Journal of Crystal Growth 249 (2003) 283–293
y- to a-phase transformation subsystem induced by
a-Al2O3-seeding in boehmite-derived nano-sized
alumina powders
Fu Su Yen*, Huei Shan Lo, Hui Lin Wen, Rung Je Yang
Department of Resources Engineering, National Cheng Kung University, No.1, University Road, Tainan 70101, Taiwan ROC
Received 5 February 2002; accepted 1 November 2002
Communicated by C.D. Brandle
Abstract
This study examines inducing an additional phase transformation subsystem using a-Al2O3 seeding techniques in a yAl2O3 powder system that undergoes y- to a-phase transformation. It is found that seeding induced subsystem occurs
independently at temperatures lower than that of the parent (original) y-powder system. The whole system is composed
of two parallel and subsequently occurring phase transformation systems during the heat treatment: the seedinginduced subsystem and the residual parent system. The fraction of the seeding induced subsystem in the whole system
increases and eventually can become predominant with greater amounts of a-Al2O3 added. The subsystem achieves
faster growth of y-crystallites up to the critical size (dcy ) of phase transformation and can begin the transformation at a
lower temperature. Thus as the seeding-induced subsystem becomes predominant, the phase transformation behavior of
the parent system is gradually obscured and is finally replaced by that of the subsystem, with an apparent reduction in
transformation temperature of the overall system. The seeding affected subsystem exhibited lower activation energy in
nucleation stage of the phase transformation. However, the activation energy of the growth stage for both seeded and
unseeded systems may have similar values.
r 2002 Elsevier Science B.V. All rights reserved.
Keywords: A1. Phase transformation; A2. Seeding; B1. Oxides; B1. Nanomaterials
1. Introduction
Effects of using nucleation aids on the formation of a-Al2O3 from boehmite (-gel) and the
sintering behavior for the derived a-Al2O3 ceramics have been extensively examined over the past
*Corresponding author. Tel.: +886-6237-4452; fax: +8866238-0421.
E-mail address: [email protected] (F.S. Yen).
twenty years [1–7], indicating that most of the aids
have positive effects. Seeds that reduce the
temperatures of a-Al2O3 formation and enhance
ceramic densification include a-Al2O3 [1–8], TiO2
[6], Fe2O3, and MgAl2O4 [7]. However, some aids,
for example, a-Cr2O3 [7,9] and MgO [4,9], may
give divergent results. The addition of a-Al2O3
seeds is the best studied [3–6]. However, the end of
the phase transformation process was unchanged
[3]. Furthermore, conceptually, the transformation
0022-0248/03/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 2 2 - 0 2 4 8 ( 0 2 ) 0 2 1 4 8 - 6
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F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
was completely affected by seeding as a whole.
These effects were attributed to the possibility that
seed particles induced the growth of a-Al2O3 by
solid-phase epitaxy during phase transformation
[10]. These observations are generally carried out
using differential thermal analysis (DTA) and
powder X-ray diffraction (XRD) techniques. The
XRD techniques generally show lower sensitivity
than DTA to reveal the transformation [2,11,12].
Boehmite undergoes dehydration at B5001C to
form g-Al2O3, which transforms to d-Al2O3 and
then to y-Al2O3 before undergoing a reconstructive phase transformation to a-Al2O3 [13]. The yto a-phase transformation of alumina is achieved
by a nucleation and growth process [11,12,14,15].
During the thermal treatment, the y-Al2O3 crystallite grows, exceeding the critical size (dcy =
B22 nm, XRD-Scherrer formula [16] on (2 0 2% )y ))
needed for the formation of a-Al2O3 nucleus
(dca =B17 nm, XRD-Scherrer formula on
(0 1 2)a ) [11,12], and nucleation of the a-Al2O3
nuclei occurs. Continuous heating leads to the
growth of a-Al2O3 nuclides to the primary crystallite by coalescence of the nuclides (dp =45–50 nm,
XRD-Scherrer formula on (0 1 2)a ), and the phase
transformation is accomplished.
The y- to a-Al2O3 phase transformation generally shows an exothermic reaction at temperatures ranging between 9001C and 13001C. The
exothermic profile can be divided into lower
ðTn Tb Þ and higher (Tb Tp To ) temperature
components [11], as shown in Fig. 1. The characteristic temperatures result from the variation of
a-Al2O3 formation [11]: Tn is the temperature at
which the first occurrence of nucleation detected
by DTA. Tb is the temperature of the beginning of
large amounts of a-Al2O3 formation. Tp is the
peak temperature and To is the offset temperature
on the profiles. The component (Tb Tp To )
generally exhibits a sharp and strong peak. This is
the common element of the phase transformation.
The peak temperature Tp was regarded as the
transformation temperature [1–5]. It also reported
that during the DTA measurement, the nucleation
Fig. 1. The relationship between a-Al2O3 formation and DTA curves during phase transformation (a) without a-Al2O3 seeded and
adding with (b) 0.05, (c) 0.25, and (d) 0.5 wt% a-Al2O3 seeds.
F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
of a-phase from y-phase persists in both the two
components and can last till the end of the
transformation [12]. The growth process of aAl2O3 crystallites starts before temperature Tb :
However, it occurs mainly in the interval Tb Tp To [12]. Similar cases of two-temperaturecomponent a-Al2O3 formation have been reported
elsewhere by XRD observations [17].
In this study the reduction in temperature of yto a-phase transformation of the nano-sized
alumina powder induced by the presence of aAl2O3 seeds was examined. It is important to point
out that, as the small amount of seed particles
added is likely to affect only the y-crystallite that
surrounds the seed particle within a limited range,
the whole powder system is eventually divided into
two phase-transformation systems: the first occurs
in the y-crystallites that surround the a-seeds,
having a lower transformation temperature (hereafter designed as the subsystem); The second
comprises the residual y-crystallites unaffected by
the seeding (designed as parent system), proceeding the transformation at the original momentum.
And the critical size phenomena present during the
phase transformations of subsystem and the
parent system must occur independently. Thus it
is possible to see that the whole system is
composed of two parallel and subsequently
occurring phase transformation systems during
the heat treatment. Discussion of the coexisting
phenomena of two transformation systems focuses
on both the thermal behaviors of the process as
analyzed by DTA, in relation to the crystallite size
variations of y- and a-Al2O3, and the quantity of
a-Al2O3 phase formation during the two phase
transformation processes. To examine the reduction in temperature of phase transformation, the
activation energies of nucleation and growth of aAl2O3 during the transformation were evaluated
separately using the Johnson, Mehl and Avrami
(JMA) equations [18–20] and the modified expressions of Matsuita [21] and Coats [22].
Boehmite mixed with a-Al2O3 particles as the
seeds of sizes smaller than 0.2 mm in diameter were
used as the starting materials. Amounts of seed
addition were 0.05, 0.25, and 0.5 wt%. The aAl2O3 seeded y-Al2O3 powders were then obtained
by calcination of the boehmite mixtures [1].
285
2. Experimental procedure
2.1. Sample preparation
y-Al2O3 powders prepared by calcination of
chemically precipitated boehmite to which a-Al2O3
seeds were added were used as the starting
materials. Boehmite was obtained using chemical
methods previously reported [23]. Four samples
were prepared: one without a-Al2O3 seed, the
other three with 0.05, 0.25, and 0.50 wt% a-Al2O3
seeds. The a-Al2O3 seed particles with Stoke’s
diameter o0.2 mm were obtained by dispersing
chemical a-Al2O3 powders (E.P., Sumito, Japan)
in distilled water (pH=3) from which the particles
were separated using Stoke’s sedimentation techniques [24], after which the size was checked by
TEM. Size distribution was obtained using a light
scattering particle size analyzer (Malvern Instrument, Zetasizer 1000), and the mean particle size
was 0.235 mm with size-range: 0.170–0.330 mm. An
aqueous solution containing Al3+ (0.1 M) was
prepared by dissolving Al(NO)3 9H2O (E.P.
Baker) in distilled water. Solutions containing
Al3+ and anticipated quantities of a-Al2O3 seeds
then were prepared by mixing the two solutions.
The boehmite precipitates with and without seedaddition were then obtained by adding NH4OH
(Merck) solution to the preparation, adjusting it to
pH=9 at 251C. Filtration and washing processes
were repeated three times with distilled water in
order to remove NO
and NH+
from the
3
4
precipitates. The filtrates were dried at 801C for
24 h and then calcined at 9001C for 100 min,
converting the solid to y-Al2O3 powder. The
calcined powders were ground in an agate mortar
with pestle until finer than 200 mesh (74 mm).
XRD-quantitative analysis showed that the yAl2O3 powders contained less than 1.5 wt% of
a-Al2O3 (including the added a-Al2O3 seeds).
These powders were then used for further thermal
treatments.
2.2. Characterization
To investigate the y- to a-Al2O3 phase transformation, the starting powders were further thermally treated to scheduled temperatures at a
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F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
heating rate of 101C/min, and then quenched to
room temperature (cooling rate>2501C/min).
Crystalline phases in the powders were identified
by XRD (Rigaku, D/MAX IIB) powder methods,
( radiausing Ni-filtered CuKa1 (l ¼ 1:540562 A)
tion, 2y ¼ 8012201; scanning rate: 41/min. The
mean crystallite sizes of y- and a-Al2O3 of the
powders were determined by the XRD-Scherrer
formula (crystallite size=0:9l=Bcos y; where
( B the width at the half-peak
l ¼ 1:540562 A,
height (WHPH) in radians, and y the Bragg angle).
It was applied to peaks of (0 1 2) and (2 0 2% ) of aand y-Al2O3, respectively. The scanning rate was
0.51/min and 2y’s for y- and a-Al2O3 were 24.5–
26.51 and 29.0–34.51, respectively. Data calculations were obtained using Siemens DIFFRAC AT
software. The fraction of a-Al2O3 transformed was
determined by quantitative XRD powder methods
using a CaF2 internal standard [11,12]. The range
of investigation was 1.5–97 wt%. The thermal
behavior of the y-Al2O3 powders was examined
by differential thermal analysis (DTA, Netzsch
STA 409C), using ignited alumina as the reference
material. The heating rate was 101C/min.
The activation energies of nucleation and
growth of a-Al2O3 during the transformation were
evaluated separately using the Johnson, Mehl and
Avrami equations (JMA) and the modified expressions of Matsusita and Coats based on aAl2O3 formation. Since the y- to a-Al2O3 phase
transformation observed on DTA profiles and the
amounts of a-phase formation can be divided into
two stages [20], indicating that the nucleation of aAl2O3 from y-phase persists in both the two stages
and the growth of a-crystallite mainly in the latter
stage, it is possible that the DTA profile accompanied with a-Al2O3 growth can be used to
differentiate the nucleation (Tn Tb on DTA)
and the nucleation+growth (Tb Tp To on
DTA) stages of a-Al2O3 crystallite formation.
3. Results and discussion
3.1. DTA profiles
Fig. 1 shows the relationships between a-Al2O3
formation and DTA profiles during phase trans-
Table 1
Characteristic temperature variations on the DTA curves as
induced by the addition of a-Al2O3 seeds (1C)
Seeds added
Characteristic temperature
wt%
Tns
Tb
Tp
To
0
0.05
0.25
0.5
1000
983
895
865
1218
1216
1225
1225
1254
1242
(1188), 1230
1220
1280
1287
1282
1270
formation of the four y-Al2O3 samples. The
temperature at which a-phase nucleation became
detectable (Tn ) of the sample without seed addition was observed at B10001C (using DTA and
TEM techniques). The temperatures for the
beginning of large amounts of a-Al2O3 formation
(Tb ), peak of a-formation (Tp ) and offset (To ) were
12181, 12541, and 12801C, respectively (Table 1).
In contrast, seeding eventually lowered the characteristic temperatures on the DTA profiles and
the degree of reduction increased with the seed
addition. Compared with unseeded sample, with
0.5 wt% seeding, the Tn was lowered from
B10001C to 8651C (DTA and TEM techniques,
Fig. 1d) and the Tp was lowered from 12541C to
12201C. However, the To temperature of the
samples remained unchanged, except for the
sample with 0.5% seed (12701C).
Detailed comparison on the lower-temperature
(Tn Tb ) and peak (Tb Tp To ) components of
DTA profiles for the samples show that seeding
results in a swelling of the lower-temperature
component, as well as a broadening of the peak
one. However, the sample with 0.5 wt% seed
(Fig. 2d) seems to have profile similar to that of
without seed addition (Fig. 2a), except for the
reduction in characteristic temperatures. Furthermore, the broadened peak shows two subsequently
connected peaks (Fig. 1c and Table 1). It is also
noted that the migration of the peak temperature
from higher to lower may be because one of the
peaks at the higher-temperature wing on the
profile was overlaid, and finally wholly replaced,
by the peak on the lower-temperature wing. These
phenomena indicate that seeding with a-Al2O3
can reduce the characteristic temperatures of
F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
287
Fig. 2. The relationship between the growth phenomena of y-Al2O3 crystallite size and corresponding a-Al2O3 formation (a) Without
a-Al2O3 seeded and adding with (b) 0.05 wt%, (c) 0.25 wt%, and (d) 0.5 wt% a-Al2O3 seeds.
Table 2
Characteristic temperatures for a-Al2O3 formation using DTA
and XRD techniques (1C)
Seeds added XRD
DTA (a-Al2O3 formation, wt%)
(wt%)
Temp. wt% Tns
Tbs
Tb
0
0.05
0.25
0.50
1050
1025
1000
1000
—
1165 (1.3)
1165 (16.2)
1165 (15.5)
1218
1216
1225
1225
1.0
1.0
1.5
2.0
—
983
895
865
(9.1)
(28.9)
(75.0)
(75.4)
Tns : nucleation temperature affected by seeds added.
Tn : nucleation temperature of without seeds added.
Tbs : blooming temperature affected by seeds added.
Tb : blooming temperature of without seeds added.
(0.25 and 0.5 wt% seeds) clearly indicates that
the parent transformation system was existing
which controlled the completion of the transformation. This is comparable to the previous
investigations [4,6] showing that the seeding may
affect the earlier 50% of transformation. Since the
fraction of the subsystem increased with the seed
addition and finally became predominant, the
characteristic temperature of the parent system
would persist at the vicinity of the same temperatures before the exothermic reaction (peak) is
replaced completely by that of the subsystem
affected by the seed (Table 1 and Fig. 1d).
3.2. a-Al2O3 formation
transformation of part of the sample, or can form
a y- to a-phase transformation subsystem that
transforms at temperatures lower than and coexisting with the parent system (Fig. 1 and
Table 2). The closeness of temperatures To ;
12801C (0% seeds), and 12871C and 12821C
The presence of transformed a-Al2O3 in the
unseeded sample observed by XRD techniques
was at 10501C (Fig. 2 and Table 2). Due to the
detection limit of XRD, it is clear that the actual
starting temperature of a-Al2O3 nucleation can be
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F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
lower than 10501C (Table 1). Examining the
amount of a-formation, at temperatures lower
than 12001C, it was less than 5 wt% and only
increased slightly as temperature rose. However,
the amount increased drastically when the heating
temperature exceeded 12181C. The residual yphase persisted until 12601C (XRD examination).
And the accomplishment of y- to a-Al2O3 phase
transformation, referring to To of DTA, was at
B12801C.
Temperatures for the presence of transformed
a-Al2O3 in the seeded samples detected using XRD
were reduced with the increase of seeding amount
(Table 2). In Table 2, Tns and Tn represent the
a-Al2O3 detected (nucleation) temperatures of the
system affected by seeding and the parent system,
respectively. Similarly, Tbs and Tb represent the
temperatures of blooming of the a-Al2O3 nuclei for
the two systems. Since the detection limit of our
XRD for the presence of a-Al2O3 in the test
sample was >1.5 wt%, Tns ’s were adopted from
the observations using DTA and TEM techniques
(Table 1). It is noteworthy that Tns was lowered
with the increase in seed addition. In contrast,
those Tb ’s that were unaffected by the seeding were
slightly shifted to higher temperatures with the
increased seed addition. It should also be noted
that the temperature Tb rose (Tn is obscured) and
the Tp was overlaid by Tps at the lower-temperature wing on the profile and finally replaced by Tps :
It is also clear that both systems made a
contribution to the a-Al2O3 formation listed in
Table 2. Fig. 2 also reveals that Tn and Tbs can be
invisible on DTA profiles. The Tbs temperatures
determined using DTA techniques are about
11651C (Table 2). With the addition of 0.25 and
0.50 wt% a-Al2O3 seeds, the Tbs temperature may
be below 11651C.
3.3. Crystallite size variations
3.3.1. y-Al2O3
Fig. 2 also shows the relationships between the
mean crystallite size variations of y-Al2O3 and the
quantity of a-Al2O3 formation during the transformation. It should be noted that the size was the
mean value contributed from both the seedinduced subsystem and the parent system. And
because the growth of y-crystallite to the size of dcy
is the prerequisite for the occurrence of a-Al2O3
nuclei, and a sufficient amount of a-Al2O3 nucleus
formation is necessary for the recognition of the
occurrence of a-Al2O3 nucleus using DTA (Tn )
and XRD techniques, therefore the mean crystallite size variation of y-Al2O3 and the a-Al2O3
formation measured using XRD and the DTA
profile will be in good accord. Thus the same
notations used in defining the characteristic
temperatures on DTA profiles are adopted to
define the corresponding temperatures explaining
the growth phenomena of y-Al2O3 crystallites.
Fig. 2a illustrates the characteristics of the
growth phenomena of y-crystallites with the
characteristic temperature on the DTA profile.
Before the beginning of nucleation (o10501C in
Fig. 2a), the mean crystallite size of y-Al2O3 was
B11 nm. During the nucleation stage (approaching 10501C, Tn ), the size was B12 nm; after this, it
grew slightly to B13 nm above the temperature
Tn : Before the blooming of a-Al2O3 (10501–
12251C, before Tb ), the size was about 13–16 nm.
0
The growth was suspended temporarily (Tb ;
B13 nm), and then started again. Once the
blooming of a-Al2O3 started (Tb ), or at the growth
stage of phase transformation forming dp -a-Al2O3
crystallites (>12251C), the size increased quickly
from 13–16 to 22 nm (Bdcy [20]). Then y-crystallites disappeared.
The mean crystallite size of y-Al2O3 exhibited
stepwise growth when the a-Al2O3 seeding was
employed, and the sample with 0.05 wt% seeds
displayed typical results (Fig. 2b). A comparison
made on the growth phenomena of y-Al2O3
crystallite size for Figs. 2a and b will show that
there exist two parallel phase transformation
0
systems: the characteristic temperatures Tn ; Tb ;
0
and Tb of the parent system; and Tns ; Tbs ; and Tbs
of the subsystem which is affected by seeding. A
detailed examination will reveal that the mean
0
sizes at points Tn ; Tb ; and Tb were slightly larger
than the sizes measured from the unseeded system
0
(Table 3). And the sizes measured at points Tbs
and Tbs were slightly smaller than that at points
0
Tb and Tb : This size variation is attributed to the
coexisting of the two systems, because seeding
brought about a temperature reduction of 20–251C
F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
289
Table 3
Growth phenomena of average y-Al2O3 crystallite size in the a-Al2O3 seeded y-a-Al2O3 phase transformation systems (nm)
Affected
Not affected
0
0.05
0.25
0.5
0
Tns
Tbs
0
Tn
a
983a
895a
865a
Tbs
Tb
1050
1050
1050
1075
(11)
(11)
(11)
(–)
—
1100 (12)
1075 (12)
1025 (12)
1125 (13)
1150 (14)
1175 (15)
1175? (–)
Tb
—
1200 (14)
1175 (15)
1150 (14)
1218 (15)
1220b(16)
1225b(18)
1225b(17)
a
Measured using DTA analysis.
The true temperature must be slightly higher than listed values.
Tn and Tns : nucleation temperatures.
0
0
Tb and Tbs : temperatures at which most dca -Al2O3 appeared.
Tb and Tbs : growth temperatures.
b
for the subsystem. The presence of dcy -Al2O3 that
transforms to a-Al2O3 nuclei in the subsystem
eventually started at a lower temperature than that
of the parent system, resulting in the seeded
sample showing a slightly larger measured mean
0
y-crystallite size at points Tn ; Tb ; and Tb than that
of the unseeded sample.
Comparing the samples with seeding, although
an increase in seeding percentages will result in the
process overlapping for both the seeding-affected
and the residual parent systems, leading to the
insensibility of some characteristic points’ determination (Figs. 2c and d), clearly, the reduction in
0
temperature of points Tns ; Tbs ; and Tbs increased
with the addition of seed; whereas the tempera0
tures of Tn ; Tb ; and Tb rose with the addition of
seed. And as the seed addition increased to
0
0.5 wt%, the points Tbs (10251C) and Tbs
(11501C) of the subsystem became predominant,
0
replacing the insensible points Tn ; Tb ; and Tb of
the parent system.
3.3.2. a-Al2O3
Fig. 3 shows the relationships between the
formation quantity and the crystallite size variations of a-Al2O3 during the phase transformation.
Fig. 3a describes the growth phenomena of aAl2O3 crystallite of an unseeded transformation
system and the temperature reduction (from Sg to
Sgs ) of a 100% seeding-affected system (dotted
lines) and the growth jump of a-Al2O3 crystallite
from dca to dp (B17–45 nm) for the two systems.
When seeding is conducted, assuming that the
fraction of the seeding affected subsystem is x (the
residual parent system is 1 x; where xo1), the
mean a-Al2O3 crystallite size, dm growth can be
expressed as:
dm ¼ dp x þ dca ð1 xÞ ðxo1Þ:
ð1Þ
The a-Al2O3 growth tracks with 0.05, 0.25, and
0.50 wt% a-Al2O3 seeding are simulated by fitting
the data to expression (1) (dash lines in Fig. 3b–d).
It was found that as the seeds increased from 0.05
to 0.25, and to 0.50 wt%, the Sgs dropped from
11501C to 11251C, and then to 11001C. The x was
about 0.6–0.8, and approached 1 when 0.50 wt%
seed was added.
3.4. Reduction of phase transformation temperature
The reduction of y- to a-Al2O3 phase transformation temperature induced by seeding with aAl2O3 crystallites is generally observed using DTA
techniques [1–7]. The reduction is measured by the
migration of the peak (Tp ) temperature with a
heating rate of 101C/min. It is fairly certain that if
a small amount of the seed is used, the peak
temperature will be lowered. However, detailed
examination would showed that, with increasing
amounts of seed addition (e.g. Fig. 1), the DTA
profile varies both in Tn Tb and Tb Tp To
components: the former swells gradually and the
peak (Tp ) migrates to lower temperatures. In
addition, the temperature reduction phenomena
are attributed to the fact that the seed induced
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F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
Fig. 3. The relationship between a-Al2O3 crystallite size and a-Al2O3 formation (a) Without a-Al2O3 seeded and adding with (b)
0.05 wt%, (c) 0.25 wt%, and (d) 0.5 wt% a-Al2O3 seeds.
subsystem becomes predominant. While the parent
system losses influence, and finally disappears.
The small amount of seed particles added can
only affect the y-Al2O3 crystallite that surrounds
the seed particle within a limited range. An
increase in added seeds will result in an increase in fraction of the seeded subsystem. It
induces a greater quantity of dcy -Al2O3 formation
and subsequent dca -Al2O3 nucleus formation
starting at a lower temperature following the
increase in fraction of the subsystem. The Tp
that migrates to a lower temperature then
results from the increase in the (DTA) peak height
of the seeded subsystem with the simultaneous
decrease in that of the parent system. And it is
clear that the new Tp is that of the subsystem
affected by seeding. This is why the y-Al2O3
powder system has a broadened peak profile on
DTA examination once the a-Al2O3 seeding is
employed.
It is noteworthy that, as the seed increases, the
segment Tn Tb on the DTA curves swells
upward (Fig. 1). The upwelling nose appearing at
the beginning of the exothermic reaction (refer to
Figs. 1b and c) implies that the subsystem affected
by seeding has a higher a-Al2O3 nucleation rate
compared to the original system starting at the
temperature Tns : Since the increase in seed addition is only to increase the fraction of the
subsystem in the overall transformation system,
it is clear that the characteristic thermal behavior
of the parent system with respect to the characteristic temperatures occurring on the DTA profile
remains unchanged during the fraction increase of
the (seeds affected) subsystem. Thus the upwelling
nose found at the temperature close to Tn also
represents an increase in a-formation at the same
temperature contributed by the seeded subsystem.
However, it is more important that the increase in
a-Al2O3 formation at the same temperature
F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
eventually leads to the possibility that once the
a-formation cannot be detected by DTA and XRD
techniques at the temperature, it is then possible,
with an apparent reduction in transformation
temperature Tn (Table 2). It is also possible that
0
slightly increase of temperatures Tn ; Tb ; and Tb
will occur. A recent study [17] of the phase
transition behavior of g-Al2O3 doped by an
alumina-sol reported that a-Al2O3 formation was
observed at temperatures as low as 6001C,
presumably supporting the current study. It
should also be noted that the previous study also
revealed that the transformation was composed of
two steps: one occurred over the temperature
region from 6001C to 9501C and the other
occurred over the region from 9501C to 11001C.
291
Fig. 4. ln[ln(1x)/T 2]-Temperature plot for y-a-Al2O3
phase transformation with 0, 0.05, 0.25, and 0.5 wt% of aAl2O3 seeds.
3.5. Activation energy for y-a-AL2O3
transformation
Since the seeding may stimulate a subsystem
with a lower transformation temperature coexisting with the parent one and both undergo phase
transformation in sequence, it is possible that
seeding may have the effect of accelerating the
kinetics of transformation [4,6,12]. An investigation of the decrease in activation energy is
conducted to determine whether it occurs in the
nucleation stage or the growth stage or both, and
what is the magnitude are significant to this study.
Fig. 4 shows the relationships of ln[ln(1 x)]/T 2
vs. (1=T) 104 according to JMA-Arrhenius
Equation. Here x is the quantity of a-Al2O3
formation during y- to a-phase transformation.
The activation energy then calculated through the
slope obtained times gas constant R: Because the
value is obtained through a-Al2O3 formation, that
may imply the difficulty of a-formation from yphase during the transformation. And because
there are two independent transformation systems
coexisting, the x can consist of the a-Al2O3 formed
from both systems during the transformation at
the some temperature.
Examining the a-formation in relation to the
temperature in Fig. 4, there is clear difference in
slope variation between the unseeded and the
seeded samples. For the former, the slope can
clearly be divided into two parts, above and below
the temperature Tb ; whereas for the latter seeded
samples, more than two slopes become visible. The
activation energies calculated for the unseeded
samples prior to and after the temperature Tb are
171.4 and 676.2 kJ/mol, respectively, for the
powder system. The two values presumably
represent the simple process of nucleation to form
a-nuclei (dca ) and a process involving both nucleation and also growth ðdca þ dp Þ: It is possible to
evaluate the energetics of nucleation and growth
during the phase transformation through these
figures. The activation energies for the nucleation
of a-Al2O3 nucleus and the growth of the nuclei to
the size of primary crystallite are then determined
to be 171.4 and 504.8 kJ/mol, respectively, for the
unseeded system (Table 4).
The activation energy of nucleation for a seeded
system derived from Fig. 4 ranges from 50 to
70 kJ/mol. The values indicate that the seeding
can lower the activation energy of a-Al2O3
nucleation. As to whether the seeding induces a
reduction in activation energy for the growth
process, the evaluation can be achieved by
comparing the determined values between the
unseeded system and the system with a higher
fraction of seeded subsystem, especially those
systems in which the parent system becomes
inferior, for examples, samples seeded with 0.25
and 0.50 wt% a-Al2O3.
F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
292
Table 4
The activation energy of y-a-Al2O3 phase transformation (kJ/mol)
Seeds added (%)
0
Tn Tb
>Tb
171.4
676.2
Nucleation stage
Growth stage
171.4
504.8
Table 4 lists the calculated activation energies for
the first and the last stages estimated from Fig. 4.
Evaluation for the reduction in activation energy
induced by seeding is performed as followings:
(1) Assume that there is x fraction of seeded
subsystem coexisting with the parent system
of (1 x) (xo1).
The activation energy of the last stage for
seeded is Ats ¼ Ans þ Acs ; and for unseeded is
At ¼ An þ Ac :
Here An and Ac are the activation energies
for nucleation and the coalescence growth.
The s represents the subsystem.
(2) The total activation energy, AT will then be
AT ¼ xAts þ ð1 xÞAt
or
At AT
ðAn Ans Þ ¼ ðAc Acs Þ:
x
ð2Þ
ð3Þ
Then let x ¼ 1; and from expression (3) At ¼
676:2 kJ/mol. Additionally from Table 4 AT ¼
ð544:6 þ 580:3Þ=2 ¼ 562 kJ/mol,
An ¼ 171:4 kJ/
mol, and Ans ¼ ð50:7 þ 55:7 þ 68:2Þ=3 ¼ 58:2 kJ/
mol. Substituting AT, An, and Ans into expression
(3) we find
Ac Acs E1:6 kJ=mol:
This indicates that the growth stage would be only
slightly affected by addition of seeds. The activation energies of growth for unseeded and seeded
samples are Ac ¼ 676:8 171:4 ¼ 505:4 kJ/mol
and Acs ¼ 503:8 kJ/mol, respectively. And since
the growth process can be performed by coalescence of the a-Al2O3 nuclei, if the removal of the
0.05
50.7
690.4
0.25
55.7
544.6
0.50
68.2
580.3
58.2
503.2
crystallite boundary is necessary for growth, then
it is possible that the removal may not be affected
by the addition of seed.
4. Conclusions
The seeding-induced subsystem started the
transformation at a lower temperature, compared
with the parent system. The two coexisting systems
then started the phase transformation in sequence
during the heat-treatment. The phenomena can be
examined through DTA profiles and growth
characteristics of y- and a-Al2O3 crystallites during
phase transformation.
The fraction of the seeding-induced subsystem
in the overall sample increased and eventually
could become predominant with greater increased
amounts of the seed addition. Thus as the seeded
subsystem became predominant, the phase transformation behavior observed on the DTA profile
of the original powder system was gradually
replaced by that of the subsystem, apparently
providing a reduction in the transformation
temperature for the whole powder system. The
seeding-induced subsystem exhibited lower activation energy in the nucleation stage of the phase
transformation. However, the activation energy of
the growth stage for both seeded and unseeded
systems may have similar values.
Acknowledgements
This study was supported by the National
Science Foundation of the Republic of China
under Contract No. NSC88-2216-E-006-015.
F.S. Yen et al. / Journal of Crystal Growth 249 (2003) 283–293
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