of kyanite and
mullite, but the former phase still predominates. The XRD
pattern of the sample fired at 1500°C reveals the two main
peaks of mullite located at 25.9° and 26.3° 2␪, although its
unsymmetrical shape indicates either that the mullite is poorly
crystallized or that there remains a small amount of undecom-
posed kyanite. This XRD pattern also has a peak of cristobalite
that crystallized from the SiO2 expelled during the kyanite
decomposition.
Fig. 4. XRD patterns of the kyanite heated to different temperatures for
1 h: (a) as-received kyanite, (b) kyanite attrition-milled 1 h, and (c) kyanite
attrition-milled 12 h; (K) kyanite, (M) mullite, (CR) cristobalite.
The XRD patterns of the samples milled for 1 and 12 h and
then heat-treated to 1200°C look similar to those in Fig. 2. The
corresponding XRD patterns of the samples fired at 1300° and
1400°C are presented in Figs. 4(b) and (c), respectively. The
sample milled for 1 h and heat-treated at 1300°C (Fig. 4(b))
shows one peak at 16.4° 2␪ and two peaks to the left of the
(200) peak of kyanite (26.6° 2␪). These two are the beginning
of the formation of mullite. It also has a cristobalite peak. At
1400°C, the peaks of mullite and cristobalite increased in size.
A small peak centered at 26.6° 2␪ suggests that this sample still
contains some undecomposed kyanite. The XRD patterns of the
samples milled for 12 h, fired at 1300° and 1400°C, are
presented in Fig. 4(c). The XRD patterns obtained after firing at
these two temperatures are similar. This sample is completely
transformed after heating to 1300°C for 1 h and has large peaks
of cristobalite.

October 2002
Mechanical Activation of the Decomposition and Sintering of Kyanite
2429
Fig. 5. XRD patterns of the as-received kyanite and of the kyanites
attrition-milled for 1 and 12 h, after firing them at 1600°C for 1 h. M ϭ
mullite.
(3) Sintering of the Attrition-Milled Kyanite
Pellets of kyanite prepared with the as-received mineral, and
with powders attrition-milled for 1 and 12 h, were fired at 1600°C
for 1 h in air. The XRD patterns of these samples only show peaks
of mullite. The cristobalite peak is not present. A characteristic
pattern is shown in Fig. 5. The cristobalite is no longer present
because the 1600°C sintering temperature is in the biphasic
liquid-mullite phase field of the binary Al2O3–SiO2 system. The
amorphous silica, which is present in all of the samples, cannot be
detected by XRD, and therefore does not appear in the XRD
patterns.
A micrograph of the fracture surface of the unground kyanite
heat-treated at 1600°C is shown in Fig. 6(a). It illustrates the poor
sintering characteristics and the presence of severe cracking that it
experienced as a consequence of the large transformation expan-
sion that occurs during the decomposition of kyanite. The prepa-
ration of polished samples of this material for observations at
higher magnification was not possible. By contrast, the samples
milled for 1 and 12 h show a dense microstructure composed of
equiaxed grains of mullite dispersed in a matrix rich in SiO2,
similar to those shown in Figs. 6(b) and (c). The mullite grain size
in both samples is on the order of about 1 ␮m. The mullite grains
appear slightly larger for the kyanite attrition-milled for 12 h and
sintered at 1600°C for 1 h. These mullite microstructures are
distinctly different from those that form during the decompo-
sition of large crystals of kyanite,18 which produce tabular
mullite plates with rounded faces. They are also different from
the typical microstructures obtained during the decomposition
of kaolinite,24 which consist of elongated mullite grains with a
very large aspect ratio.
Fig. 6. Microstructures of kyanite fired at 1600°C for 1 h: (a) as-received
kyanite, (b) kyanite attrition-milled 1 h, and (c) kyanite attrition-milled
12 h.
The densities and open porosities of the three samples fired at
1600°C for 1 h are reported in Table II. The density increases in
proportion to the milling time. Dense samples with less than 2%
open porosity are obtained with the kyanite ground for 12 h. On the
other hand, the unground kyanite is very porous.
IV. Discussion
These results clearly indicate that attrition milling induces
significant changes in the thermal decomposition and sintering
characteristics of kyanite. They can be summarized as follows.

2430
Journal of the American Ceramic Society—Aguilar-Santilla´n et al.
Vol. 85, No. 10
Table II. Density and Porosity of Samples
Sintered at 1600°C
aluminum and silicon ions in and out of the transforming regions
in the parent grains until the chemical composition of mullite is
reached.
Bulk density
(g/cm3)
Open porosity
The attrition-milled kyanite exhibits a very high sinterability.
This is a consequence of the submicrometer particle size of the
milled mineral coupled with the liberation of a silica-rich liquid at
high temperatures. The latter enhances the densification via
liquid-phase sintering. This improved sinterability also contributes
to the reduction of the transformation expansion that is observed in
the dilatometric curves of the attrition-milled kyanite. The high
degree of shrinkage that creates this increased sinterability com-
pensates for the transformation expansion of the kyanite. The
intermediate plateau that appears in the dilatometric curves when
the decomposition of kyanite is complete can be explained by the
formation of mullite and cristobalite as these two phases possess
reduced sintering characteristics at low temperatures.27 Also, the
low-temperature liquid viscosity might have increased signifi-
cantly if it dissolved at least part of the SiO2 expelled during the
decomposition of the kyanite. The presence of TiO2 impurities in
the kyanite can account for the resumption of shrinkage at higher
temperatures, because this oxide favors the formation of a liquid at
a temperature close to 1400°C.20 The consequence is that it is
possible to attain relative densities for kyanite-based ceramics in
excess of 99% by relatively simple and inexpensive ceramic
processing methods based on attrition milling.
Milling time (h)
(%)
As-received, unmilled
1.9
2.8
3.0
3.1
59.3
7.3
4.6
1.9
1
6
12
Attrition milling causes (i) a narrowing of the decomposition
temperature interval of kyanite, (ii) an acceleration of the decom-
position rate of kyanite, (iii) a significant reduction of the volume
expansion associated with the decomposition, and (iv) a greatly
enhanced sinterability. These effects make it possible to obtain
high-density ceramic bodies at relatively low firing temperatures
from kyanite.
The high-energy input of attrition milling is directed toward the
creation of more particle surface area via particle size reduction. It
also creates defects and cracks, as well as extensive deformation of
the crystal lattice, as the XRD peak broadening indicates (Fig. 2).
Several researchers have shown that the decomposition of the
sillimanite minerals starts at the grain surfaces and cracks.13–15
Therefore, it is likely that the attrition milling activates the thermal
decomposition of kyanite, because there is an increased concen-
tration of nucleation sites (particle surface area and cracks) where
mullite formation can initiate. In addition, the growing mullite
grains do not have to maintain a restrictive crystallographic
relationship with other contiguous mullite grains, as is the case
when mullite grows within large grains of kyanite. As a conse-
quence of these factors, the formation rate of mullite is increased
by attrition milling.
The formation of a liquid at high temperatures and changes in
its distribution within the microstructure are additional factors that
help to understand the high mullite formation rate in the attrition-
milled kyanite. The dilatometric curves suggest that a liquid is
formed near 1000°C in the attrition-milled kyanites, as the shrink-
age starts and before the kyanite decomposes. The alkali and the
excess of silica present in the raw kyanite can account for the
formation of this liquid. This primary liquid does not crystallize
during cooling because the XRD patterns of the samples fired
below 1300°C do not have peaks of any crystalline silica. The
lowest liquid formation temperature in the Al2O3–SiO2–Na2O
system is at 1050°C and can be as low as 985°C in the presence of
K2O. Such a liquid can probably activate the formation of mullite
by a solution-precipitation mechanism similar to the one that
operates during the transformation of kaolinite to mullite and
silica.25,26 It is evident that this liquid is more effective in the
attrition-milled kyanites because the solubility of solids in liquids
increases at smaller particle sizes and the solid–liquid interface
reaction area increases.
At higher temperatures, the kyanite decomposes and rejects the
excess silica, part of which might dissolve in the liquid. The XRD
patterns indicate that some of it crystallizes as cristobalite. This
phenomenon is more prevalent when the kyanite particle size is
smaller. Two possible mechanisms could account for the greater
crystallization of cristobalite in the attrition-milled kyanites. First,
a higher density of nucleation sites exists as a result of the smaller
particle size and defects created in the attrition-milled minerals.
Second, the low-temperature liquid may be enriched in silica as the
silica expelled from the kyanite dissolves into it, and this favors the
crystallization of cristobalite.26
V. Conclusions
Attrition milling is an effective process to enhance the thermal
decomposition of kyanite to mullite and silica. The decomposition
temperature interval of attrition-milled kyanite is reduced nearly
100°C relative to that for the as-received mineral. The large
expansion associated with the decomposition of unmilled kyanite
is drastically reduced in kyanite that has been attrition-milled.
Attrition milling also enhances the sinterability as dense glass–
mullite ceramics are formed after firing the attrition-milled
kyanite.
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Although liquid also forms in the unmilled kyanite, both at low
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segregated at the surface. Large kyanite particles do not dissolve in
the liquid as effectively as the small ones. Besides, any isolated
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