A convenient inorganic solvent thermal route to nanocrystalline tantalum diboride

Article abstract of DOI:10.1246/cl.2008.510

Nanocrystalline TaB₂ has been prepared via a simple inorganic solvent thermal route by the reaction of metallic magnesium powders with tantalum pentoxide and boric acid in an autoclave at 650°C. X-ray powder diffraction patterns indicate that the product was hexagonal tantalum diboride. Scanning electron microscopy images show that it consisted of particles with an average size of about 40 nm. The product was also studied by BET and TGA. Copyright

Full text of DOI:10.1246/cl.2008.510

510
Chemistry Letters Vol.37, No.5 (2008)
A Convenient Inorganic Solvent Thermal Route to Nanocrystalline Tantalum Diboride
Jianhua Maꢀ1 and Yihong Du2
1College of Chemistry and Materials Engineering, Wenzhou University, Wenzhou, Zhejiang 325027, P. R. China
2City College, Wenzhou University, Wenzhou, Zhejiang 325035, P. R. China
(Received January 30, 2008; CL-080104; E-mail: mjh820@ustc.edu)
Nanocrystalline TaB2 has been prepared via a simple inor-
above inorganic solvent (about 15 g). Then, the mixture was put
into a stainless steel autoclave. After sealing under argon
atmosphere, the autoclave was heated at 650 ꢁC for 10 h,
followed by cooling to room temperature in the furnace. The
product obtained from the autoclave was washed several times
with absolute ethanol, dilute aqueous HCl solution, and distilled
water to remove impurities. Finally, the product was washed
three times with absolute ethanol to remove water. The final
product was vacuum-dried at 60 ꢁC for 12 h. Black powders
were obtained.
ganic solvent thermal route by the reaction of metallic magnesi-
um powders with tantalum pentoxide and boric acid in an auto-
clave at 650 ꢁC. X-ray powder diffraction patterns indicate that
the product was hexagonal tantalum diboride. Scanning electron
microscopy images show that it consisted of particles with an
average size of about 40 nm. The product was also studied by
BET and TGA.
The obtained sample was analyzed by powder X-ray diffrac-
tion (XRD) on a Rigaku Dmax-ꢀA X-ray diffractometer using
Cu Kꢁ radiation (wavelength ꢂ ¼ 1:54178 A). The morphology
Tantalum diboride (TaB2), like borides of Ti, Zr, Cr, and
other transition metals, is well known for its high melting point,
outstanding hardness, high thermal conductivity, good chemical
stability, extreme refractoriness, and good wear resistance.1,2
Because of these attractive properties, TaB2 has extensive
applications in many fields, and it is a diboride with the same
favorable characteristics as other refractory borides. For exam-
ple, they have been used as high-temperature materials, sur-
face-protection materials, and wear-resistant materials. Besides
these properties, it is meaningful to synthesize nanocrystalline
TaB2 as the nanosized powders have other unique properties,
such as the lower sintering temperature for bulk materials.
Traditionally, tantalum diboride has been synthesized by
various methods. Crystalline TaB2 can be deposited using chem-
ical vapor deposition of TaCl5 and B2H6 in the temperature
range 773–1200 K.3 The deposits obtained with B2H6 have an
extremely small crystal size and contain amorphous boron when
the deposition temperature is below approximately 873 K but
are substoichiometric in boron above this temperature. TaB2
single crystals can be grown by floating zone and the Al solution
methods.4 Crystals grown from Al solution are hexagonal
needle-like TaB2 single crystals 0.3–3 mm in size without a
second phase. Tantalum diboride (TaB2) can be deposited on
a quartz substrate from a gas mixture of TaCl5, BCl3, H2, and
Ar at a temperature between 900 and 1300 ꢁC.5
˚
of the sample was examined on a Hitachi 800 transmission
electron microscope. The specific surface area of the sample
was measured by the Brunauer–Emmett–Teller (BET) method
(Model ASAP 2000, Micromeritics, Norcross, GA). The average
diameter of the powders (specific surface diameter) was estimat-
ed using the specific surface area. Thermogravimetric analysis
was performed on a thermal analyzer (Model: TA-50) below
1200 ꢁC in air at a rate of 10 ꢁCꢂminꢃ1 to study its thermal stabil-
ity and oxidation behavior.
Figure 1 shows the XRD pattern of the prepared sample.
There are eight obvious diffraction peaks in this pattern. And
all these diffraction peaks ((001), (100), (101), (002), (110),
(111/102), (200), and (201)) at different d space can be indexed
as hexagonal tantalum diboride. Refinement gives the cell
˚
˚
constants, a ¼ 3:090 A and c ¼ 3:266 A, which are consistent
˚
with values reported in the literature (a ¼ 3:080 A, c ¼
˚
3:270 A) (JCPDS card no. 75-1047). No evidence of impurities
such as metallic tantalum, tantalum oxides, or other tantalum
borides, can be found in this XRD pattern.
The morphology of the TaB2 sample was investigated by
transmission electron microscopy. The TEM image of the TaB2
is shown in Figure 2. In Figure 2a, the sample shows that it
consists of particles with an average diameter of 40 nm. These
particles exhibit slightly agglomerated particle morphology
due to the ultrafine size of the sample. Figure 2b shows a selected
area electron diffraction (ED) pattern of crystalline TaB2. The
ED pattern also confirms the crystallinity of TaB2, in which
In this paper, nanocrystalline tantalum diboride has been
synthesized via a simple inorganic solvent thermal route by
the reaction of metallic magnesium powders with tantalum pent-
oxide and boric acid in an autoclave at 650 ꢁC. In this route,
a mixture of sodium chloride and magnesium chloride acts as
inorganic solvent in the synthesis of nanocrystalline tantalum
diboride. Tantalum pentoxide is the tantalum source and boric
acid is the boron source.
101
100
In this research, a mixture of the same weight of anhydrous
sodium chloride and anhydrous magnesium chloride powder was
used as inorganic solvent in the reaction. All of the manipula-
tions were carried out in a dry glove box with flowing nitrogen
gas. In a typical experiment, 0.004 mol (about 1.768 g) of analyt-
ical grade tantalum pentoxide, 0.016 mol (about 0.989 g) of an-
alytical grade boric acid and 0.070 mol (about 1.701 g) of metal-
lic magnesium powders were put into a mortar, mixing with the
001
111
102
110
201
002
200
20
25
30
35
40
45
50
55
60
65
70
75
80
2 Theta / Degree
Figure 1. XRD pattern of the as-prepared sample.
Copyright Ó 2008 The Chemical Society of Japan

Chemistry Letters Vol.37, No.5 (2008)
511
TaB2 is very small. The surface energy of TaB2 particle becomes
higher in contrast with the bulk one. It can be oxidized by
oxygen at relatively lower temperature. But it has good thermal
stability below 600 ꢁC. When 0.5 g of TaB2 was added to the
1.0 molꢂLꢃ1 HCl or NaOH solution, its quality did not change
even after 48 h, and the XRD pattern was as before. Therefore,
the sample is stable either in acid medium or in alkali one.
In our experiments, as the temperature rose, the boric acid
decomposed generating boron trioxide and water vapor. The wa-
ter vapor reacts with metallic magnesium powders, forming H2
gas and magnesium hydroxide which subsequently decomposes
into magnesium oxide and water vapor. So the pressure in the
autoclave may be very high owing to the formed H2 gas. At the
reaction temperature, tantalum pentoxide and boron trioxide
can be reduced by metallic magnesium powders to produce
nascent tantalum and nascent boron. Because of the higher activ-
ity of nascent tantalum (Taꢀ) and boron (Bꢀ), they can react with
each other forming TaB2. The high pressure in the autoclave
would be helpful for reducing the reaction temperature and
enhancing the reaction speed. Therefore, the possible total
reaction process can be represented as the following,
Figure 2. TEM and ED images of the as-prepared TaB2.
50
45
40
35
30
25
20
15
10
5
0
-5
-10
100 200 300 400 500 600 700 800 900 1000 1100 1200
Temperature / °C
Figure 3. TGA curve heated under flowing air for the TaB2
sample.
Ta2O5 þ 4H3BO3 þ 17Mg ¼ 2TaB2 þ 17MgO þ 6H2 ð1Þ
the diffraction ring diameters and intensities again correspond
well to the hexagonal TaB2. These rings can be indexed as
(001), (100), (101), and (002).
In our present inorganic solvent thermal route, the mixture
of sodium chloride and magnesium chloride acts as a molten salt.
As the temperature rises, this mixture becomes liquid (melting
point: 425 ꢁC). So it can serve as an inorganic solvent to control
the reaction speed and the particle size. Since the diffusion
coefficients in the liquid state are higher than those in the solid
state, the molten salt can accelerate the kinetics by enhancing
diffusion. Besides, since the formed H2 gas may be very high
in the autoclave at the reaction temperature, it is helpful to carry
out this reaction in an autoclave. And the high pressure in the
autoclave may help to reduce the reaction temperature. The
molten salt can absorb the reaction enthalpy released from
this process and maintain a relative low reaction temperature.
And the product prepared in our route tended to be nanosized.
In summary, nanocrystalline TaB2 has been prepared via
a simple inorganic solvent thermal route in an autoclave at
650 ꢁC. It consists of particles with an average size of 40 nm.
The molten salt, the mixture of sodium chloride and magnesium
chloride, acting as inorganic solvent during the reaction, may
be important in the formation of nanocrystalline TaB2 with a
narrow particle size distribution. The product has good thermal
stability and oxidation resistance below 600 ꢁC.
We also studied the particle size of the sample by measuring
the specific surface area of the powders with the Brunauer–
Emmett–Teller (BET) method. In our result, the sample has
a value of 10.4 m2/g. So the average diameter of the powders
(specific surface diameter) is estimated according to the equation
(d ¼ 6=ꢃS, where d = the estimated average diameter of the
powders, ꢃ = the density of the sample, S = specific surface
area of the powders) using the specific surface area on the
assumption that the powder shape is spherical and that the
density of the powder is 11.15 gꢂcmꢃ3. The estimated average
diameter of the powders is about 52 nm which is a little larger
than the value observed from the TEM image, suggesting the
agglomeration of primary particles.
The thermal stability of these nanosized TaB2 powders was
investigated in air under different temperatures. A typical TGA
profile studied at temperatures below 1200 ꢁC in flowing air is
shown in Figure 3. From the TGA curve, we find that the weight
gain of the sample has not changed significantly below 600 ꢁC. A
slight weight loss indicates that this possibly arises from evapo-
ration of absorbed water on the surface of the sample. But the
quantity of the adsorbed water is very small. The onset of the
oxidation of the TaB2 sample is found to begin at about
600 ꢁC, which indicates that the sample is oxidized by oxygen
to form tantalum oxide and boron oxide. As the temperature
rises, the amount of the formed tantalum oxide and boron oxide
becomes bigger, suggesting that the oxidation rate of the sample
becomes faster. Finally, the sample can be oxidized thoroughly
at 1050 ꢁC. When the temperature exceeds 1050 ꢁC, the weight
gain becomes smaller and smaller. This is because B2O3 can
be evaporated at this high temperature (about 1000 ꢁC).6 Thus,
the evaporation of B2O3 results in the final weight gain being
less than the theoretical one (43.44%). We know that TaB2 is
recognized as the potential candidate for high-temperature struc-
tural applications. But in this paper, the size of the prepared
This work was supported by Department of Education of
Zhejiang Province of China under Grant no. 20070546.
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D. R. Lide, CRC Handbook of Chemistry and Physics, 75th
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E. Randich, Thin Solid Films 1980, 72, 517.
K. Nakano, Y. Kumashiro, E. Sakuma, J. Less Common
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S. Motojima, K. Sugiyama, J. Mater. Sci. 1979, 14, 2859.
A. Tampieri, A. Bellosi, J. Mater. Sci. 1993, 28, 649.
Published on the web (Advance View) April 5, 2008; doi:10.1246/cl.2008.510