Solution-based synthesis of submicrometer ZrB2 and ZrB 2-TaB2

Article abstract of DOI:10.1111/j.1551-2916.2008.02288.x

Zirconium diboride and a zirconium diboride/tantalum diboride mixture were synthesized by solution-based processing. Zirconium n-propoxide was refluxed with 2,4-pentanedione to form zirconium diketonate. This compound hydrolyzed in a controllable fashion to form a zirconia precursor. Boria and carbon precursors were formed via solution additions of phenol-formaldehyde and boric acid, respectively. Tantalum oxide precursors were formed similarly as zirconia precursors, in which tantalum ethoxide was used. Solutions were concentrated, dried, pyrolyzed (800°-1100°C, 2 h, flowing argon), and exposed to carbothermal reduction heat treatments (1150°-1800°C, 2 h, flowing argon). Spherical particles of 200-600 nm for pure ZrB₂ and ZrB ₂-TaB₂ mixtures were formed.

Full text of DOI:10.1111/j.1551-2916.2008.02288.x

J. Am. Ceram. Soc., 91 [5] 1469–1474 (2008)
DOI: 10.1111/j.1551-2916.2008.02288.x
r 2008 The American Ceramic Society
ournal
J
Solution-Based Synthesis of Submicrometer ZrB2 and ZrB2–TaB2
Yanli Xie, Thomas H. Sanders Jr., and Robert F. Speyerw
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332-0245
Zirconium diboride and a zirconium diboride/tantalum diboride
mixture were synthesized by solution-based processing. Zirco-
nium n-propoxide was refluxed with 2,4-pentanedione to form
zirconium diketonate. This compound hydrolyzed in a control-
lable fashion to form a zirconia precursor. Boria and carbon
precursors were formed via solution additions of phenol–
formaldehyde and boric acid, respectively. Tantalum oxide pre-
cursors were formed similarly as zirconia precursors, in which
tantalum ethoxide was used. Solutions were concentrated, dried,
pyrolyzed (8001–11001C, 2 h, flowing argon), and exposed to
carbothermal reduction heat treatments (11501–18001C, 2 h,
flowing argon). Spherical particles of 200–600 nm for pure ZrB2
and ZrB2–TaB2 mixtures were formed.
mediate reaction products during metal diboride synthesis
because of preferential carbothermal reduction of the metal oxide
by carbon. However, the metal diboride is formed eventually if
sufficient boron is available, i.e., MC(s)1B2O3(s)12C(s)-
MB2(s)13CO(g).
Chemical solution processing routes refer to methods in
which one or all of the components (i.e., metal-bearing, boron-
bearing, and carbon-bearing) are solubilized in a liquid process-
ing medium. Soluble sources that are used to provide boron
include boric acid and boron alkoxides. Numerous soluble
carbon-bearing materials have been used to produce metal car-
bides, including phenolic resins, furfuryl alcohol, sugar, corn
starch, petroleum pitch, polyacrylonitrile polymers, cellulose ac-
etate polymers, and diols. Common soluble metal/metal oxide-
bearing materials are metal alkoxides, metal diketonates, and
metal carboxylates. Metal-organic compounds are usually sub-
jected to hydrolysis and condensation reactions to produce poly-
meric or colloidal metal-oxide precursors.21–37 Depending on
the reaction conditions, the metal-organic compound may also
be a source for some of the carbon for the carbothermal reduc-
tion reaction.
The most important advantage of solution-processing meth-
ods is that more intimate mixing of components (atomic-scale or
at least molecular-scale mixing) can be achieved. However, it
should be noted that preparation of solutions with atomic- or
molecular-scale mixing does not necessarily ensure that the same
scale of mixing will be maintained during subsequent processing
steps. It is necessary to remove the liquid medium in a manner
that avoids segregation of components. For example, molecular-
scale mixing in the solutions may be maintained by first gelling
the solutions before solvent removal. The hydrolysis time, tem-
perature, and atmosphere can be altered to control the grain size
and phase distribution in the resulting product, which in turn
will factor into the properties of the resulting materials.38–40 In
this work, solution-based synthesis was used in effort to fabri-
cate submicrometer, highly sinterable powders of ZrB2, and
intimately mixed submicrometer powders of ZrB2 and TaB2.
I. Introduction
N recent years, a renewed interest has developed in borides of
groups IV and V transition metals (e.g., ZrB2, HfB2, and
I
TaB2), referred to as ultrahigh temperature ceramics. The ther-
mal and chemical stability of these compounds makes them
candidates for use in the extreme environments associated with
hypersonic flight (14001C and above in air), atmospheric reentry
(20001C and above), and rocket propulsion (30001C and
above).1–3 These materials have melting temperatures above
30001C, retain their strength at temperatures above 12001C,
and exhibit good thermal shock resistance. They can be modi-
fied with additives such as SiC to promote oxidation resistance.
In particular, ZrB2–SiC and HfB2–SiC with volume ratios of
80:20 have shown improved oxidation resistance via formation
of a passive amorphous borosilicate surface layer.4,5 Tantalum
additions in the form of TaB2 or TaSi2 have been reported to
further enhance oxidation resistance by facilitating a high-
viscosity phase-separated amorphous surface coating.6–8
Metal diborides can be synthesized by reaction between a
metal oxide and boron (borothermal reduction):9–14
MO2(s)14B(s)-MB2(s)12BO(g). The reaction temperatures
are generally lowered by having carbon present, such that the
overall boro/carbothermal reaction is given by:15 MO2(s)1
2B(s)12C(s)-MB2(s)12CO(g). Metal borides can also be syn-
thesized by reaction between a metal oxide and boron oxide
via a carbothermal reduction:16–17 MO2(s)1B2O3(l, g)15C(s)-
MB2(s)15CO(g). Excess boria is generally required for the
above reaction because of B2O3 volatilization at elevated tem-
peratures (boiling point of B2O3, i.e., 1 atm vapor pressure, is
18601C),18 as well as to compensate for the formation of gaseous
boron oxide (e.g., BO) intermediate products which are removed
from the reaction zone by the argon gas stream. Homogeneous
products with fine particle size have been reported18–20 using this
fabrication route. Metal carbides have been observed as inter-
II. Experimental Procedure
A flow chart for the solution-based synthesis of ZrB2 is shown in
Fig. 1. The starting Zr-containing material was a 70 wt% zir-
conium n-propoxide (Zr(OC3H7)4, Alfa Aesar, Ward Hill, MA)
in solution with n-propanol. The zirconium n-propoxide
(‘‘Zr(OPr)4’’) was mixed with 2,4-pentanedione (‘‘acacH’’’) us-
ing molar ratios of 3 (acacH to Zr(OPr)4) and 2-propanol was
used as a mutual diluent. This solution was refluxed (i.e., solu-
tion was heated and the vapor formed was condensed and re-
turned to the solution to be heated again) at 1701–1951C for
2 h for the purpose of reacting zirconium n-propoxide and
2,4-pentanedione to form zirconium diketonate, as illustrated
in Fig. 2. The purpose of this was to facilitate a controlled
hydrolysis reaction; zirconium n-propoxide otherwise hydrolyzes
very rapidly. Refluxing maintains constant concentrations of
constituents as reactions occur among them at temperatures at
which their volatilities vary substantially. Following refluxing,
much of the solvent (about 2/3) was evaporated using a rotary
evaporator (Rotavapor R-114, Buchi, Flawil, Switzerland)
at 351C, and then 2-propanol was added back to the sol; the
Y. Blum—contributing editor
Manuscript No. 23549. Received August 3, 2007; approved December 12, 2007.
This project was funded by the Air Force Office of Scientific Research, Contract
FA9550-04-1-0140.
Presented at the AFOSR Workshop on Ultra-High-Temperature Ceramic Materials
hosted by SRI International, July 23–25, 2007.
wAuthor to whom correspondence should be addressed. e-mail: robert.speyer@mse.
gatech.edu
1469

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Zr n-Propoxide
Journal of the American Ceramic Society—Xie et al.
Vol. 91, No. 5
which there was a buildup of species with a three-dimensional
structure. As solvent was volatilized, a residual mass formed with
a sludge-like consistency. This was then dried to powders under
vacuum conditions at 1201–1401C (2 h). These powders were
subsequently pyrolyzed at temperatures in the range of 8001–
11001C (2 h) in a flowing argon atmosphere in an alumina tube-
furnace to produce intimately mixed not-yet-reacted zirconia/
boron oxide/carbon mixtures. This temperature range was found
to permit near-complete conversion to the desired oxide or car-
bon constituents, but was not high enough to initiate carbother-
mal reduction. Pyrolyzed powders were subsequently heat treated
at temperatures in the range of 12001–14001C (2 h) in flowing
argon in a graphite tube-furnace for carbothermal reduction.
For the ZrB2–TaB2 composite, the starting Ta-containing
material was a tantalum ethoxide solution (i.e., 99.99 wt%
Ta(OC2H5)4 in ethanol, HC Starck, GmbH, Germany). The tan-
talum ethoxide was mixed with 2,4-pentanedione using molar
ratios of 3:1 and 1-propanol was used as a mutual diluent. Zir-
conium n-propoxide/acacH solution was prepared by the same
method. Tantalum ethoxide/acacH solution was added to the
zirconium n-propoxide/acacH solution to make a Zr/Ta ratio
of 3. The resulting solutions were initially refluxed at B1951C for
2 h, and then partially hydrolyzed (at 501C for 2 h) under acidic
conditions with a HNO3/Zr molar ratio of 0.27 and an H2O/Zr
molar ratio of 24. Boric acid and phenolic resin were added to the
above hydrolyzed solution to make the molar ratio of C/
(Zr1Ta) 5 4.8 and B/(Zr1Ta) 5 3.0. The mixture was then con-
centrated and dried. The dry powder was first pyrolyzed at 8001C
to obtain fine-scale mixtures of ZrO2/Ta2O5/B2O3/carbon. These
powders were then heat treated at 13001–18001C in flowing Ar in
a graphite tube-furnace in order to carry out the carbothermal
reduction reactions to produce the ZrB2–TaB2 mixture.
Boric Acid
Phenolic Resin
acac-Modified
Zr n-Propoxide
Mixed
Solution
Hydrolysis/
Condensation
Mixed Solution
Solvent Evaporation/Drying
Pyrolysis
Carbothermal Reduction
Fig. 1. Flow chart for synthesis of zirconium diboride from solution-
based precursors.
purpose of this was to remove remaining 2,4-pentanedione from
the solution. These Zr-containing precursors were then partially
hydrolyzed, in which a hydroxyl substituted for the n-propoxide
and pentanedionate (acac) by interaction with water at 501C for
2 h under acidic conditions (pH in the range 4–5) using a HNO3/
Zr molar ratio of 0.27 and an H2O/Zr molar ratio of 24.
Hydrolysis is required to facilitate later condensation reactions.
A clear solution was observed before and after hydrolysis.
Separately, a boric acid (H3BO3, Fisher Scientific, Fair Lawn,
NJ) in water solution with a concentration of 4 wt% was pre-
pared as the boron source, to make a B/Zr ratio of 3. To dilute
the boric acid solution, 2-propanol was then added with a
propanol/H2O ratio of 3. Twenty weight percent phenol–
formaldehyde resin (novolac-type; Georgia Pacific, Atlanta,
GA) in a 2-propanol solution was added to the boric acid
solution to increase the C/Zr ratio to 4.8–6.0.
X-ray diffraction (XRD, Model PW1800, PANalytical, Almelo,
the Netherlands) was used to determine the phases present after
various heat treatments. Scanning electron microscopy (SEM,
Model 1530, LEO Electron Microscopy Inc., Oberkochen,
Germany) was used to evaluate the powders for shape, size, and
extent of agglomeration. For SEM evaluation, powders were
pressed onto carbon tape adhering to an SEM stub. These were
then coated with gold via sputtering. Fourier transform infrared
spectroscopy (FTIR, Nexus 870, Nicolet Instrument Corp.,
Madison, WI) was performed on zirconium n-propoxide and
zirconium diketonate solutions, as well as a commercial source of
zirconium diketonate (Fischer Scientific, Pittsburgh, PA).
The mixture of boric acid and phenol–formaldehyde resin
was then added to the hydrolyzed Zr-containing solutions. The
solutions were concentrated in a rotary evaporator (preventing
the constituents from demixing) in a condensation reaction in
Fig. 2. Reactions for precursor synthesis.

May 2008
Solution-Based Synthesis of ZrB2 and ZrB2–TaB2
1471
evaluated by adding them before or after refluxing. In the three
evaluated solvents, ethanol, 2-propanol, and 1-butanol, the bo-
ron in solution became a member of the volatile species; no zir-
conium diboride was formed through carbothermal reduction.
Boric acid in a water and 2-propanol solution did form a non-
volatile boron species in solution.
Figure 3 shows FTIR spectra obtained from the starting
compound (zirconium n-propoxide), the solution after refluxing,
and a commercial zirconium diketonate. The figure confirms
that from the refluxing step, zirconium n-propoxide reacted with
acacH to form zirconium diketonate. In general, zirconium
n-propoxide undergoes more rapid hydrolysis reactions than the
corresponding zirconium pentanedionate, and this has resulted
in uncontrolled precipitation of relatively large precursor parti-
cles during the hydrolysis step. Therefore, zirconium n-propoxide
was first refluxed (1951C, 2 h) with 2,4-pentanedione in order to
partially or fully convert the zirconium alkoxy groups to a
chelated zirconium diketonate structure.
An idealized reaction to produce stoichiometric ZrB2 by
carbothermal reduction is given by
Fig. 3. Fourier transform infrared spectroscopy spectra of the starting
material (Zr n-propoxide), the solution after refluxing, and a commercial
zirconium diketonate.
ZrO2ðsÞ þ B2O3ðl; gÞ þ 5CðsÞ ! ZrB2ðsÞ þ 5COðgÞ
However, the necessary ratio of boron to zirconium was
found to be B3:1 instead of 2:1 as dictated by the reaction
stoichiometry. Excess boron oxide was required to form ZrB2
because of B2O3 volatilization at elevated temperatures.
III. Results and Discussion
(1) Submicrometer ZrB2
Triethyl borate was initially used as the boron source for
precursor synthesis. Solutions containing triethyl borate were
Figure 4 shows XRD patterns for dried precursor powders
that were heat treated at various temperatures. The as-dried
Fig. 4. X-ray diffraction (XRD) patterns for ZrB2 powders (as batched: B/Zr5 3, C/Zr 55.0). Patterns corresponding to 8001 and 11001C were ex-
posed to a pyrolysis heat treatment of 51C/min to the indicated temperatures, holding for 2 h in an alumina tube furnace under flowing argon. XRD
patterns corresponding to higher indicated temperatures were first exposed to the above pyrolysis step, and then placed in a graphite tube furnace for
further heat treatment. This entailed heating at 801C/min to a temperature 751C lower than final temperature, 401C/min to a temperature 351C lower
than final temperature, 201C/min to a temperature 151C lower, and 51C/min to a temperature 51C lower, and finally 11C/min to the indicated final soak
temperature. Specimens were then soaked for 2 h.

1472
Journal of the American Ceramic Society—Xie et al.
Vol. 91, No. 5
Fig. 6. Scanning electron microscopy photomicrograph a of ZrB2 sam-
ple heat treated at 13001C for 2 h in a graphite tube furnace.
Fig. 5. Plot of weight loss versus temperature for ZrB2 and ZrB2–TaB2
dried powders. Weight losses indicated for 6001–8001C are from soaks at
those temperatures for 2 h. For all higher indicated temperatures, indi-
cated weight losses are from pyrolysis heat treatments at 8001C for 2 h,
followed by heat treatments at the indicated temperatures for 2 h. Heat-
ing rates for pyrolysis were 51C/min. Higher temperature heat treatment
heating schedules were as indicated in the caption to Fig. 4.
were present in amorphous form. Initial formation of ZrB2 was
observed after heat treatment at 12001C, while the intensity of
t-ZrO2 increased compared with m-ZrO2 in the range 11001–
12501C. Zirconia phases reduced to minor levels at 13001C, and
were not observed at 14001C.
Weight loss measurements (Fig. 5) imply that the carbother-
mal reduction proceeded extensively above 11001C, and tapered-
off after heat treatments at 13001C. The calculated weight loss
from the above carbothermal reaction (in which no gaseous re-
actant escapes and only CO(g) is removed) is 55.4%. Figure 6
shows an SEM photomicrograph of a sample (initial composition
precursor was XRD-amorphous. After heat treatments at 8001
and 11001C, monoclinic zirconia (m-ZrO2) with a trace concen-
tration of tetragonal zirconia (t-ZrO2) were detected. Crystalline
B2O3 and carbon were not detected, implying that these phases
Fig. 7. X-ray diffraction (XRD) patterns for heat treated ZrB2–TaB2. XRD patterns corresponding to 6001–11001C were exposed to a pyrolysis heat
treatment of 51C/min to the indicated temperatures, holding for 2 h in an alumina tube furnace in flowing argon. XRD patterns corresponding to higher
indicated temperatures followed heating schedules as enumerated in the caption to Fig. 4.

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Solution-Based Synthesis of ZrB2 and ZrB2–TaB2
1473
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(TaZr2.75O8) was detected after heat treatment at 7001C. This
phase was at its highest concentration at 10001C and was
resorbed above 12501C. TaC was observed after heat treatments
at 11001 and 11501C. TaC was an intermediate product, which
reacted with B2O3 at B11501C to form TaB2:
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IV. Conclusions
Submicrometer spherical particles of ZrB2 and a ZrB2–TaB2
mixture were synthesized via a solution-based method. Zirconi-
um n-propoxide was refluxed with 2,4-pentanedione to form
zirconium diketonate, which facilitated controlled hydrolysis
and condensation reactions, in turn forming intimately mixed
precursors. Phenol–formaldehyde and boric acid were solution
additives providing carbon and B2O3. After pyrolysis heat treat-
ments to form oxides and carbon, carbothermal reduction heat
treatments yielded the diboride powders.
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Acknowledgments
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The authors would like to gratefully acknowledge the original principal inves-
tigator on this project, Dr. Michael Sacks. We would also like to acknowledge
Mr. Fei Peng for his help with electron microscopy. The authors would like to
express their appreciation for the helpful suggestions and support of their contract
monitor, Dr. Joan Fuller.
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