Nanocarbon-dependent synthesis of ZrB2 in a binary ZrO 2 and boron system

Article abstract of DOI:10.1016/j.jallcom.2011.06.028

Thermodynamically, ZrO₂ may react with boron to form B ₂O₃/B₂O₂ and ZrB₂ at room temperature. However, this reaction is incomplete at temperatures lower than 1550 °C, even with the use of m

Full text of DOI:10.1016/j.jallcom.2011.06.028

Journal of Alloys and Compounds 509 (2011) 8581–8583
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Nanocarbon-dependent synthesis of ZrB2 in a binary ZrO2 and boron system
Ruixing Lia,∗, Haijie Loua, Shu Yinb, Yun Zhanga, Yanshan Jianga, Bin Zhaoa, Junping Lic,
Zhihai Fengc, Tsugio Satob
a Key Laboratory of Aerospace Materials and Performance (Ministry of Education), School of Materials Science and Engineering, Beihang University, Beijing 100191, China
b Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai 980-8577, Japan
c Aerospace Research Institute of Materials & Processing Technology, No. 1 Nan Da Hong Men Road, Fengtai District, Beijing 100076, China
h i g h l i g h t s
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A reaction system of ZrO2, boron, and carbon has not been found in literature.
ZrO2 reacts with B, thermodynamic possibility at 25 ◦C, can not yet end till 1550 ◦C.
A complete reaction was achieved by adding nano-carbon at 1550 ◦C.
The metastable reactants strongly affected the kinetics of solid-state reactions.
Single crystal and plate-like ZrB2 particles were obtained.
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a r t i c l e i n f o
a b s t r a c t
Article history:
Thermodynamically, ZrO2 may react with boron to form B2O3/B2O2 and ZrB2 at room temperature. How-
ever, this reaction is incomplete at temperatures lower than 1550 ◦C, even with the use of metastable
reactants, i.e., as-synthesized amorphous hydrous nano-ZrO2 and amorphous boron powders. In this
study, a complete disintegration of ZrO2 was achieved by introducing nanocarbon to the binary system
of ZrO2 and boron at 1550 ◦C. The metastable reactants affected the temperature required for the solid-
state reactions and also strongly affected the kinetics of the transformation. Single crystal and plate-like
ZrB2 particles with a uniform distribution and a size of ca. 1.0 ␮m in two-dimensions were obtained using
5 wt.% nanocarbon and a B/Zr molar ratio of 4.
Received 31 March 2011
Received in revised form 4 June 2011
Accepted 7 June 2011
Available online 17 June 2011
Keywords:
Ceramics
Amorphous materials
Nanosize
© 2011 Elsevier B.V. All rights reserved.
Nanoparticles
Powder technology
1. Introduction
3ZrO2(s) + 10B(s) → 3ZrB2(s) + 2B2O3(l)
ZrO2(s) + 4B(s) → ZrB2(s) + B2O2(g) ↑
(2)
(3)
(4)
A
range of excellent physicochemical properties such as
high melting temperature, high electrical conductivity, superior
mechanical properties and oxidation resistance make ZrB2
2ZrO2(s) + B4C(s) + 3C(s) → 2ZrB2(s) + 4CO(g) ↑
a
Actually, B2O2 is known to be unstable and it may decompose
to B2O3 and boron [6].
promising material for use in thermal protection systems [1–3].
ZrB2 powder can be obtained by different routes [1–5] and two
main routes are used commercially: (1) the direct chemical combi-
nation of elemental zirconium and boron and (2) the borothermal
or carbothermal reduction (BCTR). The basic reactions are:
3B2O2(g) → 2B2O3(g) + 2B(s)
(5)
Reaction (1) is a simple chemical reaction and, therefore, we
focused on BCTR for this study. Many researchers have prepared
ZrB2 powder by BCTR. Submicron-ZrB2 powder has been synthe-
sized by the borothermal reduction of nano-ZrO2 with boron and
some H3BO3 under hot press conditions [3]. Millet and Hwang
[4] synthesized ZrB2 powder with residual ZrO2 using amorphous
boron and ZrO2 based on Reaction (3). They proved that mechanical
treatment was an effective way to decrease the required reaction
Zr(s) + 2B(s) → ZrB2(s)
(1)
∗
Corresponding author. Tel.: +86 1082316500; fax: +86 1082316500.
E-mail address: ruixingli@yahoo.com (R. Li).
0925-8388/$ – see front matter © 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2011.06.028

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R. Li et al. / Journal of Alloys and Compounds 509 (2011) 8581–8583
Fig. 1. (a) TG–DTA curves of the as-synthesized precursor and (b) XRD patterns of
the precursor before and after calcination.
temperature. Guo and Zhang [5] prepared ZrB2 powder based on
Reaction (4). They obtained ZrB2 at 1650 ◦C using a 20–25 wt.%
excess of B4C. In conclusion, previous studies about the BCTR
mostly followed Reactions (2)–(4). Using Reactions (2) and (3), we
intended to introduce a small amount of carbon into the binary
system of ZrO2 and boron.
There are five reasons for this. First, thermodynamic calcu-
lations show that Reaction (2) may occur at room temperature
[3] and Reaction (6) may thus ignite at higher than 1490 ◦C [7].
Consequently, it is likely that ZrO2 will react with boron to form
B2O3/B2O2 and ZrB2 [Reactions (2) and (3)] initially during heating.
With an increase in temperature, B2O3 and residual ZrO2 tend to
react with carbon to form ZrB2 and this is based on Reaction (6).
Fig. 2. XRD patterns of the samples synthesized using as-synthesized ZrO2, boron,
and (a) 5 wt.% nanocarbon; (b) without carbon at different temperatures.
the kinetics of transformation. In this study, we used nanocar-
bon, which is a popular metastable material. Additionally, hydrous
nano-ZrO2 and amorphous boron were also selected as starting
materials. We thus conducted experiments wherein nanocarbon
was incorporated into a binary ZrO2 and boron system.
2. Experimental
We used zirconyl nitrate (AR), amorphous boron (D50 = 0.82 ␮m, 95% purity) and
carbon (D50 = 30 nm, 99.5% purity) in our work. In a typical synthesis, the precursor
was prepared by dropping 0.25 mol/L zirconyl nitrate solution into stirred ammonia
at 0.5 mL/s. The precipitate was repeatedly centrifuged, washed with methanol and
then dried at 60 ◦C. Afterwards, the precipitated precursor and amorphous boron
with a B/Zr molar ratio of 4 were wet mixed for 4 h and then dried. The mixed
powder and 5 wt.% (total weight of dry hydrous ZrO2 and boron) nanocarbon were
hand ground and pressed into a disk. Finally, the disk was transferred into a crucible
with a cover and placed in an alumina tube furnace. It was initially kept at 400 and
1100 ◦C, respectively, and then at 1450 or 1550 ◦C for 2 h.
The mass and heat flow of the samples were monitored by thermal analysis
(TG–DTA, HCR-2). The crystallographic structure was identified by X-ray diffractom-
etry (XRD, D/MAX 2200 PC). The specific surface area of the powder was determined
using a NOVA-2200e analyzer. The size and morphology of the particles were char-
acterized by SEM (JEOL JSM-6700F) and TEM (JEM-2100F) microscopies.
ZrO2(s) + B2O3(g) + 5C(s) → ZrB2(s) + 5CO(g)
(6)
Second, a chemical reaction in which the forward and reverse
reactions reach equilibrium is characterized by the concentration
of the reactants and products, which do not change over time. The
decrease in product concentration would benefit the forward reac-
tion and inhibit the reverse reaction. For Reactions (2) and (3),
B2O3/B2O2 is one of the main products. If it is consumed by Reac-
tion (6) the equilibrium in Reactions (2) and (3) would change and
the forward reaction would be promoted. Third, Reactions (2) and
(3) are solid redox reactions. If carbon is introduced, Reaction (6)
tends to occur and the chemical process shifts to a gas–liquid–solid
ternary system from a unique solid system. Here, the existence of
both liquid and gaseous B2O3 is possible since B2O3 has a high vapor
pressure [8]. Moreover, the final product, CO, in Reaction (6) is an
efficient reducing agent. Fourth, B2O3/B2O2 and boron are prod-
ucts of Reactions (2), (3) and (5) can be used as recycle feeds. As a
consequence, the amount of boron used can be reduced and the
forward reactions will be more favorable. Fifth, the selection of
starting materials, e.g., metastable reactants affect the tempera-
ture required for the solid-state reaction and also strongly affect
3. Results and discussion
Fig. 1a shows the TG–DTA results of the precipitated precursor
using zirconyl nitrate and ammonia after drying at 60 ◦C. At tem-
peratures lower than 400 ◦C a loss of weight was evident as the
temperature increased, which probably resulted from the loss of
water in hydrous ZrO2. A strong endothermic peak was observed
in the DTA curve at 100 ◦C, which corresponds to the evapora-

R. Li et al. / Journal of Alloys and Compounds 509 (2011) 8581–8583
8583
Fig. 3. (a) SEM image and (b) SAED pattern of the ZrB2 powder synthesized at 1550 ◦C, the inset in (b) is a TEM image of the ZrB2 particle used to obtain the SAED pattern.
tion of physically absorbed water. The exothermic peaks centered
at 318 and 400 ◦C are attributed to the decomposition of zirco-
nium hydroxide and the formation of amorphous ZrO2 [9]. Another
exothermic peak centered at 440 ◦C was also present. We attribute
this to a phase transformation of amorphous ZrO2 [10]. An in-depth
discussion is to follow.
ature and 1490 ◦C, respectively. Therefore, Reactions (2) and (3)
might occur initially and this is accompanied by the production of
B2O3/B2O2, and then Reaction (6) is induced with a further increase
in temperature. Therefore, an understanding of the many factors
that affect the reaction process is required to understand why the
synthesis of a single ZrB2 phase might occur at 1550 ◦C over 2 h. The
three kinds of metastable reactants affect the temperature required
for the solid-state reactions and also strongly affect the kinetics of
the transformation.
Fig. 3a shows a SEM image of the ZrB2 powder synthesized using
the as-synthesized hydrous nano-ZrO2, boron and 5 wt.% nano-
carbon at 1550 ◦C over 2 h. It reveals a plate-like morphology for the
ZrB2 particles with a uniform size distribution and average particle
size of ca. 1.0 ␮m in two-dimensions. Additionally, a TEM image and
SAED pattern are shown in Fig. 3b. The ZrB2 particles are, therefore,
single crystals and well crystallized.
Further XRD analysis was conducted to understand the phase
composition of the precipitated precursor after calcination. We
found that the precipitated precursor that was dried at 60 ◦C was
amorphous and that this state was retained at up to 400 ◦C, as
shown in Fig. 1b. It then transformed to t-ZrO2 at 600 ◦C. Since we
found an exothermic peak at 440 ◦C using TG–DTA, t-ZrO2 likely
starts to form at about 440 ◦C. Additionally, the diffraction peak
centered at 28.1◦ is assigned to m-ZrO2, which was obtained after
calcination at 650 ◦C. With an increase in temperature, the diffrac-
tion intensity of m-ZrO2 became stronger whereas that of t-ZrO2
decreased. These results indicate that the precipitated precursor
initially transforms to t-ZrO2 and then to m-ZrO2. In other words,
TG-DTA and XRD show that as-synthesized hydrous ZrO2 is amor-
phous and it undergoes several changes in lattice structure during
calcination. These continuous nascent states would benefit the
synthesis of ZrB2. In addition, the specific surface area of the pre-
cipitated precursor was found to be 322 m2/g and, therefore, the
as-synthesized hydrous ZrO2 is fine.
4. Conclusions
A single phase of ZrB2 powder was successfully synthesized with
B/Zr (mol.) = 4, and 5 wt.% carbon by BCTR using as-synthesized
amorphous hydrous nano-ZrO2, amorphous boron and nanocarbon
at 1550 ◦C over 2 h. The morphology was plate-like and the average
size of the ZrB2 particles was ca. 1.0 ␮m in two-dimensions.
To clarify the above-mentioned supposition the effects of
nanocarbon on the processes of the two different reaction systems
were investigated by excluding and including nanocarbon in the
binary system of the as-synthesized hydrous nano-ZrO2 and amor-
phous boron (B/Zr (mol.) = 4). XRD patterns of the final products
synthesized at 1100, 1450 and 1550 ◦C with 5 wt.% nanocarbon are
shown in Fig. 2a. The product obtained after calcination at 1100 ◦C
showed strong diffraction peaks of ZrB2 and the peaks centered at
28.1◦ and 31.4◦ are assigned to m-ZrO2. With an increase in tem-
perature, the diffraction intensity of m-ZrO2 became weaker (see
Fig. 2a, 1450 ◦C) and finally a single phase of ZrB2 was present at
1550 ◦C.
Acknowledgments
The authors appreciate the financial support from the National
Science Foundation of China (NSFC50974007); the Scientific
Research Starting Foundation for Returned Overseas Chinese
Scholars, Ministry of Education; the Start-Up Fund for High-End
Returned Overseas Talents, Ministry of Human Resources and
Social Security, China (Renshetinghan 2010, No. 411) and the Lab-
Installation Foundation of Beihang University for New Teachers.
References
Fig. 2b shows XRD patterns of the products synthesized without
nanocarbon. ZrB2 and m-ZrO2 were observed at calcination temper-
atures up to 1550 ◦C. Therefore, we conclude that the results of the
synthesis at 1100 ◦C were independent of the presence or absence
of nanocarbon. However, an increase in the synthesis temperature
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of ZrO2 was present in the absence of nanocarbon. Therefore, the
effects of nanocarbon on the reduction of ZrO2 are evident. As noted
above, Reactions (2) and (6) can occur at higher than room temper-
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