Thermal behavior of alumina microfibers precursor prepared by surfactant assisted microwave hydrothermal

Article abstract of DOI:10.1111/j.1551-2916.2012.05448.x

Uniform alumina microfibers precursor (ammonium aluminum carbonate hydroxide, AACH) were successfully synthesized using the microwave hydrothermal method, with average length of 5-10 μm and the diameter around 300-500 nm. FT-IR spectra indicated AACH was composed of NH₄[Al(OOH) HCO ₃]A·H₂O. The thermal behaviors of the as-prepared AACH were investigated using differential scanning calorimeter and thermogravimetric analysis (DSC-TG), XRD, and SEM. The results showed that the thermal decomposition of the AACH microfibers occurred via three steps, which were respectively divided into adsorption of physical water, dehydration of crystalline water, and decomposition of AACH. The activation energies for the above three steps were calculated using Coats-Redfern method. The phase transformation sequence was found to be AACH → amorphous Al ₂O₃ → γ- and θ-Al₂O ₃ → α-Al₂O₃. It was also observed that the thermal treatment had little influence on fiber morphology of the products. The fibers morphology with high thermal stability will endow to prepare alumina microfibers with novel application potentials.

Full text of DOI:10.1111/j.1551-2916.2012.05448.x

J. Am. Ceram. Soc., 1–5 (2012)
DOI: 10.1111/j.1551-2916.2012.05448.x
© 2012 The American Ceramic Society
ournal
J
Thermal Behavior of Alumina Microfibers Precursor Prepared by Surfactant
Assisted Microwave Hydrothermal
Xuanmeng He,‡,§,† Guangjun Li,‡ Hui Liu,‡ Junqi Li,‡ and Zhenfeng Zhu‡
àSchool of Materials Science and Engineering, Shaanxi University of Science and Technology, Xi’an 710021, China
§Key Laboratory of Auxiliary Chemistry & Technology for Chemical Industry, Ministry of Education, Shaanxi University of
Science and Technology, Xi’an 710021, China
Uniform alumina microfibers precursor (ammonium aluminum
carbonate hydroxide, AACH) were successfully synthesized
using the microwave hydrothermal method, with average length
of 5–10 lm and the diameter around 300–500 nm. FT-IR
spectra indicated AACH was composed of NH4[Al(OOH)
HCO3]ꢀH2O. The thermal behaviors of the as-prepared AACH
were investigated using differential scanning calorimeter and ther-
mogravimetric analysis (DSC-TG), XRD, and SEM. The results
showed that the thermal decomposition of the AACH microfibers
occurred via three steps, which were respectively divided into
adsorption of physical water, dehydration of crystalline water,
and decomposition of AACH. The activation energies for the
above three steps were calculated using Coats-Redfern method.
The phase transformation sequence was found to be AACH?
amorphous Al2O3? c- and h-Al2O3? a-Al2O3. It was also
observed that the thermal treatment had little influence on fiber
morphology of the products. The fibers morphology with high
thermal stability will endow to prepare alumina microfibers with
novel application potentials.
size less than 5 nm using precipitation reaction.7 Qin et al.
reported the preparation of flame-retardant epoxy resins by
a mixed method using AACH as a halogen-free flame-
retardant.8
Kinetic analysis of thermal decomposition processes has
been the subject of interest for many investigators all along
the modern history of thermal decomposition, because of its
importance for designing the device in which the thermal
decomposition takes place and analyzing decomposition
mechanism.9 The Coats-Redfern method for determination
of activation energy has been extensively used in kinetic
analysis of thermal decomposition process. Sarikaya and
co-workers reported the determination of activation energy
of an alumina precursor (b-AlOOH) for four steps in the
thermal decomposition using the Coats-Redfern procedure.10
Ada et al. calculated the activation energy for dehydration
and dehydroxylation of the precursor (amorphous alumina)
according to Coats-Redfern equation.11
In this study, we presented a simple route to synthesize
uniform AACH microfibers with a composition of NH4[Al
(OOH)HCO3]ꢀH2O. The thermal behaviors of AACH
microfibers prepared by surfactant-assisted microwave
hydrothermal were investigated using differential scanning
calorimeter and thermogravimetric analysis (DSC-TG),
SEM, and XRD. The activation energy for the steps of
AACH in thermal decomposition process were determined
using Coats-Redfern equation.
I. Introduction
ECENTLY, one-dimensional (1-D) nanostructure materi-
als, such as nanowires, nanotubes, nanorods, and
R
nanofibers, have attracted considerable interests due to their
importance in fundamental research and potential wide-rang-
ing application.1,2 One-dimensional alumina nanostructure
are highly desirable in areas such as advanced high-tempera-
ture composite materials and nanodevices because of their
high dielectric constant, good thermal and chemical stability,
and high mechanical modulus.3,4 Alumina nanostructures
could be prepared using various methods, among which the
decomposition of ammonium aluminum carbonate hydroxide
(AACH) is a promising method to produce alumina. Many
efforts have been focused on the preparation of AACH
nanostructures, because it can be served as precursor for pre-
paring alumina and inorganic flame-retardants. Bai and
co-workers reported the synthesis of alumina microfibers in
the presence of AACH as precursors using the copolymer-
controlled homogeneous precipitation method under hydro-
thermal conditions.5 Ma et al. synthesized net-like AACH
particles in a reactor with a mechanical stirrer and a temper-
ature controller under different pH values.6 Sun et al.
described the synthesis of nanometric AACH with a particle
II. Experimental Procedure
(1) Preparations of AACH Microfibers
AACH microfibers were prepared through the hydrothermal
reaction of a mixture composed of aluminum sources, surfac-
tant, pH adjusting regent, and solvents. All chemicals are ana-
lytical-grade reagents without further purification. In a typical
experiment, 0.4 mmol of polyethylene glycol (PEG)-2000 was
dissolved in deionized water to form a clear solution, to which
2.0 mmol of Al(NO3)3ꢀ9H2O was added. After the alumina salt
was totally dissolved, 0.36 mmol of urea was added. The
mixed solution was further magnetically stirred for 3 h. Then
the solution was transferred into a Teflon-lined autoclave and
placed in an oven at 120°C for 24 h. After being cooled to
room temperature, the white precipitates were collected and
washed several times with deionized water and ethanol
to remove the impurities and then dried at 80°C. To study
the thermal behavior, calcination was conducted at different
temperature in a temperature-programmed muffle furnace.
R. Riman—contributing editor
(2) Characterizations
The crystal structures of as-prepared samples were character-
ized using X-ray diffraction (XRD-D/max 2200pc, Rigaku,
Manuscript No. 31148. Received April 6, 2012; approved August 15, 2012.
†Author to whom correspondence should be addressed. e-mail: hexuanmeng@sust.
edu.cn
1

2
Journal of the American Ceramic Society—He et al.
Tokyo, Japan) technology with CuKa radiation of wave-
length k = 0.15418 nm. The microscopic features of samples
were characterized using scanning electron microscope (SEM,
JSM-6700F; JEOL, Tokyo, Japan) operated at 5 KV, a
transmission electron microscope (TEM, JEM2010; JEOL)
operated at 200 KV. Thermogravimetric and differential
thermal analyses (TGA-DTA) were conducted on a thermo-
gravimetric analyzer (STA449C; Netzsch, Germany), with an
air flow rate of 30 mL/min at a heating rate of 10°C/min.
Fourier-transform infrared (FT-IR) spectrometer (FT/IR-
470; Jasco, Plus, Japan) was used for detection of the AACH
precursor and the alumina microfibers.
microfibers is about 6 nm. The high-intensity of (110) crystal
face also exhibited that the AACH crystal preferably grew
along the given direction under the synthesis condition. No
other diffraction peaks were detected, indicating that no
impurity exists in as-prepared AACH. Figure 1(D) shows the
TEM image of the as-prepared AACH microfibers. The
selected area electron diffraction (SEAD) pattern [the inset of
Fig. 1(D)] recorded from the face of the crystal having the
longest axis (for example, the circled area in TEM image)
always exhibited the same single-crystalline pattern, consis-
tent with the [001] zone of AACH crystal. The elongated mi-
crofibers shape with the exposed (110) faces demonstrated
that the AACH crystal preferably grew along the crystallo-
graphic c-axis under the synthesis conditions, which is the
same as explanation of XRD dates.
III. Results and Discussion
The AACH microfibers were prepared using hydrothermal
reaction at 120°C for 24 h. The morphologies of as-prepared
AACH microfibers are shown in Fig. 1. It can be seen that
the sample was made of uniform microfibers. These fibers
were not tangled or interwound. The length of fibers is about
5–10 lm, and the diameter is about 300–500 nm. The magni-
fied SEM micrograph [the inset of Fig. 1(B)] shows that the
fibers had a very smooth surface, and both ends of each fiber
were shrunk to needle-like structure with diameter of about
100 nm. Figure. 1(C) shows the typical XRD pattern of the
as-prepared samples, and all of the diffraction peaks can be
indexed to crystalline AACH (JCPDS card no. 42-0250). The
measured lattice constant “a,” “b,” and “c” of this ortho-
rhombic phase are 6.632, 11.947, and 5.724 A, which were in
good agreement with the theoretical values (a = 6.618,
b = 11.944, and c = 5.724 A, respectively). The high intensi-
ties and sharp peaks in Fig. 1(C) indicated that AACH phase
synthesized in this work was well crystallized. Using the
Scherer equation, the calculated crystallite size of AACH
There are two different composition of AACH docu-
mented in the literature. Erods and Altrofe12 found the cor-
rosive products of a piece of aluminum in the atmosphere
and identified it as ammonium aluminum hydroxide carbon-
ate with one water (NH4[Al(OOH)HCO3]ꢀH2O). Kato13 syn-
thesized AACH by using the reaction of NH4HCO3 and
aluminum salt and suggested that the compound had a for-
mula as NH4[Al(OOH)HCO3], but the XRD patterns and IR
were very close to Erdos and Altrofe’s NH4[Al(OOH)
HCO3]ꢀH2O. Later, Serna, Garcia-Ramos, and Pena14 also
reported the IR results of AACH, which was similar to that
reported by Erods and Alrofe’s, but the composition they
suggested had no crystal water. From then on, only NH4[Al
(OOH)HCO3] appeared in almost all of the literature, and
no related discussion has been made on the difference of the
formula. To clarify the ambiguity of the compound, FT-IR
has been done in present study. Figure 2 shows the FT-IR
spectra of (a) the as-prepared AACH; (b) AACH dried in a
vacuum dryer at 393 K for 24 h, and (c) AACH calcined at
(b)
(a)
(d)
(c)
Fig. 1. SEM micrographs of the as-prepared AACH microfibers (a and b), XRD of as-prepared AACH microfibers (c), and TEM image and
SEAD pattern of as-prepared AACH microfibers (d).

Alumina Microfibers Precusor
3
and 460 K on the DSC curve (Fig. 4). According to the
interaction between water and the as-prepared AACH, the
dehydration is divided into two steps. The first step is
adsorption of physically adsorbed water (a) The second step
is that chemical bond between water and AACH is broken,
eliminating the crystal water in the AACH (b) The second
weight-loss event (c) in the temperature range of 460–530 K,
which also can be seen from the aculeated endothermic peak
at 510 K of the DSC curve, is associated with the decompo-
sition of PEG-2000, together with the decomposition of
AACH, releasing CO2, NH3, and H2O and forming AlOOH.
The intense endothermic peak indicates that the decomposi-
tion of AACH request much energy. Therefore, AACH can
be served as a perfect inorganic flame-retardant. With the
increasing temperature in the thermal process, the TG curve
keeps decreasing in a slight manner, indicating the decompo-
sition of AlOOH and forming the alumina particles. The sole
exothermal peak which presents in DSC plot at 1157 K is
associated with the phase transition process of the alumina.
From the TG curve, it is apparent that the as-prepared
AACH contained 58.63% volatile and 41.37% fine alumina
powder.
The decrease in mass at each temperature were read from
TG curve and their ratios to the total mass loss which
remained constant at 800 K was defined as the decomposi-
tion fraction (a). The variation of the decomposition fraction
as a function of temperature is given in Fig. 5. The inset of
Fig. 5 is the magnified variation between 313 and 460 K. In
the a–T curve, the section “a” and “b” show, respectively,
the first and second dehydrations and section “c” shows the
decomposition.
Fig. 2. FT-IR spectra of (a) the as-prepared AACH; (b) AACH
dried in a vacuum dryer at 393 K for 24 h; and (c) Al2O3 calcined at
773 K for 2 h.
773 K for 2 h. In the spectrum of as-prepared AACH
shown in Fig. 2(A), the broad band round 3446 cmꢁ1 was
attributed to the O–H vibration of adsorbed water.15 The
peaks in the region 3016–3174 cmꢁ1 corresponded to sym-
metric bending-stretching vibration of NH4+, and around
1385 cmꢁ1 was due to asymmetric stretching modes of
CO32ꢁ, the obvious split of this double degenerate peak
2ꢁ
exhibited that the CO3
in the AACH belonged to the
structural carbonate group. The weak peak at 1620 cmꢁ1
was assigned to the d(H2O) bending vibration of water mole-
cules adsorbed in the product. The peaks at 1105, 978, and
858 cmꢁ1 belonged to C–O–C and -CH2- vibration, which
originated from the surfactant of PEG. After AACH dried in
a vacuum dryer at 393 K for 24 h, the broad peak at
3446 cmꢁ1 became weak, and the peak at 1620 cmꢁ1 disap-
peared, indicating gradual decrease of adsorbed water and
some water still located between crystals. The FT-IR spec-
trums of as-prepared AACH and AACH dried in a vacuum
dryer at 393 K for 24 h were similar and showed H2O peak
around 3446 cmꢁ1, which indicated that AACH and its dried
products have a similar structure, and AACH contained
some water which may be located between crystals, identified
as a composition of NH4[Al(OOH)HCO3]ꢀH2O. In the spec-
trum of AACH calcined at 773 K for 2 h, all absorption
peaks of carbonate, ammonium, and organic groups were
greatly weakened, even vanishing in the range 1300–
900 cmꢁ1, indicating complete decomposition of AACH and
formation of Al2O3 microfibers.
As the mass fraction of decomposition part of the volatile
matter in the as-prepared AACH is a, the mass fraction of
the part which is not yet decomposed is (1ꢁa). According to
Coats-Redfern equation, the chemical reaction kinetics of as-
prepared AACH decomposition can be described as follows:
Thermal properties of the as-prepared AACH were evalu-
ated using TG-DSC. Test conditions were started from 313
to 1573 K at a rate of 10 K/min in a flowing nitrogen envi-
ronment. The TG-DTG curves of the as-prepared AACH
microfibers are shown in Fig. 3. The TG curve shows that
the sample has two major weight loss events. The first weight
loss below 460 K is due to adsorption of water in the
AACH, corresponding to an endothermic peak between 313
Fig. 4. DSC curve of the as-prepared AACH microfibers.
Fig. 5. The variation of decomposition fraction as a function of
calcination temperature.
Fig. 3. TG and DTG curves of the as-prepared AACH microfibers.

4
Journal of the American Ceramic Society—He et al.
NH4½AlðOOHÞHCO3ꢂ ! AlOOH þ CO2 þ NH3 þ H2O
(2)
2AlOOH ! Al2O3 þ H2O
(3)
Combining the above reactions yields an overall reaction:
2NH4½AlðOOHÞHCO3ꢂ ! Al2O3 þ 2CO2 þ 2NH3
þ 3H2O
(4)
To identify the decomposed products of AACH microfi-
bers and the phase transformation of alumina fibers during
calcination process, the XRD patterns of calcined products
of the as-prepared AACH microfibers were measured.
Figure 7 shows the XRD pattern of the as-prepared AACH
microfibers calcined at different temperature. After calcina-
tions at 773 and 973 K, both alumina samples essentially
became amorphous. By increasing calcination temperature to
1173 K, the c- and h-Al2O3 phase were observed. The low
intensity of XRD peaks indicates that the alumina samples
calcined at this temperature has low crystalline, whereas the
broad peaks demonstrated that the alumina calcined at
1173 K are nanocrystalline. Further increasing calcination
temperature to 1373 K, the a -Al2O3 phase is observed.
When calcination temperature was increased to 1573 K,
a typical XRD pattern of the a -Al2O3 phase was obtained.
The shape and high intensities of the peaks indicate that the
Al2O3 is indeed well-crystallized. No other diffraction peaks
were detected, indicating that no impurity exists and the
AACH microfibers have completely transformed into the
a -Al2O3 phase.
Fig. 6. Fitting the decomposition data to Coats-Redfern equation.
Fig. 7. XRD pattern of the as-prepared samples calcined at
different temperatures for 2 h. (●), (■), and (▼) denote the a-Al2O3,
c-Al2O3, and h-Al2O3.
Figure 8 shows the SEM micrographs of the as-prepared
AACH microfibers calcined at 1173 K (A), 1373 K (B), and
1573 K (C), respectively. From SEM investigation, we can
see that the morphologies of the alumina microfibers calcined
at 1373 and 1573, calcinations are similar to those of their
counterparts before calcination [Fig. 1(B)]. When calcination
temperature was increased to 1573 K, some fibers became
curved, possibly because of sintering. However, the essential
fiber morphology still remains, which indicate that the heat
treatment has little influence on the fiber morphology of the
products. The fibers morphology with high thermal stability
will endow to prepare alumina microfibers with novel appli-
cation potentials.
lnf½ꢁ lnð1 ꢁ aÞꢂ=T2g ¼ ꢁE=RT þ ln½ðAR=bEÞ
(1)
ð1 ꢁ 2RT=EÞꢂ
where T is the average deformation temperature, b = dT/dt
is the heating rate, E is the activation energy of deformation,
R is the universal gas constant, and In[(AR/bE(1ꢁ2RT/E)]) is
quantity that stays nearly constant. When In [ꢁIn(1ꢁa)/T2]
values calculated using a values at different temperature were
plotted as a function of 1/T, the “a,” “b,” and “c” of straight
lines seen in Fig. 6 were obtained. By considering intervals
temperature of three straight lines, it is consistent with the
section “a,” “b,” and “c” of AACH decomposition. From
the slopes of straight line, the activation energies were,
respectively, calculated as Ea = 3.35 KJ/mol, Eb = 29.32KJ/mol,
Ec = 126.82KJ/mol. The fact that Eb was larger than Ea
showed that adsorption of physical adsorbed water was a
physical event, whereas the dehydration of crystallization
water was a chemical event. It was well-known that for the
section “c,” where the activation energy was considerably lar-
ger than Eb and Ea, NH4[Al(OOH)HCO3] is converted to
alumina by the release of H2O, NH3, and CO2 with elevating
temperature, as described in the following equation:
IV. Conclusions
The uniformly sized AACH microfibers have been success-
fully synthesized using the hydrothermal reaction of a mix-
ture composed of aluminum nitrate, PEG-2000, urea, and
solvents. FT-IR spectra indicate AACH with a composition
of NH4[Al(OOH)HCO3]ꢀH2O, different from preceding
reports. The thermal decomposition of AACH microfibers
occurred via three steps, which was, respectively, divided into
adsorption of physical water, dehydration of crystalline
water, and decomposition of AACH. All of the three activa-
tion energies were calculated using the Coats and Redfern
(a)
(b)
(c)
Fig. 8. SEM images of the as-prepared AACH microfibers calcined at 973 K (a), 1173 K (b), and 1373 K (c).

Alumina Microfibers Precusor
5
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X. S. Zhao, “Copolymer-Controlled Homogeneous Precipitaion for the Syn-
thesis of Porous Microfibers of Alumina,” Langmuir, 23, 4599–605 (2007).
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Decomposition of Ammonium Aluminum Carbonate Hydroxide (AACH),”
Mater. Chem. Phys., 72, 374–9 (2001).
procedure. In the thermal treatment process, AACH was
decomposed and formed alumina particles, meanwhile the
phase transformations of alumina occurred. The phase trans-
formation sequence was found to be AACH? amorphous
Al2O3 ? c- and h-Al2O3 ? a-Al2O3. Whereas, it was also
observed that the thermal treatment had little influence on
the fiber morphology of the products. Due to its fiber mor-
phology and high thermal stability, it will endow to prepare
alumina microfibers with novel application potentials.
7X. D. Sun, J. G. Li, F. Zhang, X. M. Qin, Z. M. Xiu, and H. Q. Ru, “Syn-
thesis of Nanocrystalline a-Al2O3 Powders From Nanometric Ammonium
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Acknowledgments
This work was financially supported by National Science Foundation of Sha-
anxi University of Science and Technology (no. zx09–17), China Postdoctoral
Science Foundation funded Project (no. 200902584), and the Special Fund
form Shaanxi Provincial Department of Education (no. 09JK355).
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