materials
Article
Preparation of Continuous Alumina Fiber with Nano
Grains by the Addition of Iron Sol
Luqun Liu, Juan Wang, Yunzhu Ma, Wensheng Liu and Shuwei Yao *
National Key Laboratory of Science and Technology for National Defense on High-strength Structural Materials,
Central South University, Changsha 410083, China; lluqun@csu.edu.cn (L.L.); wangjuan@csu.edu.cn (J.W.);
zhuzipm@csu.edu.cn (Y.M.); liuwensheng@csu.edu.cn (W.L.)
* Correspondence: shwyao@csu.edu.cn
Received: 4 November 2020; Accepted: 26 November 2020; Published: 29 November 2020
Abstract: Continuous alumina fiber exhibits excellent mechanical properties owing to its dense
microstructure with fine grains. In this study, alumina fiber was prepared by the sol–gel method using
iron sol as a nucleating agent. It was proposed that the α-Al2 O3 grain size be adjusted based on the
modification of colloidal particle size. The effect of holding temperature and reaction material ratio
on the iron colloidal particle size was studied. The microstructure of alumina fiber was characterized
by scanning electron microscopy (SEM). The experiment results indicated that iron colloidal particle
size increases with the holding temperature and the NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio. The alumina
fiber with uniform nano α-Al2 O3 grains was obtained by calcination at 1400 ◦ C for 5 min. The mean
grain size tends to rise with the mean colloidal particle size. Using the iron sol as a nucleating agent,
the fiber with a mean grain size of 22.5 nm could be formed. The tensile strength of fibers increased
with the decrease of grain size.
Keywords: alumina fiber; iron sol; holding temperature; colloidal particle size; grain size
1. Introduction
Continuous alumina fiber has been widely applied in various fields such as aerospace, military
industry and automobile, owing to its high strength, high modulus and exceptional oxidation
resistance [1–6]. The excellent mechanical properties of alumina fiber mainly depend on the dense
nanocrystalline structure. For example, NextelTM 610 fiber and FP fiber are typical alumina fibers with
high purity of α-Al2 O3 (>99 wt.%). The NextelTM 610 fiber, having a grain size of ~100 nm, exhibits
high tensile strength of greater than 3.1 GPa [3], while the tensile strength of FP fiber is only about
1.4 GPa due to its larger grains (>500 nm) [5]. Hay et al. [6] found that the tensile strength of NextelTM
610 fiber was reduced with the growth of α-Al2 O3 grains. Thus, controlling the grain size is essentially
important in order to prepare high performance alumina fiber.
Generally, alumina fiber is prepared by the sol-gel method [7–12]. In this method, complex
phase transformations of Al2 O3 take place. The fiber transfers from amorphous to metastable phases
(η, δ, γ, etc.) and then transforms into α-Al2 O3 . Since the transformation temperature of metastable
phases to α-Al2 O3 is about 1200 ◦ C, it is challenging to avoid α-Al2 O3 grain from coarsening during
the transformation and densification processes. In order to obtain fiber with fine grains, nano α-Al2 O3 ,
α-Fe2 O3 and α-Cr2 O3 serving as seeds are added in alumina fiber, which induce the heterogeneous
nucleation of α-Al2 O3 at a lower temperature [13–18]. Yamamura et al. [14] found that the alumina gel
added with 5 wt.% α-Al2 O3 particles partially crystallized to α-Al2 O3 by annealing at temperature
as low as 600 ◦ C. Li et al. [19] revealed that the addition of 3 wt.% α-Al2 O3 seeds lowered the γ-to-α
transformation temperature by 175 ◦ C. As a result, the primary α-Al2 O3 grain size decreased from
56 to 30 nm. Kumagai et al. [20] found that the introduction of nano α-Al2 O3 seeds resulted in a
Materials 2020, 13, 5442; doi:10.3390/ma13235442
www.mdpi.com/journal/materials
Materials 2020, 13, 5442
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dense microstructure with α-Al2 O3 grain size of 100–400 nm at 1200 ◦ C. In addition, NextelTM 610
fiber, the most representative alumina fiber, was also successfully produced by adding 0.7 wt.% Fe2 O3 .
The addition of Fe2 O3 provided the benefits of forming fine α-Al2 O3 grains with the uniform size
of 100 nm. In theory, increasing nucleation sites leads to the formation of more α-Al2 O3 grains,
subsequently contributing to refine grains. Li et al. [19] found that the grain size of the alumina fiber
sintered at 1100 ◦ C decreases from 86.7 to 74.3 nm, when the weight percent of α-Al2 O3 seeds rises
from 1% to 3%. However, the content of nucleating agent cannot be added indefinitely in order to
obtain continuous high purity alumina fiber.
Theoretically, reducing the seed size is an effective way to increase the amount of nucleation sites
when the content of the nucleating agent is constant. Xie et al. [21] compared the effect of α-Al2 O3 -seed
size on the transformation from κ to α phase. Employing the same content of nucleating agent,
the sample added with finer α-Al2 O3 powder exhibits a smaller average particle size with a narrow
distribution. However, the size of α-Al2 O3 , Fe2 O3 and α-Cr2 O3 powders is limited by the production
technologies, which is usually lager than 10 nm. Considering the agglomeration of nanoparticles,
the nucleation seed size is almost larger than the particle size of powders. Thus, employing powders
as nucleating agents, the seed size is difficult to further decrease. Wilson et al. [22] prepared a dense
α-Al2 O3 fiber with finer grains (200–300 nm) through the addition of iron sol. It was confirmed that
the iron sol could be transformed into α-Fe2 O3 before the formation of α-Al2 O3 and accelerating the
transformation from metastable phases to α-Al2 O3 . Compared with powders, the size of colloidal
particles can be controlled in the range of 1–100 nm. In addition, the iron colloidal particles can be
uniformly distributed in the aluminum sol. However, these factors affecting iron colloidal particle
size are unclear. The influence of iron colloidal particle sizes on the transformation of Al2 O3 and the
morphology of α-Al2 O3 grains has not been reported.
In this paper, it was proposed to prepare alumina fiber with ultra-fine grains by adjusting the
iron colloidal particle size. Fe(NO3 )3 ·9H2 O and NH4 HCO3 were employed to prepare the iron sol.
The effect of the material ratio and reaction temperature on the microstructure of iron colloidal particles
was investigated. Alumina fiber with the nano grain size was prepared using iron sol as a nucleating
agent. The relationship between the iron colloidal particle size and the grain size of α-Al2 O3 fiber was
discovered. The effect of iron sol on the microstructure of alumina fiber was discussed.
2. Materials and Methods
2.1. Iron Sol Preparation
Analytical pure Fe(NO3 )3 ·9H2 O and NH4 HCO3 provided by Aladdin (Shanghai, China) were
used to prepare the iron sol. Firstly, Fe(NO3 )3 ·9H2 O and NH4 HCO3 were dissolved in a proper amount
of water at room temperature. The weight ratios of Fe(NO3 )3 ·9H2 O/H2 O and NH4 HCO3 /H2 O were
set to be at 1:23.5 and 1:10, respectively. Then, a certain amount of NH4 HCO3 solution was slowly
added (3 drops/s) into the rapidly stirred Fe(NO3 )3 solution, which was placed in a 500 mL conical
flask and stirred by a magnetic stirrer. The molar ratios of NH4 HCO3 to Fe(NO3 )3 ·9H2 O were set to
be at 1.5, 1.75, 2.0, 2.25 and 2.5. Finally, the mixed solution was held at a certain temperature for 1 h.
The holding temperature varied from 25 to 80 ◦ C.
2.2. Preparation of Alumina Fibers
In this study, alumina fiber was prepared by the sol-gel method, which includes the preparation of
alumina precursor sol, concentration of the precursor sol and the spinning and sintering processes [23,24].
In this study, alumina powder (>99.5 wt.%, 1–3 µm), formic acid (99 wt.%), acetic acid (99.5 wt.%), nitric
acid (65 wt.%) and deionized water (lab made) were employed to prepare the alumina precursor sol.
The alumina powder, formic acid, acetic acid and nitric acid were all provided by Aladdin (Shanghai,
China). The molar ratio of starting materials was set to be at 1:0.67:0.6:0.36:28. The alumina precursor
sol was prepared at 85 ◦ C in a single-layer glass reactor. The iron sol was added into the alumina
Materials 2020, 13, 5442
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precursor sol prior to the concentration process. The additional amount of iron sol was set in order
to obtain a high-purity alumina fiber with 0.65 wt.% Fe2 O3 . The mixed solution was concentrated at
30–60 ◦ C using a rotary evaporator (Shanghai Xiande Experimental Instrument Co., LTD, Shanghai,
China) to obtain sols with a suitable viscosity (50–200 Pa·s) for spinning. The precursor fiber was
prepared using a lab-made dry spinning apparatus and collected by a bobbin winder. The precursor
fiber was first preheated at 500 ◦ C and then calcined at 1400 ◦ C for 5 min using a tube furnace
(BTF-1600C-IV-SL, BEQ, Hefei, China).
2.3. Characterization
The morphology of iron colloidal particles was observed by a high-resolution transmission electron
microscopy (TEM, JEM-2100F, JEOL, Tokyo, Japan). The colloidal particle size and Zeta potential of
iron sol were measured by a particle size analyzer (Zetasizer Nano ZS, Malvern, Malvern, England).
The thermal decomposition behavior of iron sol was analyzed using a simultaneous thermal analyzer
(STA-449C, Netzsch, Selb, Germany). The microstructure of calcined alumina fiber was characterized
by a scanning electron microscopy (SEM, Nova Nano, FEI, Hillsboro, Oregon, USA). The fiber mean
grain size was calculated by measuring the grain sizes using the image analysis software Images
J (V1.51, National Institutes of Health, Bethesda, Maryland, America). More than 800 grains were
randomly selected from the cross-section image and measured for each fiber. The tensile strength
of the fiber was measured by a fiber strength tester (XS(08)XT-3, XuSai, Shanghai, China) at room
temperatures. The gauge length of the tested fiber was set to be at 15 mm. The cross-sectional area of
the fiber was measured by SEM. In this study, thirty samples were tested for each kind of fiber in order
to acquire its strength values.
3. Results and Discussion
3.1. Effect of Preparation Conditions on the Size of Iron Colloidal Particles
In this study, iron sol was successfully prepared, which could be characterized by the color of
dark red-brown. When a laser beam penetrated the sol, the Tyndall effect could be clearly observed.
The morphology of iron colloidal particles obtained at different holding temperature is shown in
Figure 1. In these cases, the molar ratio of NH4 HCO3 to Fe(NO3 )3 ·9H2 O was set to be at 2.5. In Figure 1,
the black dots correspond to iron colloidal particles. It can be seen that the colloidal particles are
approximately spherical with a particle size of less than 10 nm. When the holding temperature rises,
the colloidal particles tend to be larger.
The colloidal particle size of iron sol was measured by a particle size analyzer. As shown in
Figure 2a, the colloidal particle size follows the lognormal distribution. The sol that was prepared at
temperatures ranging from 25 to 70 ◦ C, its colloidal particle size mainly distributed in the range of
2–10 nm. With the increase of holding temperature, the distribution curve of colloidal particle size
shifted to the right, which means that large colloidal particles tend to be formed at a higher temperature.
When the holding temperature increased to 80 ◦ C, the colloidal particle size varied from 5 to 20 nm,
which is much larger than others. Figure 2b shows the average size of iron colloidal particles. It was
also found that the average size sharply increases from 7.06 to 10.16 nm, when the holding temperature
rose from 70 to 80 ◦ C. Compared with the TEM results, the colloidal particle size obtained by the
particle size analyzer is a little larger. This is because water in iron colloidal particles was lost during
the preparation of the TEM sample. As a result, the size of iron colloidal particles decreased. Based
on the above results, it is confirmed that increasing holding temperature contributes to larger iron
colloidal particles being formed.
Materials 2020, 13, 5442
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Materials 2020, 13, x FOR PEER REVIEW
4 of 15
(a)
(b)
(c)
(d)
.
(e)
Figure 1. The morphology of iron colloidal particles prepared at different temperatures: (a) 40 ◦ C;
◦ C;iron
◦ C.
Figure
The
colloidal
particles prepared at different temperatures: (a) 40 °C; (b)
(b) 501.◦ C;
(c)morphology
60 ◦ C; (d) 70 of
(e) 80
50 °C; (c) 60 °C; (d) 70 °C; (e) 80 °C.
The colloidal particle size of iron sol was measured by a particle size analyzer. As shown in
Figure 2a, the colloidal particle size follows the lognormal distribution. The sol that was prepared at
temperatures ranging from 25 to 70 °C, its colloidal particle size mainly distributed in the range of 2–
10 nm. With the increase of holding temperature, the distribution curve of colloidal particle size
shifted to the right, which means that large colloidal particles tend to be formed at a higher
temperature. When the holding temperature increased to 80 °C, the colloidal particle size varied from
5 to 20 nm, which is much larger than others. Figure 2b shows the average size of iron colloidal
particles. It was also found that the average size sharply increases from 7.06 to 10.16 nm, when the
holding temperature rose from 70 to 80 °C. Compared with the TEM results, the colloidal particle size
Materials 2020, 13, x FOR PEER REVIEW
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obtained by the particle size analyzer is a little larger. This is because water in iron colloidal particles
was
lost during
the preparation
of theisTEM
result,
the water
size ofiniron
obtained
by the particle
size analyzer
a littlesample.
larger. As
Thisa is
because
ironcolloidal
colloidal particles
particles
decreased.
Basedthe
on the
above results,
it is
confirmed
holding
contributes
was lost during
preparation
of the
TEM
sample.that
As increasing
a result, the
size oftemperature
iron colloidal
particles
to
larger iron
colloidal
being formed.
decreased.
Based
on theparticles
above results,
it is confirmed that increasing holding temperature contributes
Materials 2020, 13, 5442
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to larger iron colloidal particles being formed.
(a)
(a)
(b)
(b)
Figure 2. The size of iron colloidal particles prepared at different temperatures: (a) size distribution;
Figure 2. The size of iron colloidal particles prepared at different temperatures: (a) size distribution;
(b)
average
size.
Figure
2. The
size of iron colloidal particles prepared at different temperatures: (a) size distribution;
(b)
average
size.
(b) average size.
Figure 3a
3/Fe(NO3)3·9H2O ratio on the size distribution of the iron
Figure
3a presents
presents the
theeffect
effectof
ofNH
NH4HCO
4 HCO3 /Fe(NO3 )3 ·9H2 O ratio on the size distribution of the
◦
colloidal
particles
prepared
at
50
°C.
As
shown
in in
this
figure,
iron
colloidal
particle
mainly
Figure
3a
presents
the
effect
of
NH
4
HCO
3/Fe(NO
3)3this
·9H
2figure,
O ratio
on the
size distribution
of the
iron
iron colloidal particles prepared at 50 C. As shown
iron
colloidal
particle size
size
mainly
distributes
in the
the range
range
of 2–10
2–10
nm.
The
distribution
curves
of colloidal
colloidal
particle size
size
are similar
similar
to the
the
colloidal particles
prepared
atnm.
50 °C.
shown incurves
this figure,
iron colloidal
particle
size mainly
distributes
in
of
The As
distribution
of
particle
are
to
iron
sols
prepared
under
different
conditions.
When
the
ratio
rises
from
1.5
to
2.5,
the
distribution
distributes
in
the
range
of
2–10
nm.
The
distribution
curves
of
colloidal
particle
size
are
similar
to
the
iron sols prepared under different conditions. When the ratio rises from 1.5 to 2.5, the distribution curve
curve
shifts
to
the
right
a
little.
Figure
3b
shows
the
relationship
between
the
average
colloidal
particle
iron
sols
prepared
under
different
conditions.
When
the
ratio
rises
from
1.5
to
2.5,
the
distribution
shifts to the right a little. Figure 3b shows the relationship between the average colloidal particle size
size
and
the NH
4HCO
3/Fe(NO
)3·9H2O3b
molar
ratio.
can be seenbetween
that the average
sizecolloidal
of iron colloidal
curve
shifts
to the
right
a little.3Figure
shows
the It
relationship
the average
particle
and
the
NH
4 HCO3 /Fe(NO3 )3 ·9H2 O molar ratio. It can be seen that the average size of iron colloidal
particles
is
only
3.61
nm
when
the
NH
4HCO
3/Fe(NO
3)can
3·9H
2Oseen
ratio
is
set
to
be
at
1.5.
With
the
increase
size
and
the
NH
4
HCO
3
/Fe(NO
3
)
3
·9H
2
O
molar
ratio.
It
be
that
the
average
size
of
iron
colloidal
particles is only 3.61 nm when the NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio is set to be at 1.5. With the increase
of
the material
material
ratio,
the
average
colloidal
particle
size
slightly
increases
from
3.61
to
5.2nm.
nm.
particles
is onlyratio,
3.61 the
nm average
when the
NH4HCO
3/Fe(NO
3)3slightly
·9H
2O ratio
is set to
be at
1.5.to
With
the increase
of
the
colloidal
particle
size
increases
from
3.61
5.2
of the material ratio, the average colloidal particle size slightly increases from 3.61 to 5.2 nm.
(a)
(a)
(b)
(b)
Figure 3. The size of iron colloidal particles prepared with different NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratios at
Figure 3. The size of iron colloidal particles prepared with different NH4HCO3/Fe(NO3)3·9H2O ratios
50 ◦ C: (a) size distribution; (b) average size. k in Figure 3a represents NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio.
at
50 °C:3. (a)
(b) average
k in Figure
3a represents
NH43HCO
3/Fe(NO
)3·9H
2O
Figure
Thesize
sizedistribution;
of iron colloidal
particlessize.
prepared
with different
NH4HCO
/Fe(NO
3)3·9H23O
ratios
ratio.
at
50
°C:
(a)
size
distribution;
(b)
average
size.
k
in
Figure
3a
represents
NH
4
HCO
3
/Fe(NO
3
)
3
·9H
2O
The stability of iron sol was studied by measuring the Zeta potential. As shown in Figure
4,
ratio.
the iron
colloidal particles are positively charged, which is the same as alumina precursor colloidal
The stability of iron sol was studied by measuring the Zeta potential. As shown in Figure 4, the
particles. The zeta potential of iron sol ranges from 15 to 34 mV, indicating that all the sols are stable.
iron colloidal
particles
charged,
which is
same
as alumina
precursor
colloidal
The stability
of ironare
solpositively
was studied
by measuring
thethe
Zeta
potential.
As shown
in Figure
4, the
Thus, the iron sol and alumina precursor sol can be mixed evenly without colloid coagulation. Among
iron colloidal particles are positively
charged,
which
is
the
same
as
alumina
precursor
colloidal
all sols, the one prepared at 50 ◦ C with the NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio of 1.5 has the smallest zeta
potential. This is probably related to its small colloidal particle size.
Materials 2020, 13, x FOR PEER REVIEW
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particles. The zeta potential of iron sol ranges from 15 to 34 mV, indicating that all the sols are stable.
Thus, the iron sol and alumina precursor sol can be mixed evenly without colloid coagulation.
Among all sols, the one prepared at 50 °C with the NH 4HCO3/Fe(NO3)3·9H2O ratio of 1.5 has the
Materials 2020, 13, 5442
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smallest zeta potential. This is probably related to its small colloidal particle size.
(a)
(b)
Figure 4. The Zeta potential of iron sols: (a) Zeta potential vs. holding temperature; (b) Zeta potential
Figure 4. The Zeta potential of iron sols: (a) Zeta potential vs. holding temperature; (b) Zeta potential vs.
vs. NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio.
NH4HCO3/Fe(NO3)3·9H2O ratio.
During the preparation process of iron sol, many complicated chemical reactions took place,
the preparation
iron sol, manyreaction.
complicated
reactions
took place,
such During
as the hydrolysis
reactionprocess
and theofpolymerization
It waschemical
discovered
that hydrous
iron
such
as
the
hydrolysis
reaction
and
the
polymerization
reaction.
It
was
discovered
that
hydrous
iron
polymers have the following formula [25],
polymers have the following formula [25],
[FeOq (OH)x (H2 O)p [3-(x + 2q)]+ ]n [counterions− ][3 − (x + 2q)]n/s
[FeOq(OH)x(H2O)p[3-(x + 2q)]+]n[counterions−][3 − (x + 2q)]n/s
(1)
(1)
where s is the charge of the counterion having a value of 1, 2 or 3 and n can be larger than
where s is the charge of the counterion having a value of 1, 2 or 3 and−n can3−
be larger than 500. The
500. The counterion can be any water-solubilizing anion, such
as Cl , NO and COOH− . These
counterion can be any water-solubilizing anion, such as Cl−, NO3− and COOH−. These complicated
complicated reactions can be simply described in the following equations:
reactions can be simply described in the following equations:
Fe3+ + H2 O
Fe(OH)2+ + H+
Fe3+ + H2O ⇌ Fe(OH)2+ + H+
)2+ +2+ +HH
Fe(OH
Fe(OH) + + H+
2O
Fe(OH)
2O ⇌ Fe(OH)22+ + H+
Fe OH
)2 + +
H2 O
Fe(OH) + H+
Fe(OH)
2+ + H
2O ⇌ Fe(OH)33 + H+
(2)
(2)
+ +
NH4NH
+
OH) ++H+H+
2O
4 +HH
2O ⇌ NH
NH44((OH)
(5)
(5)
– –
– –
HCO
HO
HCO
3 +H
H
2O ⇌ H
H22CO3 ++HO
3 +
2O
(6)
(6)
– –
++
HCO
HH
O ++CO
CO
HCO
3 +
⇌ H22O
2 2
3 +
h
i
x + 2q)]+
z)+ + H2O ⇌ [FeOq(OH)x(H2O)p[3-(x
nFe(OH)
z(3 –+
]n
nFe(OH
)z (3 – z)+
H2 O
FeOq (OH)x (H2 O)p [3−+(2q)]+
(7)
(7)
(3)
(3)
(4)
(4)
n
(8)
(8)
In this
this study,
study,adding
addingmore
moreNH
NH4 4HCO
HCO33 solution
solution in
inthe
theFe(NO
Fe(NO33))33 solution
solution will
will consume
consume more
more H
H++
In
3+
3+
3+
3+
and accelerate
accelerate the
the hydrolysis
hydrolysis reaction
reactionof
ofFe
Fe .. Since
Since the
the Fe
Fe hydrolysis reaction is an
an endothermic
endothermic
and
reaction,
the
increasing
temperature
also
promotes
the
hydrolysis
reaction.
As
a
result,
the
value of
of x
reaction, the increasing temperature also promotes the hydrolysis reaction. As a result, the value
increaseswith
withthe
theincrease
increaseof
ofholding
holdingtemperature
temperatureororNH
NH
4HCO/Fe(NO
·9H22O ratio.
ratio. In the hydrous
hydrous
increases
4 HCO
3 3/Fe(NO
3 )33)3·9H
3+ together. When the value of x increases,
iron polymer,
polymer,OH
OH–– can
can work
work as
as aa bridge
bridge joining
joining the
the two
twoFe
Fe3+
iron
the colloid
colloid particles
particles become larger.
larger. Although the polymerization reaction is an exothermic reaction,
the
largeenergy
energy
barrier
should
be overcome
during
the polymerization
[26].
Thus,
large
aa large
barrier
should
be overcome
during
the polymerization
processprocess
[26]. Thus,
large
colloidal
colloidaltend
particles
to be
at highertemperatures.
holding temperatures.
particles
to betend
formed
atformed
higher holding
3.2. Thermal Analysis of Iron Sol
Figure 5 shows the TG–DSC curves of iron sols prepared with different NH4 HCO3 /Fe(NO3 )3 ·9H2 O
ratios. It can be seen that these TG–DSC curves are similar for different samples. Thus, the iron sol
prepared at 70 ◦ C with the NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio of 2.5 was taken as an example to study
Materials 2020, 13, 5442
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its thermal decomposition behavior. These TG–DSC curves of iron sol obtained at a heating rate of
10 ◦ C/min in air atmosphere are presented in Figure 5. The TG curve reveals that the mass loss of iron
sol
occurs from room temperature to about 400 ◦ C, which can be divided into three stages. In
the
Materials 2020, 13, x FOR PEER REVIEW
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first stage, from room temperature to about 190 ◦ C, the mass decreases slowly and the weight loss is
◦ C to 270 ◦ C. In this stage, the mass sharply declines from
about
7%. slowly
The second
stage
is from
decreases
and the
weight
loss190
is about
7%. The second stage is from 190 °C to 270 °C. In this
◦
93%
64%.
In the
thirddeclines
stage (270–400
C),tothe
mass
decreases
gradually
and°C),
the the
weight
is only
stage,tothe
mass
sharply
from 93%
64%.
In the
third stage
(270–400
massloss
decreases
3%.
After
that,
the
mass
of
iron
sol
keeps
in
constant.
During
the
thermal
decomposition
process,
gradually and the weight loss is only 3%. After that, the mass of iron sol keeps in constant. During
the
weight
loss is approximately
39%.
shownloss
in Figure
5, there are39%.
threeAs
small
endothermic
the total
thermal
decomposition
process, the
totalAsweight
is approximately
shown
in Figure
peaks
andare
twothree
exothermic
peaks in the DSC
curve.
endothermic
distribute
in thecurve.
first stage.
5, there
small endothermic
peaks
andAll
two
exothermicpeaks
peaks
in the DSC
All
◦ C, respectively. Among all peaks, the one at 251 ◦ C
The
two
exothermic
peaks
are
at
251
and
380
endothermic peaks distribute in the first stage. The two exothermic peaks are at 251 and 380 °C,
is
the sharpest,
whichall
corresponds
theatsharp
decline
mass. According
to the TG–DSC
respectively.
Among
peaks, the to
one
251 °C
is theinsharpest,
which corresponds
to thecurves,
sharp
it
was
inferred
that
the
first
stage
of
thermal
decomposition
is
related
to
the
removal
of
free
water,
decline in mass. According to the TG–DSC curves, it was inferred that the first stage of thermal
bound
water and
ammonia
gas.removal
In the second
dehydration
and
condensation
hydroxyl
decomposition
is related
to the
of freestage,
water,the
bound
water and
ammonia
gas. Inofthe
second
and
the
decomposition
of Nitrate
take place.
In the third
the weight lossofisNitrate
probably
stage,
the
dehydration and
condensation
of hydroxyl
andstage,
the decomposition
takecaused
place. by
In
the removal
of
residual
hydroxyl
and
Nitrate.
third stage, the weight loss is probably caused by the removal of residual hydroxyl and Nitrate.
Figure 5. TG–DSC
TG–DSC curves
prepared
with
different
NHNH
at 70at◦70
C.
curvesofofthe
theiron
ironsolsol
prepared
with
different
4HCO
3/Fe(NO
3)3·9H
O ratio
4 HCO
3 /Fe(NO
3 )3 ·9H
2 O 2ratio
°C.
The exothermic peak at 380 ◦ C is related to the transformation from iron sol to α-Fe2 O3 . It can
◦ C. Figure 6 shows the XRD patterns of the iron sols.
be seen
α-Fe2 O3 peak
startsatto380
form
Thethat
exothermic
°C at
is about
related350
to the
transformation from iron sol to α-Fe2O3. It can be
◦ C for 12 h consists of orthorhombic NH NO (o-NH NO ), the amorphous
The
dried
at 60 to
4 XRD
3 patterns
4
seeniron
thatsol
α-Fe
2O3 starts
form at about 350 °C. Figure 6 shows the
of3 the iron sols. The
phase
and
a
small
amount
of
tetragonal
NH
NO
(t-NH
NO
).
The
amorphous
phase
corresponds
to
4
3
4NH43NO3 (o-NH4NO3), the amorphous
iron sol dried at 60 °C for 12 h consists of orthorhombic
phase
◦ C for 1 h, the thermal decomposition
the
hydrous
iron
polymers.
When
the
iron
sol
was
annealed
at
200
and a small amount of tetragonal NH4NO3 (t-NH4NO3). The amorphous phase corresponds to the
of
iron sol
occurred.
AsWhen
a result,
of these peaks
t-NH4 NO3
4 NO3 and
hydrous
iron
polymers.
thethe
ironintensity
sol was annealed
at 200related
°C for 1toh,o-NH
the thermal
decomposition
◦ C for 1 h, the iron sol completely transformed into
decreases.
When
the
sol
was
calcined
at
300
or
400
of iron sol occurred. As a result, the intensity of these peaks related to o-NH4NO3 and t-NH4NO3
α-Fe
thethe
formation
of α-Fe
is much
lower
than sol
thatcompletely
of α-Al2 O3transformed
, the iron sol
2 O3 . Since
2 O3°C
decreases.
When
sol was temperature
calcined at 300
or 400
for 1 h,
the iron
can
served
as a nucleating
agent
for aluminaoffiber.
intobe
α-Fe
2O3. Since
the formation
temperature
α-Fe2O3 is much lower than that of α-Al2O3, the iron
sol can be served as a nucleating agent for alumina fiber.
Figure 6. XRD patterns of the iron sol calcined at different temperatures for 1 h.
hydrous iron polymers. When the iron sol was annealed at 200 °C for 1 h, the thermal decomposition
of iron sol occurred. As a result, the intensity of these peaks related to o-NH4NO3 and t-NH4NO3
decreases. When the sol was calcined at 300 or 400 °C for 1 h, the iron sol completely transformed
into α-Fe2O3. Since the formation temperature of α-Fe2O3 is much lower than that of α-Al2O3, the iron
Materials 2020, 13, 5442
8 of 14
sol can be served as a nucleating agent for alumina fiber.
Figure 6. XRD patterns of the iron sol calcined at different temperatures for 1 h.
Figure 6. XRD patterns of the iron sol calcined at different temperatures for 1 h.
3.3. The Effect of Iron Sol on the Microstructure of Calcined Alumina Fibers
In this study, the iron sols prepared with the NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio of 1.5, 2.0 and 2.5 at
4 HCO3 /Fe(NO3 )3 ·9H2 O ratio of 2.5 at the temperature of 40, 50,
60, 70 and 80 ◦ C were chosen as nucleating agents to be added in alumina fibers. For simplicity, the iron
sol was labeled as T-x and K-y, where T represents the one being prepared at different temperatures
with the NH4 HCO3 /Fe(NO3 )3 ·9H2 O ratio of 2.5, K corresponds to the sol obtained with different
material ratio at 50 ◦ C and x and y are the holding temperature and the NH4 HCO3 /Fe(NO3 )3 ·9H2 O
ratio, respectively. The Al2 O3 fibers prepared in this study are listed in Table 1.
50 ◦ C and the sols obtained with the NH
Table 1. Samples prepared in this study.
Iron Sol Preparation Condition
Fiber Sample
A1
A2
A3
A4
A5
B1
B2
Iron Sol
T40
T50
T60
T70
T80
K1.5
K2.0
Holding Temperature
◦C
40
50 ◦ C
60 ◦ C
70 ◦ C
80 ◦ C
50 ◦ C
50 ◦ C
NH4 HCO3 /Fe(NO3 )3 ·9H2 O Ratio
2.5
2.5
2.5
2.5
2.5
1.5
2.0
The Fe content in the calcined Al2 O3 fibers was measured by inductively coupled plasma optical
emission spectrometer (ICP-OES). The calculated Fe2 O3 content based on the Fe content varied from
0.63% to 0.66%, which is in agreement with the setting value. The sample A4 was taken as an example
to study the phase transformation of alumina fiber. Figure 7 shows the XRD patterns of this fiber. It can
be seen that the amorphous phase exists only in the fiber preheated at 500 ◦ C. When the fiber was
calcined at 1400 ◦ C for 1 min, the amorphous Al2 O3 partially crystallized to γ-Al2 O3 . With the holding
time prolonged, α-Al2 O3 became the main phase. The fiber completely transformed into α-Al2 O3
phase, when it was calcined for 5 min. Thus, all the alumina fibers were calcined at 1400 ◦ C for 5 min.
Materials 2020,
2020, 13,
13, x
5442
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FOR PEER REVIEW
Materials 2020, 13, x FOR PEER REVIEW
of 15
14
99of
9 of 15
Figure 7. XRD patterns of Al2O3 fibers calcined at 1400 °C.
◦ C.
Figure
Figure7.7.XRD
XRDpatterns
patternsof
ofAl
Al22O
O33 fibers calcined at 1400 °C.
The morphology and cross-section microstructure of calcined Al2O3 fibers are shown in Figure
The
morphology
and cross-section
microstructure
ofofcalcined
AlAl
are
shown
inin
Figure
8.
2 O2O
3 fibers
Thebe
morphology
cross-section
microstructure
calcined
3from
fibers15
are
shown
Figure
8. It can
seen that alland
the fibers
are continuous
with a diameter
ranging
to 18
µ m. The
fiber
It
bebe
seen
that
allall
the
fibers
are
continuous
with
ranging
from
µm.
8. can
It can
seen
that
the
fibers
are
continuous
withaother
adiameter
diameter
ranging
from15
15to
to18
18microstructure
µ m. The
The fiber
fiber
surface
is smooth
without
obvious
cracks,
pores and
defects.
The cross-section
surface
is
smooth
without
obvious
cracks,
pores
and
other
defects.
The
cross-section
microstructure
surface
is smooth
cracks,ofpores
and other
defects.grains.
The cross-section
microstructure
of
alumina
fiber iswithout
uniformobvious
and consists
nano-scale
equiaxed
For different
fibers, these
of
alumina
fiber
is
uniform
and
consists
of
nano-scale
equiaxed
grains.
For
different
fibers, these
of
alumina
fiber
is
uniform
and
consists
of
nano-scale
equiaxed
grains.
For
different
these
microsturctures are similar except for their grain size. Among all fibers, the sample A1,fibers,
B1 and
B2
microsturctures
are
similar
except
forfor
their
grain
size.
Among
all fibers,
the sample
A1, B1
and
B2
have
microsturctures
are
similar
except
their
grain
size.
Among
all
fibers,
the
sample
A1,
B1
and
B2
have a significantly smaller grain size. From sample A1 to A5, the α-Al2O3 grain size increases
ahave
significantly
smaller
grain
size.
From
sample
A1
to
A5,
the
α-Al
O
grain
size
increases
gradually.
3 α-Al2O3 grain size increases
a significantly smaller grain size. From sample A1 to A5,2 the
gradually.
gradually.
(a)
(a)
(b)
(b)
(c)
(c)
(d)
(d)
Figure 8. Cont.
Materials 2020, 13, 5442
10 of 14
Materials 2020, 13, x FOR PEER REVIEW
10 of 15
(e)
(f)
(g)
(h)
(i)
(j)
(k)
(l)
Figure 8. Cont.
Materials 2020, 13, 5442
11 of 14
Materials 2020, 13, x FOR PEER REVIEW
11 of 15
(k)
(l)
(m)
(n)
(m)
(n)
Figure
8. The
morphology
and
cross-section
ofcalcined
calcined
Sample-A1;
3 fibers:
Figure
8. The
morphology
and
cross-sectionmicrostructure
microstructure of
AlAl
2O2
3O
fibers:
(a,b)(a,b)
Sample-A1;
Figure 8. The morphology and cross-section microstructure of calcined Al2O3 fibers: (a,b) Sample-A1;
(c,d) Sample-A2;
(e,f)(e,f)
Sample-A3;
(g,h)
Sample-A4;
(k,l) Sample-B1;
(m,n)SampleSample-B2.
(c,d) Sample-A2;
Sample-A3;
(g,h)
Sample-A4;(i,j)
(i,j)Sample-A5;
Sample-A5; (k),l)
Sample-B1; (m,n)
(c,d) Sample-A2; (e,f) Sample-A3; (g,h) Sample-A4; (i,j) Sample-A5; (k),l) Sample-B1; (m,n) SampleB2.
B2.
The distribution of fiber grain size is shown in Figure 9. It can be seen that the grain size of each
The follows
distribution
fiber grain
size is shown
in Figure
9. ItA1
can
seen
the value
grain size
of each from
kind of fiber
theof
normal
distribution.
From
sample
tobe
A5,
thethat
peak
increases
The
distribution
of fiber
graindistribution.
size is shown
in Figure
9.
Ittocan
bethe
seen
that
theincreases
grain size
of each
kind
of
fiber
follows
the
normal
From
sample
A1
A5,
peak
value
from
37.5 to 54.0 nm. The sample B1 and B2 exhibit finer grains, which distribute in the range of 14–35
nm
kind 37.5
of fiber
follows
thesample
normal
distribution.
From
sample
A1 to
A5, theinpeak
valueofincreases
from
to
54.0
nm.
The
B1
and
B2
exhibit
finer
grains,
which
distribute
the
range
14–35
nm
and 12–42 nm, respectively. The relationship between the fiber grain size and the iron colloidal particle
and
12–42
The B2
relationship
between
thewhich
fiber grain
size and
therange
iron colloidal
37.5 to
54.0
nm.nm,
Therespectively.
sample B1 and
exhibit finer
grains,
distribute
in the
of 14–35 nm
size is shown in Figure 9h. It was revealed that the maximum mean grain size of fiber (sample A5) was
particle
size
is
shown
in
Figure
9h.
It
was
revealed
that
the
maximum
mean
grain
size
of
fiber
(sample
and 12–42 nm, respectively. The relationship between the fiber grain size and the iron colloidal
only 54.3 nm. When the mean colloidal particle size decreased from 10.16 to 5.65 nm, the mean grain
A5)size
wasisonly
54.3 in
nm.
When9h.
theItmean
colloidal that
particle
decreased
fromgrain
10.16size
to 5.65
nm, (sample
the
particle
shown
Figure
was revealed
thesize
maximum
mean
of fiber
size also
gradually
from 54.3
to 46.5
nm.54.3
With
the further
decrease
of mean
colloidal
particle
mean
grain sizedeclined
also gradually
declined
from
to 46.5
nm. With
the further
decrease
of mean
A5) was only 54.3 nm. When the mean colloidal particle size decreased from 10.16 to 5.65 nm, the
size from
5.65 particle
to 3.61 nm,
mean
size
quickly
decreased
to 22.5
nm. Generally,
the22.5
grain
size of
colloidal
size the
from
5.65 grain
to 3.61
nm,
the mean
grain size
quickly
decreased to
nm.
mean grain size also gradually declined from 54.3 to 46.5 nm. With the further decrease of mean
alumina
fiber isthe
equal
larger
than 100
nm.
In this
the addition
ofthis
ironstudy,
sol with
small colloidal
Generally,
grainorsize
of alumina
fiber
is equal
orstudy,
larger than
100 nm. In
the addition
of
colloidal
particlesmall
size colloidal
from 5.65
to 3.61
nm, the mean
grain alumina
size quickly
decreased to 22.5 nm.
ironsize
sol with
particle
size contributed
to nano
forming
fiber with nano grains.
particle
contributed
to forming
alumina
fiber with
grains.
Generally, the grain size of alumina fiber is equal or larger than 100 nm. In this study, the addition of
iron sol with small colloidal particle size contributed to forming alumina fiber with nano grains.
(a)
(b)
Figure 9. Cont.
(a)
(b)
Materials 2020, 13, 5442
12 of 14
Materials 2020, 13, x FOR PEER REVIEW
11 of 13
(c)
(d)
(e)
(f)
(g)
(h)
The grain
distribution
aluminafiber:
fiber: (a)
(a) Sample
A2,A2,
(c) Sample
A3, A3,
FigureFigure
9. The9. grain
sizesize
distribution
ofof
alumina
SampleA1,
A1,(b)
(b)Sample
Sample
(c) Sample
(d)
Sample
A4,
(e)
Sample
A5,
(f)
Sample
B1
and
(g)
Sample
B2.
(h)
The
relationship
between
mean
(d) Sample A4, (e) Sample A5, (f) Sample B1 and (g) Sample B2. (h) The relationship between mean
grain size and mean colloidal particle size.
grain size and mean colloidal particle size.
Figure 10 shows the tensile strength of alumina fibers. The sample B1 exhibits the highest tensile
Figure 10 shows the tensile strength of alumina fibers. The sample B1 exhibits the highest tensile
strength, up to ~1400 MPa, which is related to the smallest grain size. From sample A1 to A5, the
strength, up to ~1400 MPa, which is related to the smallest grain size. From sample A1 to A5, the tensile
tensile grain size decreases, corresponding to the increase of the grain size. Thus, decreasing fiber
grain grain
size decreases,
corresponding
to thestrength
increase
of the
grain size. Thus, decreasing fiber grain size
size contributes
to higher tensile
being
acquired.
contributes to higher tensile strength being acquired.
The formation of α-Al2 O3 includes the nucleation and grain growth processes. Based on the above
results, all the iron sols can be employed as nucleating agents to promote the formation of α-Al2 O3 .
In the nucleation process, the nucleation density of α-Al2 O3 is proportional to the number of iron
colloidal particles added in unit volume. When the additional amount is consistent, the reduction of
colloidal particle size is effective to increase the colloidal particle number and the nucleation density.
Thus, decreasing the colloidal particle size contributes to forming more α-Al2 O3 grains in smaller
size. In theory, the initial grain size of α-Al2 O3 increases linearly with the colloidal particle size.
Materials 2020, 13, 5442
13 of 14
However, the α-Al2 O3 grain size rises slightly when the colloidal particle size is larger than 5.65 nm.
This probably results from the grain growth of α-Al2 O3 . As shown in Figure 7, most of α-Al2 O3 can
be formed before 3 min, while the phase transformation completes at 5 min. During this process,
the initial α-Al2 O3 grains will grow up and merge into lager grains. Since the growth rate decreases
with the increase of Al2 O3 grain size, as a result, the grain size of the samples added with large iron
colloidal
particles
tends
be constant.
Materials 2020,
13, x FOR
PEERtoREVIEW
12 of 14
Figure 10. Tensile strength of alumina fibers.
4. Conclusions
Figure 10. Tensile strength of alumina fibers.
In this study, stable iron sols were prepared with NH4 HCO3 and Fe(NO3 )3 ·9H2 O. The mean
The formation of α-Al2O3 includes the nucleation and grain growth processes. Based on the
colloidal particle size ranges from 3.61 to 10.16 nm, which increases with the holding temperature
above results, all the iron sols can be employed as nucleating agents
to promote the formation of αand the NH4 HCO3 /Fe(NO3 )3 ·9H2 O molar ratio. Sintered at 300 ◦ C for 1 h, the iron sol completely
Al2O3. In the nucleation process, the nucleation density of α-Al2O3 is proportional to the number of
transformed into α-Fe2 O3 . Adding iron sol to alumina fiber contributes to forming alumina fiber with
iron colloidal particles added in unit volume. When the additional amount is consistent, the reduction
uniform nano-scale grains. The minimum mean grain size of fiber is only 22.5 nm, when the iron sol
of colloidal particle size is effective to increase the colloidal particle number and the nucleation
with the mean colloidal particle size of 3.61 nm was used as a nucleating agent. With the increase of
density. Thus, decreasing the colloidal particle size contributes to forming more α-Al2O3 grains in
colloidal particle size, the grain size tends to rise from 22.5 to 54.3 nm. The fiber with the smallest
smaller size. In theory, the initial grain size of α-Al2O3 increases linearly with the colloidal particle
grain size exhibits the highest tensile strength, up to ~1400 MPa.
size. However, the α-Al2O3 grain size rises slightly when the colloidal particle size is larger than 5.65
nm. This
probably results
fromW.L.,
the grain
growth
of α-Al2O3. to
Asthe
shown
in Figure
7, most
of L.L.,
α-AlJ.W.
2O3
Author
Contributions:
L.L., J.W.,
Y.M. and
S.Y. contributed
investigation
of the
study.
and
preparedbefore
and analyzed
sol and
and L.L. wrote completes
the manuscript.
and Y.M.
can S.Y.
be formed
3 min,the
while
thefibers.
phaseS.Y.
transformation
at 5 J.W.,
min.W.L.
During
this reviewed
process,
the
and
gave comments. All authors have read and agreed to the published version of the manuscript.
themanuscript
initial α-Al
2O3 grains will grow up and merge into lager grains. Since the growth rate decreases
Funding:
research
was
by National
Nature
Science
China, grant
number
with the This
increase
of Al
2O3funded
grain size,
as a result,
the
grainFoundation
size of theofsamples
added
with 52001333.
large iron
colloidal
particles
tends
to
be
constant.
Conflicts of Interest: The authors declare no conflict of interest.
4. Conclusions
References
TM 6103 Fibers
In this study,
stable iron
sols were prepared
with
NH4HCO
and Fe(NO
3)3·9H
O. The mean
Schmucker,
M.; Mechnich,
P. Microstructural
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Cantonwine,
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Fiber
Strength
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Nextel™ 610ofAlumina
Fiber.
Am.
Author
Contributions:
L.L.,
J.W., T.
W.L.,
Y.M.
and S.Y.
contributed
to theininvestigation
the study.
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Ceram.
Soc. 2015,
1907–1914.
and S.Y.
prepared
and98,
analyzed
the [CrossRef]
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reviewed the manuscript and gave comments. All authors have read and agreed to the published version of the
manuscript.
1.
Funding: This research was funded by National Nature Science Foundation of China, grant number 52001333.
Conflicts of Interest: The authors declare no conflict of interest.
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