ARTICLE IN PRESS
Journal of Crystal Growth 265 (2004) 137–148
y-Crystallite growth restraint induced by the presence of
a-crystallites in a nano-sized alumina powder system
Pei Ching Yu, Fu Su Yen*, Tien Chun Lin
Department of Resources Engineering, National Cheng Kung University, No. 1 University Rd. Tainan, Taiwan 70101, ROC
Received 23 October 2003; accepted 13 January 2004
Communicated by C.D. Brandle
Abstract
The restraint of y-Al2O3 from crystallite growth induced by the presence of a-crystallites in the nano-sized alumina
powder system is examined. The y-crystallite powder was obtained by calcination of boehmite. a-Crystallites present in
the y-powder system, either through addition externally to boehmite or introduced internally by y-crystallites that
transform to a-during y- to a-phase transformation of the boehmite, affect the rate of y-crystallite growth. Small
amounts of external a-crystallite addition initiates the formation of a subsystem in the boehmite precursor in which the
formed y-crystallite grows faster, bringing about part of the powder system to experience a temperature reduction of yto a-phase transformation on DTA profiles. With large amounts of a-crystallite addition, another subsystem in which
the rate of y-crystallite growth slows down occurs. a-Crystallites introduced spontaneously during phase
transformation, although with small amounts of the a-crystallite presence, less than 10–20%, whose effect can be
eliminated, show similar effects of restraining y-crystallites from growth. Activation energy calculations demonstrated
that, once the a-crystallite appeared, more than B100 kJ/mol was required for y-crystallite growth.
r 2004 Elsevier B.V. All rights reserved.
Keywords: A1. Growth models; A1. Nucleation; A1. Phase equilibria; B1. Nanomaterials; B1. Oxides
1. Introduction
y- to a-phase transformation of nano-sized
Al2O3 powder is performed by a nucleation and
growth process [1–3]. During the thermal treatment, the y-Al2O3 crystallite grows, exceeding
the critical size (dcy ¼ 20B25 nm, XRD-Scherrer
formula [4] on ð2 0 2% Þy ) needed for the formation
of a-Al2O3 nucleus (dcy ¼ 17 nm, XRD-Scherrer
*Corresponding author. Tel.: +886-6-235-5603; fax: +8866-238-0421.
E-mail address: [email protected] (F.S. Yen).
formula on (0 1 2)a), and the nucleation of a-Al2O3
nuclei takes place. The formation of one a-Al2O3
nucleus occurs from one y-Al2O3 crystallite.
Continuous heating leads to the growth of acrystallites,
exceeding
the
primary
size
(dp ¼ B50 nm, XRD-Scherrer formula on
(0 1 2)a) [3,5,6], and the phase transformation
proceeds to accomplishment. Generally, the transformation starts at temperatures around 1000–
1150 C (using a heating rate 10 C/min), or lower
[7,8]. However, the transformation lacks a transformation temperature, Tc in the G2T (Gibbs free
energy and temperature) diagram [9], indicating
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved.
doi:10.1016/j.jcrysgro.2004.01.026
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that the transformation is triggered once the
critical size of phase transformation, dcy is
obtained and the rate of crystallite growth of
y-phase can thus affect the temperature reduction
of the phase transformation, if the kinetic requirement is satisfied.
One interesting study examined the presence of
residual y-crystallites in y- to a-phase transformation systems that underwent multiple thermal
treatments [6]. Due to the critical size phenomena,
it is inevitable that, even in a well thermal-treated
system, there can be substantial small amounts of
y-crystallites present, because these crystallites
could not reach the critical size, dcy ; during the
thermal treatment. However, effects of a-Al2O3
crystallite addition in boehmite-derived y-Al2O3
powders were examined previously [10,11]. The
reduction in y- to a-phase transformation temperatures or ‘‘seeding effects’’ [10,11] and the
co-existence of two parallel y- to a-phase transformation systems occurring in sequence [11]
in the a-Al2O3-added systems were reported.
The a-Al2O3-crystallite added externally to the
boehmite precursor will initiate a phase transformation subsystem in the original one, in which
y-crystallites grow faster, reaching the critical
crystallite size for phase transformation faster,
and thus the subsystem will experience a temperature reduction of phase transformation, compared
to that of the original. The fraction of the
subsystem increases and eventually becomes predominant with the increase of a-crystallite addition, resulting in a temperature reduction of phase
transformation for the whole system. Examinations also noted that the subsystem exhibited lower
activation energy in nucleation stage [11]. However, the phenomena occur only if a small amount
of a-crystallite addition is employed, being less
than 1.5 wt% [10,11]. Meanwhile, since y- to
a-phase transformation lacks Tc point, samples
with coarser y-crystallite sizes will experience
phase transformation at lower temperature. And
it will eventually yield more a-formation at the
same temperatures, compared with that yielded
from samples with smaller y sizes [6].
These examinations reveal that the growth rate
of y-crystallite affects the transformation temperature of a-Al2O3. It is clear that the phase
transformation of the y-crystallites to a-phase in
a y-powder system, due to the discrepancy in sizes,
cannot happen simultaneously. Moreover, the
growth rates affected by the presence of a-Al2O3
crystallite in a y-powder system seems uncertain,
depending on the present amounts of a-Al2O3
crystallite. And it has been noted that a quasihomogeneous nucleation of a-Al2O3, or a simultaneous formation of dca-Al2O3 nuclide in y- to
a-phase transformation system is requisite for the
preparation of mono-sized a-Al2O3 powders,
especially for powders with sizes near to 50 nm
(primary crystallite size) [7]. However, corresponding to it, the DTA profile normally discloses that a
time (temperature) interval is needed for the
completion of y- to a-phase transformation,
indicating that there is considerable time difference
between the first and the last appearance of aAl2O3 crystallite in the transformation. Thus it is
apparent that examining the effect on the
y-crystallite growth induced by the presence of
a-crystallite in y- to a-phase transformation
systems is crucial both to academic and to
industrial applications in terms of forming monosized nano-a-Al2O3 particles.
This study examines the effect on the rate of
y-crystallite growth induced by the presence of
a-Al2O3 particles in y-Al2O3 powder systems.
Because the crystallite growth of y-Al2O3 terminates at the critical size of phase transformation,
dcy ; above which y will transform to a, the growth
rate was examined by measuring the amounts of
a-formation accompanied with DTA profiles
[3,5,6,11] as well as the variations in crystallite
size (XRD-Scherrer formula) and activation energy of growth of the y-Al2O3. Further, because it
is difficult to evaluate whether a homogeneous
mixing of the y- and a-Al2O3 particles with
10–100 nm in diameter is obtained, the a-particles
were introduced into y-powder systems by (1)
adding a-Al2O3 particles externally as the outsider
or (2) through spontaneous occurrence internally
in the powder system by y- to a-phase transformation of the y-powder systems. Both types of effects
were examined. To the former, B10, 30, and
44 wt% of a-Al2O3 particles were added. To
the latter two groups of samples were prepared:
For DTA purposes, the a-Al2O3 contents were
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10–80 wt%; for activation energy calculation of
y-crystallite growth, the a-phase contents were
simply divided into larger and smaller than
10 wt%. But three y-Al2O3 powder samples with
crystallite sizes 12.6, 18.5, and 21.7 nm were
prepared.
2. Experimental procedures
2.1. Sample preparations
y-Al2O3 powders were prepared by calcination
of boehmite (Remet Chemical Corp USA) at
temperatures 900–1100 C for various durations.
The presence of a-crystallite in the two powder
systems was prepared by the following procedures.
2.1.1. y-Al2O3 powders with external a-crystallite
additions
Four samples were prepared: one without
a-Al2O3 crystallites, the other three with 10, 30,
and 44 wt% a-Al2O3 crystallite additions (Designated as E 0, E 10, E 30, and E 50). The
a-Al2O3 particles with Stoke’s diameter o0.2 mm
were obtained by grinding a-Al2O3 powders (A32,
Nippon Light Metal, Japan) and then dispersing in
distilled water (pH=3) from which the particles
were separated using Stoke’s sedimentation techniques [12]. Size distribution was measured using a
light scattering particle size analyzer (Malvern
Instrument, Zetasizer 1000). The mean particle size
was 0.230 mm with a size-range 0.170–0.330 mm. A
perl (bead) mill (PML-H/V, DRAIS, German)
with grinding media of 0.3–0.4 mm-sized ZrO2
beads was employed to de-agglomerate the
a-Al2O3 powders. The four a-added boehmite
samples (including the one without additions)
were calcined at 900 C for 1 h to obtain y-phase
powders. Then they were used for examining the
effects induced by a-Al2O3 crystallite additions
(Table 1).
2.1.2. y-Al2O3 powders with internal a-crystallite
additions
Two groups of y-Al2O3 powders with internal
a-crystallite addition, one for DTA purposes
and the other for activation energy calculations
139
of y-crystallite growth, were prepared. For DTA
purposes, a starting y-Al2O3 powder with mean
crystallite size 18.5 nm (XRD-Scherrer formula,
ð2 0 2% Þy ) (Designed as M sample, refer to following
description) was prepared by calcination of
boehmite at 1000 C for 2 h. Four y-powders
containing 5, 17.5, 49.5, and 83.0 wt% of
a-Al2O3 (designed as M5, M20, M50, and M80,
respectively) (Table 1) were then obtained by recalcination of the y-powder at 1200 C for variation times and quenched to room temperature
(cooling rate >250 C/min).
For activation energy calculations of y-crystallite
growth, three y-Al2O3 powders with mean crystallite sizes of 12.6, 18.5, and 21.7 nm (XRD-Scherrer
formula, ð2 0 2% Þy ) (Designated as S, M, and L,
respectively) were prepared by calcinations of
boehmite at 900 C and 1000 C/2 h, and 1050 C/
6 h, respectively. The three y-powders were thermally treated at scheduled temperatures (1130 C,
1155 C, and 1180 C) for various durations (0–
45 min) and then quenched to room temperature.
The variations in mean y-crystallite sizes and the
quantity of spontaneously formed a-ones during
thermal treatments were then measured. These
values were used in calculations of the growth
activation energies of y-Al2O3 crystallites.
2.2. Characterization
Differential thermal analysis (DTA, Netzsch
STA 409C) up to temperature 1400 C was
obtained for both samples with external and
internal a-crystallite additions. The heating rate
was 10 C/min. Ignited alumina (AKP-50, Sumitomo Chemical Co., Ltd., Japan) was used as
reference material. The crystalline phase was
identified by XRD (Siemens D5000) powder
methods using graphite monochrometer, CuKa1
radiation, 2y ¼ 20280 : The mean crystallite sizes
of y-Al2O3 in the powders were determined by the
XRD-Scherrer formula (mean crystallite size ¼
( B the broad0:9 lðB cos yÞ; where l ¼ 1:540562 A,
ening of width at the half-peak height (WHPH) in
radian, and y=Bragg angle). It was applied to
peaks of ð2 0 %2Þy of y-Al2O3. The instrument peak
width was calibrated using a well-crystallized
silicon powder. XRD powder methods were also
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140
Table 1
Physical properties of starting y-Al2O3 powders
y-Al2O3 powders
E0
E 10
E 30
E 50
M0
M5
M20
M50
M80
S
M
L
a-Al2O3 add. (%)
Crystallite sizea
(nm)
BET diameter
(nm)
12.6
12.6
12.6
12.6
18.5
25.0
25.0
25.9
27.0
12.6
18.5
21.7
NM
NM
NM
NM
20.3
NM
NM
NM
NM
18.2
20.3
26.7
0
10
30
44
0
5
17.5
49.8
82.9
0
0
0
a-Al2O3 powders
Remark
Crystallite sizea
(nm)
Mean particle
sizeb (nm)
—
>100
>100
>100
—
49.2
47.2
46.9
47.8
—
—
—
—
230
230
230
NM
NM
NM
NM
—
—
—
For DTA
For DTA
For AEC
NM: Not measured.
DTA: Differential thermal analysis
AEC: Activation energy calculation.
a
XRD-Scherrer formula.
b
Light scattering.
employed to determine quantities of y-Al2O3
formation using CaF2 as the internal standard.
Calibration line was established through calculating area ratios between a-Al2O3 (0 1 2) and CaF2
(1 1 1) measured from a series of y-Al2O3 samples
in which various amounts of a-crystallite with
10 wt% CaF2 were formulated. The range of
investigation was 1.5–97 wt%. The calculation
was assisted by a software program, XRD Pattern
Processing and Identification, Jade for Windows,
Version 5.0 developed by Material Data Inc.
Microstructure examinations were obtained by
TEM (TEM, JEOL AEM-3010) techniques.
The values of rate constant, k; were obtained by
the rate equation
2.3. Calculation for activation energies of y-Al2O3
crystallite growth
3.1. a-Crystallite addition externally
The activation energy of y-Al2O3 crystallite
growth was evaluated by isothermal experimental
methods accompanied with the Arrhenius equation.
k ¼ A expðEa =RTÞ;
ð1Þ
where k is the rate constant, T the testing
temperature, Ea the activation energy of y-crystallite growth, and A is a constant.
ðD D0 Þm ¼ kt;
ð2Þ
where D and D0 are the average y-crystallite sizes
measured at the beginning (as received) t0 and
after the sample was thermal-treated for the time
durations t at the scheduled temperature, respectively, and m is the expression of the mechanism of
grain growth.
3. Results and discussion
3.1.1. DTA profiles
Fig. 1 shows the DTA profiles of boehmite vary
with the amounts of a-Al2O3 addition for samples,
E 0, E 10, E 30, and E 50. The characteristic temperatures of y- to a-Al2O3 phase transformation on the profile [3,5–8] are shown on profile
E 0. The temperature at which a-phase nucleation became detectable (Tn ) of the sample without
a-Al2O3 addition was observed at B850 C (using
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141
because of that, all three samples, E 10, E 30,
and E 50 showed identical Tps temperatures,
although the peak heights were different. It is also
noted that the swelling of the Tn 2Tb temperature
component (lowering in Tn ), as well as a broadening of the peak (Tps ) must be attributed to the
a-Al2O3 addition. The exothermic Tpr that
occurred at temperatures higher than Tp ; being
about 1300 C for E 10, was identically shown on
the profiles of the other two samples Ex30 and
E 50 (also refer to Fig. 2). Further, it becomes
distinct as the a-Al2O3 addition increases (E 30
and E 50). And with the increase in distinctiveness of Tpr ; the height of Tps lowered and the Tp
recovered as well.
Fig. 1. DTA profile variations during y- to a-phase transformation of 4 y-powder systems, E 0, E 10, E 30, and
E 50. External a-crystallite additions are 0, 10, 30, and
44 wt%.
DTA [3]). The temperatures for the beginning of
large amounts of a-Al2O3 formation (Tb ), peak of
a-formation (Tp ), and offset (To ) were 1190 C,
1230 C, and 1280 C, respectively. In contrast,
a-Al2O3 addition eventually changed the DTA
profiles, at least in two categories: Lowering Tn
temperatures and increasing numbers of exothermic peak. The temperature reduction of Tn
increased with the a-Al2O3 addition, reducing
from 850 C to 625 C. Compared with E 0
sample, with 10 wt% a-Al2O3 addition, except
the Tn lowered from B850 C to 710 C, the Tp
became flat. Instead, two exothermic peaks, Tps
and Tpr occurred. It is noted that Tps is the peak of
a-Al2O3 formation resulting from the subsystem of
a-Al2O3 addition or seeding effects [10,11]. And
3.1.2. a-Formation varies with a-crystallite
additions
The presence of a-Al2O3 during thermal treatments of the four a-Al2O3 added boehmite samples
is shown in Fig. 2. Samples were heated to the
anticipated temperatures (10 C/min heating rate),
without holding and then quenched to room
temperatures so that the frozen a-Al2O3 formation
can presumably reflect the amounts of y-Al2O3
crystallite that reaches dcy at the temperature. It is
interesting to find that the transition phases were
present accompanying a-formation at temperature
above 1300 C. A preliminary examination on the
sample E 50 that was quenched at 1100 C,
1200 C and 1300 C showed that the amorphous
phases persisted at the temperature >1200 C
and d- and y-phases were observed at >1300 C
(Figs. 3 and 4). Moreover, the a-formation declined
with the increase in a-crystallite additions (Fig. 2).
The trends of a-formation during thermal treatments of the samples exhibit stepwise and flattened
out with the higher amounts of a-additions,
corresponding to the occurrence of exothermic
peaks on DTA profiles. This clearly indicates that
the exothermic peaks reflect the a-Al2O3 formation.
Further, it is clear that the external a-Al2O3
crystallite additions to the boehmite system presumably resulted in multiple steps of a-Al2O3
formation during thermal treatments.
Fig. 5 illustrates the variations of a-formation
observed in the four samples at the end of
temperatures Tps ; Tp ; and Tpr : All values listed
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Fig. 2. Variations in a-formation vs. DTA profiles during thermal treatments of samples (a) E 0 (without a-crystallite addition), (b)
E 10 (10% addition), (c) E 30 (30% addition), and (d) E 50 (44% addition). The heating rate was 10 C/min. The trends of aformation exhibit stepwise, corresponding to the occurrence of exothermic peaks on DTA profiles.
on the figure are recalculated from Fig. 2, based on
amounts of Al2O3 that presumably will transform
to a-Al2O3 (or 100 minus % of a-crystallite
additions). It is important to find that the
a-formation at the end of Tp was 97% for E 0.
However, the value decreased to 95.5%, to 74.3%,
then to 62.5% (solid circles in Fig. 5), and the
seeding effects (values on Tps curve) declined
drastically as the external a-addition increased
from 10% to 30%, then to 44%, indicating that
there were substantial amounts of transition
phases of alumina (other than a-Al2O3) that
remained due to the addition of a-crystallites.
Similar cases arose to the same samples, even the
samples were heated at 1400 C for 2 h (Figs. 2 and
4). The increase in a-crystallite additions from
10% to 44% resulted in the reduction in
a-formations from 96.6% to 67.9% (recalculated
from values in Fig. 2). Compare the two sets of
values obtained at Tp and at 1400 C/2 h, the
a-formation appeared at exotherms Tpr can then
be transformed from the y-crystallites that could
not coarsen, or were restrained from growing to
the size dcy at temperature Tp by the a-crystallite
addition. Further, it should be noted that, as the
a-addition increased the fraction of transition
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143
Fig. 3. Phase identifications for sample E 50 quenched at 1100 C, 1200 C, and 1300 C.
alumina that could not be transformed to a-phase
increased. This portion eventually consists of d
and y phases if the y-powder is boehmite-derived.
Thus it is clear that, the addition of larger
amounts of a-crystallite to the y-powder systems
would induce a restraining effect that results in
part of the y-crystallites in the powder system
growing sluggishly. Meanwhile, there remain
substantial amounts of transition alumina phases
that seem to expand in amount as the a-addition
increases in the powder system.
3.2. Presence of a-crystallite by spontaneous
occurrences
3.2.1. DTA profiles
Fig. 6 demonstrates that the characteristics of
exothermic peaks on DTA profiles can be altered
by the presence of a-crystallite introduced by
spontaneous occurrence during y- to a-phase
transformation. Similar to the phenomena observed by introducing external a-crystallite addition to the powder system it would induce a
restraining effect resulting in part of the y-crystallites in the powder system growing sluggishly.
Meanwhile, there remain substantial amounts of
transition alumina. Four samples, M5, M20, M50,
and M80 that were obtained by calcinations of
y-powder M, (Table 1) with a-Al2O3 presence
internally 5.0, 17.5, 49.8, and 82.9 wt%, respectively, show deviation in DTA profiles from that of
sample M. The M sample with y-crystallite size
18.5 nm and no a-Al2O3 presence showed Tp
temperature at 1200 C. The derived four samples
showing y-crystallite sizes B25 nm shifted the Tp
to 1175 C, because the y-size was coarser. However, the Tp exotherm also broadened and another
exotherm, Tpr ; appeared at temperature B1340 C.
Both phenomena demonstrate the fact that an
internal addition of a-Al2O3 to a y-powder system
would induce a restraining effect on the y-crystallites growth.
Additionally, it is noted that, in the internal
a-Al2O3-added systems, the seeding effects that
eventually initiate one faster y-crystallite growth
subsystem in a y-powder system may not occur.
Thus the swelling hill after temperature Tn (Fig. 1)
is obscured. Further, the restraining effect on
y-crystallite growth would not be provoked when
the a-formation in the powder system is less than
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Fig. 4. TEM micrographs of the sample E 50. (a) The bright field (i) and the dark field (ii) images of a single y-crystallite and the
corresponding diffraction pattern of [2 0 1] zone of the sample quenched at 1200 C. Bar=150 nm. (b) (i) TEM micrograph of the
sample E 50 quenched at 1400 C, (ii) corresponding diffraction pattern of the circled area showing presence of y and amorphous
phases.
10–15 wt % (Fig. 6), because at the early stage of
phase transformation the a-formation is achieved
absolutely through the growth to the critical size
dcy of y-crystallites. The y-growth proceeds prior
to the occurrence of phase transformation.
Clearly, the presence of a-formation in this case
neither assists nor restrains the growth rate of
y-crystallite.
3.2.2. Growth model for y-Al2O3 crystallite
It is difficult to measure the rate of y-Al2O3
crystallite growth using samples with external
a-crystallite additions. This is because size measurements using XRD-Scherrer formula for
y-Al2O3 occurring in the powder systems are
heavily interfered by the simultaneous presence
of other transition alumina phases. However, this
work is comparatively easy to access using y-Al2O3
powder samples in which the a-crystallite is
introduced by spontaneous occurrence during
y- to a-phase transformation. This is because, in
this case, the sample contains only y- and a-Al2O3
phases and the interferences can be eliminated.
However, it will encounter another problem,
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because the growth of y-crystallite terminates at
the critical size of phase transformation, dcy ; above
which y-phase will transform to a-one. Thus it is
obvious that, once y- to a-phase transformation
occurs, the value of y-size measured using Scherrer
formula can only reflect part of the true situation
and the size measured is always smaller than that
of the real case. To solve it, it is assumed that the
amounts of transformed a-Al2O3 (fraction x) was
corresponding to that of y-Al2O3 with crystallite
size dcy ; or 25 nm. Then the value of y-size was
obtained by expression (3):
100
Transition
phase(s)
90
80
Tpr
α-formation,wt%
70
60
Tp
50
40
α-Al2O3
Tps
30
20
10
0
0
10
20
30
40
50
y size ¼ ð1 xÞdy þ x25;
α-crystallites addition, wt%
Fig. 5. a-Formation variations observed at the end of
temperatures Tps ; Tp ; and Tpr vs. amounts of a-Al2O3 crystallite
additions.
Tp
α-content,
wt%
Tpr
<5
DTA, exothermal
5.0
17.5
50
83
900
1000
1100
1200
1300
145
1400
Temperature,0C
Fig. 6. DTA profiles variations during y- to a-phase transformation of y-powder systems M0, M5, M20, M50, and M80.
The amounts of a-crystallite internally formed are 0, 5, 17.5, 50,
and 83 wt%.
ð3Þ
where dy is y-size measured by Scherrer formula
(nm), x is the fraction of a-Al2O3 formation, o1
In this study, growth rates of y-Al2O3 crystallite
were evaluated using samples in which the
a-formation was less than 40 wt%. Additionally,
since the restraining effects on y-crystallite growth
may not be provoked when the a-formation in the
powder system is less than 10 wt%, the growth
model was examined separately based on the fact
that prior to and after the presence of a-Al2O3
formation was 10 wt%.
Fig. 7 illustrates the isothermal growth of
y-crystallite in the y-powder systems S, M, and L
prior to (Fig. 7a) and after (Fig. 7b) the presence
of a-Al2O3 formation was 10 wt%, considering
m ¼ 2 for expression (2). The crystallite may
undergo a coarsening effect following parabolic
growth when the presence of a-Al2O3 was
>10 wt%. And the growth seems to be dominated
by the mechanism of grain boundary migration.
Examining the value of rate constant (k) derived
from Fig. 7, y-powders with coarser crystallite
sizes would grow slower, compared with that of
finer y-powders. Once the a-crystallite is introduced, or the presence of a-Al2O3 formation is
over 10 wt%, the rate would be even lower.
3.2.3. Restraint effects during y- to a-phase
transformation
The restraining effects on y-crystallite growth
due to the spontaneous presence of a-crystallite in
the powder system can be further examined by
calculating the activation energies of y-crystallite
growth. According to values listed in Table 2 and
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Fig. 7. Isothermal y-crystallite growth in the y-powder systems prior to (a) and after (b) the presence of a-Al2O3 formation was
10 wt%, considering m ¼ 2 for expression (2).
the Arrhenius plots based on the values, the
growth activation energies of y-crystallite were
obtained. Fig. 8 displays the variations in the
growth activation energies of y-crystallite in
samples S, M, and L, including the difference
between absence and presence of a-crystallites
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Table 2
Values of rate constant, ln k derived using ðD-D0 Þm ¼ kta
T ( C)
a
the restraint of y-crystallite from growth becomes
more conspicuous (Fig. 8).
a-Formation (%)
o10%
1130
1155
1180
147
>10%
4. Conclusions
S
M
L
S
M
L
3.06
3.78
3.89
1.56
3.39
3.56
0.11
0.61
1.50
3.50
3.33
4.22
0.50
1.22
1.722
1.06
1.06
2.17
m ¼ 2:
Fig. 8. The effect of y-crystallite size and the presence of acrystallite formation in y-powder systems on growth activation
energies of y-Al2O3 crystallite.
(based on 10% a-crystallite formation) in the
powder systems. Clearly, with the presence of
a-crystallite in the y-powder system, the growth of
y-crystallite always requires higher activation
energy. The value increased with the coarsening
of y-crystallites, possibly because of the decrease in
surface energy of the y-crystallite and, more
plausibly, because of the easier presence of higher
amount of a-crystallite in coarser y-powder
systems. The inference can be demonstrated by
the differences in calculated activation energies of
the three y-powders: Coarser y-crystallites transforms to a-phase at lower temperature but more
dispersed in temperature range, compared with
finer ones as commonly observed on DTA profiles
[3,5–7]. As the amounts of a-formation increases,
This study examined the restraint of y-Al2O3
from crystallite growth induced by the presence of
a-crystallites in the y-crystallite powder system.
The y-powder was obtained by calcination of
boehmite. a-Crystallites were introduced into the
powder system through two routes: One was
added externally as the outsider to the boehmite
prior to the y-powder was obtained. The other was
presented spontaneously during y- to a-phase
transformation of the obtained y-powder system.
Both types of a-crystallite affected the rate of
y-crystallite growth. Small amounts of a-crystallite
addition to the boehmite system initiate a
y-crystallite powder subsystem in the original
powder, in which part of the formed y-crystallite
grows faster. With large amounts of a-crystallite
addition, additional subsystem in which the rate of
y-crystallite growth slows down. a-Al2O3 crystallites introduced spontaneously during y- to
a-phase transformation into a y-powder system,
although with small amounts of the a-crystallite
presence, less than 10–20%, whose effect can be
eliminated, behave similarly to that of large
amounts of a-crystallite additions externally.
With the presence of a-crystallite in the
y-powder system, the growth of y-crystallite always requires higher activation energy. The value
may increase with the coarsening of y-crystallites.
The decrease in surface energy of the y-crystallite
and, more plausibly, the easier presence of higher
amount of a-crystallite in coarser y-powder
systems can be attributed to the restraining effects.
Acknowledgements
The authors wish to thank Miss Yu-Tsan Hsu
and Mr. Jiun-Long Chang for assistance in
laboratory experiments. This study was supported
by the National Science Foundation of the
Republic of China under Contract No. NSC902216-E-006-050.
ARTICLE IN PRESS
148
P.C. Yu et al. / Journal of Crystal Growth 265 (2004) 137–148
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