Morphology evolution of ZrB2 nanoparticles synthesized by solgel method

Article abstract of DOI:10.1016/j.jssc.2011.05.040

Zirconium diboride (ZrB₂) nanoparticles were synthesized by solgel method using zirconium n-propoxide (Zr(OPr)₄), boric acid (H₃BO₃), sucrose (C₁₂H₂₂O ₁₁), and acetic acid (AcOH). Clearly, it was a non-aqueous solution system at the very beginning of the reactions. Here, AcOH was used as both chemical modifier and solvent to control Zr(OPr)₄ hydrolysis. Actually, AcOH could dominate the hydrolysis by self-produced water of the chemical propulsion, rather than the help of outer water. C₁₂H ₂₂O₁₁ was selected, since it can be completely decomposed to carbon. Thus, carbon might be accounted precisely for the carbothermal reduction reaction. Furthermore, we investigated the influence of the gelation temperature on the morphology of ZrB₂ particles. Increasing the gelation temperature, the particle shapes changed from sphere-like particles at 65 °C to a particle chain at 75 °C, and then form rod-like particles at 85 °C. An in-depth HRTEM observation revealed that the nanoparticles of ZrB₂ were gradually fused together to evolve into a particle chain, finally into a rod-like shape. These crystalline nature of ZrB₂ related to the gelation temperature obeyed the oriented attachment mechanism of crystallography.

Full text of DOI:10.1016/j.jssc.2011.05.040

Journal of Solid State Chemistry 184 (2011) 2047–2052
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Morphology evolution of ZrB nanoparticles synthesized by sol–gel method
2
a
a,
n
a
a
a
b
b
Yun Zhang , Ruixing Li , Yanshan Jiang , Bin Zhao , Huiping Duan , Junping Li , Zhihai Feng
a
Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
Aerospace Research Institute of Materials & Processing Technology, Beijing 100076, China
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Zirconium diboride (ZrB2) nanoparticles were synthesized by sol–gel method using zirconium
n-propoxide (Zr(OPr) ), boric acid (H BO ), sucrose (C12H22O11), and acetic acid (AcOH). Clearly, it
Received 16 February 2011
Received in revised form
4
3
3
was a non-aqueous solution system at the very beginning of the reactions. Here, AcOH was used as both
chemical modifier and solvent to control Zr(OPr) hydrolysis. Actually, AcOH could dominate the
hydrolysis by self-produced water of the chemical propulsion, rather than the help of outer water.
11 was selected, since it can be completely decomposed to carbon. Thus, carbon might be
accounted precisely for the carbothermal reduction reaction. Furthermore, we investigated the
influence of the gelation temperature on the morphology of ZrB particles. Increasing the gelation
7
May 2011
4
Accepted 29 May 2011
Available online 7 June 2011
12 22
C H O
Keywords:
Nanoparticles
Synthesis
2
temperature, the particle shapes changed from sphere-like particles at 65 1C to a particle chain at 75 1C,
and then form rod-like particles at 85 1C. An in-depth HRTEM observation revealed that the
Sol–gel
Zirconium diboride
Morphology
nanoparticles of ZrB
like shape. These crystalline nature of ZrB
attachment mechanism’’ of crystallography.
2
were gradually fused together to evolve into a particle chain, finally into a rod-
2
related to the gelation temperature obeyed the ‘‘oriented
&
2011 Elsevier Inc. All rights reserved.
1
. Introduction
particle size and a high purity. And it is an effective way for low
temperature synthesis of ultra-fine powders. Furthermore, the
advantage of using sol–gel method is that high chemical and
phase homogeneity can be achieved by mixing the starting
components at the molecular or colloidal level. Recently, many
2
Zirconium diboride (ZrB ) is one of the materials known as
ultra-high-temperature ceramics. It has a series of excellent
properties, such as a high melting temperature (3040 1C), an
outstanding wear, and a corrosion resistance. The unique combi-
nation of properties makes ZrB
thermal protection materials, cutting tools, high temperature
electrodes, and molten metal crucibles [1–5]. However, the
ultra-fine ceramic particles such as TiB
[14] were synthesized by sol–gel method. However, a little has
been reported about the synthesis of ZrB powder using a wet
chemical process. Yan et al. [15] synthesized ZrB powder using
zirconium oxychloride (ZrOCl O), boric acid (H BO ), and
ꢀ 8H
phenolic resin by sol–gel method. Wang et al. [16,17] synthesized
ZrB powder using ZrOCl O, boron carbide (B C), and carbon
ꢀ 8H
by wet chemical method. Additionally, Xie et al. [18] prepared
ZrB powder depending on the starting materials of zirconium
n-propoxide (Zr(OPr) ), H BO , and phenolic resin by sol–gel
2
[12], ZrC [13], and SiC
2
a promising candidate for
2
2
applications of ZrB
features of the ZrB
Traditionally, ZrB
following methods: (i) solid-state reaction synthesis [6,7];
ii) electrochemical synthesis [8]; (iii) mechanochemical synth-
esis [9]; and (iv) self-propagating high-temperature synthesis
10,11]. However, such processes need a higher temperature or
2
ceramic material have been limited by the
powder, such as size, purity, etc.
powder can be synthesized mainly by the
2
2
3
3
2
2
2
2
2
4
(
2
4
3
3
[
method. However, most alkoxides are known to react sponta-
neously with water to form a viscous precipitate; therefore, they
require to be chemically modified in order to control their
a long production period, and furthermore the synthesized
powders usually have a relatively coarse particle and a low purity.
As a result, the sinterability of ZrB
rated. Therefore, a synthesis method to reduce particle size and
increase the purity of the ZrB powder is needed. In contrast with
those traditional methods, the sol–gel method is one of the
potential methods for synthesizing ceramic materials with a fine
2
ceramic tends to be deterio-
hydrolysis and condensation. In the case of ZrB
2
, gel precursor
might be obtained by hydrolysis of Zr(OPr) chemically modified
4
2
by acetylacetone (acac) [18]. Nevertheless, most researchers
pointed out the formation of oxycarbides with relatively high
oxygen content, highly detrimental for mechanical application
using acac as a modifier. Another way to control the alkoxides
hydrolysis was to use acetic acid (AcOH) as a chemical modifier.
Such a sol–gel process was already investigated for TiC [19]
synthesis and a low oxygen content of TiC was reported.
n
Corresponding author. Fax: þ 86 10 8231 6500.
E-mail address: ruixingli@yahoo.com (R. Li).
0022-4596/$ - see front matter & 2011 Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2011.05.040

2
048
Y. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 2047–2052
However, little has been reported on AcOH as a modifier for the
ZrB synthesis.
2
This paper deals with the discussion of a new sol–gel method,
using AcOH as both a modifier and a unique solvent to control
Zr(OPr)
BO were the sources of zirconium (Zr) and boron (B), respec-
tively. Sucrose (C12 11) was a source of pyrolyzed carbon.
Here, C12 11 was used, since it can be completely decomposed
4 2 4
hydrolysis to synthesize ZrB particles. Zr(OPr) and
H
3
3
22
H O
22
H O
to carbon. Thus, carbon might be accounted precisely, as a
consequence, the molar ratio of C/Zr might be fixed. In contrast,
the char yield of phenolic resin, which is used as another source of
pyrolyzed carbon is not fixed (about 50 wt%). It is notable that no
aqueous solvent or hydrated compound was accommodated into
the starting materials. In other words, it was a non-aqueous
solution system at the very beginning of the reactions. Further-
4
more, the mechanism of stabilizing Zr(OPr) by AcOH and the
influence of the gelation temperature on the morphology of ZrB
particles were investigated.
2
2
. Experimental
2.1. Starting materials
Zr(OPr)
Shanghai Jingchun Reagents Co. Ltd., Shanghai, China. H
11, and AcOH were supplied by the Lanyi Reagents Co.
4
with a concentration of 70 wt% was supplied by the
3
BO ,
3
12 22
C H O
Ltd., Beijing, China. The grade of all the above reagents was
analytical.
2.2. Synthesis
A general flow diagram for the synthesis of ZrB
2
powder is
and 2.5 g of
12 22
C H O11 were dissolved in 35 ml of AcOH using a 80 ml beaker.
3 3
shown in Fig. 1. In a typical synthesis, 2.4 g of H BO
Then, it was heated with vigorous stirring to 80 1C and main-
2
Fig. 1. General flow diagram for synthesis of ZrB powder.
tained for 0.5 h. We refer to this solution as Mixed solution 1.
4
Afterwards, 5.7 g of Zr(OPr) were added to the Mixed solution 1,
where Dhkl is the crystallite size,
l
is the wavelength of CuK
a
which was previously cooled to room temperature. This was
Mixed solution 2. The Mixed solution 2 was heated with vigorous
stirring to 65 1C (This fixed temperature is so-called gelation
temperature to form sol/gel. 75 and 85 1C were also set in the
final section of this paper.) and maintained for 3 h to form a wet
gel. Finally, it was dried under vacuum at 120 1C for 3 h followed
by a grind process using an agate mortar and a pestle. In this way,
a precursor powder was prepared.
radiation, bhkl is the full-width at half maximum, and
y
is the
Bragg diffraction angle. The morphology of the final products was
characterized by SEM using a JEOL JSM-6700F JAPAN microscope
and TEM using a JEM-2100F microscope.
3
. Results and discussion
After that, the above precursor was firstly heated to 800 1C at a
heating rate of 5 1C/min, then to 1200 1C at 3 1C/min and main-
tained at this temperature for 2 h in argon using an alumina tube
furnace. Afterwards, the precursor powder was continued to heat
from 1200 to 1550 1C at a heating rate of 2 1C/min and kept at this
temperature for 2 h. Then, the sample was cooled to room
temperature at a cooling rate of 5 1C/min. In the end, gray powder
was obtained.
3.1. AcOH dependent mechanism
Theoretically, inorganic metal salts or metal organic compounds
such as metal alkoxides are combined with a mixture of solvents to
form a sol or a gel in sol–gel processes. As a consequence, the metal
organic compounds are partially or completely hydrolyzed and
condensed. However, metal alkoxides are well known to react
spontaneously with water and to form precipitates resulting from
2.3. Characterization
4
successive hydrolysis. In order to obtain a sol/gel, Zr(OPr) has to
be chemically modified by a complexing agent to decrease the
alkoxide reactivity with water. Several works [13,19–22] had
shown that metal alkoxides could be modified by AcOH as bridging
and chelating ligands to forming acetates by the substitution of
OAc for OPr groups. Additionally, AcOH was also used for control-
ling the sol–gel processes by self-produced water without the help
of outer water [23,24]. That is to say, the water for hydrolysis was
generated in situ by esterification of AcOH with alcohol released by
the preceding formation of acetate derivatives of the alkoxides.
The mass and heat flow of the samples were monitored by
thermal analysis (TG-DTA, Beijing Hengjiu instrument Co. Ltd.,
Beijing, China). The crystallographic constitution was identified
by means of an X-ray diffractometer (XRD) using graphite mono-
chromatized CuK
a radiation (Rigaku, D/MAX 2200 PC). Chemical
analysis was performed using infrared method (America, PC600).
Crystallite size was estimated using the Debye–Scherrer equation,
Dhkl ¼ 0:9l=bhkl cos
y
ð1Þ

Y. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 2047–2052
2049
To explore the action of AcOH on the other reactants, the
general reactions occurring during the precursor preparation are
metal chelate formation, transesterification, and hydrolysis-con-
densation. The first reaction was the formation of zirconium
2 2
propoxide diacetate Zr(OAc) (OPr) (see Eq. (2)) [25]:
PrO
PrO
OPr
OPr
PrO
OAc
OPr
Zr
+ AcOH
Zr
ð2Þ
+
PrOH
AcO
This carboxylatoalkoxide could then react following different
competitive reactions. The first one was the transesterification of
alkoxy groups in Zr(OAc)
2
(OPr)
2
, which reacted with the sucrose
OH groups according to Eq. (3):
PrO
OAc
OPr
PrO
Zr
+ HO-X-(OH)
OAc
O-X-(OH)
7
7
Zr
+
PrOH
AcO
AcO
ð3Þ
Fig. 2. TG-DTA thermal analysis curve for ZrB
2
precursor from room temperature
to 1000 1C. (a) 100 1C; (b) 240 1C; (c) 350 1C; (d) 550 1C. (Gelation temperatures:
14 3
where X is a simplified notation for C12H O .
65 1C, B/Zr (mol)¼2.3).
The second reaction occurred between the unreacted AcOH
and the propanol, which was one of the products of the reactions
(2) and (3) to form water and propyl acetate (see Eq. (4)):
Taking all results into account, the thermal decomposition
AcOHþPrOH-PrOAcþH
2
O
(4)
reactions between 100 and 550 1C may be described as follows:
2
HBO
2
(s)-B
2
O
3
(l)þH
2
O (g)m
(7)
(8)
The water and the zirconium propoxide diacetate, which was
one of the products of the reaction (2) reacted spontaneously in a
substitution reaction (see Eq. (5)), in which hydroxy groups
replaced propoxy groups to generate propanol.
C12H22O11 (s)-12C (s)þ11H O (g)m
2
In the present reaction system, HBO
2
completely decomposed
to B
100 and 240 1C (see Eq. (7)). C12
carbon and H O, accompanied with 23.6% weight loss between
40 and 550 1C (see Eq. (8)). Totally, the theoretical weight loss
2
O
3
and H
2
O, accompanied with 6.1% weight loss between
PrO
AcO
OAc
OPr
2
H O
PrO
AcO
OAc
OH
Zr
+ H
O
HO
AcO
OAc
OH
22
H O
11 completely transformed to
2
Zr
+
PrOH
Zr
+ PrOH
2
2
ð5Þ
was 29.7% in the temperature range of 100–550 1C. This magni-
As the water produced in reaction (4) was consumed for the
hydrolysis, the equilibrium of Eq. (4) was then driven to the right
as long as sufficient AcOH was present in the solution. Another
reaction, which might occur, was the generation of oxo ligands by
non-hydrolytic condensation and elimination of an ester. Once the
tude is almost in agreement with the experimental weight loss of
33% between 100 and 550 1C (see Fig. 2).
Alternatively, DTA curve in Fig. 2 reveals that an endothermic
peak at about 240 1C. It might be attributed to both the evaporation
of the bonded water and the decomposition of HBO . Another
endothermic peak at about 350 1C was due to the complete
carbonization of C12 11. The inflection in a temperature range
of 650–780 1C was due to crystallization of amorphous ZrO to
tetragonal ZrO [26]. With the increasing of the temperature, several
exothermic peaks in a temperature ramp from 820 to 880 1C could
be attributed to the delayed crystallization of ZrO [27].
2
different molecules with hydroxy groups formed Zr(OPr)
2 2
(OH) , and
Zr(OAc) (OPr)–O–X–(OH) , further condensation reactions occurred
2
7
22
H O
leading to an increase of the solution viscosity associated to the
formation of Zr–O–Zr and Zr–O–X–O–Zr bridges. A polymeric
precursor was obtained and was placed under heating at 65 1C for
2
2
3
h to evaporate the residual AcOH and the acetates products.
2
After both drying and grinding steps, a precursor powder was
obtained.
3.3. Influence of pyrolysis/calcination temperature on phase
constitution
3
.2. Thermal analysis of precursor powder
The phase constitution before and after pyrolysis/calcination
was identified based on the above results of the thermal analysis.
Fig. 3 shows XRD patterns of the precursor powder calcined at
different temperatures. Clearly, the precursor was in a typical
amorphous state without any peak in its XRD pattern (see
Fig. 3(a)). In contrast, increasing the pyrolysing/calcining tem-
perature, the powder gradually crystallized. In the sample treated
First of all, TG-DTA analysis of precursor powder in argon was
conducted to understand the processes of pyrolysis of the pre-
cursor. The complementary information obtained allows differ-
entiation between endothermic and exothermic events, which
have no associated weight loss (e.g. melting and crystallization)
and those which involve a weight loss (e.g. degradation). TG
analysis showed that a considerable weight loss occurred in a
temperature range of 100–550 1C for the precursor, as shown in
Fig. 2. An in-depth analysis showed that the weight loss was
ca. 8.0% from 100 to 240 1C, and ca. 25% from 240 to 550 1C (see
Fig. 2). To clarify why the weight loss occurred during heating, the
reaction (6) has to be considered firstly, although it had finished
during the process in which the wet gel was dried under vacuum
at 120 1C.
at 1100 1C for 2 h, m-ZrO
Fig. 3(b)). Both XRD and TG-DTA results showed that the pre-
cursor was probably transformed to ZrO , B , and carbon at
100 1C [28]. However, there was no trace of B and carbon
2 2
and t-ZrO phases were identified (see
2
2 3
O
1
2 3
O
phases in the diffraction patterns. That is to say, the temperature
was not high enough to initiate a completely carbothermal
reduction, and both B
ZrB was generated at 1300 1C as shown in Fig. 3(c). With the
increasing of the calcining temperature, the XRD patterns showed
that the diffraction intensity of ZrB increased, m-ZrO and t-ZrO
2 3
O and carbon phases might be amorphous.
2
H
3
BO
3
(s)-HBO
2
(s)þH
2
O (g)m
(6)
2
2
2

2
050
Y. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 2047–2052
decreased. Finally, a single phase of ZrB
2
was evolved at 1550 1C
3.4. Influence of gelation temperature on morphology of ZrB
2
for 2 h (see Fig. 3(e)). This means that a carbothermal reduction
process was able to complete at 1550 1C. A further study was
conducted to evaluate the average particle/grain size based on the
Debye–Scherrer equation. The result revealed that was about
It is well known that the gelation temperature is an important
factor for both nucleation and crystal feature. Here, different
gelation temperatures of 65, 75, and 85 1C were fixed to synthe-
6
2 nm.
Theoretically, an idealized reaction to produce ZrB
bothermal reduction in the present study is shown as follows:
2
size ZrB powder. Fig. 4 shows XRD patterns of the powder
2
by car-
ZrO
2
(s)þB
2
O
3
(g)þ5C (s)-ZrB
2
(s)þ5CO (g)m
(9)
Here, B
2
O
3
has an unusual low melting point (450 1C) and a
at 1527 1C
high vapor pressure. The vapor pressure of B
2 3
O
reaches 344 Pa, leading to its rapid vaporization [29]. Therefore,
a higher B/Zr ratio for the starting materials that the stoichio-
metric proportion of B/Zr (mol)¼2.0 as shown in reaction (9) is
necessary. On the basis of our investigation, the optimum ratio of
B/Zr was experimentally determined as 2.3, and this value was
used in this study (full text).
Fig. 3. XRD patterns of (a) precursor, and the precursor reduced carbothermally at
(b) 1100 1C; (c) 1300 1C; (d) 1400 1C; (e) 1550 1C. (Gelation temperatures: 65 1C,
B/Zr (mol)¼2.3).
Fig. 4. XRD patterns of the precursor powder reduced carbothermally at 1550 1C
for 2 h with gelation temperatures of (a) 65 1C; (b) 75 1C; (c) 85 1C. (B/Zr
Fig. 5. SEM images of ZrB
2
powder reduced carbothermally at 1550 1C for 2 h with
(mol)¼2.3).
gelation temperatures of (a) 65 1C; (b) 75 1C; (c) 85 1C. (B/Zr (mol)¼2.3).

Y. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 2047–2052
2051
Fig. 6. TEM images of ZrB
2
powder reduced carbothermally at 1550 1C for 2 h with gelation temperatures of (a) 75 1C; (c) 85 1C; (b) HRTEM image of ZrB
2
particle chain
marked in (a); (d) HRTEM images of rod-like ZrB
2
marked in (c). (B/Zr (mol)¼2.3).
precursor calcined at 1550 1C for 2 h with different gelation tem-
perature and a molar ratio of B/Zr¼2.3. It might be seen that all of
With the temperature further increased, adjacent particles fused
together to form strong links resulting in the evolution of ZrB rods
2
the diffraction peaks were well assigned to a single phase of ZrB
2
.
(see Figs. 5(c) and 6(c)). In the same way, the adjacent grains shared
a common crystallographic orientation, followed by joining of these
grains at a planar interface with edge dislocations (arrowhead) (see
Fig. 6(d)). It also may be found from the HRTEM image that these
particle chain/rod shared {0 0 1} lattice fringes with interplanar
spacings of 0.35 nm.
From a crystallography perspective, according to the ‘‘oriented
attachment mechanism’’, which was first explored by Penn and
Banfield [31], adjacent particles share a common crystallographic
orientation, followed by joining of these particles at a planar
interface. Bonding between adjacent nanoparticles reduces the
overall energy by removing the surface energy associated with
unsatisfied bonds. Clearly, this attachment mechanism is
Furthermore, the influence of the gelation temperature on the
morphology of the samples was also investigated, and the SEM
images of the as-calcined samples are shown in Fig. 5. It is
observed that the morphology of the samples was affected by
the gelation temperature. For the ZrB
the SEM revealed a sphere-like morphology of ZrB
(
(
2
sample gelated at 65 1C,
nanoparticles
see Fig. 5(a)). At 75 1C, the morphology showed a particle chain
see Figs. 5(b) and 6(a)). With the increasing of the temperature to
5 1C, the morphology of the ZrB particles evolved to a rod-like
shape. Additionally, the size of ZrB particle also increased
2
8
2
2
notably with the increasing of the gelation temperature.
To explain the morphology influenced by the gelation tempera-
ture requires a multidisciplinary knowledge. A further investigation,
a high-resolution TEM (HRTEM), was conducted to clarify the
2
reflected by the results in Fig. 6. The ZrB nanoparticles connected
with each other via the ‘‘oriented attachment mechanism’’ and a
reduction in surface free energy was achieved by the complete
removal of pairs of surfaces. Imperfect oriented attachment of
nanocrystals can generate dislocations. Any defects observed by
HRTEM can be attributed to the growth process.
2
microstructure and crystal feature of the ZrB particles. At a lower
temperature, the rate of nucleation was faster than that of growth,
whereas the monomer concentration was sharply decreased [30].
Such slow growth conditions favored the formation of a sphere-like
shape (see Fig. 5(a)). With increasing the gelation temperature,
2
Figs. 5(b) and 6(a) show that a particle chain of ZrB was evolved at
7
5 1C. The HRTEM image of ZrB particle chain revealed that each
2
4. Conclusions
unique particle chain was composed of several sphere-like particles.
These sphere-like particles aligned one by one to form a polycrystal-
ZrB
AcOH as both chemical modifier and solvent to control Zr(OPr)
hydrolysis in the absence of water for the starting materials. It is
notable that a single phase ZrB could be obtained with a molar
ratio of B/Zr¼2.3 for the starting materials at 1550 1C for 2 h.
2
nanoparticles were synthesized by sol–gel method using
line particle chain of ZrB
2
and shared a quasi-parallel crystal
4
orientation at the grain interfaces (see Fig. 6(a) and (b)). It is notable
that the interface between the two grains in the rectangle region
2
(magnified in Fig. 6(b)) involved edge dislocations (arrowhead).

2
052
Y. Zhang et al. / Journal of Solid State Chemistry 184 (2011) 2047–2052
Sphere-like, particle chain, and rod-like ZrB
2
particles could be
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[
The authors appreciate the financial support from the National
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