Mechanochemical processing of nanocrystalline zirconium diboride powder

Article abstract of DOI:10.1111/j.1551-2916.2011.04825.x

Nanocrystalline ZrB₂ powder was prepared by mechanochemical processing of a zirconium (II) dihydride-boron mixture with subsequent annealing from 800°C to 1200°C. The crystallite size and morphology of the synthesized ZrB₂ powders were characterized by X-ray diffractometry, scanning electron microscopy, and transmission electron microscopy. The effects of annealing temperature on powder particle size and morphology were assessed. At 900°C or above, pure ZrB₂ powder was obtained without trace quantities of residual ZrH₂. The synthesized ZrB₂ powder particles were spherically shaped, with a crystallite size between 5 and 40 nm. The crystallite size increased with increase of annealing temperature.

Full text of DOI:10.1111/j.1551-2916.2011.04825.x

J. Am. Ceram. Soc., 94 [11] 3643–3647 (2011)
DOI: 10.1111/j.1551-2916.2011.04825.x
©
2011 The American Ceramic Society
ournal
J
Mechanochemical Processing of Nanocrystalline Zirconium Diboride Powder
‡
,†
§
‡,¶
Shuqi Guo, Chunfeng Hu, and Yutaka Kagawa
‡
Hybrid Materials Unit, National Institute for Materials Science, 1-2-1 Sengen, Tsukuba, Ibaraki, 305-0047, Japan
§
Ningbo Institute of Material Technology and Engineering, Chinese Academy of Sciences, 519 Zhuangshi Road,
Zhenhai District, Ningbo, Zhejiang, 315201, China
¶
Research Center for Advanced Science and Technology, The University of Tokyo, 4-6-1 Komaba,
Meguro-ku, Tokyo, 153-8505, Japan
Nanocrystalline ZrB
cal processing of a zirconium (II) dihydride–boron mixture with
subsequent annealing from 800°C to 1200°C. The crystallite
2
powder was prepared by mechanochemi-
sources of zirconia, boron oxide, and carbon, respectively.
However, a small amount of oxygen and carbon was pre-
sented in the ZrB lattice. Nanocrystalline ZrB powder can
2
2
size and morphology of the synthesized ZrB
2
powders were
also be prepared by the solid-state reaction of Zr and
B. Camurlu and Maglia prepared nanocrystalline ZrB
2
1
4
characterized by X-ray diffractometry, scanning electron
microscopy, and transmission electron microscopy. The effects
of annealing temperature on powder particle size and morphol-
ogy were assessed. At 900°C or above, pure ZrB
obtained without trace quantities of residual ZrH
sized ZrB2 powder particles were spherically shaped, with a
crystallite size between 5 and 40 nm. The crystallite size
increased with increase of annealing temperature.
powder with starting powders of Zr, B, and NaCl using
self-propagating high-temperature synthesis (SHS). In addi-
1
5
2
powder was
. The synthe-
tion, Chamberlain et al. employed a slow heating rate
(~1°C/min) and extended isothermal holds (6 h at 600°C) to
obtain a nano-grained ZrB2 ceramic (grain size: 10 nm) by
reacting an attrition-milled mixture of Zr and B powders.
Mechanical activation, also referred to as mechanochemi-
cal processing, is another attractive method for synthesizing
2
1
6,17
materials.
powder was prepared by mechanochemical processing of a
In the present study, nanocrystalline ZrB2
I. Introduction
IRCONIUM diboride (ZrB
members of a family of ultra-high temperature ceramic
2
) is one of the most important
zirconium (II) dihydride–boron mixture and subsequent
annealing. The microstructure of the resulting ZrB powder
2
Z
materials. It has an extremely high melting point (>3000°C),
high thermal and electrical conductivities, chemical inertness
against many molten metals, excellent thermal shock resis-
was characterized by field emission scanning electron micros-
copy and transmission electron microscopy. Phases were
identified by X-ray diffraction. Also, the effects of annealing
temperature on the powder particle size and morphology
were examined.
1
,2
tance, and relatively low density. As a result, ZrB ceram-
2
ics are being considered for a variety of high-temperature
(
including furnace elements, plasma-arc electrodes, rocket
>1800°C) thermomechanical and structural applications,
II. Experimental Procedure
engines, and thermal protection structures for leading-edge
1
–5
parts on hypersonic re-entry space vehicles.
meet the strict constraints of structural applications, ZrB
However, to
The starting powders used in this study were: zirconium (II)
dihydride (ZrH ) powder (325 mesh, 99% pure, Sigma-
2
2
materials will require improved strength, fracture toughness,
and resistance to oxidation. Studies of nano-grain size cera-
mic materials previously demonstrated that nano-powders
had an excellent sinterability and nano-grained materials had
improved strength, fracture toughness, resistance to oxida-
Aldrich, Inc., Louis, MO), and amorphous boron (B) powder
(d50 = 0.8 lm, 95.9% pure, H.C. Starck, Berlin, Germany).
Figure 1 shows SEM images of as-received ZrH
phous B powders. The ZrH powder had large, irregular
2
and amor-
2
grains, whereas the B was composed of smaller, spherical
particles. The starting powders were weighed in stoichiome-
tric proportions according to the following reaction:
6–11
tion, and creep resistance.
study was the development of nanocrystalline ZrB
and nano-grained ZrB ceramic materials.
Hence, the motivation for this
2
powder
2
Currently, nanocrystalline ZrB powder has been produced
2
ZrH2 þ 2B ! ZrB2 þ H2
ð1Þ
using two main synthesis routes: chemical routes and reactive
processes. Chen et al.
1
2
synthesized nanocrystalline ZrB
2
powder (crystallite size: 10–20 nm) by reacting anhydrous
chlorides with sodium borohydride at 700°C under pressure.
The ZrH and B powders were mixed in ethanol using a sili-
2
con carbide media for 6 h. Subsequently, the as-received
ZrH –B mixture was milled in a planetary ball-mill (Model
P5; Fritsch Gmbh, Idar-Oberstein, Germany). Planetary mill-
13
Yan et al. prepared nanocrystalline ZrB powder, averaging
crystallite size 47 nm at 1500°C by a sol–gel method, using
zirconium oxychloride, boric acid, and phenolic resin as
2
2
ing was performed using hardened steel balls with a diameter
of 9.5 mm and hardened steel vials with a 65 mm inner
diameter and 45 mm inner height. A charge ratio (ball to
powder weight ratio) of 20:1 was used. Before milling, each
vial was filled with pure (99.999%) argon in a glove box.
The milling speed was 300 rpm, and the milling times were 5
and 10 h. After milling, the powder was moved to an alu-
mina crucible under an argon atmosphere in a glove box, to
avoid ignition of a self-propagating reaction. The milled
D. Butt—contributing editor
Manuscript No. 29487. Received March 22, 2011; approved July 27, 2011.
Author to whom correspondence should be addressed. e-mail: GUO.Shuqi@nims.go.jp
†
3643

3
644
a)
Rapid Communications of the American Ceramic Society
Vol. 94, No. 11
(
Fig. 2. TG/DTA curve of the as-received stoichiometric ZrH
mixture.
2
–B
(
b)
2
Fig. 1. Typical FE-SEM images of the as-received (a) ZrH and (b)
B powders.
powders were then annealed at different temperatures
between 800°C and 1200°C with a heating rate of 10°C/min
for 1 h in a flowing high-purity argon atmosphere. Field
emission scanning electron microscopy (FE-SEM) was per-
formed to characterize the particle size and morphology as a
function of temperature. The microstructures of powders
were investigated by transmission electron microscopy
Fig. 3. XRD patterns of the powders prepared by ball milling for
a, b) 5 h and (c–g) 10 h and annealing from 800°C to 1200°C.
(
peak at 785°C for ZrH
cluded that the process of thermal dissociation of ZrH
occurred at approximately 600°C–1000°C. Recently, Li
2
in argon atmosphere. They con-
2
(
2
TEM, JEOL JEM-2010F, Tokyo, Japan) operated at
00 kV. X-ray diffraction (XRD) was used to determine the
phases present in the prepared powders. The average crystal
2
0
et al. showed that there is a weak endothermic peak at
540°C and a strong endothermic peak at 730°C in argon as
1
8
size, d, was calculated using Scherrer’s formula :
a result of decomposition of ZrH
peaks observed at 126°C and 418°C in the DTA trace are
not the result of ZrH decomposition, but are presumed to
be associated with the evaporation of the bound water and
the melting of B impurities. The TG analysis showed that
2
. Thus, the endothermic
0
:9k
d ¼
;
ð2Þ
b cos h
2
where k, b, and h are the wavelength of CuKa radiation
2 3
O
˚
(
k = 1.54056 A), the full width at half maximum (FWHM,
unit: rad), and the Bragg angle (unit: rad), respectively.
a weight loss began at 500°C and the loss continuously
increased up to 800°C, indicating that the decomposition of
Differential thermal analysis (DTA) and thermogravimetry
TG) of the as-received ZrH –B mixture were performed
TGA/DTA, STA409 CD; Netzsch, Wolverhampton,
Germany) up to 1300°C using a heating rate of 10°C/min in
a flowing high-purity argon atmosphere.
2
ZrH occurred between 500°C and 800°C. The decomposi-
(
(
tion process was completed at 1000°C as indicated by several
distinguishable weak endothermic peaks in the DTA trace
up to 1000°C. In addition, the DTA trace exhibited several
exothermic peaks in the temperature range 500°C–1100°C,
2
indicating that the reaction of ZrH
occurred in multiple stages.
To synthesize a high-purity ZrB
2
with B to form ZrB
2
III. Results and Discussion
2
powder, the post-milled
(1) ZrB Synthesis
Figure 2 shows the TG/DTA curve obtained on the
precursor powders were annealed in a flowing high-purity
argon atmosphere at different temperatures between 800°C
and 1200°C. The XRD patterns for the annealed powders
2
as-received stoichiometric ZrH –B mixture. The DTA trace
2
reveals the presence of three significant endothermic peaks
at 126°C, 418°C, and 696°C, with the maximum peak at
are presented in Fig. 3. The ZrB
talline phase and a trace amount of ZrO
annealed powders. The trace quantities of ZrO
attributed to oxygen uptake during the milling and/or
handling procedures. A trace quantity of ZrH was detected
2
phase is the primary crys-
is present in all the
may be
2
6
Shemet et al. showed the presence of two weak endother-
96°C. A few small endothermic peaks are also observed.
2
19
mic peaks at 570°C and 830°C and a strong endothermic
2

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Rapid Communications of the American Ceramic Society
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only for the sample annealed at 800°C. At 900°C or above,
the peaks corresponding to the ZrH phase are non-existent,
time promoted coarsening of grains during the subsequent
annealing at higher temperature. Furthermore, the lattice
2
as a result of its decomposition. However, neither excess Zr
nor excess B was detected by XRD. Note that the absence of
peaks for B phase is a result of amorphous boron powder used.
The absence of peaks for Zr phase suggests that Zr was not
parameters of the ZrB
annealed at 1100°C or above are nearly identical to those
2
samples milled for 10 h and
˚
of the pure hexagonal ZrB
˚
2
phase (a = 3.168 A,
c = 3.530 A).
an intermediate phase during the reaction of ZrH with B to
2
form ZrB investigated in this study.
2
The samples annealed at 900°C or below exhibited a
(2) Morphology of ZrB Powder
2
broad, weak diffraction peak corresponding to ZrB
Fig. 3). Increasing the annealing temperature resulted in
the appearance of narrow and strong diffraction peaks of
ZrB phase, which indicates an increase in the crystallite
2
phase
Morphologies of the ZrB
chemical processing and annealing, were observed under
FE-SEM, as shown in Fig. 4. It was found that the ZrB
powders had a spherical shape and a broad size distribution,
with particle sizes in the range of 50–300 nm. Comparing
these micrographs with the results determined by XRD
2
powder, prepared by mechano-
(
2
2
size and concentration with increased temperature. The
average crystallite size of ZrB was calculated by Scherrer’s
2
formula [Eq. (2)] using (101) and (100) reflections. The lat-
powders pre-
2
(Table I) revealed that the size distributions of ZrB particles
tice parameters and crystallite size of ZrB
pared by high-energy milling and subsequent annealing are
summarized in Table I. It was found that the average crys-
2
determined by XRD and FE-SEM were marked differently.
This suggests that the particles of ZrB2 powder were not
single crystals but polycrystals with nano-sized grains with a
tendency to agglomerate. Interestingly, the particles size of
the sample milled for 10 h was similar to and/or slightly lar-
ger than that of the sample milled for 5 h. Similar observa-
tallite size of the synthesized ZrB
increase of annealing temperature. In addition, annealing at
00°C resulted in very similar crystallite size for the samples
2
powder increased with
9
milled for 5 and 10 h. After annealing at 1100°C, however,
the crystallite size was larger for the 10 h-milled sample
than for the 5 h-milled sample. Thus, an increase in milling
tions were previously reported for ZrB
prepared by a mechanochemical treatment of the borother-
mic reduction of titania and zirconia.
2 2
and/or TiB powders
2
1
2 2
Table I. Lattice Parameters and Crystallite Size of the ZrB Powders Prepared by Mechanochemical Processing of ZrH –B
Mixture and Subsequent Annealing
˚
Lattice parameters (A)
Processing conditions
Powders
Mixture time (h)
(°C/60 min/Ar)
Heating rate (°C/min)
a
c
Crystallite size (nm)
ZHB51
ZHB52
ZHB11
ZHB12
ZHB13
ZHB14
ZHB15
5
5
900
10
10
10
10
10
10
10
3.171
3.169
3.179
3.173
3.175
3.167
3.166
3.524
3.523
3.523
3.568
3.517
3.527
3.530
14.32
18.63
12.49
14.42
16.93
20.42
24.87
1100
800
900
1000
1100
1200
10
10
10
10
10
(
a)
(b)
(c)
(
d)
(e)
(f)
Fig. 4. Typical FE-SEM images of the powders prepared by ball milling for (a, b) 5 h and (c–f) 10 h and annealing from 900°C to 1200°C.

3
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Rapid Communications of the American Ceramic Society
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(
a)
(b)
(
c)
(d)
Fig. 5. Typical TEM images of the powders prepared by ball milling for 10 h and annealing from 900°C to 1200°C.
(
a)
(b)
(
c)
(d)
Fig. 6. High-magnification TEM images of the powders prepared by ball milling for 10 h and annealing from 900°C to 1200°C.
Figure 5 shows TEM images of the ZrB
by ball milling for 10 h and annealing between 900°C and
200°C. Here, TEM showed that the particles of ZrB were
spherically shaped and were the submicrometer agglomerates
consisting of nano-sized grains. Under high magnification
2
powders prepared
quent annealing from 800°C to 1200°C. At 800°C, the ZrH
2
–
B mixture was converted into ZrB . However, a trace quan-
2
1
2
tity of unreacted ZrH
2
was detected. At 900°C or above, the
without
the presence of any residual ZrH . The particles of the result-
ZrH –B mixture was completely converted into ZrB
2
2
2
(
Fig. 6), it is clearly revealed that the sizes of the nano-sized
ing ZrB powder consisted of agglomerates of submicrometer
2
ZrB crystallites were in the range 5–20 nm at 900°C, 10–
2
particles with an annealing temperature-dependent crystallite
size between 5 and 40 nm. An increase in annealing tempera-
ture led to an increase of crystallite size.
2
1
5 nm at 1000°C, 10–30 nm at 1100°C, and 20–40 nm at
200°C, showing a narrow size distribution. An increase in
the annealing temperature increased the crystallite size. The
average crystallite sizes estimated from TEM micrographs are
consistent with those determined by XRD.
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