Synthesis and microstructure of zirconium diboride formed from polymeric precursor pyrolysis

Article abstract of DOI:10.1111/j.1551-2916.2011.05060.x

Zirconium diboride was synthesized by pyrolysis of a novel polymeric precursor. Phase compositions and microstructures of the formed ceramic were characterized. It was found that a precursor with B/Zr molar ratio of 2 can completely transform into zirconium diboride by heating in an inert atmosphere with temperatures above 1500°C. However, the initial formation temperature of zirconium diboride was as low as 1300°C whereas zirconium oxide was also produced from the precursor at 1100°C, the mixture was finally transformed into pure zirconium diboride at the elevated temperature. Zirconium diboride particles, dispersed relatively uniformly with characteristic dimension less than 100 nm, were found to be formed following a liquid phase reaction mechanism.

Full text of DOI:10.1111/j.1551-2916.2011.05060.x

J. Am. Ceram. Soc., 95 [3] 866–869 (2012)
DOI: 10.1111/j.1551-2916.2011.05060.x
©
2012 The American Ceramic Society
ournal
J
Synthesis and Microstructure of Zirconium Diboride Formed from
Polymeric Precursor Pyrolysis
‡
,§
‡,§
‡
‡
‡,†
Changming Xie, Mingwei Chen, Xi Wei, Min Ge, and Weigang Zhang
‡
State Key Laboratory of Multiphase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences,
Beijing 100190, China
§
Graduate University of Chinese Academy of Sciences, Beijing 100049, China
Zirconium diboride was synthesized by pyrolysis of a novel
polymeric precursor. Phase compositions and microstructures
of the formed ceramic were characterized. It was found that a
precursor with B/Zr molar ratio of 2 can completely transform
into zirconium diboride by heating in an inert atmosphere with
temperatures above 1500°C. However, the initial formation
temperature of zirconium diboride was as low as 1300°C
whereas zirconium oxide was also produced from the precursor
at 1100°C, the mixture was finally transformed into pure zirco-
nium diboride at the elevated temperature. Zirconium diboride
particles, dispersed relatively uniformly with characteristic
dimension less than 100 nm, were found to be formed following
a liquid phase reaction mechanism.
cess by means of forming boron oxide and zirconium dioxide
simultaneously to make up for the defects of materials and
inhibit further oxidation damage. It has been shown that,
C/SiC composites can withstand exposure to an oxidizing
environment between 1100°C and 1650°C for aerospace
applications since SiC matrix has an inherent limit due to the
7
transition from the passive to active oxidation that occurs
8
below 1700°C. Although pure ZrB actively oxidizes above
2
2 3 2
1200°C due to intensive volatilization of B O , adding ZrB
into C/SiC composites provides more efficient oxidation resis-
tance due to the formation of borosilicate glass on exposed
surfaces above 900°C, formation of silica-enriched glass
between 1100°C and 1700°C, and formation of zirconia-
9
–11
containing scales for temperatures higher than 1800°C.
The development of ZrB2 with improved properties using
I. Introduction
low-cost processes is essential to increase the performance
and to reduce the cost of hypersonic aerospace vehicles. ZrB
LTRA-HIGH temperature ceramics (UHTCs) such as
borides, carbides, and nitrides of the early transition
metals are attractive candidates for extreme environments
which require oxidation resistance at temperature above
2
1
2,13
U
can be prepared by several techniques. Chen et al.
pre-
pared nanocrystalline TiB and ZrB by the benzene-thermal
2 2
1
4
reaction at 400°C and 700°C, respectively. Corral et al.
improved the ablation resistance of carbon-carbon composites
2
000°C, due to their unusually high melting points, high
1–3
hardness, and good oxidation resistance.
ride (ZrB ), a member of UHTCs, exhibits a unique combi-
Zirconium dibo-
at high temperature using B C and ZrB particles synthesized
4
2
by the reaction of ZrCl , NaBH , and 1,2-dimethoxyethane
4 4
2
1
5
nation of chemical stability, high thermal and electrical
conductivities which makes it suitable for the extreme ther-
mal and chemical environments associated with hypersonic
and heat treatment. Jensen et al. obtained TiB
HfB thin films by chemical vapor deposition (CVD) of the
binary tetrahydroborates Ti(BH ) , Zr(BH ) , and Hf(BH ) ,
2 2
, ZrB , and
2
4
4
4 4
4 4
4
–6
16
flight, rocket propulsion, and atmospheric re-entry. Interest
in UHTCs, especially introducing UHTCs into continuous
fiber-reinforced ceramic matrix composites (CMCs), which
generally acquire polymeric ceramic precursors for a densifi-
cation process has increased significantly in the past few
respectively. Berthon et al. used a mixture of ZrCl
and H to synthesize ZrB by CVD technique. Yan et al.
synthesized ZrB powders using hybrid precursors of zirco-
4 3
, BCl ,
1
7
2
2
2
nium oxychloride, boric acid, and phenolic resin as sources
of zirconia, boron oxide, and carbon, respectively. However,
fewer studies were conducted on the synthesis of metal
borides by polymeric solution processing.
2
,4
years.
It is well known that there are two kinds of ablation
mechanisms including mechanical denudation and chemical
erosion (oxidation) which determine ablation performances
of CMCs. During the ablation process, mechanical denuda-
tion refers to the denudations of the surfaces of materials
under high pressure and shearing forces of ablative gases.
And chemical erosion mainly means the oxidation and gasifi-
cation of carbon and forming oxide with low melting points
Using polymeric precursor pyrolysis routes has been draw-
ing significant attention in the past few years as an alterna-
tive method for preparing ceramics and more importantly for
CMCs by polymer impregnation and pyrolysis (PIP). One of
the most important reasons for this is the potential for reduc-
tions in manufacturing costs and relatively simple equipment
requirements. A new and multistep method for preparing
ZrB2 by polymeric precursor pyrolysis route is reported
herein where the phase compositions and microstructures
were characterized. Moreover, the heat treatment process for
precursors was studied and a hypothesized formation mecha-
(
B O , etc). Introducing ZrB into CMCs such as carbon
2 3 2
fiber-reinforced silicon carbide (C/SiC) composites can effec-
tively depress the oxidation reaction during the ablation pro-
nism of ZrB is also presented.
2
E. Corral—contributing editor
II. Experimental Procedure
The starting Zr-containing precursors and B-containing pre-
cursors were both synthesized and patented in our labora-
tory. The Zr-containing precursor was [(C H O)Zr(acac) ]n
4 8 2
Manuscript No. 30347. Received September 19, 2011; approved December 08, 2011.
Author to whom correspondence should be addressed. e-mail: wgzhang@mail.ipe.
18
†
ac.cn.
with molecular weight about 64870 and a softening point of
866

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Rapid Communications of the American Ceramic Society
867
Table I. Component Contents (wt%) of the Zr-Containing
and B-Containing Precursors
ucts transformed from the precursors with various B/Zr
molar ratios. Specifically, the diffraction peaks match well
with the data for hexagonal ZrB
card No. 01-089-3826) when B/Zr molar ratio was 2, which
indicates an excellent finding that the precursor could be
completely transformed into ZrB . According to XRD pat-
2
reported by the JCPDS
Component
Zr (%) O (%) N (%) C (%) B (%)
H (%)
(
Zr-containing
precursor
B-containing
precursor
29.82 26.40
–
37.90
–
5.88
2
terns, the precursor with B/Zr ratio of 1 may transform into
ZrB , ZrO , and ZrN during heat treatment. Meanwhile, as
–
–
34.41 29.89 27.25 8.45
2
2
B/Zr molar ratio increased, there was also a small quantity
of cubic ZrC produced other than ZrB with a ratio of 4.
ZrC mixed with a bit of ZrO and C was obtained with the
2
2
1
70°C. The B-containing material was 2, 4, 6-tri-methylami-
no borazine with the chemical formula (NHCH ) B N H
3
and molecular weight ~600. The component contents of these
precursors are shown in Table I. Then ZrB polymeric pre-
cursor solution were prepared by dissolving Zr-containing
polymeric precursor and B-containing precursor into dimeth-
ylbenzene, and the solvent was removed by the reduced pres-
ratio of 6 and more ZrC, ZrO
ratio reached 8.
2
, and C were obtained as the
3
3
3
3
To understand transformation mechanism, FTIR was per-
formed on the ZrB2 precursor. According to FTIR spectra
shown in Fig. 2, the peaks at about 2930 cm were assigned
2
ꢀ1
to be C–H stretching whereas the humps at 1720 and
ꢀ1
1
respectively. The humps observed at 1070 and 960 cm
610 cm were due to stretching and bending of C=O bond,
sure distillation technology at 80°C. Therefore, ZrB
2
19
ꢀ1
precursors with various B/Zr molar ratios, namely 1, 2, 4, 6,
and 8, were prepared. The subsequent heat treatment was
performed at a heating rate of 2°C/min to the desired tem-
were believed to be due to C–N and N–H stretching, respec-
ꢀ1
tively. A set of two bands appeared at 2340 and 660 cm
which could be attributed to antisymmetric and symmetric
,
2
perature of 1500°C and then held for 2 h in a MoSi furnace
stretching vibration of O=C=O, respectively. Therefore, it
under flowing argon atmosphere.
standard X-ray diffractometer (XRD; D/max-rb,
could be deduced that gaseous products such as CH
and NH were produced and the CO generating rate went
up gradually with increasing temperature.
To characterize the generative process of ZrB
4 2
, CO ,
A
3
2
Rigaku Corporation, Tokyo, Japan) with CuKa radiation
was used to determine phase compositions and crystalline
state of the products after heat treatment. On-line Fourier
transform infrared spectroscopy (FTIR; Bruker Tensor 27,
Bruker, Ettlingen, Germany) detection was performed over
the range from 4000 to 400 cm from the outlet of the tube
furnace during heating process with a rate of 10°C/min in
2
, the phase
compositions and crystalline state of the products after heat
treatment at various temperatures were characterized using
XRD analysis. Figure 3 shows XRD patterns of pyrolysized
products of ZrB polymeric precursor heat treated at various
2
temperatures. After heat treatment at 1100°C, tetragonal zir-
ꢀ1
2
nitrogen atmosphere, using the ZrB precursor powders pre-
conia (t-ZrO ) was detected. B O and carbon were not
2
2
3
pared using grinding method to analyze the pyrolysis pro-
cess. Yields of solids were calculated as a ratio of the mass
of products after heat treatment to the mass of precursors at
detected, implying that these phases were present in amor-
phous form or in minute amount. Initial formation of ZrB
was observed at 1300°C, while monoclinic zirconia (m-ZrO
2
2
)
1
500°C, and the components were determined by chemical
elemental analysis. Specifically, the content of boron was
determined by alkali-based titration and the relative standard
deviation was less than 5%. The content of zirconium was
tested using a gravimetric method with a relative standard
deviation less than 1%. To determine the contents of oxygen
and nitrogen, inert gas pulse infrared thermal conductivity
method was used (QB-QT-33-1997). The content of carbon
was determined by high-frequency combustion infrared
method (ASTM E 1941-2004) while that of hydrogen was
tested using an inert gas pulse infrared method (ASTM E
2
1447-2009). Microscopic structures of ZrB were investigated
using a scanning electron microscope (SEM; Quanta 200
FEG, FEI, Eindhoven, the Netherlands).
III. Results and Discussion
Fig. 2. FTIR spectra of the ZrB
temperatures: (a) 120°C; (b) 220°C; (c) 400°C; and (d) 900°C.
2
precursors at various
The XRD analysis of the ZrB2 polymeric precursors after
heat treatment at 1500°C with various B/Zr molar ratios are
shown in Fig. 1, which shows that there were several prod-
2
Fig. 3. XRD patterns for the ZrB polymeric precursors that were
Fig. 1. XRD patterns of samples produced by precursors with
various B/Zr molar ratios: (a) 1; (b) 2; (c) 4; (d) 6; and (e) 8.
heat treated at various temperatures: (a) 1100°C; (b) 1300°C; and (c)
1500°C.

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Rapid Communications of the American Ceramic Society
Vol. 95, No. 3
Table II. Component Contents (wt%) of the Synthesized
ZrB Powders at 1500°C
ZrO2ðsÞ þ B2O3ðlÞ þ 5CðsÞ ! ZrB2 þ 5COðgÞ
CðsÞ þ 4H ! CH4ðgÞ
ð1Þ
ð2Þ
2
Component
Zr (%)
B (%)
N (%)
C (%)
O (%)
H (%)
ZrB powders 79.04 18.39 0.37
2
0.78
0.91
<0.1
From the elemental analysis of the precursors (Table I),
the theoretical yield of ZrB should be 30.1% when the B/Zr
ratio of the precursor was 2. During heat-treatment process,
a great deal of gases including CO and CH were produced
and escaped away. As a result, the yield of ZrB powders
was measured to be 31.9%. Within the limits of experimental
error, the calculated result coincides with the experimental
result. Further chemical elemental analysis was characterized
2
was produced and t-ZrO
precursor was heat treated at 1500°C, zirconia phases disap-
peared and transformed into ZrB completely. Therefore, it
2
was detected as well. Whereas the
4
2
2
could be deduced that zirconia was an intermediate product
which reacted with B O at 1500°C to form ZrB completely.
2
3
2
According to the chemical thermodynamics of the system,
methane is a very stable phase. Excess carbon must be gasi-
fied by the excess of hydrogen in the system. The following
reactions occurred:
2
on the synthesized ZrB powders. The component contents
are shown in Table II. It could be found that the final pow-
ders consist of Zr, B, and a little amount of O, N, C, and H.
The B/Zr molar ratio is about 1.97, which indicates that the
2
synthesized ZrB powders have a high purity.
Microstructures of the formed ceramic are shown in
Figs. 4(a) and (b). As shown in Fig. 4(c), only Zr and B are
detected using EDS analysis, indicating that the precursor
(
a)
2
has almost entirely been transformed into ZrB . This result
agrees well with that of XRD analysis and chemical elemen-
tal analysis. According to the SEM micrographs, a multitude
of ZrB
formly, with the sizes of a majority of ZrB
2
particles were formed and dispersed relatively uni-
particles being
2
on the order of 50–100 nm, which is hypothesized to be the
consequence of the homogeneous nucleation and crystal
growth of ZrB
that the ZrB particles tended to be spherical and aggregated.
There are a host of spherical droplet ZrB2 particles whose
grain sizes are diverse around the large ZrB aggregate,
which indicates that the formation of ZrB particles was via
2
. The large magnification micrograph shows
2
2
2
2
0
the liquid phase reaction mechnism. At higher tempera-
tures, the spherical droplet ZrB2 particles thus formed are
ready to proceed by liquid diffusion and have the tendency
of aggregating together to form large particles.
(
b)
IV. Conclusions
In summary, ZrB
2
polymeric precursors were prepared using
[(C H O)Zr(acac) ]n and (NHCH ) B N H , and the precur-
sor with a B/Zr molar ratio of 2 can be completely trans-
4
8
2
3 3
3
3
3
formed into hexagonal ZrB
in argon for 2 h. ZrC, ZrO
2
during heat treatment at 1500°C
, and even C could be obtained
2
if molar ratios of B/Zr are increased larger than this value.
Keeping a constant B/Zr molar ratio of 2 and decreasing the
(
c)
temperature as low as 1100°C, t-ZrO
precursor, while large amounts of ZrB
were formed at increased temperatures up to 1300°C. The
precursor could only transform into pure ZrB at tempera-
2
was formed from the
2
with a little m-ZrO
2
2
tures above 1500°C, with the formed diboride particles dis-
persed uniformly and sizes around 50–100 nm. It was also
assumed that the formation of ZrB2 particles was via the
liquid phase reaction mechanism with an aggregation phe-
nomenon occurring during pyrolysis.
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