Unique preparation of hexaboride nanocubes: A first example of boride formation by combustion synthesis

Article abstract of DOI:10.1111/j.1551-2916.2010.03853.x

Nanocubes of LaB₆ and Sm_0.8B₆ have been synthesized using low-temperature combustion synthesis, a technique that had only been used previously for the preparation of oxides. The hexaboride nanocubes were prepared using lanthanum nitrate or samarium nitrate, carbohydrazide, and boron powders. The furnace temperature for synthesis was kept at 320°C, lower than the typical temperature values used in combustion processes for the preparation of oxides. After synthesis, the nanocubes were characterized using X-ray diffraction, scanning electron microscopy, and X-ray photoelectron spectroscopy. The combustion process was analyzed using differential scanning calorimetry, which shows that the formation of the carbohydrazide and nitrate melts as well as the formation of a complex between metal ions and carbohydrazide are crucial steps for the reaction. The technique results in high-purity powders with a unique cubic morphology, in which the corners of the cubes can be used as point sources for efficient electron emission

Full text of DOI:10.1111/j.1551-2916.2010.03853.x

J. Am. Ceram. Soc., 93 [10] 3136–3141 (2010)
DOI: 10.1111/j.1551-2916.2010.03853.x
r 2010 The American Ceramic Society
ournal
J
Unique Preparation of Hexaboride Nanocubes: A First Example of
Boride Formation by Combustion Synthesis
Raghunath Kanakala,*,z,y Gabriel Rojas-George,y and Olivia A. Graeve*,w,z
zKazuo Inamori School of Engineering, Alfred University, Alfred, New York 14802
yDepartment of Chemical and Metallurgical Engineering, University of Nevada, Reno, Nevada 89557
Nanocubes of LaB6 and Sm0.8B6 have been synthesized using
low-temperature combustion synthesis, a technique that had
only been used previously for the preparation of oxides. The
hexaboride nanocubes were prepared using lanthanum nitrate or
samarium nitrate, carbohydrazide, and boron powders. The fur-
nace temperature for synthesis was kept at 3201C, lower than
the typical temperature values used in combustion processes for
the preparation of oxides. After synthesis, the nanocubes were
characterized using X-ray diffraction, scanning electron micros-
copy, and X-ray photoelectron spectroscopy. The combustion
process was analyzed using differential scanning calorimetry,
which shows that the formation of the carbohydrazide and ni-
trate melts as well as the formation of a complex between metal
ions and carbohydrazide are crucial steps for the reaction. The
technique results in high-purity powders with a unique cubic
morphology, in which the corners of the cubes can be used as
point sources for efficient electron emission.
always significant, with the lowest synthesis temperature of
4001C achieved only by using long processing times (B4 h) in
an autoclave.20 Our combustion synthesis process for producing
borides has a series of advantages, not the least of which are the
low temperatures required for processing. The technique is
highly efficient involving fast heating rates, short reaction times
of just a few seconds, and no need for a protected inert atmo-
sphere, while at the same time being easily scalable. For this
reason, it has been used very effectively for the synthesis of ox-
ides, when it is sometimes referred to as the glycine-nitrate pro-
cess. In this type of synthesis, an exothermic reaction occurs
between metal nitrates and a fuel in an oven, muffle furnace, or
hot plate. For clarification, our synthesis process is not a subset
of the well-known self-propagating high-temperature synthesis
technique,22 which makes use of self-sustaining solid-state reac-
tions between metal and boron powders for making ceramics
such as TiB2 and ZrB2, both of which result in very high exo-
thermic heats of reaction. Our technique makes use of nitrate
salts and an organic fuel mixed in the form of a slurry, resulting
in intimate atomic level mixing upon formation of a melt, as will
be described later, thus, permitting the formation of very ho-
mogeneous compounds. Thus, our process for making borides is
truly unique and with the additional advantage that the reac-
tants are easy to handle and do not require protected atmo-
spheres during synthesis.
I. Introduction
ORIDE compounds have proven to be essential for myriad
B
applications such as reinforcing phases in metal–matrix
composites1–3 and armor materials.4 One particular subgroup
in this large class of materials includes the hexaborides, which
are being used as additives for wear- and corrosion-resistant
engineering components,5 for decorative and thermionic coat-
ings,6 and for electron emission sources.7 Here, we demonstrate
the first instance of the preparation of boride materials by an
efficient combustion synthesis technique at a temperature of
3201C. Combustion synthesis has been utilized since the 1990s
for preparing many types of nanocrystalline oxides, as has been
shown in countless studies.8–12 However, boride materials were
thought to be unfeasible by this technique. We have selected two
compounds, the hexaborides LaB6 and Sm0.8B6, as model mate-
rials for representing the effectiveness of the process for the
preparation of borides. The technique results in high-purity pow-
ders with a unique cubic morphology, in which the corners of the
cubes can be used as point sources for efficient electron emission.
Significant efforts have been directed toward the preparation
of boride materials,13 which for hexaborides include the molten
salt technique,14 borothermic reduction of oxides at varying
pressures,15–17 aluminothermic reduction using aluminum
melts,18 synthesis in autoclaves,19,20 and thermal plasmas.21
However, the synthesis temperatures and processing times are
II. Experimental Procedures
The reactant chemicals for the synthesis included La(NO3)3 ꢀ
6H2O (99.99%; Alfa Aesar, Ward Hill, MA), Sm(NO3)3 ꢀ 6H2O
(99.99%; Alfa Aesar), CO(NHNH2)2 (98%; Sigma-Aldrich,
St. Louis, MO), and boron powders (Noval Industrial Group,
Shandong, China). These compounds were thoroughly hand-
mixed using a mortar and pestle in the order of nitrate, car-
bohydrazide, and boron powders. Once mixed, the reactant
chemicals were spread as a thin layer at the bottom of a ce-
ramic crystallization dish. The crystallization dish, along with
the precursor materials, was introduced into a muffle furnace at
room temperature. The furnace was heated along with the re-
actant chemicals at a moderate rate of B301–351C per minute
until the combustion reaction occurred at B3201C. A typical
reaction consists of 10.625 g of La(NO3)3 ꢀ 6H2O, 1.592 g of
boron, and 0.553 g of carbohydrazide, for the case of LaB6
synthesis, and 10.326 g of Sm(NO3)3 ꢀ 6H2O, 1.507 g of boron,
and 0.523 g of carbohydrazide, for the case of Sm0.8B6 synthesis.
The fuel-to-oxidizer ratio, f, for our reactions was 0.08, which is
below the stoichiometric ratio of 0.53, making our mixtures
significantly fuel lean. These reactant amounts correspond to
stoichiometric quantities for obtaining 5 g of either LaB6 or
Sm0.8B6 after the reaction is completed. The washing process
involved a series of steps: (1) cleaning with 80 mL of HCl and
20 mL of H2O for every 1 g of the powder, and centrifuging at
4000 rpm for 30 min, (2) washing with 100 mL deionized water
and centrifuging, (3) cleaning with 50 mL H2SO4 and 50 mL H2O
for every 1 g of the powder, followed by centrifuging, (4) washing
K. Watari—contributing editor
Manuscript No. 27482. Received January 29, 2010; approved April 8, 2010.
*Member, The American Ceramic Society.
This work was financially supported by grants from the U.S. Army Research Office
under contract numbers W911NF-06-1-0226 and W911NF-08-1-0330 with Dr. William
Mullins and Dr. David Stepp as project managers, by request of the Defense Advanced
Research Projects Agency, with Dr. Leo Christodoulou and Dr. Cindy Daniell as program
managers.
wAuthor to whom correspondence should be addressed. e-mail: graeve@alfred.edu
3136

October 2010
Unique Preparation of Hexaboride Nanocubes
3137
Fig. 1. Crystal structure and X-ray diffraction (XRD) patterns of hexaboride nanocubes. (a) Cubic crystal structure of hexaboride compounds,
(b) XRD of as-synthesized powders of Sm0.8B6, (c) XRD of as-synthesized powders of LaB6, (d) XRD of acid-washed powders of Sm0.8B6, and (e) XRD
of acid-washed powders of LaB6.
with deionized water and centrifuging, and (5) air drying. We
used diluted hydrochloric acid (12.1M, Fisher Scientific, Pitts-
burgh, PA) and diluted sulfuric acid (18M, Fisher Scientific).
Characterization of the powders was carried out by X-ray
diffraction (XRD) using a Philips PW 1800 system (Koningkl-
bohydrazide fuel resulting in an exothermic reaction, according
to the following chemical reaction:
D
LnðNO3Þ þ 6B þ XðH2NNHÞ2C ¼ O ꢁ! LnB6 þ xN2
3 þ yH2O þ zCO2 þ other gases Ln ¼ La or Sm
yke Phylips Electronics N.V., Eindhoven, the Netherlands),
¨
scanning electron microscopy (SEM) using an FEI Quanta
200F system (FEI Company, Hillsboro, OR), X-ray photoelec-
tron spectroscopy (XPS) using a PHI Quantera SXM system
(Physical Electronics Inc., Chanhassen, MN), and differential
scanning calorimetry (DSC) using a TA Instruments DSC Q10
(TA Instruments, New Castle, DE). XRD measurements were
completed using CuKa radiation without subtraction of the Ka2
peak by placing powders on a zero-background holder and
slightly tapping them into place to form a smooth surface. The
SEM samples were prepared by dispersing the powders in
acetone, drop coating the slurry onto a silicon wafer, and
sputter-coating with carbon. The XPS measurements were per-
formed using a 200 mm diameter/50 W/15 kV beam. The depth
Our interest in hexaboride materials stems from their unique
crystal structure (Fig. 1(a)). The lattice is simple cubic and con-
sists of boron octahedra at each corner of the cube bonded at the
apexes. The octahedra have a coordination number of six and
consist of the boron atoms, with four adjacent neighbors in ev-
ery octahedron for every boron atom, and one on the main axis
of the cube. The valence electrons of boron are distributed over
five bonds. The metal atom is located in the middle of the unit
cell with no bonds to the boron atoms. Thus, the metal atoms
provide free electrons imparting a metallic character to most of
these materials, although some are known to be semiconductors.
This, together with the strong bonds between the boron atoms in
the framework, produces a series of compounds that have high
thermal and chemical stabilities, and low work functions for
electron emission applications.
Figures 1(b)–(e) illustrates the XRD patterns for our as-syn-
thesized and washed LaB6 [PDF #00-0340427] and Sm0.8B6
[PDF #00-056-1206] powders. [Correction added after online
publication June 9, 2010: corrected first PDF # in preceding
sentence.] From Figs. 1(b) and (c), we can confirm the presence
of the desired hexaboride powders. Both sets of powders also
contain borate phases of LaBO3 or SmBO3, as noted on the
diffraction pattern. Also, a small amorphous hump is observed
˚
profile for the XPS equipment is B20 A. DSC analysis was
carried out at a 201C/min heating rate in alumina cups. The
boron powders were also characterized by BET surface area
analysis using a Micromeritics (Micromeritics, Norcross, GA)
Tristar surface area and porosity analyzer.
III. Results and Discussion
For the synthesis of the LaB6 and Sm0.8B6, lanthanum nitrate or
samarium nitrate and boron powders were mixed with car-

3138
Journal of the American Ceramic Society—Kanakala et al.
Vol. 93, No. 10
Fig. 2. Scanning electron microscopy of (a) rhombohedral boron powders, (b) as-synthesized LaB6 powders, and (c, d) acid-washed LaB6 powders.
between B401 and 551 and is characteristic of the as-received
boron powders, of which there are some that are left unreacted
after synthesis. Compound stability between La, B, and O, can
result in an orthorhombic phase [PDF #00-012-0762] at lower
temperatures, and a monoclinic phase at temperatures greater
than approximately 15001C [PDF #01-073-1149 or PDF #00-
gonal phase [PDF #00-013-0479] is stable at lower tempera-
tures, and a low-symmetry phase [PDF #00-013-0489], of un-
known structure, at higher temperatures.23 During the synthesis
of the LaB6 phase, we obtain powders that contain a mixture of
LaB6, orthorhombic LaBO3, and monoclinic LaBO3 (minor
phase). The orthorhombic LaBO3 phase is predominant over
the monoclinic LaBO3 phase because, even if the flame temper-
ature during synthesis is high (typical combustion synthesis re-
actions exceed 10001C), the reactants are at the higher
temperatures only for a very short time. During synthesis of
the Sm0.8B6 phase, we obtain powders with a mixture of Sm0.8B6
and the high-temperature SmBO3 phase. The formation of the
high-temperature phase of SmBO3, instead of the hexagonal
form, is not clear at this point.
013-0571]. For the case of Sm, B, and O,
a hexa-
Borates are soluble in hydrochloric acid, whereas hexaborides
only dissolve in concentrated sulfuric and/or nitric acid.24,25
Thus, the as-synthesized powders can be cleaned with hydro-
Fig. 3. X-ray diffraction patterns for LaB6 powders prepared using
higher fuel-to-oxidizer, f, ratios of 0.25 and 0.33. Note that these f
values are still below the stoichiometric f value of 0.52.
Fig. 4. Differential scanning calorimetry of the combustion reaction
between lanthanum nitrate, boron, and carbohydrazide.

October 2010
Unique Preparation of Hexaboride Nanocubes
3139
chloric acid in order to remove the small amount of borate
impurities and then in diluted sulfuric acid to remove the un-
reacted boron and obtain cuboids of single crystals with cube
size dimensions around 500 nm, as illustrated in Figs. 2(c) and
(d). Hexaborides slowly decompose in sulfuric acid26; thus, some
loss of surface material occurs during exposure to sulfuric acid,
with the outcome that only the stable cubic morphology is re-
tained, with the faces corresponding to the lower energy {100}
surfaces.27 Once the acid washes are complete, the LaB6 pow-
ders are free of impurity phases (Fig. 1(e)), while the Sm0.8B6
powders exhibit only an extremely small residue of SmBO3
(Fig. 1(d)). The powder yield is about 10%. We are currently
working on increasing the yield by improving the cleaning pro-
cess, so as not to loss as much powder during the acid washing
steps.
Fig. 5. Two different representations of the carbohydrazide/metal
complex, which forms during heating of the reactants at a temperature
of approximately 1501–2001C.
The synthesis of the hexaborides was accomplished using
significantly fuel-lean (f 5 0.08) mixtures, which are close to
Fig. 6. Proposed mechanism for the preparation of hexaborides using the combustion synthesis process.

3140
Journal of the American Ceramic Society—Kanakala et al.
Vol. 93, No. 10
Fig. 7. X-ray photoelectron spectroscopy of (a) La 3d signal in LaB6, (b) B 1s signal in LaB6, (c) Sm 3d signal in Sm0.8B6, and (d) Sm 1s signal in
Sm0.8B6.
seven times lower than the stoichiometric fuel-to-oxidizer ratio
of 0.52. As f is increased from 0.08 to 0.25 and 0.33 (Fig. 3) for
the case of LaB6 synthesis, the peaks corresponding to LaBO3
become much more significant, compared with the LaBO3 peaks
found for the samples prepared using f 5 0.08 (Fig. 1(c)). Fur-
ther increasing f to the stoichiometric value results in the ex-
pected oxide material (i.e., La2O3), as is typically seen in
combustion synthesis reactions (not shown). Thus, we confirm
that the heat of reaction must be kept as low as possible in order
to promote the formation of the borides. The morphologies of
the powders at different stages of the process are illustrated in
Fig. 2. Figure 2(a) corresponds to the as-received rhombohedral
boron powders. The boron powders have a fibrous morphology
with a surface area of 17.1270.08 m2/g, which we determined
from surface area analysis. The as-synthesized powders
(Fig. 2(b)) containing the unreacted boron, borate, and hexabo-
ride phases are large, with particle sizes between 1 and 5 mm.
After acid cleaning, the cubic morphology is evident (Figs. 2(c)
and (d)).
furnace temperature than is lower than what we used. However,
we did not test for this because of the large errors associated
with temperature measurements in our large muffle furnace,
which has errors as high as 7501C. Subsequent exothermic
peaks represent the combustion and crystallization processes.
The significant exothermic peaks observed at 3391 and 3791C
are due to completion of the combustion. All of these steps need
to be completed in turn and with enough time for the hexaboride
formation to become favorable. If the heating rate is too high,
then the final combustion reaction occurs faster than what is
required for melting and full mixing of the reactants. Also, the
mixing of the reactant powders in the mortar and pestle has to
be accomplished in a specific order. If the boron powders are
mixed with the nitrates before adding the fuel, the boron covers
the nitrate salts, preventing direct contact between the nitrate
oxidizer and the carbohydrazide and impeding the reaction.
However, if the powders are mixed in the correct order: nitrate
and fuel first, then boron, the reaction proceeds successfully.
Our proposed mechanism is illustrated fully in Fig. 6.
In a conventional combustion synthesis reaction, the reac-
tants are placed in a preheated muffle furnace at temperatures
B5001C; thus, the heating rate is extremely high and results in a
reaction within a couple of minutes, depending on the amount
of water that needs to be evaporated, because the reaction does
not begin until all the water is removed from the system. In our
case, if the experiment is completed using the above procedure,
we obtain La2O3 and borates, with no evidence of hexaborides.
However, if the heating of the reactants is attempted at a max-
imum furnace temperature of 3201C, the reaction results in the
desired hexaboride compounds. This mechanism can be ex-
plained from DSC analysis (Fig. 4). During heating, a series
of both endothermic and exothermic peaks are observed. First,
an endothermic peak is observed at 761C due to melting of the
nitrates,28 followed by a significant endothermic peak at 1701C
due to melting of the carbohydrazide,28 the formation of a com-
plex between carbohydrazide and metal ions,29 and dehydration.
The carbohydrazide has two amine groups (one on each end of
the molecule) and can form a complex with La31 ions, as illus-
trated in Fig. 5. If this mechanism describes our process accu-
rately, then the combustion process can probably occur at a
To determine surface species on the powders, we have used
XPS. The representative spectra for LaB6 (Figs. 7(a) and (b))
illustrates the XPS signal of La 3d at 837.9 (La 3d5/2) and 854.5
(La 3d3/2), and the B 1s peak at 188.2 eV, in agreement with the
data obtained in earlier reports19,20,30 for lanthanum and boron
bonding in LaB6. Based on the scaling of the peak area, the
atomic ratio of La to B is 1 to 5.96, which confirms the chemical
stoichiometry of LaB6. Small amounts of nitrogen, oxygen, and
carbon, are also detected and are likely due to the adsorption of
nitrogen, oxygen, and carbon dioxide gases, on the surfaces of
the powders. The signal for lanthanum and boron is very strong,
which implies that the oxygen signal is from the very top surface
layer, and if there is an oxide coating, it is thin enough that it
does not interfere with the electrons from the underlying ions to
emerge unaltered. The representative spectra for Sm0.8B6 (Figs.
7(c) and (d)) illustrate the XPS signal of Sm 3d at B1084 eV and
the B 1s peak at 188 eV for boride bonding in SmB6. The peak(s)
at B193 eV can be possibly evaluated as B–O.
In summary, a low-temperature combustion synthesis meth-
odology for the synthesis of nonoxide ceramics has been devel-
oped. Combustion synthesis is a technique that has been

October 2010
Unique Preparation of Hexaboride Nanocubes
3141
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hexaboride materials has been investigated. Our results show
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