New borothermal reduction route to synthesize submicrometric ZrB 2 powders with low oxygen content

Article abstract of DOI:10.1111/j.1551-2916.2011.04869.x

The ZrB₂ powders with submicrometric particle size and low oxygen content were synthesized by a new borothermal reduction route using ZrO₂ and excess boron as raw materials. The conventional process only contained the borothermal red

Full text of DOI:10.1111/j.1551-2916.2011.04869.x

J. Am. Ceram. Soc., 94 [11] 3702–3705 (2011)
DOI: 10.1111/j.1551-2916.2011.04869.x
© 2011 The American Ceramic Society
ournal
J
New Borothermal Reduction Route to Synthesize Submicrometric
ZrB2 Powders with Low Oxygen Content
Wei-Ming Guo and Guo-Jun Zhang*,†
State Key Laboratory of High Performance Ceramics and Superfine Microstructures,
Shanghai Institute of Ceramics, Shanghai, 200050, China
ZrO2ðsÞ þ 4BðsÞ ! ZrB2ðsÞ þ B2O2ðgÞ
ð4Þ
ð5Þ
The ZrB2 powders with submicrometric particle size and low
oxygen content were synthesized by a new borothermal reduc-
tion route using ZrO2 and excess boron as raw materials. The
conventional process only contained the borothermal reduction
of ZrO2 with boron at 1550°C. By exploring the mechanism of
ZrB2 particle coarsening during conventional process, a new
borothermal reduction route was proposed. This route included
the borothermal reduction of ZrO2 with boron at 1000°C,
washing by hot water, and the removal of residual boron oxides
at 1550°C, namely, two-step reduction plus intermediate water-
washing (RWR). Using conventional process, the particle size
and oxygen content of ZrB2 powder were about 2–3 lm and
0.68 wt%, respectively. Based on the new route, the particle
size and oxygen content of ZrB2 powder were about 0.4–
0.7 lm and 0.40 wt%, respectively.
2ZrO2ðsÞ þ B4CðsÞ þ 3CðsÞ ! 2ZrB2ðsÞ þ 4COðgÞ
The particle size is the first important factor assessing
the characteristics of ZrB2 ceramics. Ultra-fine powders can
increase the driving force for sintering and can improve the
densification behavior. To reduce the particle size of com-
mercial ZrB2 powder, ball milling and attrition milling are
often carried out before sintering. These processes could
effectively reduce the particle size. At the same time, how-
ever, they also introduce impurities,10–12 especially in terms
of oxygen content,10 which has an adverse effect for densifi-
cation. The particle size of synthesized ZrB2 powders is
influenced by the particle size of raw materials, holding
time, and synthesis temperature. Zhao et al. prepared ZrB2
powder by boro/carbothermal reduction of ZrO2 with B4C
and carbon in an inert atmosphere, and indicated that the
particle size of ZrB2 increased with increasing synthesis
temperature.8 Ran et al. synthesized the ZrB2 powders by
borothermal reduction of ZrO2 with boron, and found that
the average particle size increased from 0.15 to 0.66 lm
with the increasing synthesis temperature from 1000°C to
1650°C.7
Oxygen content is another important factor to evaluate
the quality of ZrB2 powders. It is well known that the oxy-
gen impurities, in the form of B2O3 and ZrO2, are present
mainly on the particle surfaces of ZrB2 powders. To remove
the oxygen impurities and to promote the densification of
ZrB2, many additives were used, such as carbon,11 boron,13
B4C,10 transition metal carbides,14–16 and silicides.17 Hence,
the direct synthesis of ZrB2 powders with small particle size
and low oxygen content is very important.
I. Introduction
UE to its high melting temperature, high strength, high
thermal and electrical conductivity, ZrB2 is especially
D
promising for high-temperature structural applications, such
as cutting tools, thermal protection structures for hyper-
sonic vehicles, and molten metal crucibles.1,2 Currently, the
ZrB2 powder can be mainly obtained by the following
synthesis routes: direct reaction of elemental precursor pow-
ders [reaction (1)],1 self-propagating high-temperature
synthesis [reaction (2)],3 polymer-precursor route,4 carbother-
mal reduction method [reaction (3)],5 borothermal reduction
method [reaction (4)],6,7 and boro/carbothermal reduc-
tion method [reaction (5)].8,9 To limit the introduction of car-
bon or metal impurities, the borothermal reduction method
is the most interesting way.
In this work, the key problem in the conventional boro-
thermal synthesis of ZrB2 powders was first analyzed, and
then a novel process was proposed to produce ZrB2 powders
with small particle size and low oxygen content. The new
process includes three steps: (1) borothermal reduction at
around 1000°C; (2) water-washing; and then (3) a second
reduction stage at 1550°C to remove residual oxygen.
ZrðsÞ þ 2BðsÞ ! ZrB2ðsÞ
ð1Þ
ZrO2ðsÞ þ 2H3BO3ðsÞ þ 5MgðsÞ !
ZrB2ðsÞ þ 5MgOðsÞ þ 3H2OðlÞ
ð2Þ
ð3Þ
ZrO2ðsÞ þ B2O3ðsÞ þ 5CðsÞ ! ZrB2ðsÞ þ 5COðgÞ
II. Experimental Procedure
The raw materials used in this study were ZrO2
(D50 = 0.6 lm, CSG Holding Co., Ltd, Shenzhen, China)
and amorphous boron (average particle size <1 lm, Dan-
dong Chemical Engineering Institute Co., Ltd, Dandong,
China) with purities of 99.8% and 96%, respectively. The
starting mixtures of ZrO2 and excess boron were mixed for
24 h in a polythene bottle using ethanol and Si3N4 balls,
and dried by rotary evaporation. The dried powder mix-
tures were dry-pressed into disks (about 10-mm diameter
and 10-mm thickness). Then, the disks were placed in a
L. Silvestroni—contributing editor
Manuscript No. 29947. Received July 05, 2011; approved August 29, 2011.
*Member, The American Ceramic Society.
This work was financially supported by the Chinese Academy of Sciences under the
Program for Recruiting Outstanding Overseas Chinese (Hundred Talents Program), the
National Natural Science Foundation of China (No. 50632070), and the Science and
Technology Commission of Shanghai (No. 09ZR1435500).
†Author to whom correspondence should be addressed. e-mail: gjzhang@mail.sic.
ac.cn
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Rapid Communications of the American Ceramic Society
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When the temperature is above 1200°C, ZrO2 reacted with
boron to form gaseous B2O2 phase according to reaction (4).
This should be the reason why B2O3 phase existed in the
ZB2 sample, and disappeared in the ZB3 sample.
Table I. Synthesis Process and Oxygen Content of the
ZB1, ZB2, ZB3, ZB4, ZB5, and ZB6 Samples
Oxygen
Sample
Process
content (wt%)
The oxygen contents of ZB3 and ZB4 powders are shown
in Table I. It can be seen that the oxygen content of ZB3
and ZB4 was 0.68 and 2.68 wt%, respectively. In spite of the
water-washing process, the oxygen content of ZB4 was far
higher than that of ZB3 sample. This indicated that high-
temperature treatment was more effective to decrease the
oxygen content compared with the water-washing process.
The SEM images of ZB3 and ZB4 powders are shown in
Fig. 2, and the TEM image of ZB4 is shown in Fig. 3. Com-
pared with ZB3, the particle size of the ZB4 product was
obviously finer. Based on the SEM observation, the particle
size of ZB3 and ZB4 samples was about 2–3 lm and about
0.3–0.5 lm, respectively.
The above result indicated that ultra-fine ZrB2 powders (0.3
–0.5 lm) could be obtained at low temperature (~1000°C).
Although most of the oxygen can be removed by the water-
washing process, its content (2.68 wt%) was still too high for
the preparation of ZrB2-based ceramics. Increasing synthesis
temperature could decrease the oxygen content. For example,
ZrB2 powder with low oxygen content of 0.68 wt% was
obtained at high temperature of ~1550°C. However, this
powder had a coarser particle size of 2–3 lm. Accordingly,
the key issue in the borothermal synthesis is how to solve the
above conflicting problems and produce a ZrB2 powder with
small particle size as well as low oxygen content.
ZB1
ZB2
ZB3
ZB4
ZB5
ZB6
900°C/2 h
1000°C/2 h
1550°C/1 h
1000°C/2 h and water-washing
1000°C/2 h and 1550°C/1 h
1000°C/2 h, water-washing and
1550°C/1 h
–
–
0.68
2.68
0.69
0.40
graphite crucible, and heated at a rate of 10°C/min to the
final temperature in a graphite element furnace (ZT-60-22Y,
Shanghai Chen Hua Electric Furnace Co. Ltd., China).
The detailed reduction processes of different sample are
shown in Table I. The thermally treated disks were
immersed into hot water to remove the boron oxides. Then,
the powders were washed with de-ionized water and etha-
nol before drying.
Phase composition was determined by X-ray diffraction
(XRD, D/max 2550 V, Tokyo, Japan). The morphology of
the synthesized powder was characterized by scanning elec-
tron microscopy (SEM, Hitachi S-570, Tokyo, Japan) and a
high-resolution transmission electron microscope (HR-TEM,
JEM-2010, JEOL Ltd, Tokyo, Japan). Oxygen content was
determined by a nitrogen/oxygen determinator (TC600, Leco
Corporation, St. Joseph, MI).
(2) Synthesis of Submicrometric ZrB2 Powders with Low
Oxygen Content
III. Results and Discussion
As mentioned above, the particle size of synthesized ZrB2
powders is influenced by both particle size of raw materials
and reduction processes. Coarse raw materials, long holding
time, or high synthesis temperature would lead to the coars-
ening of ZrB2 powders. Based on the XRD results, boro-
thermal reduction could proceed to completion at 1000°C.
The high synthesis temperature (~1550°C) has also induced
the coarsening of the raw materials before the ZrO2 reacts
with the boron to form the ZrB2 phase, which was very
similar to the synthesis process of HfC ultra-fine powder.18
To prevent the coarsening of the starting ZrO2 particles, in
this study. the borothermal reduction was first held at low
temperature to synthesize ZrB2 phase at where the coarsen-
ing of ZrO2 and boron was very restricted, and then was
held at high temperature to remove the oxygen impurity.
By this two-step process, ZB5 sample was prepared. As
shown in Table I and Fig. 2, the oxygen content and parti-
cle size of ZB5 were 0.69 wt% and 2–3 lm, respectively.
The oxygen content of ZB5 sample was lower than that of
ZB4 sample, whereas the particle size of ZB5 sample was
bigger than that of ZB4 sample, and close to that of ZB3
sample.
(1) Key Problem in the Borothermal Synthesis of ZrB2
Powders
Figure 1 shows the XRD patterns of the ZB1, ZB2, ZB3,
and ZB6 powders. In addition to ZrB2 phase, ZrO2 phase
was detected in the ZB1 sample, whereas ZrO2 phase disap-
peared in the ZB2 and ZB3 samples. This showed that the
borothermal reduction of ZrO2 with boron could be com-
pleted at 1000°C or above. The B2O3 phase was observed in
the ZB2 sample, but no B2O3 phase was detected in the ZB3
sample. Previous thermodynamic calculations showed that
ZrO2 reacted with boron to form liquid B2O3 phase below
1200°C based on the following reaction:7
3ZrO2ðsÞ þ 10BðsÞ ! 3ZrB2ðsÞ þ 2B2O3ðlÞ
ð6Þ
Previous studies on densification of boron-containing
ceramics, such as B4C, TiB2, and ZrB2, have indicated that
the existence of B2O3 is a very important factor in promoting
coarsening through evaporation–condensation kinetics.19–21
To reduce the adverse effect of the B2O3 phase for the
ZrB2 particle coarsening,
a water-washing process was
added before the second-step reduction was carried out. By
two-step reduction plus an intermediate water-washing pro-
cess (RWR, reduction-wash-reduction), the ZB6 powder
was obtained. The XRD pattern of the ZB6 sample showed
that the RWR route could obtain pure phase ZrB2 powders
(Fig. 1). As shown in Table I, Figs. 2 and 3, the oxygen
content and particle size of ZB6 were about 0.43 wt% and
about 0.4–0.7 lm, respectively. The particle size of ZB6
sample was considerably smaller than that of ZB3 sample,
whereas the oxygen content was comparable. This result
demonstrated that the two-step reduction plus intermediate
Fig. 1. XRD patterns of the ZB1, ZB2, ZB3, and ZB6 samples.

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Fig. 2. SEM images of the ZB3 (a), ZB4 (b), ZB5 (c), and ZB6 (d) samples.
Fig. 3. TEM images of the ZB4 (a) and ZB6 (b) samples.
water-washing process was very effective to prepare the
ZrB2 powder with both small particle size and low oxygen
content. At the same time, this result also further con-
firmed that the coarsening of ZrB2 particles mainly depend
on the presence of boron oxides. In addition, the TEM
images reveal that the synthesized ZrB2 powders have dif-
ferent morphologies. The ZB4 sample has faceted morphol-
ogy, whereas ZB6 sample displays spherical morphology
(Fig. 3).
boron to form gaseous B2O2 based on the following reac-
tion:
2B2O3ðlÞ þ 2BðsÞ ! 3B2O2ðgÞ
ð7Þ
In our experiment, the furnace pressure remained below
20 Pa during the whole reduction process. Here, assuming a
B2O2 partial pressure of 20 Pa, thermodynamic calculation
shows that reaction (7) is thermodynamically favorable above
1250°C. Hence, the residual B2O3 could be eliminated by the
formation of gaseous B2O2. One mole of ZrO2 needs only
Next, the removal mechanism of the residual B2O3 phase
in the second-step reduction after water-washing will be
explored. At high temperature, B2O3 could react with

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ð8Þ
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new borothermal reduction route was developed, namely,
two-step reduction plus intermediate water-washing (RWR).
The ZrB2–B2O3 powder mixtures were first prepared by
borothermal reduction of ZrO2 with excess boron at 1000°C.
Then, most of the B2O3 phase was removed from the ZrB2–
B2O3 powder mixtures by water-washing process. Finally, the
residual B2O3 was further eliminated by its evaporation and
the formation of gaseous B2O2 at 1550°C. Using conven-
tional process, the particle size and oxygen content of ZrB2
powder were about 2–3 lm and 0.68 wt%, respectively.
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0.40 wt%, respectively.
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