A new route for the synthesis of NdB6 powder from Nd2O3-B4C system

Article abstract of DOI:10.1016/j.jallcom.2006.05.084

The neodymium hexaboride NdB₆ powder has been synthesized by the reduction of Nd₂O₃ with B₄C from Nd₂O₃-B₄C system under the conditions of high temperature and high vacuum. Thermodynamic and dynamic analyses show that synthetic temperature and partial pressure in the furnace are the most important determinants during the synthesis. According to the results of thermodynamic calculation and differential scanning calorimetry (DSC), the synthetic process was determined. The compositions of products achieved at different temperatures and different holding times were identified by X-ray diffraction (XRD). The characters of powder were observed by scanning electronic microscope (SEM). It is concluded that after a series of pretreatments, NdB₆ powders are fabricated from Nd₂O₃-B₄C system under the conditions of 10^-2 Pa vacuum and 4 h holding time with synthetic temperature above 1773 K. The diameter of NdB₆ ranges from 2 to 4 μm.

Full text of DOI:10.1016/j.jallcom.2006.05.084

Journal of Alloys and Compounds 431 (2007) 337–341
A new route for the synthesis of NdB powder from Nd O –B C system
6
2 3
4
∗
Y. Liu, W.J. Lu , J.N. Qin, D. Zhang
State Key Laboratory of Metal Matrix Composites, Shanghai Jiao Tong University, 1954 Huanshan Road, Shanghai 200030, PR China
Received 3 May 2006; received in revised form 22 May 2006; accepted 23 May 2006
Available online 30 June 2006
Abstract
The neodymium hexaboride NdB
6
2 3 4 2 3 4
powder has been synthesized by the reduction of Nd O with B C from Nd O –B C system under the
conditions of high temperature and high vacuum. Thermodynamic and dynamic analyses show that synthetic temperature and partial pressure
in the furnace are the most important determinants during the synthesis. According to the results of thermodynamic calculation and differential
scanning calorimetry (DSC), the synthetic process was determined. The compositions of products achieved at different temperatures and different
holding times were identified by X-ray diffraction (XRD). The characters of powder were observed by scanning electronic microscope (SEM). It
−
2
is concluded that after a series of pretreatments, NdB
and 4 h holding time with synthetic temperature above 1773 K. The diameter of NdB
2006 Elsevier B.V. All rights reserved.
6
powders are fabricated from Nd
2 3 4
O –B C system under the conditions of 10 Pa vacuum
6
ranges from 2 to 4 ␮m.
©
Keywords: Nd2O3; B4C; NdB6; High temperature; High vacuum
1
. Introduction
electrolysis method [10]. The element sintering method is syn-
thesis of a mixture of elemental boron and rare earth under the
condition of extra high temperature with low efficiency. The
reaction time is usually more than 10 h. The molten salt elec-
trolysis method is a synthesis of depositing the molten salts
like boron oxide and rare earth oxide. However, it is diffi-
cult to separate the product of the molten salt because there
are too many impurities. Furthermore, floating zone method
[10,11] and aluminium flux method have been studied for
Borides have many specific physical and chemical proper-
ties because of electron deficiency of boron. In particular, rare
earth hexaborides (RB ) have attracted much attention in both
scientific and technical fields. RB show lots of different spe-
cial properties, such as superconductivity [1], high efficiency
thermionic emission and narrow band semiconductivity, etc.
6
6
[
[
2,3], which have been widely used in many industrial sectors
4,5]. Therefore RB play important roles in industries and their
obtaining crystals of NdB . But powders of NdB need to
6
6
6
synthesis is significant.
Neodymium hexaboride (NdB ) has a cubic CaB type struc-
be prepared prior to the crystal growth by the floating zone
method. Although high quality NdB could be fabricated from
6
6
6
ture, space group Pm-3m, with simple cubic Nd atoms centered
by a boron atom octahedron [2,6,7]. Fig. 1 shows this unique
these methods, they are too expensive to be widely used in
industry.
structure of NdB . Especially, there are two different B–B bonds
In conclusion, each synthetic method mentioned above is not
6
in this structure, inter-octahedron and intra-octahedron, on the
edge of the octahedron and between the octahedron which dif-
fer in B–B distance. Considering about vacancies of Nd atom,
suitable for producing NdB powder in industry. People expect
6
to prepare NdB powder with low price and high purity at the
6
sametime. Twofeasiblemethodsareconsidered:oneistoreduce
the cost to fabricate, the other is to use cheap reactants. Com-
pared to substance neodymium, with tremendous advantages
in price, Nd2O3 is a judicious selection of reactants. Including
frame element boron and element carbon with high reducibility,
B4C is another fitting reactant, which could be easily found with
NdB could exist in a homogeneity region with compositions
6
ranging from NdB6.01 to NdB8.2 [8,9].
Many synthetic methods have been developed for NdB pow-
6
der, such as the element sintering method and the molten salt
low cost. With Nd2O3 and B4C, NdB could be directly formed
6
∗
from elements Nd and B. The other elements exist in the form of
CO gas without any solid residues, which can be easily removed
Corresponding author. Tel.: +86 21 62933106; fax: +86 21 62822012.
E-mail addresses: luweijie@sjtu.edu.cn, lvweijie@yahoo.com (W.J. Lu).
0
925-8388/$ – see front matter © 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2006.05.084

3
38
Y. Liu et al. / Journal of Alloys and Compounds 431 (2007) 337–341
Fig. 2. The changes of Gibbs free energy ꢀG as a function of temperature for
reaction (1).
Fig. 1. Crystal structure of neodymium hexaboride (NdB6).
5
T ≥ 1395 K, ꢀG = 6.951 × 10 − 228.164T
−
3 2
+
22.235T ln T − 8.324 × 10 T − 3.825
by the procedure of self-volatilization in order to get pure NdB6
powder.
PCO
−1
×
105T + 3RT ln
5
1
.01 × 10
In this paper, a new route to synthesize pure NdB powder
6
from Nd2O3–B4C system by the reduction of Nd2O3 with B4C
has been proposed, which may provide a new way to fabricate
NdB powder in industry. The synthesis mechanism was dis-
cussed. The microstructure and phase were investigated in this
paper.
The changes of Gibbs free energy versus synthetic temper-
ature for reaction (1) under the conditions of different partial
pressure of CO (g) is plotted in Fig. 2. ꢀG decreases sharply by
increasing synthetic temperature or by reducing the partial pres-
sure. Therefore, synthetic temperature T and partial pressure
of CO (g) in the furnace are the most important determinants
of feasibility of reaction (1). In addition, the temperature of
equilibrium (ꢀG = 0) decreases notably by reducing the partial
pressure. In the view of dynamics, dismissing carbon monoxide
gas under the condition of low partial pressure gives advantages
to synthesis, which makes reaction move toward right side.
6
2
. Theoretical basis
In this work, NdB powder is synthesized utilizing Nd2O3
6
powder and B4C powder by the following reaction:
−2
Nd2O3 + 3B4C = 2NdB + 3CO (g)
(1)
Therefore, this work took high vacuum 10 Pa as process
condition.
While increasing the synthetic temperature under the condi-
tion of 10 Pa, ꢀG remains in positive until 1446 K. It indicates
6
Furthermore, the thermodynamic feasibility of reaction (1) has
been considered in this paper, which depends on ꢀG. ꢀG is
defined as:
−2
that NdB could be synthesized above 1446 K. Namely, NdB
6
6
would be synthesized by reaction (1) under the condition of
ꢀ
G = 3GCO + 2GNdB6 − 3GB C − G
4
Nd2O3
−2
10
Pa with the synthetic temperature above 1446 K.
PCO
+
3RT ln
(2)
(3)
5
3
. Experimental procedure
1.101 × 10
Nd2O3 powder(99.0%, purity)andB4Cpowder(99.0%, purity)wereusedas
ꢀ
ꢀ
G = ꢀH − TꢀS
raw materials. Since the strong affinity to water, two powders were dried at 423 K
for 2 h at first. Then they (which were weighed on ratio of Nd2O3:B4C = 2:1)
were mixed together uniformly by ball milling (200 rpm for 4 h). Columnar sam-
ples (Ø38 mm × 15 mm) were made on Omnipotence Experimental Machine.
The samples were placed in a vacuum carbon furnace and heated electrically
with a carbon heater. The high vacuum synthesis procedure was to decrease the
pressure in furnace to the required level, then increase the temperature to the
required value. During high temperature synthesis, the pressure in the furnace
Based on theory of thermodynamics, the Gibbs free energy
G of reaction (1) has been calculated using the thermodynamic
data from Refs. [12–14]. The results of the Gibbs free energy
ꢀ
G for reaction (1) are listed as following:
5
T ≤ 1395 K, ꢀG = 7.489 × 10 − 159.622T
−
2
was held on 1 × 10 Pa level. After holding the required high temperature and
desired high vacuum conditions for hours, the samples were cooled to room
temperature prior to the release of the applied pressure. Finally, target powders
were made after grinding process. The synthetic temperature and holding time
of products are listed in Table 1.
−
3 2
+
12.705T ln T − 4.764 × 10 T − 5.245
PCO
−1
×
105T + 3RT ln
5
1
.01 × 10

Y. Liu et al. / Journal of Alloys and Compounds 431 (2007) 337–341
339
Table 1
Nomenclature and process conditions of Nd2O3 and B4C mixtures
Sample
Synthetic
temperature (K)
Holding
time (h)
Partial
pressure (Pa)
−
−
−
−
−
2
2
2
2
2
1
2
3
4
5
1573
1673
1673
1673
1773
4
1
2
4
4
10
10
10
10
10
The measure of heat transformation during synthesis was carried on NET-
ZSCH differential scanning calorimetry 404 (DSC). Phase identification of the
products was performed by a D/max 2550 V X-ray diffraction analysis (XRD).
The characters of powder were observed by Philips XL30 W/TMP scanning
electronic microscope (SEM) with energy dispersive X-ray analysis (EDX).
4
. Results and discussion
Fig. 3. DSC curve of the mixture of Nd2O3 powder and B4C powder under the
condition of 10 Pa vacuum.
−
2
4
.1. DSC and dynamic morphology analysis
Nd(BO2)3 and Nd2CO5 are fabricated around 561 K when
NdBO3 is fabricated around 696 K, which give us a clearly view
of the dynamic morphology during synthesis.
Fig. 4 illustrates the morphology model of reaction (1).
Firstly, phase A (neodymium oxide) and phase B (boron carbide)
were mixed uniformly together at stage (a). Then phase B dis-
missed into phase A to form new intermediate phases including
DSC analysis of the mixture of Nd2O3 powder and B4C pow-
−
2
der under the condition of 10 Pa vacuum was performed in
order to find the actual temperature of equilibrium and to get a
clearly view on morphology of reaction at different steps, which
is shown in Fig. 3. From the DSC curve of the mixture of Nd2O3
powder and B4C powder, two obvious endothermic peaks were
detected at the temperatures of 561 and 696 K. Moreover, the
most obvious endothermic peak appears around 1658 K. It illus-
trates that Nd2O3 and B4C will react vigorously around 1658 K.
Reaction (1) is carried out at the temperature in which
neodymium oxide, boron carbide and their intermediate phases
are not melted. Namely, reaction (1) is a traditional type of
solid–solid reaction.
Due to the solid–solid reaction type of reaction (1), the dif-
fusion speed between two phases is very slow. Therefore, the
actual temperature of equilibrium is much higher than what we
got from calculations of thermodynamics. Concern about the
results from thermodynamic calculations and DSC curve, pure
Nd(BO2)3, Nd2CO and NdBO3. After the complete function
5
of diffusion between these phases, phase C (NdB ) was finally
6
synthesized at stage (c). Stage (b) took the most part of the whole
procedure, which was the most important stage during synthe-
sis. So controlling of the process conditions during stage (b)
is a key to synthesize NdB , which includes temperature con-
6
trol, holding time control, resultant gas’s partial pressure control.
This paper explored the effect of different process conditions on
synthesis.
4.2. Effect of different process conditions on synthesis
−
2
NdB may be fabricated under the conditions of 10 Pa vacuum
with synthetic temperature above 1658 K from the mixtures of
Nd2O3 powder and B4C powder.
Fig. 5 shows the X-ray diffraction patterns of samples 1–5
6
−
2
fabricated under the conditions of 10 Pa vacuum with differ-
ent synthetic temperatures for hours’ holding time. Diffraction
According to the former researches on La2O3–B4C system
peaks of NdB , Nd2O3, B4C and NdBO3 are found in samples
6
[
15] and Nd2O3–B system [10], the intermediate phases dur-
1–4. Moreover, NdB diffraction peaks without any other impu-
6
ing synthesis are Nd(BO2)3, Nd2CO and NdBO3. Namely,
rity can be observed in the XRD patterns of sample 5. Products
5
Fig. 4. Morphology model of reaction (1). (a) Mixing stage: phase A (Nd2O3) and phase B (B4C) are mixed uniformly together; (b) synthesizing stage: phase B
dismissed into phase A to form new intermediate phases; (c) final stage: phase C (NdB6) was finally synthesized.

3
40
Y. Liu et al. / Journal of Alloys and Compounds 431 (2007) 337–341
Table 2
Products obtained for each synthetic temperature and holding time
Sample
Synthetic
temperature (K)
Holding
time (h)
Products
1
2
3
4
5
1573
1673
1673
1673
1773
4
1
2
4
4
NdB6, Nd2O3, B4C, NdBO3
NdB6, Nd2O3, B4C, NdBO3
NdB6, Nd2O3, B4C, NdBO3
NdB6, Nd2O3, B4C, NdBO3
NdB6
different samples resulted from a decrease of the holding time.
Furthermore, reaction (1) is a traditional type of solid–solid reac-
tion. An appropriate holding time is of great advantage to get
NdB withoutanyenergywasting. Comparedwiththeothers, 4 h
6
holding time would be the most optimal value we adopted. It is
suitable for producing NdB in industry, which is long enough to
6
Fig. 5. XRD patterns of samples fabricated under the conditions of 10 2 Pa
vacuumwithdifferentsynthetictemperaturesforhours’holdingtime:(1)1573 K
for 4 h; (2) 1673 K for 1 h; (3) 1673 K for 2 h; (4) 1673 K for 4 h; (5) 1773 K
for 4 h.
−
complete synthesizing NdB6 without any excess energy wasting.
Concerning XRD patterns of samples 1, 4, 5 fabricated under
−2
the conditions of 10 Pa vacuum at 1573, 1673 and 1773 K for
4
h holding time in Fig. 5, it is evident that there is significant
increasing in the NdB peak intensities of different samples,
6
obtained for each synthetic temperature and holding time are
shown in Table 2.
which resulted from an increase of synthetic temperature. It can
be concluded that less intermediate phase NdBO3 exist in the
As to samples 2–4, which were fabricated under the condi-
products and more target phase NdB are fabricated when the
synthetic temperature increase.
6
−2
tions of 10 Pa vacuum at 1673 K for 1, 2, and 4 h, it can be
clearly seen that compositions of their products depend on the
holding time during synthesis. There is reducing in the NdB6
peak intensities and increasing in the NdBO3 peak intensities of
From Table 2 and Fig. 5, it can be concluded that the optimal
process conditions to fabricate pure NdB powder by reaction
6
−
2
(1) from Nd2O3–B4C system are 10 Pa vacuum, 4 h holding
Fig. 6. (a) SEM micrographs of NdB6 particles in sample 5; (b) magnified image of large-sized particle in (a); (c) magnified image of large-sized particle in (a); (d)
EDX on spectrum 1 in (a).

Y. Liu et al. / Journal of Alloys and Compounds 431 (2007) 337–341
341
time and synthetic temperature of 1773 K which is applied to
dynamic analysis and is proved by experiment. The syn-
sample 5. After a series of pretreatments, NdB powders can
thetic temperature for synthesis of NdB decreases notably
6
6
be fabricated from Nd2O3–B4C system under the conditions of
by reducing the partial pressure of CO (g).
−2
1
0
Pa vacuum and 4 h holding time with synthetic temperature
above 1773 K.
Acknowledgements
4
.3. Microstructure of NdB particles
We would like to acknowledge the financial support provided
by A Foundation for the author of National Excellent Doctoral
Dissertation of PR China under Grant no: 200332, Research
Fund of Science and Technology Commission of Shang-
hai Municipality under Grant nos: 04DZ14002, 04QMX1412,
0452nm045 and Aeronautics Foundation of China under Grant
no. 03G57007.
6
Fig. 6 shows the scanning electronic microscope micrographs
and energy dispersive X-ray analysis of NdB particles, which
6
are fabricated from sample 5. Fig. 6(d) shows the results of
EDX with elements of Nd and B. Combined with XRD patterns
shown in Fig. 5, the particles shown in Fig. 6(a) are confirmed
to NdB . In Fig. 6(a), particles of NdB in different diameters
6
6
are observed, ranging from 1 to 8 ␮m. Then magnified image
of large-sized particle is taken, which is shown in Fig. 6(b).
Magnified image of small-sized particle is shown in Fig. 6(c).
They demonstrate that the large-sized particles with diameter
from 4 to 8 ␮m are set up by small-sized particles with diameter
from 2 to 4 ␮m. Furthermore, phenomenon of sintering is found
between these small-sized particles, which make these particles
like a single one. According to the investigations of Fig. 6(c),
it can be concluded that the diameter of small-sized particles
shown in Fig. 6(a) ranges from 2 to 4 ␮m.
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(
(
(
1) After a series of pretreatments, NdB powders are fabricated
6
[
−
2
from Nd2O3–B4C system under the conditions of 10 Pa
vacuum and 4 h holding time with synthetic temperature
above 1773 K.
(
[
2
2) The investigation of SEM shows that the diameter of small-
[
sized particles of NdB range from 2 to 4 ␮m. The large-
6
1
sized particles with diameter from 4 to 8 ␮m are set up by
these small-sized particles.
[
[
13] E.K. Storms, J. Phys. Chem. 85 (1981) 1536–1540.
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3) NdB will be synthesized easily by increasing synthetic
6
[
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temperature, which is advanced by thermodynamic and