Preparation of borides in Nb-B and Cr-B systems by combustion synthesis involving borothermic reduction of Nb2O5 and Cr2O3

Article abstract of DOI:10.1016/j.jallcom.2009.10.007

An experimental study on the preparation of metal borides in the Nb-B and Cr-B systems was conducted by self-propagating high-temperature synthesis (SHS) involving the reduction of Nb₂O₅ and Cr₂O₃ by amorphous boron. The starting stoichiometry of the reactant compact was shown to make a great impact on the combustion behavior and the phase composition of the final product. For the powder compacts of Nb₂O₅ and boron, self-sustaining combustion was performed under a molar ratio of B/Nb₂O₅ between 5 and 10, but complete reduction of Nb₂O₅ was achieved when B/Nb₂O₅ ≥ 8. Partial reduction of Nb₂O₅ caused a decrease in the combustion temperature and velocity, and was responsible for the presence of NbO₂ in the final products. For the samples with stoichiometry of 6 ≤ B/Nb₂O₅ ≤ 8, three boride phases NbB, Nb₃B₄, and NbB₂ were synthesized. An increase in the boron content up to B/Nb₂O₅ = 8.5-10 resulted in not only full reduction of Nb₂O₅, but also formation of single-phase NbB₂. On the other hand, the SHS process involving Cr₂O₃ and boron was feasible for the powder compacts of 4 ≤ B/Cr₂O₃ ≤ 9, wherein the highest combustion temperature and the fastest reaction front were observed in the compact with B/Cr₂O₃ = 6. During combustion Cr₂O₃ was fully reduced, leading to the formation of three borides Cr₅B₃, CrB, and CrB₂ in either monolithic or composite form. With a boron content more than the stoichiometric amount, the powder compacts of B/Cr₂O₃ = 4, 5, and 9 yielded single-phase Cr₅B₃, CrB, and CrB₂, respectively.

Full text of DOI:10.1016/j.jallcom.2009.10.007

Journal of Alloys and Compounds 490 (2010) 366–371
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Preparation of borides in Nb–B and Cr–B systems by combustion synthesis
involving borothermic reduction of Nb O and Cr O
2
5
2
3
∗
C.L. Yeh , H.J. Wang
Department of Aerospace and Systems Engineering, Feng Chia University, 100 Wenhwa Rd., Seatwen, Taichung 40724, Taiwan, ROC
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2009
Received in revised form
An experimental study on the preparation of metal borides in the Nb–B and Cr–B systems was conducted
by self-propagating high-temperature synthesis (SHS) involving the reduction of Nb2O5 and Cr2O3 by
amorphous boron. The starting stoichiometry of the reactant compact was shown to make a great impact
on the combustion behavior and the phase composition of the final product. For the powder compacts
of Nb2O5 and boron, self-sustaining combustion was performed under a molar ratio of B/Nb2O5 between
3
0 September 2009
Accepted 1 October 2009
Available online 9 October 2009
5
and 10, but complete reduction of Nb2O5 was achieved when B/Nb2O5 ≥ 8. Partial reduction of Nb2O5
caused a decrease in the combustion temperature and velocity, and was responsible for the presence of
NbO2 in the final products. For the samples with stoichiometry of 6 ≤ B/Nb2O5 ≤ 8, three boride phases
NbB, Nb3B4, and NbB2 were synthesized. An increase in the boron content up to B/Nb2O5 = 8.5–10 resulted
in not only full reduction of Nb2O5, but also formation of single-phase NbB2. On the other hand, the SHS
process involving Cr2O3 and boron was feasible for the powder compacts of 4 ≤ B/Cr2O3 ≤ 9, wherein
the highest combustion temperature and the fastest reaction front were observed in the compact with
B/Cr2O3 = 6. During combustion Cr2O3 was fully reduced, leading to the formation of three borides Cr5B3,
CrB, and CrB2 in either monolithic or composite form. With a boron content more than the stoichio-
metric amount, the powder compacts of B/Cr2O3 = 4, 5, and 9 yielded single-phase Cr5B3, CrB, and CrB2,
respectively.
Keywords:
Ceramics
Solid-state reactions
X-ray diffraction
Combustion synthesis
Transition metal borides
©
2009 Elsevier B.V. All rights reserved.
1
. Introduction
NbB2 by heating amorphous boron and niobium powders of their
◦
equivalent compositions at 1000 C for 60 min and obtained Nb B
3
4
◦
Borides of the transition metals are very attractive materials,
under a higher temperature of 1800 C. According to Iizumi et al.
due to a unique combination of favorable properties like high melt-
ing point, high hardness, high mechanical strength, high electrical
conductivity, good chemical stability, and excellent wear and cor-
rosion resistance [1–3]. Most transition metal borides feature many
stoichiometric compositions. For example, five niobium borides
[12,13], both CrB and CrB have been synthesized in pure form by
2
milling chromium and boron powders in a planetary ball mill for
◦
20–40 h, followed by annealing at 900–1000 C. Peshev et al. [14,15]
performed a series of experiments on the borothermic reduction
◦
of metal oxides between 1000 and 1750 C to prepare the corre-
(
Nb B , NbB, Nb5B , Nb B , and NbB ) and six chromium borides
sponding metal borides, including CrB , Mo B5, W B5, VB , NbB ,
3
2
6
3
4
2
2
2
2
2
2
(
Cr B, Cr5B , CrB, Cr B , CrB , and CrB ) have been reported in the
and TaB . In addition, Sonber et al. [16] fabricated CrB2 through
2
3
3
4
2
4
2
◦
Nb–B and Cr–B systems, respectively [4]. Among them, NbB2 is
promising for high-temperature structural applications [5] and has
been recognized as a superconductor [6,7]. CrB2 shows potential
not only as a high-temperature structural material [3], but also as
a hard coating or a protective layer on tools and materials exposed
to wear and corrosion [8,9].
Many transition metal borides have been fabricated by the solid-
state reaction between boron and metal powders [10–13], and by
the reduction of metal oxides with boron [14,15] or boron carbide
the reaction of Cr O3 with boron carbide at 1500–1700 C in the
2
presence of carbon.
In contrast to the aforementioned manufacturing routes that
are generally time-consuming and energy-intensive, combustion
synthesis in the mode of self-propagating high-temperature syn-
thesis (SHS) takes advantage of the self-sustaining merit from
highly exothermic reactions and hence has the potential of time
and energy savings [17–19]. The SHS technique has been effec-
tively applied to produce a variety of advanced materials, including
borides, carbides, nitrides, hydrides, silicides, and intermetallics, as
well as composites on their bases [17–19]. Production of the transi-
tion metal borides by SHS is typically through the direct reaction of
mixed constituent elements in a sample compact, by which numer-
[
16] as the reducing agent. Matsudaira et al. [10] produced NbB and
∗ Corresponding author. Tel.: +886 4 24517250x3963; fax: +886 4 24510862.
E-mail address: clyeh@fcu.edu.tw (C.L. Yeh).
ous borides such as TiB , ZrB , NbB , TaB , HfB , NbB, MoB, and
2
2
2
2
2
0
925-8388/$ – see front matter © 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2009.10.007

C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 490 (2010) 366–371
367
Fig. 1. Recorded combustion images illustrating SHS processes of powder compacts with stoichiometric ratios of (a) Nb2O5:B = 1:8.5 and (b) Cr2O3:B = 1:6.
TaB have been obtained [20–25]. However, when direct combus-
tion between the metal and boron is not feasible, the SHS process
involving borothermic reduction of a metal oxide has been consid-
ered as an alternative of preparing the metal boride. For example,
three borides of molybdenum (Mo B, MoB , and Mo B5) were pro-
(Nb3B2, NbB, Nb5B6, Nb3B4, and NbB2) in the Nb–B system, the powder mixtures of
Nb2O5 and boron were formulated with proportions of a broad range. As expressed
in reactions (1) and (2), Nb3B2 and NbB2 can be stoichiometrically formed from the
samples with a molar ratio of B/Nb2O5 = 14/3 and 22/3, respectively.
3
Nb2O5 + 14B → 2Nb3B2 + 5B2O3
(1)
(2)
2
2
2
duced by SHS from the reactant compacts composed of MoO , Mo,
3Nb O + 22B → 6NbB2 + 5B O
3
2
5
2
3
and B powders under different stoichiometries [26].
In order to cover the scope denoted by reactions (1) and (2), this study takes on
a span of the stoichiometric ratio from B/Nb2O5 = 4 to 10 for the synthesis of various
niobium borides. In the case of preparing chromium borides, the existence of six
different phases (Cr2B, Cr5B3, CrB, Cr3B4, CrB2, and CrB4) in the Cr–B system signifies
a requisite of the proportion of B/Cr2O3 from 3 to 10, as described in reactions (3)
and (4).
The objective of this study is to investigate the production of
metal borides in the Nb–B and Cr–B systems through the reac-
tion of metal oxides (Nb O5 and Cr O ) with amorphous boron in
2
2
3
the form of self-propagating combustion. In contrast to niobium
borides which have been produced by SHS from elemental powder
compacts [24], direct combustion between Cr and boron is not fea-
sible. However, formation of borides of niobium and chromium has
not been well characterized by SHS involving borothermic reduc-
tion of the relevant metal oxides. For both combustion systems,
this study gives emphasis to establish the relation between the
boride phase formed and the initial sample composition (i.e., the
Cr2O3 + 3B → Cr2B + B2O3
Cr O + 10B → 2CrB4 + B O
(3)
(4)
2
3
2
3
The reactant powders with a prescribed composition were dry mixed in a ball
mill and then cold-pressed into cylindrical test specimens with a diameter of 7 mm
and a height of 12 mm. The compaction density was 50% relative to the theo-
retical maximum density (TMD) for the Nb2O5–B samples and 45% TMD for the
Cr2O3–B compacts. The SHS experiment was conducted in a stainless-steel win-
dowed chamber under an atmosphere of high-purity argon (99.99%). The sample
holder is equipped with a 600 W cartridge heater used to raise the initial temper-
ature of the sample prior to ignition. It was found that a preheating temperature
molar ratios of B/Nb O5 and B/Cr O ). In addition, the effects of
2
2
3
sample stoichiometry were explored on sustainability of the com-
bustion process, propagation velocity of the combustion front, and
combustion temperature.
◦
of 300 C was required for both types of the samples to assure self-sustaining com-
bustion. Moreover, a Ti–C pellet made up of the powder blend with an atomic ratio
of Ti:C = 1:1 was placed on the top of the test specimens to serve as an ignition
enhancer which was triggered by a heated tungsten coil. Details of the experimen-
tal setup and measurement approach were reported elsewhere [27]. It should be
noted that according to Ref. [26], the by-product B2O3 yielded from the displace-
ment reaction is voluntarily expelled in the form of very tiny liquid droplets from
the porous sample during the SHS process, thus resulting in the absence of B2O3 in
the final products.
2
.
Experimental methods of approach
Niobium oxide Nb2O5 (Strem Chemicals, 99.9% purity), chromium oxide Cr2O3
(
Showa Chemical Co., 99% purity) and amorphous boron (Noah Technologies Corp.,
2% purity) were employed as the starting materials. Similar to typical amorphous
boron, the major impurities in the boron powders include magnesium (Mg) about
.0%, water soluble boron (0.50%) and moisture (0.50%). In view of five boride phases
9
5

3
68
C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 490 (2010) 366–371
Fig. 3. Effect of sample stoichiometry on flame-front propagation velocity of
Cr2O3–B powder compacts.
Fig. 2. Effect of sample stoichiometry on flame-front propagation velocity of
Nb2O5–B powder compacts.
for the reactant compacts containing boron higher than the pro-
3
. Results and discussion
portion of B/Nb O5 = 8.5. Such a decrease is believed to be a result
2
of the dilution effect produced by an excessive amount of boron.
The influence of sample stoichiometry on the flame-front veloc-
3.1. Observation of combustion characteristics
ity of the Cr O –B powder compact is presented in Fig. 3. With
2
3
Fig. 1(a) and (b) illustrates typical SHS sequences associated
the increase of the molar ratio of B/Cr O3 from 4 to 9, the com-
2
with solid-state combustion of the Nb O5–B and Cr O –B pow-
2
2
3
bustion wave velocity increases from 0.86 mm/s to a peak value
of 2.56 mm/s at B/Cr O = 6 and decreases afterward to about
der compacts, respectively. It is evident in Fig. 1(a) and (b) that
a distinct combustion front forms upon initiation and propagates
along the sample in a self-sustaining and nearly parallel manner.
Moreover, a brighter burning glow and a faster combustion front
2
3
1
.25 mm/s. No combustion was achieved for the samples with com-
positions out of this range. Unlike the reaction of Nb O5 with boron,
2
Cr O was completely reduced by boron during combustion. There-
2
3
are observed for the Nb O5–B powder compact, implying a more
2
fore, the composition dependence of combustion front velocity
of the Cr O –B sample is attributed primarily to an alteration in
exothermic reaction between Nb O5 and boron when compared to
2
2
3
that of Cr O with boron.
2
3
the reaction exothermicity associated with formation of different
borides of chromium in the final products.
However, the experimental observations indicated that the
reaction was quenched shortly after the ignition for the powder
compacts with B/Nb O5 = 4.0 and 4.67. This is due most likely
to a lack of sufficient reaction heat to sustain the combustion
for the samples of such low boron contents. On the other hand,
2
3
.3. Measurement of combustion temperature
Five temperature profiles recorded from solid-state combustion
the Cr O –B samples were shown to possess a flammability limit
2
3
of the Nb O5–B powder compacts with different initial stoichiome-
of self-sustaining combustion residing in 4 ≤ B/Cr O ≤ 9, beyond
2
2
3
tries are depicted in Fig. 4, where the abrupt rise in temperature
signifies rapid arrival of the combustion front and the peak value
represents the reaction front temperature. As shown in Fig. 4, the
which combustion ceased to propagate.
.2. Measurement of flame-front propagation velocity
The propagation velocity (V ) of the combustion front was deter-
3
◦
combustion front temperature increases from 1048 to 1380 C with
f
mined from the recorded SHS images. It was found for both types
of the powder compacts that the combustion velocity was signif-
icantly affected by the reactant composition. As shown in Fig. 2,
the reaction front velocity of the Nb O5–B compact first increases
2
with increasing B/Nb O5 ratio, approaches to a maximum about
2
3
.9 mm/s at B/Nb O5 = 8.5, and then decreases with further increase
2
of the boron content in the reactant mixture. It is believed that for
the samples of Nb O5 and boron, the extent of borothermic reduc-
2
tion of Nb O5 is responsible for the variation of combustion velocity
2
with sample stoichiometry. Based upon the XRD analysis of synthe-
sized products to be presented later, an incomplete reduction of
Nb O5 occurred in the samples of B/Nb O5 < 8. Partial reduction of
2
2
Nb O5 yields another metal oxide NbO and liberates less reaction
2
2
heat, which causes a decrease in the propagation rate of the com-
bustion wave. The degree of borothermic reduction of Nb O5 was
2
enhanced by increasing the proportion of boron to Nb O5 in the
2
powder mixture. This accounts for an increase in the flame-front
velocity with increasing B/Nb O5 ratio from 5.0 to 8.5. However, a
moderate decrease in the combustion velocity is observed in Fig. 2
2
Fig. 4. Measured combustion temperature profiles of Nb2O5–B powder compacts
with different stoichiometric ratios.

C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 490 (2010) 366–371
369
Fig. 5. Measured combustion temperature profiles of Cr2O3–B powder compacts
with different stoichiometric ratios.
increasing B/Nb O5 ratio from 5.0 to 8.5, but declines slightly to
2
◦
1
343 C as the stoichiometric ratio further increases to 9.0. As men-
tioned above, the rise is due to an improvement in the borothermic
reduction of Nb O5 and the fall is caused by the dilution effect of
2
excessive boron.
When compared with those of the Nb O5–B samples, Fig. 5
2
reveals considerably lower flame-front temperatures varying
◦
between 675 and 830 C for the Cr O –B samples. This confirms
2
3
that the reaction of Cr O with boron is less exothermic than that
2
3
Fig. 6. XRD patterns of products synthesized from solid-state combustion of powder
compacts with molar ratios of Nb2O5:B = (a) 1:5, (b) 1:7, and (c) 1:9.
of Nb O5 with boron. In agreement with the composition depen-
2
dence of combustion velocity, the combustion front temperature of
the Cr O –B sample increases with boron concentration to a max-
2
3
imum at B/Cr O = 6 and then decreases with further increase of
boron.
given in reaction (2). The need of an excessive amount of boron
was partly to compensate the relatively low purity of amorphous
boron about 92%, and in part to account for the loss of boron due to
evaporation during the SHS process.
2
3
3.4. Composition and morphology analysis of combustion
For the solid-state combustion involving Cr O and boron,
products
2
3
Fig. 7(a), (b), and (e) indicates that single-phase Cr5B , CrB, and
3
CrB2 were practically produced from the powder compacts of
Fig. 6(a)–(c) plots XRD spectra of the products obtained from
B/Cr O = 4, 5, and 9, respectively. In agreement with that observed
the reactant compacts of Nb O5:B = 1:5, 1:7, and 1:9. The presence
2
3
2
in the synthesis of NbB , an additional amount of boron was
of NbO2 in Fig. 6(a) and (b) signifies partial reduction of Nb O5.
2
2
required for the production of pure Cr5B , CrB, and CrB when com-
A noticeable decrease in the peak intensity of NbO2 is observed
in Fig. 6(b), implying a substantial improvement in the reduction
of niobium oxides by the sample with a larger amount of boron.
In addition, as shown in Fig. 6(a), NbB is the only boride phase
formed from combustion. Fig. 6(b) indicates the formation of multi-
3
2
pared to the corresponding stoichiometric quantity. In addition,
Fig. 7(c) and (d) reveals that the reactant compacts of B/Cr O = 6
2
3
and 7 yielded the mixtures of CrB and CrB2.
As summarized in Table 2, the SHS process of the Cr O –B sam-
2
3
ples produces three boride phases, Cr5B , CrB, and CrB , and each of
ple boride compounds, including the dominant phase NbB and two
3
2
2
them can be obtained in pure form as mentioned above. It is useful
to note that the sample of Cr O :B = 3:14 produces a CrB–Cr5B
secondary phases NbB and Nb B . For the sample of Nb O5:B = 1:9,
3
4
2
Fig. 6(c) shows not only a full reduction of the metal oxide, but also
a complete conversion to single-phase NbB2.
2
3
3
composite rather than single-phase Cr B4 that represents the
3
A summary of the product composition with respect to the reac-
tant stoichiometry for the Nb O5–B samples is given in Table 1. An
2
Table 1
increase in the molar ratio of B/Nb O5 was found to enhance the
2
Summary of phase composition of synthesized products with respect to their initial
molar ratio of Nb O to B.
borothermic reduction of Nb O5, but a complete reduction was not
2
2
5
observed until the stoichiometric ratio reached up to B/Nb O5 = 8.
2
StoichiometryNb2O5:B
Phase composition of synthesized products
For the samples with a low boron content like Nb O5:B = 1:5 and
2
Dominant boride
Secondary boride(s)
Oxide
3
:16, NbB is the only boride formed along with a significant amount
of NbO . Within a composition range of B/Nb O5 = 6–8, the synthe-
1:5
3:16
NbB
NbB
NbB
NbB2
NbB2
NbB2
NbB2
NbB2
NbB2
NbO2
NbO2
NbO2
NbO2
NbO2
2
2
sized products consist of three boride compounds (NbB , Nb B ,
2
3 4
1
1
3
:6
:7
:22
Nb3B4, NbB2
Nb3B4, NbB
Nb3B4, NbB
Nb3B4, NbB
and NbB) and the dominant phase changes from NbB to NbB2 as
the ratio of B/Nb O5 exceeds 7. Further increase of the boron con-
2
tent to B/Nb O5 = 8.5–10 led to formation of single-phase NbB . It
2
2
1:8
1:8.5
is interesting to note that in this study the lowest amount of boron
required for the production of pure NbB is B/Nb O5 = 8.5, which is
1
1
:9
:10
2
2
greater than the stoichiometric quantity of B/Nb O5 = 7.33 (22/3)
2

3
70
C.L. Yeh, H.J. Wang / Journal of Alloys and Compounds 490 (2010) 366–371
boride phase based upon stoichiometric balance under such a start-
ing composition. For the powder compacts of B/Cr O = 6, 7, and
2
3
8
, the other composite product composed of CrB and CrB2 was
obtained.
A typical SEM micrograph is illustrated in Fig. 8, which reveals
the morphology of niobium borides synthesized from a sample
compact of Nb O5:B = 1:9. As displayed in Fig. 8, the product is
2
porous and consists of softly agglomerated boride grains with a
small particle size about 1–3 ␮m. The morphology of chromium
borides obtained from the Cr O –B samples is very similar to that
2
3
shown in Fig. 8.
. Conclusions
This study presents an experimental investigation on the prepa-
4
ration of borides of niobium and chromium through combustion
synthesis involving borothermic reduction of Nb O5 and Cr O .
2
2
3
The starting stoichiometry of the reactant compact was shown to
considerably affect the combustion behavior and the phase com-
position of the final product.
For the production of niobium borides, the SHS process was con-
ducted with Nb O5–B powder compacts formulated by a molar
2
ratio of B/Nb O5 between 5 and 10. The degree of borothermic
2
reduction of Nb O5 was improved by increasing the boron content
2
in the reactant mixture, which enhanced the reaction exother-
micity and hence accelerated the combustion wave. Complete
reduction of Nb O5 was achieved in the samples of B/Nb O5 ≥ 8,
2
2
below which the partial reduction yielded another oxide NbO2 in
the final products. Upon a full reduction of the niobium oxide, the
solid-state combustion of the sample with B/Nb O5 = 8 resulted in
2
formation of an NbB -dominated product with trivial amounts of
2
Fig. 7. XRD patterns of products synthesized from solid-state combustion of powder
compacts with molar ratios of Cr2O3:B = (a) 1:4, (b) 1:5, (c) 1:6, (d) 1:7, and (e) 1:9.
NbB and Nb B . Moreover, single-phase NbB was obtained from
3
4
2
the powder compacts with proportions of B/Nb O5 = 8.5, 9, and 10.
2
For the powder compacts of Cr O3 and boron, self-sustaining
2
Table 2
Summary of phase composition of synthesized products with respect to their initial
molar ratio of Cr2O3 to B.
combustion was feasible within a stoichiometry range from
B/Cr O = 4 to 9. Both combustion front temperature and velocity
2
3
increased with boron content, reached their maxima at B/Cr O = 6,
2
3
Stoichiometry Cr2O3:B
Phase composition of synthesized products
and decreased with further increase of the B/Cr O3 ratio. Dur-
2
Dominant boride
Secondary boride
ing the SHS process, Cr O3 was fully reduced by boron and three
2
1:4
3:14
1:5
1:6
1:7
1:8
1:9
Cr5B3
CrB
CrB
boride phases Cr B , CrB, and CrB were produced in either mono-
lithic or composite form, depending upon the initial composition
of the sample. Based upon the XRD analysis, the powder compacts
5
3
2
Cr5B3
CrB
CrB2
CrB
CrB
of B/Cr O = 4, 5, and 9 yielded single-phase Cr5B , CrB, and CrB ,
CrB2
CrB2
CrB2
2
3
3
2
respectively. Two types of the composite products, CrB–Cr5B3 and
CrB–CrB , were obtained from the samples of other stoichiometries.
2
This study demonstrates the feasibility of producing niobium
andchromium borides throughthe SHS process involvingborother-
mic reduction of the relevant metal oxides. This approach is
especially appropriate for the reaction system, in which formation
of the metal borides from direct combustion between metallic ele-
ments and boron is not feasible. In the future, this method can be
worthily applied to prepare the borides of iron, nickel, tungsten, and
manganese from the borothermic reduction of Fe O , NiO, WO ,
2
3
3
and MnO , respectively.
2
Acknowledgement
This research was sponsored by the National Science Council of
Taiwan, ROC, under the grants of NSC 97-2221-E-035-097.
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